Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 192047

Area Forecast Discussion
National Weather Service Saint Louis MO
347 PM CDT FRI AUG 19 2016

.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 18z TAFs through 18z Saturday Afternoon)
Issued at 1236 PM CDT Fri Aug 19 2016

Isolated to widely scattered showers and thunderstorms look to
stay south of the terminals w/ exception of KCOU this
afternoon/early evening. Cold front will approach the region late
tonight/early on Saturday and will likely bring a round of
showers/storms along ahead of it. Behind the front...MVFR
ceilings appear possible along with a wind shift from the south to
the west is expected.


Believe most of diurnal shower/storm activity to stay south of the
terminal so took out previous mention of VCTS this afternoon. Cold
front will approach the terminal Saturday morning with a round of
showers/thunderstorms expected out ahead of it. After the frontal
passage...borderline MVFR ceilings are possible along with a wind
shift from the south to the west by Saturday afternoon.





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