Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 231729
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1129 AM CST Thu Feb 23 2017

.UPDATE...
For 18Z TAFS.

&&

.NEAR TERM...(Rest of Today)
Issued at 920 AM CST Thu Feb 23 2017

Fog has dissipated across the area, leaving low clouds over all but
the central portion of the TN Valley. Insentropic analysis was
handling the two areas of low clouds fairly well, and based on the
near term guidance, these clouds will stick around through the
remainder of the morning. Despite these clouds, upper ridging and
strong southerly flow will help our temperatures rise through the
day, with forecast max temps in the low to mid 70s. No major changes
were made to the previous forecast package, except to update hourly
temps and keep clouds around a bit longer.

.SHORT TERM...(Tonight through Saturday)
Issued at 400 AM CST Thu Feb 23 2017

A broad, partially closed upper low currently in the Western CONUS
will move into the Plains states as it phases with a northern stream
trough in central Canada tonight and Friday. In response, a surface
low will develop and briefly strengthen along the baroclinic boundary
stretching across the Plains and Upper Midwest while a surface cold
front moves into the Mississippi Valley region. In the warm sector
in advance of this front, temperatures may soar into the upper 70s
across many locations in the Tennessee Valley. Temperatures as high
as 80 degrees may occur particularly in NW Alabama closer to the main
axis of strong temperature advection from the SW. Record high
temperatures are likely to fall at some locations on Friday.

Showers and thunderstorms are expected to develop along the advancing
front on Friday, but a significant capping inversion centered around
700-800 mb will tend to keep development at bay in our area until
late in the day. To our north, where better instability and
dynamical forcing will be present, thunderstorm development is
expected to occur earlier. Showers and storms are expected to then
develop gradually southward along and ahead of the cold front as it
moves across the area Friday evening. So, the best chances for
shower/thunderstorm activity will be in the north. CAPE values may
reach ~1000 J/KG while deep layer shear in the vicinity of ~50 kts
suggests some storm organization will be possible. Sounding profiles
suggest "thin" CAPE up through 700 mb, with most of the CAPE above
700 mb to the EL, and dry mid-level conditions with a relatively
moist boundary layer. The threat for damaging winds and large hail
will be present, but is marginal. Looping hodographs, marginally
high effective inflow helicity, and LCLs around 1 km suggest a
slight tornado threat during the evening. The threat for strong to
severe storms will subside as the cold front moves across the area
quickly during the overnight period.

Much cooler, but sunny conditions will return for Saturday as a
modified Canadian airmass moves into the area.

.LONG TERM...(Saturday night through Wednesday)
Issued at 400 AM CST Thu Feb 23 2017

Medium range guidance from the global models is in agreement that a
zonal flow regime in place over the southern CONUS at the beginning
of the extended forecast period will become more amplified on Sunday
as a 500-mb trough digs southeastward into WA/OR. A shortwave
disturbance crossing the southern Rockies late Sunday is forecast to
weaken and eject east-northeastward into the upper OH valley by late
Monday afternoon, as prevailing flow downstream from the western
CONUS trough backs to the southwest.

At the surface, the center of a Canadian surface high will be
located across the TN valley on Sunday morning providing ingredients
for strong radiational cooling. After morning lows in the u20s/l30s,
highs will rebound into the u50s/l60s as the ridge shifts eastward
off the Atlantic coast and southeasterly winds strengthen. Return
flow will continue to increase Sunday evening, as the southern
Rockies disturbance ejects into the Plains and a weak surface low
forms near the Red River valley of OK/TX. Strong low-level
warm/moist advection east of the low will support widespread showers
from the southern Plains into the mid-South region early Monday
morning, with this regime expected to impact our forecast area
during the day. A few thunderstorms are possible based on amount of
elevated CAPE in forecast soundings, but overall instability should
be too meager to support a threat for severe convection even with
deep-layer shear around 60 knots.

The Red River valley surface low will likely begin to degenerate
into a baroclinic trough as the parent wave weakens and lifts
further into the OH valley late Monday. This boundary may serve as a
focal point for additional convection on Monday night/Tuesday, but
the influence of this feature should also dissipate with time as the
western CONUS longwave begins to push eastward and a new surface low
forms in the lee of the central Rockies. Due to uncertainties on
where the boundary may lie, we have maintained a chance POP both
periods. If clouds and precip are less than anticipated on Tuesday,
temperatures could easily be several degrees warmer than values in
the current forecast.

Convection will likely become fairly widespread to the north/west of
the region Tuesday night beneath an intense low-level jet attendant
to deepening cyclone ejecting through the Great Lakes and into
southern Ontario/Quebec. The trailing cold front is forecast to
cross our region Wednesday afternoon, with showers and thunderstorms
likely throughout the day. Although shear/instability parameters
appear favorable for severe storms both Tuesday night and Wednesday,
coverage/intensity of frontal convection will be dictated by
coverage of precipitation Tuesday night. A cooler/drier airmass of
north Pacific origin will be advected into the region in the wake of
this front and linger through the end of the forecast period.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1129 AM CST Thu Feb 23 2017

Low clouds, at or slight below 3000 ft, will hold together at the
KMSL and KHSV terminals through at least 20Z. Confidence on exact
timing of when the clouds lift is low, so will maintain a SCT deck
through 03Z. After 03Z, cigs will lift and skies will become mostly
clear. Lower MVFR clouds will then build back into the terminals
after 11Z, lingering through the end of the period. Winds will
generally remain from the south and may become gusty after 15Z on
Friday.

&&

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

$$

NEAR TERM...73
SHORT TERM...KDW
LONG TERM...70/DD
AVIATION...73


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