Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 141206
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
706 AM CDT Mon Apr 14 2014
.SHORT TERM: (Through Late This Afternoon)
Issued at 304 AM CDT Mon Apr 14 2014
Cold front continues to push southeast across the CWA early this
morning. FROPA has just occurred at STL, and at the current speed
front should clear our far SE counties before 12z.
Interesting spring forecast for today as unseasonably cold air
surges into the mid-Mississippi Valley. There should be a bit of a
lull in the wet weather this morning as the region is between
shortwaves, but expect precipitation to ramp back up later this
morning and especially into the afternoon as shortwave enters the
bottom of the trof and pushes into the area.
There will be some ptype issues today as the cold air deepens across
the area. Critical thickness values drop fairly rapidly over the
northwest half of the CWA already this morning, but forecast
soundings indicate that the morning lull in precip will allow for a
slight moderation in surface/low level temps as well as a slight
increase in the freezing level. However, the next round of afternoon
precipitation will produce some evaporative cooling that will cause
low level temps to drop once again. Will take the mention of snow
as far east as a PPQ-VIH line this afternoon, but bottom line is
that ptypes will be messy due to the localized effects of
evaporative cooling on low level thermal profiles...especially this
Temps in our far SE counties will likely fall all day today as
they are just now getting into the heart of the CAA. Our NW
counties may see temps rebound several degrees this morning with
the lull in precip, but will likely fall once again this afternoon
as round 2 of the precip develops.
.LONG TERM: (Tonight through Sunday)
Issued at 304 AM CDT Mon Apr 14 2014
There are three primary forecast issues for the extended forecast:
a hard freeze tonight, a rapid warm-up on Wed, and a possible
frontal system during Fri-Sat.
Hard freeze tonight...Expect some lingering precipitation before
midnight tonight due to a vort max in the base of an upper level
trough as the trough moves east of the region. BUFKIT profiles
suggest that some of the precip may fall as snow (possibly mixed
with rain), but no accumulation is expected because the ground is
still warm. Temperatures are expected to fall into the mid 20s to
lower 30s due to the unseasonably cold air mass. Temperatures are
expected to remain several degrees below freezing for at least 4-6
hours, therefore a Freeze Warning is warranted because the
growing season has started with the recent warm weather.
Warm-up on Wed...Winds turn southerly by 12z Wed around the back
side of a high pressure center, and the pressure gradient will
tighten ahead of a developing surface low over SD/NE with a
secondary surface low taking shape over the OK/TX panhandles. The
tightening pressure gradient will produce breezy south winds on Wed,
contributing to a rapid warm-up. Went a few degrees warmer than
guidance since guidance has not been warm enough on days with
favorable warming signals. The frontal system discussed below
would tend to keep temperatures warm through Thu.
Possible frontal system late in the week...The upper vort max
associated with the aforementioned northern surface low will quickly
lift toward the Great Lakes on Wed night and Thu. Models depict a
cold/quasistationary front stretching from northern KS through
extreme northwest MO into central IA towards WI with very little
southwestward progress due to the absence of upper forcing once the
system has lifted to the northeast. This front then lifts north as a
warm front once another vort max crosses the Rockies over CO/NM and
the entire southern system lifts northeastward. Models continue to
disagree on how much interaction occurs between a vort max over
southern Canada/northern Plains and the southern system as it lifts
into the central Plains. Chance PoPs look reasonable for now.
.AVIATION: (For the 12z TAFs through 12z Tuesday Morning)
Issued at 700 AM CDT Mon Apr 14 2014
MVFR cigs and strong NW winds, with periodic IFR cigs/vsbys in
precip, can be expected across the forecast area today. More
specifically regarding precip trends...CWA will be in somewhat of
a minima this morning which will limit the precip to patchy
drizzle in most areas. However, secondary shortwave entering the
base of the trof will swing into the region this afternoon,
bringing a fairly widespread band of rain mixed with snow to the
area. Explicit models are in very good agreement with this trend,
and have timed the introduction of steady rain at all TAF
locations based on latest HRRR/NCEP 4km WRF, with periodic IFR
conditions as this rain attempts to mix with snow as evaporative
cooling drops freezing levels enough to support frozen precip.
Expect a rapid improvement in conditions late this afternoon and
early evening hours as this shortwave exits and drier air in its
wake erodes the low cloud deck.
Specifics for KSTL: Cigs of 1500-2500 ft expected this morning,
with occasional IFR conditions over the next few hours as the much
colder air surging into the region produces patchy drizzle.
Looking for a steady rain to develop by mid afternoon with the
approach of the next shortwave, with this rain possibly mixing
with a bit of snow by early this evening which would also produce
periodic IFR cigs/vsbys. The latest forecast soundings and plan
view RH progs indicate a rapid improvement to VFR conditions in
the 02-04z time frame.
MO...FREEZE WARNING from 11 PM this evening to 9 AM CDT Tuesday FOR
Audrain MO-Boone MO-Callaway MO-Cole MO-Crawford MO-
Franklin MO-Gasconade MO-Iron MO-Jefferson MO-Knox MO-Lewis
MO-Lincoln MO-Madison MO-Marion MO-Moniteau MO-Monroe MO-
Montgomery MO-Osage MO-Pike MO-Ralls MO-Reynolds MO-Shelby
MO-St. Charles MO-St. Francois MO-St. Louis City MO-St.
Louis MO-Ste. Genevieve MO-Warren MO-Washington MO.
IL...FREEZE WARNING from 11 PM this evening to 9 AM CDT Tuesday 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-Randolph IL-St. Clair IL-