Area Forecast Discussion
Issued by NWS Huntsville, AL

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873
FXUS64 KHUN 262007
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
307 PM CDT Mon Jun 26 2017

.NEAR TERM...(This afternoon and Tonight)
Issued at 307 PM CDT Mon Jun 26 2017

A band of mid-level cloud cover associated with a passing mid-level
shortwave trough is currently moving across the TN Valley this
afternoon with light precipitation/virga occurring well above the
surface. This area of clouds also marks a differential heating zone
where `cooler` air is present to the north/northwest of the cloud
cover with temperatures mostly in the mid to upper 70s with dewpoint
temperatures in the mid to upper 50s. Currently, expecting no
precipitation for this afternoon and evening as these clouds
gradually pass. The primary impact will be a temporary hiatus for
rising daytime highs. Otherwise expect a generally pleasant evening
with much less humidity than normally experienced this time of year.

Along those lines, for later this evening and tonight, an additional
mid-level shortwave trough rotating around a parent broad upper
trough axis centered over Great Lakes region will move southeast.
This may bring rainfall over portions of southern middle TN and far
northern AL later tonight but with antecedent low-level dry air mass
(below 700 mb), rainfall will likely be rather light. May need to
make some adjustments to the forecast for this evening depending on
the timing of this trough. Even then, kept POPs low for tonight and
concentrated to the extreme portions of northern Alabama and all of
southern middle Tennessee.

.SHORT TERM...(Tuesday through Wednesday night)
Issued at 307 PM CDT Mon Jun 26 2017

The threat for rain should pass quickly during the morning hours on
Tuesday with most hi-res models showing all activity
dissipating/moving off to the east by 15-18Z. In conjunction with the
passage of this shortwave trough axis, a reinforcing `cool` front
will bring a surface anticyclone from the Corn Belt Region. This
approaching high pressure system will act to keep the rest of
Tuesday/Tuesday night dry with another pleasant day as this dry/cool
airmass arrives. Daytime high temperatures should rise into the
upper 70s/low 80s which is 10-11 degrees below climatic normals.

Overnight lows should be in the upper 50s to low 60s on Wednesday
morning. Then, the upper flow pattern shifts with a more zonal flow
aloft and the polar jet lifts well to the north. Meanwhile, a low to
mid-level ridge builds over the Southeast from the Bahamas/Florida
as an upstream trough and decreasing mid-level level heights and
increasing LLJ across the Plains states brings the potential for
severe weather over the Plains. As this trough shifts east and the
low to mid-level ridge shifts east, low-level flow will veer to the
south/southwest bringing back higher humidity and warmer overnight
low temperatures for Wednesday night into Thursday morning.

.LONG TERM...(Thursday through Sunday)
Issued at 307 PM CDT Mon Jun 26 2017

The suite of global models continue to indicate the likelihood of a
quasi-zonal flow pattern developing across the SC and SE CONUS by the
middle of next week. On Thursday, the remains of a sheared short
wave along/near the Gulf Coast will begin to move NNEWRD as an upper
ridge builds off the East Coast. Boundary layer moisture will rise on
Thursday in response to the gradually deepening SW flow over the TN
Valley. The lower level instability combined with the effects of the
northward moving upper vorticity will lead to the generation of
isold/sctd showers/storms in the region especially on Thursday
afternoon. Development may be a little more favored farther to the
south, closer to the dynamic/thermodynamic support of the upper wave.

The overall low-lvl pattern will be relatively stationary over our
region into the weekend, although a trough will tend to dig into
portions of the Southern Plains and the TN Valley by Saturday-Sunday.
Deepening SWRLY flow will lead to gradual increases in overall
column moisture and PW values may be as high as about 1.8 to 1.9
inches as mentioned in previous forecast discussions. While not
overly unusual, these values would represent around the 90th
percentile for this time of year. In the maritime subtropical air
mass expected to set up across the region, with high WBZ values
(~13-14 kft) each day from Thurs-Sun, some locally heavy rainfall
will be possible. Relatively low-shear will result in fairly typical
pulse thunderstorm activity though, but ML CAPE values approaching
~1500-2000 J/Kg and the presence of some mid-lvl dry air (mainly
Thurs-Sat) will allow for the presence of some strong/marginally
severe wind gusts in the most vigorous convection. Overall though,
the threat for severe weather during the period appears to be rather
small.

A surface front may move into the OH/TN Valley region by Sunday night
or Monday, but it is still not clear how far south this front will move.
Even so, the general consensus stalls the front well to our north
and our immediate area never quite leaves the influence of the West
Atlantic sub-tropical ridge. So, typically humid and warm summer
weather with chances for showers and storms will linger into the
early part of next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1238 PM CDT Mon Jun 26 2017

VFR conditions should prevail across north Alabama/southern middle
Tennessee for the next 24 hours as an upper level disturbance move
across the region. There could be some light precipitation over
southern middle Tennessee between 27/06Z-12Z but not enough
confidence to include at this point. Fog potential looks low tonight
given the possibility of low-mid level cloud cover.


&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...SL.77
SHORT TERM...SL.77
LONG TERM...KDW
AVIATION...SL.77


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