Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 231620

Area Forecast Discussion
National Weather Service Huntsville AL
1120 AM CDT Sun Apr 23 2017

.NEAR TERM...(Rest of Today)
Issued at 1120 AM CDT Sun Apr 23 2017

Cloudy skies and much cooler conditions prevailed across the greater
Tennessee Valley late this Sunday morning. A system that brought the
widespread showers and thunderstorms yesterday (at least its surface
reflection) has moved east of the area. It`s surface low was across
east TN and far western NC. However its upper reflection was moving
to the south, SE of the Missouri Bootheel, with 1000/500 thickness
values nearing 550 decameters, indicative of much cooler air aloft.
As such, steep mid level lapse rates were moving southward with this
upper low across the western TN/KY border. A few lightning strikes
were noted with some of the showers in this unstable regime.

Experimental GOES-16 lower level water vapor (~7.34um or Channel 10)
indicated some of the lower clouds from the western Ohio Valley and
south of the Mid South. The visible view indicated extensive low
altitude clouds covering an area from ~1000 feet above the average
ground level in place in a similar area, mainly east of an Ohio
Valley to western Louisiana line. Most of the cloud bases over our
region were around 1000` above average ground level.

The above noted upper systems should move SSE for the remainder of
today, keeping cloudy skies and chilly conditions going. As such,
scaled back a tad on high temperatures - lowering them to around 60
our western areas to lower 60s east. Given the cold air aloft, cooled
our mountain spots more than a usual adiabatic scale. The 12Z NAM
and HRRR were indicating shower development later today, while the
RAP stayed on the drier side. That being said, given an unstable
mid/upper atmosphere; a few lightning strikes could occur over our
area later this afternoon. At this time, will keep our forecast free
of thunder for now; as the lapse rates should moderate somewhat with
time; when the coldest air reaches this region.

.SHORT TERM...(Monday through Tuesday night)
Issued at 249 AM CDT Sun Apr 23 2017

Broken cloud cover will likely remain in place as the now cutoff low
meanders over GA sending low clouds southward from TN/KY on its NW
flank. Though the mid-levels are mostly dry could see some very
isolated showers over northeast Alabama/southern middle Tennessee.
Left dry weather in the forecast for now. With the potential for
cloud cover remaining, kept daytime high temperatures near 70
degrees. However, this may need adjustment if cloud cover becomes
more scattered. Fog may develop again on Monday night with the cloud
cover clearing almost completely while temperatures drop into the
low 50s.

Shortwave ridging behind this cutoff low will move over the Southeast
by Monday night into Tuesday. Because of this, southwesterly flow
aloft and warm air advection will allow temperatures to rise quickly
into the low 80s on Tuesday afternoon while subsidence keeps any
precipitation from occurring. Quiet conditions will continue on
Tuesday night with overnight lows expected to be warmer in the mid to
upper 50s.

.LONG TERM...(Wednesday through Saturday)
Issued at 249 AM CDT Sun Apr 23 2017

On Wednesday, the upper ridge position will shift east of the area
while a large amplitude trough digs into the southern U.S.  A good
fetch of southerly flow will precede a cold front arriving Wednesday
night as an initial shortwave rapidly ejects northeast through mid
MS valley and western Great Lakes. However, despite the flow,
moisture recovery in low levels will be slow to evolve. As the front
arrives during the nighttime hours, just a narrow tongue of 60- 63F
dew points is projected. Other factors which may limit thunderstorm
strength will be stronger QG dynamics lifting well north, and a
weakening of the front as it arrives.

The forecast on Thursday into Friday remains uncertain with regard
to coverage and intensity of convection along the northward moving
warm front through the Ozarks through the TN Valley. The ECMWF
appears to be more bullish with MCS clusters along the front Thursday
Night into early Friday, but then comes into line with the GFS with
a rather capped warm sector airmass on Friday Night into Saturday.
Will lower PoP from suggested blends during this period. Also raising
highs on Days 6-7 above suggested blends with upper 80s looking
quite probable.


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 628 AM CDT Sun Apr 23 2017

IFR conditions will continue through 21Z-00Z as a surface cold front
and upper level weather system moves across the region. Expect -SHRA
to occur especially over northeast Alabama. Category improvements to
MVFR may occur around 21Z especially over central Alabama as
potential for rain showers moves off to the east. However, after
04Z-05Z, IFR conditions and possibly LIFR/below alternate minimums
cigs may develop once again with fog formation possible near areas
that have more broken cloud cover.





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