Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 220518 AAC
AFDHUN

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
1118 PM CST Wed Feb 21 2018

.UPDATE...
For 06Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 916 PM CST Wed Feb 21 2018

A tight temperature gradient, wind, and pressure change is in place
as a cold front stretching along a line from Halleyville AL to
Fayetteville TN has stalled. Overrunning southerly flow across this
front and an embedded shortwave trough within the subtropical jet is
causing mostly anafrontal rapidly moving convective updrafts to
develop. Showers and a few lightning strikes have already moved
across northwest Alabama and expect NW AL to be the main focus for
convective activity through at least 06Z when the front begins
shifting back to the NW as a warm front.

Hi-res mesoscale models do depict modest wind shear with these cells
but mainly over N MS/W TN as a quick moving shortwave trough moves
across the area between now and 06Z. After 06Z the boundary moves
northwest, and the hi res models hint at the area of convection over
MS/TN merging into a line and shifting east overnight arriving by
12Z. However, the differences in timing and challenges with the
surface front movement make the progression of this line difficult to
ascertain. The timing of the strengthening ridge to the SE is also a
challenge.

Noticing some cells have broad rotating updrafts over N MS which is
in line with 0-3 km SRH values spiking up to 300 M2/S2 over N MS/W TN
as the weak embedded shortwave moves northeast. With the retreat of
the front could still see some cells with rotating updrafts over NW
AL between 06-12Z but would likely only cause gusty winds given the
lack of surface based storms and weaker low-level shear. Will keep
the gradient of POPs/Wx in the forecast with a very gradual
decrease. Also, made some adjustments to the sky cover with partly
cloudy conditions present over NE AL.

Otherwise, made modest changes to the temperature and dewpoint
forecast retaining the gradient along the front and actually warming
temperatures overnight over NW AL after front moves northwest.

.SHORT TERM...(Thursday through Friday night)
Issued at 307 PM CST Wed Feb 21 2018

Lingering showers and thunderstorms will be possible through the
morning hours on Thursday, mainly across northwest Alabama and
southern middle Tennessee as a cold front remains stalled near the
area. The subtropical ridge is expected to strengthen through the
day, which will help lift the cold front further northward, along
with reducing rain chances through the late morning and afternoon
hours. Along with reducing rain chances, the increasing low and mid
level heights will combine with continued southerly surface flow to
allow temperatures to warm into the upper 70s once again. If we can
lose some of the cloud cover tomorrow, a few spots may break 80
degrees for the 2nd or third day in a row.

Although the forecast area should remain mostly dry Thursday night,
areas north of the TN River may see some isolated showers if the
aforementioned cold front does not retreat as far north as currently
forecast. Any light showers that do develop will be short-lived,
with Friday expected to be mostly dry across the TN Valley. The above
normal temps will continue, with lows Thursday night in the 60s once
again, and highs on Friday in the lower to mid 70s. By Friday night,
a system well to our west will begin to approach the area, and push
the cold front well north of the area. The subtropical ridge will
begin to breakdown as an upper trough moves into the Plains. Weak
shortwaves may be able to propagate around the northern periphery of
the ridge and bring isolated to scattered showers across the TN
Valley late Friday night.

.LONG TERM...(Friday night through Tuesday)
Issued at 307 PM CST Wed Feb 21 2018

High pressure off the southeast Atlantic coast will gradually
flatten and shift further to the south and east on Saturday, as a
deepening upper-trough over the Central Plains ejects northeastward
into the Great Lakes region. In response, a surface cold front will
move east from the Southern Plains into the Mid-South and lower
Mississippi Valley during the day on Saturday, and eventually into
the Tennessee Valley Saturday night. THE ECMWF and GFS differ
slightly with the magnitude of this feature, with the GFS the more
aggressive solution in showing a stronger, sharper trough (and thus
greater wind shear and dynamics). ECMWF continues to trend with a
broader upper-trough and potentially less dynamics/lift.

Ahead of the front, southwesterly flow will continue to advect a
warm, moist air mass into the region, with dewpoints climbing into
the mid 60s by Saturday afternoon. Instability will somewhat be in
question, as morning cloud cover will likely limit daytime heating
and potential destabilization of the atmosphere. Additionally, lapse
rates, especially low-level appear to be fairly weak. Model
soundings do suggest some SBCAPE values around 1000 J/kg, with
ML/MUCAPE around 700-800 J/kg around 00z Sunday as there will be some
breaks in the clouds. Coinciding with this, a 60+ kts LLJ will ramp
up along the Mississippi River by 00z and eventually shift into
Tennessee Valley around 06z. This feature will generate more robust
convection along and ahead of the front as it pushes east.
Additionally, 0-1 km shear values will increase to 40 kts, with 0-3
and 0-6 km values ramping up to 50-60 kts -- and effective inflow
shear roughly progged to be around 45 mph. This will help to organize
these storms into a MCS/QLCS feature that could potentially maintain
a few rotating mesocyclones. As a result, at least a limited severe
thunderstorms threat is possible, with damaging straight-line winds
appearing to be the primary impact at this time.

The good news is that the front and squall-line should be moving
into a likely less favorable thermodynamic environment by 06z Sunday,
so a weakening trend is possible as it moves across Northern Alabama
and Southern Middle Tennessee during the nighttime hours on Saturday
night. This frontal boundary will drop just south of the region on
Sunday, but with continued WSW flow aloft and the appearance of a
couple of weak shortwave troughs, continued opportunities for
isolated to scattered showers will continue through Monday. A
slightly cooler air mass will establish itself early next week with
highs remaining in the 60s for most of the next several days. Ridging
will occur late in the period as a upper-high over the Gulf of
Mexico noses its way into the area. This would support a drier day
with fair weather to close out the period.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1118 PM CST Wed Feb 21 2018

VFR conditions are expected to prevail for most of the next 4-5
hours mainly over northeast Alabama and portions of north central
Alabama. However, a front that is now moving NW across NW AL may
cause MVFR cigs mainly over NW AL along with some showers close to
KMSL. The wind direction change from NE to SE is taking place now and
should continue through 06Z. Oscillations in the cigs between VFR
and MVFR are likely between 06-14Z at KMSL. Reduction to MVFR may
also occur at KHSV as well. Lifting of these low clouds may occur
around 15Z before the cigs scatter after 18Z at HSV and maybe by 20Z
at MSL. Light showers and possibly isolated storms will be likely
especially at KMSL through at least 14Z. Then the front is expected
to cross over NW AL again after 23Z.

&&

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

$$

NEAR TERM...SL.77
SHORT TERM...73
LONG TERM...AMP.24
AVIATION...SL.77


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