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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

                            
536 
FXUS64 KHUN 210439
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1139 PM CDT Thu Sep 20 2018

.UPDATE...
For 06Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 833 AM CDT Thu Sep 20 2018

Quiet wx conditions prevail across the cntrl TN Valley this Thu
evening, as temp trends fall into the lower/mid 80s from their near
record highs earlier today. With sfc winds fairly light, these temps
should continue to fall into the 70s overnight, with lows early Fri 
mainly in the upper 60s/near 70F for most spots. Moisture is also 
gradually spreading into the area from the southeast along a weak sfc
trough located west cntrl AL into extreme NW AL. This weak boundary 
may be a focus for a few showers/tstms later tonight across NW AL, 
although the prob is fairly low. Showers/tstms may then develop a bit
more to the east heading into Fri morning, although rain chances 
will remain somewhat low for now.

.SHORT TERM...(Friday through Saturday)
Issued at 250 PM CDT Thu Sep 20 2018

During the day on Friday, convergence increases, as the 
surface high moves further east and a stronger cold front in the 
western Great Lakes region gets closer. Should see a better coverage
of showers and thunderstorms. Kept 30 to 40 percent chances of 
activity in the forecast. 

This trend continues into Friday night, as the front edges slightly
closer. Model guidance is split concerning how far south the more
concentrated/heavier shower and thunderstorm activity sinks. At this
point went a bit below Superblend and closer to GFS with 30 to 50 
pop. Expect higher chances of rain to be in northwestern Alabama and
southern middle Tennessee. Could see a few stronger storms capable 
of producing gusty winds and heavier rainfall as shear between 25 and
35 knots impinges on areas near the TN/AL border and in southern 
middle Tennessee. 

This doesn't change much on Saturday night, as the front seems to
stall in westerly flow. Shower and thunderstorm chances don't change
much and heavy rain and gusty winds look to be the biggest threats
with stronger thunderstorms.

.LONG TERM...(Saturday night through Wednesday)
Issued at 250 PM CDT Thu Sep 20 2018

Some changes coming to the forecast in the extended period as a cold
front will continue to slowly push ESE from the lower Ohio Valley and
Mid-South into the Tennessee Valley. With the upper-ridge sliding off
to the east of the Carolinas, this will promote a deeper
southwesterly flow, and thus good moisture flux off the Gulf of 
Mexico. This can be seen with dewpoints which are progged to return 
to the lower 70s this weekend. This boundary will approach 
Tennessee/Alabama border on Sunday and serve as a focus for scattered
to perhaps numerous showers and thunderstorms along and near it. 
This activity will be somewhat tied to heating, even with a boundary 
in place. Therefore, would expect this activity at it's greatest 
coverage in the afternoon hours -- even with the forcing mechanism in
place. The most favored areas for storms will be nearest to the 
front, across far Northwest Alabama and Southern Middle Tennessee, 
with this activity becoming more widely scattered along and south of 
the Tennessee River. The added cloud cover will make for a cooler day
as highs in the mid 80s will be common -- around 15 degrees cooler 
than what we've experienced of late.

Models are hinting that the boundary may actually retreat back
northward on Monday as the Bermuda high temporarily tries to nose 
it's way back into the Southern Appalachians. The result, will 
likely be less clouds and a warmer, drier day. Still, with southwest 
flow and abundant moisture in place, there will be plenty of 
ingredients for some scattered showers and storms. The deepening 
upper-trough will then swing southeast from the Canadian prairies 
into the Great Lakes and Ohio Valley from Tuesday into Wednesday. 
This will force a stronger, secondary cold front south into the 
Tennessee Valley sometime during this timeframe. Thus, more 
widespread shower and thunderstorm activity, and hopefully some much 
needed rainfall is possible during the middle of next week. The best 
dynamics, however, will be well displaced of the region and thus am 
not expecting any severe risk with this activity. Behind this front, 
some cooler (more autumn-like) and drier air will filter into the
area, which may actually support highs in the upper 70s to lower 80s
come late next week! 

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1139 PM CDT Thu Sep 20 2018

No changes to previous aviation forecast reasoning. With a weak area
of high pressure remaining in place across the srn Atlantic Basin, 
VFR conds prevail at both main airports. Little change in these conds
is xpcted into the day Fri, with mid/high clouds developing across 
the area as moisture spreads into the area from the south. Some -shra
may also begin to develop across parts of the area late tonight into
Fri, and in addition to a prevailing VCSH, a PROB30 group was added 
late in the TAF to account for any precip.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...09
SHORT TERM...KTW
LONG TERM...AMP.24
AVIATION...09


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