Area Forecast Discussion
Issued by NWS Huntsville, AL
FXUS64 KHUN 200459
Area Forecast Discussion
National Weather Service Huntsville AL
1059 PM CST Sun Feb 19 2017
For 06Z TAFS.
Issued at 905 PM CST Sun Feb 19 2017
The warm temps and good mixing today really helped to dry out the air
in the lower levels. This is most notable when comparing the 00Z BMX
sounding to yesterdays 00Z sounding. (Picked BMX sounding as it`s
slightly more representative of what we can expect tonight).
Forecast soundings show surface dewpoints and temps dropping tonight
with the dewpoints decreasing a bit faster than the temp. If this
does happen and dewpoint depressions end up a bit higher tonight,
any potential for fog formation will be quite limited. Both the SREF
and NARRE probs for surface visibility are near zero, meaning very
limited fog potential for the TN Valley. The reasons for this make
sense as the surface high has shifted eastward and winds just above
the surface have turned to the east as represented in latest VAD wind
profile from HTX. This easterly component in winds should help to
re-enforce the dry air just above the surface and could restrict fog
to low lying areas and near bodies of water.
Now with all of that said, find it extremely difficult not to expect
some fog tonight. Virtually clear skies and light winds are a perfect
setup for radiational cooling and many areas in TN and NE AL within a
few degrees of the lows. Will be watching surface obs and satellite
trends closely tonight to see if indeed fog is developing.
Made some adjustments to the temps and dewpoints for tonight. Also,
limited the patchy dense fog wording to valleys and bodies of water.
Other locations will likely see fog but it is not expected to be
dense in nature. Again will watch things closely tonight.
.SHORT TERM...(Monday through Tuesday night)
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
.LONG TERM...(Wednesday through Saturday night)
Issued at 302 PM CST Sun Feb 19 2017
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 06Z TAFS through 06Z Monday night)
Issued at 1059 PM CST Sun Feb 19 2017
Latest guidance has backed off on fog potential for tonight and have
decided to raise the visibility forecasts to MVFR instead of LIFR
conditions. A very brief period between 11-12Z of IFR Vsbys is
possible but confidence is too low to keep in the forecast.
Any fog that is in place Monday morning will quickly dissipate
leaving VFR conditions for the remainder of the TAF period. Winds
will favor a SE direction with speeds of 5-10kts.
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