Area Forecast Discussion
Issued by NWS St. Louis, MO

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685
FXUS63 KLSX 171641
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
1041 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 12z TAFs through 12z Wednesday Morning)
Issued at 512 AM CST Tue Jan 17 2017

Low cigs will be the main focus of this TAF period. Surface cold
front has pushed well east of sites this morning. In its wake,
MVFR and IFR cigs have pushed back into the region. Worst
conditions are currently at COU/UIN where low-end IFR cigs
prevail. Expect these IFR cigs to continue to push east through
the morning hours, but there is some uncertainty as to how far
east they are able to get before they lift to low-end MVFR. For
now, will keep IFR cigs mentioned at the St. Louis metro sites,
but there is a chance some of these sites could remain MVFR
through the day. All sites should improve to at least MVFR by
tonight as drier air works in. If areas are able to clear out,
then fog could become an issue tonight especially at UIN/COU, but
will leave out for now given the uncertainty.

SPECIFICS FOR KSTL:
High confidence in at least fuel-alternate MVFR cigs prevailing
throughout the day today. Only moderate confidence in IFR cigs
getting into the terminal late this morning into early this
afternoon, as these lower cigs may stay just off to the northwest.
Otherwise, expect improving conditions overnight but if skies are
able to clear out for a period of time, then fog and/or additional
low stratus may become an issue by Wednesday morning.

KD

&&

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

$$

WFO LSX



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