Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 210543

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1143 PM CST Sat Jan 20 2018

Issued at 915 PM CST Sat Jan 20 2018

A couple forecast items this evening we are dealing with. Stratus
is once again slow to regroup and head northward but it is still
expected to envelop our region overnight--reaching much of the
I-70 corridor by midnight and the remainder to the north a couple
hours later.

As a result of this clearing and a surface frontal boundary draped
just to the north of I-70, temps have fallen off rather well into
the 30s but will eventually be placed in check and rise again back
into the 40s once the clouds return and the front moves to the
north of the area. Needless to say, the min temp forecast is
considerably more challenging because of this!

There is still a chance for some drizzle, but the timing has been
pushed back to correspond better with the onset of the low cloud
bases late tonight and primarily in the presence of the surface
front which should be in northern MO and central IL or where there
are some topo influences, such as southeastern MO.



.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 06z TAFs through 06z Sunday Night)
Issued at 1143 PM CST Sat Jan 20 2018

The gap in the stratus clouds is slowly closing, with the northern
group of clouds holding steady and digging in over northern MO and
central IL, as the southern group steadily moves north and fills
the gap. Much of this is MVFR category cloud bases with the
highest likelihood of further deterioration into IFR being at UIN.
Drizzle will be hard to handle throughout late tonight and Sunday
daytime, and have for the most part, left it out until we see
better development. Gradual improvement towards hi-MVFR or VFR
should occur during late morning and afternoon Sunday before a
descent into lower CIGs again Sunday evening. SE-S surface winds
will continue thru the valid period with some strengthening
heading towards Sunday evening as a deep low pressure system heads
our way. The high prob pcpn will hold off until late Sunday
evening where it was placed in all the TAFs, save UIN, where
location and timing issues are enough to hold off.

SPECIFICS FOR KSTL: Stratus has returned and should lower below
2000ft soon, with improvement for the afternoon, possibly into
VFR, before deterioration again to MVFR for Sunday evening. Good
chance for rain late Sunday evening for several hours into the
overnight with thunder in vicinity.





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