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to the NWS Wakefield, VA ![nwslogo](img/nwslogo.gif)
Severe
Weather Information Page
Severe
Thunderstorm Watches and Warnings-
What is the difference
between a Severe
Thunderstorm Watch and a Severe
Thunderstorm Warning?
The
definition of a Severe
Thunderstorm Watch
Atmospheric
conditions over a large area, usually the size of a large state or multiple
states, support the development of severe thunderstorms for 4 hours or longer:
- The Storm
Prediction Center in Norman, Oklahoma issues a Severe
Thunderstorm Watch for
an area defined by a parallelogram usually 10,000 mi2 or larger,
which can cover more than one state. SEVERE
THUNDERSTORMS CAN OCCUR IN OR NEAR THE WATCH AREA.
- The Severe
Thunderstorm Watch is
usually issued before severe thunderstorms have developed, when conditions
in the atmosphere favor development of organized or relatively widespread
severe thunderstorms.
- Sometimes,
but not often, isolated severe thunderstorms can develop in an area not affected
by a Severe
Thunderstorm Watch or
an event can begin to occur just before the Severe
Thunderstorm Watch is
issued. There are numerous variables that must come together to produce severe
weather, some of which depend on features that can't be resolved within the
surface or upper-level data networks, but as soon as organized severe weather
is expected, a watch is issued.
- DEFINITION
OF ORGANIZED SEVERE WEATHER-Severe Thunderstorms that are either initiated
or supported by weather features characterized by well-defined temperature
or moisture gradients. In other words, if their is a warm or cold front at
the surface, midlevels, or upper-levels, and other variables look favorable
for thunderstorm development, then the movement of the front(s) could enhance
severe weather over a large area.
The
definition of a Severe Thunderstorm Warning
A Severe
Thunderstorm Warning is issued by the National Weather Service in Wakefield,
Virginia when severe weather is either occurring or will occur within a specified
time (usually within 60 minutes) within one or several counties:
- A thunderstorm on radar
is determined, after analysis to be capable of producing 3/4" hail or larger,
or produce damaging winds capable of blowing down trees or damage to permanant
structures.
- Someone has reported
severe weather and the thunderstorm is maintaining or increasing it's strength.
National
Weather Service definition of a severe thunderstorm-
- Hail
3/4" or larger, or basically the size of any coin or larger (a dime is 11/16"
which the NWS accepts as 3/4")
- Fallen
tree limbs with a minimum diameter of an average adult's wrist (if your over
18 and a tree limb is the same diameter as your wrist or larger, then it's
worth reporting).
- Living
trees uprooted or blown down ("Living trees" are important since dead, questionably
rotted trees require much less wind to blow them down).
- Any
part of a permanant, well-built structure damaged or destroyed
- Measured
wind gust from a calibrated anemometer of 58 MPH (50 knots) or greater
How can severe weather
occur before a warning is issued if the NWS is continuously monitoring the
radar?
*IMPORTANT
CONSIDERATIONS*
- The WSR-88D (Doppler
radar) is the BEST operational tool in the world for thunderstorm analysis.
- The NWS IS continuously monitoring the radar, even when the sky is clear!
- The radar offers over
100 "snapshots" or "products" (every 5 to 10 minutes depending on the type
of weather occurring) showing different parts and characteristics of thunderstorms.
Most of these products are composed of pixels, with minumum "size" or "resolution"
of one degree in width and depth, and 1km in length. The resolution decreases
with distance as the radar beam spreads out to as much as >1.5 nautical miles
at 120 nautical miles from the radar. A few products can have resolutions
of one degree in width and depth, and .5 nautical miles in length within 30
nautical miles of the radar.
- Due to the differences
in resolution of products at different distances, and the continuously changing
nature of thunderstorms, complete, 100% sampling and analysis of every thunderstorm
is impossible.
- Severe weather such
as 3/4" hail and downburst winds often BUT NOT ALWAYS occur on a scale of
one square nautical mile or less, which sometimes is smaller than the radar
product resolution. The organized severe weather outbreaks will ALMOST ALWAYS
produce severe weather over areas larger than one square nautical mile.
- Local surface temperature
inversions (temperatures increasing with height from the surface to some height
above the ground) can prevent damaging winds from reaching the ground.
- Public reports of severe
weather can be subjective at times. There is often a mix of hail sizes in
a thunderstorm, much of which is irregularly-shaped, so a wide variety of
perspectives from a wide variety of people will result in a wide variety of
reports for the same storm. Magnitude of wind is also a very subjective element
as is description of damage.
Why don't I witness damage
or large hail every time my county is under a Severe
Thunderstorm Warning?
- It
is important to remember that Severe Thunderstorm
Warnings are issued for COUNTIES, meaning severe weather is expected
SOMEWHERE WITHIN ONE OR SEVERAL COUNTIES.
- The
effects of any thunderstorm are very localized. Often, large hail or damaging
winds will affect a neighboring town, or maybe just part of a town, and could
just miss your location.
- The
continuously changing nature of thunderstorms sometimes results in a quick
shift in the movement of the thunderstorms, causing a shift in the track of
the severe weather, or even end the severe weather abruptly.
So what is the bottom
line, when it comes to Severe
Thunderstorm Watches and Severe
Thunderstorm Warnings ?
- !!!!
IF A SEVERE THUNDERSTORM WARNING IS ISSUED FOR YOUR COUNTY, SEEK SAFE SHELTER
AND TAKE NECESSARY PRECAUTIONS, BECAUSE YOU NEVER WANT TO TAKE THE CHANCE
OF BEING A VICTIM OF A SEVERE THUNDERSTORM.
- Listen to media, local
television stations, NOAA weather radio, or go to our website to access statements,
warnings and local storm reports. You will know what severe weather has occurred,
where it has occurred, where it is moving and when the threat of severe weather
has ended. It is always wise to stay updated on the latest information.
*The
importance of reporting severe weather to local
law enforcement or the National Weather Service.*
- The
National Weather Service needs to know what type of severe weather is occurring,
so that we can correlate radar signatures to observed severe weather.
- If
the radar indicates strong winds just above the ground surface, we need
to know if damage is occurring to trees or buildings. The damaging wind
doesn't always reach the ground surface, and sometimes the radar indicates
winds that are just on the borderline of possibly being damaging.
- If
the radar indicates hail, we need to know the size, so we can correlate
the radar signature with your reported hail size. Reporting hail of any
size is important since any hail falling at your location could mean larger
hail is falling nearby.
- YOUR
REPORTS MAY RESULT IN WARNINGS THAT COULD SAVE SOMEONE ELSE'S LIFE OR PROPERTY.
- WE
ARE GREATLY LACKING IN SPOTTERS IN MANY TOWNS IN OUR COUNTY WARNING AREA,
WHICH CONCERNS US.
- WE
NEED SPOTTERS IN EVERY TOWN AND CITY IN OUR COUNTY WARNING AREA. IF YOU HAVE
FRIENDS, RELATIVES, OR ANYONE
- YOU
KNOW IN ANY CROSSROADS OR SMALL TOWN, PLEASE HAVE THEM CONTACT Bill
Sammler AT OUR OFFICE TO BECOME A SPOTTER.
Tornado
Watches and Warnings-
What is the difference
between a Tornado Watch and a !TornadoWarning!?
The
definition of a Tornado Watch-
Atmospheric
conditions over a large area, usually the size of a large state or multiple
states, support the development of tornadoes for 4 hours or longer:
- The
Storm Prediction Center in Norman, Oklahoma issues a Tornado Watch for
an area defined by a parallelogram usually 10,000 mi2 or larger,
which can cover more than one state. TORNADOES
CAN OCCUR IN OR NEAR THE WATCH AREA.
- The Tornado
Watch is usually issued before tornadoes have developed, when conditions
in the atmosphere favor development of severe thunderstorms that could produce
tornadoes.
- Sometimes,
but not often, isolated severe thunderstorms with tornadoes can develop in
an area not affected by a Tornado Watch or
an event can begin to occur just before the Tornado
Watch is issued. There are numerous variables that must come together
to produce tornadoes, some of which depend on features that can't be resolved
within the surface or upper-level data networks, but as soon as tornadoes
are expected, a watch is issued.
- DEFINITION
OF A TORNADO - Rotating column of air, with some degree of vertical orientation, that
is not always visible, and causes damage to objects in contact with the Earth's
surface, or shows evidence that part or all of the circulation contacted the
earth's surface. For a MUCH more detailed definition, see Chuck Doswell's Definition
of a Tornado. A FUNNEL CLOUD is a visible tornadic circulation
that does not affect the Earth's surface.
The
definition of a !TornadoWarning!-
A !TornadoWarning! is issued by the National Weather Service in
Wakefield, Virginia when tornadoes are either occurring or will occur within
a specified time (usually within 60 minutes) within one or several counties:
- A thunderstorm on radar
is determined, after analysis to be capable of producing a tornado.
- Someone has spotted
a tornado, funnel cloud, or reported damage characteristic of a tornado.
How can a tornado occur
before a warning is issued if the NWS is continuously monitoring the radar?
*IMPORTANT
CONSIDERATIONS*
- The
WSR-88D (Doppler radar) is the BEST operational tool in the world for thunderstorm
analysis.
- The NWS IS continuously
monitoring the radar, even when the sky is clear!
- The radar offers over 100 "snapshots" or "products" (every 5 to 10 minutes
depending on the type of weather occurring) showing different parts and characteristics
of thunderstorms. Most of these products are composed of pixels, with minumum
"size" or "resolution" of one degree in width and depth, and 1km in length.
The resolution decreases with distance as the radar beam spreads out to as
much as >1.5 nautical miles at 120 nautical miles from the radar. A few products
can have resolutions of one degree in width and depth, and .5 nautical miles
in length within 30 nautical miles of the radar.
- Due to the differences in resolution of products at different distances, and
the continuously changing nature of thunderstorms, complete, 100% sampling
and analysis of every thunderstorm is impossible.
- The diameter of nearly all tornadoes are less than one mile wide, and large
majority of tornadoes are one-quarter mile or less in diameter, which is much
smaller than the radar product resolution. Rotation can occur in some particularly
severe thunderstorms, which may indicate the possibility of a tornado, and
can sometimes be too small to appear on any radar products.
- Local surface temperature inversions (temperatures increasing with height
from the surface to some height above the ground) can prevent the damaging
tornadic winds from reaching the ground.
- Public reports of tornadoes and funnel clouds can be subjective at times.
The base of a thunderstorm can be very ragged, and sometimes a piece of the
thunderstorm's base can be moving in such a way that someone may believe it
is a funnel cloud or tornado, when in fact, it may not be. Some downburst
wind damage can also sometimes be interpreted as tornadic damage.
Why don't I witness damage
or a tornado every time my county is under a !TornadoWarning!?
- It
is important to remember that !TornadoWarnings! are issued for COUNTIES, meaning a tornado is expected SOMEWHERE WITHIN ONE
OR SEVERAL COUNTIES.
- The effects of any thunderstorm are very localized, and tornadoes are on a
scale much smaller than a thunderstorm. Often, some effect of a thunderstorm
will occur in a neighboring town, or maybe just part of a town, and could
just miss your location.
- The continuously changing nature of thunderstorms sometimes results in a quick
shift in the movement of the thunderstorms, causing a shift in the track of
the severe weather, or even end the severe weather abruptly.
So what is the bottom line,
when it comes to Tornado Watches and !TornadoWarnings!?
- !!!
IF A !TornadoWarning! IS ISSUED FOR YOUR COUNTY,
SEEK SAFE SHELTER AND TAKE NECESSARY PRECAUTIONS, BECAUSE YOU NEVER WANT TO
TAKE THE CHANCE OF BEING A VICTIM OF A TORNADO.
- Listen to media, local
television stations, NOAA weather radio, or go to our website to access statements,
warnings and local storm reports. You will know what severe weather has occurred,
where it has occurred, where it is moving and when the threat of severe weather
has ended. It is always wise to stay updated on the latest information.
*The
importance of reporting severe weather to local law enforcement or the National
Weather Service.*
- The
National Weather Service needs to know what type of severe weather is occurring,
so that we can correlate radar signatures to observed severe weather.
- If
the radar indicates strong winds just above the ground surface, we need
to know if damage is occurring to trees or buildings. The damaging wind
doesn't always reach the ground surface, and sometimes the radar indicates
winds that are just on the borderline of possibly being damaging.
- If
the radar indicates hail, we need to know the size, so we can correlate
the radar signature with your reported hail size. Reporting hail of any
size is important since any hail falling at your location could mean larger
hail is falling nearby.
- Report
any rotation seen at the base of the thunderstorm, including funnel clouds,
so that we can correlate rotation we see on the radar to tornadoes, or developing
tornadoes. Many rotations,
that we see within thunderstorms on the radar, never become tornadoes, so
we need your help in reporting tornadoes and funnel clouds to us.
- YOUR
REPORTS MAY RESULT IN WARNINGS THAT COULD SAVE SOMEONE ELSE'S LIFE OR PROPERTY.
- WE
ARE GREATLY LACKING IN SPOTTERS IN MANY TOWNS IN OUR COUNTY WARNING AREA,
WHICH CONCERNS US.
- WE
NEED SPOTTERS IN EVERY TOWN AND CITY IN OUR COUNTY WARNING AREA. IF YOU HAVE
FRIENDS, RELATIVES, OR ANYONE YOU KNOW IN ANY CROSSROADS OR SMALL TOWN, PLEASE
HAVE THEM CONTACT Bill Sammler AT OUR OFFICE TO BECOME A SPOTTER.
Special
Marine Warnings-
The
definition of a Special
Marine Warning-
A Special
Marine Warning is issued by the National Weather Service in
Wakefield, VA when the following conditions exist in the Atlantic Waters between
Fenwick Island, DE and Currituck Beach Lighthouse in Corolla, NC, and the
Chesapeake Bay south of Smith Point:
- Convective wind gusts
of 34 knots or greater, usually associated with thunderstorms.
- Waterspouts
It is important to remember
that when a Severe
Thunderstorm Watch or Tornado
Watch is issued for coastal locations, the appropriate areas of the
Atlantic Waters and Chesapeake Bay are also included in the watches.
Why do waterspouts and
34+ knot wind gusts sometimes occur before a Special
Marine Warning is issued?
- Convective
wind gusts of 34 knots or greater
- A
large majority of Special
Marine Warnings are
issued before the strong winds reach the water. Often, strong or
severe thunderstorms will have a history of strong or damaging winds on
land, so the radar operator can anticipate the effects on the water.
- Most
commercial and pleasure boaters receive their marine weather forecasts and
information from the U.S. Coast Guard, listening to specific radio frequencies.
- After
the National Weather Service issues a Special
Marine Warning the U.S. Coast Guard (among others) receives
the warning and retransmits it over their specific radio frequency, a
process that can take up to 10 minutes. Usually thunderstorms with strong
convective wind gusts move at approximately the same speed as the wind
gusts themselves, which can be a considerable distance.
- Consider
a thunderstorm that is moving at 50 knots, it could track 9 miles in the
10 minutes it may take the U.S. Coast Guard to retransmit the warning.
Most small craft and pleasure boaters stay within 10 miles of the shore.
- Waterspouts
- Often
associated with elatively weak showers and thunderstorms, many of which
have little or no lightning and thunder.
- Often
associated with showers and thunderstorm with very little rain and/or affect
a very localized area.
- NWS
Doppler radar often has difficulty detecting such small, relatively weaker
rotations of waterspouts (compared to land-based tornadic circulations).
Why don't I witness winds
greater than or equal to 34 kt or a waterspout every time a Special
Marine Warning is issued for my marine area?
- Special
Marine Warnings are issued for areas between specific
points of the Atlantic Waters out to 20 nautical miles offshore, and the Chesapeake
Bay. The severe weather may be occurring in another part of the specified
area. The breakpoints are as follows:
- Chesapeake
Bay
- Smith
Point
- Windmill
Point
- New
Point Comfort
- Cape
Henry
- Atlantic
waters
- Fenwick
Island, DE
- Chincoteague,
VA
- Parramore
Island, VA
- Cape
Charles, VA
- Virginia
Beach, Virginia
- Virginia/North
Carolina border
- Currituck
Beach Lighthouse in Corolla, NC
- The continuously changing
nature of thunderstorms sometimes results in a quick shift in the movement
of the thunderstorms, causing a shift in the track of the severe weather,
or even end the severe weather abruptly.
So what is the bottom
line when it comes to Special
Marine Warnings?
- !!!
IF A SPECIAL MARINE
WARNING IS ISSUED FOR YOUR LOCATION, SEEK SAFE HARBOR, AND PREPARE
FOR STRONG WINDS, CHOPPY SEAS, AND/OR WATERSPOUTS, BECAUSE YOU NEVER WANT
TO TAKE THE CHANCE OF BEING A VICTIM OF A SEVERE THUNDERSTORM OR WATERSPOUT.
- Listen to media, local
television stations, NOAA weather radio, or go to our website to access statements,
warnings and local storm reports. You will know what severe weather has occurred,
where it has occurred, where it is moving and when the threat of severe weather
has ended. It is always wise to stay updated on the latest information.
*The
importance of reporting severe weather to local law enforcement, The Coast
Guard, or the National Weather Service.*
- The
National Weather Service needs to know what type of severe weather is occurring,
so that we can correlate radar signatures to observed severe weather.
- If
the radar indicates strong winds just above the ground surface, we need
to know if damagin gwinds are occurring, that could capsize boats and build
waves. The damaging wind doesn't always reach the water surface, and sometimes
the radar indicates winds that are just on the borderline of possibly being
damaging.
- Report
any rotation seen at the base of the thunderstorm, including funnel clouds
and waterspouts, so that we can correlate rotation we see on the radar to
waterspouts, or developing waterspouts. Waterspouts
are somtimes too small or weak to be detected by radar. Additionally, many
rotations, that we see within thunderstorms on the radar, never become waterspouts,
so we need your help in reporting waterspouts and funnel clouds to us.
- YOUR
REPORTS MAY RESULT IN WARNINGS THAT COULD SAVE A FELLOW BOATER'S LIFE OR PROPERTY.
- WE
ARE GREATLY LACKING IN MARINE OBSERVATIONS IN THE CHESAPEAKE BAY AND ATLANTIC
WATERS, WHICH CONCERNS US.
WE
NEED MARINE OBSERVATIONS OVER OUR ENTIRE MARINE AREA. IF YOU HAVE FRIENDS, RELATIVES,
OR ANYONE YOU KNOW THAT GOES BOATING ON THE CHESAPEAKE BAY OR ATLANTIC WATERS,
PLEASE ENCOURAGE THEM TO REPORT WIND AND SEA CONDITIONS TO THE COAST GUARD,
SPECIFICALLY REQUESTING THAT THE INFORMATION BE RELAYED TO THE NATIONAL WEATHER
SERVICE.
Flash
Flood Watches and Warnings-
What is the difference
between a Flood
Watch and a Flash
Flood Watch?
- The major difference
between a Flood Watch and
a Flash Flood Watch, is the duration
of the rainfall needed to produce flooding.
- A Flood
Watch implies a longer period of relatively lighter rains, adding
up to a large amount of rain, while a Flash
Flood Watchimplies a shorter period of heavier rain. Generally, if
flooding is expected within 6 hours of the onset of rain, a Flash Flood
Watch is most appropriate.
- If flooding is expected
beyond 6 hours after the onset of rain, a Flood
Watch is most appropriate. The 6 hour threshold is very subjective
and varies from forecaster to forecaster. REGARDLESS OF THE TYPE OF WATCH,
FLOODING CAN OCCUR AT ANY TIME, SO BE ALERT.
- Another difference
between Flood Watches and Flash
Flood Watches is the type of flooding it implies.
- Flash flooding by
definition suggests rapidly rising water, such as a surge of water heading
rapidly downstream in a creek or small river. It could also be rapidly rising
water on roadways, which can cause motorists to become stranded in vehicles,
or even worse, washed into creeks and small rivers due to rapid runoff.
- Longer-term flooding
implies a slower or more steady rise in the water levels of creeks, streams
and larger rivers. Roads can also become flooded, but it is usually more
gradual, allowing motorists to monitor conditions more closely.
What is the difference
between a Flash Flood Watch and a Flash
Flood Warning?
The
definition of a Flash Flood Watch-
Atmospheric
conditions over a large area, varying in size from multiple counties to multiple
states, support the development of heavy rain and/or thunderstorms that are
capable of producing flash flooding:
- The National Weather
Service in Sterling, Virginia and Raleigh, North Carolina issue Flash
Flood Watches for
an area consisting of multiple counties, which can cover more than one state.
Flash flooding generally occurs within 6 hours of the beginning of the heavy
rain.
- The Flash
Flood Watch is usually issued before heavy rain and/or thunderstorms
have developed, when conditions in the atmosphere favor development of heavy
rain and/or thunderstorms over an area consisting of multiple counties.
- Sometimes, but not
often, flash flooding can occur in an area not affected by a Flash
Flood Watch or an event can begin to occur just before the Flash
Flood Watch is issued. There are numerous variables that must
come together to produce flash flooding, some of which depend on features
that can't be resolved within the surface or upper-level data networks, but
as soon as flash flooding is expected, a watch is issued.
The
definition of a Flash Flood Warning-
A Flash Flood Warning is issued by the National Weather Service in Wakefield,
Virginia when heavy rain has been occurring, and flash flooding is either
occurring or will occur within a specified time (usually within 60 minutes),
within one or several counties:
- A thunderstorm on radar
is determined, after analysis, to be capable of producing rainfall rates that
exceed our flash flood guidance.
- Someone has reported
flash flooding and the heavy rain and/or thunderstorm rainfall rates are not
decreasing.
National
Weather Service definition of flash flooding-
- Rapid rise (generally
within 3 to 6 hours from the onset of rain) of water in creeks or rivers that
produces life-threatening flooding.
- Major roads closed
or impassable.
- Dam break.
How can flash flooding
occur before a warning is issued if the NWS is continuously monitoring the
radar?
*IMPORTANT
CONSIDERATIONS*
- The WSR-88D (Doppler
radar) is the BEST operational tool in the world for thunderstorm analysis.
- The NWS IS continuously monitoring the radar, even when the sky is clear!
- The radar offers over
100 "snapshots" or "products" (every 5 to 10 minutes depending on the type
of weather occurring) showing different parts and characteristics of thunderstorms.
Most of these products are composed of pixels, with minumum "size" or "resolution"
of one degree in width and depth, and 1km in length. The resolution decreases
with distance as the radar beam spreads out to as much as >1.5 nautical miles
at 120 nautical miles from the radar. A few products can have resolutions
of one degree in width and depth, and .5 nautical miles in length within 30
nautical miles of the radar.
- Due to the differences
in resolution of products at different distances, and the continuously changing
nature of thunderstorms, complete, 100% sampling and analysis of every thunderstorm
is impossible.
- The radar estimates
rainfall, but can only offer a rough calculation of how much rain has fallen
over any given area. Even though rainfall estimation is questionable, the
radar is the BEST operational tool in the world for rainfall estimation..
- Public reports of life-threatening
flooding can be subjective at times. Nuisance urban flooding and water across
secondary roads can be described in a variety of ways.
Why don't I witness flash
flooding every time my county is under a Flash
Flood Warning?
- It is important to
remember that Flash Flood Warnings are
issued for COUNTIES, meaning flash flooding is expected SOMEWHERE WITHIN ONE
OR SEVERAL COUNTIES.
- The effects of any
thunderstorm or heavy rain band are very localized. Often, flooding thunderstorm
rains will affect a neighboring town, or maybe just part of a town, and could
just miss your location.
- The continuously changing
nature of thunderstorms sometimes results in a quick shift in the movement
of the thunderstorms, causing a shift in the track of the heavy rain, or even
end the heavy rain abruptly.
So what is the bottom
line, when it comes to Flash Flood Watches and Flash
Flood Warnings ?
- !!!
- IF A FLASH FLOOD WARNING IS ISSUED FOR YOUR COUNTY, SEEK HIGH GROUND AND
TAKE NECESSARY PRECAUTIONS, BECAUSE YOU NEVER WANT TO TAKE THE CHANCE OF BEING
A VICTIM OF FLASH FLOODING.
- Listen to media, local
television stations, NOAA weather radio, or go to our website to access statements,
warnings and local storm reports. You will know what heavy rain and flash
flooding has occurred, where it has occurred, where it is moving and when
the threat of flash flooding has ended. It is always wise to stay updated
on the latest information.
*The
importance of reporting flash flooding and 2.00"+ rainfall in less than 3
hours to local law enforcement or the National Weather Service.*
- The
National Weather Service needs to know what type of heavy rain and flash flooding
is occurring, so that we can correlate radar signatures to observed heavy
rain and flash flooding.
- If
the radar indicates a thunderstorm or heavy rain that is producing large
rainfall estimates, we need to know if the radar estimates are accurate,
and if flash flooding is occurring. Flash flooding doesn't always occur
when very heavy rain is observed, and sometimes the radar overestimates,
or underestimates rainfall.
- If
the radar indicates heavy rain, we need to know how much has fallen, so
we can correlate radar signatures with your reported rainfall amount. Reporting
flash flooding on roadways, or rising creeks and rivers is very important,
it could mean that more serious flooding could be nearby, or developing.
- YOUR
REPORTS MAY RESULT IN WARNINGS THAT COULD SAVE SOMEONE ELSE'S LIFE OR PROPERTY.
- WE
ARE GREATLY LACKING IN SPOTTERS IN MANY TOWNS IN OUR COUNTY WARNING AREA,
WHICH CONCERNS US.
WE
NEED SPOTTERS IN EVERY TOWN AND CITY IN OUR COUNTY WARNING AREA. IF YOU HAVE
FRIENDS, RELATIVES, OR ANYONE YOU KNOW IN ANY CROSSROADS OR SMALL TOWN, PLEASE
HAVE THEM CONTACT Bill Sammler AT OUR OFFICE TO BECOME A SPOTTER.
Flood
Watches and Warnings-
What
is the difference between a Flood
Watch and a Flash
Flood Watch?
- The major difference
between a Flood Watch and
a Flash Flood Watch, is the duration
of the rainfall needed to produce flooding.
- A Flood
Watch implies a longer period of relatively lighter rains, adding
up to a large amount of rain, while a Flash
Flood Watch implies a shorter period of heavier rain. Generally,
if flooding is expected within 6 hours of the onset of rain, a Flash
Flood Watch is most appropriate.
- If
flooding is expected beyond 6 hours after the onset of rain, a Flood
Watch is most appropriate. The 6 hour threshold is very subjective
and varies from forecaster to forecaster. REGARDLESS OF THE TYPE OF WATCH,
FLOODING CAN OCCUR AT ANY TIME, SO BE ALERT.
- Another difference
between Flood Watches and Flash
Flood Watches is the type of flooding it implies.
- Flash flooding by
definition suggests rapidly rising water, such as a surge of water heading
rapidly downstream in a creek or small river. It could also be rapidly rising
water on roadways, which can cause motorists to become stranded in vehicles,
or even worse, washed into creeks and small rivers due to rapid runoff.
- Longer-term flooding
implies a slower or more steady rise in the water levels of creeks, streams
and larger rivers. Roads can also become flooded, but it is usually more
gradual, allowing motorists to monitor conditions more closely.
- The rivers for which
NWS Wakefield, Virginia issues River Flood
Warnings are as follows:
- Rivanna River at
Palmyra, affecting Fluvanna County
- James River at Cartersville,
Westham and City Locks, affecting Fluvanna, Goochland, Powhatan, Henrico
and Chesterfield Counties including the City of Richmond
- Appomattox River
at Farmville, Matoax, Matoaca and Petersburg, affecting Prince Edward, Cumberland,
Amelia, Powhatan, Chesterfield and Dinwiddie Counties including the cities
of Farmville and Petersburg
- Nottoway River at
Rawlings, Sebrell, and Stony Creek, affecting Brunswick, Dinwiddie, Greensville,
Sussex and Southampton Counties
- Meherrin River at
Lawrenceville and Emporia, affecting Brunswick
and Greensville Counties including the city of Emporia.
What is the difference
between a Flood Watchand a Flood
Warning?
The
definition of a Flood Watch-
Atmospheric conditions
over a large area, varying in size from multiple counties to multiple states,
support the development of heavy rain and/or thunderstorms that are capable
of producing flooding:
- The National Weather
Service in Sterling, Virginia and Raleigh, North Carolina issue Flood
Watches for an area consisting of multiple counties, which can
cover more than one state. Flooding generally occurs 6 to 24 hours after the
beginning of the heavy rain.
- The Flood Watch
is usually issued before heavy rain and/or thunderstorms have developed, when
conditions in the atmosphere favor development of heavy rain and/or thunderstorms
over an area consisting of multiple counties.
- Sometimes, but not
often, flooding can occur in an area not affected by a Flood
Watch or an event can begin to occur just before the Flood
Watch is issued. There are numerous variables that must come together
to produce flooding, some of which depend on features that can't be resolved
within the surface or upper-level data networks, but as soon as flooding is
expected, a watch is issued.
The
definition of a Flood Warning-
A Flood Warning is issued by the National Weather Service in Wakefield,
Virginia when heavy rain has been occurring, and flooding is either occurring
or will occur within a specified time (usually within 60 minutes), within
one or several counties:
- A thunderstorms and/or
heavy rain on radar are determined, after analysis, to be capable of producing
rainfall rates that exceed our flood guidance.
- Storm-total rainfall
estimates from the radar exceed our flood guidance.
- Someone has reported
flooding and the heavy rain and/or thunderstorm rainfall rates are not decreasing.
National
Weather Service definition of flooding-
- Gradual rise (generally
6 hours or more from the onset of rain) of water in creeks or rivers that
produces life-threatening flooding.
- Major roads closed
or impassable.
How can flooding occur
before a warning is issued if the NWS is continuously monitoring the radar?
*IMPORTANT
CONSIDERATIONS*
- The WSR-88D (Doppler
radar) is the BEST operational tool in the world for thunderstorm analysis.
- The NWS IS continuously monitoring the radar, even when the sky is clear!
- The radar offers over
100 "snapshots" or "products" (every 5 to 10 minutes depending on the type
of weather occurring) showing different parts and characteristics of thunderstorms.
Most of these products are composed of pixels, with minumum "size" or "resolution"
of one degree in width and depth, and 1km in length. The resolution decreases
with distance as the radar beam spreads out to as much as >1.5 nautical miles
at 120 nautical miles from the radar. A few products can have resolutions
of one degree in width and depth, and .5 nautical miles in length within 30
nautical miles of the radar.
- Due to the differences
in resolution of products at different distances, and the continuously changing
nature of thunderstorms, complete, 100% sampling and analysis of every thunderstorm
is impossible.
- The radar estimates
rainfall, but can only offer a rough calculation of how much rain has fallen
over any given area. Even though rainfall estimation is questionable, the
radar is the BEST operational tool in the world for rainfall estimation..
- Public reports of life-threatening
flooding can be subjective at times. Nuisance urban flooding and water across
secondary roads can be described in a variety of ways.
Why don't I witness flooding
every time my county is under a Flood Warning?
- It is important to
remember that Flood Warningsare issued
for COUNTIES, meaning flooding is expected SOMEWHERE WITHIN ONE OR SEVERAL
COUNTIES.
- If a River
Flood Warning is issued, flooding is expected to be confined primarily
along the rivers.
- The effects of any
thunderstorm or heavy rain band are very localized. Often, flooding thunderstorm
rains will affect a neighboring town, or maybe just part of a town, and could
just miss your location.
- The continuously changing
nature of thunderstorms, or areas of heavy rain, sometimes results in a quick
shift in the movement of the thunderstorms, causing a shift in the track of
the heavy rain, or even end the heavy rain abruptly.
So what is the bottom line,
when it comes to Flood Watches and Flood
Warnings ?
- !!!
- IF A FLOOD WARNING IS ISSUED FOR YOUR COUNTY, OR FOR A NEARBY RIVER, SEEK
HIGH GROUND AND TAKE NECESSARY PRECAUTIONS, BECAUSE YOU NEVER WANT TO TAKE
THE CHANCE OF BEING A VICTIM OF FLOODING.
- Listen to media, local
television stations, NOAA weather radio, or go to our website to access statements,
warnings and local storm reports. You will know what heavy rain and flooding
has occurred, where it has occurred, where it is moving and when the threat
of flooding has ended. It is always wise to stay updated on the latest information.
*The
importance of reporting flooding and 2.00"+ storm-total rainfall to local
law enforcement or the National Weather Service.*
- The
National Weather Service needs to know what type of heavy rain and flooding
is occurring, so that we can correlate radar signatures to observed heavy
rain and flooding.
- If
the radar indicates a thunderstorm or heavy rain that is producing large
rainfall estimates, we need to know if the radar estimates are accurate,
and if flooding is occurring. Flooding doesn't always occur when very heavy
rain is observed, and sometimes the radar overestimates, or underestimates
rainfall.
- If
the radar indicates heavy rain, we need to know how much has fallen, so
we can correlate radar signatures with your reported rainfall amount. Reporting
flooding on roadways, or rising creeks and rivers is very important, it
could mean that more serious flooding could be nearby, or developing.
- YOUR
REPORTS MAY RESULT IN WARNINGS THAT COULD SAVE SOMEONE ELSE'S LIFE OR PROPERTY.
- WE
ARE GREATLY LACKING IN SPOTTERS IN MANY TOWNS IN OUR COUNTY WARNING AREA,
WHICH CONCERNS US.
WE NEED
SPOTTERS IN EVERY TOWN AND CITY IN OUR COUNTY WARNING AREA. IF YOU HAVE FRIENDS,
RELATIVES, OR ANYONE YOU KNOW IN ANY CROSSROADS OR SMALL TOWN, PLEASE HAVE THEM
CONTACT Bill Sammler AT OUR OFFICE
TO BECOME A SPOTTER.
Coastal
Flood Watches and Warnings-
What is the difference
between a Coastal Flood Watch and
a Coastal Flood Warning?
The
definition of a Coastal Flood Watch-
Strong
onshore winds are expected to produce minor coastal flooding, and possibly
moderate to severe coastal flooding, along some portion of the Atlantic or
Chesapeake Bay shoreline:
- The National Weather
Service in Sterling, Virginia and Raleigh, North Carolina issue Coastal
Food Watches for an area consisting of multiple coastal counties,
which can cover more than one state. Coastal flooding generally occurs during
the high tide after the Coastal Flood Watch is
issued.
- Sometimes, but not
often, minor coastal flooding can occur in an area not affected by a Coastal
Flood Watch or an event can begin to occur just before theCoastal
Flood Watch is issued. There are numerous variables that must
come together to produce coastal flooding, some of which depend on features
that can't be resolved within the surface or upper-level data networks, but
as soon as coastal flooding is expected, a watch, warning or statement is
issued.
The
definition of a Coastal Flood Warning-
A Coastal Flood Warning is issued by the National Weather Service in
Sterling, Virginia or Raleigh, North Carolina when moderate or greater coastal
flooding is either occurring or will occur during high tides, within one or
several coastal counties:
- An extended period
of strong onshore winds can push water into the coast much like a hurricane
storm surge.
- Strong north winds
can push Chesapeake Bay water into Hampton Roads.
- Strong east or southeast
winds can push Chesapeake Bay water into the mouth of the York, James, Rappahannock
and Potomac Rivers, producing flooding on the Peninsula, Middle Peninsula
and Northern Neck
- Strong west or east
winds can produce flooding along the north shore of the Albemarle Sound.
National
Weather Service definition of minor, moderate and severe coastal flooding-
- Minor flooding is described
as tides 2 feet or less above minimum flood stage, causing nuisance flooding.
- Moderate flooding is
described as tides 2 to 3 feet above minimum flood stage, often producing
some damage to permanent structures along the water's edge, closing some roads
and producing significant beach erosion.
- Major or Severe flooding
is described by tides 4 feet or more above minimum flood stage causing significant
damage to buildings, widespread road closures and significant erosion.
What are some of the estimated minimum flood levels along the Maryland, Virginia and North Carolina Shores?
Tide levels are measured
feet above Mean Lower Low Water (MLLW), which is
the average of all of the lowest tides each day (there are usually 2 low tides
each day, one of which is lower than the other). Mean Sea Level (MSL) is the
average of all high and low tides.
- Virginia - Atlantic
Coast
- Chincoteague - 3.0
feet MLLW
- Watchapreague - 7.5
feet MLLW
- Cape Charles - 3.0
feet MLLW
- Virginia - Eastern
Chesapeake Bay
- Tangier Island -
2.0 feet MLLW
- Muddy Creek - 2.0
feet MLLW
- Virginia - Western
and Southern Chesapeake Bay
- Urbanna - 3.0 feet
MLLW
- Gwynn Island - 5.5
feet MLLW
- Gloucester Point
- 5.0 feet MLLW
- Yorktown - 4.0 feet
MLLW
- Norfolk - 5.0 feet
MLLW
- Maryland
- Ocean City - 5 to
6 feet MLLW
- North Carolina - Atlantic
Coast of Currituck County
- Corolla - 4.0 to
5.0 feet MLLW
- North Carolina - Albemarle
and Currituck Sounds (no tidal effects on the sounds)
- Edenton - 4.0 feet
- Harvey Point - 4.0
feet
- Hertford - 4.7 feet
- Winfall - 2.8 feet
- Elizabeth City -
3.5 feet
- Wade Point 2.7 feet
- Tip of Camden County
- 3.4 feet
- Knotts Island area
- 3.0 feet
If you would like historical
tide data for the Hampton Roads area from past significant storms, click
here .
Why don't I witness flooding
every time my county is under a Coastal
Flood Warning?
- It is important to
remember that Coastal Flood Warnings are
issued for COUNTIES, meaning flooding is expected SOMEWHERE WITHIN ONE OR
SEVERAL COUNTIES.
- If a Coastal
Flood Warning is issued, flooding is expected to be confined primarily
along the immediate coast in unprotected areas. Some areas of the Virginia
Eastern Shore are protected by smaller bays and islands. All counties along
the coast of Virginia, Maryland and North Carolina have their areas that are,
to some degree, protected, and some areas more prone to coastal flooding.
So what is the bottom line,
when it comes to Coastal Flood Watches and Coastal
Flood Warnings ?
- !!!
- IF A COASTAL FLOOD WARNING IS ISSUED FOR YOUR COUNTY, SEEK HIGH GROUND AND
TAKE NECESSARY PRECAUTIONS, BECAUSE YOU NEVER WANT TO TAKE THE CHANCE OF BEING
A VICTIM OF COASTAL FLOODING.
- Listen to media, local
television stations, NOAA weather radio, or go to our website to access statements,
warnings and local storm reports. You will know what coastal flooding has
occurred, where it has occurred, and when the threat of coastal flooding has
ended. It is always wise to stay updated on the latest information.
*The
importance of reporting coastal flooding to local law enforcement or the National
Weather Service.*
- The
National Weather Service needs to know what type of coastal flooding is occurring,
so that we can correlate winds and tide levels to actual flooding.
- Once
we can correlate flooding in certain areas with tide levels and winds, we
can taylor coastal flood threat information to more specific locations in
future storms.
- Reporting
flooding on roadways, or damage to homes and businesses is very important.
It could mean that more serious flooding could be nearby, or developing.
- YOUR
REPORTS MAY RESULT IN WARNINGS THAT COULD SAVE SOMEONE ELSE'S LIFE OR PROPERTY.
- WE
ARE GREATLY LACKING IN SPOTTERS IN MANY TOWNS IN OUR COUNTY WARNING AREA,
WHICH CONCERNS US.
WE
NEED SPOTTERS IN EVERY TOWN AND CITY IN OUR COUNTY WARNING AREA. IF YOU HAVE
FRIENDS, RELATIVES, OR ANYONE YOU KNOW IN ANY CROSSROADS OR SMALL TOWN, PLEASE
HAVE THEM CONTACT Bill Sammler AT
OUR OFFICE TO BECOME A SPOTTER.
References
Weather
Service Operations Manual, Chapter C-40
Federal
Meteorological Handbook No. 11 - Doppler Radar Meteorological Observations,
Parts A-D. U.S Department of Commerce, National Oceanic and Atmospheric Administration,
Office of the Federal Coordinator for Meteorological Services and Supporting
Branch, Washington, DC.
Advanced
Spotter's Training Guide, NOAA PA 92055 (Available from local NWS Offices)
Thunderstorms
and Lightning, the Underrated Killers, NOAA PA 92053, January 1994 (Available
from local NWS Offices)
Tornadoes,
Nature's Most Violent Storms, NOAA PA 92052, September 1992 (Available from
local NWS Offices)
Marine
Information and Services Provided by the National Weather Service in Wakefield,
Virginia (Available from the NWS in Wakefield, Virginia)
Flash
Floods and Floods, the Awesome Power, NOAA PA 92050, July 1992 (Available
from local NWS Offices)
Link
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