Area Forecast Discussion
Issued by NWS St. Louis, MO

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629
FXUS63 KLSX 200544
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1144 PM CST Fri Jan 19 2018

.UPDATE...
Issued at 915 PM CST Fri Jan 19 2018

A shield of thick cirrus clouds will maintain mostly cloudy skies
the remainder of the night, but most sites should still slip into
the 30s, albeit barely in some cases.

Stratus clouds have been slow to move north from Arkansas this
evening and are not expected to enter the forecast area until
after 09z, and make most of its headway between 12 and 15z.

A general model consensus continues to exist on some drizzle
development soon after onset of the stratus, particularly for
areas north of Interstate 70 from late morning onward, with a
broader development elsewhere on Saturday night.

Temps on Saturday could be quite challenging, balancing the higher
dewpoints and warm air moving northward with the weak sun of
January and anticipated stratus clouds.

TES

&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 339 PM CST Fri Jan 19 2018

Clockwise flow around the surface high currently over the
southeastern CONUS combined with a developing low over the western
Plains is producing south-southwest flow over the Mid Mississippi
Valley today.  The southerly flow will continue tonight and Saturday
keeping mild temperatures over the area.  Temperatures have already
hit the low to mid 50s across much of the area as of 300 PM, and
similar if not slightly warmer temperatures are likely on Saturday.
The one potential problem for getting much warmer than today is
clouds.  Current satellite pics show an expansive area of clouds
over eastern Texas, Louisiana, and Arkansas.  Short range guidance
is pretty insistent that this area of clouds will expand northward
tonight to spread into the Mid Mississippi Valley.  Latest thinking
is that the clouds will reach central and southeast Missouri an hour
or two before sunrise and continue spreading north or northeast
through the morning.  The clouds will likely put a bit of a damper
on Saturday`s warmup, but warm advection will continue through the
day so think there`s a good chance temperatures will warm back up
well into the 50s.

Carney

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 339 PM CST Fri Jan 19 2018

A strong shortwave trof will move from the West Coast across the
Rockies on Saturday night.  The trof will move into the western
Plains on Sunday and pick the surface low up...forcing cyclogenesis
and pushing it northeast from the Oklahoma Panhandle Sunday morning
into Iowa by Monday.  The low will push a strong cold front through
Missouri and Illinois Sunday night and Monday morning.  Temperatures
will fall from the mid 50s to around 60 on Sunday to the 40s behind
the front on Monday and down to the upper 30s to mid 40s on Monday.
There should be plenty of low level moisture over the Mid
Mississippi Valley ahead of the front to produce precipitation...so
guidance PoPs in the likely to categorical range Sunday night and
Monday morning as the front sweeps through look very reasonable.
Thunderstorms slight chance of thunderstorms still looks reasonable
as MUCAPE remains in the 200-400 J/Kg range Sunday night.  There`s a
ton of deep layer shear in the atmosphere ahead of the front as one
would expect with a cool season storm system...but the low MUCAPE
values should keep the threat for severe weather at a minimal level.
 That being said, the storm system bears watching because it would
take a whole lot more instability to really increase the severe
chances significantly.

The pattern aloft remains quasi-zonal in the wake of the Sunday
night/Monday storm system.  The surface system will lift northeast
into the Canadian Maritimes by Thursday and ridging aloft will build
over the central U.S.  This pattern will keep the coldest air behind
the system bottled up over the Upper Midwest and southern Canada.
While temperatures will be colder behind the system on Monday and
Tuesday...this won`t be an Arctic deep freeze like we`ve seen behind
cold fronts in the last few weeks.  Current medium range guidance
puts lows in the 20s Tuesday and Wednesday morning with highs
warming into the 40s for most locations.  Warm advection ramps up
again ahead of the next downstream trof on Thursday and continues
Friday.  Lows Thursday morning look likely to drop to just below
freezing in the upper 20s and low 30s with highs rebounding into the
50s both Thursday and Friday.

Carney

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1144 PM CST Fri Jan 19 2018

VFR conditions will prevail at the TAF sites thru about 10z, with
MVFR CIGs overspreading all sites from 10z thru 13z and then
likely persist for the remainder of the valid time. Mainly dry
weather will prevail thru the early part of the period, with
patchy drizzle entering the picture after 15z/Sat for areas north
of Interstate 70, but hard to necessarily add to TAF at this time
given low prob of occurrence at any one site. Otherwise, a steady
S-SW surface wind around 10kts is anticipated, backing SE by
Saturday night. LLWS conditions remain marginal and expected to
be just below thresholds and so have continued with the no
mention.

SPECIFICS FOR KSTL: Other than the above, wind directions should
remain below 210 thru the valid period, with best potential for
any drizzle currently expected to the north, so no mention at this
time.

TES

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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