Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 271438

Area Forecast Discussion
National Weather Service Huntsville AL
938 AM CDT Tue Jun 27 2017

.NEAR TERM...(Rest of Today)
Issued at 938 AM CDT Tue Jun 27 2017

A quick moving shortwave trough rotating around the base of the
primary trough axis over the Great Lakes has caused enough lift for
light to moderate rain showers to develop and continue east over
central TN. The trajectory of this area of showers is actually
different than the hi-res model output which has a more ESE
movement to it. As a result, no real changes to the POP/WX forecast
as the S Middle TN and far northern Alabama areas expected to receive
the rainfall are still on track. Meanwhile, the area of showers is
overrunning a drier sfc airmass advecting southward today as a
surface high pressure system shifts SE from the Corn Belt Region to
the OH River Valley. This is occurring in response to the synoptic
longwave trough axis finally moving over the TN/OH River Valleys
today. Once the showers pass, dry air should continue advecting south
with dewpoints currently in the low 60s across most of north Alabama
and Tennessee, while dewpoints are in the low to mid 50s over

Made some minor adjustments to the dewpoint temperature and air
temperature trends mainly for drier air expected later this
afternoon and additional increase in daytime highs after the cloud
cover passes. In addition, tweaked the wind forecast as winds are
already higher than originally forecast.

.SHORT TERM...(Tonight through Thursday night)
Issued at 303 AM CDT Tue Jun 27 2017

With dewpoints in the 50s and a surface high just to our north, we
can expect our last cool night for awhile. The northern half of the
area should see temps dip into the mid to upper 50s while the
southern half remains just above the 60 degree mark.

The surface high moves quickly off to the east during the day
Wednesday turning winds to the S/SE and a return of gulf moisture.
Sunny skies and WAA will result in highs Wednesday climbing back
towards normal for this time of year with values in the mid to upper
80s. Overnight lows Wednesday night into Thursday are going to be
about 10 degrees warmer compared to Tuesday nights values due to the
influx of better moisture.

Models have been in good consistency in showing some energy that has
been trapped along the Gulf coast lifting back to the north on
Thursday. This energy coupled with the better surface moisture
should result in scattered showers and thunderstorms developing
Thursday afternoon. The highest chances will be in NE AL closer to
where the energy is forecast to be located. With little shear and
CAPE between 1,000-1,500 J/kg storms should generally remain below
severe limits and be pulse in nature. All activity will quickly end
as we lose daytime heating leaving mostly clear conditions Thursday
night with lows around 70 degrees.

.LONG TERM...(Friday through Monday)
Issued at 303 AM CDT Tue Jun 27 2017

An upper-ridge situated across the Southeastern United States will
slide east off the Atlantic coast late this week. This will put the
southern Tennessee Valley and Southern Appalachians in position for
a good moisture return from the Gulf of Mexico by Friday as
southwesterly winds advect 70s dewpoints back into the area. Thus a
return to a humid, subtropical air mass will result in very warm and
muggy days with good chances for scattered diurnal convection --
thanks to the abundant moisture, instability, and a weaker CAP. With
virtually no shear to work with on Friday, this activity will be
quite pulse-like and short-lived (likely favoring the terrain of
northeast Alabama).

However, by Saturday, coverage is expected to become more widespread
as a shortwave trough rotates around a broader low pressure system
moving into the Great Lakes/Ohio Valley. This feature should help to
kick off more widespread convection Saturday afternoon and evening
(some of which may linger into the night). Given good model
continuity on this the past couple of days, have felt confident
enough to go with likely PoPs for Saturday across the entire area
(and for part of the area Saturday night). A notable jump in PWATs
close to 2 inches has also been noticed, increasing the threat for
localized flooding from the slower moving cells. Instability
parameters also continue to look good, with ML CAPE values progged
to be around 1800 J/kg (SB/MU CAPE close to 3000 J/kg). Shear/lapse
rates are not very impressive, but given the higher CAPE values,
scattered pulse/multi-cell clusters are expected, with a localized
downburst threat and heavy rainfall being the primary threats.

Instability parameters for Sunday and Monday look even better, with
ML CAPE values a at/over 2000 J/kg and very high SB/MU CAPEs over
4000 J/kg, per the GFS. Coverage will be similar to Saturday and
will generally be tied to heating and any lingering outflow
boundaries in place. Will maintain wording for strong to severe
thunderstorms in the HWO for this weekend and into the Holiday week
ahead. One other note will be high temperatures which should be
upper 80s to around 90 degrees in most spots, with heat index values
in the mid 90 to potentially upper 90s by Sunday/Monday. Those with
outdoor plans should monitor the potential for thunderstorms and be
prepared for the heat and humidity expected.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 620 AM CDT Tue Jun 27 2017

Showers moving across west TN should remain north of both terminals.
However, the cloud cover associated with those will spread into
the area and lower cigs to around 5,000 ft. By 18z clouds begin to
scatter out leaving only a few high level clouds. Winds remain out of
the north with speeds at or below 10 kts. VFR conditions are forecast
for the TAF period.





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