Area Forecast Discussion
Issued by NWS St. Louis, MO

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203
FXUS63 KLSX 171733
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1133 AM CST Tue Jan 17 2017

.UPDATE...
Issued at 1041 AM CST Tue Jan 17 2017

Expect cloudy skies to remain over the area into tonight.
Extensive area of low level moisture remains across the region in
cyclonic curvature underneath an inversion. Temperatures will
remain steady the rest of the day or may fall a degree or two.
This is because of the clouds and the cold air advection behind
the front that moved through the region last night that has
already moved into the Ohio Valley.

Britt

&&

.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.

KD

.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 18z TAFs through 18z Wednesday Afternoon)
Issued at 1127 AM CST Tue Jan 17 2017

MVFR and some IFR ceilings will continue through this afternoon
and this evening before drier air causes them to break up and VFR
ceilings develop by later this evening or overnight. Do expect
some fog with MVFR and local IFR visibilities to develop between
08-16Z as the clouds clear and the winds turn light. This fog will
burn off leaving scattered MVFR cloud bases and light winds
tomorrow morning.

SPECIFICS FOR KSTL: Expect MVFR ceilings to continue through 06Z.
Then ceilings will rise to VFR and eventually scatter out as drier
air moves into the area. Some MVFR visibilities is expected to
develop early tomorrow morning under the clearing skies with light
winds. Dry and VFR conditions are expected on Wednesday.

Britt

&&

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

$$

WFO LSX



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