Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 281809
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1209 PM CST Sat Feb 28 2015

.UPDATE:
Issued at 1206 PM CST Sat Feb 28 2015

Snowfall tracking east about as expected, and changes for this
afternoon are largely cosmetic.

May need to make some adjustments to snowfall totals in the
afternoon package, especially in the south where latest guidance
seems to be suggesting snow hanging on a bit longer than earlier
expected. However, will try to get a better handle on that over
the next few hours before making any significant changes to
forecast.

Truett

&&

.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 18z TAFs through 18z Sunday Afternoon)
Issued at 948 AM CST Sat Feb 28 2015

Snow gradually moving into the area. Looking at isentropic
surfaces it looks like the best lift will be this afternoon up
until about 06z. Ligth snow still beyond that but significant
accumulating snow may taper off. Timing for COU around 18z and UIN
about 21z. Will back off on the quick drop to IFR ceilings based
on what is out there. Plenty of sites going 3 - 5sm -sn with VFR
ceilings. But once it sets in, IFR will stay around through 18z
Sunday. Southeast wind will remain until front moves in Sunday.

Specifics for KSTL: At this time, I don`t see IFR right off the
bat at the beginning of the snow. That has not happened at sites
to the southwest so will back off a bit and bring in IFR more
gradually, by 00z. After that, not much change with light snow and
IFR ceilings likely through 18z Sunday if not longer.

JPK

&&

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

     WINTER STORM WARNING until 6 PM CST Sunday FOR Lincoln MO-St.
     Charles MO-St. Louis City MO-St. Louis MO-Warren MO.

     WINTER WEATHER ADVISORY until 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 until 6 PM CST Sunday FOR Calhoun IL-Greene
     IL-Jersey IL.

     WINTER WEATHER ADVISORY until 6 PM CST Sunday FOR Adams IL-Brown
     IL-Pike IL.

     WINTER STORM WARNING until 6 PM CST Sunday FOR Bond IL-Clinton
     IL-Fayette IL-Macoupin IL-Madison IL-Montgomery IL-St.
     Clair IL.

     WINTER WEATHER ADVISORY until 6 PM CST Sunday FOR Marion IL-
     Monroe IL-Washington IL.

     WINTER WEATHER ADVISORY until Noon CST Sunday FOR Randolph IL.

&&

$$

WFO LSX






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