Alan Gerard, Corey Mead, Stephen Miller, Russell Pfost, Peter Wolf
National Weather Service Forecast Office
Jackson, MS
1. Introduction
In spite of all of our sophisticated models, up-to-date computer methods, better
communications, and exhaustive training, there are times in operational weather forecasting
when everything seems to fail. The heavy snowstorm which surprised everyone across northeast
Louisiana, central Mississippi, and extreme west Alabama was just one of those times. Light to moderate
snow began falling shortly after midnight Sunday, 14 December, 1997, as a deep upper level storm system moved
slowly along the northern Gulf coast. By the time the snow ended about 7 hours later, 8 inches of snow was
common from the Jackson-Vicksburg metro area of Mississippi east to just north of Meridian
into west central Alabama (figures 1, 2).
The storm was not a disaster situation - indeed most residents of Mississippi seemed
almost jubilant about the winter weather. Complaints about the terrible forecasting of the event
were scarce. Since the storm occurred on Sunday, no children were in school and most regular
office workers were at home and not traveling the roads. Even though the snow was heavy, it was
a very wet snow with the surface temperature remaining near 32 degrees Fahrenheit. Thus, roads
remained wet and still passable in spite of the intensity of the precipitation.
Even so, we at the National Weather Service Forecast Office in Jackson were a little
frustrated with our performance. How could such a heavy event occur with little advance notice?
How could we have not forecast such an event with our new, improved numerical models and
our much better communications and gridded data? True, our forecasts had included a mention of
snow and possible accumulations as early as the forecast package issued Thursday afternoon. But
that was for the Friday and Friday night time period, and no snow actually occurred then. The
only mention of snow we had in the forecast for Saturday night and Sunday was "flurries".
4.8 inches of snow fell officially at the Jackson International Airport, which was the 8th
largest on record, and the largest so early in the winter (actually late fall). It was the second
largest one day snowfall in the month of December (the largest December snow was 7.5 inches
on 29 December, 1929).
This brief paper will try to address what happened and how we may be able to forecast
such an event better in the future.
2. Model Performance General Remarks
The three NWS operational models, the Eta, Nested Grid Model (NGM), and aviation
(AVN), all did poor jobs forecasting the depth of the 500 hPa low as well as the distribution of
moisture with the system. This was surprising, especially in the case of the new Eta model,
because the Eta is often touted as a much superior model due to closer grid spacing, the Eta
vertical coordinate, and better physics packages. Actually, the Eta made the worst errors in the
height of the 500 hPa low center, followed by the NGM and the AVN.
The following table shows relative height errors (+/- 3 m) for the center of the 500 hPa
low from the three operational models at 12 UTC Saturday December 13 (48km Eta, 80 km
NGM, and AVN) and the 15 UTC meso-eta (29 km resolution) verifying at 00 UTC and 12 UTC
Sunday, 14 December, 1997 (the meso-eta was verified with the NGM's analysis):
|
AVN |
Eta |
Meso-Eta |
NGM |
| 12/13 12Z F12 |
5485 |
5550 |
F09 5500 |
5505 |
| 12/14 00Z F00 |
5470 |
5455 |
NGM 5445 |
5445 |
| Error |
15 |
95 |
55 |
60 |
|
AVN |
Eta |
Meso-Eta |
NGM |
| 12/13 12Z F24 |
5460 |
5555 |
F21 5505 |
5470 |
| 12/14 12Z F00 |
5420 |
5420 |
NGM 5395 |
5395 |
| Error |
40 |
135 |
110 |
75 |
Even more amazing is that if Slidell, Louisiana's, observed 500 hPa height at 5395 m for 12
UTC 14 December is taken as a close approximation to the actual minimum height for the center
of the 500 hPa low (which is pretty close to the NGM's verifying analysis), the 24 hour forecast
of the 500 hPa low center height is actually 160 m too high!
Moisture analysis was a second and more serious problem with the models. Both the Eta
and NGM were consistently drier than the very wet AVN (fig. 3). While the AVN did verify
better for the snowstorm event, it had been so wet and cold for the previous three days and did
not verify well that Jackson forecasters believed that the Eta and NGM model runs were more
accurate.
The 13 December, 1997, 15 UTC run of the meso-eta, in spite of its flaws in forecasting
the intensity of the storm system, did a much better job with the moisture distribution. The 15
UTC meso-eta was the only model to correctly depict in advance an axis of moisture coming
from the Atlantic off the east coast of Florida across Georgia and Alabama directly into central
Mississippi. It also was the only model to show precipitation falling in central Mississippi,
although the amounts were much less (generally about 0.15 inch water equivalent) than what
actually occurred and the axis of precipitation was displaced too far southeast (fig. 4). This model
run, which was not available in real time to Jackson forecasters for communications problems
unknown, was the only reasonable model run from which a forecaster might have made a
decision to issue a watch for Sunday morning with any respectable lead time. However, a watch
issued from the 15 UTC meso-eta likely would have been in the wrong place and likely would
not have specified 8 inches of snow.
The model runs from 14 December, 1997, 00 UTC, were much better with the moisture
distribution around the storm system. The storm relative isentropic vertical motion was also
impressive on the 00 UTC model runs, as is described in detail later. However, snow was already
beginning to fall across the Jackson CWA by the time the midnight shift arrived Sunday
morning, and little time was therefore available for analysis. Forecasters at Jackson issued a
Winter Weather Advisory at 3 AM Sunday, followed by a Heavy Snow Warning at 7 AM
Sunday. Fortunately, most of the snow fell after the Heavy Snow Warning was issued.
3. Isentropic Potential Vorticity Considerations
Potential vorticity is defined as the absolute vorticity divided by a static stability
parameter.
Potential vorticity is found in abundance in the stratosphere and is created through stratospheric
warming associated with the ozone layer (FDC course book, 1996). Ageostrophic circulations
associated with upper level jet streaks are a means by which this stratospheric potential vorticity
can enter into the troposphere. These "stratospheric intrusions" are often seen as warm pockets
on 200 and 300 hPa charts in the vicinity of intense upper level storm systems.
In 1940, Rossby determined that potential vorticity, when evaluated on isentropic surfaces,
was conserved for frictionless, adiabatic flow. From the equation above, it can be seen that when
stratospheric air descends along an isentrope into a less stable environment, the absolute vorticity
of this air parcel must increase to conserve the potential vorticity. Numerous studies have
attempted to trace upper tropospheric pockets of potential vorticity and link them to cyclogenesis
at the surface. As an example, Barnes and Colman (1994) investigated the possible role of
isentropic potential vorticity (IPV) on the rapid development of a storm system over Colorado
that was not well handled by short term numerical models.
Since the Mississippi snowfall event began at roughly 0600 UTC on 14 December, the
1200 UTC 13 December and 0000 UTC 14 December runs of the Eta and AVN were used to
analyze IPV and its potential effect on the Gulf of Mexico storm system:
a. 1200 UTC 13 December Run
The 1200 UTC 13 December run of the Eta gave no indications of high IPV air in the
vicinity of the upper low that was forecast to move across the northern Gulf Coast. Conversely,
the 18-hr forecast of the AVN, valid 0600 UTC on the 14th, showed a significant pocket of warm
air over southern Louisiana being advected into southern and central Mississippi. To verify
whether or not this warm pocket was a source of potential vorticity, a cross section was
constructed through this feature, as denoted by the heavy black line (fig. 5). Indeed, IPV values
in excess of 6 Potential Vorticity Units (1 PVU = 10-6 m2 s-1 K kg-1 ) were found extruding into
the troposphere in the vicinity of the upper level low (fig. 6). IPV values of 1.5 PVUs or greater
usually are associated with stratospheric air (Bluestein, 1993). Values of this magnitude were
found as low as 650 hPa at this time. Model-derived potential vorticity advection fields and
trajectory analysis (not shown), suggested that this "high IPV" air was being advected
northeastward into central Mississippi. This would signal the deepening of the entire storm
system through increasing vertical velocities and the associated decreased static stability.
b. 0000 UTC 14 December Run
By the 0000 UTC 14 December run, the 6-hour forecast of the Eta had slowed the
progression of the upper level low considerably and developed a pocket of stratospheric air over
southern Louisiana. This was a significant change from the previous run and was in good
agreement with the 18-hour forecast from the 1200 UTC run of the AVN on the 13th.
Meanwhile, the 6-hour forecast of the AVN, valid 0600 UTC 14 December, continued the trend
of strong 250 hPa warm advection over southern and central Mississippi.
The Eta appeared to lose the stratospheric air on the 12-hour forecast, as it cooled
temperatures considerably to the southeast and east of the 250 hPa low center. The AVN,
however, continued to maintain the integrity of this feature on its 12-hour forecast with
temperatures in the stratospheric intrusion warming 2 degrees C. As it turned out, the 00-hour
forecast of 1200 UTC run on the 14th for both the Eta and AVN, showed that the trend of AVN
was the correct solution.
IPV is commonly considered to be a potential tool for forecasting the deepening rate of
surface cyclones. However, it may also be used to gauge the deepening potential of the entire
storm system, from the surface to the upper troposphere. Forecasters who accepted the AVN
solutions for these consecutive model runs and understood the potential synoptic role of IPV may
have correctly anticipated continued deepening of the storm system. Although these concepts do
not address the amount of moisture available to the system, they do directly diagnose vertical
motion and static stability potential.
4. Isentropic Considerations
a. 1200 UTC 13 December Eta Run
The 1200 UTC 13 December run of the Eta model provided few clues to assist in
forecasting the impending snow event, even when viewed in an isentropic framework. On the
290K and 295K surfaces, some upward motion was forecast over central and southern
Mississippi and northeast Louisiana at 0600 UTC and 1200 UTC 14 December. This vertical
motion was more impressive when viewed in a system-relative sense, using the model forecast
for system motion of 270 degrees at 15 knots. Using this motion, an axis of 7 to 10 microbars s-1
of upward motion was forecast over southern Mississippi at 0600 UTC and 1200 UTC 14
December. However, this area of vertical motion was forecast to move quickly to the east by
1800 UTC, and was also placed south of the area where the precipitation actually developed.
Even with the upward motion forecast by the 1200 UTC 13 December Eta run, the
amount of moisture forecast by the model did not appear nearly sufficient to support significant
snow. Condensation pressure deficits during the period of maximum upward motion were
forecast to be between 100 and 180 hPa over the area concern. These values are very dry, and
would generally be insufficient to support cloud development even in the presence of upward
motion. Mixing ratio values on the 295K surface, which the model forecast to be around 750
hPa over the area of concern (the critical surface to examine for snowfall forecasting based on the
Garcia technique (1994)), were forecast to be less than 2 g kg-1 between 0600 and 1800 UTC 14
December. The combination of the low mixing ratios and upward motion for only a fairly short
period of time would imply very little in the way of snowfall accumulation based on the Garcia
technique.
b. 0000 UTC 14 December Eta Run
The 0000 UTC 14 December Eta run indicated a totally different scenario for Mississippi
and Louisiana than did the previous run. When using the forecast system motion of 270 degrees
at 20 knots, one obtained a well defined axis of system-relative upward vertical motion on the
290K surface from about Natchez to near Jackson, then east to north of Meridian (fig. 7). The
axis was slightly further south on the 295K surface. The upward motion was forecast to persist
by the Eta from 0600 to around 1800 UTC, and be strongest between 0600 and 1200 UTC with
values of up to 12 microbars s-1 on the 295K surface. The area of upward motion forecast by the
model correlated quite well to the position of the observed snowfall, particularly on the 290K
surface.
In contrast to the 1200 UTC 13 December run, this run of the Eta indicated that sufficient
moisture would be present to support significant snowfall during the time in question. In fact,
the Eta forecast that an axis of condensation pressure deficits of 10 hPa or less on the 290K and
295K surfaces would be collocated with the upward motion axis described above through the
event. This forecast of near saturation at these levels was a total reversal of the very dry airmass
forecast by the previous run of the Eta, and when present with upward motion these low
condensation pressure deficit values indicated that precipitation development was likely.
The Garcia technique when used with this run also indicated a much greater snowfall
potential than did the previous run. The 0000 UTC run of the Eta showed that the critical surface
to examine for moisture per the Garcia technique was again the 295K surface. The average
mixing ratio on this surface during the 0600 UTC to 1800 UTC time period using the technique
outlined by Garcia was 4 g kg-1. This mixing ratio value in the presence of strong, persistent
upward motion yielded a forecast maximum snowfall accumulation during the 0600 UTC to
1800 UTC time period of 8 inches. This matches the observed snowfall very well, and
demonstrates that the 0000 UTC 14 December run of the Eta gave a much clearer indication of
the heavy snowfall potential during the morning of 14 December than the 1200 UTC 13
December run did.
6. Satellite Considerations
From the World Wide Web (WWW) page maintained by the Cooperative Institute for
Meteorological Satellite Studies (CIMSS), University of Wisconsin - Madison
(here) the heavy snow band apparently
was associated with a deformation zone (fig. 8) that formed just to the north and northwest of the
cold cloud shield "pivot point" associated with the nearly barotropic (vertically stacked) Gulf
closed low. Since deformation zones are normally associated with frontogenesis at some level
(Moore, 1998), an in depth study of the frontogenetical aspects of this system would be a good
topic for a future paper.
7. Conclusions
It's tempting to brush this event off as a failure of the models, as shown previously.
However, as professional operational meteorologists, it is our job to maintain a weather watch
over the defined area of responsibility whether the models are good or not. Snow, especially
heavy snow, in the south is relatively rare and residents of our region anxiously anticipate each
occurrence. Because winter weather forecasts in the south are always subject to intense scrutiny
whether they are right or wrong, we must strive to meet the challenge and do an even better job
with them than more routine situations.
Looking back, synoptic scale forecasters at Jackson had few real clues from the models
that might have resulted in a better forecast for the snow event in the fourth, third, or second
periods. Only older, cliche-type, rules like "There's often a 'surprise' with a closed 500 hPa low"
led Jackson forecasters to even keep a mention of snow flurries in the forecast at all. Closer
synoptic scale attention to IPV concepts, satellite depiction of moisture distribution and it's
agreement with model forecasts, and storm relative isentropic vertical motion will help with
future cases like this, but it's still doubtful that a watch for this event would have ever been
issued with the model guidance available. A first look by the 'R' shift forecaster at the better
model runs at 00 UTC on 14 December could have provided the information necessary for a
watch for Sunday, but a watch issued at 10 or 11 PM CST on Saturday night would not have the
desired lead time or media distribution as one issued Saturday afternoon.
Mesoscale forecasters, however, had an abundance of information that could have been
used if time was available to forecast the amount of snow. The meso-eta model run at 15 UTC on
13 December was the first model run to reasonably depict the vertical motion and moisture
fields, and meso-eta model soundings each hour are routinely available at NWSFO Jackson.
Satellite imagery in time lapse distinctly showed the deformation zone, and the WSR-88D
showed obvious banding structures that led forecasters finally to issue the Heavy Snow Warning
and 7 AM CST Sunday, before most of the snow fell.
To sum up the entire experience in a few sentences:
- �The old rule of 'surprises' with a closed 500 hPa low is true. The challenge is for
forecasters to watch for possible 'surprises' and hopefully make them 'expected'.
- �The AVN did the best job forecasting the intensity and deepening of the 500 hPa low.
- �None of the models did a very good job with the moisture distribution until the 15 UTC
meso-eta run on 13 December.
- �We have learned a healthy respect for deformation zones on the north side of 500 hPa
lows but we want to understand more how they affect vertical motion and the
development of heavy precipitation.
- �The Eta model was disappointing in its forecasts of the 500 hPa low until 00 UTC Sunday
14 December.
- �Storm relative isentropic vertical motion was the most valuable tool in assessing lift in
this situation.
- �Forecasters must keep a closer watch for stratospheric intrusions as a tipoff to the
deepening of storm systems.
8. References
Barnes, S. L., and B. R. Colman, 1993: Quasigeostrophic Diagnosis of Cyclogenesis Associated
with a Cutoff Extratropical Cyclone--the Christmas 1987 Storm, Mon. Wea. Rev., 121,1613-1634.
Bluestein, H., 1993: Synoptic-Dynamic Meteorology in Midlatitudes, Volume II, Observations
and Theory of Weather Systems, Oxford University Press, pg. 182.
Garcia, Jr., C., 1994: Forecasting Snowfall Using Mixing Ratios on an Isentropic Surface, U.S.
Dept. of Commerce, National Oceanic and Atmospheric Administration, National
Weather Service, NOAA Technical Memorandum NWS CR-105.
Moore, J. T., 1998: Personal Communication.
Rossby, C.-G., 1940: Planetary Flow Patterns in the Atmosphere, Quart. J. Roy. Meteor. Soc.,
66, Suppl., 68-87.
U.S. Dept. of Commerce, National Oceanic and Atmospheric Administration, National Weather
Service Training Center Forecaster's Development Course booklet, 1996: Potential
Vorticity.
CAPTIONS FOR FIGURES
Fig. 1 Storm Total Snowfall Depth Reports (inches and tenths) at 00 UTC 15 December
1997
Fig. 2 Visible Satellite imagery 1545 UTC 15 December 1997
Fig. 3 Average 1000-500 hPa Relative Humidity from the Eta (dotted), NGM (heavy
dash-dot), and AVN (thin solid) models for 12 UTC 14 December
Fig. 4 Quantitative Precipitation Forecast for 06-18 UTC 14 December from the 15 UTC
13 December meso-eta model
Fig. 5 250 hPa heights and temperatures for 06 UTC 14 December 1997
Heavy black line shows cross section for Fig. 6
Fig. 6 Cross section of IPV for 06 UTC 14 December 1997
STJ = subtropical jet PFJ=polar jet
Fig. 7 Storm relative upward vertical motion (microbars/s) on the 290K isentropic
surface (heavy black lines) with pressure (thin black lines) and storm relative
winds (assuming storm motion vector of 27020)
Fig. 8 Infrared satellite picture 14 UTC 14 December 1997 showing deformation zone
associated with heavy snow band across central Mississippi (courtesy CIMSS)
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