Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 011758
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1258 PM CDT Wed Apr 1 2015

.UPDATE:
Issued at 1051 AM CDT Wed Apr 1 2015

Made a few tweaks to the forecast this afternoon into the evening
hours. MCV over northeast Oklahoma is drifting northeast into
Missouri. Showers and a few storms are ongoing over over southwest
Missouri at this time, and it looks possible that they could
spread into parts of central Missouri by late this afternoon.
Most short range guidance does overspread some rain from the
southwest into the CWFA, so have bumped up PoPs to chance/slight
chance category generally along and south of I-70 in Missouri.

Carney

&&

.SHORT TERM:  (Through Late This Afternoon)
Issued at 354 AM CDT Wed Apr 1 2015

An upper ridge axis will pass overhead today. At the surface, a
boundary which was located near the MO/AR border last night will
be drawn northeastward as a warm front during the day in response
to a developing low pressure system over the northern plains. The
thermal ridge axis will be over central MO and good mixing
conditions are forecast. Based on these factors, people in MO/IL
can expect another day of unseasonably warm temperatures with
afternoon highs reaching the mid to upper 70s. Attention then
turns to fire wx. Although winds will be rather breezy at times
due to the tightening pressure gradient, increasing dew points
throughout the day will probably keep RH values too high to meet
Red Flag Warning criteria but low enough to meet criteria for
Heightened Fire Danger. This will be mentioned in the Fire Weather
Forecast (FWF) and Hazardous Weather Outlook (HWO) products.

Kanofsky

.LONG TERM:  (Tonight through Tuesday)
Issued at 354 AM CDT Wed Apr 1 2015

One area of convection is possible late this afternoon and early
tonight near the eastern Ozarks where instability will be a bit
higher compared to farther north. Another area of convection is
expected to develop tonight near a cold front which will stretch
from KS through NEB into IA. As decaying upstream convection moves
southeastward, precipitation chances across the LSX CWA will
increase overnight, especially after midnight and especially
across northeast MO/west central IL. PW values are forecast to be
around 0.8-1" tonight, which is near the 75th percentile for April
in this part of the country.

The cold front will sag southward on Thu, eventually becoming
oriented W-E. Ahead of the front, PW values rise to around 1.3" by
18z, which is at +2SD for April. Despite the cloud cover which
will inhibit diurnal destabilization, an increasingly moist air
mass will be spreading northeastward ahead of the cold front. Any
breaks in the clouds will add to the instability. Another round of
thunderstorms is expected to develop on Thursday afternoon,
especially south of I-70. Some of the storms could be strong to
severe based on forecasts of 40-60 kts of 0-6km bulk shear. The
shear vectors are forecast to be oriented nearly parallel to the
approaching cold front.

The cold front`s southward progress on Thu night may be delayed
by a new surface low which develops on the tail end of the front
in KS in response to an approaching shortwave. The surface low
then rides along the boundary on Thu night and Fri, and the cold
front finally sinks south of the area late Thu night or early Fri
morning.

Compared to the late spring warmth of the last few days, Fri`s highs
in the 50s and Fri night`s lows in the mid-30s will seem like a
blast of winter. Temperatures will begin to rebound slightly over
the weekend after a surface high pressure center shifts southeast
of the area and low-level flow becomes southwesterly around the
back side of the high. The area remains beneath northwest flow
aloft this weekend. Another low pressure system could bring
precipitation to parts of MO/IL early next week.

Kanofsky

&&

.AVIATION:  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1242 PM CDT Wed Apr 1 2015

Area of showers with embedded thunderstorms over southwest
Missouri will continue moving northeast this afternoon. Not sure
what the coverage will be as it gets into our forecast area, but
scattered seems likely. Flight conditions should remain VFR except
under heavier rain/thunderstorms. Rain should dissipate slowly
through mid to late evening as it continues moving northeast.
Still expecting LLWS conditions tonight as the low level jet
increases ahead of an approaching cold front. Thunderstorms are
expected to develop tonight over Nebraska and move southeast ahead
of the front and should push into northeast Missouri between
10-12Z. Storms should be weakening as the move into our area, but
will likely still be strong enough to produce MVFR and possibly
brief IFR conditions. As the storms continue to move east into
eastern Missouri and southwest Illinois they will continue to
weaken until there will be little if any thunder left by 14-16Z.
Another round of strong storms is expected ahead of the front
Thursday afternoon.

Specifics for KSTL:

Area of showers with embedded thunderstorms over southwest
Missouri will continue moving northeast this afternoon. Believe
the rain will be dissipating as it moves toward the STL Metro area
this evening, but several pieces of guidance do show thunderstorms
in the vicinity of the terminal for a short time this evening. Flight
conditions should remain VFR unless a thunderstorm hits the terminal
directly. Still expecting LLWS conditions tonight as the low level
jet increases ahead of an approaching cold front. Thunderstorms
are expected to develop tonight over Nebraska and move southeast
ahead of the the front. By the time the storms reach Lambert
around 14Z they should be weakening. Another round of strong
storms is expected ahead of the front Thursday afternoon.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX






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