Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171730
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1230 PM CDT Wed Sep 17 2014

.UPDATE:
Issued at 1230 PM CDT Wed Sep 17 2014

Main batch of rain from MCS should remain to our southwest the
remainder of the day with scattered showers being generated
further north thru the MS river area, in part from influence from
the MCS, slow to dissipate and wind down. In fact, it will
probably take much of the afternoon, and it is not a given that
something may still linger into the evening although looking
pretty meager at the present time.

Clouds will be very slow to clear out today, and probably only
most effectively in the far eastern counties of our CWA (southwest
IL) late this afternoon. Look for this clearing trend to continue
from the east as high pressure`s easterly flow keeps chipping
away, and once the rain clears out, should also be more
effective in booting the clouds out too. Only in central and
southeast MO will it be very tough, probably too tough, to fully
clear out tonight.

Temps will end up being cooler than previously thought, due to
clouds, and have most max temps remaining in the 60s today.

TES

&&

.SHORT TERM:  (Through Late This Afternoon)
Issued at 347 AM CDT Wed Sep 17 2014

No changes to the Dense Fog advisory issued earlier for our south
central IL counties.  Some patchy dense fog is occurring further to
the west into east central MO  but this appears to be primarilyl
confined to river valleys, and high clouds should be moving in
shortly to limit the dense fog threat along the Mississippi River.

Otherwise, concern for today is shower and thunderstorm threat, and
increasing WAA and moisture advection finally initiated convection
between 07-08z over east central KS. Low level theta-e progs suggest
that this area will continue to grow in coverage (and intensity)
over the next few hours, with the activity making an e-se surge
during the morning generally along and especially south of the
Interstate 70 corridor. While forcing and moisture advection with
the low level jet would suggest most of the precipitation threat
would be this morning, there is some indication in both the hi-res
and synoptic models that additional development will occur this
afternoon as the increasingly unstable air over the Plains interacts
with the rain-cooled dome from this mornings convection.  Because of
this will continue PoPs this afternoon, especially over southern
sections of the CWA.

Still working on temps, but believe much of the area may struggle to
warm up due to clouds and the potential of rain-cooled air,
especially if the convection can linger for any length of time this
afternoon.  Welcome to the cool season...

Truett

.LONG TERM:  (Tonight through Tuesday)
Issued at 347 AM CDT Wed Sep 17 2014

There will be a continued potential for convection mainly across
southeast MO tonight in an area of weak low-mid level warm air
advection north of a weak warm front, and where ample low level
moisture and instability exists.  The threat for any convection
should end by Thursday afternoon as a strong surface ridge builds
southward through the Great Lakes region and into MO and IL with
slightly drier and more stable air filtering southwestward through
our forecast area.  Warmer temperatures can be expected on Friday as
an upper level ridge shifts eastward into MO with southerly surface
winds on the western periphery of the surface ridge extending
southwest into southeastern MO.  Shortwaves will break down and
flatten the upper level ridge on Saturday, and this coupled with a
deepening upper level trough across the northern Plains and Great
Lakes region will allow a cold front to drop southeastward through
our forecast area late Saturday night and Sunday.  The ECMWF model
is now slower with the progression of this front compared to the GFS
model.  The chance for convection will return beginning Saturday
afternoon as the shortwave approaches, then continue Saturday night
and Sunday as the cold front moves through our area.  Most of the
precipitation should end by Sunday night, although the ECMWF model
does still have some precipitation continuing until Monday across
our area due to the slower progression of the surface cold front and
850 mb front and as shortwaves drop southeastward through our area
on the backside of the Great Lakes upper level trough.  Much cooler
temperatures can be expected beginning Sunday night and continuing
through Tuesday night as a fairly strong surface ridge moves slowly
eastward through the region with upper level troffing centered over
the Great Lakes region leading to upper level northwesterly flow
over our area.  The models drop the 850 mb temperatures to 4-8
degrees C across the forecast area by 12z Tuesday.

GKS

&&

.AVIATION:  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1230 PM CDT Wed Sep 17 2014

Region of sub-VFR ceilings, whose eastern edge is near the MS
river, will begin to erode this afternoon and shrink back to the
west as easterly flow combined with dissipating SHRA kick in more.
This will result in some intermittent MVFR ceilings across STL
metro sites with much more persistent sub-VFR conditions at COU,
unfortunately. May eventually see a break in COU very late this
afternoon and into parts of this evening, but looking like IFR
conditions are likely once again later tonight there. Depending on
how well we clear out further east into STL metro and UIN, may see
some fog development in an otherwise favorable setup with light
east winds and damp ground. Threw out initial numbers on VSBYs
with this being understood conditionally--if clearing is more
substantial in those areas, will need to go more widespread IFR
VSBYs. A return to VFR is anticipated across all sites by midday
Thursday, except perhaps COU.

Specifics for KSTL: Intermittent MVFR ceilings likely for the next
2-3 hours before easterly flow and time of day should push the
eastern edge backward. Period of VFR then expected for late this
afternoon and early this evening before some fog is expected to
develop. How well we clear out will determine how bad the fog gets
as this is an otherwise good setup for it given light easterly
flow, lack of previous day mixing, and recent rainfall. A return
to VFR then expected late Thursday morning.

TES

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX




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