Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 191033

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
533 AM CDT Thu Sep 19 2019

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 404 AM CDT Thu Sep 19 2019

Early this morning, an upper level RIDGE continued to dominate the
Lower and Mid-Mississippi Valley, while Tropical Depression Imelda
was barely much further north into extreme eastern Texas than it was
24 hours ago.  At the surface, a large area of high pressure
centered over the New England states had a large swath of influence
extending over much of the eastern CONUS, resulting in a light
southeast flow over our region.  Skies were clear over our region
with temperatures ranging from the mid 60s in southern IL to the
lower 70s in northeast and central MO.  Convection IA was occurring
in the vicinity of a surface cold front and where upper level
support was greatest from a shortwave disturbance.

A slight weakening of the upper RIDGE is expected through Friday
afternoon but the main structure should remain intact.  The
convection to our north in Iowa in association with the surface
front and upper level disturbance is expected to remain to our north
with flow expected to push it very slowly eastward.  Down our way, a
similar setup as to what we have seen the past few days should occur
again sans the "backdoor" surface front:  MLCAPEs around 2000 J/kg,
very weak 0-6km bulk shear of less than 20kts, and no CINH.  The
main wildcard is where the focus could be with models showing
largely noise for favored moisture convergence.  Have broad-brushed
mentionable slight chance PoPs over a larger area for midday and
this afternoon with most locations not expected to get rain.

Tonight should be dry as diurnal convection dissipates early and
remnants of "Imelda" are still too far south to have any impact.

The remnants of "Imelda" with respect to lift may never get any
further north than Arkansas but it should still have some impact on
our region heading into Friday as the rich moisture gets shunted
northward into our region on the backside of the upper RIDGE.  PWs
are expected to exceed 1.5" by late Friday afternoon.  During the
afternoon, upper level disturbances will also edge into the area
with scattered showers and thunderstorms expected to develop.

Will continue to forecast on the high side of MOS with upper 80s and
lower 90s for most locales this afternoon and mid to upper 80s for
much of the area on Friday afternoon.  Min temperatures tonight
should be similar to the last several nights, with mid 60s to lower


.LONG TERM...  (Friday Night through Wednesday)
Issued at 404 AM CDT Thu Sep 19 2019

Active, wet weather pattern this weekend as a moist, southwest flow
regime becomes established with low-mid level warm air advection and
increasing upper level divergence ahead of weak southwest flow
shortwaves.  Heavy rainfall possible for northeast MO and west
central IL Saturday night as a deepening upper level trough moves
eastward through the northern Plains with a cold front sagging
southeastward into northwest MO.  Precipitable water values will be
quite high with a deep moisture profile.  The axis of heavier rain
will shift southeastward on Sunday as the upper level trough
approaches and the cold front continues to shift slowly
southeastward through our forecast area.  The ECMWF model is a
little stronger/deeper with the upper level trough and has a better
defined cold front compared to the operational GFS, and hence also
has a little more QPF Sunday afternoon and night. With increased
cloud cover and precipitation the daytime highs will be cooler this
weekend, and closer to seasonal normal values for late September.
The showers/storms should shift south-southeast of our forecast area
by Monday morning as a surface ridge builds into our area behind the
cold front.  The ECMWF model keeps us dry through Wednesday morning
with both surface and upper level ridging over the region.  The
operational GFS brings convection back into our area by late Monday
night and Tuesday morning as a warm front shifts northeastward into
our area, then keeps rain chances going through Tuesday night as a
weak surface low moves eastward through central MO.  With
significant model differences, for now will just include slight
chance to low chance pops for parts of our area mainly from late
Monday night through Tuesday night.



.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 533 AM CDT Thu Sep 19 2019

VFR conditions, dry weather, and light southeasterly surface
winds will prevail at the TAF sites through the valid period.
Once again, there is some small potential for a few showers and
storms this afternoon but probs remain too low to mention at this





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