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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 182313

Area Forecast Discussion
National Weather Service Huntsville AL
613 PM CDT Wed Jul 18 2018

For 00Z TAFS.


.NEAR TERM...(Tonight and Thursday)
Issued at 225 PM CDT Wed Jul 18 2018

The "cold" front has dropped south into southern middle TN and will
continue to make slow progress southwestward this evening. Dew 
points will lower into the 60s as a result, for all but western 
portions of northwest AL. This will also be an area where a slight 
chance of thunderstorms will be maintained this evening and again 
late tonight and early Thursday morning as residual MCS activity 
forming over the mid South may enter these locations. Otherwise, a 
brief but welcome drop in dew points and mugginess should be felt 
later tonight and early Thursday morning.

.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 00Z TAFS through 00Z Thursday evening)
Issued at 613 PM CDT Wed Jul 18 2018

VFR conditions are expected thru 00Z/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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