Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 202130

Area Forecast Discussion
National Weather Service Saint Louis MO
330 PM CST Sat Jan 20 2018

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 320 PM CST Sat Jan 20 2018

Clouds and temperatures have been a challenge today as a stratus
deck overspread the eastern Missouri and southern Illinois this
morning, and then slowly receded northward as warm advection melted
the southern edge away this afternoon.  Warm southerly flow is
expected to continue tonight which will keep mild temperatures over
the forecast area.  Expect a quick drop into the 40s this evening,
and then guidance is insisting that the stratus will redevelop over
those areas that cleared out this afternoon.  The insulating blanket
of clouds combined with southerly flow will produce steady or slowly
rising temperatures overnight.  Short range guidance continues to
spit out a few hundredths of spotty QPF.  Some light rain or drizzle
isn`t unreasonable in this situation, but I`m noticing the low level
isotherms continuing to warm up several degrees overnight which says
to me that the southwest flow is going into warming the atmosphere
rather than isentropic lift.  Sunday`s temperatures should warm a
few degrees above today`s as warm advection continues.  Seeing spots
of QPF in the short range models for Sunday as well, but the
synoptic warm front is forecast to be well up over Iowa so I`d think
that any precip that falls will be spotty and light.


.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 320 PM CST Sat Jan 20 2018

GFS and ECMWF are still showing that the upper low that will be over
the central high Plains on Sunday evening will move into the eastern
Great Lakes by Tuesday morning.  As it does it will bring a cold
front across Missouri and Illinois late Sunday night into Monday.
Still expect a band of showers to develop on Sunday night in the
strong low level moisture convergence beneath increasing mid level
ascent ahead of the approaching upper low.  This band of showers
will move west to east across the area during the late evening and
overnight hours, so will continue with 80-100% PoPs given the strong
forcing. Will also keep with a chance of thunderstorms over the area
on Sunday night as the SPC SREF mean MUCAPES are showing between 100-
250 J/kg over the area between 03-12Z.  Will need to monitor parts
of central and southeast Missouri for a few severe thunderstorms
late on Sunday night as it appears that a line a thunderstorms will
be moving in from western Missouri.  However, the main limiting
factor for severe weather will be the decreasing instability as the
line moves east into the LSX CWA.

The front will move across the area on Monday with colder air moving
in behind it.  The global models continue to show that there will be
some wrap around precipitation late Monday into Monday night mainly
along and north of I-70.  This looks like just a light precip event
with just chance PoPs for now.  Expect rain changing to light snow
with little if any accumulation.

Then mainly dry weather is expected Tuesday into Friday as
subsidence sets in behind the upper low and an upper ridge builds
into the area by mid-late week.  While temperatures will initially be
cooler behind the front on Tuesday and Wednesday, they will still be
above normal with highs in mainly in the 40s.  By later in the week,
low level flow will turn out of the southwest allowing highs to
climb back into the 50s.

Global models are in good agreement that a trough will dig into the
central CONUS by next weekend with moisture moving northward from
the Gulf ahead of it.  This will bring a chance of rain to the area
next Saturday.



.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1155 AM CST Sat Jan 20 2018

Primary aviation concern for the next 24 hours is MVFR ceilings
and potential for IFR late tonight. Latest observations show that
the stratus has lifted above 2000ft in most locations. Expect this
lifting trend to continue...though I doubt it will make it above
3000ft this afternoon. There is an area of cloud-free space over
the eastern Ozarks and southern Illinois, but it looks like the
clear slot will fill in over the next few hours from the
southwest. Ceilings will likely lower during the late evening to
low MVFR...between 1000-1500ft. Could see IFR ceilings as well as
MVFR fog before sunrise, but am not confident enough to include
this in the TAFs at this time. Some light rain or patchy drizzle
is also possible tonight...primarily across northern Missouri into
west central Illinois...but the greatest impact to aviation will
likely be from the aforementioned low clouds.


MVFR flight conditions are expected to prevail at Lambert through
Sunday. The southern edge of the stratus deck currently over the
terminal is making some slow progress northward...and it may
scatter out briefly this afternoon. However, expect more stratus
to move in from the southwest this afternoon and evening. Model
guidance is consistent in lowering the ceilings to between
1000-1500ft late this evening or overnight. There`s a chance
ceilings could go down to IFR and there could be some fog as well,
but confidence in that scenario is not high at this time. Most
likely scenario is that the stratus will drop to aforementioned
levels and stay there into Sunday morning with some lifting
likely in the afternoon.





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