Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 280338
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
938 PM CST Fri Feb 27 2015

.UPDATE:
Issued at 928 PM CST Fri Feb 27 2015

Area of snow is across northeast OK/southwest MO/northwest AR is
having trouble pushing into south-central and southeast Missouri
due to dry low-level air. Top-down saturation has been very slow
and believe this will continue through tonight. Cannot rule out
some light snow across southeast Missouri...but probability of
measuring precipitation still appears quite low. Tweaks to rest of
tonight`s forecast included incresing sky cover for the southern
half of the area and correspondingly boosting mins a degree or two
for these areas. Near steady temperatures appear likely for some
of these locations.

Regarding the winter storm beginning tomorrow...did speed up the
onset of snow approximately 3 hours for most areas. Latest
guidance continues to indicate strong isentropic ascent in
conjunction with weak PVA and UL divergence aloft. Given that
models are notoriously slow with onset of precipitation with a
strong component of forcing via WAA...believe a quicker onset is
the way to go. With a quicker onset of snow...did boost snow
amounts for portions of the area tomorrow afternoon. Axis of
heaviest snow for the event as a whole may also need to be
adjusted slightly southward if latest 0Z NAM is correct as it has
come in colder/further southward.

Gosselin

&&

.SHORT TERM:  (Through Late Tonight)
Issued at 354 PM CST Fri Feb 27 2015

Not too many issues for the forecast overnight. Increasing high
and mid level clouds with a light wind transitioning to easterly
Saturday morning. Despite the clouds, have lowered low temps just
a bit along our eastern border mainly due to deeper snow depth. A
solid 4 - 6 inches on the ground there should keep low temps on
the low side, which matches the MAV guidance pretty good.

JPK

.LONG TERM:  (Saturday through Next Friday)
Issued at 354 PM CST Fri Feb 27 2015

Primary focus remains on the Sat/Sun system and another one early
next week. A four-segment Winter Weather Advisory will be issued
for the Sat/Sun system to handle progressive onset of pcpn (west
to east), p-type variation (snow in the north vs. mix in the
south), and very flexible headline criteria for collaboration.

Sat/Sun: Widespread pcpn expected especially Sat night into Sun.
Favorable factors include strong isentropic ascent between
290-305K, strong frontogenesis in H9-H8 & H8-H7 layers, mconv H85
LLJ on Sun, and PW values of 0.6-0.8". Initial thoughts are around
3-5" of snow across the northern CWA and about 1-3" across the
southern CWA plus a light glaze of ZR and sprinkling of IP at
times across the southern half when warmer air spreads northward
on Sun morning. Transition to liquid across the south with
continued low level warming then back to wintry mix with cooling.
Further refinements likely. Many repeated adjustments were needed
to many individual grids for near seamlessness at the expense of
other fcst details.

Quasizonal flow aloft becomes swly aloft on Sun/Mon after a piece
of the large cutoff low over CA/Baja CA ejects ewd in response to
an upstream kicker trof. Most areas probably see a break in pcpn
on Sun night/Mon between systems.

Mon/Tue: Pcpn expected with the lifting wmfnt on Mon night and
also with the sfc low on Tue/early Tue night. Favorable factors
include backbuilding H5 jet and RER dynamics on Tue, strong
mid/upper vort max, PW values in excess of 1" (AOA +2SD for Mar),
and strong mconv with H85 LLJ. Marked warmup on Tue-- highs reach
mid to upper 50s on the warm side of the warm front. Conditions
are favorable for scattered thunderstorms at times. Colder temps
return behind the cdfnt and remain across the region through the
middle/end of the week.

42

&&

.AVIATION:  (For the 00z TAFs through 00z Saturday Evening)
Issued at 537 PM CST Fri Feb 27 2015

Clouds will continue to spread newd tonight with winds remaining
aob 10 kts thru the period. Cigs will gradually lower overnight
and thru Sat. Onset of SN may need to be moved up with future
updates, but confident on timing at all sites for now. That said,
some guidance suggests SN will be possible much sooner at COU, but
only briefly. Some guidance suggest in the 09 to 12z timeframe,
however, confidence is much lower. Will continue to monitor trends
and update as needed.

Tilly

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER WEATHER ADVISORY from Noon Saturday to Noon CST Sunday
     FOR Crawford MO-Iron MO-Reynolds MO-Washington MO.

     WINTER WEATHER ADVISORY from 6 PM Saturday to Noon CST Sunday
     FOR Madison MO-St. Francois MO-Ste. Genevieve MO.

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

     WINTER WEATHER ADVISORY from 6 PM Saturday to 6 PM CST Sunday
     FOR Jefferson MO-St. Charles MO-St. Louis City MO-St. Louis
     MO.

IL...WINTER WEATHER ADVISORY from 6 PM Saturday to Noon CST Sunday
     FOR Randolph IL.

     WINTER WEATHER ADVISORY from 6 PM Saturday to 6 PM CST Sunday
     FOR Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-Fayette
     IL-Greene IL-Jersey IL-Macoupin IL-Madison IL-Marion IL-
     Monroe IL-Montgomery IL-Pike IL-St. Clair IL-Washington IL.

&&

$$

WFO LSX





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