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Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
1254 PM CDT sun Sep 15 2019

Short term... (through late Monday afternoon)
issued at 350 am CDT sun Sep 15 2019

Elevated convection across northwest MO and southeast Iowa will move
through northeast MO and west central Illinois early this morning. This
convection is in a region of low-mid level warm air advection ahead
of a weak shortwave over eastern Nebraska and western Iowa. This
convection is also in a region of low level moisture convergence on
the nose of a 40-45 knot southwesterly low level jet. This
convection should weaken and shift east of our forecast area by late
morning. Warmer and more humid conditions can be expected today as
an upper level ridge builds over the central US, along with
southwesterly surface winds. Highs today will be about 10 degrees
above normal. A very weak cold front will sag southward into
northeast MO and west central Illinois by this evening, and then drop into
east central MO and southern Illinois Monday morning as surface ridging
builds over the Great Lakes region. At this time it appears that
the atmosphere will be too capped for any convection to develop
along this weak front tonight. Unseasonably warm conditions will
continue on Monday with afternoon heat indices approaching 100
across parts of the forecast area.


Long term... (monday night through saturday)
issued at 350 am CDT sun Sep 15 2019

On Tuesday, an upper level ridge will continue to dominate our
region. A large area of high pressure at the surface will be
centered over the eastern Great Lakes and northeast Continental U.S. But its
influence will extend all the way westward into our region,
resulting in a light east or southeast surface wind for most
locales. Despite the east-southeast wind direction, the influence from the
airmass to our east is only expected to have a slight impact, with
forecast Max temps a couple degrees cooler for most areas, or from
the mid 80s to near 90. While rain chances look minimal on Tuesday,
they are not nil, as there is a weak signal for an isolated shower
or thunderstorm from the interface of the high pressure area
centered to our northeast with the warmer airmass to our west, and
this could create enough weak convergence in a weakly capped
environment for these isolated cells.

Most model guidance then weakens the grip of the upper ridge on our
area--only slightly--heading into Wednesday with this setup carrying
through to Saturday. At the same time, the influence of the cooler
airmass to our east should fade, with southerly flow kicking back
in. Max temperatures for most of these days should return to much
above normal (10-15 degrees higher than mid-late September values),
ranging from the mid 80s to the lower 90s, with the highest readings
tending to congregate in the stl Metro area. Fortunately, another
weak area of high pressure to our south will prevent a direct flow
from the Gulf of Mexico, and dewpoints should struggle in getting
much above 70-72f. This stands to translate into afternoon peak
heat index values mainly in the 90s, and approaching but probably
falling just short of 100 in the stl Metro area.

A result of the slightly weakening upper ridge would be to steer
upper level disturbances from the monsoonal flow to our west into
our region, increasing rain chances at least into mentionable range
(20%+) for a chunk of this period primarily for areas to the north
of Interstate 70. The main surface frontal boundary appears likely
not to be a factor in aiding convective development, however, with a
Stout SW flow aloft remaining in place during this period and will
probably limit pops to low chance values at best. Another
contingency for late week will also depend on convection realized--
and could take a cut at temps ultimately with additional clouds
coming into play.


Aviation... (for the 18z tafs through 18z Monday afternoon)
issued at 1251 PM CDT sun Sep 15 2019

Dry and VFR flight conditions expected through the forecast
period. Otherwise, southwest winds will diminish by sunset. Only
other concern is a weak frontal boundary that will sink south
across region and washout by mid morning on Monday. Passage should
remain dry with light and variable winds becoming west to
northwest behind boundary.


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