Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 170918
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
418 AM CDT Fri Apr 17 2015

.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 06z TAFs through 06z Friday Night)
Issued at 1147 PM CDT Thu Apr 16 2015

The main forecast issue continues to focus on fog development
overnight. The lingering patches of clouds from 5000-6000 ft and
high clouds spreading across the region lower confidence some, but
not enough to remove the mention from the terminals. Anticipating
that during the overnight hours, areas of fog should develop,
especially from the St. Louis area southeastward into southern IL.
I have delayed the onset from the previous forecast by a few
hours, thinking that it will be after 08-09z before visibilities
lower with potential for IFR flight conditions from around 10-14z.
By mid-morning the fog should dissipate and VFR flight conditions
will dominate with scattered diurnal cu and some high cloudiness.


Specifics for KSTL:

The main forecast issue continues to focus on fog development
overnight. The lingering patches of clouds from 5000-6000 ft and
high clouds spreading across the region lower confidence some, but
not enough to remove the mention from the terminal. Anticipating
that it will be after 09z before visibilities lower with potential
for IFR flight conditions from around 10-14z. By mid-morning the
fog should dissipate and VFR flight conditions will dominate with
scattered diurnal cu and some high cloudiness.

Glass

&&

.PRELIMINARY POINT TEMPS/POPS:
Saint Louis     82  60  77  60 /  10  10  40  80
Quincy          79  58  74  58 /  10  10  40  80
Columbia        80  60  74  59 /  10  30  70  80
Jefferson City  79  60  73  59 /  10  30  70  70
Salem           79  57  75  60 /  10  10  30  80
Farmington      78  59  73  60 /  10  10  50  80

&&

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

$$

WFO LSX





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