Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 290818

Area Forecast Discussion
National Weather Service Huntsville AL
318 AM CDT Mon May 29 2017

.NEAR TERM...(Today)
Issued at 318 AM CDT Mon May 29 2017

Latest water vapor imagery shows an upper level low over the
northern Great Lakes, between a fairly amplified ridge along the
Intermountain West and a weaker ridge over the northeastern CONUS.
The TN Valley remained under the quasi-zonal flow aloft, with the
subtropical high just off the FL coast. At the surface, a cold front
extended from its parent low over the eastern Great Lakes,
southwestward through NE Arkansas and into the ArkLaTex. Area radars
show two areas of precip near the TN Valley; a decaying MCS moving
into central Alabama and a line of developing showers along the cold
front. Light to moderate rain was also moving across NW Alabama
early this morning, and will continue to push northeastward over the
next hour or so. Enhanced by an upper level shortwave moving around
the base of the upper trough, the showers and thunderstorms along the
front should maintain themselves through the overnight period and
impact NW Alabama around daybreak.

Coverage along the front is expected to remain isolated to scattered
through at least mid morning. Ahead of the front, the TN Valley will
remain within the warm sector and dewpoints are expected to climb
back into the upper 60s. Temperatures will warm into the lower 80s,
but may be impacted a bit by cloud cover and any rainfall this
morning. Forecast guidance has slowed down the timing of the front
actually making it into the forecast area, with many models keeping
it just to our north through the day. This makes timing out the
better chances of precip for today somewhat challenging. However,
several shortwaves are expected to continue to move east-northeast
across the region today as the upper trough amplifies just a bit and
digs southward into the lower MS and TN Valley. This will enhance
convection across the area, with the highest coverage expected to be
along and south of the TN River, where the better forcing will be.
There may be an isolated strong to severe storm given the instability
increasing, however not expecting a widespread severe threat.

.SHORT TERM...(Tonight through Wednesday night)
Issued at 318 AM CDT Mon May 29 2017

Showers and thunderstorms will diminish in coverage and intensity
tonight as the forcing moves off to the east/northeast and we lose
our daytime heating. There may be some lingering showers through the
overnight period, as the front moves slightly to the south.
Overnight lows will be dependent on if we see any clearing, but
should remain just above normal for this time of year.

The front will remain stalled to the north on Tuesday but may drift
slightly southward. Guidance shows several shortwaves continuing to
move across the region in the quasi-zonal flow along. These impulses
will enhance any convection along the stalled surface boundary,
especially during the peak heating hours. An upper level shortwave
will move from the northeast and into the area overnight Tuesday,
which should help push the front to our south by daybreak on
Wednesday. However, scattered showers and thunderstorms will
continue to be possible once again on Wednesday given the ample
moisture and the lack of subsidence. There may be a strong storm or
two on both Tuesday and Wednesday, but organized convection does not
appear likely given the lack of wind shear. Any activity that
develops will diminish after sunset. Temperatures will generally be
near normal for this time of year, but may be dependent on when and
if rain falls over a given location.

.LONG TERM...(Thursday through Sunday)
Issued at 318 AM CDT Mon May 29 2017

An unsettled pattern will continue for the latter half of the work
week, continuing into the upcoming weekend. Big picture wise, most
effects from the northern stream influence on the Tennessee Valley`s
sensible weather will for the most part remain north of the region.
Weak but broad upper level ridging parked off of the east coast will
be a controlling factor in our sensible weather. A southwesterly
flow around this ridge will continue into the weekend. Surface high
pressure in a similar position will also bring a good supply of
moisture laden air from the Gulf inland. This moisture, along with
heating and resulting instability will bring chances of showers and
thunderstorms across the forecast area.

Big questions for the extended include how high will the rain
chances go? Given that we are getting into a summer like pattern,
limited rain chances to scattered (less than 55%). Given that this
precipitation are showers, aka hit/miss, stayed that route. Even so,
the best rain chances will be during the Friday to Saturday time
frame, as deeper moisture (precip water per the GFS increase into the
1.5 to 1.7 inches range) during this period. This will produce CAPE
values ranging up into the 1500 J/kg range during Fri/Sat afternoons.
Wind shear values for the most part should remain low. Thus mainly
"general" intensity convection is expected - with the usual gusty
winds, locally heavy rainfall and occasional to frequent lightning
occurrence. With the moisture and more clouds than sun, highs should
only warm into the lower 80s late next week. Usual values in early
June are in the mid 80s.

A frontal boundary will make a move southward across the region early
next week in the Sunday/Monday time frame. The Canadian model was the
most aggressive and coolest of the global models looked at, with the
ECMWF the warmest and the GFS in between. Even with the frontal
boundary lurking about, rain chances should continue early next week.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1210 AM CDT Sun May 28 2017

VFR conditions are still prevailing across N AL/Southern Middle TN
with an area of storms well to the southwest over central MS. The
northern portions of those storms may affect the terminals later
tonight especially after 29/08Z-09Z as a cold front to the NW also
gradually moves towards the region. Models are also showing the
possibility of MVFR cigs for a few hours between 29/13Z-29/18Z. High
clouds are also moving across the Southeast which should mitigate any
fog issues. Anticipating the threat for VCTS to prevail through the
remainder of the TAF period.





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