Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 192358
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
658 PM CDT Tue May 19 2015

.SHORT TERM:  (Through Late Tonight)
Issued at 322 PM CDT Tue May 19 2015

Shortwave over Colorado will move east across the central Plains
tonight.  Resulting low level wave will develop and move along the
850mb baroclinic zone tonight into early Wednesday morning.  Strong
low level jet ahead of the low level circulation will be pointing
directly at the baroclinic zone out over the eastern Plains and into
western Missouri late tonight.  Expect there to be plenty of rain
west of our area tonight, approaching the Columbia/Jefferson City
areas toward 12Z Wednesday as the wave moves into eastern Kansas.
Don`t think there will be much more than some light rain or
sprinkles, but probably enough to get the ground wet.  MOS
temperatures for tonight in the upper 40s to low 50s look reasonable.

Carney

.LONG TERM:  (Wednesday through Next Tuesday)
Issued at 322 PM CDT Tue May 19 2015

(Wednesday through Friday)

Still looks like tomorrow will be cool with showers as upper trough
now over the southwest CONUS moves northeast and shears out as it
moves into confluent flow over the the Midwest.  The GFS still looks
have a bit too much gridscale feedback with this system while the
ECMWF looks like it has better continuity the past 24 hours, so have
followed it.  There is still quite a bit of ascent with the trough
along with low level moisture convergence ahead of it which warrants
categorical PoPs.  Some limited ascent will linger over the
southeast half of the CWA into Wednesday evening before subsidence
sets in over the area behind the trough for Thursday.  An upper
ridge will be building into the region from the west on Friday which
will continue to keep the area dry.

Still expect cooler than normal weather during this period because
of the cool, dry northerly winds.  This will be particularly
noticeably tomorrow with the rain when highs will be around 15
degrees below normal. There will be some moderation thereafter as
high pressure moves over the area and sunshine returns to the
area. A compromise on MOS guidance looks reasonable the next few
days.

(Saturday through next Tuesday)

ECMWF/GFS/UKMET look in a little better agreement today with the
timing of the upper ridge axis that will not move across the area
until Saturday now.  This means that scattered showers and
thunderstorms will likely be delayed until Saturday afternoon over
mainly Missouri now.  The global models are also in better agreement
that upper low over southwestern CONUS will open up and lift
northeastward over the holiday weekend which will allow for an a
series of vort maxes to move over the area with an open Gulf.  Will
likely see a few rounds of showers and thunderstorms over the
weekend that will be timed out by the aforementioned vort maxes and
low level moisture convergence.  Will keep high chance or likely
pops until the upper trough passes through the area on Tuesday.
Temperatures over the weekend will warm up back closer to normal
despite the rain chances because of the surge of warm, moist air
from the south and +15C 850mb temperatures.

Britt

&&

.AVIATION:  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 658 PM CDT Tue May 19 2015

Deteriorating conditions are expected beginning late tonight and
continuing into Wednesday morning from west to east across our
region as first rain, then lower CIGs move in. At some point, we
should see IFR conditions at COU, with STL metro continuing to be
on the threshold and have held off with this issuance. Otherwise,
low-end MVFR will be the rule for much of Wednesday. Prefer the
faster timing presented with the 18z GFS model run, which brings
rain in a few hours earlier than the previous TAF package, but
also exits it a bit quicker too, with a likely hand-off from light
to moderate rain from VFR CIGs to drizzle with low-end MVFR or
IFR CIGs during the day. Otherwise, NE surface winds will prevail
for much of this period.

Specifics for KSTL: Have sped up the timing by several hours in
light of several new model runs as well as based on the main
driving forces of the rain itself. Now expecting rain to move in
shortly after sunrise and should be a bit heavier initially, and
this expected to be followed by low-end MVFR CIGs and drizzle
during the afternoon. Models are showing KSTL being right on the
edge of IFR CIGs and want to take another look before adding this
in, and with the expected timing not to be until afternoon anyway,
have time to watch this.

TES

&&

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

$$

WFO LSX



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