Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 240510

Area Forecast Discussion
National Weather Service Huntsville AL
1110 PM CST Tue Jan 23 2018

For 06Z TAFS.


.NEAR TERM...(Tonight)
Issued at 811 PM CST Tue Jan 23 2018

RGB imagery shows a stratus deck based at 025-035agl slipping
southeast into northern AL at this hour. This trend will continue,
with most clouds staying along and north of MSL-HSV-4A9. The western
edge of the clouds was clearing far western TN and this trend should
also continue eastward overnight. Have made some modifications to
account for the near term cloud trends. Temperatures have already
dipped into the middle to upper 30s this evening, and have made
adjustments to hourly temps. Morning low forecasts look good if the
clearing trend presents itself as indicated.

.SHORT TERM...(Wednesday through Thursday night)
Issued at 233 PM CST Tue Jan 23 2018

An amplifying mid-tropospheric shortwave trough is expected to drop
southeastward from western NE into MO overnight, and spread rapidly
southeastward across TN/KY tomorrow before reaching the southern
Appalachians by 00Z Thursday. Although guidance does suggest that
weak elevated vertical motions will develop as this system crosses
the region tomorrow morning, moisture in the same layer will be
insufficient to support more than an increase in cloud cover.
Stronger low-level subsidence in the wake of the departing trough
will allow any lingering stratocumulus clouds to dissipate shortly
after sunrise, leaving sunny skies during the afternoon, and this
should allow temps to warm into the lower 50s for most of the area.

Northwest flow aloft will increase in the wake of tomorrow morning`s
trough, providing mostly clear skies on Wednesday night/Thursday
morning. At the surface, a broad ridge will translate northeastward
across the TN Valley providing calm winds, and the combination of
these factors should allow for strong radiational cooling. Lows will
likely fall into the mid 20s for most outlying areas, and (although
not explicitly mentioned in the forecast) conditions will be
favorable for patchy freezing fog in river valleys and near large
bodies of water. Light/variable winds will continue on Thursday, with
abundant sunshine anticipated as a mid-level shortwave ridge
approaches the region from the southwest, and this should provide
even warmer max temps in the mid 50s. A slight warming trend will
continue Thursday night, as stronger southerly return flow begins to

.LONG TERM...(Friday through Tuesday night)
Issued at 233 PM CST Tue Jan 23 2018

During the day on Friday, southwest return flow around the surface
high (over the Carolinas and off the eastern seaboard) increases,
especially near and west of the I-65. Further east, winds may remain
a bit more southeasterly keeping highs a few degrees cooler (52 to 56
degrees). Not expecting much cloud cover, so couldn`t rule out a few
highs reaching 60 degrees, but think higher temperatures in the west
will mainly remain below 60 degrees. Models tighten up the pressure
gradient in response to the storm system pushing into Kansas and the
western Great Lakes region. Thus kept gusts up to around 20 mph in
the forecast in the afternoon.

Models remain consistent from last night keeping the best
convergence and moisture content west of Alabama through Friday
night, maybe into Saturday morning. Thus only have isolated to widely
scattered chances of light showers in the forecast after midnight on
Friday into the early morning hours on Saturday. There will be deep
enough moisture and ample convergence for mostly cloudy conditions to
develop around midnight across northern Alabama. Newest guidance
looks to be coming into a little better agreement on the evolution of
the next storm system moving through southeastern Canada during this
period and into the weekend. Models tend to develop stronger
convergence and deeper moisture ahead of the longwave trough axis
associated with this system Saturday afternoon into Saturday night. A
complicating factor is that models continue to show some upper level
forcing/energy pushing northeast from the central Gulf of Mexico at
the same time. However, they are very different with the speed and
movement of this feature. This may help to either increase rain
chances or keep them lower (ECWMF solution) or delay high
precipitation chances (GFS solution). At this point went a bit below
blended guidance given the uncertainty with 30 to 40 percent pop
Saturday afternoon. Highs in the lower 60s look reasonable given the
925 mb temperatures and mixing of boundary layer.

There is quite a spread in model precipitation guidance given the
differing solutions with the energy pushing northeast or east-
northeast from the Gulf of Mexico Saturday night through Sunday. Thus
keeping a bit below guidance, with high chance or likely pops
Saturday night into Sunday. Timing of heavier rainfall and if it will
hang around through Sunday is very uncertain. Due to limited
instability and the uncertainty with this forecast, leaving
thunderstorms out of the forecast with through Sunday.

Somewhat cooler air returns Sunday night into early next week, as
the storm system pushes east of the area. Highs return to the mid 40s
to lower 50s and lows drop back into the upper 20s to 30s.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1110 PM CST Tue Jan 23 2018

Low clouds have pushed southward and into the KMSL and KHSV terminals
over the last hour. These clouds, currently around 3000 feet, have
eroded from the west, and are now east of KMSL. Trends suggest that
the clouds will continue to erode over the next couple of hours and
push out of the KHSV terminal by 08Z. This will leave VFR conditions
through the remainder of the period. Winds will remain from the
north-northwest and may become gusty during the afternoon on





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