Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 270803

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

.NEAR TERM...(Today)
Issued at 303 AM CDT Tue Jun 27 2017

A broad trough remains across the eastern half of the US with a
shortwave rounding the base of this trough. The shortwave is now
moving through Missouri and a band of showers and storms have
developed just ahead of the trough along an axis of enhanced mid-
level convergence. The past few scans of radar show some weakening of
the activity as it moves into western TN. Based on mesoanalysis these
might be riding and moving along a mid-level Theta-E ridge, which
could put them into northern MS/NW AL by 5am. GOES-16 WV imagery is
showing an axis of dry air advecting eastward just ahead of these
showers and might help to weaken them further. Hires guidance hasn`t
been much help this morning as the timing in intensity of the
activity varies from run to run. Even though the thinking is that
these should weaken a bit more before arriving into the area, the
current radar depiction is hard to ignore. So, have adjusted PoPs
upward for the immediate near term and taken some of these higher
values through morning hours. All activity should end by mid to late
morning as the shortwave driving the showers moves off to the east.
Any rainfall amounts will be light as there is still drier air
in the lower levels which shows up in 00z soundings.

For today, the cloud cover from this activity will linger through
the morning hours but begin to push off to the east as we approach
the noon hour. By early afternoon, the western half should be mostly
sunny and starting to warm up. We could see a slight gradient in temps
this afternoon between the east/west due to the departing cloud cover.
The west should warm up into the mid 80s again today, while the
eastern half of the area may not make it much above the 80 degree

Surface high pressure builds in behind the departing trough this
afternoon and evening, pushing a weak front across the area. The
only change we may see would be in dewpoints as they fall into
the 50s.

.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 06Z TAFS through 06Z Tuesday night)
Issued at 1213 AM CDT Tue Jun 27 2017

VFR conditions are forecast through the period. A band of showers
continues to develop over southern Missouri/northern Arkansas and
will drop E/SE towards the area tonight. Not expecting any impact to
either terminal except for increased mid-level cloud cover and
possibly some light rain. Winds will favor a northerly direction with
speeds at or below 10 kts.




NEAR TERM...Stumpf

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