Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 271154 AAA

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
554 AM CST Mon Feb 27 2017

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 415 AM CST Mon Feb 27 2017

An unsettled weather pattern is in the forecast for the next few
days, as an active semi-split flow regime prevails across the
lower-48. Over the Pacific Northwest, an upper level system moving
across that region, along with another upper low moving towards the
California coast will help produce progressive troughing across the
CONUS the first half of the week. A few upper level systems in the
southern portion of the westerlies was bringing additional moisture
and support to the southern CONUS. High pressure now off of the Mid
Atlantic coast was producing a SE flow across the southern states.
This southerly flow has resulted in rather warm conditions (compared
to last night) with lows so far in the mid 40s to lower 50s.

An area of showers and embedded thunderstorms noted by regional
radars was approaching the Tennessee Valley from the ARLAMS region.
Short term model trends were in reasonable agreement with this
activity, bringing likely to definite rain chances today. Given
lightning strikes noted above and elevated instability present today,
kept the precipitation character mainly as showers with a few
thunderstorms. The highest rain chances will tend more to the
southern portions of our forecast area into the afternoon. Even with
clouds and good rain chances, near seasonably cool temperatures are
forecast with highs in the mid 50s to around 60.

.SHORT TERM...(Tonight through Wednesday)
Issued at 415 AM CST Mon Feb 27 2017

Scattered rain chances should continue tonight. Added thunder to the
mix after midnight as greater instability is realized. Milder
temperatures are in the forecast with lows only in the low/mid 50s.
Another system moving in will bring likely shower/thunderstorm
chances on Tuesday. Rain chances again should be reduced somewhat
Tuesday evening, but should pick up late in the night, as the main
system approaches. Some of the storms Tuesday evening/overnight
especially out west could become strong, with gusty winds the main

The upper system noted above over the western CONUS will track
eastward and spawn a surface low east of the Rockies today. This
next low will move towards the Great Lakes during Wednesday, and
bring a strong cold front eastward. This front as it nears will help
produce widespread showers and thunderstorm. Most of the convective
parameters are on the high side as they have been the past few runs;
all suggestive of a severe weather episode for the Tennessee Valley -
with damaging wind gusts the main threat. Large hail and a few
tornadoes are also possible too, especially if breaks occur in a
squall line as it moves across the region on Wed. Dewpoint values
during Wed should rise into the lower 60s area wide, with 0-3km
helicity values between 200-400 m/s (at times higher), and CAPE
values around 500 j/kg. The Day-3 Convective Outlook shows similar
trends yesterdays Day-4 outlook indicated. The GFS again was the
quicker of the models concerning timing with the front/Quasi-Linear
Convective System or QLCS. But the ECMWF was indicating a further
southward and deeper surface low than it had yesterday, which could
enhance overall storm strength. The severe threat will end from NW to
SE during the course of the day as the QLCS moves across the region.
Timing from the GFS has it well SE of the forecast area before dusk,
while the EC was delayed by 3-5 hours or so.

.LONG TERM...(Wednesday night through Sunday)
Issued at 415 AM CST Mon Feb 27 2017

Most of the very active wx should be just about thru NE AL with the
onset of this part of the forecast package, as showers/tstms continue
to quickly taper off from the w with the passage of the cold front.
The latter half of the model suites point to the sfc boundary moving
ewd into the mid/srn Atlantic basins late Wed evening, with the
associated convective line in tow. This in turn will allow a strong
dome of high pressure out of the mid/srn Plains states to build ewd
into the SE region into early Thu, as clouds begin to gradually sct.
High pressure out of the nrn Plains will actually become reinforced
across the cntrl TN Valley by Fri, under mostly clr/sunny skies. This
should allow for a brief cooling trend going into the weekend period,
as overall temps trend closer to seasonal values. Rain chances may
then work their way back into the forecast by the end of weekend
period, as the sfc high translates ewd into the srn Atlantic and
return/sly flow develops across much of the region. With the slow
increase in returning moisture, weak isentropic lift may provide
enough support for sct showers into the new week.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 554 AM CST Mon Feb 27 2017

VFR weather across the region will deteriorate this morning, as an
area of showers affecting north and central Mississippi moves towards
and across the Tennessee Valley. An embedded thunderstorm cannot be
ruled out today within the heaviest showers, but chances of them are
too low for inclusion into the TAF. CIG/VIS values should be reduced
into the MVFR range in shower activity today. Higher moisture values
and low clouds should continue tonight after the showers exit to the
east - continuing MVFR flying weather. IFR CIG reductions look
possible across the area late in the TAF.





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