Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 201201
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
601 AM CST Mon Feb 20 2017

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 435 AM CST Mon Feb 20 2017

An amplified ridge axis extending northward from a mid-level
subtropical anticyclone over the western Caribbean sea continues to
translate eastward across the forecast area this morning. Near term
model guidance suggests that the ridge axis will maintain a steady
eastward progression across the southeastern CONUS today, with flow
aloft across the local forecast area expected to back to the
southwest and increase between the departing ridge and a 500-mb
trough predicted to spread eastward from the Rockies into the central
Plains. At the surface, a ridge currently encompassing much of the
central Appalachians is forecast to shift eastward into the mid-
Atlantic region today, with low-level southeasterly return flow
across our region gradually strengthening as this occurs. A notable
increase in cirroform cloud cover has already begun over the TN
valley, and severely limited the coverage of nocturnal fog. Although
the coverage of high-level clouds will likely remain broken-overcast
for majority of the day, warm temps this morning coupled with
strengthening southeasterly flow should allow for another day of
highs in the l/m 70s.

.SHORT TERM...(Tonight through Wednesday)
Issued at 435 AM CST Mon Feb 20 2017

Models are in general agreement that a vorticity maxima within the
base of the central Plains trough will begin to close off across
southeastern TX late tonight/early Tuesday morning. This feature is
then predicted to drift east-southeastward across southern LA and the
adjacent Gulf waters on Tuesday -- while becoming increasingly
detached from the northern portion of the trough ejecting rapidly
northeastward into Ontario/Quebec. In the lower-levels, southeasterly
flow will continue to slowly strengthen late tonight as a surface
trough/very weak cold front pushes slowly eastward across the
lower/mid MS valley. The combination of overcast high-level clouds
and elevated southeasterly winds this evening should allow for only a
very gradual drop in temperatures after sunset, and with forecast
soundings indicating a strong signal for the development of a lower
stratus deck by sunrise lows were increased a couple of degrees above
blended guidance.

Conditions will become more favorable for precipitation Tuesday
morning -- and especially during the afternoon -- as PWAT values rise
into the 1.3-1.5 inch range and flow aloft becomes increasingly
diffluent to the north/east of the Gulf coast closed low. With these
factors in mind, subtle low-level streamline confluence should be
sufficient to focus the development of at least a couple of bands of
rain that will track northeastward through the region on Tuesday.
Lingering dry air in the low-levels noted on point forecast soundings
may limit overall precipitation amounts, but high POPs for at least
some measurable rainfall are justified. Although temperatures on
Tuesday will be a few degrees cooler due to clouds and rainfall,
highs should still manage to reach the m/u 60s. Soundings depict
nearly moist adiabatic lapse rates with MLCAPE generally AOB 150
J/kg, and thus we will mention stratiform rainfall as the primary
precip type.

Based on latest NWP guidance, the closed mid-level cyclone will
continue to track southeastward into the north central Gulf on
Tuesday night and into the eastern Gulf on Wednesday. A continuation
of the regime responsible for Tuesday`s precipitation will likely
linger into the evening hours, before winding down from northwest-to-
southeast after Midnight. However, deep saturated profiles indicate
that a combination of drizzle/low clouds/fog will be possible for
most locations in the wake of the more widespread precipitation.
Light southerly winds will persist on Wednesday, with a few light
showers possible throughout the day. However, the potential does
exist for partial clearing of low stratus clouds during the
afternoon, and for this reason we have increased highs into the
u60s/l70s.

.LONG TERM...(Wednesday night through Sunday)
Issued at 435 AM CST Mon Feb 20 2017

Lingering light rain/light showers associated with the departing
cutoff upper low to our S/E will finally end from W to E across the
area Wednesday night into Thursday. At the same time, however, a weak
sfc boundary associated with a quick-moving sfc low over the Great
Lakes will enhance convergence along the Appalachians, which may
spark an additional shower here or there through Thursday morning. As
such, have retained POPs for mainly the Ern zones during this time,
spreading nearly area-wide on Thursday thanks to a warm front to our
N (associated with a developing sfc low to our W). As has been the
case in recent days, a stout cap remains in place for Thursday, so no
thunder is expected.

WAA will be the real story for Thursday and Friday ahead of the
developing system to our W, with temperatures soaring into the middle
70s Thursday to perhaps upper 70s by Friday thanks to W/SW flow
aloft. 850mb winds will be between 30 and 40kts with 925mb winds
~30kts, translating to a windy day across the area Friday. Sustained
sfc winds of 15-20mph are likely, with gusts possibly approaching
Wind Advisory criteria in the higher elevation locations (~30kts).
Given this, record high temperatures Friday may be in jeopardy of
being tied/broken. They are as follows: Huntsville: 77F (2000), and
Muscle Shoals: 78F (1930).

A disturbance aloft will induce sfc cyclogenesis by Thursday night
across the TX/OK Panhandles, ejecting NE as the upper low begins to
deepen through the day Friday. While much of the forcing/upper level
support remains displaced to the N, there is a window from 18Z Friday
through 00Z Saturday where a few stronger storms are possible,
assuming the stout cap in place can be eroded. Gusty winds would be
the primary threat along the cold front itself if that were the case.
That said, am not sold on the `cap being completely eroded` solution
just yet, so have prevailing showers with a slight chance of thunder
to account for this uncertainty.

The weekend ahead looks to be a nice one, albeit a little chilly.
After several days in the middle 70s, Saturday`s high will struggle
to reach the 60-degree mark, with morning lows starting out in the
middle/upper 40s. Sunday morning will be about 10 degrees cooler than
Saturday morning, with lows falling into the middle 30s. Afternoon
highs will reach the lower/middle 60s. Winds will be light/variable
on Saturday and will gradually become SErly as the high shifts E by
Sunday.

Another disturbance will come ashore in the PACNW/NORCAL regions,
racing Ewrd across the Srn Plains by early next week. Long-range
models differ on their strength/placement of upper level/sfc features
during this time frame quite a bit. Whereas the ECMWF shows more
potential for moderate/heavy rainfall for N AL/Srn Middle TN, the GFS
shows more of an organized round of showers and storms set to affect
the area Monday afternoon/evening. Will make minimal changes this
far out for now, awaiting better agreement before any more
significant changes are made.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 600 AM CST Mon Feb 20 2017

VFR conditions are expected at both HSV/MSL today and tonight,
although bkn-ovc cirrostratus can be expected in advance of an
approaching storm system. Clouds will begin to thicken/lower at the
end of the valid TAF period as moisture transport increases more
rapidly, with a bkn deck of stratocu arnd 3500 ft psbl by 21/09Z. Sfc
winds will veer to se shortly after sunrise this morning, and remain
from this direction at speeds of 5-10 kts thru end of forecast
period.

&&

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

$$

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


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