Area Forecast Discussion
Issued by NWS Huntsville, AL
FXUS64 KHUN 220535
Area Forecast Discussion
National Weather Service Huntsville AL
1135 PM CST Tue Feb 21 2017
For 06Z TAFS.
Issued at 903 PM CST Tue Feb 21 2017
A southerly flow around an area of low pressure at the surface and
aloft, stacked across the lower Mississippi Valley, continued to
bring periods of light rain, along with some light drizzle and fog.
This system showing a nice comma head from a satellite view (water
vapor through infrared bands) was centered near Greenville MS. A
notable dry slot aloft was working its way northward towards SW
Alabama, while this system itself headed towards the NW Gulf.
Precipitation from this system has edged further to the east as has
been previously advertised, now extending from south of Apalachicola
to NE Alabama (mainly east of I-65 and south of the Tennessee River).
Radar trends and short term model output has this next batch of rain
overspreading the area (more to the east) during the overnight. The
drier air SW of the area will tend to put an end to rain during the
late night. However with plenty of moisture remaining, drizzle and/or
fog will at times affect most of the area. Enough clouds remaining
should help minimize widespread and/or dense fog for the most part.
Made small adjustments, delaying fog/drizzle wording until after
midnight (given that it`s generally more likely then). Temperatures,
cloud coverage, winds, rain chances, etc. otherwise looked mighty
fine at this time.
.SHORT TERM...(Wednesday through Thursday Night)
Issued at 300 PM CST Tue Feb 21 2017
Low clouds, drizzle and light rain pockets will persist Wednesday
morning as the upper low travels southeast into the northeast Gulf of
Mexico. Moist low level southeasterly upglide will keep at least some
pockets of light showers in the area through Wednesday, particularly
in our eastern counties. Partial breaks in the cloud cover will
develop during the afternoon across northwest AL into western
portions of southern middle TN. This will enable highs in these areas
to reach around 70 degrees. The low clouds and spotty drizzle and
rain should end by Wednesday night, however, the NAM is indicating
some hold over into Wednesday evening, but will leave out for now.
Morning low cloudiness will eventually become more cellular in
nature Thursday afternoon as heating/mixing ensues. We will forecast
highs in the m-u70s, although this is quite conditional on the cloud
trends. Low clouds could tend to redevelop Thursday Night as well as
925 mb streamlines and RH fields suggest.
.LONG TERM...(Friday through Monday)
Issued at 246 PM CST Tue Feb 21 2017
Despite cloud cover Friday morning, fairly strong low level winds (~
30 knots at 925 mb and 30-40 knots at 850 mb) are still forecast by
models. Most models continue to show a very weak pre-frontal trough
convergence area ahead of the main front further west of the
Tennessee Valley. However, given the continued strong cap shown in
both ECMWF and GFS, it will be hard for more than cloud cover to
develop for much of the day on Friday. Kept a 20 percent chance in
the afternoon for much of the area, but this is very uncertain even
given the strong cap. If an isolated storms does develop though late
in the afternoon, they could be strong to severe. The main threats
with this convection would be large hail and marginally damaging
winds. Models actually hint at the cloud deck thinning toward noon,
especially south of the Tennessee River. Strong low level warm
advection ahead of the front (925 mb temps between 16 and 18 degrees)
and possibly some sunshine should allow the highs to climb into the
upper 70s if not lower 80s. It will be very breezy on Friday, with
sustained winds 10 to 15 mph with gusts around 25 mph possible, given
the strong low level winds.
Winds 10 to 20 mph with gusts around 25 mph will continue Friday
night in higher elevations. Below 1000 feet, winds will be lighter,
but still between 10 and 20 mph. The big question continues to be
whether the upper level forcing with the front dips south far enough
to trigger more widespread convection given the strong CAP ahead of
the main front Friday evening. Elevated CAPE values in the evening
are forecast by most models to remain between 700 and 1300 J/KG, with
surface based CAPE values between 400 and 900 J/KG. Wet-bulb zero
heights between 7000 and 9000 are progged by models. So some storms
could be strong to severe producing marginally severe winds and large
hail. Some model guidance is showing just enough low level helicity,
that an isolated tornado is not out of the question. However, this
is a very low threat Friday evening. At this point due to some
uncertainty with convective initiation, this may the more pessimistic
solution. Since most models are showing at least scattered
convection firing, as the CAP break just ahead of/with the front,
kept 40 to 60 chance of showers and storms in the forecast in the
evening. Expect to lose the instability fairly quickly after
midnight. However, kept 30 to 60 chance of just rain showers, as the
forcing pushes through eastern Alabama overnight.
Much colder air pushes in behind the front on Saturday. Highs in the
lower to mid 50s look reasonable. Winds of 10 to 15 mph with gusts
around 20 mph are possible through the day with clearing skies.
This strong cold air advection will advect much drier air into the
area as well (dewpoints drop into the 20s to ~ 30 degrees in the
afternoon). A surface high settles into the Tennessee Valley Saturday
night into Sunday morning. Light winds coupled with low
dewpoints/cold air mass should allow temperatures to fall into the
upper 20s to lower 30s. Sunday should remain fairly cool (but warm
slightly) with highs remaining in the 50s primarily.
By Sunday night, the surface high slides east to just off the
Atlantic coast. Most model guidance does bring some upper level
energy into the Ohio Valley/Kentucky areas north of the departing
high. It will still be too dry for any precipitation. Low level
southwesterly flow will begin to develop via return flow around the
high. This will continue the warming trend as highs climb back into
the lower to mid 60s and cloud cover returns by Monday.
.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1128 PM CST Tue Feb 21 2017
MVFR to IFR conditions are expected to prevail across all of N
AL/Southern Middle TN for the next 10 hours as an upper low
continues to send low clouds/-RA/DZ northward. It is difficult to
time exactly when the -DZ will end and the low cigs break between
16Z- 20Z. So not as much certainty on that particular time frame. Did
incorporate a SCT of the MVFR cigs and return to VFR cigs after
23/00Z once the upper low finally moves off to the southeast.
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