Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 151114 AAA
AFDHUN

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
514 AM CST Fri Dec 15 2017

.UPDATE...
For 12Z TAFS.

&&


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

Satellite imagery indicates that the northern extent of a large veil
of CI/CS associated with the 160kt 250mb jet continues to stream
eastward across the TN valley. Model indications are that this will
will exit through the day as the jet core shifts east and NVA
develops behind the passage of a positive tilt trough. The mid level
ceilings to our north in central and northern TN are showing no signs
of making it this far south either. Cold advection will become
neutralized rather quickly by this afternoon per 850 mb temp progs.
Nevertheless, highs will be several degrees colder today in the
40s. 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
strengthens.

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.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

SHORT TERM...17
LONG TERM...SL.77
AVIATION...17


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