Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 232235

Area Forecast Discussion
National Weather Service Huntsville AL
535 PM CDT Fri Aug 23 2019

For 00Z TAFS.


.NEAR TERM...(Tonight)
Issued at 256 PM CDT Fri Aug 23 2019

As of this writing, scattered shower and thunderstorm activity was
mostly confined to extreme northwest AL and areas south of the
Tennessee River where an axis of convergence could be seen stretching
northward along the Tennessee River. Expect this activity to
continue through the early evening and early overnight hours.
Frequent lightning and wind gusts to 40 mph will be the primary
hazards with these storms in addition to brief heavy downpours. PW
values generally range from 1.7-1.9", so will have to monitor the
potential for flash flooding in any areas where training
thunderstorms persist.

With the loss of diurnal heating after sunset, expect coverage of
storms to generally decrease although some lingering shower activity
is possible overnight. Expect an increase in cloud cover overnight,
keeping lows in the low 70s and fog potential to a minimum. A weak
cold frontal boundary will continue to drop south from the Ohio
Valley, and this will ultimately serve as a source of convection
throughout the weekend.

.SHORT TERM...(Saturday through Sunday night)
Issued at 256 PM CDT Fri Aug 23 2019

An active pattern will continue through the weekend as a weak frontal
boundary stalls over southern Tennessee and northern Alabama on
Saturday. Expect shower and thunderstorm activity to be maximized
during the afternoon and evening hours, where diurnal heating
influences are maximized. Overall, lapse rates appear to be on the
weaker side, so organized severe weather is not expected with these
thunderstorms. With that being said, will not rule out the potential
for a strong storm or two, especially on Saturday where thermodynamic
profiles look to be a little more favorable. Frequent lightning will
remain a hazard with any thunderstorms that develop, and will be
something to monitor for those with outdoor plans. In addition, PW
values will be near or exceed 2", so will have to monitor the
potential for flash flooding in areas that receive consecutive days
of heavy rainfall or training thunderstorms especially in vicinity of
the frontal boundary. Have placed high chance/likely PoPs on Sunday
afternoon where an upper level disturbance is expected to provide
additional support for numerous thunderstorms.

An increase in persistent cloud cover will keep high temperatures
ranging from the middle to upper 80s Saturday and Sunday, and dew
point temperatures will remain in the upper 60s/lower 70s. Overnight
lows will likewise drop into the upper 60s/lower 70s.

.LONG TERM...(Monday through Thursday)
Issued at 256 PM CDT Fri Aug 23 2019

By Monday morning, there is good model consensus that a mid/upr
trough will be poised just to the west of the area. Moisture
advection and enhanced lift downstream from the trough across our
area will lead to the development of showers and thunderstorms.
Overall dynamic forcing and instability suggests numerous showers and
thunderstorms are possible and likely POPs were carried for the
afternoon during peak heating as the trough axis passes. Marginally
steep lapse rates and fcst MLCAPE values ~1500-2000 kJ/kg suggest
some strong wind gusts are possible, but the threat for severe
weather appears rather small at this time with limited shear present.

Shower and thunderstorm activity will likely diminish with the loss
of daytime heating and as the trough passes the area Monday night,
but a broad long wave trough will be taking shape in the upper
Midwest late Monday night into Tuesday. Frontogenesis will take shape
in the Great Plains, and the latest suite of 00Z guidance brings this
cold front into the TN Valley region by late Tuesday. The details
with respect to the front at this time still need to be resolved as
there are lingering discrepancies among timing. Similarly, run-to-
run consistency among models is still relatively weak and the latest
00Z guidance appears to have backed off a bit with the degree of dry
air advection. Nevertheless, it does appear that the cold front will
at least cross into the area on Tuesday with an associated sfc
convergence axis and wind shift. Enhanced lift along/ahead of the
front will lead to showers and thunderstorms once again on Tuesday
afternoon/evening (pending timing of course). The core of the drier,
continental air mass may not really begin to filter into the area
until later on Wednesday or especially by Thursday. Some of the
operational 00Z guidance (especially ECMWF) suggests 50s dew points
are possible, but due to the lack of consistency and some question
regarding the degree of dry air advection here, forecast dew points for
for now were left in the 60s. The broad long wave pattern continues
to favor ridging in the West CONUS, with troughing in the Great Lakes
to NE during the latter part of next week. Near zonal flow in the TN
Valley at the base of the long wave trough indicates the region may
be located within a region of increased baroclinicity and low-level
convergence. This would lead to an unsettled pattern, but the
location of the sfc front still remains to be resolved.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 535 PM CDT Fri Aug 23 2019

Scattered thunderstorms in northwest through north central AL will
remain just north and west of the terminals through 02Z. Barring
additional development, will just carry VCTS at both locations, then
remove after 02Z. Visibility may be reduced to 5SM late tonight,
although the risk of denser fog will depend on rainfall this evening.
Will carry VFR ceilings on Saturday, but leave thunderstorms out at
this time until confidence in timing and location at either terminal
becomes more certain.





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