Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 272000

Area Forecast Discussion
National Weather Service Huntsville AL
300 PM CDT Mon Mar 27 2017

.NEAR TERM...(Tonight)
Issued at 300 PM CDT Mon Mar 27 2017

An active convective situation continued to take shape across the
Tennessee Valley this afternoon, as strong/severe thunderstorms
begin moving across far NW Alabama. As such, Severe Thunderstorm
Watch #90 is now in effect for all of our Alabama counties until
9 PM CDT; and #91 for our Tennessee Counties until 10 PM.

Atmosphere wise, high temperatures so far have warmed into the
mid/upper 70s, with average dewpoint values ranging from the mid
50s to mid 60s (higher dewpoints generally to the west). This has
resulted in surface based CAPE values up into the 2000 J/kg range.
But lower level shear values were on the low side, mainly below 150
m/s except our far NW areas (per SPC meso-analysis). A warm frontal
boundary, thanks to a strong southerly flow has moved northward to
north of the Ohio Valley. A warm front denoting this change was in
place from low pressure along the central AR/MO border. A cold front
extended SW of this low across far NE Texas.

For tonight, various mesoscale models, as well as the NAM/ECMWF/
Canadian have convection rolling in a west east manner across the
region. As noted above, some of these storms could become strong to
severe --- with large hail and damaging winds the primary threats. A
tornado or two cannot be totally ruled out in the strongest storms.

With evening cooling after dusk, instability will remain but should
become more elevated. This will overall help temper tornado
potential, but large hail and strong to damaging wind gusts threats
remain. The storms per mesoscale models have the strongest convection
mostly to the south and east of the area and/or on a weakening trend
after midnight. Night low temperatures should fall to around 60.

.SHORT TERM...(Tuesday through Wednesday)
Issued at 300 PM CDT Mon Mar 27 2017

Isolated to scattered thunderstorms remain possible over areas south
and east of the NW Alabama Quad Cities Tuesday morning. These storms
will end from NW to SE as a cold front moves across the region during
the day. Another mild to warm spring day is forecast with highs
warming into the low/mid 70s. The above noted frontal boundary will
become stationary; oriented west to east south of the area later
Tuesday and Tuesday night. Slightly cooler and drier air filtering in
from the NW will produce lows into the lower/mid 50s. There could be
patchy fog formation late Tuesday night, mainly within and near wind
sheltered areas and locations that were affected by recent heavy

Wednesday, the boundary south of the area will return northward as a
warm front, and bring a slight chance of showers/thunderstorms in
the afternoon. Given limited amounts and timing of instability, did
not continue rain chances into Wednesday night. The frontal boundary
will move ever so further north during the night-time, setting up
another busy day convective wise on Thursday...

.LONG TERM...(Wednesday night through Sunday)
Issued at 300 PM CDT Mon Mar 27 2017

Models continue to diverge a bit concerning the eastward progression
of the storm system moving from Kansas/Oklahoma towards the
Tennessee Valley on Thursday. Based on the strong ridge it is moving
into over the Great Lakes region into the Mid-Atlantic region, think
the slower solution is more likely. Thus kept precipitation chances
isolated east of I-65 early on Thursday. West of I-65 enough forcing
and low-level moisture convergence should be in place in northwestern
Alabama for widespread shower and thunderstorm development.

Models diverge a bit concerning how far north the surface low and
thus how far north the warm front with this system sets up. This
would affect the amount of instability in place along and ahead of
the approaches storm system. Models are in fairly good agreement that
surface based instability around 500 J/kg is in place in the
morning. Along with 40 to 50 knots of Bulk Shear in place and some
low level turning of winds, severe storms producing damaging winds
are possible. These parameters continue and spread slowly east into
the late afternoon hours ahead of the system near and west of I- 65.
Surface based instability climbs to between 1000 and 1500 J/KG in
models (maybe a bit higher depending on how far north the warm front
makes it). Morning precipitation should keep the best instability in
place near and east of I-65. This would enhance damaging wind
potential and tornado potential as the afternoon would progress in
those areas. Luckily, low level helicity values forecast at this time
do suggest any tornadoes that develop would be fairly weak in
nature. As even stronger forcing moves into northern Alabama ahead of
a fairly strong cold front, strong to severe storms could last into
the early evening hours. However, by midnight the atmosphere should
be too worked over by precipitation to produce more than ordinary
showers and thunderstorms. Models diverge how quickly showers push
east of the area on Friday. Forecast models indicate a blend of this
timing and leans towards the later exiting time into Georgia late
Friday afternoon.

High pressure builds into the Tennessee Valley behind the front
Saturday night into Sunday, as another storm system develops
somewhere over southwestern Texas or Oklahoma. The surface high
should keep forcing and better moisture advection to our west in
Mississippi until Sunday evening. This should also keep highs and
lows cooler (highs in the upper 60s to lower 70s/lows in the upper
40s to lower 50s) Friday night through Sunday. By Sunday evening
into Monday, these parameters should become stronger over northern
Alabama and Southern Middle Tennessee. At this time, this system
does not look as unstable as previous systems. Strong storms look
possible, but surface based instability does not looks as good. Rain
chances really pick up Monday into Monday night as this systems
pushes east through the area.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1242 PM CDT Mon Mar 27 2017

Scattered to numerous showers/thunderstorms will affect the area
this afternoon and evening. Some of the storms could become strong to
severe in intensity, with large hail, damaging winds, and a tornado
or two possible (the strongest storms occur in areas west of I-65).
Started prevailing convection at KMSL at 27/1900Z and KHSV an hour
later, with erratic wind gusts and lowered CIG/VIS values into MVFR.
Even lower minimums are possible in the strongest showers/storms.
Shower activity should wind down as the activity progresses in a NW-
SE manner this evening. Winds should remain high enough to keep fog
formation minimized late tonight.





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