Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 200453

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1153 PM CDT FRI AUG 19 2016

Issued at 920 PM CDT Fri Aug 19 2016

Have updated the grids this evening to reflect the latest
observational and guidance trends. Loosely organized QLCS
continues to progress across eastern KS and western MO this
evening. This line has weakened considerably over the past 30-45
minutes. This trend should continue, as sites across central MO
have dipped into the middle 70s, suggesting a nocturnal near-
surface inversion setting in. Despite the weakening trend, looks
rather likely central MO will get in on some pecip later tonight,
so have upped pops to categorical in this region.

It is more of a question mark as if this line will make it to
eastern MO and particularly into the Saint Louis metro. There
will be a response in the low-level jet tonight as the synoptic-
scale shortwave ejects out of the Central Plains, but this will
likely focus mainly to the south and west of the LSX CWA.
Therefore, with the main synoptic forcing to the north and the
low-level jet focused to the south, will nudge pops back slightly
for eastern MO/far western IL, but still keep them in the 50 percent
range for now.



.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 324 PM CDT Fri Aug 19 2016

Isolated to widely scattered showers and thunderstorms primarily
over the southern and western sections of the forecast area are
expected to weaken and dissipate with loss of diurnal instability.
Attention will then turn to an approaching cold front and the
chances of showers and thunderstorms overnight tonight. Shortwave
currently near the NE/KS border has already helped initiate a
cluster of thunderstorms along the cold front. This shortwave is
expected to travel largely west to east overnight reaching
western MO just after midnight. Combination of upper-level forcing
for ascent ahead of this feature and low-level moisture
convergence along/ahead of cold front should result in a round of
showers/thunderstorms for most locations late tonight and into the
day on Saturday.

The front will continue to progress southeastward through the day
on Saturday. Could be a period between the morning and early
afternoon hours where coverage of storms lessens as stronger low-
level forcing wanes coincident with minimum instability.
However...expect an uptick in activity by mid afternoon for
portions of southeast Missouri and southwest Illinois as
instability increases ahead of the cold front. Cannot rule out a
couple of strong to severe thunderstorms with wind gusts a
concern...though fairly middling instability ahead of boundary
due to expected cloud cover and better shear to the north
suggests only an isolated threat of strong/severe thunderstorms.


.LONG TERM...  (Saturday Night through Next Friday)
Issued at 324 PM CDT Fri Aug 19 2016

SHRA/TSRA will continue during the early evening hours on
Saturday until the aforementioned cold front moves through the
area and post-frontal subsidence brings an end to the precip. The
cooler air mass will be noticeable right away: overnight
temperatures on Saturday night will be in the upper 50s to low
60s, which will be around 10 degrees cooler compared to the
previous night.

A surface high pressure center builds into the region behind the
front, bringing quiet wx conditions to the region for Sunday and
Monday. Highs on Sunday will be around 5-10 degrees cooler than
average, and Sunday night still looks quite cool. Although the
speed of the surface high is slightly faster on the 19/12z model
runs compared to 24hr ago, conditions on Sunday night/early Monday
morning still look favorable for radiational cooling over most of
the area given nearly clear skies and light winds. Widespread
overnight lows in the 50s still look reasonable. The exception may
be across parts of central MO if winds become southerly earlier
than fcst.

Return flow around the back side of the retreating surface high
will act to sharpen a baroclinic boundary over the plains ahead of
at least one approaching mid/upper trough. There is some model
disagreement regarding whether the southernmost trough is a
separate shortwave or whether it is an extension of the parent
shortwave moving near the US/Canada border. Regardless, the sfc
reflection(s) of the upper disturbance(s) eventually lift the sfc
bdry northeastward across the area on Wed as a warm front followed
by the passage of the trailing cold front on Thu/Fri. The presence
of low-level boundaries and the passage of at least one trough
supports a chance of rain for mid/late next week.



.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1147 PM CDT Fri Aug 19 2016

Complex forecast this TAF package as thunderstorms, low cigs, and
a wind shift are all in the cards. Have highest confidence in
thunderstorms at COU and UIN, but less confidence in the storms
holding together as they push east into the other terminals.
Therefore, will carry prevailing TSRA at these sites, and leave
only shower/VCTS mention in the others for now. Behind the
convection, a period of MVFR cigs appear likely mainly at COU and
UIN, along with a wind shift to the WNW and some gusts to near 20
knots at times by Saturday afternoon.


Main forecast challenge will be whether storms hold together into
the morning hours. Will leave just -SHRA mention in for now, but
will continue to monitor storm trends to the west over the next
several hours. Frontal passage looks to occur in the early
afternoon Saturday, swinging winds from SSW to WNW and bringing
some gusts to around 20 knots.





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