Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 280959
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
359 AM CST Sat Feb 28 2015

.SHORT TERM: (Through Monday Night)
Issued at 358 AM CST Sat Feb 28 2015

A busy forecast stretch thru Monday Night, primarily concerned with
wintry wx.

A small area of flurries is currently occurring over central and
southeast MO and is mainly associated with an area of decent
broadscale lift that is also getting a bit of WAA and moisture
convergence as well to produce the pcpn.  Short-term progs indicate
that this "perfect blend" of lift will weaken or exit the area by or
shortly after 12z to result in this ending early this morning.  In
any event, this is not the main wx item we are concerned with this
morning.

The general overview remains the same:  two areas/episodes of pcpn
will affect our region thru Monday night in response to pieces of
energy ejecting from a large storm system over the southwest CONUS.
The first will occur today thru Sunday and the second will occur
Monday night and beyond.

What has changed:  the models have come in a bit further south with
the max QPF axis and are faster with the onset today.  They are also
colder, and as a result, snowier for much of the first episode of
pcpn thru Sunday and for more of the CWA than before.  There is good
agreement between the NAM/GEM/EC on the placement of the max QPF
axis following up I-44 in MO and I-70 in IL, and likewise reducing
values from the previous forecast most substantially in northeast
MO.  The GFS shows something similar but has a much broader swath
and reduces the northern QPF much less.

The GFS seems to adequately take in the new southern trends while
retaining some continuity with the previous forecast, plus prefer
its handling of Sunday, where decent frontogenesis will remain over
I-70 in MO and IL thru much of the day and should result in
continued pcpn, whereas the NAM was much too quick to sweep it out
and rejected.

Vertical temp profiles now show a fairly high warm wedge of air
aloft late tonight and into Sunday that should be more generous to
frozen pcpn with the deeper cold layer and so was reluctant to go
liquid pcpn anywhere until some area further south of the 0C line at
H850, again maintaining some continuity with previous forecast on
pcpn types but given these new vertical temp profile numbers, may
struggle to see FZRA hardly anywhere with the tonight-Sunday event.

Bottom line:  snow amounts were upped to 4-7" in STL metro and areas
just to their N and NE, enough to consider a warning upgrade, and
they were also upped in much of SE MO as well with less ice as a
result, but enough to probably still get a light glaze.  Sleet will
be the wildcard in southeast MO where amounts there may force a
warning ultimately.  Snow amounts were edged down in northeast MO
and may need to be further knocked down as confidence on this
southward shift grows.  At least Winter Wx Advisories to continue,
with earlier start times, for the entire area thru early Sunday
evening.

It now appears like more of a break for later Sunday night and into
early Monday evening before Round 2 appears.  But colder solutions
thru that time now favor a bit more FZRA further south into STL
metro but even then it looks to be pretty brief with rising temps
thanks to a strengthening southerly flow with warm FROPA.

TES

.LONG TERM: (Tuesday through Friday)
Issued at 358 AM CST Sat Feb 28 2015

Longwave trof bottoming out over southern California will provide
southwest flow through the depth of the atmosphere over the Mid-
Mississippi Valley on Tuesday. Medium range guidance has strong
return flow with an open Gulf of Mexico providing plenty of
moisture; although the GFS is much more aggressive than the ECMWF
in this regard. A shortwave moving off the Rockies in the
southwest flow forces lee-side cyclogenesis over Colorado and
western Kansas Tuesday morning and the low moves rapidly east-
northeast to near Chicago by 00Z Wednesday. GFS QPF is impressive
ahead of the cold front, perhaps overdone due to its stronger
moisture return, but ensemble PoPs of likely or higher look
reasonable due to the strong moisture convergence ahead of the
front. Have continued the chances for thunderstorms due to the
strong lift and good potential for unstable air to be drawn up
into the area from the south. Temperatures should fall sharply
behind the cold front as the full-latitude trof moves east into
the Great Plains Tuesday night opening the door for another very
cold Canadian airmass to move south. The low level baroclinic zone
does not move south very fast in the current model runs, and
another shortwave ripples northeast out of the base of the trof
late Wednesday night, moving over the baroclinic zone and
generating another round of precip. This precipitation looks to
fall the cold air behind the front...mainly along and south of the
I-70 corridor. Kept high chance to likely PoPs for snow in these
areas Tuesday night into Wednesday. The longwave trof will sweep
east Wednesday night and cold high pressure will finally move over
the area Thursday bringing drier weather to the area for the end
of the week. Temperatures should be well below normal for at least
Wednesday and Thursday as the Canadian high builds overhead.
Friday looks to warm up to near normal as southerly flow develops
when the high moves east of the area.

Carney

&&

.AVIATION:  (For the 06z TAFs through 06z Saturday Night)
Issued at 1143 PM CST Fri Feb 27 2015

Only minor changes to timing were made compared to prev TAF. More
uncertainty exists regarding visbys at UIN as main wave of precip
may remain just S of the terminal. However, believe that IFR
conditions are likely either thru visbys or cigs. Elsewhere,
expect main band of SN to orient itself over COU and gradually
sink S to impact SUS/CPS.

Specifics for KSTL: Only minor changes in timing were made to the
prev TAF. Believe initial band will move ewd just N of the
terminal, but will gradually sink swd. While uncertainty remains
regarding visbys beyond 00z, believe conditions will remain in IFR
cat.

Tilly

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER WEATHER ADVISORY from Noon today to Noon CST Sunday FOR
     Crawford MO-Iron MO-Madison MO-Reynolds MO-St. Francois MO-
     Ste. Genevieve MO-Washington MO.

     WINTER STORM WARNING from Noon today to 6 PM CST Sunday FOR
     Lincoln MO-St. Charles MO-St. Louis City MO-St. Louis MO-
     Warren MO.

     WINTER WEATHER ADVISORY from Noon today to 6 PM CST Sunday FOR
     Audrain MO-Boone MO-Callaway MO-Cole MO-Franklin MO-
     Gasconade MO-Jefferson MO-Knox MO-Lewis MO-Marion MO-
     Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls
     MO-Shelby MO.

IL...WINTER STORM WARNING from Noon today to 6 PM CST Sunday FOR
     Calhoun IL-Greene IL-Jersey IL.

     WINTER WEATHER ADVISORY from Noon today to 6 PM CST Sunday FOR
     Adams IL-Brown IL-Pike IL.

     WINTER STORM WARNING from 3 PM this afternoon to 6 PM CST Sunday
     FOR Bond IL-Clinton IL-Fayette IL-Macoupin IL-Madison IL-
     Montgomery IL-St. Clair IL.

     WINTER WEATHER ADVISORY from 3 PM this afternoon to 6 PM CST
     Sunday FOR Marion IL-Monroe IL-Washington IL.

     WINTER WEATHER ADVISORY from 3 PM this afternoon to Noon CST
     Sunday FOR Randolph IL.

&&

$$

WFO LSX





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