Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 221423

Area Forecast Discussion
National Weather Service Huntsville AL
923 AM CDT Sun Oct 22 2017

.NEAR TERM...(Rest of Today)
Issued at 923 AM CDT Sun Oct 22 2017

Latest water vapor imagery shows a fairly amplified longwave trough
extending southward across much of the eastern Plains this morning.
Meanwhile, a cold front extends from its parent low over the Great
Lakes, south-southwestward through the mid Mississippi Valley and
into the Arklatex. Showers and thunderstorms have been ongoing ahead
of this front, along a prefrontal trough axis extending through
southern Mississippi. Across the TN Valley, temperatures were warming
fairly well, with temps as of 9 am in the mid to upper 60s. Low and
mid level cloud cover was slowly beginning to increase across the
western half of the area, which may limit the diurnal heating through
the day, however temps should still be able to reach into the mid to
upper 70s given the southerly flow.

Latest hires guidance seems to be handling the current precip
activity fairly well, with a cluster of storms moving along the central
Gulf and more scattered activity further north. Given drier air in
place across northeast Mississippi and northern Alabama, activity has
been slowly diminishing. However, as low level winds veer to the
south late this morning, moisture will quickly increase, with an
uptick in activity expected by mid afternoon. Most of this activity
will remain west of I-65 through the near term period, with
increasing coverage across the remainder of the area this evening
(more on that in the short term section below). Looking at
predominately showers and isolated thunderstorms this afternoon,
despite the precip moving in during peak heating. This is mainly due
to the increase in cloud cover limiting the surface based instability
and meager lapse rates. If the clouds do not fill in as quickly as
expected, we may realize more instability, so can`t rule out a strong
storm. But heavy rainfall looks to be the main threat with any of
the precip.

.SHORT TERM...(Tonight through Tuesday)
Issued at 359 AM CDT Sun Oct 22 2017

Coverage of showers and storms will increase ahead of a cold front
tonight into Monday as a 40-50 kt LLJ ramps up later this evening
across the TN/OH River Valleys. The orientation of this LLJ would
place much of Northern/Central Alabama in the right entrance region
of the jet, enhancing the lift and potential for deeper convection.
Think coverage will peak across the region during this time period
between 03-09z tonight, with lingering chances for sustained heavy
precipitation over NE Alabama through 12-15z as the upper-trough
moves into Middle TN and Northern AL and negatively tilts. Currently
favoring the faster ECMWF and NAM solutions -- though models are in
much better agreement on timing compared to last night. Have
categorical PoPs over the entire region this evening and overnight,
before tapering them off early this morning (rather quickly over NW
Alabama) from west to east. Though we have been dry of late, the
threat for flooding will exist -- mainly due to excessive rainfall
from training, especially in Northeast Alabama.

Think we`ll see some lingering scattered showers in areas east of
I-65 for most of the day on Monday, along with a mostly cloudy sky --
so went more pessimistic there. However, drier will move in behind
the front and scour out the moisture Monday night, putting an end to
precipitation chances. Some clouds in the eastern zones may be a
little stubborn to erode overnight Monday, but mostly clear
conditions are expected by Tuesday morning and the cool, dry autumn
air mass takes hold.

Primary noticeable change will be the air mass and temperatures,
which will drop into the mid to upper 60s on Monday and struggle to
climb much above 60 degrees on Tuesday.

.LONG TERM...(Tuesday night through Saturday)
Issued at 359 AM CDT Sun Oct 22 2017

An upper-level trough will dive into the Southeast and filter in
additional CAA on Wednesday to reinforce the cooler conditions
already in place. Daytime highs in the upper 50s and lows in the
upper 30s/lower 40s across the TN Valley will be around 10 degrees
below normal! Wednesday night, depending on how quickly clouds move
out, radiational cooling would allow for temps to fall a few more
degrees. With this in mind, frost could be possible in a few
locations but will watch additional trends as we get closer in the

If you are looking for a break in the cold weather, the trough
quickly lifts northeast and is replaced by a weak ridge on Thursday
and Friday. This will allow temperatures to warm up into the upper
60s with overnight lows ranging from the low to upper 40s. However,
even the ridge is short lived, as it is pushed eastward by the next
system moving across the Midwest. This system will bring another
cold front with our next round of showers over the weekend. Models
are still in disagreement and even have run to run inconsistency.
For example, there is a 24 hour timing difference between the GFS
and the ECMWF. The GFS is still running faster bringing the precip
through on Saturday compared to the ECMWF that moves it through the
TN Valley on Sunday. With the consistent model disagreement, went
with a blend of the GFS/ECMWF for this forecast package. So if
planning any outdoor events for the weekend, stay tuned to the
forecast for additional updates.


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 636 AM CDT Sun Oct 22 2017

Clouds will steadily increase this morning as a deepening upper-level
trough races toward the area. Ahead of the cold front, winds will
also become gusty at times by late morning through the day. Late
this afternoon into early this evening a broad cluster of SHRA with
isolated TSRA will move into KMSL and eventually KHSV late this
evening and overnight. This precipitation will create reduced MVFR
visibilities and lowered ceilings after 00z through the remainder of
the period.





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