Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 201414 AAB

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
914 AM CDT Wed Jun 20 2018

.NEAR TERM...(Rest of Today)
Issued at 914 AM CDT Wed Jun 20 2018

A pattern change is underway as high pressure flattens and moves
south into the northern Gulf Coast region. This has allowed cloud
cover and deeper tropical/Gulf moisture to filter into the Tennessee
Valley from Texas along the northern edge of this ridge. Some denser
mid to upper-level clouds have already begun to filter into the
western 2/3 of the area this morning, creating a mostly cloudy to
overcast sky. Some partial cloudiness still remains over Northeast
Alabama, but this will quickly change by early afternoon. The result
will be a cooler day across the region, with highs struggling to
reach the 90 degree plateau for the first time in several days. In
fact, peak temperatures in the mid to upper 80s seem plausible in
most locations -- and have lowered temps just a hare. A few spots,
especially the HSV, DCU, and MSL ASOS sites may hit 90 degrees where
there are some breaks in the clouds and where current readings in
the lower 80s were noted.

Some light elevated showers have also developed across the region,
mainly in areas west of I-65 in this very moist air mass. Do expect
these scattered showers to continue throughout the day, with the
potential for a few isolated thunderstorms as well, thanks to some
weak elevated instability. Overall, QPF amounts will be low and most
locations will remain dry.

.SHORT TERM...(Tonight through Friday)
Issued at 320 AM CDT Wed Jun 20 2018

Models are in consistent agreement that the aforementioned trough
over the Plains will continue to migrate eastward, shifting the
subtropical ridge further to the southeast tonight. Meanwhile, the
low over Texas will continue to send additional waves northeastward
and rounding the northern periphery of the ridge. The combination of
these waves, along with the upper height falls will mean a greater
chance for showers and thunderstorms than we typically see, with the
first round moving in around daybreak on Thursday. While the main
upper low will remain over the upper Mississippi Valley on Thursday,
a shortwave will move eastward through the TN Valley, bringing
numerous showers and thunderstorms during the day. Given the tropical
nature of this initial wave, moisture will surge northward as well,
giving PWs well above climatological norms. This may lead to locally
heavy rainfall, though flooding is not anticipated given the quick
movement of the activity. Forecast soundings show a moist adiabatic
profile, so not expecting much in the way of strong storms as this
activity moves through during the day.

By Thursday night, this initial shortwave will be east of the area,
though the upper low will not have moved much and will still be
located over the Northern Mississippi Valley. A weak southwest flow
will persist aloft, with southerly surface flow. The steady stream
of moisture, along with the weaker upper level heights will keep a
higher coverage of showers and thunderstorms over the area Thursday
night. Temperatures will be mild and only fall into the upper 60s to
lower 70s. The upper low will pick up some momentum on Friday and
move into the Ohio Valley. With the trough swinging into the forecast
area, another round of numerous showers and thunderstorms are
expected. The strength of these storms will really depend on what
happens in the morning and if we are able to see some decent
destabilization by the afternoon. Given that this is the day 3
timeframe, will not get into too many of the mesoscale details, but
we could see a higher likelihood of organized thunderstorms Friday

.LONG TERM...(Friday night through Tuesday)
Issued at 320 AM CDT Wed Jun 20 2018

The upper level low will move through the Ohio Valley and into the
eastern Great Lakes by daybreak on Friday. Flow aloft will become
more zonal by Saturday, with several additional shortwaves
continuing to move through the region over the weekend. Given some
uncertainty on how strong the forcing will be and on the exact timing
of each wave, did not go higher than chance pops for Saturday and
Sunday. However, the shortwaves will aid in diurnal coverage so
higher chances of thunderstorms are possible.

Models are coming into better agreement for the Monday through
Tuesday time frame, though inconsistencies still exist to some
extent. Upper ridge will begin to build back into the region,
limiting our thunderstorm chances to our typical summertime
convection. The GFS is still hinting at a higher coverage of precip
on Monday, but this will depend on how quickly the ridge builds.
Therefore, kept close to the model blends given the uncertainty.

Temperatures in the long term period will greatly depend on when the
precip moves through. For now, it looks like a return to seasonal
conditions during the extended period, with highs in the upper 80s
to lower 90s.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 612 AM CDT Wed Jun 20 2018

VFR conditions will prevail through the first 18 hours of the
forecast period at the KMSL and KHSV terminals. Isolated to scattered
showers and thunderstorms are possible through much of the day time
hours at each terminal, however given a low confidence on coverage
and timing, do not have any mention of weather in the TAFs at this
time. Looking upstream, showers should have very little impact on the
terminals and thunder will likely not occur until the afternoon
hours, if at all. However, amendments may be necessary. An increase
of moisture is expected later today as a more tropical airmass moves
into the region. This increase in moisture may lead to reduced
visibilities. At this point, have included MVFR visibilities at each
terminal after 09Z, but lower cigs will be possible as well. A wider
coverage of showers and thunderstorms are expected just beyond the
forecast period on Thursday.





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