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Area forecast discussion...updated aviation
National Weather Service Saint Louis MO
1139 am CST Sat Dec 14 2019

Short term... (through late tonight)
issued at 359 am CST Sat Dec 14 2019

Dense fog over southern Illinois this morning continues and may lead to
icy conditions on elevated surfaces and untreated roads,
especially bridges and overpasses. Conditions have been improving
as clouds move over the area. Temperatures have also been rising
slightly ahead of the clipper system mentioned below. Believe
visbys will continue to improve thru the morning, but will be
closely monitoring trends.

Clipper system continues to dig into the area this morning
bringing rain to the western half of the County Warning Area. So far, the precip
has been ending as the cold air arrives, keeping everything rain.
Also of concern is the cold air over southern Illinois as mentioned
above. Trends have been that temps continue to rise ahead of the
precip. Model guidance suggests this trend will continue, but will
need to closely monitor trends thru the morning.

Otherwise temps will likely remain somewhat steady and cool thru
the day as strong cold air advection persists as the surface ridge builds into
the region. Temp trends tonight are actually a little on the warm
side compared to MOS. Clouds and weakening cold air advection shud help keep
temps warmer, but will need to monitor model trends as the going
forecast may be too warm.

Tilly

Long term... (sunday through friday)
issued at 429 am CST Sat Dec 14 2019

Confidence is increasing that an impactful system will affect the
County Warning Area sun and Mon. While amounts have changed slightly, the overall
message remains similar. Mostly snow likely roughly for areas
along and north of I-70 in MO and IL, a mix of all precip types
for the middle sections of the cwa, predominantly freezing
rain/drizzle for the southern third of the County Warning Area with ice
accumulations nearing two tenths of an inch. Across far southern
and southeast portions of the cwa, more rain is likely with
temperatures rising above freezing at times. Also similar to
yesterday, the warm air advection event with primarily snow, may be the main
event for the area.

However, there is increasing confidence that additional snow will
occur on Mon with a def zone/ aided by deep frontogenesis. Gefs
plumes depict a clustering of solns that suggest this precip
occurring. There is also support from the Gem and to some degree
the ECMWF, although the European model (ecmwf) opens the upper trof with lessening
precip as it approaches the County Warning Area. Given the inconsistency of the
NAM with this event, have largely discounted this soln.

Have increased pops late Sun morning. Models have sped up arrival
of precip, but trying to account for precip generally arriving
sooner than expected in strong warm air advection setups. Models, including
ensembles, are in fairly good agreement placing a band of precip
thru the middle portions of the County Warning Area. That said, there is some
disagreement regarding placement with the European model (ecmwf) solns keeping
higher amounts mainly across western portions of the County Warning Area. Much of
the snow is expected to change to freezing rain/drizzle as the
thermal profile warms and cloud ice is lost as a dry slot moves
into the area. Precip shud change back to snow thru Mon as cold
air moves back into the region. However, with the warmer temps
aloft lingering longer into Mon afternoon, more ice is possible
later into Mon, with snow possible later into Mon evening.

Have the watch currently ending at 00z tues with the bulk of the
precip expected to be ending by then. However, headlines may need
to be extended into at least Mon evening if trends continue to
slow the exit of precip.

Cold temps continue into Wed, however going forecast may be too
warm. Have kept forecast slightly cooler than the median thru Wed.
Temps shud begin to warm as the surface ridge builds east allowing
southerly flow to return to the region.

Tilly

&&

Aviation... (for the 18z tafs through 18z Sunday afternoon)
issued at 1137 am CST Sat Dec 14 2019

Lower cigs will be much slower to clear out than originally
anticipated, with clearing after sunset at KUIN and around or
after midnight for kcou and the stl Metro sites. Otherwise dry
conditions will prevail until late Sunday morning or early
afternoon (beyond the valid period for all but kstl) when snow is
expected to build eastward. As is usual for snow events, bring
the visibilities rapidly down at onset.

Tes

&&

Lsx watches/warnings/advisories...
MO...Winter Storm Watch from Sunday afternoon through Monday
afternoon for Audrain MO-Boone MO-Callaway MO-Cole MO-
Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson MO-
Knox MO-Lewis MO-Lincoln MO-Madison MO-Marion MO-Moniteau
MO-Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-
Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis
City MO-Saint Louis MO-Sainte Genevieve MO-Shelby MO-Warren
MO-Washington MO.

Illinois...Winter Storm Watch from Sunday afternoon through Monday
afternoon for Adams Illinois-Bond Illinois-Brown Illinois-Calhoun Illinois-Clinton
Illinois-Fayette Illinois-Greene Illinois-Jersey Illinois-Macoupin Illinois-Madison Illinois-
Marion Illinois-Monroe Illinois-Montgomery Illinois-Pike Illinois-Randolph Illinois-Saint
Clair Illinois-Washington Illinois.

&&

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