Scientific Forecaster Discussion
fxus63 klsx 191121
Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
621 am CDT Sat Oct 19 2019
Short term... (through late Sunday afternoon)
issued at 257 am CDT Sat Oct 19 2019
Main changes were to cool temps slightly, mainly tomorrow, as well
as small changes to pops.
The approaching trof, and associated cold front, continue to support
showers across eastern Kansas and far western MO. Still expect precip to
move into western portions of the County Warning Area. As the forcing shifts north
and low level forcing weakens, expect the showers to slowly
dissipate through the afternoon. The precip should come to an end by
sunset this evening.
Focus then turns to potential for fog development tonight. Winds
will be light and variable tonight as a weak surface ridge builds
into the area. With a lack of dry air advecting into the region,
expect moisture within the boundary layer. The main question will be
if clouds can clear out enough to allow enough cooling and fog
development. Have added mention of light fog, but updates will need
to monitor trends.
Long term... (sunday night through friday)
issued at 257 am CDT Sat Oct 19 2019
An upper longwave trough is expected to prevail over the central Continental U.S.
Through this period, steering storm systems through our region every
few days. A strong storm system will be in the process of deepening
as it moves into the Central Plains late on Sunday and becomes
vertically stacked by Monday as it turns northward through the upper
MS valley. While the most widespread forcing with this system will
be well northwest and north of our region, a surface cold front is
expected to slide through late Sunday night and Monday morning and
should have enough convergence along it, coupled with a
strengthening upper jet streak, to produce a wave of precipitation.
The main model consensus is to position this upper jet streak into
the more favorable position to maximize divergence aloft when the
precipitation is east of our region, but there should be enough
broadscale lift to promote higher pops of likely or higher category
for most areas. This lift will interact with an atmosphere that is
weakly unstable but it will be enough to result in a conditional
severe threat for tornadoes: strong 0-6km and 0-1km bulk shear, low
local levels, and surface dewpoints nearing 60f.
Heading later into Monday, the system dry slot will take hold and
substantially reduce precipitation potential but as we see often in
these well wound systems, heating of the day could result in a few
thunderstorms to maintain a low mentionable pop for much of the day.
A pair of dry days then look on tap for Tuesday and Wednesday with
the next cold front and storm system affecting our region set for
late week. The models still disagree on the timing this far out,
with it ranging from Wednesday night to Friday, but there is
slightly more tightening of the time range than there was 24hrs ago.
Airmasses of a Pacific origin will prevail until the second storm
system and associated cold front can cross through, with a colder
airmass of a more Canadian origin can build in for next weekend.
High temperatures each day until Friday should be within 5 degrees
of seasonal normals, with a noticeable dip in temperatures on Friday
with maxes mainly in the lower 50s.
Aviation... (for the 12z tafs through 12z Sunday morning)
issued at 616 am CDT Sat Oct 19 2019
Southeast winds will gradually veer to become SW today expected to remain
under 9 kts. Showers will slowly move east today and should impact
kcou, and perhaps KUIN this afternoon. Chances are lower that much
rain will reach KUIN as the rain should diminish as it moves east.
Clouds will linger behind the front with cigs dropping into the
MVFR range. Believe at least some light fog is possible late
tonight where clouds can clear out. The best chances for this is
kcou and KUIN.
Specifics for kstl/ksus/kcps: the bulk of the precip should
dissipate before reaching terminals today. Still think MVFR cigs
will move in late tonight and slowly lift out of the region Sun