Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 190520
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1120 PM CST Wed Jan 18 2017

.UPDATE...
For 06Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 844 PM CST Wed Jan 18 2017

Temps as of 8PM were ranging from the middle 40s in central middle TN
to the middle 50s in northern AL. A stationary front is sitting just
south of Birmingham and hasn`t made much northern progress over the
past few hours. Hires guidance is still suggesting this will make a
slow push to the north after midnight as it`s very weak parent
surface low over E. Texas strengthens and moves northeast. Temps may
fall another deg or two through the next couple hours, even under
cloudy skies, before the front pushes north and temps remain steady
or rise slightly into the morning hours.

An area of stronger showers are developing along the front in S. MS
with lighter showers to the north. These showers are slowly moving to
the northeast with a few very light radar returns finally showing up
in NW AL. Hires guidance has continued to slow these showers down so
have slowed the overnight pop`s as well. Also noticed the HRRR/RAP13
soundings are showing some drying in the low levels, right above the
surface, after midnight. This could confine much of the area just to
sprinkles/light rain until closer to the morning hours when the atm
finally saturates.

Also went ahead and removed the mention of thunderstorms through 12z.
Hires guidance is showing very little instability until that time,
with low lapse rates and a decently strong capping inversion
developing. This is likely due to the slower progression of the warm
front and therefore, the cooler temps. Cannot rule out a rumble but
it is not worth a 15% coverage at this time.

.SHORT TERM...(Thursday through Friday night)
Issued at 301 PM CST Wed Jan 18 2017

The warm front should by the late morning be located across our
northern areas, placing the forecast area in a rich southerly flow
form the Gulf. This increase of moisture and instability, and the
approach of a cold front will bring more widespread showers and
thunderstorms during Thursday. Model output overall is in reasonably
good agreement, bringing the heaviest axis of rainfall across our
western areas in the later morning and early afternoon, and the
central and eastern area during the afternoon hours.

Convective numbers suggest some of the storms could become "strong"
with locally heavy rainfall and wind gusts in the 40-50 mph.
Thunderstorms into the morning will be generally embedded within
widespread showers, then more numerous during the afternoon/evening
as the cold front approaches from the west. This front should move
across the region during the overnight of Thursday night/Friday
morning. Rain coverage and intensity should tapering off during the
day Friday. Rainfall totals should range into the 1-2 inch range
with this system. Only have a slight chance of rain in for our SE
area Friday afternoon. Also kept a slight chance of rain in for
Friday night, mainly for areas south of the Tennessee River.

.LONG TERM...(Saturday through Tuesday)
Issued at 301 PM CST Wed Jan 18 2017

Models continue to forecast various shortwaves pushing northeast
over the Tennessee Valley on Saturday. This is all ahead of another
upper low that swings northeast from Oklahoma into Iowa/Nebraska
during the day on Saturday. However, at the same time, the front to
the south of the Tennessee Valley continues to remain over the
Florida panhandle west into southern Mississippi. This should keep
deeper moisture advection well south of northern Alabama through
Saturday. 925 mb temperatures are progged to climb to between 15 and
16 degrees, so highs in the upper 60s to around 70 degrees look
good.

By Saturday night, models are in better agreement that a shortwave
impulse deepens into a surface low as it moves from central Alabama
northeast into northern Georgia/extreme eastern Tennessee. This
could produce some strong to severe storms during the evening and
overnight hours, as wet-bulb zero values around 7000 feet and CAPE ~
between 500 and 1000 J/KG is forecast to be in place by most models.
Luckily, not much helicity is forecast, so tornadoes do not look very
possible Saturday night. More of this activity may remain east of
I-65 and closer to Cullman to Dekalb counties, but this could change.

Models are in fairly good agreement snaking a warm front from near
northeastern Alabama southwest into central Mississippi before
extending it further northwest to the occluded surface low near
Memphis by Sunday morning. Not much instability shown by models, due
to the location of the warm front. However, as the surface low near
Memphis pushes east later in the morning, some models do show a band
of shower/storms developing just ahead of the surface low. If this
occurs, there is a brief window for additional strong thunderstorms,
but severe threat does not look very good. Some storms could produce
gusty winds to 50 mph and small hail, but instability and helicity
are not impressive. Regardless an additional 1 to 2 inches of
rainfall looks possible Saturday through Sunday, especially east of
I-65. Due to continued cloud cover and additional shower/storm
development, highs will be hard pressed to climb above the lower 60s.
Due to the slow moving and large area of upper level forcing progged
by the models Sunday night into Monday, keeping likely to a good
chance of showers in the forecast. Cooler and drier air will move in
behind the front Monday and Tuesday with highs in the 50s to lower
60s.

Expect a break from the rainfall by Tuesday Tuesday night, as high
pressure builds over the Tennessee Valley.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1120 PM CST Wed Jan 18 2017

Complicated TAF forecast through the next 24 hours. Ceilings are
currently VFR with CIGS around 5kft. CIGS and visibilities will
lower through the overnight and into Thursday, getting to IFR (around
1000ft) by Thu afternoon. They could drop even lower but confidence
is not high enough to go much lower at this time. They should stay
down through the end of the TAF period once they drop.

Light rain will move in after midnight and be off/on through the
morning. Heavier rain will move in after 16-18z west to east.
Thunderstorms are possible after 19-20z and have included a Tempo
group at this time.

&&

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

$$

NEAR TERM...LN
SHORT TERM...RSB
LONG TERM...KTW
AVIATION...LN


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