Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 031741

Area Forecast Discussion
National Weather Service Huntsville AL
1141 AM CST Sat Dec 3 2016

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 1016 AM CST Sat Dec 3 2016

Not much of a change to overall forecast thinking. Broad surface high
pressure sprawled from the mid Mississippi Valley to the central
Appalachians was building eastward, as a mass of moisture which
originated from the NE Pacific subtropics moves towards the region.
This moisture moving under an upper low situated east of the Gulf of
California was spreading rain towards the Tennessee Valley.

Per surface observations and spotter reports, the eastern edge of
the rain (at least reaching the ground) had reached parts of NW
Alabama, being reported in Haleyville. The new 12Z NAM sounding
indicated a dry layer below the cloud base level and cold enough
temperatures could produce sufficient cooling, with maybe an ice
pellet or two, mainly over NW Alabama this morning as the rain
commences. Otherwise an all rain event is expected from SW to NE as
the precipitation spreads across the region.

Made minor adjustments to high temperatures, mainly down a degree or
two in most spots given denser clouds and the fact that rain is on
the way. That being said, think rain will be more likely further to
the west later today, with odds of rain increasing even for our NE
areas later this afternoon.

.SHORT TERM...(Tonight through Monday)
Issued at 247 AM CST Sat Dec 3 2016

A prolonged light to moderate rainfall event will be underway this
evening as isentropic lift and strong moisture transport persist
through tonight into Sunday. Beneficial rain may fall for over 24
hours straight from late today through Sunday afternoon. The synoptic
pattern favors a heavy rainfall event with upper ridge position
across the southeast with upstream moist southwesterly flow ahead of
closed upper low in southwest TX. Ample moisture advection will occur
atop the cool low level airmass, with PWs climbing over an inch by
this evening, and inflow PWs of 1.5 inches+ Sunday afternoon into
Sunday night. A very strong 850 mb warm front is evident across
southern AR into northern MS into central AL by 09-12Z Sunday
morning. Showalter indices will drop to zero or just below south of
this front as it lifts northeast. After this front passes, it is
followed by 55-60kt SW flow thru the day. I expect an elevated MCS
with heavy rain and lightning to develop to our west and southwest
along this front, which will shift slowly east and northeast through
the day on Sunday. There is a surface reflection of this in the
models in the form of an inverted surface trough and weak bubble
high. Rain should become more stratiform in nature during the late
afternoon into early evening hours Sunday before ending.

How long the rain will cease is still uncertain before pressures
fall again and southerly flow redevelops. At this point, it appears
that scattered to numerous showers should redevelop by midday Monday
in our southern counties, then spread northeast through the
afternoon. This begins round two of a rather prolonged period of
showers and thunderstorms as the upper low opens up and ejects
northeast through the MS and TN valleys Monday Night into early
Tuesday. At this time, the warm sector may remain bottled up to our
south, with an occluded front moving through during the early morning
hours of Tuesday. This should keep the risk of severe weather to our
south. Projected rain totals for both events will be on the order of
1.75-3 inches based on WPC forecasts, which appear reasonable. Of
course, convective elements may yield heavier totals in spots.
Although I wouldn`t call our soils saturated, we could see some areal
flooding develop in a few locations due to prolonged rainfall,
especially during the second event.

.LONG TERM...(Tuesday through Friday)
Issued at 247 AM CST Sat Dec 3 2016

As mentioned above, the shortwave trough will begin moving within
the subtropical jet across the southern Plains on Monday into the MS
River Valley by Monday night into Tuesday. Forecast soundings and
plan view products show low elevated instability on Monday night into
early Tuesday morning. In addition, the shortwave trough looks to
develop a negative tilt resulting in a deepening low pressure system
as it crosses just to the northwest leading to stronger shear. As a
result, thunderstorm development is likely with gusty winds up to 50
MPH may accompany this thunderstorm activity. Convective showers
should end during the late morning hours on Tuesday with broken to
overcast cloud cover lingering through Tuesday night/Wednesday.

Daytime high temperatures should rise into the low 50s on Wednesday.
Then, a longwave trough moving across the Pacific Northwest on
Tuesday will move east on Wednesday over the Southern Plains. A
few intriguing characteristics with this trough are that the
height anomalies with this trough are between -260-300m and -2 std
deviations off the normal. This longwave trough will push a
significantly cold Arctic airmass across much of the country.
Before its arrival, southwest winds will increase to 10-15 MPH along
with saturation of the low and mid-levels on Wednesday. Rainfall
should begin on Wednesday night into Thursday morning, and could
exacerbate any flooding that could occur earlier on Monday into

As the vertical profile cools aloft just prior to the Arctic front
passage, there could be snow mixing with the rain on Wednesday night
into Thursday, but little to no accumulation is expected due to the
quick development of subsidence on Thursday. With the arrival of the
cold front and strong cold air advection, temperatures are forecast
to drop significantly from daytime highs in the mid 50s on Tuesday to
daytime highs in the low 50s on Wednesday and low 40s on Thursday.
This is definitely below 30-year climatological normals in the mid
50s. Note: The record low maximums look to be out of reach for both
December 8th (Thursday, KMSL: 34, KHSV: 33) and December 9th (Friday,
KMSL: 30, KHSV: 30).

Wind chills will also be a concern (possibly in the single digits) on
Thursday night due to the pressure gradient associated with the 1045
mb surface high over the Plains the prevailing surface northwest
winds and very cold overnight lows in the teens/20s. Otherwise, the
daytime high on Friday are forecast to be in the upper 30s with
overnight lows in the low 20s again on Friday night. Look for the
possibility of these post Arctic front temperatures to be adjusted in
either direction over the coming days due to the strength and
possible change in trajectory of the trough.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1141 AM CST Sat Dec 3 2016

VFR weather should continue across northern Alabama and adjacent
southern middle Tennessee into the late afternoon, before CIG/VIS
values lower as rain moves across the region. Stayed with similar
timing regarding the rain, with CIG/VIS values falling into the MVFR
range this evening. Moderate to locally heavy rain will result in
IFR, and at times lower CIG/VIS reductions late tonight and Sunday
morning. Easterly winds around 5kt this afternoon should gradually
veer to an ESE component during Sun morning.





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