FXUS64 KHUN 180239 AAB

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
939 PM CDT Fri Aug 17 2018

.NEAR TERM...(Tonight)
Issued at 939 PM CDT Fri Aug 17 2018

The main line of storms that became strong to severe after crossing
I-65 this evening is currently pushing out of the forecast area and
into NW GA and SE TN. There is a moisture boundary just ahead of the
line with PWATs ~2.1 inches across NE AL. The highest instability has
been to our SE all evening, however with ~1,000 J/kg still over the
area, a scattered thunderstorm behind the line could still become
strong. Not to mention the boundary collision that is about to occur
near NW AL between two lines of storms that derived from earlier
outflow. Will keep an eye on this area for storm strength and
additional development. Adjusted POP/Wx to accommodate for current
radar trends, but keeping scattered chances in overnight. Precip
chances will start to increase early tomorrow morning, however with
model disagreements on the timing of development, have scattered
chances in through sunrise.

.SHORT TERM...(Saturday through Sunday)
Issued at 251 PM CDT Fri Aug 17 2018

The upper level system that has been near the Great Lakes begins to
shift east on Saturday, but the main trough axis will extend W/SW
into the TN Valley. This should be the focus for another round of
showers and storms to develop mainly during the afternoon and evening
hours Saturday. The airmass is expected to be similar to what we had
today, although the upper level winds begin to weaken and this should
keep storms below severe limits. The frontal boundary tied to the
upper level system is expected to stall near the area and could lead
to additional rounds of showers and storms on Sunday. However, there
is some slight uncertainty in just where this boundary will stall
out. The latest ECMWF has it setting up just south of the area and
the result would be a gradient in PoPs with the higher chances
occurring over the E/SE half of the area with drier conditions in NW
AL. Ended up going with a bit of a blend in guidance due to the
lingering uncertainty in where this boundary will set up. The larger
forcing is expected to move off to the east and thunderstorms should
be more typical of summertime pop- up storms Sunday afternoon.

Highs each afternoon should be in the mid to upper 80s. A few
locations across the western half of the area could jump above the 90
degree mark both Saturday and Sunday afternoons. Lows each day will
be around 70.

.LONG TERM...(Sunday night through Thursday)
Issued at 251 PM CDT Fri Aug 17 2018

The stalled frontal boundary begins to shift back to the north in
response to a dropping E/SE out of the northern Rockies Sunday night
into Monday. Models are all in decent agreement in generating
widespread showers and storms over the area Monday afternoon. Per the
latest GFS forecast sounding the environment will be similar to what
we've had the past few days and a few stronger storms are possible
Monday afternoon. On Tuesday the system lifts N/NE across the Great
Lakes and a cold front extending S/SW from this system will move
across the area Tuesday afternoon. Upper level winds increase
slightly but the stronger jet core will be focused further north
closer to the system, so bulk shear values are forecast to remain
below 25-30kts. The wind profile and orientation of the cold front
should allow a line of storms to develop during the day Tuesday and
will quickly move across the area. Drier air advecting in aloft could
result in some dry air entrainment into storms that form along the
front and more widespread gusty winds are possible with the line.

Drier and cooler air settles over the area from Wednesday into
Thursday resulting in below normal temps for both highs and lows.
A few of the colder guidance have lows Thursday morning dropping
below 60 degrees in TN and the more sheltered valleys. However, still
a bit far out to go that low but something that would be a welcome
break from the warmer temps we've had recently. Isolated to scattered
storms are possible Wednesday afternoon as the main trough axis moves
across the area but it's possible that we'll see a drying trend in
the guidance for Wednesday. Have kept the region PoP free for
Thursday as a surface high settles in over the area.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 654 PM CDT Fri Aug 17 2018

The majority of thunderstorms have moved east of the terminals,
however a few showers could still impact KHSV and KMSL over the next
few hours. A low level cloud deck will move in around 09Z with
rainfall chances returning shortly after by 13/14Z lasting through
the TAF period. Did not include thunderstorms at this time due to
low confidence on exact time they will affect the terminals.




LONG TERM...Stumpf

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