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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

                            
856 
FXUS64 KHUN 202002
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
202 PM CST Tue Feb 20 2018

.NEAR TERM...(Tonight)
Issued at 202 PM CST Tue Feb 20 2018

Another warm day with a mid level ridge and surface high pressure 
off the southeast coast, continuing to usher in a warm southerly flow
and low level moisture. With a little more moisture and also some 
instability today, clouds are a little more widespread than what we 
saw yesterday. This will impact the highs today but Muscle Shoals 
already reached 77 as of 1PM and Huntsville reached 76. Both records
for today are 79 so it's going to be tough with the cloud cover out 
there. 

Winds are gusty like yesterday, thanks to the sufficient mixing. 
Gusts of 20-25mph will continue through sunset. Winds remain elevated
overnight, around 10mph so fog formation won't be an issue. The 
southerly flow keeps lows tonight even warmer, in the middle 60s. 
Clouds will also be on the increase overnight, or stay steady 
depending on how overcast the clouds become this afternoon. With the 
increased moisture and weak isentropic lift, a few showers will be 
possible this evening and have kept the slight chance of showers in 
the forecast. 

.SHORT TERM...(Wednesday through Thursday Night)
Issued at 202 PM CST Tue Feb 20 2018

The mid level ridge remains strongly in place on Wednesday. A 
surface low north of Maine will have a cold front dragging south all 
the way through south TX. This front will be inching east through 
tonight and into the short term, leaving us firmly placed in the warm
sector. Models are in decent agreement on keeping most of the precip
confined to NW AL through the day Wednesday with a few showers 
popping up in the rest of the area. There isn't much lift ahead of 
the system but with the moisture present and CAPE values in the 
500-600 J/kg range, we could get a few showers out of the cumulus 
clouds that should develop by afternoon. There are still some 
discrepancies in the high temperatures for Wednesday. The sustained 
southerly flow and similar 850mb temps to previous days will keep it 
in the upper 70s, near 80. However, we will see more clouds and rain 
so will cap the highs in the 76-78 range at this time. 

Multiple waves of low pressure will ride along the previously
mentioned frontal boundary, giving areas to our west plenty of
rainfall. The front continues to inch east Wed night into Thursday
with it essentially dissipating over the area Thursday afternoon,
without even a noticeable wind shift. Overall, Dekalb/Jackson
counties may see no precip from this front. The tale of extremes in 
regards of precip for Thursday with the NAM very dry, GFS very wet 
and ECMWF somewhere in the middle. The drier solution seems more
reasonable due to the ridge but there is going to be plenty of low
level moisture so showers still won't be out of the question east of
the frontal boundary. In terms of thunderstorms, pulse like
thunderstorms Wednesday, maybe a few at most, would be most likely
with the higher instability and moisture although little lift. 
Storms Wed night would be more lift driven, again a few storms at 
best, nothing strong. Same story for Thursday, if we get anything at 
all, the storms wouldn't be strong, especially with a more saturated 
sounding. The biggest impact would be potential flooding issues in NW
AL. Thursday night the front will be lifting back north and it may 
be dry after midnight.

Highs on Thursday will be a touch cooler, in the middle 70s with the
additional cloud cover. Lows both nights will be in the lower 60s. 

.LONG TERM...(Friday through Monday)
Issued at 202 PM CST Tue Feb 20 2018

The mid level ridge strengths once again and builds west on Friday, 
keeping the frontal boundary moving off to the northwest. Friday 
should be dry and with no change in air mass with the front, highs 
will be in the mid to upper 70s. Dew points drop to the upper 
50s/around 60 on Friday and don't get into the lower 60s until Sat 
afternoon.

The bad thing about the strong ridge and warm weather is that a
trough has been deepening into the southwestern states through the 
last few days. Surface cyclogenesis will occur near TX/OK Saturday 
and the low will track northeast, along the upper ridge, through
Sunday. 

Ahead of the system locally on Saturday, in the warm sector, dew
points increase and we do get a few hundred J of CAPE in the
afternoon. There is no lift to speak of outside of weak isentropic
lift. So it looks like we could have a widespread mid level cloud
deck even with a dry mid level. Could be showery and cloudy all day,
therefore lessening a severe threat later in the evening. The one
change with this forecast package (still looking at 00z ECMWF) is
that the GFS is about 6-12 hours faster. A pre-frontal
trough/decaying squall line would arrive in NW AL around 6z and be
out of NE AL by 18z. ECMWF has the line at NW AL around 12z, making
Sunday a little more interesting once it heats up. Shear will be
sufficient, even abundant, with this system. Given the fairly 
significant shortwave with it, only a few hundred J of CAPE would be 
necessary to sustain damaging winds in a squall line, even overnight.
This needs to be monitored and will keep the mention of possibly 
severe storms in the HWO. The biggest concern would be between 00-06z
Sunday, in NW AL.

Monday/Tuesday should be dry but will maintain a low pop Monday as
the front may hover just south of the area. 

Highs on Saturday will still be in the lower 70s but drop to the mid
to upper 60s behind the front on Sunday. Lower 60s highs return for
Mon/Tue. Lows that have been in the upper 50s/lower 60s will drop
to the upper 40s for the start of the week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1109 CST Tue Feb 20 2018

Scattered to broken ceilings are currently being observed but luckily
the broken cigs are VFR. KMSL dropped to BKN015 temporarily but those
clouds have moved off to the west. There's a chance both terminals 
could dip to around BKN030 at times but not expected to go too much 
lower than that this afternoon. Winds will be gusty again today, 
slightly more frequent than yesterday. Gusts 20-25mph are possible 
this afternoon.

Another low stratus deck moves in overnight with once again,
questions on coverage and how low the cigs get. For now, will bring
the 2kft clouds back in around 7z at both terminals but there is a
chance it drops lower before sunrise tomorrow. Should see the cigs 
mix out once again by late morning. There's also a 20% chance of a 
shower overnight so chances are too low to include in the TAFs at 
this time.


&&

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

$$

NEAR TERM...LN
SHORT TERM...LN
LONG TERM...LN
AVIATION...LN


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