Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171037
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
437 AM CST Tue Jan 17 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 352 AM CST Tue Jan 17 2017

The synoptic pattern early this morning features broad
southwesterly flow continuing over the Midwest, due to a pesky
longwave trough hanging back across the southwest CONUS into
northern Mexico. This regime will largely persist through the
period.

A surface low that plagued the region with rain and a few storms
yesterday has now pushed northeast and is currently centered over
the Chicago area. A trailing cold front has now pushed through the
entire LSX CWA and has brought slightly cooler temperatures this
morning, albeit still well above normal for this time of year with
readings in the 40s.

The surface low will continue to push off to the northeast today.
However, a weak PV anomaly noted in water vapor imagery will slide
northeast across northeast MO and western IL this morning into
early this afternoon. Guidance suggests plenty of low-level
moisture will remain in place today (as evidenced by the copious
cloud cover upstream), which coupled with the passing PV anomaly,
may be enough to induce some drizzle across northeast MO and
western IL. Have gone ahead and added patchy drizzle into the
forecast for this morning. Otherwise, expect another gloomy day.
With at least weak cold air advection continuing on the backside
of the departing surface low, have undercut MOS guidance a bit for
highs today, putting readings in the low 40s in the northwest CWA
and low 50s in the southeast.

For tonight, expect low-level clouds to continue to plague much
of the region. Guidance suggests that some drying occurring in the
mid-levels could make it all the way to the surface helping to
scour out the low clouds, but have doubts on this actually
occurring. If we are indeed able to see some clearing overnight,
then fog development would certainly be a possibility given light
winds and the limited daytime mixing expected today. Will leave
out of the forecast for now but will continue to monitor trends
through the day today.

Deitsch

.LONG TERM...  (Wednesday through Monday)
Issued at 352 AM CST Tue Jan 17 2017

Unseasonably warm temperatures will persist through the weekend,
and highs will be 10-20 degrees warmer than average at times.

The GFS and ECMWF are in disturbingly good agreement through 210
hrs. According to the models, the upper air pattern becomes more
active by the end of the week after a large but slow-moving low
pressure system lifts out of the southwestern CONUS. This feature
will bring a chance of rain to the area from Wednesday night
through Thursday night. Several mid/upper disturbances are
expected to reach the west coast and eventually develop into
mature low pressure systems as they move across the country. If
the latest model runs are correct, then one of these low pressure
systems will intensify as it moves from the southern plains into
the Ohio valley, bringing a chance of rain to the area from
Saturday night through Monday.

Kanofsky

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1103 PM CST Mon Jan 16 2017

Surface low just east of UIN with a trailing cold front
approaching the St Louis metro area. The low will continue moving
northeastward and drag a cold front through the St Louis metro
area overnight. Band of showers moving through UIN and the St
Louis metro area will shift east of this area overnight. Although
the cloud ceilings have been improving this evening, wrap-around
low level moisture and cloudiness behind the surface low will
advect back into the taf sites late tonight along with at least
patchy fog and possibly a little drizzle. Cloud ceilings should
drop back below 1000 feet late tonight. Surface winds will become
westerly and strengthen in the St louis metro area late tonight like
they have already in UIN and COU after fropa. The cloud ceilings
should gradually improve Tuesday afternoon and evening as a
surface ridge over the Plains builds into our area.

SPECIFICS FOR KSTL: Surface low just east of UIN with a trailing
cold front approaching STL. The low will continue moving
northeastward and drag a cold front through STL overnight. Band of
showers moving through UIN and the St Louis metro area will shift
east of STL overnight. Although the cloud ceilings have been
improving this evening, wrap-around low level moisture and
cloudiness behind the surface low will advect back into STL late
tonight along with at least patchy fog and possibly a little
drizzle. Cloud ceilings should drop back below 1000 feet late
tonight. Surface winds will become westerly and strengthen in STL
late tonight like they have already in UIN and COU after fropa.
The cloud ceilings should gradually improve Tuesday afternoon and
evening as a surface ridge over the Plains builds into our area.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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