Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 250309
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
909 PM CST Sat Jan 24 2015

.UPDATE:
Issued at 835 PM CST Sat Jan 24 2015

The radar was showing relatively weak reflectivities across
southwest IA, northeast KS, and into extreme northwest MO this
evening. Some of this light rain was evaporating before making it
to the ground due to an initially dry surface/boundary layer. This
rain was ahead of an approaching shortwave over southeast SD and a
weaker shortwave over eastern KS. This light rain should spread
into portions of northeast and central MO around midnight per the
latest HRRR model run. The latest NAM model run appears a little
deficient in its QPF tonight across MO. The light rain will spread
southeastward into much of the rest of our forecast area late
tonight/early Sunday morning as the main shortwave amplifies into
an upper level low and the surface low now over northwest IA and
southwest MN drops southeastward to just west of UIN by 12z
Sunday. Low temperatures tonight will be well above normal for
late January due to the lowering and thickening cloud cover along
with southwesterly surface winds for much of our area for most of
the remainder of tonight.

GKS

&&

.SHORT TERM:  (Through Late Tonight)
Issued at 322 PM CST Sat Jan 24 2015

Elongated shortwave extending from Saskatchewan southeast into
Montana will dive southeast into Missouri by late tonight.  The
associated low level circulation will drive southeast across Iowa
tonight and into extreme northeast Missouri by 12Z.  All models are
printing out fairly widespread/light QPF ahead of the shortwave
along and north of the I-44 corridor from the Ozarks north to the
I-70 corridor.  Another area of light QPF is showing up over
northern Missouri closer to the low level circulation.  GFS and NAM
show a pretty broad area of 850-500mb omega ahead of the shortwave,
and a band of 850mb moisture convergence which sweeps through
Missouri into western Illinois ahead of the 850mb cold front.  These
features appear to be the primary drivers for precipitation
tonight.  4KM ARW and NMM WRF members develop a broad area of
simulated reflectivity across the aforementioned parts of the CWFA
and MOS PoPs are likely to categorical in these areas as well.
Given all this, have increased PoPs from I-44 north to Quincy to
likely or better late tonight...with much of southwest Illinois east
of the STL Metro still staying in the high chance category.  Kept
temperatures pretty mild in the mid 30s to near 40 with clouds and
southwest flow.

Carney

.LONG TERM:  (Sunday through Next Saturday)
Issued at 322 PM CST Sat Jan 24 2015

(Sunday-Tuesday)

Over the last few days the synoptic guidance has been trending a bit
stronger/a bit more southwesterly with Sunday`s clipper system.
12z guidance has also followed this trend, with associated MOS also
indicating a sizeable jump in PoPs as well for the day on Sunday.

It would appear that band of rain that develops overnight will sweep
east into southern/eastern sections of the CWA early Sunday, along
and east of system`s cold front and in area of strong UVV with
initial shot of vorticity with upper system.   This area of rain
will be followed by additional pockets of precip sprialling around
the upper low as it works into southern Illinois during the day.
While 850mb temps drop fairly quickly (model consensus indicates
0C isotherm will advect into southern sections of the CWA by
18z), forecast sounding from both NAM and GFS indicate cooling of
low level AMS and the drop of the freezing level to support snow
will be much slower than suggested by 850 mb temps alone. This
makes sense given the strong low level warming that should occur
ahead of the clipper later tonight and into early Sunday. Started
a changeover to snow in our far north by mid morning, with the
changeover dipping into the Ozarks in MO and to near the I70
corridor in IL by 00z Monday. Given how warm we are getting today
and the fact that it appears that the precip will be fairly light
once the changeover to snow occurs, it appears that any
accumulations will be quite minor...generally less than one-half
an inch and primarily near/east of the UIN area.

Precip in the form of light rain and light snow will wind down
across southeast sections of the CWA Sunday evening, with the quick
shot of cold air in the wake of the Clipper dropping temps into the
lower to middle 20s.

All of the 12z guidance indicates a very rapid rebound in temps on
Monday as the upper ridge over the Rockies begins to build into the
Plains.  30s will linger over our eastern counties but 40s seem
likely for areas along and west of the Mississippi...with some
locations in mid MO possibly pushing the 50 degree mark.

Shortwave partially driving Monday`s moderation will drop into the
east coast trof Monday night, with the CAA in the wake of the
shortwave allowing a weak cold front to drop into the mid
Mississippi Valley.  However, greatest push of cold air will be well
east of our CWA, so Tuesday`s temps should be in the 40s over
most of the FA.

(Wednesday-Saturday)

Unseasonably warm airmass over the Plains should work its way east
at midweek as the UA pattern over the CONUS becomes progressive and
upper ridge works into the mid-Mississippi Valley.  Given the
intensity of the warm air have gone a bit above initialization
numbers with a few locastion in mid MO pushing 60.

Next system in the increasingly active flow regime passes through
the area on Wednesday night.  Cold air in the wake of the system
should return temps to more normal levels on Thursday, with temps
dropping a few more degrees on Friday.

While medium range solutions are in very good agreement into
Friday, they diverge considerably heading into the end of the
week. Both GFS and ECMWF develop a cut-off low over the southwestern
U.S. but mid level flow over the central CONUS varies considerably,
with GFS indicating a nearly zonal flow regime from the Rockies
into the east coast while the ECMWF suggests trof deepening over
the eastern half of the country...a much colder solution for our
area by next Saturday (ECMWF 850mb temps are about 10 degrees
colder than those of the GFS). For now have leaned towards the
warmer GFS, with a chance of rain or snow as broad overrunning is
progged across the region.

Truett

&&

.AVIATION:  (For the 00z TAFs through 00z Sunday Evening)
Issued at 607 PM CST Sat Jan 24 2015

Mid and high clouds moving in ahead of next weather system. Cigs
to lower to low end vfr between 08z-11z Sunday with rain moving
in. Rain will be on the increase with cigs/vsbys lowering to mvfr.
Then as system slides southeast through region will see winds
pickup and veer to the northwest to north gusting to near 25kts at
times. Will see a bit of a break in activity by mid to late
morning for taf sites along I-70, so kept vcsh mention. Then as
colder air filters in rain to become mixed with snow Sunday
afternoon and evening.

Specifics for KSTL:
Mid and high clouds moving in ahead of next weather system. Cigs
to lower to low end vfr by 11z Sunday with rain moving
in. Rain will be on the increase with cigs/vsbys lowering to mvfr.
Then as system slides southeast through region will see winds
pickup and veer to the north gusting to near 25kts at times. Will
see a bit of a break in activity by mid to late morning for taf
sites along I-70, so kept vcsh mention. Then as colder air filters
in rain to become mixed with snow by 21z Sunday.

Byrd

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX






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