Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 181202
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
602 AM CST Sat Nov 18 2017

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 444 AM CST Sat Nov 18 2017

Winds aloft across the TN Valley will continue to back to the
southwest and strengthen throughout the day, as an amplified 500-mb
trough shifts eastward into the northern/central Plains this morning,
and across the mid/upper-MS Valley region later today. At the
surface, flow will gradually increase and veer to the south through
sunrise, as the low related to the mid-level trough ejects
northeastward into eastern MO/western IL. Recent satellite imagery
from GOES-16 indicates that a lower stratus deck is rapidly expanding
across the region in the strengthening low-level warm/moist
advection regime downstream from the approaching trough, and the
combination of clouds/elevated winds will keep temperatures stable or
allow them to slowly rise through 12Z.

Guidance from a consensus of near term models suggests that the
stratus layer will remain overcast for much of the day, as the rate
of low-level moistening should continue to increase. Although the
base of the cloud deck may tend to slowly rise by late this
morning/early this afternoon, cloudy skies are expected for the
majority of the period. As the synoptic surface cyclone continues to
deepen and lift northeastward into the eastern Great Lakes region,
deep-layer southwesterly flow will strengthen, and conditions remain
favorable for sustained wind speeds in the 20-25 mph range with
frequent gusts to around 35 mph from late this morning through early
this evening. Thus, the Wind Advisory will remain in effect for all
zones between 18/12Z-19/03Z. This advisory may need to be extended
through 19/06Z for portions of northeast AL/southern TN.

A strong cold front attached to the aforementioned surface low is
still expected to accelerate southeastward this afternoon, and should
cross the northwestern half of the CWFA between 18/22Z-19/00Z, and
the southeastern half between 19/00-02Z. Both global models and
convection-allowing model guidance indicate that a band of
showers/thunderstorms will develop along the cold front across
southern IL/southeastern MO between 17-18Z, and expand southeastward
as the arrival of stronger deep-layer ascent erodes a low/mid-level
capping inversion. With guidance now suggesting that the intensity of
the low-level WAA will boost max temps into the u60s/l70s in spite
of cloud cover, MLCAPE in the 250-750 J/kg will be possible given
dewpoints in the l60s -- and this will support a slightly greater
threat for a few strong storms. Locally damaging gusts in the 50-60
mph range still appear to be the main threat with this activity.

.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 12Z TAFS through 12Z Sunday morning)
Issued at 602 AM CST Sat Nov 18 2017

A thick deck of low stratus clouds has developed across nrn AL
ahead of an approaching cold front, and ovc cigs arnd 1500 ft are
expected to persist thru the morning as sfc winds veer more to the SW
and increase into the 20G30 kt range. Strong boundary layer
flow/mixing may allow the base of the stratus layer to lift to btwn
1500-2500 ft early this aftn, with an increasing threat for sct shra.
Current guidance suggests that wind shift to NW associated with the
cold front will arrive at MSL/22-00Z and HSV/23-01Z, accompanied by a
band of mod-hvy shra and embedded tsra. Stratus cigs will become ovc
and quickly descend to arnd 1500 ft once again in the wake of the
front, with NW flow in the 20G30 kt range continuing thru late
evening. Winds will begin to slowly subside shortly before Midnight,
with stratus beginning to disperse from W-to-E closer to sunrise on
Sunday.

&&

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


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