Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 251120

Area Forecast Discussion
National Weather Service Huntsville AL
620 AM CDT Thu May 25 2017

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 454 AM CDT Thu May 25 2017

Area radars show the last of the light to moderate showers moving
east of the forecast area this morning. This activity is in response
to a fairly deep upper low, as noted in latest GOES-16 upper and mid
level water vapor imagery. At the surface, the associated low was
over the Great Lakes region, with high pressure building along the
northwestern Gulf of Mexico, keeping winds generally from the
southwest. Temperatures have fallen into the mid to upper 50s, and
would expect lows to only bottom out in the low to mid 50s.

After daybreak, the axis of the upper low will shift east of the
area, with drier air filtering across the region. Showers will be
well east of the area, making for a dry and relatively nice day
across the TN Valley. Despite skies clearing this morning,
temperatures will still be roughly 10 to 15 degrees below normal,
thanks to the upper level system and cooler airmass currently over
the area.

.SHORT TERM...(Tonight through Friday night)
Issued at 454 AM CDT Thu May 25 2017

The dry conditions will continue into Friday across the TN Valley.
Upper level ridging will begin to build in overnight. Meanwhile,
surface high pressure will shift eastward across the northern Gulf of
Mexico, shifting our winds to the south overnight. Despite mostly
clear skies across the area, the combination of height rises and
southerly surface flow will keep Friday morning lows in the mid to
upper 50s.

The upper ridge will continue to build in on Friday, with southerly
flow increasing through the day. This will begin quite a warm up as
we head into the weekend. Under plentiful sunshine, temperatures will
warm into the mid to upper 80s Friday afternoon. Clouds will begin
to increase Friday night as moisture increases in advance of the next
system. Thus, overnight lows will only fall into the mid to upper
60s early Saturday morning.

.LONG TERM...(Saturday through Wednesday)
Issued at 454 AM CDT Thu May 25 2017

A mid-level subtropical ridge -- initially centered across the
northwestern Gulf of Mexico -- is forecast to weaken and shift
eastward across the southern FL peninsula and into the western North
Atlantic over the course of the extended forecast period. To the
north of our region, a northern stream trough located across
Saskatchewan/Manitoba on Saturday morning is expected to push
eastward into Ontario by 12Z Monday and gradually become more
amplified early next week as it begins to lift slowly northeastward.
Across the TN Valley, seasonably strong westerly flow aloft will be
maintained between the ridge to our south and trough to our
north...with several lower amplitude waves embedded in this regime
expected to periodically enhance the coverage of showers and

As mentioned by the previous long term forecaster, model soundings
from the GFS/ECMWF continue to suggest the presence of a rather
strong capping inversion during the day on Saturday -- which will
likely limit the coverage of convection during the day despite
moderately unstable conditions (MUCAPE in the 1500-2500 J/kg range
generated by dewpoints in the u60s/l70s beneath a plume of steep
lapse rates aloft). Any storms which manage to develop will have a
high likelihood of producing severe wind/hail based on sufficient
vertical wind shear for storm organization, but coverage appears low
at this point. A greater coverage of thunderstorms is still expected
to evolve on Saturday night/Sunday morning, as the southwesterly low-
level jet strengthens after sunset -- effectively allowing an
organized cluster of storms originating from MO/IL Sat afternoon to
propagate southeastward into our region. Based on little change in
elevated CAPE profiles and a gradual increase in shear, this activity
will also have a high likelihood of producing damaging winds/large
hail. How far southward into the region this activity is able to
penetrate is still a bit uncertain, but current guidance suggests
that this activity will persist across at least the northeastern
portion of the CWFA late Sunday morning before a lull occurs Sunday

By Sunday evening, a weak cold front to our northwest will finally
drift into the region -- providing an additional, more widespread
round of showers and thunderstorms. Both shear and instability will
be decreasing with time on Sunday night/Monday morning, which
suggests the greatest threat for severe will be with activity the
previous night. However, parameters will remain favorable for at
least a few strong storms on Sunday night. Based on a slow
southeastward progression of the cold front...this activity will
likely continue for at least the southeastern half of the forecast
area on Memorial Day...with the primary threat transitioning to
locally heavy rainfall/flash flooding.

As the primary northern stream trough begins to lift northeastward
on Monday night/Tuesday, mid-level flow across the TN Valley will
become slightly more confluent and less conducive for precipitation.
That said, deep layer moisture will be sufficient to support isolated
convection during the peak of the diurnal heating cycle on
Tuesday/Wednesday afternoons as temps warm into the l-m 80s. However,
the advection of a drier airmass in the wake of the front will
provide dry and pleasantly cool nights with lows in the u50s/l60s.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 620 AM CDT Thu May 25 2017

VFR conditions will prevail at the KMSL and KHSV terminals through
the period. Low clouds ~1000 feet may impact KHSV over the next hour,
however latest obs and satellite imagery show the BKN and OVC cigs
have moved east of the terminal. Otherwise, expect clearing skies and
winds from the west. There may be some gusts this afternoon between
15 and 20 kts, however confidence is somewhat low on how long the
gusty winds will last. Winds will become light and variable





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