Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201020

Area Forecast Discussion
National Weather Service Saint Louis MO
420 AM CST Tue Feb 20 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 331 AM CST Tue Feb 20 2018

Focus continues to be precip today, turning to falling temps and
precip tonight.

The sfc front currently extends from just west of KUIN to west of
KCOU. A sfc wave is actually expected to move NE along the front,
which will help to move the front west slightly this morning.

A lot of similarities from yesterday for today. Very strong LLJ
across the region to pump moisture into the area. The only
difference is that winds will be slightly more sly today and
perhaps not quite as strong. The approaching deep trof and back
building ULJ will help provide broad lift across the region.

Believe showers will increase in activity, roughly from I-44 in
MO and I-55 in IL, through sunrise. An approaching s/w will help
precip fill in across much of the CWA north and west of this
corridor. The rain will gradually move SE with the front thru
today. While showers will continue ahead of the front, believe the
more widespread precip will remain just ahead and behind the
front. That said, much of the guidance suggests the precip will
end as the sfc temps drops to freezing and below. While the front
shud be roughly from KVIH to KSTL to K3LF at 00z this evening,
temps shud be be reaching the freezing point in far nwrn portions
of the CWA as the precip comes to an end. This trend shud
generally continue as the cold air continues to move SE into the
region. With a warm ground and anticipating only a brief period
of freezing rain, have held off issuing any headlines for now.
However, some uncertainty still exists regarding this scenario.
The 06z NAM just arriving continues to suggest more precip further
back into the cold air.

Have also held off issuing any flood headlines at this point.
While there is expected a prolonged period of rain thru tonight
with the area expected to receive 1 to 2 inches by sunrise Wed,
believe that it will fall over a long enuf period of time that
rivers will be able to handle the water. This rain will help to
reduce the ongoing drought across the area and may increase the
flooding potential later this week.


.LONG TERM...  (Wednesday through Monday)
Issued at 331 AM CST Tue Feb 20 2018

The primary forecast issue in this period continues to be the
coverage and duration of freezing rain on Wed night and Thu
morning. There is little doubt that at least some freezing rain
will occur on Wed night and Thu morning (perhaps mixed with sleet
at times) but there is a moderate level of uncertainty regarding
the duration of FZRA and therefore the amount of additional ice
accretion which can be expected on Wed night and Thu.

Lingering freezing rain on Wed morning should transition to rain
as surface temperatures warm into the mid to upper 30s. Many
areas will see a lull in precipitation during the day, especially
over parts of central and northeast MO as well as west central
IL. Additional precipitation is expected to develop and spread
northward on Wed night and Thu due to isentropic ascent, and this
is the time period with uncertainty regarding ptype. The NAM
remains a moist and cold outlier and would suggest several hours
of moderate to heavy FZRA. The 20/00z GFS is still warmer than
the NAM (although it is slightly colder than the 19/00z GFS), but
it also ends the precipitation much earlier compared to the NAM
and would suggest a much shorter period of FZRA. There is as yet
no strong signal to suggest that either solution is superior,
although the NAM remains quite an outlier compared to other
guidance and it is not the preferred solution at this time.

Therefore, this forecast package follows a middle ground which is
coincidentally similar to the SREF: a broad area of FZRA
overnight on Wed night (with a zone of mixed precip along its
northern edge) which transitions to rain from south to north on
Thu as a stalled frontal boundary begins lifting northward and
surface temperatures warm into the mid/upper 30s to 40s. The
expectation of at least some FZRA on Wed night is consistent with
the expected synoptic pattern where isentropic ascent creates
precipitation which then falls into a colder air mass at the
surface. This conceptual pattern matches most of the FZRA events
in the CWA.

The southwest flow pattern aloft will bring additional chances for
precipitation to the area between Thu night and Sat night. A brief
period of upper ridging is expected on Sunday ahead of yet another
developing trough over the western half of the CONUS. Models are
in poor agreement regarding the strength, timing, and evolution
of this feature, therefore SChc PoPs look reasonable for early
next week.

The coolest day during the extended period is likely to be Wed
when highs are in the 30s. Temperatures will rise each day between
Wed and Sat followed by a dip on Sun due to the passage of a cold



.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1140 PM CST Mon Feb 19 2018

The main corridor of showers and scattered thunderstorms will
remain from southwest MO through central and northeast MO and
west central IL overnight, with showers becoming more numerous
east of this zone across the St. Louis region in the predawn
hours. Flight conditions have been quite variable thus far
tonight. I think KUIN has the highest probability of predominately
MVFR fight conditions and elsewhere VFR may dominate. The coverage
of showers and scattered thunderstorms should increase throughout
the day on Tuesday with flight conditions deteriorating to
predominately MVFR. A cold front will advance slowly east across
the area Tuesday afternoon and into the evening and flight
conditions will lower to IFR in the wake of the front. Gusty
southerly winds will continue ahead of the front with gusty
northwest winds after the cold frontal passage.


I think predominantly VFR flight conditions will prevail overnight
until around mid-morning on Tuesday. Early on during the overnight
period, showers will be rather spotty however they will become
more numerous prior to daybreak. The coverage of showers and
scattered thunderstorms should continue to increase throughout
the day on Tuesday with flight conditions deteriorating to
predominately MVFR. A cold front will move through the terminal
on Tuesday evening with flight conditions lowering to IFR in the
wake of the front. Gusty southerly winds will continue ahead of
the front, with gusty northwest winds after the cold frontal
passage Tuesday evening.





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