Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 270520

Area Forecast Discussion
National Weather Service Huntsville AL
1220 AM CDT Sun May 27 2018

For 06Z TAFS.


.NEAR TERM...(Tonight)
Issued at 701 PM CDT Sat May 26 2018

Convection continues to make its final hurrah across the TN Valley
this evening, with just a few showers/storms of note on KHTX. The
trend will likely continue for the next hour or so, with only a rogue
shower/storm expected overnight. Unlike last night where we had
widespread rain and lingering stratus during the overnight/morning
hours, with convection being more sparse today, fog formation is
likely by sunrise. The fog will be more dense in locations that
received heavy rainfall this afternoon/evening, but have included the
chance for at least patchy fog area-wide in the grids. Will monitor
obs through midnight and will make updates as warranted to more
dense-wording in the official forecast (if need be).

Several changes were necessary for the update this evening as a
result of this thinking, with PoPs decreased and Sky grids updated as
well. The HRRR was doing a fairly good job with the coverage of
isolated/scattered convection at the moment, and so those Sky grids
were implemented for the rest of the evening/overnight hours. With
more clearing noted in the grids than before, this better supports
the thinking for fog formation (as opposed to 80-100% coverage from

Temps are on track to bottom out in the upper 60s to around 70F again
for Sunday morning.

.SHORT TERM...(Sunday through Monday night)
Issued at 159 PM CDT Sat May 26 2018

A narrow mid to upper level ridge position builds into place north
of Alberto as the upper trough remains over the lower OH and MS
valleys on Sunday. The ridge gives way quickly during the afternoon
as the first of several spokes of vorticity rotate north and
northwest into the TN valley. By late in the day, expect numerous
showers and thunderstorms along this feature that will translate
northwest and west during the evening. A narrow dry gap behind this
band is quickly filled in by a potentially heavy band of showers and
thunderstorms late Sunday night into early Monday. Have carried the
highest PoP in our southeast (Likely), with chance further north. The
models have shown a sharp gradient to this particular rain band in
successive runs, making the PoP forecast challenging. It`s possible
that our TN counties may see very little from this band, while north
AL gets a period of heavy rain Monday morning. Then, this abruptly
diminishes and/or dissipates during day as a strong dry slot becomes
ingested north-northwestward into the circulation.

On Monday Night, depending on the track of Alberto either into
southwest AL or southeast MS, a compact precipitation area around
the circulation may affect parts of our area as early as late Monday
night. The best chance may be our southwest counties based on the 12Z

.LONG TERM...(Tuesday through Friday)
Issued at 159 PM CDT Sat May 26 2018

As one would expect, some model discrepancies are noted with the
track of Alberto (or the remnants of Alberto) after it makes
landfall. The operational GFS/Euro track the system across the heart
of Alabama Tuesday and to near western Kentucky by early Wednesday.
The NAM is a bit faster and has more of a westward track, and is
supported by a large number of GEFS members. The official NHC track
is a blend of the two camps and that seems quite reasonable.

For now, that would place the highest potential QPF across western
Alabama Tuesday underneath the core/proximity of the lower. Depending
on the track as well some areas may be in the favorable right front
quadrant for rotating convective bands early Tuesday. Despite the low
lifting north of the Tennessee Valley by Wednesday, expect scattered
to numerous storms again especially in the afternoon with broad
troughiness and high PWATs in place.

Not much notable change on Thursday, and in fact we may see another
uptick in precipitation with the passage of an upper s/w trough
during the afternoon/evening hours. In general, have trended toward
the lower end of the diurnal ranges (warmer mins/lower maxes) given
the tropical airmass and likely early cumulus/convective initiation.
The upper flow becomes a bit more northwesterly by late in the work
week/toward next weekend as the upper ridge begins to build into the
Plains. The high amplitude nature of the ridge will bring potentially
extreme heat to the southern/central plains late next. Closer to
home, will continue chance pops for Friday/Saturday as there`s some
indications of another piece of northern stream energy working its
way across the Tennessee Valley during that time frame.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1220 AM CDT Sun May 27 2018

Isolated convection firing up along outflow boundaries across the
area will continue off/on overnight, keeping at least isolated
chances for a shower/storm through sunrise. Confidence in convection
directly affecting the terminals is low, so no official mention was
included with the 06Z TAF issuance. Fog formation is likely
overnight (compared to more stratus coverage last night/this
morning), so have retained the IFR conds possible between 27/10-14Z
at both KHSV and KMSL. Fog will dissipate shortly after 14Z with VFR
conds returning/prevailing thru the end of the TAF period.
Showers/storms will begin developing by this time, so a VCTS was
mentioned at both sites beginning at 27/17Z.





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