Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 310357

1057 PM CDT Sat Aug 30 2014

Issued at 845 PM CDT Sat Aug 30 2014

Very spotty convection continues to linger along and ahead of
weak mid-level shear axis, with remnants of weak surface boundary
also aiding a bit of development attm from just south and se of
UIN into central IL. PoPs should continue to ebb into the late
evening and overnight and slowly shift east with the shear axis, but
in spite of very spotty coverage/absence of radar returns it may
be that total cessation of the convective threat will be slow to
work its way east. Other sensible weather concern for tonight is
fog potential. Very humid low level ams will remain draped over
area, and while some higher level clouds may limit radiation in
the se part of the FA, very little cloud cover and fairly light
winds over the northwest half of the CWA should mean a greater
potential for fog development in this area. Will get update out
the door after 02z ob trends are ingested.



.SHORT TERM:  (Through Late Tonight)
Issued at 329 PM CDT Sat Aug 30 2014

Weakening trof of low pressure will move over the area tonight.  The
vast majority of the mid-upper level energy associated with this
system is well north of the area over the Upper Midwest, and what
little vorticity there is in our vicinity is moving northeast away
from the area.  The surface trof is already quasi-stationary, and is
providing a nice area of surface moisture convergence over eastern
Missouri and southwest Illinois.  Expect convection to continue to
develop in this area of moisture convergence through late afternoon
into early evening.  Convection should loose steam within a couple
of hours after sunset though.  Some isolated showers or storms could
continue to survive through the evening as a weak low level jet
develops and interacts with left over boundaries, but the vast
majority of the area should remain dry overnight.  Think there will
be some fog development before sunrise Sunday, though am not sure
how dense it will be.  First thoughts are 3-5 miles with some spots
dropping lower.  With weak southerly flow over the area, expect
overnight lows to remain in the mid 60s to low 70s.


.LONG TERM:  (Sunday through Next Saturday)
Issued at 329 PM CDT Sat Aug 30 2014

(Sunday - Tuesday)

Seasonably warm conditions with drier weather is expected on Sunday.
Tropical disturbance with origins in the Gulf of Mexico looks to track
southeast of area. Cannot rule out some showers/storms over portions of
southwest Illinois...but feel that the bulk of the precipitation should
be across KY/TN.

Fairly vigorous late summer shortwave trough will quickly eject out of
the lee of the Rocky Mountains and move northeastward into the Upper
Midwest by 1200 UTC Monday. Combination of strong DCPVA and diffluence
aloft should help force an area of widespread showers and thunderstorms
Sunday night somewhere across the mid-Missouri valley. Convective
evolution is difficult to ascertain heading into Monday...but current
thought is that MCS should be able to maintain itself east/southeastward
into at least portions of central and northeastern Missouri as well as
west-central Illinois as 30-40 kt LLJ veers. Uncertainty continues to
increase thereafter as best dynamics head toward the US-Canadian border.
Trailing cold front or effective boundary from prior night`s
convection will likely stall out somewhere over the CWFA and could
provide a focus for convection in the late afternoon. Shear/CAPE
combination certainly is supportive of organized severe
thunderstorms including supercells due to orientation of deep-
layer shear vector with respect to aforementioned boundary. Risk
does look quite conditional however due to many factors including:
1) degree of instability due to antecedent clouds/precipitation,
2) weak convergence along sfc fold front, and 3) UL
dynamics/forcing displaced well to the north of CWA. Temperatures
also will be a challenge on Monday and leaned toward the cooler
guidance for highs across northern areas and more of a blend of
model guidance for southern areas. However... actual high
temperatures do have the potential of being quite a bit cooler if
the overnight MCS from Sunday night propagates further
southeastward than currently forecast.

Monday night looks wet as frontal boundary will interact with strengthening
nocturnal LLJ should produce widespread showers and thunderstorms over
portions of the mid-Mississippi Valley. Main uncertainty is to exactly where
this would transpire across the area as exact positon of sfc boundary will
be important. For those reasons...upper PoPs a bit over some areas but did
not go with categorical PoPs just yet. Heavy rain may also be a threat
with convection Sunday night - Monday night due to anomalous precipitable
water values and deep warm cloud depths.

Did lower/shift PoPs on Tuesday a bit to the south and east as latest
guidance suggests cold front or effective boundary will finally clear
CWA by Tuesday morning. Skies should at least scatter out or partially
clear from north to south into early Tuesday afternoon. Consequently...
did bump up high temps a degree or two with afternoon temps topping out
in the mid 80s to near 90 degrees.

(Wednesday - Saturday)

Active weather with chances of showers and thunderstorms looks to return
to the region Tuesday night through Wednesday as frontal boundary moves
back northward across area as upper-level ridge begins to amplify once
again. Weather pattern will essentially revert to what we have seen
predominatly over the past two weeks with upper ridging across the central
United States and a trough in close proximity to the west coast. What it
means for us...above to well above normal temperatures day and night
along with mainly dry conditions. Look for lows largely in the low 70s
and highs in the upper 80s to low 90s to end the work week. Slightly
cooler conditions may occur by next Saturday as upper ridge flattens
out a bit allowing precipitation chances and cloud cover to increase
across the region.



.AVIATION:  (For the 06z TAFs through 06z Sunday Night)
Issued at 1037 PM CDT Sat Aug 30 2014

Most of the showers/storms have dissipated this evening, although
there was patchy clouds around 5000 feet in height along with mid-
high level clouds streaming northeastward through the St Louis
metro area. As the mid-upper level cloudiness shifts slowly
southeast of the taf sites late tonight and the surface wind
becomes light the MOS guidance is hinting at the formation of
stratus clouds and fog late tonight/early Sunday morning. The
stratus clouds and fog should dissipate by late Sunday morning,
although there will likely be scattered diurnal cumulus clouds in
the St Louis metro area during the late morning and afternoon.
S-swly surface winds can be expected on Sunday and Sunday evening.

Specifics for KSTL: Much of the mid-high level cloudiness may
shift southeast of STL late tonight with light fog developing
towards morning as the s-swly surface wind becomes light. The NAM
MOS guidance was also hinting at the possible formation of stratus
clouds as well. The fog and any stratus clouds should dissipate by
late Sunday morning, although there will likely be enough residual
low level moisture for the formation of scattered diurnal cumulus
clouds late Sunday morning and afternoon. The surface wind will
increase again to around 8 kts in the afternoon from a s-swly





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