425 
FXUS64 KHUN 280521
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1121 PM CST Sun Jan 27 2019

.UPDATE...
For 06Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 740 PM CST Sun Jan 27 2019

Very quiet weather in the near-term as the Tennessee Valley remains
under the influence of high pressure at the surface. With the weak
trough that clipped the region earlier in day shifting well east of
the area, zonal flow returns aloft. Thus, other than a few passing
high clouds, a mostly clear sky will continue for the remainder of
the overnight hours. After peaking in the mid 50s in most locations,
temperatures have dropped 15-20 degrees into the upper 30s to lower
40s, thanks to the good radiational cooling setup. Dewpoints in the
upper 20s to lower 30s will prevent them from falling more than an
additional 5-6 degrees, however. This will result in most locations
remaining at or just above the freezing mark by early Monday 
morning. All in all, the forecast remains on track and only a few 
tweaks were needed. 

.SHORT TERM...(Monday through Wednesday night) 
Issued at 233 PM CST Sun Jan 27 2019

An upper trough will dig into the Central Plains on Monday, with a
surface low moving into the Great Lakes region. A cold front trailing
south of this parent low will make its way eastward and arrive into
the Mississippi River by sunset on Monday. Ahead of this, southwest 
flow through the vertical column will begin to filter moisture 
northward and into the TN Valley. Given ample sunshine and WAA, 
temperatures will feel quite warm, with highs in the mid to upper 50s
during the day. A secondary piece of energy dropping SE out of the 
northern Rockies and into the ArkLaTex region will trigger pressure 
falls over southern MS/LA and a weak surface wave is progged to 
develop. This secondary wave will work in concert with the parent 
upper level trough to increase vertical motions over the area between
06-12z Tuesday and coincide with our best chances for snowfall. 
Thinking snow could start as early as 06z in NW AL and be covering 
much of the TN Valley by 09z. Then as the front and upper wave shift 
E/SE expect snow to end from NW to SE after 12z Tuesday. 

South to southwesterly winds ahead of this feature will allow for 
PWATs upwards of 0.75 inches to advect into the area. While not 
overly anomalous for this time of year, these values are on the 
higher side for a winter weather type of an event. As the forcing 
tied to the features mentions above overspread the area, expect a 
band of rain to develop as early as 9 PM along the MS River and W TN. 
Then as the Arctic front and secondary trough shift south this band 
of precip will move into NW AL. Forecast soundings suggest that 
enough of a warm layer will exist near the surface to keep this 
initial band of rain in liquid form. However, the strength of the 
Arctic front will send temps quickly below freezing early Tuesday 
morning and we can expect a rapid transition to all snow. Ensemble 
probability guidance has shown high confidence in receiving at least 
a quarter inch of QPF but low probability of anything more than 0.5 
inches. So, have adjusted our QPF based on this thinking and values 
come in around 0.3 inches. Not all of this will be falling as snow 
and it's possible that up to 0.1 inch could fall as rain before the 
change over. Snow ratios will be close to 10 to 1 and that puts 
snowfall amounts on the low end at 1 inch and on the high end 2.5 to 
3 inches. 

While the speed of the front might limit the overall length/time
period for snow, time-height analysis from various data sets were 
all indicating a stronger vertical motions either in the dendritic 
snow growth zone or just below. This would suggest the potential for 
some brief heavier snowfall rates in any mesoscale banding that 
develops. Total snowfall plumes from various data sets were trending
the mean snowfall towards the 1.5 to 2 inch range. Our current 
forecast fall inline with these values with some locations in 
southern middle TN receiving upwards of 2.5 to 3 inches. 

These amounts occurring either just before or possibly during the 
morning commute on Tuesday has prompted the need for a Winter Storm
Watch to highlight the potential for travel impacts. Even though we
could see some melting of snow initially, the fact that temps will
remain below freezing until Wednesday afternoon, any melting that
does occur should lead to refreezing and icy roads. 

The shot of Arctic air will be brief and temps only fall into the
teens to lower 20s Tuesday night. High pressure will move across the
Gulf on Wednesday and allow for a winds to switch to the SW for a 
brief period and highs Wednesday will make it into the low to mid 
30s. A much stronger push of cold/dry air comes Wednesday evening 
with dewpoints dropping into the single digits and possibly below 
zero. High pressure moving into the OH Valley during the overnight 
period should allow for clear skies and light winds Thursday morning.
This is a good setup for a strong radiational cooling night and 
models could be underdoing overnight lows on Thursday a tad. Knocked 
temps down with lows Thursday morning in the single digits in TN and 
lower teens for the rest of the area. However, as mentioned above we 
may need to nudge these values down further. Any light breezes we do 
end up having would result in Wind Chill values possibly dropping 
below zero. 

.LONG TERM...(Thursday through Saturday) 
Issued at 233 PM CST Sun Jan 27 2019

Some dangerously cold arctic air will be in place to start the day on
Thursday morning, with wind chill values starting off in the 0 to 5 
degree range across much of the Tennessee Valley. Thanks to some 
decent insolation and a southeasterly wind component, some warming 
will take place by the afternoon and allow temperatures to climb 
above the freezing mark and into the mid to upper 30s. The dry and 
mostly clear weather will be short-lived as a weak frontal boundary 
approaches the region from the Ohio Valley late Thursday night into 
Friday. There will be limited moisture available and fairly modest  
lift/forcing. However, think that we may see a window for a light 
mix of rain and snow Friday morning through early afternoon as this 
wave passes. Locations along and north of the Tennessee River will be
more favored to experience precipitation. QPF is very light so would
not expect there to be any impacts, and more than a few hundreths of
liquid accumulation. Temperatures will also warm into the mid to 
upper 40s by Friday afternoon as this system pulls away from the 
area, and winds veer to the south. 

High pressure will begin to nose into the region Friday night into
Saturday, promoting dry and fair weather for the first half of the
weekend. Morning stratus will dissipate by early Saturday afternoon,
resulting in a mostly sunny sky and our first chance for 
temperatures to warm above normal in six days, with highs expected to
peak in mid to upper 50s. An area of low pressure will quickly shift
northeast from South Texas and the lower Mississippi Valley late 
Saturday night into Sunday, bringing a potential for scattered to 
perhaps numerous light rain showers to close out the weekend. There 
is uncertainty on the timing of this system, so it's possible the 
onset of precipitation may be delayed later into Sunday depending on 
which solution verifies. 

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1121 PM CST Sun Jan 27 2019

VFR conditions will prevail at each terminal overnight and into the
day on Monday under a mostly clear sky. Clouds will increase by the
late morning and afternoon ahead of the next frontal system. Winds
will also increase and become gusty at times from the SW. A gradual
lowering of 4-5 kft BKN to OVC ceilings will occur by Monday evening
as the atmosphere moistens. Any precipitation should hold off until 
after 06z when a strong cold front will push into the region. 

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Winter Storm Watch from late Monday night through Tuesday morning 
     for ALZ001>010-016.

TN...Winter Storm Watch from late Monday night through Tuesday morning 
     for TNZ076-096-097.

&&

$$

NEAR TERM...AMP.24
SHORT TERM...Stumpf
LONG TERM...AMP.24
AVIATION...AMP.24


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