Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171208
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
708 AM CDT Wed Sep 17 2014

.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 12z TAFs through 12z Thursday Morning)
Issued at 648 AM CDT Wed Sep 17 2014

Main area of convection that fired in zone of isentropic ascent
and moisture advection over western Missouri should continue to
work ESE with time. While this movement would take the main
centroid of convection south of our TAF sites, broad low level
jet is causing some development on the northern fringes of the
complex that should clip the COU area over the next few hours, and
have gone with VCSH for now. Also believe most of this activity
should remain just south of STL area TAFs, however it could be a
close call if northward development continues past mid morning.
Will be monitoring radar trends during the morning for possible
adjustments at COU and in the STL area. Meanwhile, development of
southerly flow has advected low level moisture back across the
area, with MVFR cigs now into eastern MO and IFR cigs over mid MO.
With southerly flow increasing see no reason that these MVFR cigs
will not reach UIN and STL area TAFS over the next few hours,
however do expect this MVFR cloud deck to break up late this
morning and into the afternoon. Area of IFR over mid MO may be
longer lasting due to persistent precipitation over the Ozarks
through at least late morning.

Specifics for KSTL: Expecting leading edge of MVFR cigs around
2kft that now extends from east of MJY to UVV to push into the
area by around 15z, with this lower cloud deck then scattering out
by early afternoon with additional cloudiness aoa 5kft persisting
into the evening. As mentioned in the main aviation AFD, believe
most of the convection will be south of the TAF area, but will be
monitoring upstream radar trends over mid-MO for nowcast adjustments.

Truett

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...DENSE FOG ADVISORY until 10 AM CDT this morning FOR Bond IL-
     Clinton IL-Fayette IL-Marion IL-Washington IL.

&&

$$

WFO LSX





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