Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171654

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1154 AM CDT Sun Sep 17 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 356 AM CDT Sun Sep 17 2017

SHRA/TSRA will slowly spread across the area today due to an
approaching cold front. As the last several forecast discussions
have noted, the lack of a strong surface high behind the front as
well as the largely parallel wind field aloft will make it difficult
for the front to make much southeastward progress today/tonight
before it lifts back northward as a warm front (see discussion
below). Today`s high temperatures will likely exhibit a strong
gradient, ranging from the mid/upper 70s across northern MO to the
mid/upper 80s in southeastern MO and southwestern IL. The cooler
temperatures reflect the influence of clouds and precipitation for
areas which are closest to the front, while the warmer temperatures
reflect the limited cloud cover and warmer air mass ahead of the
front. Overnight lows will be in the 60s areawide with some local
variations depending on cloud cover and the location of the front.


.LONG TERM...  (Monday through Saturday)
Issued at 356 AM CDT Sun Sep 17 2017

Main concern this week is the timing and chances of rain this week.

Upper flow will become more amplified through the week.  The global
models show shortwave troughs moving across Missouri and Illinois on
Monday into early Tuesday as a warm front moves north of the area
early on Monday morning.  With the ascent from the shortwave and
decent low level moisture convergence over the area, there should be
a chance of showers and thunderstorms, particularly along the front
Monday morning.

The chance of rain will lower late Tuesday into Wednesday as a
shortwave ridge moves across the area.  The chance for showers and
thunderstorms will increase again by Wednesday afternoon and
Wednesday night over central and northeast Missouri as well as west
central Illinois as a cold front moves into Missouri from the west.
This will occur as a shortwave trough moves across the Midwest ahead
of a deepening longwave trough over the western CONUS.  This front
will get pushed back north by early Thursday.  Rain chances from
Thursday into Saturday will be lower as a longwave ridge builds
over the area in response to the trough deepening over the west.

Temperatures this week will be well above normal given the front
staying north the region and upper ridge building over the area.
850mb temperatures will generally be between 15-18C which favors
temperatures between 85-90 degrees outside of showers.



.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1139 AM CDT Sun Sep 17 2017

Slow moving cold front in a predominately southwest flow aloft
will likely make little progress south. Wind shifts to the north
possible due to rain area and weak pre-frontal low pressure trof.
Based on current radar trends all of the short term models seem
too aggressive with the rain fall over the area, although
scattered showers and thunderstorms have rapidly developed across
the southern part of the area. It doesn`t look like this will
affect any terminals for this forecast. Some MVFR ceilings look
likely tonight and early Monday. Feel COU and UIN will have the
best shot. it looks like the CONSHORT and GLAMP models have the
best trends in this regard.

SPECIFICS FOR KSTL: Biggest question will be whether MVFR ceilings
make their way into STL. This may very well depend on how far
south the front travels. The two short term models that seem to be
handling the current weather the best keep MVFR out of STL, so I
will go with that for now. Otherwise VCSH seems to be the best way
to go for now with an eye on any new development this afternoon
that could have thunder. Next short wave moves across late tonight
and Monday so it looks like the next berst time for precipitation
will be Monday morning into early afternoon.





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