Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 181720
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1220 PM CDT Thu Sep 18 2014
.SHORT TERM: (Through Late This Afternoon)
Issued at 347 AM CDT Thu Sep 18 2014
Passage of weak shortwave overnight is going to give the surface
ridge extending from the upper Great Lakes into the mid-Mississipi
Valley a bit of a southwest nudge today. The resultant
intensification of the surface high will mean an increasing easterly
low level flow in the lowest 5kft of the AMS, which should advect
drier air from s IL into our area. Impacts of this dry air can be
seen in ceiling trends indicate that lower stratus is having a
difficult time forming over s IL.
Low cloud IR imagery early this morning is also showing the impacts
of this drier air, as northern edge of cloud deck is remaining
quasi-stationary from just south of PPQ to near Effingham/1H2.
However, ground fog is forming in the clear skies on the northern
fringes of this cloud deck, and after checking with several county
sheriffs in ne MO/w central IL, will be issuing a dense fog advisory
til 9am this morning for areas along and north of a Paris-Bowling
With the increasing easterly low level flow, believe there should be
a gradual decrease in clouds from west to east during the day.
Given low cloud tendencies over the CWA the last few weeks I hate to
be too optimistic, but aforementioned trends as well as good
agreement in the models regarding the low level moisture profile
seem to indicate this clearing trend. However, the emphasis will be
on "gradual", as we will also be contending with some high level
cloudiness spreading east from the convection over eastern KS/sw MO,
at least during the morning.
As far as showers and thunderstorm threat is concerned...axis of
convection is gradually sagging south as redevelopment continues on
the west/south side of the rain-cooled boundary. With this boundary
remaining active, the shower and thunderstorm threat should remain
southwest of our CWA.
.LONG TERM: (Tonight through Wednesday)
Issued at 347 AM CDT Thu Sep 18 2014
A ridge of high pressure extending from the Great Lakes region
southwest into eastern MO should keep any precipitation west of our
forecast area tonight. Warmer temperatures can be expected on
Friday as an upper level ridge shifts eastward into MO, and surface
winds become southerly on the western periphery of the surface ridge
extending southwest into southeastern MO. The chance of convection
will return Saturday afternoon and night as the upper level ridge
flattens as an upper level trough deepens over the northern Plains
and Great Lakes region. As the trough deepens it will send a cold
front southeastward through our forecast area Saturday night and
Sunday morning. At least scattered showers/storms are expected
along this front. It appears that most of the precipitation will
shift south of our forecast area by Sunday afternoon as a strong
surface ridge builds into the region behind the cold front with
cooler and drier air filtering southeastward into our area. Both
the GFS and ECMWF models drop the 8 degree C 850 mb isotherm
southward to STL by 12z Monday. A dry and cool period of weather is
expected for the remainder of the extended forecast with
northwesterly upper level flow and as the surface ridge moves slowly
eastward through the region.
.AVIATION: (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1207 PM CDT Thu Sep 18 2014
Stratus breaking up as drier air advects into the region from the
east and is aided by September sunshine. Will see this trend
continue into the afternoon with most locations becoming clear by
evening. Some concern for valley fog tonight and have kept some
MVFR visibility at a few of the favored terminals. Otherwise,
light east/southeast wind to continue.
Specifics for KSTL:
Stratus breaking up and should see sky clear by this evening.
Light east wind will become southeast on Friday.