Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 242348

Area Forecast Discussion
National Weather Service Huntsville AL
648 PM CDT Mon Apr 24 2017

For 00Z TAFS.


.NEAR TERM...(Tonight)
Issued at 326 PM CDT Mon Apr 24 2017

Skies have begun to clear across the TN Valley, as drier air filters
in around the northwest periphery of an upper low. Visible satellite
imagery, along with surface observations, so many locations west of
Interstate 65 mostly clear, while clouds across the east thin out.
Areas that are already cleared have warmed into the upper 60s, with
lower 60s in locations under mostly cloud skies. Based on latest
trends, the clouds will continue to erode and most areas should
become mostly clear toward sunset.

The upper low will continue to push to the northeast through the
overnight hours, as an upper level ridge builds across the TN
Valley. Clear skies and calm winds will allow temperatures to cool
into the lower 50s. This will combine with rather moist surface
conditions and patchy fog is expected to develop during the predawn
hours. Some areas may see dense fog, however due to some uncertainty
in how much we cool overnight, do not have dense wording in the
forecast. Any fog that does develop will quickly dissipate around

.SHORT TERM...(Tuesday through Wednesday)
Issued at 326 PM CDT Mon Apr 24 2017

The upper ridge will continue to build through the short term
forecast. Skies are expected to remain clear as the strong
subsidence persists across the area. Meanwhile, winds will become
southerly by Tuesday afternoon as high pressure builds to our south.
This will help temperatures warm each day, and highs on Tuesday will
reach into upper 70s to lower 80s. The upper ridge will begin to
push east of the area on Tuesday as a rather strong upper trough
digs southward across the Central Plains. Despite some upper level
height falls, clear skies will persist and strong WAA will help
temperatures warm into the low to mid 80s, with a few spots
reaching into the upper 80s.

.LONG TERM...(Wednesday night through Sunday)
Issued at 326 PM CDT Mon Apr 24 2017

The extended portion of the forecast is shaping up to be fairly
active, with a little uncertainty mixed in here/there, which is
typical for this time of year.

The period begins 00Z Thursday with an H5 shortwave and associated
sfc low tracking E/NE into the Great Lakes region. The cold front is
draped S/SW back into the ArkLaMiss, with a broad area of showers
and storms ahead of this feature, set to enter NW AL around 9Z. At
the same time, convection looks to get better organized on the Gulf
Coast, which introduces some uncertainty into the forecast for later
Thursday morning and into the afternoon hours for our local area.
Add to this drier air aloft filtering into the region from the W/NW,
and it looks to be a repeat of a few events ago (when we had decent
coverage of convection forecast and nothing really happened due to
these two things happening). For now, have kept the Superblend PoPs
in place, with greater coverage generally along/E of I-65, but am
not sold on anything other than some scattered showers/storms
occurring Thursday afternoon. To wishcast: assuming the dry air
arrives later than anticipated and the convection on the Gulf Coast
doesn`t materialize as forecast, we could see strong to severe
storms develop that afternoon. ~200 m2/s2 of 0-3km helicities are
available, along with about 1500 J/Kg ML CAPE. ~30-35kts of wind at
H85 would lead to some gusty winds, but nothing severe.

Temperatures Thursday should top out near 80 degrees across the
area, falling into the upper 50s/lower 60s Friday morning in the
wake of the front that pushed across the area.

Models are hinting at another trof to our W advancing E on Friday,
spurring cyclogenesis the Central Plains, moving NE into the Great
Lakes region. This should lift a warm front northward across the
local area on Friday, which may spur a few showers/storms. Much like
Thursday, there is some uncertainty on the front making it this far
N, but models are painting quite a bit of QPF across N AL/Srn Middle
TN Friday, assuming this is correct. Storms would have quite a bit
of CAPE to work with (~3000 J/Kg ML CAPE), and a bit more shear than
Thursday (~300 m2/s2 0-3km helicities). Highs Friday will climb into
the middle to upper 80s, with convective temps in the lower 80s, so
we should see *something* Friday. Gusty winds up to 40-50mph would
be the primary threat, should convective initiation ensue.

The bigger story for the extended revolves around the late-weekend
system taking shape quickly on the heels of the departing system on
Friday. Saturday will be a transition day, of sorts, with one system
departing to the E and another approaching from the W. Highs will
again top out in the upper 80s.

As the upper low traverses the Central CONUS during this time, sfc
cyclogenesis will occur over TX/OK Friday, deepening as it moves NE
into the OH River Valley. The upper trof itself takes on a negative
tilt, with widespread showers and storms developing ahead of the
front to our W Sunday afternoon ~18Z, crossing the MS River shortly
thereafter. The line of strong to severe storms would continue
pushing E, arriving on the doorstep of NW AL around or shortly
before 00z Monday. Damaging winds would be the primary threat, but
it`s too far out to pinpoint much else. This will be a system to
watch for sure, but will give the models a few more days to come
into better agreement before including much more than that in the
official forecast.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 648 PM CDT Mon Apr 24 2017

A few areas of lower clouds/cigs aoa 3K ft continue to affect parts
of the cntrl TN Valley this late Mon evening, as an upper level storm
system continues to translate ewd into the srn Atlantic Basin. The
lingering low clouds should gradually diminish with the onset of the
evening hrs. With light/var winds, patchy -br/MVFR vis should then
develop overnight. Vis may briefly lower into the IFR cat at times
going into the early morning period Tue. Patchy -br will then lift
shortly after sunrise, with VFR conds prevailing for the remainder of
the TAF period.





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