Warning Decision Process
WFO RAH winter weather
philosophy includes a targeted lead time for Winter Storm Watches of 24 hours
and for Winter Storm Warnings of 12 to 18 hours. These lead times can be
increased if confidence is unusually high, if sociological factors require it
(such as a holiday weekend or first storm of the season) and if collaboration
among adjacent offices necessitates it.
The Watch/Warning/Advisory decision
making during this event was very difficult. Generally, forecasters knew that
the arctic cold air arriving from the west with the northern stream trough
would be chasing the moisture associated with the southern stream upper trough and
distant surface low. This was a classic case of relatively limited
precipitation in the cold air over the western Piedmont, with a lack of cold
air (at least initially) where moisture was more plentiful over the Coastal
Plain. Model thicknesses and forecast soundings indicated a transition from
rain to snow from northwest to southeast during the day Saturday, as cooling in
the lowest 5 thousand feet pushed the thermal profile to slightly sub-freezing
isothermal.
One weak winter storm was already
threatening central North Carolina on Tuesday January 15 when forecasters
recognized the upcoming weather pattern would potentially lead to southern
stream short waves that could produce cyclogenesis across the Gulf of Mexico
and East Coast during the weekend and early the next week. On
Wednesday, forecasters monitored the system and it appeared
that the track of the surface low and moisture would remain largely southeast
of the area.
Forecasters
on Thursday recognized that this storm would fit the “Miller A” pattern of
limited freezing precipitation (freezing rain and sleet) and a narrow
transition zone. Forecasters accepted a GFS/ECMWF solution that brought system closer
to the coast but kept the center offshore. Model
guidance was inconsistent and portrayed a variety of solutions. Details
regarding the potential phasing of the southern stream trough with the northern
stream were a significant unknown and forecasters kept the forecast
conservative. Snow was first
introduced to the weekend forecast for the RAH CWA in the 400 PM ZFP issuance
on Thursday with a rain/snow mix in the north an northwest portion of the
CWA in the morning and then with the rain mixing with snow eastward during the
day Saturday. The 18Z
HPC snow accumulation guidance included a potential for 4 to 6 inches of
snow accumulation across parts of the RAH CWA.
During the midnight shift on
Friday morning, the event was close enough to warrant the issuance of a Winter
Storm Watch for the northern Piedmont and Coastal Plain region for the
potential of 2 to 4 inches of snow. Snow accumulation guidance
from HPC issued at around 07Z agreed with the potential for up to 4 inches
of snow from the NC mountains to the Piedmont largely because of concern over
an intense 150 to 170 knot jet at 250 mb. The Heavy Snow Discussion from HPC
issued at 451 AM on Friday morning noted
the continued inconsistency and the southeastward trend in the NWP guidance.
In addition, the
HPC Model Diagnostic Discussion issued Friday morning noted the importance of
the potential phasing of the northern and southern jets.
Forecasters
during the day shift on Friday were impressed with the intense dynamics observed
with the system (note the impressive upper
trough south of Texas in the water vapor imagery at 1815Z.) This short wave
trough over the Mexico was still digging south during the midday hours and
convection was ongoing across the Gulf of Mexico. This in part led forecasters
to believe that the developing storm system had the potential to intensify more
than the NWP guidance was suggesting; resulting in stronger cyclogenesis and
greater amounts of precipitation (closer to the NAM solution.) Colder air was
expected to be edging into the area during the day from the northwest and the
low levels were expected to be cooled by diabatic processes (evaporation and
melting.) As noted
by HPC in the 412 PM Heavy Snow Discussion, the 12Z Friday model guidance
was coming into better agreement (the NAM was still an outlier) with a slower and
more amplified solution. Interestingly, the HPC Heavy Snow Discussion mentioned
that snow
accumulations would be limited because of above freezing surface temperatures.
After collaborating with neighboring offices, WFO RAH issued
a Winter Storm Warning for portions of the northern Piedmont and northern
Coastal Plain with a Winter Weather Advisory for the remainder of the RAH
CWA. Snow accumulations were expected to be greatest in the Interstate 95
corridor when amounts were forecasted to range between 3 and 5 inches.
During the evening shift on Friday, forecasters noted some potential points
where the predicted accumulations could be overdone. Snow to liquid
equivalent ratios were expected to be 8:1 or even 6:1 because of the relatively
warm atmosphere. In addition, the above freezing temperatures at and just above
the surface were expected to initially melt the snow and a portion of the falling
snow would go toward cool the near ground temperature and the ground itself.
During the midnight shift on
Saturday morning, forecaster
confidence at RAH was low and decreasing despite the fact that the event
was about to begin. The diminished confidence stemmed from several factors,
including an evolving depiction of QPF values across central NC with each model
run and the fact that the 00Z models were having great difficulty in
forecasting the observed weather just 6 hours after model initialization time.
The 00Z Saturday GFS/NAM were too slow and too far south with the precipitation
across the Carolinas (note
the difference between the 3 hour NAM20 QPF ending at 12Z and the 1 hour RTMA
observed precipitation analysis over SC and NC which already showed up to
0.10 inches in 1 hour.) Forecasters realized that the primary lifting
mechanisms will be divergence aloft from a strong upper level jet and mid level
frontogenesis, but the various ways these fields were progged by the models was
disconcerting. In addition, observed temperatures and dew points were still in
the upper 30s to lower 40s during the predawn hours.
With the poor model performance
in handling the short term and uncertainty in the QPF, forecasters decided to
leave the Warnings and Advisories as they were. The midnight shift did note
that this would be a borderline Warning event and that there was potential for
Warning criteria snow in the southern Coastal Plain. The ZFP
issued at 407 AM on Saturday morning generally included 1-2 inches in the
advisory area with 2-3 inches in the Warning area with potentially higher
amounts.
Forecasters
on Saturday morning were troubled with mixed signals when analyzing the
observed and guidance data. The 12Z proximity soundings came in on average
3 to 6 m higher with 1000-850 and 850-700 mb thicknesses than progged by the
models. In addition, the NAM and GFS BUFR soundings were too cool in the lowest
5,000 feet relative to the observed 06Z and 12Z KGSO soundings as well as too
cool over RDU relative to AMDAR aircraft soundings between 11 and 13z. Colder
and drier air was believed to be on its way with the observed soundings
displaying an increasingly backing wind profile in the lower layers. Dew points
over southwestern VA and northwestern NC were dropping into the teens which
meant the potential for further near surface cooling via evaporation.
The subsidence on the back
side of the short wave that produced the initial round of precipitation across
central NC was noted during the late morning hours in the drying
shown on satellite imagery and a large
lull in the precipitation across the western Carolinas. This break in the
precipitation allowed surface temperatures to climb into the lower 40s before
the onset of the second round of precipitation.
The Advisory was allowed to
stand across the Triad because of the models poor performance with
precipitation amounts upstream, indications that the cold air is showing signs
of infiltrating the Triad and snow ratios will accordingly increase closer to
9:1 or 10:1, and forcing for ascent was expected to continue into the late
afternoon hours.
arrival of the colder air
will co-exist with the higher QPF made it difficult for forecasters to trim
back the Warning. In fact, the locations in which the forecasters contemplated
downgrading the Warning (the northwestern tier of counties – Person, Orange,
Alamance, and Chatham) were the locations
that received the most snow. Finally, the forecasters realized that most of
the accumulation will occur on grassy surfaces, cars, and rooftops given the
above freezing near surface temperatures and already wet ground.
Across the Warning and the
remainder of the Advisory areas, the forecast rationale and resultant snowfall
amounts were generally unchanged from the previous package. It was noted that
the accumulations would likely be on the lower end of the Advisory/Warning range.
Uncertainty in the location of an expected sweet spot where the arrival of the
colder air will co-exist with the higher QPF made it difficult for forecasters
to trim back the Warning. In fact, the locations in which the forecasters
contemplated downgrading the Warning (the northwestern tier of counties –
Person, Orange, Alamance, and Chatham) were the locations
that received the most snow. Finally, the forecasters realized that most of
the accumulation will occur on grassy surfaces, cars, and rooftops given the
above freezing near surface temperatures and already wet ground.
Forecasters
during the afternoon Saturday were relieved to see the second area of
precipitation develop and spread across central NC. The precipitation had
already developed or changed to snow in the Triad area around 200 PM and the
changeover was moving into the western portions of the Triangle area by 300 PM.
Snow accumulations on the order of a coating
to a third of an inch were already reported in the Triad area by 300 PM.
With conditions rapidly changing over to snow, forecasters were still hesitant
on pulling the warning despite the fact they realized it may be a long shot. While
it was not clear at the time the warning decision was made (1-3 PM), the fact
that the surface temperature was failing to fall below 33 degrees in the RAH
CWA during the afternoon and early evening hours was a chief reason Warning
criteria snow accumulations were never realized. The Winter Weather Advisory
was dropped across the Triad area at 752 PM and the remainder of the Advisories
and Warnings were updated. All of the Advisories and Warnings were cancelled at
1158 PM.
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