Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 210859

Area Forecast Discussion
National Weather Service Huntsville AL
359 AM CDT Sat Oct 21 2017

.NEAR TERM...(Today)
Issued at 359 AM CDT Sat Oct 21 2017

Quiet weather for one more day in the near term, with only some
passing high cirrus to deal with across the Tennessee Valley early
this morning. These clouds are moving in ahead of a cold front and
developing low pressure system along the leeside of the Rocky
Mountains. For today, the high pressure system that has brought us
the mild and dry weather will slide off to the east off the SC
coastline. This will setup SW flow at the surface and aloft, bringing
in some Gulf moisture -- and help warm highs into the upper 70s to
at/just above 80 degrees in much of the Valley. Cirrus clouds will
filter across the area for most of the day, but there will be plenty
of breaks and opportunities for heating. No threat for rainfall as
the forecast remains dry through tonight.

.SHORT TERM...(Tonight through Monday)
Issued at 359 AM CDT Sat Oct 21 2017

Complicated forecast for Sunday and Monday as the pattern quickly
changes. The aforementioned high will continue to move well east into
the Atlantic and a deepening upper-trough will quickly swing into the
Tennessee valley on Sunday, bringing scattered showers and
thunderstorms across most of the region by the afternoon. This
activity will become more widespread and numerous by the evening and
overnight as a cold front advances into the area.

Some model solutions (GFS, NAM) indicate this trough will strengthen
into a cut-off low over south MS/AL as it quickly advances through
the Deep South Sunday Night into Monday. It is during this period
where PoPs and QPF will be greatest, especially on Monday when a LLJ
ramps up ahead of the deepening upper-cyclone. Given the moisture in
place (with PWATs increasing to near 2 inches) and the dynamic lift
associated with this storm system, locally heavy rainfall is expected
with this storm system -- especially in the Sunday night through
Monday afternoon period. Though we have been fairly dry of late, the
need for some flash and areal flooding will need to be monitored in
the coming days. This is especially true given the abnormally high
PWATs and the potential for training of cells along and parallel to
the advancing cold front. Storm total rainfall of 2 to 3 inches seems
likely, with some locally higher amounts.

The primary uncertainty lies with the timing. The ECMWF is weaker
and more progressive, still showing an open wave quickly shifting NE
of the region by 00z Tuesday. The NAM is slowest and doesn`t want to
move in the best precip chances until Monday afternoon, evening --
while the GFS is somewhere in the middle. For now, have favored more
the GFS/ECMWF solutions which have had better continuity -- though
will need to watch the notable slower model trend of the NAM and
some of the other mesoscale guidance.

One last note: it going to feel noticeably colder behind this
frontal system. Highs may run 10 degrees to perhaps 15 degrees
lower on Monday, in the upper 60s to around 70 degrees.

.LONG TERM...(Monday night through Friday)
Issued at 359 AM CDT Sat Oct 21 2017

The coolest air this season is on the way to the TN Valley. As
previously mentioned in the short term, a cold front will pass
through the TN Valley on Monday. A few wrap around showers from the
backside of the system are possible early Tuesday as it lifts
northeast. Otherwise the rest of the day will be quite breezy with a
northwesterly wind 10-15kts and gusts up to 20kts filtering in
cooler and drier air. A second, stronger, front will arrive Tuesday
afternoon, bringing in a reinforcing shot of cooler air. Daytime
highs for Tuesday will be quite cool and only reach the lower 60s!
Overnight lows will be chilly in the upper 30s to lower 40s across
the TN Valley.

To further reinforce the cooler air, an upper-level trough will dig
all the way into the Southeast Tuesday night into Wednesday. Daytime
highs on Wednesday will be in the upper 50s to lower 60s with
overnight lows also in the upper 30s to lower 40s. If you are not a
fan of the cooler weather, temperatures will warm up a little bit on
Thursday and Friday as a weak upper-level ridge builds into the
region. Temps will reach the upper 60s, maybe 70 degrees with
overnight lows in the upper 40s. The ridge is quickly pushed eastward
by the next system moving across the Midwest. This system will bring
another cold front and our next round of showers and thunderstorms
to the area next weekend, however there is still a lot of model
disagreement to overcome.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1220 AM CDT Sat Oct 21 2017

VFR conditions will prevail through the period at the KMSL and KHSV
terminals. Light/variable winds will increase to around 10 kts from
the southeast by 15Z Saturday. Other than high clouds, skies will
remain clear.





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