Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 271956

Area Forecast Discussion
National Weather Service Huntsville AL
256 PM CDT Sun May 27 2018

.NEAR TERM...(Tonight)
Issued at 256 PM CDT Sun May 27 2018

Isolated to widely scattered convection has developed this afternoon
(mainly along the terrain areas of the Cumberland Plateau). With
modest instability parameters, weak steering flow, and no other
lifting mechanism of note, this activity has remained weak and short-
lived. A few outflow boundaries should help to keep this activity
going through early this evening, before begins to wane after sunset.
With this moist air mass in place (PWATs up to 1.9 inches), the
threat for locally heavy downpours will exist with any of this
activity -- along with frequent lightning. In locations that have
destabilized more, the potential for a sub-severe, strong storm will
exist, briefly from 21-01z this evening -- with gusty winds to 40-50
mph being the main threat.

Scattered showers and storms will be possible overnight -- mainly on
any residual outflow boundaries. Clouds will also continue to filter
in overnight as Subtropical Storm Alberto moves northward toward the
FL/AL coastline. These denser clouds (and the boundary layer
moisture) will keep the sky overcast, helping to keep lows quite mild
in the upper 60s to around 70 degrees. Some very localized patchy
fog will be possible tonight, but the cloud cover (and any lingering
shower activity) will likely limit coverage. As such, have kept a
mention of this out of the forecast for now.

.SHORT TERM...(Monday through Tuesday)
Issued at 256 PM CDT Sun May 27 2018

Attention turns to Subtropical Storm Alberto as this system
continues to push north into Florida Panhandle by midday Monday.
Ahead of this system, clouds will continue to expand and deep
tropical moisture from the Gulf of Mexico and Western Caribbean will
advect into the Tennessee Valley. PWATs will continue to rise and
approach 2.1 to 2.3 inches by Monday night into Tuesday, essentially
record values for late May for this area. Showers and storms will be
possible again in this tropical air mass ahead of this system again
on Monday (Memorial Day), but limited instability will once again
keep this activity sub-severe, with only an isolated strong storm
possible. As one might expect in this environment, the main impact
with this activity would be locally heavy rainfall/downpours.

A broad upper-trough over Great basin will swing onto the Central
Plains Monday Night into Tuesday. This system will help to pick up
Alberto as it moves into Central Alabama Monday evening and
accelerate north into the Tennessee Valley late Monday night into
Tuesday. Given the deep, tropical moisture in place, locally heavy
rainfall and flash flooding will be possible, especially on Tuesday
when the remnant "core" of Alberto moves across the area. There is
still uncertainty in the track of this storm with some models veering
slightly to the west in recent runs. This track will have a big
impact on which areas will experience the heaviest rainfall and
potentially something more (in the form of an isolated, weak tornado
or two). At this time, feel comfortable that between 2 to 3.5 inches
of rain over the region is possible given how primed we`ll be for
rainfall in this tropical environment. Should the remnant center of
Alberto veer further west, the region will be in a more favorable
position for rainfall and a tornado or two due to favorable
speed/directional shear profiles. Most of the thunderstorm activity
will be very low-topped, so am not expecting damaging winds/hail to
be much of a threat. But shear profiles, as mentioned, will help keep
some of this activity little more organized and capable of producing
a quick spin-up.

Most guidance quickly lifts the remnant center of Alberto and the
heavy rainfall potential up into TN/KY and the Ohio Valley after
00-06z Wednesday. However scattered to numerous showers and storms
are possible through the evening, before tapering off after midnight
as this best forcing lifts to the north and east. Given the forecast
clouds and rainfall, have maintained a below normal temperature
forecast. Highs on Memorial day will likely only reach the lower 80s,
and struggle to climb out of the mid to upper 70s on Tuesday.

.LONG TERM...(Tuesday night through Saturday)
Issued at 256 PM CDT Sun May 27 2018

In the wake of Alberto, I will favor the ECMWF solution as the GFS
appears to be  in allowing shortwaves to penetrate the growing
Plains ridge. The area will be in a warm and moist sector on
Wednesday as the trough axis pushes east. The blended PoP guidance
may need to be subtracted if this moves by even faster than progged
as the wave passage may precede any appreciable destabilization
Wednesday midday into afternoon. On Thursday, I have discounted the
blended PoP guidance in favor of a more reasonable ECMWF solution.
The growing Plains ridge will amplify the flow from the Great Lakes
into the Southeast, but the cap may edge most of the precipitation
and "ring of fire" just to our northeast over eastern KY/TN/north GA
on Friday into Saturday. Have mapped the PoP from low chance east to
slight chance west with this in mind. The ECMWF suggests a better
shortwave penetrating the ridge and diving southeast into the TN
valley Saturday night into Sunday (Day 8). Have also increased max
temps a degree or two above guidance.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1204 PM CDT Sun May 27 2018

VFR to high-end MVFR conditions will prevail at each terminal this
afternoon and evening, as TCU and scattered SHRA/TSRA develop across
the region. Have maintained a TEMPO for 20-24z/21-01z at KHSV/KMSL
respectfully during the window of greatest coverage and a VCTS thru
02-03z before convection wanes after sunset. Mid/High clouds will
continue to increase overnight ahead of Subtropical Storm Alberto. An
isolated storm is possible late in the period, but coverage will be
isolated and a low confidence in this kept a mention out of the TAF
at this time.





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