Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 262255

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
555 PM CDT MON SEP 26 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 315 PM CDT Mon Sep 26 2016

Clear, dry and much cooler conditions for tonight as a strong
surface ridge continues to build into our area from the Plains with
northwesterly upper level flow due to the deep upper level low just
north of the Great Lakes region.  The 850 mb temperatures late
tonight will range from around 8 to 11 degrees C across our forecast
area.  There will be good radiational cooling due to a crystal clear
sky, surface dew points only in the lower to middle 40s, and
diminishing surface winds this evening.  Although it will feel much
cooler than the past several nights, lows tonight will only be
around 5 degrees below normal for late September.  There will be
some steam fog in the river valleys late tonight and early Tuesday
morning due to the cool air over the warm river water, but this
should be mainly confined to these areas due to the dry
surface/boundary layer conditions.


.LONG TERM...  (Tuesday through Next Monday)
Issued at 315 PM CDT Mon Sep 26 2016

The synoptic pattern at the start of the long term period will
feature a strong upper-level low over the northern Great Lakes, with
ridging across the western CONUS.  This low will meander to the
southeast through the period, becoming cut off from the upper-level
flow over the Ohio Valley by this weekend.  This regime will keep
seasonably cool and dry conditions in the forecast through the

Tuesday will be fairly similar to today, although it will likely be
a couple of degrees warmer ahead of a cold front slated to move
through on Wednesday.  Mixing to around 850mb will once again yield
fairly breezy conditions across northern MO/western IL given around
25 knots of flow in this layer.  Look for sustained winds of 15-20
mph with a few gusts around 25 mph.

A rather strong PV anomaly will dig southeast around the upper-level
low on Wednesday.  This will help drive a cold front through the
region Wednesday morning.  The strong mid-level ascent associated
with the impulse will likely bring some enhanced cloudiness to the
region Wednesday afternoon, but forecast soundings are just too dry
to squeeze out any measurable precipitation. Behind the front and
with at least some cloudiness (especially in IL), Wednesday`s
temperatures will be the coolest of the period (and of this young
fall season), with highs only expected in the upper 60s/low 70s.

Canadian high pressure will then remain in control for the remainder
of the week into the upcoming weekend.  This will keep dry
conditions in the forecast but temperatures will slowly climb
through the 70s each day Thursday through Monday of next week.



.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 550 PM CDT Mon Sep 26 2016

The only aviation concern is the potential for some fog at KSUS.
Conditions overnight favor river fog and given a light westerly
wind component, fog along the Missouri River could advect into
KSUS. Confidence is not high and therefore just kept with a TEMPO
group for MVFR conditions. Otherwise VFR flight conditions will
prevail at the terminals through the forecast period.


VFR flight conditions will prevail through the forecast period.



Issued at 317 PM CDT Mon Sep 26 2016

Winds around 15 mph and minimum relative humidity between 25 and
30 percent will combine with dry fuels to produce an elevated
fire danger across parts of northeast Missouri and west central
Illinois again on Tuesday afternoon. Increasing relative
humidities and lower wind speeds should mitigate the fire danger
across the rest of the area.





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