Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 230446
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1046 PM CST Thu Feb 22 2018

.UPDATE...
For 06Z TAFS.

&&

.NEAR TERM...(Rest of tonight)
Issued at 928 PM CST Thu Feb 22 2018

Seasonably strong deep-layer southwesterly flow continues across the
TN Valley this evening, as the region remains along the western rim
of a 500-mb subtropical ridge centered off the southeastern Atlantic
coast. To our northwest, a mid-level vort max is forecast to eject
northeastward from the high plains of CO into the upper MS Valley in
the flow downstream from an amplified longwave trough over the
western CONUS.

At the surface, a low related to the northern stream vort max is
expected to slowly deepen and lift north-northeastward from the Ozark
plateau into the western Great Lakes overnight, resulting in a
subtle increase in south-southeasterly winds across the local area.
It appears as if an axis of widespread rain and elevated convection
tied to the lows trailing confluence axis will remain just to the
north/west of the region through at least sunrise tomorrow, and we
will maintain only a silent 10 POP in the forecast for all areas
overnight.

Even with scattered high-level clouds spreading over the region from
the southwest, conditions will become increasingly favorable for the
development of fog after Midnight, and especially between 09-13Z
when the boundary layer may decouple. Although not noted in the
official grids, locally dense fog may also briefly develop during
this period in wind-protected valleys. Forecasted min temps were
adjusted downward in a few locations to reflect both current obs and
the potential for greater radiational cooling early Friday morning.

.SHORT TERM...(Friday through Saturday)
Issued at 315 PM CST Thu Feb 22 2018

Models push the front as far northwest as northern Kentucky
southwest into western Arkansas by Friday. Could see an isolated
shower or storm develop near the MS/AL border, but this may be
stretching it. This activity may remain in Mississippi.

However, by Friday night, models move this upper level energy
northeast and combined with good deep moisture produce a round of
showers and isolated thunderstorms over northern Mississippi and
northern Alabama. Good shear is shown by models, but not the
strongest low level jet. However, enough bulk shear and just enough
surface based instability may be present for a few strong storms
producing locally heavy rainfall and gusty winds. This activity will
likely decrease in coverage and become scattered Saturday morning.
An additional half and inch to one inch of rainfall looks possible
with this system. A strong cold front pushes east from the Rockies
into Missouri/Texas during the day on Saturday. Stronger low and mid
level jets are forecast by models with this storm system. In fact,
a 50 knot 850 mb jet sets up and is oriented so that the Tennessee
Valley is in the right entrance region of this low level jet. We
could see some scattered showers/storms begin to redevelop on
Saturday (especially it the afternoon). With ample bulk shear ahead
of this system by then and CAPE values around 500 J/KG or possibly
more, a decent setup for strong to severe storms looks possible. THe
main threat would be damaging winds and additional heavy rainfall.
Cannot rule out brief, weaker tornadoes, but uncertainty is very high
given differences in surface based CAPE values.

.LONG TERM...(Saturday night through Wednesday)
Issued at 315 PM CST Thu Feb 22 2018

As the front moves east into Mississippi Saturday evening, guidance
depicts a fairly solid line of showers and thunderstorms developing
as it pushes into northwestern Alabama night around or just after
midnight. Although, CAPE decreases in models we will continue to have
at least some surface based instability. With very strong forcing
along and ahead of the front, strong to severe storms will remain
possible, as the main upper jet pushes in behind the front. The same
threats look possible as during the day. This could produce an
additional 1 to 2 inches of rainfall. Some isolated flash flooding
and river flooding could result given our very Saturday conditions.

The rest of the extended is very uncertain, as models are having a
hard time agreeing on the timing and strength of a amplified upper
level trough axis next week. Thus, kept close to blended guidance
through Wednesday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1046 PM CST Thu Feb 22 2018

VFR conditions will prevail at HSV/MSL early Friday morning,
although with only sct high-lvl clouds and winds expected to become
lgt/vrbl btwn 09-13Z patchy fog should develop across most of the
region. Winds will veer to the SSW and increase to 8G16 kts shortly
after sunrise tomorrow, with development of sct-bkn cu expected in
the 3500-4500 ft layer by 16Z. An increase in the coverage of as/cs
clouds is anticipated by late afternoon as a subtle upper-level
disturbance approaches the region in strengthening SW flow aloft.
This wave will initiate development of sct-nmrs shra and tsra to the
west-southwest of the region early Friday evening, with this activity
forecast to spread newd into the vicinity of the terminals by
24/04Z.

&&

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

$$

NEAR TERM...70/DD
SHORT TERM...KTW
LONG TERM...KTW
AVIATION...70/DD


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at weather.gov/huntsville.



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