Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service St Louis MO
410 PM CST Fri Feb 27 2015

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 h9-800 mb & 800 mb-700 mb layers, mconv 850 mb
low level jet on sun, and precipitable water values of 0.6-0.8". Initial thoughts are around
3-5" of snow across the northern County Warning Area and about 1-3" across the
southern County Warning Area plus a light glaze of freezing rain and sprinkling of ice pellets 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 California/Baja California 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 500 mb jet and rer dynamics on Tue, strong
mid/upper vort Max, precipitable water values in excess of 1" (aoa +2sd for mar),
and strong mconv with 850 mb low level jet. 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 18z tafs through 18z Saturday afternoon)
issued at 1050 am CST Fri Feb 27 2015

VFR forecast for the most part. MVFR ceilings may make their way
into the area depending on the speed of the next system. Looks
like a pretty good warm air advection snow for Saturday night
into Sunday.

Specifics for kstl: VFR through 18z Saturday, then will bring in
some MVFR ceilings late in the period as the weekend system moves
closer. Timing right now is a question, so will mainly introduce
as an "alert"feature.

Jpk

&&

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.

Illinois...Winter Weather Advisory from 6 PM Saturday to noon CST Sunday
for Randolph Illinois.

Winter Weather Advisory from 6 PM Saturday to 6 PM CST Sunday
for Adams Illinois-Bond Illinois-Brown Illinois-Calhoun Illinois-Clinton Illinois-Fayette
Illinois-Greene Illinois-Jersey Illinois-Macoupin Illinois-Madison Illinois-Marion Illinois-
Monroe Illinois-Montgomery Illinois-Pike IL-St. Clair Illinois-Washington Illinois.

&&

$$

Weather forecast office lsx

National Weather Service Glossary of Abbreviations