Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 271122

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
622 AM CDT Thu Jul 27 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 321 AM CDT Thu Jul 27 2017

Northwest flow aloft prevails over our region early this morning
with the upper level RIDGE continuing to retreat away from us and is
now centered over the southern High Plains.  A pair of MCS are seen
on the radar mosaic, with the eastern one having exited our CWA to
the east as it enters southern Indiana while the western one has
pushed into central MO.  This western MCS is where the heavy rain
axis was thought to setup further north 24hrs ago but instead
preferred something closer to the MO river.  Flash Flood Warnings
have been issued for parts of west-central MO where this heavy
rainfall has been realized and will likely enter parts of central MO
during the next few hours.  In between these two MCS, convection has
filled in much of the gap.  Despite the synoptic cold front still
located just northwest of our CWA from near Kansas City to southeast
IA, the effective boundary is much further south near the southern
edge of the current convection:  just south of I-70.  This effective
boundary will likely govern the temperature regime for today.
Speaking of which, temps remain warm across the region with readings
in the 70s and a few spots around 80F.

Evolution of rain chances and their effects on temps are the two
main forecast issues.

Should see main rain shield with the western MCS slowly roll E-SE
thru parts of central MO and into southeast MO this morning and also
should slowly drag southward the areas of convection that have
sprouted downstream along the I-70 corridor.  The effective boundary
is expected to push even further south into southern MO and far
southern IL by afternoon as a result, and it is here where new
convection with the heating of the day should fire.  Further to the
north, showers and storms are expected to be much more spotty during
the afternoon hours.  All of this should very slowly push south and
decrease heading into tonight with the last of the rain exiting the
southern CWA after midnight.

Temps are expected to struggle to rise much today with the effective
front being so far south already and rain and clouds expected to
persist thru much of the morning with limited clearing if at all,
during the afternoon.  This should yield max temps from 80-85 for
most areas with some areas in the eastern Ozarks of southeast MO not
getting out of the 70s.  With the synoptic front finally getting
thru this evening, but clouds likely sticking around longer, min
temps a few degs below persistence in the 60s look reasonable.


.LONG TERM...  (Friday through Wednesday)
Issued at 321 AM CDT Thu Jul 27 2017

The cold front should be well south of the CWA at daybreak Friday
with surface high pressure building southward through the MS Valley
and low level northeast flow advecting cooler and drier air into the
region. The large scale pattern across the CONUS will continue to
amplify heading through the weekend as the eastern upper trof
deepens and the western ridge builds. As a result the large surface
high pressure system will gradually ooze southward through the mid-
upper MS Valley region and dominate the sensible weather, bringing
pleasant late July conditions featuring below average temps/

The general pattern aloft will persist into the middle of next week
with the western upper ridge continuing to build and shifting
slightly westward. Meanwhile the eastern trof will weaken overall
but will persist and allow the expansive surface high to continue,
weakening with time. The result should be predominately dry weather,
lower than normal humidity, and temps gradually moderating to near



.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 622 AM CDT Thu Jul 27 2017

Area of low clouds, mostly MVFR CIGs, has developed around the
rain and will be slow to clear out over the next 18 to 24 hours
for the TAF sites near I-70 (COU, STL, SUS, CPS). Surface cold
front about to move thru UIN and this should be the first site to
become dry and lose the low CIGs by afternoon. Further south,
persistent rain will continue for much of the morning, with
thunder looking less and less of a possibility, with rain
scattering out in the afternoon. Low clouds will likely hang on
thru most of tonight, finally clearing out after midnight. Light
southwest winds will exist ahead of the front with winds becoming
NW-N with FROPA.





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