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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 210508

Area Forecast Discussion
National Weather Service Huntsville AL
1108 PM CST Wed Feb 20 2019

For 06Z TAFS.


.NEAR TERM...(Tonight)
Issued at 916 PM CST Wed Feb 20 2019

The surface front that passed through the Tennessee Valley this 
afternoon has shifted south and east of the region, and precipitation
has tapered off in its wake -- save for some isolated light showers 
and drizzle. The low clouds remain in place, however, with ceilings 
around 1500 feet or less. This low stratus and the moist boundary 
layer conditions will create patchy fog conditions across the region,
with visibilities dropping as low as 1/2 mile or so in some areas. 
Will have to watch for the development of locally dense fog along the
ridgetops of the Cumberland Plateau due to these potential low 
ceilings developing in these elevated areas. Temperatures have 
remained quite mild and in the low to mid 50s as of 03z. Will 
probably see some additional cooling overnight, but not much -- and 
would expect most areas to bottom out in the upper 40s by early 
Thursday morning. Most areas should remain dry, save for some 
occasional drizzle underneath the low cloud decks. Some minor tweaks 
made to the forecast, but otherwise everything remains on track. 

.SHORT TERM...(Thursday through Friday)
Issued at 251 PM CST Wed Feb 20 2019

On Thursday, the 850 mb warm front and associated showers and
possible thunderstorms to our south will lift rapidly north-northwest
into northwest AL and TN. This may leave our southeast counties
relatively rain-free through Friday, while areas from middle and
southern middle TN through far northwest AL into northern MS stay
under the gun for potentially excessive rainfall. Thus, agree with
the WPC excessive rain outlooks for Day 2-3. The question is how
much. Will stick with WPC guidance, but acknowledge that convective
enhancement may yield higher totals. Will keep the entire Flood Watch
in tact through Friday, but may be able to trim back some of the
south and southeast counties in later updates given rain will be
limited until this weekend in these areas. After lows in the 40s to
around 50 again Friday morning, highs should jump into the 60s with
the warm sector lifting northward.

.LONG TERM...(Friday night through Tuesday)
Issued at 251 PM CST Wed Feb 20 2019

Friday night, models continue to build upper level ridging east into
the area. This should force strong upper level forcing along an axis
stretching from eastern Arkansas into the Nashville and southeastern 
portions of Kentucky. This is good news, because it should give us a 
break from persistent and heaviest rainfall as this activity shifts 
northward and remains in northeastern Arkansas, northern Tennessee, 
and southeastern Kentucky (at least a good portion of northern 
Alabama). The exception to this will be the southern edge of some 
moderate rain along with a few thunderstorms which could linger in 
northwestern Alabama and southern middle Tennessee Friday night. At 
this point, the thunderstorm activity looks elevated with MUCAPE 
values only around 400 J/KG or less. So only expecting gusty winds 
and frequent lightning with any thunderstorms. Strengthening 
southerly flow originating from the Gulf of Mexico should advect much
warmer and more moist air into portions of northern Alabama and 
southern middle Tennessee (especially in locations near and west of 
I-65) overnight Friday into Saturday morning, despite abundant cloud 
cover and scattered to more widespread precipitation. Lows in the
upper 50s to lower 60s look reasonable. 

This activity should lift north and much of southern middle Tennessee
on Saturday morning. However, a strong upper low and surface low 
moves from Kansas into Missouri on Saturday afternoon. Models quickly
develop a line of strong convection over eastern Arkansas or 
Mississippi ahead of an associated dry line ahead of this storm 
system. This line of showers and thunderstorms push east into 
northwestern Alabama either in the late afternoon or evening hours. 
Models are in some disagreement with the timing of the arrival of 
this activity. ECMWF is a bit quicker with the arrival of this 
activity than NAM or GFS, which is a bit unusual. 

Even stronger southerly flow on Saturday and maybe some thinning of 
clouds, especially south of the Tennessee River, should allow high 
temperatures to climb into the lower to mid 70s near and west of 
I-65. Further east wedging effects of drier/cooler air should keep 
highs cooler in the upper 60s to around 70 degrees. Strong shear sets
up ahead of this dryline on Saturday afternoon and evening. Models 
are showing between 200 and 600 J/KG of CAPE developing west of I-65.
With wet-bulb zero heights dropping to around 10000 feet, some large
hail seems possible. With strong 850 mb and 500 mb jets and plenty 
of helicity, models indicate that severe storms could produce 
damaging winds and some brief tornadoes as well. An even bigger 
impact might be additional flash flooding and flooding concerns on 
area rivers and streams. A wind advisory may be needed as mixing to 
925 mb winds between 45 and 50 knots could allow sustained winds 
to reach that criteria. However, for now forecasting winds just 
below criteria with sustained winds between 15 and 20 mph and gusts 
up to 35 mph. Strong to severe thunderstorm parameters don't drop off
much in the evening hours on Saturday, but should end as the front 
pushes through the area into Georgia after midnight. 

A much longer break from precipitation looks to setup finally Sunday
into early next week, before a weaker front pushes through towards
midweek, which may draw a few showers north into the area. Highs will
feel much cooler Sunday into early next week, only reaching the
upper 50s to around 60 degrees. A slower warming trend is expected 
towards the middle of next week. 


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1107 PM CST Wed Feb 20 2019

IFR to LIFR conditions will continue overnight and early this
morning from low ceilings, and IFR to MVFR fog will develop continue
at each terminal thanks to the very moist boundary layer conditions.
Some gradual improvements will occur at both terminals after 
sunrise, but a weak front will generate some -SHRA/SHRA (and maintain
sub-1000 ft decks) across the region from the late morning through 
most of the afternoon. This activity will taper off by this evening 
as the boundary lifts north, with scattered VCSH remaining through 
the period. 


AL...Flood Watch through Friday afternoon for ALZ001>010-016.

TN...Flood Watch through Friday afternoon for TNZ076-096-097.




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