Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 171159
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
559 AM CST Sun Dec 17 2017

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 300 AM CST Sun Dec 17 2017

Temperatures are in the mid 30s to lower 40s in most locations,
except in far northeastern Alabama (Jackson and DeKalb counties).
In these locations temperatures are still in the upper 20s.

A shortwave trough axis continues to move northeastward through
Kansas and into Missouri at this time (seen well in water vapor
imagery). Also, a surface low/stalled frontal boundary are in place
over the Gulf Coast region. The afore-mentioned shortwave is helping
to increase low and mid level forcing along an inverted trough axis
extending to the northeast from the storm system over southwestern
Louisiana. A large area of moderate to heavy rainfall extends
northeastward from Louisiana into Arkansas/western Mississippi as a
result of strong moisture and warm air advection. Low and mid level
jet dynamics (ahead of the shortwave trough axis) are helping to
increase lift along and ahead of this inverted trough axis, producing
the moderate to heavy rainfall at times. This area of rainfall will
continue to push eastward across northern Alabama today, as an area
of low pressure moves eastward along a stalled front over the Gulf
coast states.

At this point based on radar trends, it looks as though some virga
will push into northwestern Alabama between now and 6 AM. Given the
very dry air in the lowest few thousand feet of the atmosphere, this
should remain primarily virga through 6 or 7 AM, even in northwestern
Alabama. Low dewpoint values do give a pause for concern given the
strong forcing expected to move into the area below 700 mb with the
strong jet dynamics, mainly east of I-65. Farther west, temperatures
look high enough that even if temperatures drop a bit due to wet-
bulbing, precipitation should remain primarily rain. A brief mix of
sleet or snow is not out of the question with the onset of
precipitation reaching the ground even west of I-65, but temperatures
should be warm enough to melt any brief mix of frozen precipitation
(especially given the fairly robust warm nose aloft seen in
soundings).

East of I-65, trends in temperatures and dewpoints will need to be
monitored a bit more closely early this morning. Luckily, most
guidance advects higher dewpoints and temperatures from the southwest
into those areas around and just after daybreak. Again, temperatures
could lower a bit due to wet-bulbing, but given the fairly robust
warm nose aloft show in model soundings and the strong warm
air/moisture advection expected, do not think that frozen
precipitation will be a factor by the time the precipitation moves
into that area around or after 9 AM.

As the shortwave continues northeast and weakens significantly early
this afternoon, lift associated with weakening low/mid level jet
dynamics will drop noticeably, except in southern and central
Alabama. There, a surface low is forecast to continue east along the
front (this is what is also helping to move the precipitation
eastward through the area). The abundant cloud cover and rainfall
will make for a raw day with highs only climbing into the lower 40s
east of I-65 to the upper 40s in northwestern Alabama. Between 1/2
and 3/4 of an inch of rain looks possible given the strong lift
across much of the area.

.SHORT TERM...(Tonight through Tuesday)
Issued at 300 AM CST Sun Dec 17 2017

Guidance continues to push the shower activity with the passing
inverted trough axis through the area by early this evening. Cannot
rule out a few showers in Cullman, Marshall, or Jackson counties
until just before midnight. Although high and mid level cloud cover
should push east of the area behind the front, low clouds look like
they will linger overnight. This, despite lighter winds, should help
keep temperatures from dropping much lower than the upper 30s to
lower 40s.

Model guidance continues to keep the colder air swinging well north
of the area, as westerly flow continues. No precipitation is
expected as the front drops southward into southern Alabama and
Georgia, where it stalls. Not really much of a northerly component to
the flow behind the front. This should allow temperatures to
moderate more on Monday. Upper 50s to around 60 degrees look
reasonable with mainly thin high cloudiness remaining in place. Low
clouds early should dissipate in the late morning/early afternoon
timeframe, but high clouds will persist.

After midnight on Monday, as another storm system pushes into Texas.
This system pulls the stalled front north as a warm front north over
northern Alabama or Southern Middle Tennessee. This brings rainfall
back into the forecast. Models continue to show some elevated CAPE
between 700 mb and 500 mb developing along this warm front. This
could produce some isolated thunderstorm activity late Monday night.
At this point, strong storms do not look possible. However, decent
low and mid level jet dynamics, climbing PWAT values, and elevated
instability would point to some heavier rainfall potential on
Tuesday. An additional 3/4 to one inch look reasonable. With the
strong warm air and moist advection, think highs in the 60 to 65
degree range are possible, especially if 925 mb temperatures are
mixed up to.

.LONG TERM...(Tuesday night through Saturday)
Issued at 330 AM CST Sun Dec 17 2017

At the beginning of the long term period, the large scale will
feature a quasi-zonal upper flow pattern across the CONUS, but an
embedded upr wave will provide focus for a potential heavy rain
event in our area. Confidence has increased in the last 24-hours on
the timing and location of heavy rainfall. Another rainfall event
still looks likely for the Friday-Saturday period, and forecast temps
have increased due to increased confidence in a slower frontal
progression. Details follow below.

At the beginning of the long term portion of the forecast (00-12Z
Wednesday), the area will already be under the influence of a
mid/upr wave ejecting EWRD out of the Southern/Central Plains.
Moderate to locally heavy rainfall will already be affecting much if
not all of the area by 00Z Wednesday and this is expected to linger
and potentially intensify during the 00-12Z period on Wednesday. As
the upr wave moves into the lower Miss Valley region late Tuesday
night into early Wednesday, frontogenesis is expected to take shape
W-E from near the Arklatex into the TN Valley. The suite of
regional/global guidance suggests the developing surface warm front
will move northward into our area during this time. The exact
northern extent/movement of the front remains to be seen, however,
the suite of guidance indicates the most favorable location
essentially cutting W-E right across northern AL. Strong, deep layer
ascent will be focused along a relatively narrow axis in association
with the warm front and aided by favorable upr jet dynamics.
Moisture advection will likewise be aided by 40-50 kt SW flow in the
850-700 mb layers. This will set the stage for relatively heavy rain
event from Tuesday into Wednesday. Total rainfall amounts from this
event primarily ending 00Z Thursday (Wednesday 6 PM CST) may total
around 2.5 to 3.5 inches. The WPC has placed nearly the entirety of
the HUN forecast area (and also locations especially to our west in
a Day-3 Excessive Rainfall Outlook. (Thanks for the coordination
WPC.)

The upr wave and associated sfc low will track eastward in the
progressive flow pattern, with most sensible weather effects coming
to an end late Wednesday. A slight chance POP lingers in the
forecast for about the eastern half of the area after 00Z Thursday,
otherwise, dry weather is still expected for the remainder of
Thursday before the next rain event begins.

By late Thursday, the large scale pattern is expected to become
highly amplified across western North America, with a pronounced
ridge taking shape across parts of Alaska and the West Coast,
forcing a deep trough to develop in the interior West CONUS. Cold
Canadian air, followed by Arctic air will being spilling southward
into the northern Plains late in the week. Significant height falls
and cold advection will lead to cyclogenesis in the lee of the
Rockies Thursday into Friday, with a cold front setting up across
the Central/Southern Plains. The southward/eastward progression and
timing of the sfc cold front is still uncertain however, as the
upper forcing largely remains centered over the Northern
Plains/Upper Midwest, and the upr trough axis may remain to our
west. Although the sfc cold front may cross the area as early as
Saturday morning, mid/upper flow may remain out of the southwest.
Nevertheless, forcing along the front and relatively strong, deep
moisture advection could lead to another moderate to heavy rain
event across the area beginning Friday and continuing into Saturday,
with elevated instability leading to at least a slight chance for
thunderstorms. Previous mention of the possibility for any wintry
weather seems to be more uncertain now. However, the larger scale
pattern would favor a sfc front essentially stalling in the region
by late in the weekend, with waves of low pressure developing along
the stalled front in the deep SW flow aloft. A number of the
ensemble members including operational models favor such an outcome,
however the location of the stalled front and thus the location of
precipitation still remain far too uncertain at this time.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 557 AM CST Sun Dec 17 2017

VFR cigs are still in place at both terminals. However, between 12Z
and 13Z, would expect this precipitation to start making it to the
ground at KMSL and mvfr vsbys to affect that terminal. Expect MVFR
vsbys to continue from 12Z to 15Z at KMSL and occur primarily from
14Z to 17Z at KHSV. VSBYS or cigs could drop into the IFR realm
during these periods at times when RA affects both terminals. -RA
should linger as cigs lower below 700 feet through 19Z at KMSL and
20Z at KHSV. Expect fog to form late tonight with light winds. May
have IFR or lower VSBYS with fog late tonight.

&&

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

$$

NEAR TERM...KTW
SHORT TERM...KTW
LONG TERM...KDW
AVIATION...KTW


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