Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 101956

Area Forecast Discussion
National Weather Service Saint Louis MO
256 PM CDT Thu Aug 10 2017

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 252 PM CDT Thu Aug 10 2017

A cold front will push into northern Missouri early this evening
with scattered showers and thunderstorms will be possible along the
boundary over the western half of the region. The last several runs
of the HRRR generate scattered showers and storms but placement
varies run to run. The GFS/NAM/ECMWF are in consensus with moving
the cold front to the St Louis metro area or just to the south of
the city.  Winds will switch from the south southwest to the
northwest but will remain light. The frontal boundary will push into
southeast Missouri and southwestern Illinois late Friday morning. A
series of upper level impulses will ride along the front providing
enough ascent for scattered showers and thunderstorms through Friday
afternoon. Temperatures will be at or just below climatological


.LONG TERM...  (Friday Night through Next Thursday)
Issued at 252 PM CDT Thu Aug 10 2017

A surface cold front will extend from the Ohio Valley across extreme
southern IL through southeast MO early Friday evening. This front
will continue to push southward on Friday night as the eastern U.S.
upper trof sharpens up and surface high pressure builds into the
upper and middle MS Valley. There will be a threat of some residual
isolated to widely scattered showers and thunderstorms in the
vicinity of the frontal zone during the evening, but the main
elements of interest with be cooler and drier air overspreading the
region in the wake of the cold front. The large scale pattern
continues to show a broad but deamplifying mean trof from the
Rockies eastward into the eastern U.S. through the weekend, and this
will maintain surface high pressure as the controlling feature
Saturday through Saturday night.

Greater model variability and poor run-to-run model consistency
exists Sunday through next week leading to less confidence in the
forecast details. The ECMWF has backed off the previous "wet"
forecast for Sunday, but both the GFS and NAM show a progressive
short wave trof moving across the region within the broad upper
trof. Lift with this wave and low level WAA/MCON attendant with a
weak southwesterly LLJ on the backside of the retreating surface
high are forecast to support showers and thundestorms, primarily for
central and northeast MO.

Differences on Monday with the strength and speed of a digging
shortwave trof once again result in uncertainty in the forecast. The
GFS is slower/stronger with QPF nearly everywhere within the 12h
period, while the ECWMF is faster and weaker and predominately dry.
Predicability continues to diminish as we head into midweek and the
pattern flattens. We see progression of a low-amplitude upper level
ridge/trof system, the models diverge on details and timing of any
precipitation chances. Below average temps still look on target
through at least the first few days of next week.



.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1224 PM CDT Thu Aug 10 2017

An upper level disturbance currently in south central Missouri
will push east into western Illinois this afternoon. A cold front
will push into central Missouri later this afternoon. Scattered
showers and thunderstorms are possible late this afternoon over
the western 1/2 of the area. The cold front is expected to push
through late tonight into the early morning hours tomorrow. The
front will clear the region by early tomorrow afternoon. Winds
will shift from south southwest to northwest behind the frontal
boundary but should remain light.


A cold front will approach the area late tonight into early
tomorrow morning may provide a period of MVFR conditions with
lower ceilings. Winds will switch from the south southwest to the
northwest with the frontal passage but remain light . Otherwise,
VFR conditions are expected.





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