Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 302035

335 PM CDT Sat Aug 30 2014

.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 frontor 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 18z TAFs through 18z Sunday Afternoon)
Issued at 1240 PM CDT Sat Aug 30 2014

A trough of low pressure extending from north central Missouri
southwest into southeast Kansas will remain nearly stationary
today. This trough will provide a focus for thunderstorm
development across the area through the afternoon into early
evening. Expect scattered to numerous showers and thunderstorms
this afternoon...primarily east of the trough. MVFR/IFR flight
conditions can be expected in the areas of rain. Expect VFR flight
conditions and light south-southwest flow outside of thunderstorms
except for a few patches of higher MFR CIGS north of the STL Metro
area. Showers and storms will dissipate within an hour or two
after sunset. VFR conditions are expected until the pre-dawn hours
of Sunday morning when fog will develop. Areal coverage and
density of the fog is not certain at this time, but at least
patchy MFR fog is expected. Fog should dissipate within 2 hours
after sunrise Sunday.

Specifics for KSTL:

Expect showers and thunderstorms to increase in coverage this
afternoon. Timing and intensity is uncertain at Lambert, and there
is a definite chance there could be some intermittent heavy rain
which could reduce the visibility to IFR. However, expect VFR
conditions to prevail for the vast majority of the afternoon.
Showers/storms should dissipate quickly after sunset. With moist
air in place over the STL Metro, think there will be some light
fog at the terminal before sunrise Sunday. Fog should dissipate
within an hour or two after sunrise.



Saint Louis     71  89  74  89 /  50  10  10  60
Quincy          65  87  72  83 /  20  10  40  70
Columbia        67  91  72  87 /  30  10  20  60
Jefferson City  68  90  72  87 /  40  10  20  60
Salem           69  85  71  88 /  50  30  10  40
Farmington      69  87  71  89 /  40  20  10  30




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