Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 191507 AAB

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
1007 AM CDT Sat Aug 19 2017

.NEAR TERM...(Rest of Today)
Issued at 1007 AM CDT Sat Aug 19 2017

A quiet morning across the southern Tennessee Valley with a high
pressure ridge at the surface and aloft building into the region. Am
watching a broad area of scattered to broken decks of AC clouds
currently pushing across southern Middle Tennessee into Northern
Alabama (mainly north of the River). These clouds should push into
East Tennessee and dissipate by midday/early afternoon. Stronger
subsidence from this building ridge should really diminish any
potential for diurnal convection this afternoon. The mostly clear
conditions (along with some high thickness values) will allow highs
to reach the lower 90s in most areas (a degree or so warmer than
yesterday). With the drier boundary layer conditions in place, RHs
values will bottom out at more comfortable 45-50 percent this
afternoon as well.

.SHORT TERM...(Tonight through Monday)
Issued at 203 AM CDT Sat Aug 19 2017

For tonight, conditions should remain favorable for patchy valley fog
once again. And lows should once again slip into the upper 60s in
southern TN and northeast AL, but stay around 70 in north central and
northwest AL. As the trough axis in the Great Lakes and OH valley
slides east, a more or less zonal pattern develops to our north,
while an upper ridge at 500 mb bridges across the TN valley in a west
to east fashion. Forecast soundings off the GFS (which looks more
reasonable) indicate a good subsidence inversion around 650-700 mb.
This leads me to keep a dry forecast going Sunday and even into
Monday as 500 mb heights increase to 595-596 dam and the inversion
holding. Thus, just a few cumulus clouds are expected at this time
for the solar eclipse period. High clouds associated with the
advancing TUTT low will remain to our southeast and with the zonal
flow well to our north and northwest. Forecast maxes on Monday should
still reach the lower 90s, but will indicate a slight decrease in
hourly temps during the eclipse.

.LONG TERM...(Tuesday through Friday) Issued at 203 AM CDT Sat Aug
19 2017

As a ridge begins to break down across the Southeast, a shortwave
trough of varying amplitude will eject off a parent Canadian upper
low and push southeast on Tuesday. As it moves southeast, the
surface response will be to push a continental polar airmass
southeast. This time of year, the polar airmass tends to be modified
as it is slower to move south. This is indeed the case with this
cold front which should arrive sometime on Wednesday. Prior to its
arrival expecting showers and storms to develop ahead of the cold
front on Tuesday, but these should be scattered in nature and will
need to overcome antecedent dry air. Early analysis of convective
parameters show potential for frequent lightning and microbursts as
the vertical profile becomes saturated on Tuesday night.

More widespread showers and storms are anticipated on Tuesday night
as the cold front that initially stalled/slowed just to the
northwest receives an extra push as a more amplified shortwave
trough moves southeast. Higher instability is present on Wednesday
and may need to watch for strong to severe storm potential if models
show frontal passage during peak heating. Did at least keep the
overall higher trend in POPs/WX for Wednesday with this current

Once the front finally pushes south of the area on Wednesday night
into Thursday, a cooler and drier airmass will impinge across the TN
Valley with daytime highs in the low to mid 80s on Thursday. This
will occur as a ridge amplifies over the Intermountain West and a
longwave trough prevails across the eastern half of the country.
Lows should be cooler as dewpoints drop into the low to mid 60s on
Thursday. Further drops in dewpoints are possible as the dry airmass
continues to advect southeast underneath amplified NW flow aloft. In
fact, there could be some areas over southern TN that experience
lows in the upper 50s on Thursday night. Little change in the
weather is expected on Friday before the upper flow pattern begins
to shift.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 600 AM CDT Sat Aug 19 2017

VFR flight weather conditions are expected during the next 24 hours
at KHSV and KMSL. Patchy early morning river valley fog is possible
elsewhere in north AL and southern middle TN.  Light and variable





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