Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181158

658 AM 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
Green line.

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 12z TAFs through 12z Friday Morning)
Issued at 655 AM CDT Thu Sep 18 2014

While deep moisture is causing IFR cigs/vsbys to linger over mid MO,
slow but steady increase of easterly low level flow appears to be
advecting somewhat drier air into eastern sections of the FA, as
ceilings from east central MO into S IL have held steady or risen
a bit over the last several hours. This should mean ceilings
pushing above 3kft throughout the STL metro no later than 15z,
with this cloud deck finally scattering out in the 19-20z time
frame. Improving trend is also expected in mid MO during the day,
although it will likely be a bit slower since this area in deeper
into the low level moisture. Meanwhile at UIN, pockets of dense
fog formed during the predawn hours in northern sections of the
FA near UIN on the northern fringes of the ST/SC deck, but a
slight increase in higher level cloudiness appears to have prevented
the dense fog from becoming more widespread, and the remaining fog
should lift in the next hour or so with VFR conditions the
remainder of the period.

With the westward advection of the dry air today, expect the SC field
to be pushed west of the FA by tonight. Did add a bit of predawn
fog with MVFR vsbys to the low-lying TAF locations (SUS, CPS)

Specifics for KSTL: Ceilings should be climbing above 3kft no
later than 14z, with the SC deck finally scattering out by early
afternoon. VFR conditions should prevail tonight with a mostly
clear sky, and although some fog is possible in low lying areas
of the STL metro believe STL terminal area will not be impacted by



MO...DENSE FOG ADVISORY until 9 AM CDT this morning FOR Knox MO-Lewis
     MO-Marion MO-Monroe MO-Pike MO-Ralls MO-Shelby MO.

IL...DENSE FOG ADVISORY until 9 AM CDT this morning FOR Adams IL-
     Brown IL-Pike IL.



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