Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 270824

Area Forecast Discussion
National Weather Service Saint Louis MO
324 AM CDT TUE SEP 27 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 315 AM CDT Tue Sep 27 2016

Still expect a sunny day as the area will continue to lie under
northwesterly flow aloft with little in the way of moisture through
a deep layer.  Went with highs closer to the warmer NAM MOS
temperatures which supports the RAP forecast soundings that show
mixing up into the 850-800 layer by this afternoon.  Both the GFS
and NAM are showing a weak upper trough moving through Missouri and
Illinois from the northwest which will bring an attendant surface
cold front into the CWA between 06-12Z. This trough will produce
some weak mid-level ascent, but have no moisture to work with, so
only expect partly cloudy skies with its passage and no rain. Lows
tonight should be close to the MOS lows which match up nicely.


.LONG TERM...  (Wednesday through Monday)
Issued at 315 AM CDT Tue Sep 27 2016

Wednesday morning will see the strong cutoff low moving from near
Chicago southeast across Indiana possibly into western Ohio.  The
cold front associated with the surface low should be through our
CWFA by 18Z Wednesday.  Models are showing a tremendous amount of
mid-upper level vorticity dropping south over the area on the
western side of the low.  Forecast soundings are showing steep low
level lapserates and RH plan views are showing RH values in excess
of 85% between 900-850mb during the late morning and afternoon
behind the front.  This suggests there will probably be a decent
strato-cu field behind the front.  While MOS PoPs are very low, and
there is no model QPF, I wouldn`t be surprised to see some light
rain showers or sprinkles develop as well...primarily along and east
of the Mississippi.  With Strong cold air advection and strato-cu,
ensemble guidance temperatures are probably too am leaning
heavily toward the cool side of MOS.  With the cool start to the
evening and likely clearing sky Wednesday night, lows look likely to
dip into the upper 40s and low 50s which will be the coolest
readings since May for most locations.

Remainder of the week should remain seasonably cool under the
influence of a sprawling Canadian high pressure system. The mid
and upper level cutoff is still forecast to meander over the Ohio
Valley and mid-Appalachians. This will keep the Mississippi Valley
in northwest flow aloft, and block up the pattern so not much will
change probably through at least Sunday. Temperatures should
moderate a bit from Wednesday and Thursday`s coolest readings, but
guidance temps stay a bit below normal through the period and this
looks reasonable given the pattern. Also...would not be at all
surprised to see a few showers or sprinkles east of the
Mississippi each afternoon with the cutoff lurking over the Ohio
Valley. Medium range models have the cutoff finally ejecting
northeast Sunday night and Monday, but am not sure there will be
much change in sensible weather other than a more east-southeast
direction to the surface flow as another trof begins developing
over the Great Plains.



.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1137 PM CDT Mon Sep 26 2016

Still looking at the potential for some river valley fog at KSUS
overnight however confidence is marginal. Otherwise VFR flight
conditions will prevail through the forecast period.


VFR flight conditions will prevail through the forecast period.



Issued at 317 PM CDT Mon Sep 26 2016

Winds between 12 and 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 this afternoon. Higher relative humidities
and lower wind speeds further south should limit the fire danger
across the rest of the area.





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