Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 212032
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
332 PM CDT Fri Apr 21 2017

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 330 PM CDT Fri Apr 21 2017

Another batch of showers a few thunderstorms across south central MO
will move eastward through southeast MO and southwest IL late this
afternoon and early this evening.  Cloud tops were warming so the
intensity of the rain may weaken.  More rounds of showers or rain
are expected later tonight across central and southeast MO and
southwest Illinois as the upper level trough over the central Plains
approaches with the upper level divergence increasing ahead of the
trough due to upper level jet coupling between the upper level jet
diving into the southern Plains and the upper level jet in the Great
Lakes region.  A surface wave along the nearly stationary front
south of the forecast area will move east-northeastward across the
southern Plains tonight and into the Tennessee Valley region on
Saturday. The best QPF will occur across southeast MO and southwest
IL tonight in a region of low level warm air advection, theta-e
advection, and moisture convergence on the nose of a south-
southwesterly low level jet ahead of the 850 mb low.  Will hold off
on a flood watch for now across the southern portion of the forecast
area, although localized flooding is possible if my QPF for tonight
is underdone.  Right now have around 1.5 inches of rain for tonight
and Saturday morning across parts of southeast MO and southwest IL.
The intensity of the precipitation and QPF should lessen late
Saturday morning and afternoon with lingering rain in the
deformation zone. Temperatures will be cooler tonight and Saturday.
The coolest temperatures on Saturday will be across southeast MO in
the eastern Ozarks where low level cloud cover and rain is expected.
 Lows tonight will be slightly below normal, then about 10 to 15
degrees below normal on Saturday.

GKS


.LONG TERM...  (Saturday Night through Next Friday)
Issued at 330 PM CDT Fri Apr 21 2017

(Saturday Night)

Bulk of light rainfall should have concluded across the area by
Saturday evening. Mid/upper level ridging will build across the
Plains helping to accelerate the mid/upper level low affecting the
bi-state area further to the south and east. Lingering chances for
light rain will continue across southeastern sections of the CWFA on
Saturday night, mainly during the evening hours. Combination of a
cold start to the period and decreasing clouds from northwest to
southeast should yield a chilly night with lows about 10 degrees
below normal. Cannot rule out some frost potential in favored valley
areas given the high dewpoints, clearing sky, and light northerly
winds. Will hold off on any mention for now however given low
temperatures just a bit too warm in the upper 30s, but something to
keep an eye on as there is at least a low probability of occurrence
given the setup.


(Sunday - Monday Night)

Fairly benign weather pattern will at least temporarily take hold to
end the weekend and start off the new work week. Look for dry
weather with moderating temperatures from slightly below normal on
Sunday to back to slightly above normal already by Monday.


(Tuesday - Next Friday)

Weather pattern already begins to become more active by midweek next
week with an increasingly likelihood of active weather in the form
of showers and thunderstorms by the end of the weekend and likely
peeking ahead into the following weekend. Way too early to get into
specifics as deterministic models such as the GFS/ECMWF will likely
show high variability (and already have over the past 1-2 days) in
the details this far out. However, does appear that with anomalous
deep troughing to our west and ridging to our east that bi-state
area will be in between these two areas with several rounds of
showers/thunderstorms possible. Some of these rounds appear to have
at least the potential for strong to severe thunderstorms and/or
heavy rainfall so something to keep abreast of going forward.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1111 AM CDT Fri Apr 21 2017

Surface ridge extends from the northern Plains southeast into MO
and IL. A developing surface low over eastern OK will move east-
northeastward along a nearly stationary front and bring periods of
showers or rain to COU and the St Louis metro area this forecast
period. Current activity on radar staying mainly south of the St
Louis metro area with only a few sprinkles, but another batch of
showers across southwest MO should spread into COU and the St
Louis metro area later this afternoon and this evening. Cloud
ceilings will lower, dropping into the MVFR category this evening
in COU and the St Louis metro area, and possibly down to around
1000 feet late tonight and early Saturday morning. Northeast
surface wind will strengthen this afternoon and tonight as the
surface pressure gradient tightens.

SPECIFICS FOR KSTL: Surface ridge extends from the northern Plains
southeast into MO and IL. A developing surface low over eastern
OK will move east-northeastward along a nearly stationary front
and bring periods of showers or rain to the STL area this
forecast period. Current activity on radar staying mainly south of
STL with only a few sprinkles, but another batch of showers
across southwest MO should spread into STL late this afternoon
and this evening. Cloud ceilings will lower, dropping into the
MVFR category this evening, and possibly down to around 1000 feet
late tonight and early Saturday morning. Northeast surface wind
will strengthen this afternoon and tonight as the surface pressure
gradient tightens. The cloud ceiling should gradually improve
Saturday afternoon with the rain tapering off.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX


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