Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 282202
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
402 PM CST Sat Feb 28 2015
.SHORT TERM: (Tonight through Sunday)
Issued at 400 PM CST Sat Feb 28 2015
Going forecast trends still looking good, with latest guidance
still indicating two shots of snow across the FA over the next 24 hours.
Have made some changes to snowfall accumulations (discussed
below), but current headlines still depict this winter weather
threat quite well so no changes are needed at this time.
Intial surge of snowfall with strong WAA/isentropic ascent has now
worked across the entire CWA. Radar trends are supporting short
range high res guidance which depicts strongest isentropic lift
and heaviest precip/snowfall surging from the Ozarks into
southern Illinois into the evening hours. Based on regional radar
composite and reports over SGF`s CWA, fairly heavy snowfall and
rapid accumulations can be expected across the southeast half of
the CWA, with the STL metro on the northern edges of this
With the latest guidance indicating the AMS remaining colder over
southern sections of the CWA any significant mix of ptypes should
be confined to our far southern counties, and it may be that any
"mix" will be snow transitioning to a bit of freezing drizzle as
the strongest lift exits the region later this evening. Based on
the fairly heavy shot of snow this evening, as well as the ptype
precip type remaining as snow longer than earlier expected, have
boosted snowfall totals across most of our southern counties by
several inches. Forecast is depicting a very tight snowfall
gradient between FAM and CGI, and evening shift will be watching
trends to tweak this gradient.
While there will likely be a lull in the snowfall overnight,
another round of accumulating snowfall is expected to develop later
tonight and into at least Sunday morning....associated with W-E
band of frontogenetical forcing. While intensity of this forcing
varies depending on which model you are viewing, there is
surprisingly good agreement in QPF fields on the development of
this snowfall. I`ve attempted to trim PoPs a bit in our far north
based on position of the forcing and QPF, while forecasting
another several inches of snow in this band primarily north of
I-70. It`s certainly possible that there could be a narrow
corridor of even heavier snowfall, but will leave this fine-tuning
to short range updates when the meso-scale features become defined
on radar trends.
Precipitation should wind down over southeast sections of the CWA
on Sunday evening, followed by dry weather Monday and Monday night.
.LONG TERM: (Sunday through Next Saturday)
Issued at 400 PM CST Sat Feb 28 2015
A shortwave ejects from the cutoff low near CA on Monday and induces
surface cyclogenesis near the CO/KS border on Monday night. The
surface low lifts quickly northeastward Monday night through early
Tuesday night, first swinging a warm front northeastward and then
sending a cold front southeastward through MO/IL. This low pressure
system will bring a period of active weather to the CWA, especially
on Monday night and Tuesday.
Strong moisture convergence on the nose of the H85 LLJ interacting
with the lifting warm front aloft should produce an area of
precipitation on Monday night, especially in the presence of large
scale ascent from RER H3 jet dynamics. The combination of cold air
at the surface and strong WAA aloft is favorable for freezing rain
and sleet. Sleet should be limited to the northern half of the CWA
where the depth of cold air will be greater and the warm nose will
be more shallow, at least at first. As low level temps warm up on
Tuesday morning due to a combination of WAA and diurnal rises, the
precipitation will transition from ZR or ZR/IP to RA.
Tuesday will likely be the warmest day of the week due to the
lifting warm front. Temperatures will rise into the mid 40s to mid
50s. On Tuesday, PW values rise to between 1" and 1.3", which is
around +2 standard deviations above normal for March. These values
are notable because the PW normals for March were calculated over
the entire month, and Mon/Tue will only be a couple of days into the
month. In other words, there will be an unusually high amount of
moisture available. RER H5 jet dynamics will add to the large-scale
ascent ahead of the upper vort max. Expect widespread rain with
scattered embedded thunderstorms on Tuesday into early Tuesday
night. Some of the precipitation may change back over to RA/SN or SN
behind the cold front within the colder air.
The aforementioned cold front should be moving through the CWA by
00z Wed. An Arctic high pressure center then spills into the region
behind the cold front, bringing temperatures back down into the 20s
on Wednesday and Thursday. The remainder of the cutoff low over CA
will become absorbed into the flow and lift eastward on Wednesday
night and Thursday. Temperatures may rebound back towards more
seasonable values on Friday after the Arctic high pressure center
shifts eastward and return flow commences.
.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.
.PRELIMINARY POINT TEMPS/POPS:
Saint Louis 26 35 22 35 / 100 80 10 10
Quincy 21 31 14 32 / 90 90 10 10
Columbia 25 34 20 37 / 100 80 10 10
Jefferson City 27 35 20 38 / 100 80 10 10
Salem 27 34 23 35 / 100 80 20 10
Farmington 27 37 23 37 / 90 70 20 10
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 WEATHER ADVISORY until Noon CST Sunday FOR Randolph IL.
WINTER STORM WARNING until 6 PM CST Sunday FOR Bond IL-Calhoun
IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-
Madison IL-Montgomery IL-St. Clair IL.
WINTER WEATHER ADVISORY until 6 PM CST Sunday FOR Adams IL-Brown
IL-Marion IL-Monroe IL-Pike IL-Washington IL.