Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 181801
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1201 PM CST Sat Nov 18 2017

.UPDATE...
For 18Z TAFS.

&&

.NEAR TERM...(Rest of Today)
Issued at 1018 AM CST Sat Nov 18 2017

A combined circulation rounding the western side of a surface high
situated east of the Outer Banks, and a deepening cyclone moving
across south/central Illinois was setting the stage for a breezy and
windy day for the Tennessee Valley into the evening. For much of the
area, southerly winds were ranging in the 15-25 mph range. Wind gusts
over 35 mph have been reported in the higher terrain. To our west,
even stronger winds in the 20-30 mph range were occurring. These
stronger winds, occurring as the parent low moves towards Lake Erie
by the early evening warrants a Wind Advisory for the Tennessee
Valley, now in effect until 9 PM this evening. Output from the 12Z
NAM, as well as the hourly HRRR/RAP runs were hinting that we may
need to extend the Wind Advisory for a few more hours over our more
eastern areas later today if other models trend this way too.

Scattered to numerous showers, and a few to scattered thunderstorms
are forecast this afternoon and early evening, as a strong cold
front moves towards the region. The general atmosphere is becoming a
high shear and low instability setup. Strong winds were occurring at
the surface and aloft as the pressure gradient tightens with the
approach of the cold front. 850mb winds are already in the +50kt
range, and in the 40-45kt range at 925mb. These winds mixing down in
shower activity to the surface, along with already higher winds at
the surface could cause wind damage issues.

Strong enough shear and marginal instability could also result in
brief QLCS-type tornado formation in the late afternoon and early
evening. This will coincide with the frontal boundary as it rushes
across the region in a WNW-ESE manner. Point soundings indicate a
decent inversion aloft that breaks down as the frontal boundary moves
by - would hint at a brief window for severe storms and/or quick
tornado formation. The precipitation with this front will be post
frontal or anafrontal, with showers tapering off from NW-SE this
evening and overnight. After highs today in the lower 70s a few
spots, low temperatures by midnight will be falling through the mid
40s with gusty 15-25 mph winds from the northwest.

.SHORT TERM...(Tonight through Monday night)
Issued at 444 AM CST Sat Nov 18 2017

The cold front will continue shifting southeastward and away from
the region early this evening, although based on expected location of
front at 19/00Z, high POPs are warranted for the southeastern zones
during the early evening period. However, any lingering threat for
light postfrontal rain across the southeast will rapidly diminish by
Midnight (if not sooner), as the mid-level trough axis crosses the
region and deep-layer northwest flow begins to advect a much drier
airmass into the region. Regardless of the influx of drier air aloft,
low stratus clouds will be slow to dissipate and should only
gradually begin to clear the region from NW-to-SE shortly before
sunrise. Based on expected coverage of clouds and moderately strong
northwest winds, lows will only fall into the m/u 30s.

Highs tomorrow will depend on how quickly the lingering stratus deck
dissipates, and with clouds expected to persist a bit longer across
southern TN/northeast AL, temps have been adjusted to reflect cooler
values in this region. The center of the parent surface ridge will
spread northeastward across the Arklatex region on Sunday afternoon,
and into the TN Valley on Sunday night. Conditions still appear very
favorable for a hard/killing freeze on Monday morning, with clear
skies, calm winds and low dewpoints leading to lows in the m20s for
most of the forecast area. Flow aloft will quickly back to the
southwest during the day on Monday, as a weak mid-level trough shifts
eastward across the southern Plains. This will allow for a gradual
increase in high clouds through the day, but morning/early afternoon
insolation should still be sufficient for temps to rebound into the
m50s. Lows on Monday night/Tuesday morning are a bit tricky due to
the approach of the trough from the southwest and an expected
increase in high cloudiness, and we used a blend of available
guidance to generate lows in the m30s.

.LONG TERM...(Tuesday through Friday)
Issued at 444 AM CST Sat Nov 18 2017

As a ridge builds just off the coast of Baja California a mean
trough pattern will remain over much of the eastern CONUS. With the
mean trough, typically what happens is that several shortwave troughs
move along the prevailing NW flow from Canada to the Plains to the
Southeast. It also means that little in the way of warm up will occur
before reinforcing cold fronts bring additional cool/dry intrusions.
Because of these fronts, anticipating temperatures for the extended
to run below 30 year averages for this time of year.

The first cold front (for the extended) approaches the region on
Tuesday with an antecedent dry and cool airmass in place before its
arrival. The latest med range guidance has this particular front
arriving on Tuesday night as a deepening mid-level shortwave trough
moves across the Midwest and Great Lakes region. With the cold air
advection of the continental polar airmass, temperatures will
struggle to reach into the mid 50s for daytime highs on Wednesday
even with mostly sunny conditions. Should also see a drop to or below
freezing on Wednesday night.

Little change is expected on Thanksgiving Day with highs forecast to
be in the low 50s as cold air advection remains in place and the
surface high pressure center moves over the TN/MS River Valleys.

By Thursday night into Friday, the southern flank of the trough
becomes nearly cutoff from the prevailing flow as the northern flank
of the trough moves rapidly off shore over New England. Though
typical variableness exists with the med range models/ensembles on
the cutoff (or nearly cutoff mid-level low), and possible shower
activity, will keep the forecast dry on both Thanksgiving and Friday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1201 PM CST Sat Nov 18 2017

A widespread MVFR (1800-2500 ft AGL) CIGs persisted across the
greater Tennessee Valley, as a powerful storm system moves from
Indiana to Lake Erie. It will bring a strong cold front across this
area in the very late afternoon and evening. This boundary should
reach the KMSL terminal around 6 PM, with SW winds becoming W, then
NW - still in the 20-30kt range. The boundary should reach KHSV
around 7 PM, with a similar wind direction change. Numerous showers,
along with isolated/scattered t-storms will briefly precede, accompany
the front. Post frontal showers are expected after frontal passage,
which should taper off a few hours later from west to east, along
with wind speeds. CIG/VIS values could briefly lower to into the IFR
range with the front. Timing of this fast moving front precluded
including that in the TAF. MVFR CIGs otherwise should improve into
the VFR range after midnight, continuing for Sunday morning.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Wind Advisory until 9 PM CST this evening for ALZ001>010-016.

TN...Wind Advisory until 9 PM CST this evening for TNZ076-096-097.

&&

$$

NEAR TERM...RSB
SHORT TERM...70/DD
LONG TERM...SL.77
AVIATION...RSB


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