Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 170500

Area Forecast Discussion
National Weather Service Huntsville AL
1200 AM CDT Thu Aug 17 2017

For 06Z TAFS.


.NEAR TERM...(Rest of tonight)
Issued at 959 PM CDT Wed Aug 16 2017

A deep-layer west-southwesterly flow regime will continue across the
CWFA overnight...between a 500-mb high centered over the northeastern
Gulf coast and a shortwave trough pivoting east- northeastward
across the northern Plains. Gentle ascent associated with a weak
disturbance embedded within this flow is likely responsible for the
persistent nature of the isolated convection still impacting the
region at this hour. However...despite several outflow boundaries
still indicated in regional radar data...lift with the mid-level
disturbance should not be sufficient to maintain showers for more
than 1-2 more hours.

Thus, the forecast will be updated to carry isolated showers through
Midnight...and we will also include patchy early morning fog in
larger river valleys based on the persistent nature of this synoptic
pattern. Forecast soundings once again indicate the development of
scattered-broken very low stratus after weak boundary
layer ascent should occur based on the low-level veering profile. Sky
cover grids will be adjusted to reflect this, and minimum temps will
also be increased by a few degrees for most of the region.

.SHORT TERM...(Thursday through Friday night)
Issued at 256 PM CDT Wed Aug 16 2017

The upper-level ridge will be established across the Southeast on
Thursday, warming up temps but not keeping us precip free. Highs will
reach the lower 90s with scattered showers/storms possible by the
afternoon. Thursday will be the warmest day of the week, but it will
feel even warmer with lingering moisture, so look for heat index
values around 100 degrees across the TN Valley. As an upper-level
trough and sfc low move across the Great Lakes, an associated weak
front will glide into the TN Valley Thursday night/Friday. This will
provide a lingering chance for showers/storms. It will not bring a
cool down, just a little bit of drier air behind it.

.LONG TERM...(Saturday through Tuesday)
Issued at 223 PM CDT Wed Aug 16 2017

The axis of an upper level trough with swing through the TN Valley
on Saturday, bringing northwest flow aloft. Surface winds are
expected to quickly shift to the south early Saturday morning,
bringing moisture back into the region. However, the moisture
recovery through the vertical column may not respond quickly enough
for showers and thunderstorms to develop Saturday, especially given
the lack of significant forcing. Therefore, removed the slight
chance pops from the forecast.

Models diverge on their how quickly the upper trough shifts east and
the strength of a building ridge. Where the GFS keeps northwest flow
across the region on Sunday, the ECMWF builds the ridge into the
region by Sunday. Despite these differences, ample moisture will
return to the area, with PW values rising to around 1.75 inches by
Sunday afternoon. This will bring diurnally driven showers and
thunderstorms back to the TN Valley, though coverage is expected to
remain isolated. Models keep some semblance of the upper ridge in
place over the area during the beginning of the next work week.
Moisture will continue streaming into the area so that will not be
an issue. So, rain chances will depend on just how strong the
subsidence associated with the ridge becomes. However, with the lack
of any large scale forcing, when and where thunderstorms develop
will be contingent on mesoscale features that cannot be resolved this
far in advance. While this model run brings a lesser chance of rain
during the eclipse on Monday, this could certainly change as we get
closer to the event. Given these uncertainties, kept close to the
blended precip guidance for the extended, with scattered showers and
thunderstorms each day.

Temperatures through the period will be near seasonal values, with
highs in the upper 80s to lower 90s and lows in the lower 70s.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1200 AM CDT Thu Aug 17 2017

VFR conds are currently observed at both HSV/MSL. However...based
largely on a persistence forecast...a deck of lower stratus clouds is
expected to develop in the very moist boundary layer by 17/09Z. At
this point, we will keep prevailing cigs broken arnd 2500 ft...with a
tempo for 1500 ft cigs and minor vsby reductions in br/hz btwn
09-13Z. The stratus will gradually lift tomorrow morning...becoming
based arnd 5 kft for the majority of the aftn/evening. Isolated-
widely scattered tsra will be possible invof each terminal beginning
early tomorrow aftn...but expected coverage only warrants VCTS at
this point. With a weak cold front expected to enter the region late
tomorrow evening, the threat for shra/tsra will persist thru end of
the TAF period. Sfc winds will remain from the southwest, with speeds
increasing to arnd 10 kts late tomorrow morning.





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