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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 190542

Area Forecast Discussion
National Weather Service Huntsville AL
1242 AM CDT Thu Jul 19 2018

For 06Z TAFS.


.NEAR TERM...(Tonight)
Issued at 848 PM CDT Wed Jul 18 2018

The little convection that did occur across the TN Valley this 
afternoon died very quickly with sunset. There has been some more
persistent scattered activity focused to the west and south, 
associated with the first remnant cold front. A 2nd front is now 
draped from the Shoals to near Cullman. There is little air mass 
change with this front right away, but some spots in middle/east TN 
have seen dew points fall into the low-mid 60s.

Our first window for showers/storms has closed, but rain chances are
not over for the night. Several operational models continue to hint 
at MCS formation over the MS Valley later this evening, then push 
this into N MS and central AL. Model consensus is not great, so
confidence is not high, but it is a feasible solution based upon 
conceptual models. The MCS track would be close enough to ramp PoPs 
back up (only to slight chance) towards daybreak across NW AL and 
Cullman County. The forecast already represents this well, and only 
minor adjustments were needed. With so much cloud cover across the 
area, fog should be a non-issue. The rest of the forecast is in good 
shape as well; minor tweaks were made to nudge dew points higher.

.SHORT TERM...(Thursday night through Friday night)
Issued at 225 PM CDT Wed Jul 18 2018

As usual with NW flow in mid summer, the models are giving varied
solutions synoptically and convectively induced. In general, a 
rather potent northwest flow shortwave will be dropping southeast 
through the eastern corn belt and mid MS valley Thursday afternoon 
and evening. This will induce a return to southerly flow across the 
MS into the lower TN valley. Clusters of thunderstorms, possibly of 
the HP supercell variety due to the increasing bulk shear values, 
will develop along the wave Thursday afternoon and evening across the
mid MS valley. These will likely feed off increasing low and mid 
level wind fields. NW flow will enhance sr-inflow for these storms 
with lengthening curved hodographs and deep layer bulk shear values 
of 40-60kt. So, one round of thunderstorms may arrive late Thursday 
night into Friday morning. There are indications that an MCS could 
propagate southwestward into the inflow and theta-e axis over the 
lower MS valley. Then, a second round of convection and possible 
supercells/MCS may develop Friday afternoon into Friday night as the 
next shortwave drops into the base of the deepening trough. At this 
point, will favor higher (likely) PoPs northeast and lower (chance) 
PoPs southwest into Friday Night. Given the amount of bulk shear and 
sr-helicity present in the forecast hodographs, severe HP storms may 
be possible both Friday into Friday night.

.LONG TERM...(Friday night through Tuesday)
Issued at 225 PM CDT Wed Jul 18 2018

Latter half of the global models continue to indicate a fairly 
active wx pattern in place across the cntrl TN Valley heading into 
this part of the forecast package, as a potent upper level low/trough
axis drops into the Midwest/OH Valley states. Dynamic forcing at the
base of this trough pattern is xpcted to be quite strong, with mid-
level flow around 50kt. With an active nwly flow regime in place, 
some of the models continue to hint at a series of MCSs affecting the
area beginning early Sat, and continuing into the afternoon. With 
SBCAPE values increasing into the 3-4K J/kg range, some of the tstms 
Sat could be strong/marginally svr capable of producing damaging wind
gusts. This could especially be the case within embedded bowing 
clusters/segments, if these MCSs do in fact impact the mid TN Valley 
on Sat and actually provide enough time between complexes for the air
mass to recover. Even with the rainfall/subsequent cloud cover, 
temps may still be able to climb well into the upper 80s/near 90F, 
mainly during breaks in the convection or perhaps in locations which 
do not receive any rainfall.

Overall conditions should then improve heading into the evening hrs
Sat night, as a cold front associated with an area of low pressure
to the north moves into the region. Rain chances may linger into Sun,
as the front continues to gradually move ewd, before showers/tstms
come to an end by Sun evening, with the trough axis translating into
the Appalachians and weakening. While the passage of the front may
not offer much in the way of cooler air, with highs Sun/Mon still
xpcted to predom reach the upper 80s for most areas, some drier air
will briefly filter into the region from the north for a few days. 
Rain chances though will work their way back into the forecast near 
the end of the period, as a subtle increase in moisture over the 
cntrl TN Valley results in the prob for iso showers/tstms on Tue.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1242 AM CDT Thu Jul 19 2018

No changes to previous aviation forecast reasoning. VFR conditions 
are expected thru 06Z/20 as slightly drier air filters into the area.
Some guidance suggests a chance of fog at KMSL, but this will be 
discounted for now due to thick cirrus clouds in place.





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at weather.gov/huntsville.