Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 220529 AAC

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
1129 PM CST Sun Jan 21 2018

For 06Z TAFS.


.NEAR TERM...(Tonight)
Issued at 807 PM CST Sun Jan 21 2018

High level cirrus continues to spill into the region as upper
divergent flow from a powerful upper trough entering the lower MS
valley. Lower clouds are still several hours away. The high clouds
are just entering our far eastern counties where significant drainage
cooling was underway. For example, Fort Payne, Valley Head and
Scottsboro were already down to 43 degrees. Have made some
adjustments to hourly and minimum temperatures as a result. Southeast
flow remains rather light thus far, but that will pick up late
tonight. Otherwise, no other significant changes were needed to
tonight`s forecast.

.SHORT TERM...(Monday through Tuesday night)
Issued at 241 PM CST Sun Jan 21 2018

Models are in good agreement through the short term period,
indicating that the primary synoptic trough will continue to deepen
and assume a slight negative-tilt tomorrow as it lifts east-
northeastward across northern MO and into western IL. A prefrontal
surface trough trailing southward from the associated surface cyclone
is predicted to push eastward into northwest AL between 14-16Z, and
shift eastward for the remainder of the late morning/afternoon hours
before exiting into northwest GA around sunset. Although a fairly
solid QLCS should enter the forecast area tomorrow morning, it now
appears that the strongest vertical motions related to DCVA will
likely bypass our region to the northwest, and this along with an
increasingly veered component to the 0-6 km bulk shear vector should
allow convection to evolve from an initial linear mode into more of
a mixed mode featuring broken line segments throughout the day.

Forecast soundings still depict fairly limited surface-based
instability, which will continue to mitigate the overall threat for
severe weather. However, due to very strong environmental flow ahead
of the convective system, locally damaging winds in the 55-65 mph
range can be expected with the heaviest showers and any storms that
develop. Across the southeastern portion of the forecast area,
dewpoints may reach the upper 50s, which would support greater
instability and the potential for low-topped mini-supercells. With
guidance still suggesting 0-1 km shear in the 30-40 knot range
beneath a 50-60 knot low-level jet, any supercellular structures in
this region would need to be monitored closely for development of
brief tornadoes. The threat for convection will end with the
departure of the prefrontal trough, although some lingering light
rain will be possible during the late afternoon and evening prior to
the passage of the true Pacific cold front between 21-03Z. As a side
note...conditions will be borderline for a Wind Advisory during the
period from 14Z-02Z, but due to the fact that this is marginal and
confined mainly to the higher terrain of southern TN/northeast AL, we
will not issue one at this point.

The mid-level trough will begin to accelerate northeastward as it
crosses the southern Great Lakes and ejects into western New England
by 00Z Wednesday. However, the trailing trough axis extending
southwestward into the TN Valley will be much slower to lift
northeastward, and should support a high coverage of clouds and
perhaps even a few light showers during the day on Tuesday. Although
cold air advection will be rather weak, max temps have been lowered
due to the expected coverage of clouds. Clearing skies on Tuesday
evening will set the stage for strong radiational cooling as the
center of the north Pacific anticyclone builds further eastward into
the region, with lows forecast to reach the u20s/l30s.

.LONG TERM...(Wednesday through Sunday night)
Issued at 241 PM CST Sun Jan 21 2018

Northwest winds aloft will continue on Wednesday, as the center of
the surface high shifts eastward and over eastern Texas and into some
portion of northern Alabama. Fairly light northerly winds around 5
mph should pick up a bit in the afternoon, as stronger winds aloft
are mixed down to the surface. Some gusts in the afternoon could be
around 15 mph. Otherwise, some neutral or slight warm air advection
looks possible. With plenty of sunshine, it looks like highs could
climb into the mid to upper 40s in most spots. Maybe some lower 50s
in our southwestern most areas (such as Franklin into Cullman
counties in Alabama). The highest elevations of Southern Middle
Tennessee may only reach the lower 40s, with a decent lapse rate in
place, as temperatures aloft remain a bit colder. Overall forecast
temperatures may still be a bit warm, but ample sunshine and some
slight warm air advection may keep highs closer to current forecast.

Wednesday night, the high is forecast to be centered right over
northern Alabama and Southern Middle Tennessee. Along with clear
skies, this will allow for very efficient radiational cooling. Thus
could see one more night with lows in the upper 20s to around 30

As the surface high shifts further east on Thursday, southerly
return flow develops west of I-65. This warm air advection along with
continued abundant sunshine should help to warm highs into the mid
to upper 50s. Cooler temperatures in the lower 50s may hold on in
northeastern Alabama, as southerly return flow will still be very
weak that far east.

This southerly low level flow increases in strength Thursday night
into Friday morning. This should bring more moist air into the area.
Combined with some warm air advection, this should allow lows to only
bottom out in the lower to mid 30s. Cloud cover should increase on
Friday, as some upper level energy ahead of a developing storm system
over the Rockies and Upper Midwest pushes just northwest and into
the area. At this time, moisture looks too meager for any
precipitation this far east, especially given the amplified nature of
the departed surface high off the Atlantic coast. GFS wants to pull
some precip into northwestern Alabama, but leaving it our for now due
to the strong nature of the ridge further east.

As model guidance pushes the main forcing with this system into the
western Great Lakes region Friday night, better convergence along
with deeper moisture advection occurs near and west of the area. Some
deeper moisture pushes into northern Alabama in most models. GFS is
still the most aggressive and wettest. At this time, left a blend of
POP, producing scattered showers in the forecast and have mostly
cloudy conditions across northern Alabama Friday evening.

Even better forcing pushes into the area Friday night into Saturday,
as the storm system moves from the Great Lakes towards the Tennessee
Valley. This energy along with continued moisture advection could
hang around for a bit, as it will wait for the main storm system
approaching the area to push the precipitation and cloud cover out of
the area. This could make precipitation linger through Sunday
morning or a bit later. Thus a good dousing of rainfall looks
possible. One to two inches of rainfall looks very possible Friday
night through Sunday morning. At this point, dynamics look a bit
weaker than previous model runs. Also, instability looks non-
existent, even aloft. So for now kept thunderstorms out of the
forecast with this system. Some highs in the 60s still look possible
sometime next weekend, likely on Saturday.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1129 PM CST Sun Jan 21 2018

VFR flight weather conditions will continue until early Monday
morning. Ceilings of 020-030agl arrive ~12Z at KMSL and ~14Z at KHSV.
Southeast to south flow will increase to 15-25kt with gusts near 30kt
by 12-14Z. A broken line of showers and thunderstorms will move into
northwest AL including KMSL between 15-18Z and KHSV between 17-20Z.
Wind gusts of over 40kt are possible with this line, but will
continue to hold off on mentioning in TAFs until this becomes more
certain. South winds will shift to the southwest behind this line of
showers and thunderstorms late Monday afternoon and early evening.





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