Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 010820
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
320 AM CDT Fri Aug 1 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 320 AM CDT Fri Aug 1 2014

Primary forecast issue for today is PoPs.  Dirty surface ridge
remains in control of the weather today.  A shortwave will pass over
the area on northwest flow aloft.  Looks like a combination of broad
mid-level lift ahead of the shortwave and weak low level moisture
convergence east of the surface-850mb reflection of the shortwave
which is causing the precip currently over northwest Missouri.
Expect these features to continue to migrate southeastward through
the day, and short-range guidance has the shortwave sharpening up as
it approaches the base of the longwave trof east of the
Mississippi.  All short-range guidance has precip over the area to a
greater or lesser extent today, so have increasing PoPs through the
day from slight chance to chance category as diurnal heating becomes
a greater factor.  However, with the lack of a true surface trigger
have kept PoPs at 30% or less.  This may ultimately need to be
increased if afternoon thunderstorms can get organized into
clusters.  This is certainly possible if forecast SBCAPE 1000-2000
J/kg is realized.  Highs today will be highly dependent on cloud
cover and afternoon precip.  Used a combination of persistence and
MOS which yielded temperatures a degree or two cooler than yesterday
in the low to mid 80s.  Still relatively mild for August 1st.

Carney

.LONG TERM:  (Tonight through Thursday)
Issued at 320 AM CDT Fri Aug 1 2014

A longwave upper TROF will persist over eastern North America thru
Tuesday, although it will slowly de-amplify heading into early next
week.  Some upper level disturbances from this resultant NW flow can
be expected early, with one this evening--a leftover from today--and
another possibly rolling down for Saturday.  What does develop today
should carryover a bit into this evening and maintained a
mentionable slight chance PoP until about 9pm with the loss of
heating and upper level support moving downstream then should shut
things down.  Another upper level disturbance may slide down on
Saturday as well, although the signals for this are considerably
less clear and moisture should be a bit more limited than with the
first disturbance today with the latter taking some of the moisture
with it as they are oft to do in the wake of passage of an upper
TROF axis.  Kept some low PoPs around for Saturday, maintaining the
timing of late morning and afternoon, but trimmed its expanse and
confined more to the eastern Ozarks and southwest IL.

Otherwise, lack of upper level support and surface fronts with
marginal levels of moisture for August will result in PoPs largely
being a wash and favor values below climo--climo being around 20%.

By mid-late next week, a storm over the upper Midwest appears to get
caught in the upper level "undertow" and stalls as the overall
regime becomes more zonal and split, and so not as progressive as
one might initially think.  A warm frontal boundary appears to
extend into the area late Wednesday with approach of a cold front on
Tbursday representing the best widespread chance for rain over the
next seven days--with PoPs to show for it--about 40%.

Max temps on Thursday were already within 2-3 degrees of normal and
should effectively be at normal values by the latter half of the
weekend and will even edge above normal for a period next week
before another return of some cooler air for late week.

TES

&&

.AVIATION:  (For the 06z TAFs through 06z Friday Night)
Issued at 1109 PM CDT Thu Jul 31 2014

Still expect some areas of fog to develop between 09-13Z tonight
as they have the past few nights. Then an upper level disturbance
will move through the area on Friday which will bring a slight
chance of thunderstorms to the area. Think the best chance is
still over west central Illinois, so have continued the VCTS
between 17-22Z. Still too much uncertainty at the St. Louis metro
TAF sites, so will leave the VCTS mention out for now. Otherwise,
expect VFR conditions.

Specifics for KSTL: Have gone with a dry and VFR TAF with light
winds. There is a slight chance of thunderstorms on Friday
afternoon, though there is enough uncertainty in the timing to
leave them out of the forecast for now.

Britt

&&

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

$$

WFO LSX




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