|
Frequently Asked Questions
FREQUENTLY
ASKED QUESTIONS
|
When
you ask for WEATHER, do you want: What it will be? What it
is right now? Or what it was last week/month? |
Here
are definitions to help you choose where to find what you want.
FORECAST = What the weather is
expected to be in the future. The short range forecast covers a 1-2
day period. The extended forecast covers a 3-5 day period.
OBSERVATIONS = Current weather conditions
(temperature, precipitation, dew point, relative humidity, and wind)
reported from a specific location - usually updated hourly.
CLIMATE = 1, 6, and 24
hour precipitation maps and tables, as well as latest 24 hour highs
and lows.
HISTORICAL DATA = Archive
of past maximum, minimum, and 24 hour precipitation for selected stations
(from May 1997 to present). Data is arranged in a daily format. Also
station climate summaries, based on data from past years. |
|
|
What
is GMT, Zulu (Z), and UTC time? |
GMT
stands for Greenwich Mean Time and UTC stands for Universal Coordinated
Time. All three refer to same time, which is a standard used around
the world by the military and other agencies. It can also be thought
of as the time at the Prime Meridian, or zero degrees longitude. Greenwich
is a borough of London that the Prime Meridian runs through. Hence
the name Greenwich time. In short, to convert this time to PDT, subtract
7 hours. To convert to PST, subtract 8 hours. Here's a few examples:
Convert 2030Z to PDT:
2030 - 7 hours = 1330 (1:30 pm PDT)
Convert 0000 GMT October 8, 2001 to PST:
Note: 0000 GMT is equivalent to 2400 GMT
2400 - 8 hours = 1600 (4:00 pm PST October 7 , 2001)
Remember: This would be midnight in England, so that's why it's October
8 in GMT and October 7 in PST.
Convert 0830 UTC to PDT:
0830 - 7 hours = 0130 (1:30 am PDT) |
|
|
Where
can I get more detailed climate information? |
Aside
from our Climate
Page, there are several other sources of climate information:
The National Climatic Data Center
151 Patton Ave, Room 120
Asheville, NC 28801-5001
Phone: (828)271-4800
Fax: (828)271-4876
Homepage: http://www.ncdc.noaa.gov/ncdc.html
The Western Region Climate Center
Phone: (775)674-7010
Fax: (775)674-7016
Homepage: http://www.wrcc.dri.edu/index.html
The State Climatologist
Phone: (916)574-2614
Keep in mind that it usually takes several weeks for data to be processed
and made available for public use. Thus, if you are looking for data
that is only a couple weeks old, you might be out of luck. On the
other hand, if all you need is a high or low temperature, or how much
precipitation fell at one of our observing sites in Southern California,
first try our online archive
of temperature and precipitation data. If you still can't find
what you need, contact our office during normal business hours at
(805) 988-6610.
Please be aware that if you call during a particularly active weather
situation, don't be offended if we ask you call back at a later time.
This is because we need to focus our resources on the situation at
hand. We are glad to fulfill your request, but
our mission of saving lives and property must come first! |
|
|
What
is the National Weather Service and what do we do? |
For
a thorough description of the NWS and it's mission, go to the
National Weather Service Headquarters homepage. |
|
|
What
are Short Term Forecasts? |
A Short Term
Forecast is issued only during significant weather. Generally, the
Short Term Forecast will discuss current thunderstorms, showers,
fronts, fog, and wind. If these phenomena become more of a threat
to life or property, appropriate watches, warnings, or advisories
will be issued. The Short Term Forecast is intended to replace the
Special Weather Statement during weather events. Special Weather
Statements will still be issued for weather events that are more
than 6 hours away.
EXAMPLE SHORT-TERM
FORECAST (NOW, FPUS46)
LAXNOWLAX
TTAA00 KLAX 011754
SHORT TERM FORECAST
NATIONAL WEATHER SERVICE OXNARD CA
1000 AM PST FRI NOV 1 2000
CAZ041-046-047-054-059-087-088-020000-
LOS ANGELES COUNTY-
1000 AM PST FRI NOV 1 2000
.NOW...
LOW CLOUDS AND FOG WILL BURN OFF TO THE COAST
BY MIDDAY. SKIES OVER LA COUNTY THIS AFTERNOON WILL BE MOSTLY SUNNY
INLAND AND PARTLY CLOUDY ALONG THE COAST. HIGH TEMPERATURES WILL
BE IN THE LOWER 70S AT THE COAST...THE LOWER 80S DOWNTOWN AND IN
THE MOUNTAINS...AND MID 80S TO MID 90S OVER THE VALLEYS.
$$
CAZ040-044-045-053-020000-
VENTURA COUNTY-
1000 AM PST FRI NOV 1 2000
.NOW...
LOW CLOUDS AND FOG WILL BURN OFF THE COAST
BY MIDDAY.
$$
|
|
|
How
to read the pressure gradient tables |
First of all,
what is a pressure gradient? The term "gradient" implies a difference.
Therefore, simply put, "pressure gradient" means "pressure difference."
Meteorologists use pressure gradients in many ways, but primarily
for determining wind speed and direction. Without going into too
much detail or technicalities, in the upper atmosphere, air generally
flows clockwise around areas of high pressure and counter-clockwise
around areas of low pressure. However, at the surface, due to friction
and other smaller scale effects, air flows more from high pressure
to low pressure.
In Southern
California, pressure gradients are critical in forecasting the weather.
When reading weather related documents, you will often see Meteorologists
refer to "onshore" and "offshore" flow. Onshore flow refers to air
flow from the water onto the shore. In other words, west to east.
A majority of the time, the pressure gradients in Southern California
are onshore. Why is this? Well, that would take a little while to
explain in detail. However, to put it simply, climatologically speaking,
there is an area of high pressure in the eastern Pacific and low
pressure in the desert southwest. The low pressure in the desert
intensifies during the day because when the air heats up, it rises,
causing the air pressure at the surface to lower. This increases
the pressure gradient. When the gradient increases, so does the
wind. This is why we so often get afternoon sea-breezes at the coast
and why it seems to increase as the day goes along. The mountains
provide a very effective block, keeping the cooler marine air to
the west. However, the breezes due filter through some of the passes,
which is why the Antelope Valley gets breezy in the afternoons.
"Offshore"
flow occurs when the pressure is higher over the land than over
the ocean, often resulting in north-east winds over Southern California.
This is also referred to as the Santa Ana effect, or the Santa Ana
winds. The terrain here often enhances the offshore breezes because
as the winds are forced through the narrow canyons, they increase
in speed. This is referred to as the Bernoulli effect. This is why
during Santa Ana's, some places will have winds exceeding 50 mph
and others will have almost nothing.
Having said
that, what do the three columns in the table represent? The first
column is the actual difference in the pressure between the two
listed stations, in millibars (mb). So, for example, a 3.2 mb. gradient
means that the surface pressure at LAX (Los Angeles International
Airport), is 3.2 mb. higher than at (DAG) Barstow-Daggett airport.
If the value was -3.2 (negative 3.2 mb.), that would mean the pressure
at DAG is 3.2 mb. higher than at LAX.
The second
column and third columns are a little more complicated, but equally
as important. The second column is the difference between the gradient
today and the gradient yesterday, at the same time. This is also
referred to as the 24 hour change. So for example, if the number
in the first column today at 16z is 3.2, and the number yesterday
at 16z was 2.4, then the value in the second column today would
be 0.8. To put it another way, if the gradient at 16z today was
3.2, and the second column showed a 0.8, then that would mean that
the gradient yesterday at 16z was 2.4. Meteorologists use the second
column to determine trends; that is, whether the gradient is becoming
more onshore or offshore. Using the above example, the 0.8 would
indicate that the gradient is becoming slightly more onshore (an
onshore trend). A negative value would imply an offshore trend.
Keep in mind, though, that an offshore trend doesn't necessarily
imply offshore flow. It can simply imply a weaker onshore flow.
The third column
is even more complicated. To put it as simply as possible, it is
the difference in the difference. In mathematical terms, it is an
acceleration, or the second derivative. More definitively, it is
the difference between the 24 hour change now and the 24 hour change
3 hours ago. So, for example, if the 24 hour change at 16z was 1.4
and at 13z is was 2.4, the third column value would be -1.0. What
does this tell us? It tells us whether the change is increasing
or decreasing. If, for example, we have offshore flow, and the third
column is negative, this would tell us that the offshore flow is
intensifying (becoming more negative).
|
|
|
What
do each of the columns represent in the marine observations listing
|
Here is an example:
- 46054
2518 / 3224 / 15-17 / 57 / 54 / 1018.3 W. SANTA BARBARA CHNL
- 46054 - Station
ID number
- 2518 - Observation
time, day/hour (UTC) (in this case, Feb. 25 at 1800Z)
- 3224 - Wind
direction and speed (here, from 320 degrees at 24 kts.)
- 15-17 - Swell
height and period (here, 15 ft at 17 second intervals). If there
is a third number, that will be the direction from which the swells
are travelling (26 would represent 260 degrees).
- 57 - Sea
surface temperature in degrees F.
- 54 - Air
temperature in degrees F.
- 1018.3 -
Air pressure in millibars.
- W. SANTA
BARBARA CHNL - Name of buoy or site of observation.
Back to Top
|
|
|
|
Webmaster
US Dept of Commerce
National Oceanic and Atmospheric Administration
National Weather Service
Los Angeles/Oxnard Weather Forecast Office
520 North Elevar Street
Oxnard, CA 93030
Tel: (805) 988-6610
|
Disclaimer
Information Quality
Credits
Glossary |
Privacy Policy
Freedom of Information Act
About Us
Career Opportunities
|
National Weather Service Mission: "The
National Weather Service (NWS) provides weather,
hydrologic, and climate forecasts and warnings for
the United States, its territories, adjacent waters
and ocean areas, for the protection of life and
property and the enhancement of the national economy.
NWS data and products form a national information
database and infrastructure which can be used by
other governmental agencies, the private sector,
the public, and the global community." |
|
|