Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 220138

Area Forecast Discussion
National Weather Service Huntsville AL
838 PM CDT Thu Sep 21 2017

.NEAR TERM...(Tonight)
Issued at 838 PM CDT Thu Sep 21 2017

Afternoon convection has quickly diminished with the loss of heating
across the Tennessee Valley. A stable nocturnal boundary layer has
set up across the region, underneath a clear/mostly clear sky. The
result has been some very good radiational cooling as temperatures
have dipped into the lower 70s as of 8 PM CDT. With dewpoints in the
mid to upper 60s, lows should remain fairly mild in the 65-70 degree
range for most locations.

Moist boundary layer conditions, particularly along area rivers and
lakes and areas that received rainfall, will result in some patchy
dense fog in those areas after midnight. Those locations may begin
to experience visibility reductions as early as 03-04z late this
evening. Additionally, with winds expected to be very light to calm
overnight, patchy light fog is possible after 06z over much of the
region. Thus, have made this adjustment to the forecast after 06z.
Will need to monitor trends to see if an Advisory and/or Special
Weather Statement will be needed later tonight. Very minor
adjustments made to the forecast, but otherwise everything is on

.SHORT TERM...(Friday through Saturday night)
Issued at 240 PM CDT Thu Sep 21 2017

The jet/upper level diffluence will be well to the SW of the area
and the high is forecast to shift a little further east on Friday.
Models are also in pretty good agreement in keeping the forecast
dry with little in the way of convection noted. We may see a few
isolated storms develop in the afternoon over mainly the southern
half of the area but again with limited shear, storms won`t be very
organized. Main impacts will be wind gusts to 40 mph and brief heavy

Another vort max is forecast to move across the area on Saturday and
could result in a slight uptick in storm coverage. But with a few
degrees of cooling (still in the mid 80s) CAPE is forecast to be
lower, around 1,000 J/kg. Shear remains below 15kts, so storms will
continue to be pulse in nature.

Temperature wise, we are in the Dog Days of Summer with temps well
above normal close to the 90 degree mark. Normal for this time of
year is in the lower 80s. Fall officially starts at 302 pm Friday
with the Autumnal Equinox but won`t quite feel like fall.

Fog is possible both Friday night and Saturday night with dense fog
possible in the valley locations.

.LONG TERM...(Sunday through Wednesday)
Issued at 240 PM CDT Thu Sep 21 2017

The extended portion of the forecast begins 12Z Sunday, with an
inverted trof in place from SW to NE across the area. Further NE
across the DelMarVa/Northeast, an upper ridge remains anchored just
north of the approaching TC Maria. To our W, a longwave trof
enveloped much of the Intermountain West. This was rapidly being
approached by an upper ridge building ashore on the W Coast, and
it`s this feature that will finally give the trof the push it needs
(along with the aforementioned upper ridge over the DelMarva/NE
breaking down and shifting offshore). This will bring a cold front
toward/through the area by Day 7/8 (Thursday/Friday) of next week.
More details to follow...

With the inverted trof in place across our area to begin the
extended, PoPs will generally be erratic as diurnal convection
bubbles up by mid/late morning and fizzles out shortly after sunset
both Sunday and Monday. While the previous model runs had a bit more
coverage across the entire area, the 12Z runs mainly confined the
convection to mainly W/SW areas, so PoPs have been scaled back as
such. Again, this will probably change quite a bit between now and
then as much of this activity will be defined by mesoscale/outflow
boundaries that are yet to be determined. Daytime highs
Sunday/Monday will top out in the middle/upper 80s with morning lows
starting out in the lower/middle 60s.

Tuesday/Wednesday will be the warmest/driest of the period, with the
inverted trof having shifted quickly E/SE of the area and having
become absorbed by TC Maria. With daytime highs in the upper 80s to
around 90, we should be thankful that dewpoint temps are only in the
lower/middle 60s, which will make Heat Index values top out in the
lower 90s.

Thankfully a pattern change is on the horizon, though, with the trof
previously over the Intermountain W finally shifting E as an upper
ridge builds quickly into the W Coast. The initial cold front with a
departing low riding up and over the ridge well NE of our local area
will drag a cold front across the TN Valley Thursday into Friday,
with showers/storms overspreading the area from the NW sometime
Thursday morning. Post-frontal rainfall may linger into Friday
before the upper trof swings across the area by Saturday, bringing
cooler and drier air to the region to properly welcome the arrival
of Fall (albeit about a week late). It`s still a little far out on
the horizon, and at this point it`s total wish-casting, but there
are a couple of models hinting at daytime highs next weekend in the
middle 60s (with morning lows flirting with the upper 40s). Though
it`s not a definite bet just yet, it`s well within the realm of
possibility with a 1025mb sfc high building into the area in the
wake of the fropa during this time. Fingers-crossed, and stay tuned!


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 607 PM CDT Thu Sep 21 2017

A mostly clear to clear sky is expected this evening, with the loss
of heating quickly dissipating any isolated -TSRA that may be about.
The combination of a clear sky, decoupled winds, and a moist
boundary layer (from earlier rainfall) will result in light MVFR fog
formation at KMSL as early as 04z, becoming locally dense late
tonight (08-12z). Light fog is also possible at KHSV late tonight.
Thereafter, a return to VFR conditions is expected thru the rest of
the period.





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