Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 242012

Area Forecast Discussion
National Weather Service Huntsville AL
212 PM CST Tue Jan 24 2017

.NEAR TERM...(Tonight)
Issued at 212 PM CST Tue Jan 24 2017

Generally quiet and unseasonably warm temperatures will prevail
tonight as a broad ridge builds over the Southeast. The ridge will
be moving east fairly quickly as a broad trough over the western half
of the country begins to move towards the Southern Plains. By 12Z
(6AM CST) Wednesday morning the TN Valley should be underneath the SW
flank of the ridge with warm air advection (WAA) developing between
06Z-12Z. As a result, overnight low temperatures should drop to the
mid to upper 40s as southwesterly sfc winds gradually increase by
early Wednesday morning to 10-15 MPH with gusts up to 20 MPH mainly
across the higher elevations. Mid-level cloud cover should also
begin to increase from west to east mainly around 10Z-12Z.

.SHORT TERM...(Wednesday through Wednesday night)
Issued at 212 PM CST Tue Jan 24 2017

Breezy southwesterly winds will continue through the day on Wednesday
as PVA associated with an embedded trough, oriented from NE/SW over
the eastern flank of the approaching longwave trough, moves over the
Corn Belt region to the Southern Plains. The trajectory of the
embedded shortwave trough is mainly due to the strength of the ridge
in place across the Southeast. As the ridge gradually moves east, the
shortwave trough opens as a wave, but it gains a more positive and
elongated orientation by Wednesday afternoon as it pushes a strong cP
airmass towards the MS River Valley and TN Valley. The frontogenesis
in combination with the TN Valley below the center of a strong polar
jet will mean strong (40-50 kts) of bulk shear with rain showers
developing. Though instability is non-existent, could see some gusty
winds in the vicinity of the strongest showers despite the lack of
convective updrafts. The moisture profiles are quite dry tomorrow
especially above 700 mb. So, whatever precipitation does occur should
be on the light side. Something to note with the onset of the
precipitation is that the model trend has been slower with the
progression. As a result, have trended a little slower with the
precipitation onset closer to late morning with a NW to SE gradient
in POPs.

An additional impact to the approaching cold front and strong WAA
preceding its arrival will be daytime highs in the low to mid 60s
with the highest temperatures over Cullman County and the SE portions
of north Alabama. This is where a greater amount of sunshine may be
realized before cloud cover completely envelopes the TN Valley and
also more has the greatest amount of time within the warm sector.
This will be the last `warm` day for a while as this front brings
unseasonably cold daytime highs but more seasonable overnight lows.
Wednesday night lows should drop pretty quickly into the low to mid
30s. More details on the coming cold conditions are found in the
below extended discussion.

.LONG TERM...(Thursday through Monday)
Issued at 212 PM CST Tue Jan 24 2017

Synoptic models are in fairly good agreement with the longwave
pattern on Thursday. Behind the departed frontal boundary, several
upper lows become established over the Ohio Valley and into the
southeastern Canada. Fairly strong upper level energy extends
southwest courtesy of a series of longwave troughs extending into the
Mid-Atlantic and the Rockies.

Models keep best forcing and moisture mainly just northeast of our
Southern Middle Tennessee counties, as these longwaves swing east-
southeast. Therefore, keeping isolated chances of rain out the
forecast through Saturday. With mainly zonal flow forecast by
models, and colder temperatures remaining primarily to our north,
expect highs in the mid to upper 40s most of that period. Could see
a few isolated showers edge into our Tennessee counties Friday
afternoon/evening, but models are showing better moisture still
remaining to our northeast. Despite stronger lift moving across the
area courtesy of the main longwave trough pushing east, think we
will be too dry.

As stronger upper level energy is progged to move south from Canada
into the Ohio Valley on Sunday, primarily zonal flow should
continue. Models are similar dropping good forcing down to between
Southern Middle Tennessee and the Great Lakes region during the day.
At this point though, it looks as though temperatures in the morning
will be too warm for any wintry precipitation, and kept isolated pop
out of the forecast. As we go into the afternoon hours on Sunday,
brought isolated rain/snow showers into the forecast, as strong
forcing could combine with enough moisture to produce some light
precipitation. Based on GFS sounding profiles, cannot rule out some
light snow.

However, models suddenly are quite different this run with their
solutions Sunday afternoon into Sunday evening. GFS develops
stronger upper level forcing and moves it about due south into
western TN/NE Arkansas. This means much colder temperatures and the
possibility of a few snow showers. ECMWF pushes this forcing much
more to the southeast rather than south. This would be much warmer
and possibly not allow any light snow development, just light rain
showers. As a result, which solution ends up being correct will have
a big impact on where/what type of light precipitation could
develop. For this update, stayed close to previous model run
indications, and a bit lower in pop than GFS, given ECMWF further
east solution for precipitation. Either way, this will likely change,
but keeping primarily a light mix of rain/snow showers Sunday
afternoon into Sunday evening. If GFS is right, this could be all
snow (but still isolated) after midnight on Sunday. Based on
Superblend temperatures, kept a period of light snow showers after
midnight on Sunday through daybreak on Monday.

The colder air pushes into the area behind the departing front
Sunday night into Monday with either solution. It will just be a
matter of how cold it gets on Monday. For now, as indicated, kept
with colder solution and highs in the upper 30s to lower 40s. Given
the wide variance of solutions with models at this point, went with
Superblend temperatures.

Both extended models hint at warmer temperatures again by Tuesday,
as upper level ridging builds quickly over the Tennessee Valley
Monday night into Tuesday (highs 50s).


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1125 AM CST Tue Jan 24 2017

VFR conditions are expected to prevail for most of the period until a
a cold front approaches the region late in the period between 12Z-
15Z. Low MVFR clouds may accompany the passage of this front but due
to the uncertainty on the timing of the low cloud cover will include
a scattered FM group with VCSH for now.





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