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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 171538

Area Forecast Discussion
National Weather Service Huntsville AL
1038 AM CDT Mon Sep 17 2018

.NEAR TERM...(Rest of Today)
Issued at 1038 AM CDT Mon Sep 17 2018

The big weather maker from late last week to the present had come 
from now Tropical Depression Florence. It was located approximately 
30 miles northwest of Huntington WV moving to the NE around 15 mph. 
Moisture wrapping around the system extended SW to across much of the
central Tennessee Valley. The lower clouds (< 5000' AGL) were mainly
affecting parts of far northern Alabama east of I-65 and points 
northward. Along with the clouds, our Doppler radars indicated a few 
showers moving in a NNW-SSE manner across parts of the forecast area.
As of this writing, those showers were mainly across Middle Tennessee
and points northward, but a few showers were noted just west of the 
Hytop AL radar. Those showers were moving into a drier environment 
further to the south across northern Alabama, resulting in little to
no precip in much of this region at this time.

The satellite view confirmed a sharp gradient between the southern 
extent of the clouds, and generally clear skies to the south. That 
line as of this writing was roughly from the Quad Cities of NW AL, to
Moulton and Cullman. Some sun was making it through breaks in the 
clouds north of this line. Strong late summer heating today (with 
high temperatures ranging from the lower 80s southern middle TN to 
lower 90s in NW AL, will result in destabilization this afternoon. 
Given higher instabilities forecast by the shorter range models, left
isolated thunder in for this afternoon in NE Alabama and our 
southern middle Tennessee group. Except for minor temperature 
adjustments, the rest of the forecast looked fine for the rest of 

.SHORT TERM...(Tonight through Wednesday night)
Issued at 218 AM CDT Mon Sep 17 2018

Showers or storms that do form during the afternoon hours should
quickly dissipate after sunset and as the remnants of Florence
continue to shift E/NE away from the area. With calm winds overnight
and a clearer sky it's possible we may see fog develop in the valleys
in NE AL and in TN. Lows will dip into the mid to upper 60s by
Tuesday morning. An upper level ridge centered over the southern 
Plains will begin to shift east towards the area Tuesday and then
will be centered over the TN Valley on Wednesday. Northerly winds 
continue as this ridge builds east and will reinforce the dry air 
aloft already in place across the area. The combination of this dry
air and increasing subsidence will keep the area rain free both
Tuesday and Wednesday. Guidance has been consistently underdoing the
high temps by 2-4 degrees the past few days and have nudged temps
above guidance. This would put highs both Tuesday and Wednesday in
the low to mid 90s but with the strong subsidence wouldn't be
surprised if some of the warmer locations made it into the upper 90s.
Thankfully, we mix down some of this drier air both days and this
will help to keep Heat Index values just below 100 degrees. 

Lows both Tuesday and Wednesday nights will be in the mid 60s to near
70 degrees. 

.LONG TERM...(Thursday through Sunday)
Issued at 218 AM CDT Mon Sep 17 2018

By Thursday the ridge mentioned above starts to shift east of the
area and as a result winds become more southerly. This should allow
Gulf moisture to gradually be advected into the area as we head into
weekend. While subsidence is in ongoing Thursday think that just
enough moisture will have streamed north into the western half of the
area for a few isolated storms. With dewpoints back in the 70s and 
slightly steeper lapse rates, expecting an increase in shower/storm 
coverage for Friday afternoon. In terms of highs for Thursday and
Friday, with weakening subsidence models are going with temps more 
in the upper 80s to lower 90s. Think those are too low for Thursday 
and have kept highs in the mid 90s for much of the area. The slightly
better rain chances for Friday should allow for temps closer to the 
90 degree mark. 

A shortwave ejecting out of the desert SW on Thursday night/Friday
morning will spawn a low pressure system that is forecast to more 
into the upper Midwest/Great Lakes region over the weekend. This 
system will push a frontal boundary towards the area. However, models
continue to waver on just how strong the frontal feature will be and
the influence the ridge to our east will have on the front. We've
gone from the front will be completely south of the area to stalling
to the north over TN, to now showing it stalling over the TN Valley.
With all of this uncertainty have kept lower end PoPs in the forecast
for the weekend as even it remains north of the area we should still
be able to see isolated to scattered afternoon storms. Based on the
dry pattern we've had the past few weeks and the weak forcing
associated with the front, think the drier solution might be the
better bet at this point. In any case the convergence associated with
the front should result in a bit more cloud cover and highs over the
weekend will be in the mid 80s to around 90 degrees. 


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 620 AM CDT Mon Sep 17 2018

VFR conditions are forecast through the period. Winds will be N/NW
with speeds at or below 10kts. A mid-level cloud deck between 
110-160 associated with the remnants of Florence have overspread the 
area. These are forecast to linger through much of the morning before
scattering out this afternoon. Additional low level moisture could 
result in a 4,000ft deck of clouds developing by 15z. Showers and 
storms are expected to stay east of both terminals today. 




LONG TERM...Stumpf

For more information please visit our website 
at weather.gov/huntsville.