Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 192102

Area Forecast Discussion
National Weather Service Huntsville AL
302 PM CST Sun Feb 19 2017

.NEAR TERM...(Tonight)
Issued at 302 PM CST Sun Feb 19 2017

With high pressure drifting east into the upper Ohio Valley thru the
lower Appalachians tonight, conditions will be nearly ideal for fog
development again. Both NAM/GFS MOS and Bufkit fog tools all point to
this as well. Will incorporate patchy dense fog in the forecast
areawide. Lows will dip into the middle to upper 40s.

.SHORT TERM...(Monday through Tuesday)
Issued at 302 PM CST Sun Feb 19 2017

The upper ridge will shift east of our area by late Monday afternoon
with 12-14C 850 mb temperatures forecast. Mixing will not reach that
depth due to a rather distinct warm and dry layer aloft, likely an
elevated EML above 700 mb. Forecast highs in the middle 70s appear
on target at this point, although the NAM MOS is running considerably
warmer than the GFS (3-5 degrees). Tuesday will be a bit cooler, but
still mild, due to increasing cloud cover. The short range models
continue to cutoff an upper low over the western Gulf of Mexico while
northern stream flow brings a weaker positively tilted trough in
Tuesday night into Wednesday. This will keep QG forcing rather weak
over the TN valley, and with a strong high based capping inversion,
even elevated convection will be difficult to develop. The NAM
appears over done on QPF, possibly due to convective feedback. Will
go with the more likely solution of shallow isentropic
ascent/moistening with a gradual increase in shallow light shower
activity and/or drizzle and low cloud development, especially by
Tuesday Night.

.LONG TERM...(Tuesday night through Saturday)
Issued at 302 PM CST Sun Feb 19 2017

Wednesday through Saturday night...

Models continue to show a weak boundary along with some upper level
energy lingering over northern Alabama/Georgia on Wednesday. This
energy lingers behind the main surface front, that pushes into
southern AL/GA. Better forcing will be present the further south you
go. There might be enough moisture combining with enough lift for
some isolated showers to develop as a result. Cloud cover will keep
highs from rebounding as much on Wednesday. However, 925 Boundary
layer winds will remain primarily from the west to southwest,
despite the frontal passage. This will not allow much colder air to
push into the area (some drier air, but not colder). Models prog 925
mb temperatures to rise into the 15 to 17 degree range across much
of the Tennessee Valley and over northern AL on Wednesday as well. A
low level lapse rate should allow mixing of this warmer air to the
surface. Thus highs in the lower to mid 70s look reasonable on
Wednesday, in spite of the cloud cover. Despite some low level
instability forecast by models on Wednesday, a strong CAP is also
forecast to be in place aloft around 700 mb. With forcing not very
strong over most of northern Alabama, this CAPE should keep any
thunderstorm activity from being realized. Therefore, leaving out
thunderstorm potential on Wednesday.

By Wednesday night, most models guidance moves the weak surface
boundary in place over southern AL/GA northeast, as the main surface
high just off the Atlantic coast quickly moves further east. Very
warm temperatures aloft around 700 mb continue. This should keep any
precipitation from developing, despite very weak forcing associated
with this surface boundary as it moves north across the region.
Cloud cover and stronger warm air advection should keep lows from
dropping below the mid to upper 50s in most locations (the exception
being NE AL, where 51-56 degrees lows are possible).

By Thursday into Thursday night, several models show weak
convergence extending southwest along the Appalachians into
northeastern Alabama. This convergence is associated with a weak
trough axis extending southwest from the warm front developing north
of the area. This warm front extends west to a developing area of
low pressure over western Texas or Kansas. Again the CAP aloft will
be in place and strengthen more on Thursday. Both ECMWF and GFS30
show this. Therefore, only included a 20 pop for possible convective
development on Thursday/Thursday night for much of northern Alabama
and Southern Middle Tennessee. At this point, despite some low level
instability being in place, the elevated CAP should only allow
showers to develop, not thunderstorms. Very strong warm air
advection is forecast by models on Thursday. A few models even warm
925 mb temperatures to 18 degrees. If this happens lower 80s will be
very possible for highs. At this point, since convergence of models
are closer to between 15 and 17 degrees for 925 mb temperatures,
raising highs into the mid to upper 70s.

As the 993 surface low tries to move eastward in earnest on Friday,
the strong surface high off the Atlantic does not move much.
However, the surface high does steer the surface low to the
northeast rather than due east. This should keep the best forcing
associated with this low and its associated surface front north of
the Tennessee Valley. However, it will increase winds energy ahead
of the front over northern Alabama and southern middle Tennessee.
During the day on Friday, stronger 30 to 40 knot 850 mb winds are
forecast to develop along with 30 knot 925 mb winds. This will
produce breezy conditions over the area on Friday. Sustained winds
10-15 with gusts to around 25 mph look reasonable. They may be a bit
higher than this. Low level convergence will increase, but not much
forcing really expected otherwise. Thus will an even stronger CAP in
place around 700 mb, not sure we will see convection, but kept a 30
percent chance of rain/storms in the forecast lingering through
Friday night. Enough instability will be available for some strong
storm development if the convection can overcome the CAP, but this
is very uncertain.

Friday night models push the front and stronger forcing aloft into
the Ohio Valley. However, again most of the forcing with the front
(even with the stronger ECMWF solution) is well north of the area in
Kentucky. Given strong CAP and expected location of forcing, keeping
pop on the low side (30-40 percent). Models are producing some QPF,
although light. Could see no precipitation, if the CAP wins out.
Highs in the 70 to 75 degree range are possible, maybe even higher
despite abundant cloud cover expected ahead of the front.

Much cooler (compared to earlier temperatures) will drop in behind
the front Saturday. It will feel chilly with highs in the 55-60
degree range, after highs between 70 and 80 degrees.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1106 AM CST Sun Feb 19 2017

VFR flight weather conditions will prevail this afternoon into the
evening hours as high pressure moves east across the region. Light
flow and a clear sky tonight will set the stage for another fog event
late tonight. Have dropped visibility to LIFR 1/2sm at KMSL and
3/4sm at KHSV in tempo groups from 09-13Z. Rapid improvement to VFR
is then expected by ~15Z.





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