Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 171755
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1255 PM CDT Fri Apr 17 2015

.UPDATE:
Issued at 1255 PM CDT Fri Apr 17 2015

Another day with a healthy diurnal cumulus cloud field which
should slow the temp rise to a crawl for the remainder of the
afternoon, but will still yield temps around 80 for most
locations.

Best chances for convective rain this afternoon, small as they
are, will be in central MO where band in southwest MO will attempt
to hold together as it moves north. Air not very favorable for
this to happen and so have maintained dry forecast but will bear
some watching over the next few hours.

Taking a sneak peak at tonight...once the leftovers of the band of
showers and storms weakens or dissipates by early this evening, it
may be quite a while before the next chance for rain makes a run
on our area. Some of the latest model indications are suggesting
limiting any mentionable PoPs tonight to just central and some
parts of northeast MO and only in the hours immediately preceding
dawn.

TES

&&

.SHORT TERM:  (Through Late This Afternoon)
Issued at 327 AM CDT Fri Apr 17 2015

Deep cutoff low over the Four Corners has amplified the upper level
ridge over the mid-Mississippi Valley.  This ridge should hold fast
today as the low moves slowly east into Colorado.  The ridge should
provide enough subsidence to keep the area dry today, and with 850mb
temperatures in the 12-14C range the MOS highs in the upper 70s to
low 80s range look good.  The low will continue drifting northeast
tonight and should be in eastern Colorado by 12Z Saturday morning.
Have slowed PoPs down marginally with only a slight chance edging
into central Missouri this evening, and then low chance overnight.
With increasing clouds and southerly flow, should see overnight lows
only dip into the upper 50s to low 60s.

Carney

.LONG TERM:  (Tonight through Thursday)
Issued at 417 AM CDT Fri Apr 17 2015

Should see a general increase in shower and thunderstorm activity
through Saturday afternoon into Saturday evening as the ridge pushes
east into the Ohio Valley and the low opens up and begins moving
more rapidly east-northeast.  There is some discrepancy between the
NAM and ECMWF which keep much of the CWFA dry on Saturday vs. the
GFS which spreads a fairly large swath of light QPF across the
entire CWFA.  In the end, certainly cannot rule out thunderstorms
over the area, but think the best chances will be over central and
northeast Missouri which will be deeper into the broad ascent ahead
of the low.  Saturday looks a bit cooler due to increasing clouds
and rain; but temperatures should still hit the mid to upper 70s,
especially further east where rain is less likely.

Sunday`s weather looks complex as the low finally opens up fully and
the resulting highly amplified wave moves into Missouri.  The
surface reflection of the wave with what looks like a fairly
complicated set of low pressure centers moves through
Sunday afternoon/evening with another round of showers and
thunderstorms.  DY3 SPC outlooks is still 15%..  GFS forecast is
perhaps a little weaker with the instability than yesterday`s
runs...showing 500-1200 J/kg MLCAPE but still showing 30-40kts of
0-6km shear Sunday afternoon.  NAM is showing similar parameters,
with a little more instability especially across southeast Missouri.
Am feeling a little more confident in the possibility of severe
weather given the decent continuity between models and the
run-to-run consistency noted over the past couple of days  Will
therefore go ahead and start mentioning the possibility of severe
thunderstorms on Sunday in the hazardous weather outlook.

Still looks like a cool and dry start to next week as another deep
cutoff low moves across the Upper Midwest/south central Canada
allowing cool Canadian air to drop through the northern Plains into
the Midwest.  Guidance is still advertising temperatures around 5-10
degrees below normal at least through Tuesday.  Some warmer
temperatures and higher moisture filters in Wednesday into Thursday
as low level flow turns back to the south-southwest.  Still looks
like a chance for more thunderstorms...most likely Wednesday night
into Thursday as  warm advection and lift from a passing shortwave
combine over the area.  GFS is much more aggressive than the ECMWF
with this precip, starting it as early as Tuesday evening.  ECMWF
holds off until Wednesday afternoon.  Have held PoPs to slight
chance/low chance category because of the discrepancies.

Carney

&&

.AVIATION:  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1255 PM CDT Fri Apr 17 2015

VFR conditions are expected at the TAF sites thru the valid
period. Small rain chances exist for COU both around 00z as what
should be a weakening band of SHRA approaches and during Saturday
morning with the outer edges of a more organized complex edging
thru. The rain chances are enough in doubt for around 00z that we
went dry at this time, but did add VCSH mention for Saturday
morning. Otherwise, rain chances to minimal elsewhere to mention
in TAFs. Easterly flow at or below 10kts will gradually veer
southeast on Saturday.

Specifics for KSTL: VFR and dry thru the period. Rain chances too
small to mention but outside chance will be during the day on
Saturday but indications are increasing on keeping this west thru
00z/Sun. Otherwise E winds becoming SE.

TES

&&

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

$$

WFO LSX



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