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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 141458 AAA

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
958 AM CDT Sat Jul 14 2018

.NEAR TERM...(Rest of Today)
Issued at 958 AM CDT Sat Jul 14 2018

An upper-ridge will remain almost directly overhead across North 
Central Alabama this afternoon. Morning fog has lifted, but there's
plenty of moisture to go around underneath this ridge as seen by the
lingering low to mid cloud cover on visible/IR satellite. These 
clouds make for a slightly more complicated forecast as they may 
impact how quickly temperatures "jump" into the lower 90s by midday. 
The very moist air mass is still in place and models have continued 
to underestimate dewpoints as readings in the mid 70s to upper 70s 
are common across much of the region. With little wind/flow to speak 
of, do not think there will be much air mass modification. Therefore,
even if we "mix out" to dewpoints of 73-76 degrees this afternoon, 
it will only take low 90s highs to reach Heat Advisory criteria in 
many locations -- especially in Northwest Alabama and the HSV-DCU 

Where as yesterday an expansion of the Heat Advisory was essentually
a sure thing, today is very much forecast dependent on what clouds 
do and how high temperatures can climb. However, do feel an expansion
is warranted for two reasons. First, the very oppressive tropical- 
like air mass in place will make it easier for HIs of 105 or greater 
to be reached once heating gets going. Secondly, while there should 
be slightly better coverage of showers/storms, the impact of 
subsidence underneath the high pressure ridge will limit their 
coverage and thus keep the local boundary layer undisturbed. Thus, 
will expand the Advisory to the same area as yesterday and include 
Lawrence, Cullman, Limestone, Morgan, and Madison Counties. Given 
the latest temperature trends will start the Advisory an hour earlier
at 11 AM CDT. 

Speaking of storms, once some breaks in the clouds develop, the
latest HiRes guidance continues to show widely scattered pulse 
convection developing this afternoon. Latest HRRR indicates around
2500 J/Kg of SBCAPE and 1500 J/Kg of MLCAPE to work with, but
virtually no shear. A strong storm is possible, but this will be the
exception. Lightning, however, will continue to be a danger with any
storm that develops. If you hear thunder or see lightning -- move

.SHORT TERM...(Tonight through Monday)
Issued at 322 AM CDT Sat Jul 14 2018

A loss of daytime heating/instability should help dissipate coverage
of showers/storms tonight. Locations that receive a "good" wetting
rain would be more apt to be affected by fog late Saturday night into
Sunday morning. Muggy conditions otherwise are expected, with low
temperatures cooling only into the lower/mid 70s.

An almost repeat performance as far as warmth and rain chances to 
start a new week during Sunday. Say almost, as high temperatures 
should trend down another degree or two, which effectively lessens a 
risk of heat advisory conditions. That being said, heat indices will 
still range from the upper 80s to 104 degrees. 

Somewhat higher rain chances are expected on Sunday, and especially
during Monday, as lift preceding an approaching frontal boundary is
realized. Model precip water forecasts ranging in the 1.7" range 
today, will increase into the 1.9" on Sunday and over 2" by Monday.
Model soundings, especially on Sun/Mon both show an inverted-V look;
which could be indicative of a microburst potential (especially on
Mon with higher instability and moisture values). Thunderstorms in
these high moisture regimes can become prolific lightning producers
as well. 

.LONG TERM...(Monday night through Friday)
Issued at 322 AM CDT Sat Jul 14 2018

An upper trough well into the Canadian provinces will start to drag 
a cold front through the area Tuesday morning with the front through 
most of the area by the late afternoon. The mid level trough is more 
pronounced than the 500mb trough and will provide the synoptic scale 
lift for the front. Even so, there is very little support for this 
front outside the convergence along the surface feature. This will 
still be able to support numerous thunderstorms all day. Looking at 
GFS forecast soundings, nothing is catching my eye to support severe 
storms. The CAPE is high as usual, above 2500 J/KG but the DCAPE, 
Lapse Rates, WBZ/FZL heights are all marginal or less and there is 
little to no shear or helicity. PW values near 2 inches combined with
the instability and lift could produce a strong storm or two but at 
this point, severe looks unlikely. Clouds and rain will cool highs a 
bit; in the mid to upper 80s. 

Dry air will be quick to move in Tuesday night with surface high
pressure building in along with northerly winds. Storms should end
from north to south shortly after midnight. With dew points dropping
and skies clearing, lows will be in the upper 60s to around 70.

Should see mostly sunny days Wed and Thu with high pressure firmly 
in control with dew points falling into the upper 60s. This seems 
high but when you compare it with the mid 70s of late, the upper 60s 
dew points will feel pleasant! But given that it is mid July and 
heights don't lower much, it's still going to be hot with highs 
around 88-91 degrees both Wed and Thu. Honestly, left the slight 
chance pops in the south for Wed, the east for Thu and most of the 
area Thu Night but think this is too high and we won't have much of 
anything. Lows will be the coolest Wed night, in the mid to upper 60s
and warming to around 70 for Thu night.

A trough starts to deepens aloft on Friday between two strong high 
pressure ridges, one over the Atlantic and one over the southwestern 
states. With dry air remaining at the mid levels, looks like the
feature won't give us much precip till later Friday and mostly in 
the east. Both GFS/ECMWF are showing something forming along the Gulf
Coast in response to an upper jet streak that could cut off our 
moisture as the winds turn southerly on Saturday. There are still 
some differences this far out so we'll wait and see as we get closer.
With the clouds increasing at least, highs will be in the upper 80s 
both Fri/Sat.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 648 AM CDT Sat Jul 14 2018

Under high pressure at the surface and aloft, variable cloud 
coverage prevailed across the region. Patchy light fog that was 
affecting a few terminals should dissipate shortly after 12Z. Modest 
atmospheric moisture and strong daytime heating will result in the 
development of isolated to scattered showers and thunderstorms. Did 
not include precipitation given uncertain model placement and timing.
Showers/t-storms if they occur, should be in the afternoon hours. 
Any showers that form will move slowly, and capable of producing 
locally heavy rainfall; gusty winds and frequent lightning. Shower 
activity should wind down this evening. More late night fog 
development is possible, especially in/near areas that are impacted 
by precipitation on Saturday. 


AL...Heat Advisory until 6 PM CDT this evening for ALZ001>003.




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