Area Forecast Discussion
Issued by NWS Huntsville, AL

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782
FXUS64 KHUN 180104 AAB
AFDHUN

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
704 PM CST Tue Jan 17 2017

.NEAR TERM...(Tonight)
Issued at 651 PM CST Tue Jan 17 2017

Early evening surface analysis shows the cold front extending from
the Cumberland Plateau through northwest Alabama and into east
Central Mississippi. Ahead of the front, scattered shower activity
was noted across the region. Temperatures continue to run quite warm
for this time of year ranging from the middle/upper 60s across
northeast Alabama to the middle 50s just behind the front.

Latest soundings and MSAS/LAPS analysis indicates a narrow wedge of
very limited instability across east central and northeast Alabama.
However, overall radar trends have been for weakening and more
stratiform type echoes across the HUN CWA so will remove the thunder
wording. Based on the current movement of the front, which matches
fairly well with the HRRR, will taper pops quite notably from east
(highest) to west (lowest) and decrease across the board temporally
as the front moves through. Temperatures overall look fairly good
and have made only minor adjustments using latest blended guidance.

.SHORT TERM...(Wednesday through Friday)
Issued at 255 PM CST Tue Jan 17 2017

The cold front now south of the region will result in a slightly
cooler mid week, with highs only warming to around 60 (mid 40s north
to lower 60s south). Seasonable norms are around 51 this time of
year. Residual moisture plus weak upglide above the surface will make
for more clouds than sun, and a lower end chance of rain (especially
areas SW of Huntsville). This front should begin returning northward
as a warm front during Thursday, in response to low pressure forming
over the central plains. The return of this boundary along with
additional lift/moisture convergence will lead to a likely to
widespread showers, especially Thursday afternoon through predawn of
Friday. Model soundings and the broad view of stabilities indicate
convection will accompany more widespread shower activity. Only a
modest amount of lift/instability will be present within a more
sheared environment - typical for this time of year. At this time,
think the main threats posed by the stronger storms will be wind
gusts to 50 mph and locally heavy rains. Small hail is also possible
in the stronger storms.

The frontal boundary moving across the region before sunrise Friday
will result in rain tapering off from west to east during Friday.
Also behind the front, a modified Pacific high will move eastward, so
temperature falls west of this boundary will only be minimal. High
temperatures both Thursday and Friday will be in the mid/upper 60s.

.LONG TERM...(Friday night through Monday night)
Issued at 255 PM CST Tue Jan 17 2017

Models continue to forecast various weak shortwaves pushing
northeast over the Tennessee Valley Friday night. This is all ahead
of another upper low that swings northeast from Oklahoma into
Iowa/Nebraska. However, the air mass over the Tennessee Valley will
be very dry. Likely will see mostly cloudy conditions produced by
this lift, but low/mid levels look to be too dry for any
precipitation development. These high clouds may not be very thick,
until Saturday morning. So some radiational cooling could occur
Friday night with overnight lows, but this will be limited by winds
remaining between 5 and 10 mph west of I-65. Lows in the lower 50s
look look reasonable east of I-65, where winds might be less than 5
mph.

Some upper level ridging builds back in a bit more Friday night into
Saturday as well, but shortwaves continue to be progged by models
near/over the area. This upper level ridging/somewhat drier air near
the surface initially should help to limit rain chances on Saturday.
However, isolated to widely scattered showers and thunderstorms look
possible as lower level moisture increases, particularly later in
the day via increasing southerly flow. Could see some wind gusts
around 45 mph with some of these storms, as models show ~ 500 mb
surface based CAPE developing and even more elevated CAPE. At this
point though, this activity does not look severe in nature.

By Saturday night, as a potent and large upper level low deepens
over eastern Texas, a strong shortwave develops somewhere between
Mobile, AL and Memphis, TN. This strong shortwave quickly moves east
ahead of the developing upper low overnight and northeast of the area
by Sunday morning. Models differ the trajectory of this shortwave.
Given the flow, would expect it to be more northeastward, but that
far out, there is quite a bit of uncertainty, that I cannot discount.

Either way, strong to severe storms look possible, but likely will
be more organized/widespread if the more northeastward movement of
the shortwave is realized. At this point, given sounding and surface
CAPE forecast by most models, damaging winds look reasonable with
this system either way, possibly along with large hail. Could see a
window late Saturday night for some isolated tornado activity, but
again, this will greatly depend on how far north the shortwave moves.

A very strong 536 mb upper low/992 surface low is forecast by all
synoptic models to be near northwestern Lousiana or central Arkansas
Sunday morning. Subsidence provided by a more stabilized atmosphere
should be behind the departed warm front over most of northern
Alabama/southern middle Tennessee at that time. However, very strong
forcing, good shear, and decent low level helicity should develop
ahead of this upper low over Mississippi and Arkansas and move east
during afternoon and evening hours into the Huntsville Forecast area.

How much moisture advection we will see is in question though, as
better surface instability/moisture may be cut-off to our southeast
over the Florida panhandle into Georgia. This could make a big
difference concerning how severe and what threats we see on Sunday
afternoon and night over northern Alabama and Southern Middle
Tennessee. If we do not get cut-off, this could be a more
significant event that could include tornadoes. However, confidence
is very low concerning which solution will actually evolve. Either
way, another round of strong to severe storms looks possible Sunday
afternoon into Sunday night.

Luckily, the pattern becomes less unsettled by Monday, but the slow
moving upper low, will keep rain chances in the forecast through
Monday, before tapering off. Not very cold behind the front (due to
Pacific origin of the air mass). However, it will be cooler, as
highs in the 50s to lower 60s return.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 508 PM CST Tue Jan 17 2017

Cold front continues to approach the region from the west this
evening. Ahead of the front, scattered showers are impacting portions
of North Alabama. Based on current projection of the front and
precipitation shield will continue periodic light rain/showers at
both TAF sites through the remainder of the evening. Ceilings have
been variable all day long, although primarily MVFR at KMSL and VFR
at KHSV. Will slowly lower CIGS tonight, though still in MVFR
category, based on lower cloud deck noted on IR/VIS imagery in the
vicinity of the front and post-frontal. Winds will also gradually
swing around to the west and eventually a northerly direction by
morning, although speeds will be generally light.


&&

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

$$

NEAR TERM...15
SHORT TERM...RSB
LONG TERM...KTW
AVIATION...15


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