Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 151529

Area Forecast Discussion
National Weather Service Huntsville AL
929 AM CST Fri Dec 15 2017

.NEAR TERM...(Rest of Today)
Issued at 929 AM CST Fri Dec 15 2017

A weak mid level trough will move eastward across the region today,
while high pressure builds at the surface. Clouds continue to stream
across the area in the southwest flow aloft as moisture filters
through the mid levels. Winds will generally remain from the
northwest for the remainder of the day, helping to bring drier air at
the surface. Weak CAA, along with the cloud cover, will keep
temperatures cool today, with highs only warming into the lower to
mid 40s. By the afternoon, the axis of the upper trough will shift
east of the area, with weak northwest flow aloft helping to clear
skies by early evening. Previous forecast was on track so no major
changes were made.

.SHORT TERM...(Today through Saturday Night)
Issued at 202 AM CST Fri Dec 15 2017

Lows tonight will be in the 20s again with the surface high
translating east across the area. Then, as has been advertised for
several days, mid to upper level ridging and a turn to south-
southwest flow in low levels will bring a rebound in temperatures on
Saturday. With 850 mb temps in the 5-6C range, highs in the lower to
middle 50s are expected. As a strong shortwave begins to eject
northeast through TX, a broad fetch of southerly flow off the Gulf of
Mexico will develop over the central and eastern Gulf Coast Saturday
afternoon and evening. Although trending a bit slower now,
isentropic lift and moisture transport will be significant over the
lower MS valley into the mid South region Saturday night. This should
spread rain showers and elevated high based convection across areas
just to our west through 12Z. Despite the shortwave dampening as it
arrives, deep southwesterly flow will bring a steady feed of moisture
(PW of greater than 1 inch) and isentropic ascent. Our eastern
counties may need a bit more time to saturate due to the very dry low
level dew points. But expect this to be overcome quickly by the
strong UVV and large scale precipitation area that will be spreading
eastward. This first "wave" of rainfall will diminish and/or end from
west to east late in the day into Sunday evening.

If the GFS and NAM are correct, the synoptic pattern will become
quite favorable for heavy rainfall again Sunday night with a weak
upper ridge position and ample south- southwesterly Gulf inflow.
Meanwhile, the ECMWF is remaining the outlier keeping a slower
approach and dry weather Sunday Night into Monday. Will side closer
to the wetter solutions for now given so many GFES plumes are
suggesting this. The low level boundary will be fortified with
Sunday rainfall, producing a good thermal/convergence zone for
renewed development Sunday night with the aid of a nocturnal increase
in the low level jet (40-50kt). The best chances of heavy rainfall
will occur from central MS into north central AL, so have gone with
our higher PoP south of highway 72.

.LONG TERM...(Sunday night through Thursday)
Issued at 202 AM CST Fri Dec 15 2017

A progressive prevailing westerlies pattern shift will be well
underway by Monday as a ridge axis centered over the Bahamas
continues to bring southerly flow across the TN Valley while
successive shortwave troughs impact the ridge. The differences in
ridge strength between the medium range models make the
Monday/Tuesday timeframe particularly uncertain. So, additional
refinements to this forecast are likely. A couple of things that do
seem to be consistent:

1) Temperatures are definitely warmer due to the warm air advection
in place with daytime highs in the low 60s on Mon/Tues with some
slight fluctuations on Wed/Thu in the 50s/near 60 degree highs.
2) Ensembles (GEFS) are showing a wetter pattern this next week with
some QPF amounts as high as 2-4 inches during the extended forecast.

With the caveats in mind, the strong ridge stretching from the
Bahamas to the Southeast may cause an oncoming shortwave trough to
deamplify and with the resulting weaker lift result in lighter
rainfall on Monday. However, the Canadian/ECMWF are showing weak
undulations moving across the MS River Outlet of LA to central AL/GA
which may concentrate rainfall over the SE portions of the forecast
area. Have kept the overall trend in higher POPs over central and
northeast Alabama. An additional noticeable trend is steepening
lapse rates aloft from the Gulf of Mexico to just south of the TN
River. Though the instability is rather weak (100-300 J/kg), there
may be enough to support thunderstorms on both Monday and Monday
night ahead of a shortwave trough moving across the Southern
Plains/Ozarks. The thunderstorm threat may spread north over the
rest of the forecast area on Monday night as warm air advection

This oncoming shortwave trough`s progression definitely differs
among the medium range models with the ECMWF showing a much weaker
and further north solution due to the stronger ridge solution.
However, will keep chance of rain in the forecast on Tuesday before a
modified cold front arrives ahead of this shortwave trough and
brings slightly cooler and dry air for Wed. Another broader polar jet
trough arrives on Thursday with a cold front. This trough and
accompanying front may provide additional showers and have kept the
Superblend POPs in the forecast.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 515 AM CST Fri Dec 15 2017

VFR flight weather conditions are expected during the next 24 hours.





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