Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 231145

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

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 400 AM CST Thu Feb 23 2017

Moisture advection and shallow isentropic ascent has led to low
stratus development around 1.5 to 2.5 kft this morning in eastern
areas, generally from Franklin County (TN) southward through much of
Jackson and DeKalb Counties. Patchy fog had developed by late in the
evening as indicated by several observation sites, however
MODIS/VIIRS RGB satellite images have shown that the fog is very
limited in coverage and largely located in narrow valleys adjacent to
the plateau. Since the low stratus has developed and moved across
the area, some, if not most, of this valley fog has dissipated.
Elsewhere, patchy fog was present along/near the TN River channel
around Wheeler/Wilson Lakes and nearby low-lying communities.
Nevertheless, the fog does not appear to be sufficiently dense or
widespread to warrant a dense fog advisory at this time. Temperatures
have fallen into the mid to upper 50s at most locations early this
morning, and will remain near stationary until sunrise.

Any remaining fog will quickly dissipate after sunrise this morning.
The low stratus currently in the east and in areas to our south will
gradually spread farther westward, perhaps into the
Huntsville/Decatur metro area later this morning. However, this cloud
deck will tend to disperse during the afternoon. Temperatures this
afternoon are likely to reach the low/mid 70s for most locations.
The most likely adjustments made to the forecast later may be to
address issues with cloud cover and impacts to temps though, since
the timing of dissipation and extent of the low cloud deck remains a
little uncertain.

.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 12Z TAFS through 12Z Friday morning)
Issued at 535 AM CST Thu Feb 23 2017

Radiation fog developed overnight due to ideal conditions in some
locations. The KMSL terminal has been affected overnight with
visibility down to 1/2 SM mostly and even briefly touching 1/4 SM.
However, the lack of apparent fog in satellite imagery, even
including higher-res polar orbiting imagery suggests the fog there
is likely thin and should dissipate quickly with the onset of
sunrise/mixing. So, the TAF will bring vis at KMSL from 1/2 SM
prevailing at 12-13Z to MVFR (3 SM) at 13Z and to VFR at 14Z.
Conditions at KHSV are likely to remain VFR through the period.





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