Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 260450

Area Forecast Discussion
National Weather Service Huntsville AL
1150 PM CDT Sun Jun 25 2017

For 06Z TAFS.


.NEAR TERM...(Tonight)
Issued at 757 PM CDT Sun Jun 25 2017

The broad surface high centered across the Missouri Valley continues
to build into the region this evening with sub 60 degree dewpoints
and light northerly flow. Satellite imagery shows that the mid level
clouds that brought the earlier light rain/sprinkles have been
shunted to the south with clear skies across the region early this
evening. Having said that, there is another band of mid/high clouds
located from western Arkansas/Missouri back into Kansas/Oklahoma
that will eventually make their way into the Tennessee Valley.

Current thinking is that most of the night will be cloud free with
these clouds moving across toward morning and part of the day on
Monday. Current grids have this handled pretty well. Not overly sold
on fog formation tonight given the rather robust drying we saw today
and likely sustained dewpoint depressions that we will see
overnight. Will likely leave in the isolated/very patchy fog for the
river valleys as is for just a few hours. All in all, not planning
many changes to the forecast.

.SHORT TERM...(Monday through Wednesday)
Issued at 312 PM CDT Sun Jun 25 2017

Global models suggest that little will change in the overall
synoptic pattern on Monday/Tuesday, as a broad longwave trough will
encompass much of eastern North America between subtropical ridges
centered east of the Bahamas and west of the Baja peninsula. Another
strong 250-mb speed max and associated mid-level vort lobe embedded
within the longwave trough are progged to dig south-southeastward
across the Dakotas/MO Valley on Monday...before spreading
southeastward into the OH Valley Monday night and central/southern
Appalachians by 00Z Wednesday. This feature appears to be a bit more
amplified than the disturbance which resulted in a fairly widespread
coverage of light rain earlier this morning. Stronger large scale
ascent associated with this feature is expected to lead to a similar
coverage of light rain between 06-18Z Tuesday...based on PWATs rising
back into the 1-1.2 range by this point. At the minimum, forecast
soundings suggest that an increase in mid-level clouds should lead to
temperatures several degrees warmer on Monday night/Tuesday morning.
Any lingering precipitation and cloud cover will end from west-to-
east fairly rapidly on Tuesday afternoon as the disturbance advances
eastward into the Appalachians...with highs predicted to be a bit
cooler compared to Monday.

During the period from Tuesday night into Wednesday, northwest flow
aloft in the wake of the departing system will quickly transition to
a weaker zonal flow as a mid-level shortwave trough shifts eastward
across the northern Plains and the Atlantic subtropical ridge
retrogrades across FL and into the southeastern CONUS. Conditions
appear favorable for nocturnal radiational cooling once a
surface ridge settles across the southern Appalachians. However,
southerly return flow is expected to develop quickly on Wednesday as
the low-level ridge axis translates eastward...and this coupled with
mostly sunny skies will result in highs warming back into the

.LONG TERM...(Wednesday night through Saturday)
Issued at 312 PM CDT Sun Jun 25 2017

A quasi-zonal flow pattern will likely be present across much of the
central/eastern CONUS by Thursday. However, as a deepening
subtropical ridge takes shape off the East Coast on Thursday,
southerly flow will advect low/mid level moisture northward into the
TN Valley. Surface dew point temperatures will begin the long climb
by late Wednesday night and continue into Thursday. The former
modified Canadian air mass will move off to the NE, to be replaced
gradually by a modified maritime tropical air mass. A weak, sheared
short wave will skirt by the region largely to our north, but may
provide sufficient lift/instability along with the increasing low-
lvl moisture to produce isolated/scattered showers and thunderstorms
in the area. The better chances for showers/storms will be in eastern
portions of the area during the afternoon, which may be better
positioned for the weak dynamic forcing.

Shower and thunderstorm chances will increase generally each day
through Saturday and perhaps into Sunday, as a broad, but relatively
shallow trough is expected to take shape in the eastern CONUS. The
closer proximity of synoptic scale forcing in association/w short
wave maxima rotating around the upper ridge/trough interface will
help to instigate this activity, along with continued low- lvl
moisture advection from the S-SW. Thunderstorm activity during this
period will partially be driven by mesoscale boundaries and without
substantial deep layer shear. ML CAPE values may approach 2000 J/kg
by Saturday afternoon, but the vertical distribution of CAPE suggests
mainly strong thunderstorm this time. PW values do
climb to around 1.9 inches by late in the weekend, and along with the
potential for slow-moving storms in the relatively weak wind
environment and rather high freezing/WBZ levels (~13-14 kft) may
preclude a small threat for localized flash flooding.


.AVIATION...(For the 06Z TAFS through 06Z Monday evening)
Issued at 1150 PM CDT Sun Jun 25 2017

Northerly flow continues to usher in relatively cooler and drier air
into the Tennessee Valley tonight. Mainly clear skies were noted
across the region. Additional mainly mid/high clouds will approach
the region toward Monday morning and may lower/thicken late in the
day ahead of a weak upper level system. There is a narrow window of
fog potential early Monday morning but any fog should be light and
short lived. Otherwise, VFR conditions are anticipated through late
Monday evening.





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