Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181219

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
619 AM CST Sun Feb 18 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 353 AM CST Sun Feb 18 2018

The combination of clear skies, calm winds, and moisture from
yesterday`s rain and melted snow was promoting radiational fog
development across parts of the area early this morning. METARs and
traffic cameras depicted patchy dense fog at times, especially near
the rivers. Although the dense fog does not appear to be widespread
at this time, we will continue to monitor the need for a Dense Fog
Advisory. Fog should begin to dissipate after sunrise, although it
may take until mid-morning for the fog to completely mix out.

The tightening pressure gradient between a departing surface high
pressure center and a developing surface low over CO/WY/SD will
produce breezy conditions today across most of the CWA. Persistent
southerly winds will help boost temperatures into the 50s to near 60
degrees, which is around 10+ degrees warmer than yesterday and 5-15
degrees above normal for this time of year. Temperatures will not
fall too far tonight, and they may even start rising before sunrise
after the passage of a warm front.

A LLJ interacting with the lifting warm frontal boundary will
produce areas of rain showers and isolated thunderstorms tonight.
The greatest precipitation coverage is expected after midnight, but
it is possible that some rain could occur over southeastern MO and
southwestern/southern IL as early as this afternoon.


.LONG TERM...  (Monday through Saturday)
Issued at 353 AM CST Sun Feb 18 2018

Focus thru this period continues to be the active pattern this week
with the main focus thru Tues night.

Mdls are in fairly good agreement thru much of the period. At the
beginning of the period, the region is under deep SW flow with a
deep trof over the Rockies. While this trof transitions ewd and
lifts into the Plains on Tues, another trof digs into the region to
replace it, keeping the CWA under SW flow aloft.

With a wrmfnt lifting nwd thru the area by 12z Mon, or shortly
thereafter, have trended aoa the warmer MOS and may still be too
cool. Some guidance suggests much of the area reaching around 80.
Have trended warmer, but with some question regarding amount of rain
during the day, kept forecast below the warmest guidance for now. If
a lull in precip does occur during the late morning and much of the
afternoon hours, future updates may need to trend warmer. This setup
is also suggestive of a rather windy day. Have also trended wind
speed and gusts higher. If deeper mixing does occur, expect much of
the area to see wind advisory level gusts with a quite strong LLJ
across the region.

As for the rain during this period, not much change overall in PoPs.
The main focus for heavier rain is still Tues afternoon thru at
least the evening hours. That said, the 06z NAM is just arriving
with a much more precip back into the cold air. Have kept PoPs
lingering longer due to lower confidence in ending time of precip
with lack of run to run consistency. Some differences still exist
regarding the placement of some of the heavier rain. Main focus
continues with 1.50 to nearly 3.00 inches of rain across the area
with the bulk of that being Tues afternoon thru evening hours. With
rivers at such low levels and dry ground, do not currently
anticipate flooding issues with the precip being spread out over a
longer period of time. While some convection is expected, do not
expect enuf to cause issues either.

The next concern also remains p-types as the precip comes to an end.
The ECMWF has come into better agreement with the GFS/GEM. However,
the ECMWF/GEM still have more precip back further into the cold air.
Due to lower confidence as discussed above, will continue to keep
mention of freezing rain, sleet and perhaps some snow as precip

Mdl differences increase with the ECMWF/GEM depicting a more classic
freezing rain setup with overrunning WAA precip Wed afternoon into
the night. Have kept PoPs lower for this until mdls come into better
agreement. Mdls suggest yet another round of precip as a wrmfnt
lifts north across the area late this week. Best chances still
appear to be Fri into Fri night.



.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 542 AM CST Sun Feb 18 2018

SPECIFICS FOR KCOU, KUIN: Incoming cirrus and rising surface wind
speeds have interfered with radiational fog formation, and
visibilities have risen at sites near KCOU and KUIN. Wind speeds
and gusts will increase today due to the tightening pressure
gradient. Wind shear is expected after 03z. Rain showers with a
few isolated embedded thunderstorms are expected after 06z.

SPECIFICS FOR KSTL, KSUS, KCPS: Patchy dense fog was still
occurring in some river valleys, but visibilities have risen in
the last few hours at the St. Louis metro area terminals.
Lingering fog will dissipate this morning and VFR conditions are
expected thereafter. Rain showers with a few isolated embedded
thunderstorms are expected after 04z, although there is a slight
chance that rain could occur as early as 21z at KCPS.





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