Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 222310 AAA

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
610 PM CDT Sun Oct 22 2017

For 00Z TAFS.


.NEAR TERM...(Tonight)
Issued at 222 PM CDT Sun Oct 22 2017

Showers and thunderstorms have been slow to develop across the area
so far this afternoon. This is due in part to the better forcing
still to our west, and showers/storms to the south intercepting some
of the moisture. Area radars show isolated to scattered light rain
beginning to develop across northeast Mississippi, moving to the
northeast. However, much of this is not reaching the ground due to
the drier air through the vertical column. With that said, moisture
is still expected to increase through the afternoon, with coverage
increasing across northwest Alabama within the next few hours.

The main cold front remains just west of the Mississippi River and
will continue to inch closer to the TN Valley during the overnight
hours. Meanwhile, the upper trough currently over the Plains will
deepen and quickly move eastward tonight, swinging into the
Mississippi Valley by early Monday morning. The low level jet will
increase during the early evening hours and move to the northeast
through the overnight period. The forcing associated with the
approaching trough and jet, along with a vast increase in moisture,
will increase the coverage of showers generally after 00Z, with
numerous to widespread moderate rain expected across the area. Still
not looking at much coverage of thunderstorms given the meager
instability diminishing after sunset, but an isolated thunderstorm
is possible owing to the strong forcing. Heavy rainfall embedded in
the more widespread activity is possible late this evening,
especially across northeast Alabama. Storm total rainfall amounts
will generally be between 1 to 3 inches, with locally higher amounts
possible. Although this could pose a localized flash flooding
threat, especially with any training cells, the area remains fairly
dry and should be able to handle that much rainfall.

The front will reach NW Alabama around midnight and then slowly move
eastward during the predawn hours. The front will likely reach I65
by the end of the near term period, with the widespread precip
continuing across northeast Alabama.

.SHORT TERM...(Monday through Tuesday night)
Issued at 222 PM CDT Sun Oct 22 2017

The axis of the upper trough will move into northwest Alabama Monday
morning, with the surface cold front over northeast Alabama.
Lingering showers will begin to taper off by the late morning hours,
with much of the precip expected to be out of the area by the
afternoon. There is still some uncertainty with the timing of the
front. Additionally, wrap around moisture associated with the upper
low will move through the area during the day, so kept in isolated
to scattered light rain into the afternoon hours. Lingering cloud
cover, along with the lowering heights will keep temperatures on
Monday in the mid to upper 60s.

Dry air will quickly filter into the area Monday night, with
dewpoints dropping into the low to mid 40s. Skies will also clear
through the overnight hours and combine with light winds to create
good radiational cooling conditions. Overnight lows will drop into
the mid to upper 40s, which is roughly 10 to 15 degrees cooler than
those expected tonight. A reinforcing front is also expected to move
through late Monday night/early Tuesday morning as a second upper
trough digs southward and through the area. This will send even
cooler and drier air into the area, with strong cold air advection
during the day Tuesday keeping high temps in the upper 50s to lower
60s despite plenty of sunshine. Another good radiational cooling
night on Tuesday will send temperatures down into the upper 30s.
Will have to watch for the potential frost development across
portions of the area early Wednesday morning.

.LONG TERM...(Wednesday through Saturday)
Issued at 222 PM CDT Sun Oct 22 2017

A rather complicated and potentially messy pattern to persist into
the extended period. By mid week, the medium range guidance is in
fair agreement with a rather deep trough positioned across the
eastern CONUS with generally northerly flow across the Tennessee
Valley. This will pull down rather cool and dry air though it`s
often hard to time/predict cloud cover in that type of flow regime.
Either way, expect temperatures will be quite a bit below normal on
Wednesday with some recovery expected on Thursday ahead of the next
upstream system.

The pattern gets quite interesting toward the end of the week as you
have a rather potent northern stream system diving southeast across
the Midwest and toward the Ohio Valley. This will aid in
cyclogenesis across the Great Lakes and help push a cold front
toward the area in the Friday/Saturday time frame. The medium range
guidance is not exactly on the same page with the GFS pushing the
front through on Friday night with the Euro slower with more energy
noted in the southern stream. To add to the confusion, the Canadian
has similar timing to the GFS but essentially a dry front with the
primary energy off the Florida coast.

For now have trended with essentially a blend of the GFS/Euro for
timing and discounted the dry Canadian solution. Also trended temps
down a notch for Saturday/Sunday given the expected cloud cover with
the upper trough passage. Have also kept thunder out of the forecast
given the rather limited gulf return and overall cool
statically stable lower troposphere ahead of the approaching front.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 610 PM CDT Sun Oct 22 2017

Bands of light shower activity will occur this evening before a more
solid band arrives after 04Z at KMSL and 06Z at KHSV. This is when
VFR ceilings of 040-080 will drop to around 015agl (MVFR). Winds will
shift from southeast to southwest or west after this rain band and
cold front move through. Low clouds around 015agl will likely last
into much of Tuesday, but with the probability of showers dropping
through the day.





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