Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 251847

Area Forecast Discussion
National Weather Service Huntsville AL
147 PM CDT Thu May 25 2017

.NEAR TERM...(Tonight)
Issued at 146 PM CDT Thu May 25 2017

The Tennessee Valley remains sandwiched between a low positioned
across the Ohio Valley today and a a ridge of high pressure situated
across the northern Gulf Coast. Overall, this has resulted in quite
favorable weather for the area after some fairly active weather on
Wednesday evening. Visible/infrared imagery both show scattered to
occasional broken stratocu rotating around the base of an upper low
across the Tennessee Valley and lower Appalachians. Despite the
clouds and a rather cool airmass in place in the wake of the upper
low, temperatures have been able to warm into the upper 60s to lower
70s in most locations. Given the gradient in place, we have seen
some gusts approaching 20 knots today.

For tonight, expect the mainly diurnal cu/stratocu field to
dissipate and leave mostly clear skies around sunset. The boundary
layer overall remains fairly dry so at least one more "cool" night
will be in store with lows dropping into the lower to middle 50s.

.SHORT TERM...(Friday through Saturday)
Issued at 146 PM CDT Thu May 25 2017

The flow should back to the south/southwest on Friday as the surface
high weakens and slides eastward across the Florida peninsula. This
should set the stage for a gradual moistening of the boundary layer.
However, overall mid level temperatures will be rather warm as the
upper level heights increase across the southeast United States.
The limited moisture along with warm temperatures aloft will keep
conditions dry and rather pleasant on Friday.

Conditions begin to change gradually toward the first half of the
weekend as shortwave energy ejecting out of the Plains begins to
undercut the ridge and lower heights across the region. Dewpoints
will also steadily rise reaching the middle to upper 60s making
things feel much more humid than today/Friday. However, a rather
pronounced cap will remain in place as indicated on all the model
guidance. Hard to see any discernible way we can initiate and sustain
surface based convection given the thermodynamic profile in place.
An isolated elevated cell may not be out of the question so will
maintain a low end (20%) pop for Saturday afternoon. Highs on
Saturday may be a touch cooler than Friday, but not much different.

.LONG TERM...(Saturday night through Wednesday)
Issued at 146 PM CDT Thu May 25 2017

This part of the forecast is complex and likely to change over the
coming 24-48 hours.

The primary focus will involve the interaction of a compressed ridge
centered over the GOMEX and a southeastward approaching mid-level
shortwave trough within a parent closed low/upper trough. The
induced subtropical jet and southerly flow will act to saturate the
low and upper levels, initially. In fact, a strong LLJ will advect a
maritime tropical airmass on Saturday. Though capped aloft, the cap
gradually weakens as cooler air aloft and further saturation of the
mid-levels results in a nearly completely unstable environment late
Saturday night. However, the latest 2 model runs of both NAM/GFS
continue to show a slower progression of the development of deep
severe convection further to the northwest--closer to the surface
low pressure system and frontal boundaries. There may even be 2 MCS`
moving across TN/OH River Valleys, if the GFS is correct, sometime
late on Saturday night with gusty LLJ providing very warm/moist
airmass as inflow. One crosses north MS and the other crosses KY/E
TN. Steering flow could send the one moving over N MS/W TN into NW
AL. These MCS` may begin as multi-cell clusters (and perhaps
discrete cells) that would merge into a line.

As mentioned above, the timing of the convective system arriving
over the TN Valley is slower and looks to arrive sometime between
10Z-15Z on Sunday morning. As it arrives, a high amount of MUCAPE is
present (~2000-3000 J/kg) which combined with deep bulk shear values
between 35-45 kts, would be conducive to severe weather. Most of the
instability is elevated and well within the hail growth zone. So,
very large hail and damaging wind gusts look possible especially as
the MCS is decaying over NW AL. This is where the resulting
mesoscale environment will determine the subsequent impacts on
Sunday, which is quite uncertain. If the airmass is allowed to
realize daytime heating without much convective overturning, a
surface based very unstable environment is likely on Sunday. If this
materializes, any convective outflow boundaries/cold pools from the
morning activity could provide just enough lift and steepening lapse
rates for deep convective updrafts to the EL. With some models
showing SBCAPEs of 3500-4000 J/kg there could be a risk for severe
weather again with discrete and multi-cell clusters on Sunday
afternoon. With the uncertainty on these details, will trend towards
mentioning severe potential on both Saturday night into Sunday with
daytime highs in the low to mid 80s and overnight lows in the upper
60s to low 70s.

The mid-level shortwave trough rotating around the parent upper low
will then continue pushing southeast on Sunday sending a cold front
with another line of convection towards the TN Valley on Sunday night
into early Monday. Given that the bulk of the mid to upper forcing
is to the north of the region, as expected, the sfc cold front
passage will likely continue to slow its forward momentum. Because
of this, thunderstorms will likely continue into Memorial Day in the
morning (over NW AL) and afternoon hours (elsewhere). It appears that
the surface front should push over the SE portions of the area on
Monday evening/night as another shortwave trough rotates around the
parent upper trough/closed low.

Then, a drier airmass/profile should prevail on Tuesday into
Wednesday, but with the upper flow still zonal (lack of subsidence),
there could be lift to support isolated diurnally driven
thunderstorms. Dry air aloft may limit this activity. So, could see
adjustment to POP/storm forecast for those periods as well. The
upper flow pattern then becomes rather murky with either mean
troughing or zonal flow prevailing by the middle of next week. Will
keep at least isolated storm potential in the rest of the extended.
Daytime highs should be in the low 80s but with the dry airmass
overnight lows will be low 60s (cooler than the weekend).


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1204 PM CDT Thu May 25 2017

VFR conditions are anticipated through the period. Broad high
pressure at the surface will slowly push east allowing for westerly
flow to eventually back to a more southwesterly fetch by early
Friday. The pressure gradient will remain somewhat moderate allowing
for minor gusts during the daytime mixing periods. Otherwise expect
mainly clear skies with fair weather cumulus today and a few more
high clouds on Friday.





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