Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 242123

323 PM CST Sat Jan 24 2015

.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.


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


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.


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.



.AVIATION:  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1151 AM CST Sat Jan 24 2015

VFR flight conditions will continue to prevail across the area
this afternoon and into this evening. A storm system will move
into the Midwest later tonight bringing an area of light rain with
it. Ceilings will drop to MVFR overnight as rain overspreads the
area. Think areas of rain with MVFR ceilings and visibilities will
prevail through the day Sunday, with areas of IFR...primarily across
northern Missouri into Illinois. Rain may mix with and turn to snow
Sunday afternoon across northern portions of the area.

Specifics for KSTL:

VFR flight conditions will continue to prevail at Lambert at least
through the evening into the overnight hours. Expect a VFR ceiling
to develop this evening, and then lower to MVFR as precipitation
overspreads the Metro Area late tonight. Guidance is hinting at
some periods of IFR around 12Z with the rain, but confidence is
not high in IFR conditions at this point. Prevailing rain will
likely end during the mid to late morning, but the chance for
showers will continue all day. MVFR and possibly IFR conditions
are also expected to continue all day.





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