Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 231752

Area Forecast Discussion
National Weather Service Huntsville AL
1252 PM CDT Thu Mar 23 2017

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 1108 AM CDT Thu Mar 23 2017

The near term forecast is in good condition this morning, with only a
few minor changes made in this update. Current regional radar mosaic
continues to indicate a large area of stratiform precipitation across
western KY which has likely developed in response to a strengthening
warm air advection regime within 1500 ft of the surface. Although
some of the precipitation has spilled southeastward into northern
middle TN...the very dry nature of the low-level airmass from I-40
southward will limit the threat for any light rainfall to reach our
forecast area...and we will maintain a 0 POP at this time. However,
scattered mid/high-level cloud cover related to this feature will
spread southeastward across the TN valley through the day...and sky
cover grids have been update to match current trends in IR satellite
data. At the surface, a gradual increase in southeasterly flow is
expected to continue throughout the afternoon...and based on current
temp trends and expectations of mostly sunny skies, forecasted highs
have been raised by a few degrees for most locations.

.SHORT TERM...(Tonight through Saturday)
Issued at 402 AM CDT Thu Mar 23 2017

A southerly flow will become more established across the area
tonight as the high moves a bit further to the east. This will start
the process of bringing deeper Gulf moisture towards the forecast
area. Along with lower level moisture, clouds preceding the weather
system to our west will also overspread the region, resulting in
mostly cloudy skies. Despite the clouds, a milder and dry night is
expected with lows ranging from the upper 40s east (with cold air
damming fading there) to the mid 50s west.

Stayed with a dry forecast Friday and Friday evening, while the
system to our west continues to develop. Output from the various
models have the surface low "bottoming out" in the upper 990s mb
surface pressure Friday morning east of the Colorado front range,
before it gradually weakens as it moves further eastward. With a
deeper southerly flow, high temperatures on Friday should warm into
the lower 70s most locations under mostly cloudy skies.

The upper trough associated with the system progged to affect the
forecast area during Saturday will take on a neutral, to maybe a
slight negative tilt as it progresses eastward. Showers and
thunderstorms accompanying this system should start overspreading the
area from west to east, beginning after midnight. Given rather
consistent output from the models, stayed with a slower timing
regarding how fast the shower activity advances Friday night into

Saturday appears to be a wet day, with numerous showers and
thunderstorms in the forecast, especially during the afternoon and
evening. Model differences change a bit during Sat, with the ECMWF`s
depiction of the surface low a bit further to the south compared to
the NAM/GFS/Canadian. It was also a little slower bringing showers
eastward, with the GFS/Canadian the quickest. The NAM/GFS and
Canadian at this time were hinting at possible development ahead of,
or south of the forecast area. The first scenario especially from the
NAM perspective; could result in storms forming east of the main
line in the afternoon. Those storms and ones with the main line could
become strong to severe. Storms forming to our south would intercept
some of the moisture and instability, resulting in weaker storms
north of them. In any case, the main threats at this time appear to
be strong to damaging wind gusts and marginally severe hail (in the
stronger storms). A tornado threat still looks to be low, but would
increase if more discrete storms are realized Saturday afternoon.

.LONG TERM...(Saturday night through Wednesday)
Issued at 402 AM CDT Thu Mar 23 2017

The medium to long range is marked by an active pattern: one
shortwave trough departing, one moving through early next week, and
one potentially affecting the region at the very end of the period.
Given the amplitude of these features, confidence is moderate.

The main trough axis from System #1 will cross the area at the start
of the period. Depending on the outcome of convection earlier
Saturday, this could be the main event for the system. There is
sufficient instability and shear for a severe event; at this stage
this looks like primarily a damaging wind threat. Tornadoes can`t be
ruled out, particularly as one moves west towards Mississippi where
low-level wind shear is greater. Indeed, models/ensembles diminish
the storm coverage and intensity as the low moves into the mid-

Little will change behind this first system, with S-SW flow
continuing. That sets things up for System #2, which will already be
moving into the central Plains as System #1 exits. There is some
amplitude spread here, but the consensus points to weaker wind and
wind shear, but perhaps greater instability. There are some timing
differences here as well but the main timing looks to be Monday
night. Timing the associated frontal passage is problematic but
NAEFS/etc suggest a slower progression is better. The post-frontal
airmass will not be that much cooler (maybe ~5 degrees) than pre-
frontal anyway.

As System #2 slowly departs the stage, System #3 will be digging
deeply over the Desert SW. Model/ensemble solutions begin to diverge
here, as there is greater uncertainty regarding the overall pattern
and its blockiness. Given the pattern, the forecast will reflect a
slower solution more similar to the GFS than the ECMWF, which will
favor lower PoPs through Thursday. This could lead to a warmer and
drier forecast than what is indicated, but the forecast will stick
closely to default model blends for the time being.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1252 PM CDT Thu Mar 23 2017

VFR conditions currently observed at both HSV/MSL should persist thru
the early evening...with few-sct mid and high level clouds and a
prevailing se flow of 10G18 kts. A southerly low-level jet will
strengthen rather quickly after sunset...with elevated warm advection
and moisture return supporting a solid mvfr stratus deck by 24/05Z.
Cigs should remain in the 1500-2500 ft range throughout the early
morning hrs, before the base of the stratus layer begins to slowly
lift to arnd 2500 ft btwn 14-16Z. Sfc winds will gradually veer from
se to sse by end of TAF period, with speeds dropping off to 8G16 kts
overnight before increasing to 12G20 kts once again by 15Z. Based on
anticipated strength of the low-level jet, LLWS has been introduced
btwn 05-15Z.





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