Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 251657 AAC

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
1157 AM CDT Fri May 25 2018

For 18Z TAFS.


.NEAR TERM...(Today and Tonight)
Issued at 350 AM CDT Fri May 25 2018

Isolated pulse showers/storms have been developing/dissipating this
early morning along a moisture convergence axis that is slowly
moving north across northern Alabama into eastern Tennessee. In
between this convective activity, patchy fog may develop. Then,
anticipating that little in the way of shower/storm activity should
develop until late this morning as the convergence axis shifts north
and a northward moving mesoscale vortex moves north from central
GA/AL. This will result in more storm activity during the afternoon.
Forecast soundings and plan view displays show a completely unstable
environment (2000-3000 J/kg). With a lifting mechanism in place, and
saturating profile aloft, this could mean strong wind gusts up to
40-50 MPH, very heavy rainfall, and frequent lightning. Small hail is
a more questionable impact given the high freezing layer (12-14kft).
In addition, steering flow aloft is weak (typical for summer like
conditions) which could mean some varying storm movements or storms
remaining nearly stationary. Given the lifting mechanism and unstable
environment, have made minor adjustments to the prevailing POP/Wx
forecast mainly to the timing as CAMs are showing convective
initiation around 20-21Z.

.SHORT TERM...(Saturday through Sunday)
Issued at 350 AM CDT Fri May 25 2018

By Saturday, a shortwave trough currently moving across the Central
Plains/Midwest region will be moving across the MS/MO River Valleys
bringing more extensive moisture transport and more convective
initiation/storms. In fact, PWATs may register close to 1.5-1.6
inches which is well above normal for this time of year. Numerous
thunderstorms are expected during the day as result of this
shortwave trough approaching. Meanwhile, as has been advertised the
past several days, an incipient tropical disturbance over the bay of
Campeche will be moving northward into the Gulf of Mexico. The latest
model trends have shown a slower and generally westward track. As
the vorticity wind field approaches, it will likely cause the
southern flank of the shortwave trough to become stalled over NW AL
on Saturday afternoon/evening.

Storms may continue on Sunday mainly with the lingering MCV from the
shortwave trough, but could also see some inverted troughs/"bands"
eject off the tropical disturbance as it moves northward. These may
also provide enough lift for showers/storms within a tropical
environment. Due to the saturated (PWATS near 1.8 inches) on Sunday,
expect that these storms will be efficient rainfall producers with
large hydrometeors. Could also see just about any cloud produce at
least light rainfall with such low dewpoint depressions in the lowest
levels. Even then, if the slower trend in the model holds, there
could be some breaks in the storm activity/cloud cover, and still
very humid conditions in place. As a result, have slightly higher
daytime highs in the mid to upper 80s for Sunday.

.LONG TERM...(Sunday night through Thursday)
Issued at 350 AM CDT Fri May 25 2018

Omega type blocking over the European region at the start of the
extended period will be assisted by a high latitude Rex block setting
up across the Alaska. This will keep a "generalized" troughing west
and ridging east pattern in place for the extended period.

Another item of consideration is the formation of a tropical low over
the western Caribbean, moving northward and bringing even deeper
moisture inland. This will help keep chances of showers and
thunderstorms in the forecast. At the present, an area of
convection was in progress just east of the Mexican Yucatan. It was
in a sheared environment not conducive for development, at least in
the next day or so. This morning`s models were in overall better
agreement with one another regarding the direction this next system
will go. The GFS this go around has maintained a more western track,
with the Canadian being the furthest east (in comparison to the ECMWF
and NAM). But the GFS was by far the fastest model bringing its
center of circulation ashore Sunday afternoon/evening - and was
weakest regarding overall storm strength. The other models were
hinting at this system becoming more organized, at least as a
tropical or subtropical low early next week. Water temperatures in
the Gulf between the Yucatan and central Gulf coast are now just warm
enough to support tropical systems. Stay abreast of the latest
statements regarding the development of this system.

Sensible weather for the Tennessee Valley will remain unsettled for
the extended period. Deep tropical moisture already in place will
really become even more "tropical" as the above noted system nears
the region. For this forecast, stayed closer to an ECMWF and Canadian
solution. For Memorial Day, although deep moisture will be present,
the ECMWF as well as the Canadian hint at some subsidence over this
region. So lowered rain chances into the "high chance" range or 50
percent. That being said, any rain that occurs in this very moist
atmosphere (with precipitable water amounts in the 1.7 to 1.9 inch
range) could be very heavy. Better rain chances should be realized on
Tuesday and Tuesday night, as the system moves northward across the
Deep South. At this time, am hedging that the system`s center should
remain west of the region, which will place this forecast in an area
of maximum rain potential. A further east location would reduce
overall precipitation totals. Even though we are firmly in the
growing season, rain from this tropical system (which are notorious
for producing excessive amounts) could range above 3 inches by
Thursday. Hydrological issues such as rises in stream/river levels,
as well as areal or flash flooding cannot be ruled out.

High temperatures on Memorial Day should warm into the mid 80s or
close to seasonable norms. More clouds than sun and higher rain
chances should produce slightly cooler high temperatures for the
Tue/Wed time frame. Temperatures should warm to and above normal
levels on Thursday, with the highest moisture associated with the
system north of the region then.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1157 AM CDT Fri May 25 2018

VFR flight weather conditions are expected for a majority of the
period. Isolated to scattered thunderstorms are possible after 22Z
through around 02Z, and have included a VCTS at both KHSV and KMSL
during this period. Brief heavy rain will lower visibility into the
IFR or MVFR range (1-3 miles) along with locally gusty winds and
cloud to ground lightning. The thunderstorms should dissipate by
around 02Z, although a low chance of additional development is
possible through the night into Saturday morning. Lower ceilings of
010-020agl are possible between 08-12Z along with light BR with
visibility of ~5sm.





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