Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 221702 AAC

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
1102 AM CST Wed Feb 22 2017

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 921 AM CST Wed Feb 22 2017

Band of deformation precipitation continues to shift east northeast
into TN and has nearly ended completely in northeast AL and southern
middle TN. Mid level clouds will be the first to exit or dissipate,
while low level moisture and clouds will be slower to clear off from
west to east until tonight. Partial sunshine will be possible this
afternoon, mainly in northwest AL.

.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

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


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1102 AM CST Wed Feb 22 2017

Ceilings of 015-025agl (MVFR) are expected to scatter out, lift and
dissipate by this evening. Light and variable flow is expected
through tonight, with BR and/or FG developing by 04Z. It is still
unclear on how widespread and thick the fog will become, so have only
gone with visibility down to 2SM at both KMSL and KHSV in a tempo
group from 08-12Z. There is a possibility of dense fog and LIFR
conditions, so please stay aware for later amendments. VFR conditions
are then anticipated after 15Z after the BR and FG dissipates.





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