Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 221223
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
623 AM CST Wed Feb 22 2017

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 505 AM CST Wed Feb 22 2017

A closed upper low currently across southern MS will continue to
shift southeastward into the north central Gulf of Mexico this
morning, before turning east-southeastward toward the west coast of
the FL peninsula today. In the wake of this feature, a shortwave
ridge extending from southwestern TX into the Ozarks early this
morning will gradually spread southeastward across the TN valley.

As this occurs, the synoptic scale processes responsible for the
light rainfall and patchy light drizzle/fog we have observed
overnight will come to an end by late morning...with low stratus
clouds beginning to lift during this period as well. Forecast
soundings suggest that PWAT values -- currently around 1.3 inches --
will slowly fall throughout the day as mid-level subsidence
associated with the shortwave ridge results in pronounced drying
above the 700-mb level. Despite light/variable low-level flow and
little mixing, the advection of drier air aloft should allow clouds
to scatter more efficiently this afternoon, with temperatures
expected to rise into the u60s/l70s.

.SHORT TERM...(Tonight through Friday)
Issued at 505 AM CST Wed Feb 22 2017

Global models are in agreement that the previously mentioned upper
low will continue to dig east-southeastward across the southern FL
peninsula tonight before ejecting northeastward into the western
Bahamas tomorrow. Meanwhile, the shortwave ridge axis to its north
will gradually shift to the south of the local area...allowing winds
aloft to back to more of a westerly direction late this evening. A
weak 500-mb disturbance across OK/KS today will spread east-
northeastward over the TN valley tomorrow morning, but lift
associated with this feature appears to be too weak to support
precipitation based on expected moisture profiles. However, forecast
soundings still suggest a favorable setup for radiational cooling
this evening and subsequent development of fog late tonight/early
Thursday morning. Although some guidance indicates the development
of light southeasterly return flow prior to 12Z due to a deepening
lee cyclone across eastern CO, we feel that winds will remain
sufficiently light to support at least a threat for patchy dense fog
before sunrise Thursday. Morning fog should lift rather quickly after
sunrise, with light-moderate southerly winds and more late
morning/afternoon sunshine leading to highs in m70s for most
locations.

During the period from Thursday night into Friday, models suggest
that the mid-level ridge to our south will gain amplitude and build
northeastward across the southeastern Atlantic coast as an initial
shortwave trough begins to eject out of a broader longwave over the
western CONUS. This wave is forecast to deepen as it travels east-
northeastward across NE/IA on Friday, with attendant surface cyclone
shifting northeastward from western KS into the Lake Michigan region
by late Friday afternoon. Low-level winds across our forecast area
will respond to the increasing pressure gradient, with a
strengthening southwesterly low-level jet resulting in warmer low
temps Thursday night/Friday morning. Forecast soundings suggest that
at least some low-level stratocumulus will be advected northeastward
with the low- level jet, but should scatter fairly quickly allowing
temps to warm into the m70s-around 80 based on magnitude of deep-
layer southwest flow. A fairly substantial inversion in the 850-900
mb layer should severely limit the threat for convection through
early Friday evening -- with no evidence of any source of mid-level
ascent to erode the cap. However, a few showers/thunderstorms may
develop during the afternoon as convective temperatures are
approached or slightly exceeded.

.LONG TERM...(Friday night through Tuesday)
Issued at 505 AM CST Wed Feb 22 2017

As the aforementioned sfc low continues to track NEwrd from the
Central Plains into the Great Lakes region, a cold front will be
dragged across the MS River Valley. While the better forcing/lift
will remain well north of the local CWFA, a line of showers and
storms will push across the area Friday night, with a few strong
storms possible. This mainly hinges on breaking the fairly stout cap
at 700mb. If that should occur, mainly a gusty wind threat would be
the primary hazard, with winds ~40 (to perhaps 50) mph possible in
the stronger storms that develop. Am not really impressed with
anything more than that, really, so will be trimming back wording in
the HWO with its issuance later this morning.

Showers/storms will push across the area within a couple of hours of
midnight (3-6Z West, 6-9Z East), and will have pushed fully E of the
area by 12Z Saturday. The bigger story with this fropa will be the
CAA advecting in much colder temperatures than in days past. Compared
to morning lows around 60 and highs in the upper 70s/lower 80s,
morning lows Saturday will bottom out in the middle 40s, with daytime
highs in the middle 50s (~20-25 degrees F cooler than on Friday).
Sfc high pressure will build into the area Saturday, settling nearly
overhead by Sunday morning, and making for the coldest morning of the
forecast period due to ideal radiational cooling expected. Morning
lows Sunday will bottom out near the freezing mark, with temperatures
flirting with the upper 20s in sheltered valley locations in NE
Alabama, and in notoriously colder spots across the border in
Southern Middle Tennessee. Daytime highs Sunday will be a few degrees
warmer than Saturday, as the sfc high shifts E of the region, and
winds become more southerly.

A more active weather pattern takes shape for much of next week,
with several disturbances slated to traverse the Mid-MS River Valley
by midweek. ECMWF/GFS have their differences regarding the
timing/strength of these disturbances, but a general consensus of the
two has slight chances for rain Monday (with better chances N closer
to the disturbances as they track E across the SE). Higher rain
chances arrive Tuesday and especially Wednesday as an upper low comes
ashore in SOCAL and weakens into an open wave across the Central
Plains. This coincides with a stationary boundary that will be draped
SW to NE across our area associated with a departing low over New
England and its elongated parent trof aloft. The subtropical high to
our S will also play a role in slowing down the progression of this
front, basically steering any disturbances across our area Wednesday.
Thunderstorms will be possible with this final wave of convection
Wednesday/Wednesday night along a cold front, with moderate to heavy
rainfall expected (on the order of 2- 3" possible from Tuesday night
through Wednesday night). Temperatures during this time will be
around 70 degrees for aftn highs, with morning lows falling from the
lower 60s Wednesday morning to the middle 40s by Thursday. A dry end
to the week is expected with a sfc ridge parked nearly overhead by
Friday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 623 AM CST Wed Feb 22 2017

Upper-lvl storm system will continue to move southeastward and away
from the region today...and although sct patches of lgt rain will be
psbl thru late this morning, the expected coverage/potential impacts
are too low to include -ra in either TAF. Otherwise, an overcast
stratus deck will prevail thru 22/17Z, with cigs remaining arnd 1500
ft. The stratus should begin to lift/scatter late this morning, with
SKC anticipated by late afternoon. Abundant low-level moisture and
lgt/vrbl winds will be very favorable for the development of fog...
and MVFR vsby was introduced at each terminal beginning at 23/04Z.
IFR-LIFR vsby in locally dense fog is psbl btwn 08-12Z, but have only
included 2SM in tempo group at this point.

&&

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

$$

NEAR TERM...70/DD
SHORT TERM...70/DD
LONG TERM...12
AVIATION...70/DD


For more information please visit our website
at weather.gov/huntsville.



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