Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 100344
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
944 PM CST Mon Dec 9 2013
Issued at 938 PM CST Mon Dec 9 2013
Have updated the forecast to basically push the northern bounds of
higher pops and measurable snowfall a bit south. Overall this
evening, the snowfall has been slower to spread east northeastward from
southwest Missouri into the southern CWA. Also where it has
developed the coverage and intensity has been less than expected - thus
far spotter reports indicating less than a half inch in Crawford
County. Radar trends combined with the recent runs of the RAP/HRRR
and new NAM suggest the northern edge will be just south of St.
Louis across northern sections of Jefferson...Monroe...and St.
Clair counties. Still thinking we will see a band of 1-2 inches
generally along a Steelville-Sparta-Nashville line where mid level
frontogenesis and large scale ascent are maximized for the next
3-5 hours...with maybe a few embedded higher amounts.
.SHORT TERM: (Through Tonight)
Issued at 233 PM CST Mon Dec 9 2013
Focus tonight will be approaching system with another quick shot at
Latest mdl guidance has shifted sn a little further nwd tonight and
have adjusted POPs nwd somewhat. SN is beginning to reach the ground
across sern KS and nrn OK and will continue to push ene into swrn MO
this afternoon. Light sn may reach srn portions of the CWA by late
this afternoon, but accumulations are expected to be minimal thru
the afternoon hours. This current band is expected to diminish over
swrn MO with another redeveloping just to the south. This recycling
band will probably occur again as the upper trof approaches the
Mdls also increased amount of liquid precip with this system with
some areas across the ern Ozarks possibly receiving around one tenth
of an inch of liquid precip. With a fairly sizable portion of the
atmo within the DGZ, have used a high SLR. Cobb algorithm suggests
an SLR of 15 to 20 to one. To account for the entire area, have used
a conservative 16:1 ratio.
Would not be surprised to have a couple locations end up with around
three inches of snowfall from this system. However, believe most
areas will receive two inches of sn or less. Have therefore held off
on any headlines for now.
As for temps tonight, with snowpack over much of the area and with
clouds expected to clear late tonight, have trended twd coldest
guidance again tonight.
.LONG TERM: (Tuesday through Next Monday)
Issued at 304 PM CST Mon Dec 9 2013
Next shortwave expected to move north of the CWA Tuesday night,
sending a cold front across the region on Wednesday. Believe
snowfall with this feature will remain north of the CWA. Another
cold surface ridge will build into the Midwest Wednesday night and
Thursday morning providing ideal radiational cooling conditions.
Have went below guidance for low temperatures, especially across
areas that have snow on the ground.
Southerly flow ramps up ahead of the next shortwave Thursday night
with clouds and precipitation rapidly developing northeast across
the CWA on Friday. Have tried to nail down some precipitation type
trends with the lastest model guidance. Conceptially it appears that
area will still have cold/dry airmass overhead when precipitation
develops, which will support snow/sleet at the onset.
Temperatures will try to warm on Friday, but 2m temperatures
forecast by the models show a persistent dip in the isotherms
across the CWA indicating the power of evaporative cooling.
Precipitation will probably transition to freezing rain or rain
along and south of I-70, depending on surface temperatures, Friday
afternoon as 850mb temperatures briefly rise above freezing.
Colder air will begin to transition the winter mix back to snow from
northwest to southeast Friday night as the inverted trof/surface low
passes east of the CWA. Tad early to determine amounts of any
winter precipitation but it certainly looks like at least an
advisory type event. Have increased PoPs to likely and will include
in the Hazardous Weather Outlook.
.AVIATION: (For the 00z TAFs through 00z Tuesday Evening)
Issued at 536 PM CST Mon Dec 9 2013
Primary concern for tonight is the band of snow overspreading the
area from the southwest. Expect primarily MVFR flight conditions
in light snow along and south of the I-44 corridor in Missouri and
southeast of the I-70 corridor in Illinois. There will likely be
bands of heavier snow where VSBYS will drop below 2SM and CIGS
will drop below 2000ft. Expect the snow to taper off and end after
06Z. The remainder of the forecast should be VFR with the wind
backing to the west-southwest and becoming gusty Tuesday morning.
Specifics for KSTL:
The primary concern at Lambert tonight is the band of snow
overspreading the area from the southwest. Lambert looks to be on
the northern edge of the potential snowfall gradient once again.
Think flight conditions will remain VFR throughout the event, with
only light snow or flurries. I cannot rule out heavier snow, but
it looks unlikely at this time. The snow should taper off and end
by 05Z. VFR flight conditions will continue to prevail through
Tuesday. The wind will back to the west-southwest and become gusty