Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 221000

Area Forecast Discussion
National Weather Service Saint Louis MO
400 AM CST Thu Feb 22 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 327 AM CST Thu Feb 22 2018

Main focus thru tonight will be the ongoing freezing rain/drizzle
event, turning to PoPs for this evening and overnight.

Had hoped to cancel sern portions of the ongoing advisory with
this issuance. However, a heavier band of DZ, largely below the
radar beam, moved into the STL metro area and helped to drop temps
to the freezing mark again. With temps warmer further south and
east, believe this cooling effect is expected to dampen as it
moves SE. So, the going headline will remain as-is for now, tho
still expect areas to be canceled before Noon.

The light rain and drizzle is expected to gradually lift north
this morning as the 850mb front lifts north of the area.
Meanwhile, a weak sfc wave will be pulling ENE out of the area
allowing sfc winds to become ely. These two features will help
temps to rise above freezing during the morning hours. This temp
climb is expected to be slow, but expect the entire CWA to be
just above freezing by 18z today, at which point the precip is
expected to have ended anyway.

The next s/w, currently just being ejected into the srn Plains,
will bring showers with a couple of thunderstorms to the area
early this evening. Expect temperatures to be above freezing this
evening and slowly warm thru tonight. Looks like two areas of
focus for this round of rain. One area of lighter showers is
expected across portions of central and northeast MO into west-
central IL. Heavier showers with a few storms is expected across
southeast and into east-central MO and southwest IL. Mdls differ
exactly where this band of precip will be and how much rain it
will contain. The 06z NAM just arriving wud suggest a more nly
soln as well as higher amounts than the 00z soln. If this trend
continues and supported by other guidance, will need to consider
flood headlines for tonight. For now, have held off on issuing a
flood headline with uncertainty in the placement of the heavier


.LONG TERM...  (Friday through Wednesday)
Issued at 327 AM CST Thu Feb 22 2018

The active southwest flow pattern aloft will steer a few more
disturbances through the region, bringing periods of showers and
thunderstorms to the area from Fri evening through Sunday
morning. A period of dry weather is then expected from Sun
afternoon through Tue due to upper ridging aloft ahead of another
large low pressure system developing in the southwestern CONUS.
This low pressure system is expected to bring another period of
rain to the area during the middle and end of the week, but there
is considerable model variation regarding the details of how that
system evolves.

The primary forecast issue between Fri evening and Sunday morning
is the amount of rainfall. Recent precipitation has helped to
saturate the soils despite the lengthy drought conditions prior
to pcpn onset. Taken together, high PW values (1-1.3") and a
boundary orientation which is nearly parallel to the upper flow
suggests that there will be at least one and possibly two separate
periods of moderate rainfall over parts of the CWA, especially
the southeastern one-half to one-third. Isolated to scattered
thunderstorms are also expected on Fri night through Sat night due
to the instability, and this will help to locally increase
rainfall amounts. Furthermore, models have been consistently
showing plenty of upper lift (albeit from different sources)
during this time. Initial thoughts are that a widespread 1-3" is
possible across the southeastern half of the CWA, although the
forecast axis of heaviest rainfall will likely shift over the next
few forecast cycles. No flood watches or flash flood watches were
issued with this forecast package due to uncertainty about
location of the axis of heaviest rainfall, but future shifts will
be monitoring the need for flood headlines.

Temperatures will generally be warm between Fri and Wed,
especially on Sat after a warm front has lifted into the region. A
cold front moves through the region on Sat evening, but the
coolest night will probably be on Sunday night after high
pressure has built into the area and skies have partially cleared
within the cooler post-frontal air mass.



.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1155 PM CST Wed Feb 21 2018

IFR CIGs will prevail at the TAF sites for much of the valid
period. Pcpn just off to the S and E of STL metro (not counting
drizzle) will continue to slide east away while a new area of pcpn
forms to the west of our area and moves back in to many TAF sites
overnight and Thursday morning. Much of this will be FZRA for COU
and UIN with temps now warm enough for mostly RA at STL metro
sites. This area of pcpn will then exit to the northeast during
mid-late Thursday morning. This should result in another pcpn
break for much of the afternoon until the next round moves in as
rain showers Thursday evening. Surface winds from the NE will
gradually veer E and SE during the valid period.

SPECIFICS FOR KSTL: IFR thru the period with possible improvement
to MVFR during the dry period Thursday afternoon. Temps should be
warm enough from here on out to keep pcpn type all rain.



MO...Winter Weather Advisory until noon CST today 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.

IL...Winter Weather Advisory until noon CST today for Adams IL-Brown
     IL-Calhoun IL-Greene IL-Jersey IL-Macoupin IL-Madison IL-
     Monroe IL-Montgomery IL-Pike IL-Randolph IL-Saint Clair IL.



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