Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 172324

Area Forecast Discussion
National Weather Service Huntsville AL
524 PM CST Sun Dec 17 2017

For 00Z TAFS.


.NEAR TERM...(Tonight)
Issued at 302 PM CST Sun Dec 17 2017

Weak upper-trough has shifted well NE of the region into the Ohio
Valley, with the surface low and associated LLJ pushing well into
the Southern Appalachians of NC/TN. As a result, the rain shield has
dissipated as this lift has moved off with it. The result late this
afternoon has been some lingering low clouds and perhaps some light
drizzle that will persist in a few areas (especially the higher
terrain of the Cumberland Plateau) through this evening. A light
southerly fetch off the Gulf of Mexico will help to maintain this
warmer, moist air mass overnight. Model soundings show the potential
for low stratus and fog development after sunset -- with some
localized patchy dense fog possible after midnight. Thus, have added
a mention of this to the forecast from Midnight through 14z Monday
morning. Other than perhaps some lingering drizzle, dewpoints in the
mid to upper 40s will keep temperatures quite mild and they will
struggle to drop off more than a few degrees tonight.

.SHORT TERM...(Monday through Wednesday)
 Issued at 302 PM CST Sun Dec 17 2017

Attention quickly turns to another bigger (potentially much bigger)
rain maker later this week. Monday should be fairly dry and
seasonable -- though it may feel damp and chilly underneath the
lingering stratus for most of the day. A couple of breaks in the
cloud cover in the afternoon may allow for some limited heating to
get us in the 55 to 60 degree range for highs by mid-afternoon. Any
precipitation chances will be displaced well to the south as forcing
will be tied to activity that develops along weak boundary along the
Northern Gulf Coast. Thus, the broad subtropical ridge over the Gulf
of Mexico that is trying to nose its way into the area will play a
much more dominant role in the weather here for Monday.

A change in the pattern will occur toward the middle of the week as
another stronger mid-level shortwave trough is progged to track from
the Ozarks into the Mid South and eventually the Tennessee Valley.
Ahead of this feature SW flow will continue to advect Gulf moisture
into the region. The aforementioned boundary over the Northern Gulf
Coast Monday will lift NE into the Tennessee Valley on late Tuesday
morning into Tuesday afternoon. This feature will serve as a focus
for rain showers, but also bring in this richer moisture as PWATs
climb to around 1.5 to 1.6 inches by 18-00z Tuesday afternoon and
evening. There will likely be a few breaks in the cloud cover Tuesday
afternoon which should allow for some modest elevated instability to
develop. Thus, have maintained a chance for thunderstorms in this
widespread shower activity. Total QPF forecast for Tuesday is
expected to be 1/2 to 3/4 of an inch, with heavier rains to come
Tuesday Night into Wednesday.

Model soundings (in this moist air mass) will favor a heavy rainfall
potential, given the warm cloud layer, a small amount of CAPE and an
advancing cold front moving in late Tuesday Night into Wednesday.
The upper-trough will become negatively tilted as it tracks into the
Mid South Tuesday Night. In response, a strong 70 kt LLJ will ramp up
from Central Mississippi into Northern Alabama late Tuesday night
into Wednesday morning. This once again places much of the Tennessee
valley in the favorable right entrance region of the jet, as the
surface cold front swings in from the west later in the day. The
combination of deep moisture and strong synoptic and mesoscale lift
will result in numerous to widespread heavy showers (and isolated
storms) late Tuesday Night through Wednesday morning. Given the storm
motion parallel to this boundary, the threat for training will
exist. Thus, a heavy rainfall and flooding threat is increasing for
this midweek time period -- with some model/WPC QPF approaching 1.5
to 2 inches for the 06-12z Wednesday time period alone. Total
rainfall may be closer to 2 to 3 inches for this midweek over much of
the region. This certainly warrants a mention in the HWO and we may
potentially need to issue a Hydrologic Outlook later tonight for
flooding impacts midweek.

The cold front will pass through the area Wednesday afternoon,
bringing in some drier air and an end to precipitation by that
evening and especially the overnight period. As one might imagine in
this pattern, mild but cloudy continues continue with highs in the
low 60s and lows in the 50s.

.LONG TERM...(Wednesday Night through Saturday)
Issued at 302 PM CST Sun Dec 17 2017

The progressive and wet pattern will continue late into next week.
We`ll start the long term period in the wake of storm system with
another wave encroaching by the weekend. Global models are in decent
agreement Wednesday night and Thursday, showing zonal flow
establishing over in the mid/upper levels.  This will likely be our
driest and most benign day of the long term given that we`re in
between two systems. The main feature we`ll be keeping on eye for
Thursday will be the development and progression of a system coming
out of the Rockies. Global models are continuing to show some
disparities with timing and strengthen of this system by Friday.
Regardless, the attendant cold front and prefrontal precipitation
will be spreading into the TN Valley by the afternoon. Mid level
support and upper level jet dynamics look to wane in both the GFS
and and Euro so have decided to remove thunder wording on Friday.
However, while the parent system is quick to shift through the Great
Lakes area by Friday night, the front appears to slow as it slides
through the area. This solution will support the previous QPF
forecasts of around 2" through Sunday.

It is definitely worth noting that amid the disparities in the GFS
and Euro, ensemble members (specially GFS) are showing a wide range
of solutions with the placement of the front over the weekend.  It
is also worth noting that GFS (and ensemble members) and Euro have
showed warmer temps through the atmosphere as we head into the
weekend. Regardless of the final placement of the front over the
weekend, all lingering precipitation will be in liquid form, but
confidence remains low.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 520 PM CST Sun Dec 17 2017

A mixture of MVFR to IFR cigs are present over the TN Valley this
evening. A strong inversion just above the surface will trap moisture
near the surface and result in cigs lowering down to IFR (between
700-900 ft) this evening. As winds become calm after 06z cigs/vis
lower further towards LIFR conditions with bases around 200-300feet
and visibilities possibly as low as 1/4SM at times. Not confident
enough to include 1/4SM in the TAFs at this time but may need to
add for the 06z TAFs. Cigs/Vis improve slightly with ceilings ranging
from 1,000-1,500 feet after 13z Monday morning.




LONG TERM...Barron

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