Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 251804
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
104 PM CDT MON APR 25 2016

.SHORT TERM...  (Through Late This Afternoon)
Issued at 355 AM CDT Mon Apr 25 2016

Main focus today will be chances for thunderstorms.

Convection from early this morning continues to dissipate as the
upper level trof shears out and a loss of low level forcing. As the
trof lifts nwd, height rises build into the area, which shud
suppress convection thru at least the morning hours.

Mdls depict a outflow bndy or pre-frontal trof draped across the
nrn portions of the CWA later today. With the cdfnt remaining N of
the area, this bndy will be the focus for convection this afternoon.
Heating shud be sufficient to erode the cap in place, allowing
storms this afternoon. That said, there is some question on
placement of storms given the lingering convective debris. Still, it
appears there should be enuf MLCAPE, around 1000 J/kg according to
the more conservative GFS, as well as deep layer shear to support a
few potentially severe storms, lingering into the evening hours.
These values seem appropriate given the warm dewpoint bias in the
NAM.

As for temps, the sfc bndy and convective debris mentioned above
lead to uncertainty in the temps for today. Believe the cool spot in
the CWA will be mainly the nrn counties. Believe the warmer areas
will be generally from KJEF to KPPQ and areas to the SE with warmest
temps along a KVIH to KSTL line, just ahead of the surface bndy.
These temps are well depicted in the MOS and have trended at the
warmer MOS for the area. The RAP and local 4km WRF suggest even
warmer temps, but too many questions in cloud cover today to go that
warm attm.

Tilly

.LONG TERM...  (Tonight through Sunday)
Issued at 355 AM CDT Mon Apr 25 2016

The weather is about to get a lot more active for this week, with a
sustained period for the threat of severe storms.

An upper level Rex Block covering much of North America will
continue to prevail thru much of this week, with a RIDGE over the
Canadian Prairie and TROFs flanking it on the coasts.  Complimenting
this will be a series of strong Pacific storm systems that will run
just south thru the CONUS, being dropped off from the west coast
TROF and being picked up by the east coast TROF a few days later,
with an overall track to the E-NE thru our region.  Model progs show
that each successive storm system will track a bit further south
than its predecessor.

The first storm system will be well on its way to becoming absorbed
into the main flow by this evening, but the main focus at this point
will be where an old convective surface boundary will be.  Several
models are in good agreement on firing this up either just before
00z/Tue or right at, when the CINH aloft can weaken just enough, and
once it gets going, should be able to sustain development for a good
portion of the evening, if not into the late night.  Enough
instability may be able to be tapped during a small window of time
early this evening for isolated severe storms possible.

A cold front will drop down very late tonight into Tuesday and then
stall at some point over our forecast area, probably just north of I-
70, and will maintain a chance for storms.  Should be decent
moisture pooling near the front by Tuesday afternoon, and while
shear is sub-par, MLCAPE near the stalled frontal boundary will be
around 3000 J/kg and would support some severe storms thru magnitude
of instability alone.

The second Pacific storm system will be more of a direct hit as it
will initially correspond well with the surface frontal boundary
buckling north as a warm front Tuesday night into Wednesday morning.
Along with an increase of broadscale lift, this should result in
elevated storms, with hail as their primary severe threat during
that time.

Heading into Wednesday afternoon and evening, things look much more
muddled than they did 24-48hrs ago, with the severe threat and mode
still heavily dependent on how far north the warm front gets by this
time.  Models continue to show enough variation with frontal
position for low-medium confidence, which for forecast purposes for
now will be near or just north of the I-70 corridor.  If the rain
coverage is as high as what several model solutions are suggesting,
this may also work to lower the severe potential on this day,
limiting instability more than previously thought.  Either way,
shear will increase substantially as the storm system approaches and
instability may not need to be as high as what it was Tuesday anyway
to justify a continuation of a severe risk.

Things dry out briefly from late Thursday thru early Friday before
the third storm system approaches from the west for the upcoming
weekend and rain chances ramp up again.  But with this storm system
expected to track the furthest south, this next episode of rain
could very well be mostly showers with imbedded storms if this track
holds, with any severe potential remaining to the south.

Otherwise, daytime max temps will begin much above average this
week, but then slide back towards average for late week and the
upcoming weekend.  If the models showing the loss of the western
TROF is accurate for late week, this should result in a slide
towards more of a NW flow aloft pattern for the following week, and
could yield a period of more tranquil wx.

TES

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1245 PM CDT Mon Apr 25 2016

Main concern through Tuesday afternoon will be on chances of
showers and thunderstorms. Cold front will move into the northern
sections of the forecast area late this evening and eventually
become stationary overnight tonight. Chances of showers/storms
hard to pin down tonight and especially on Tuesday but believe
scattered activity will develop by mid evening and to persist
through the night. Given the uncertainty in timing/placement of
this expected activity...just have VCTS groups in for now.

Consensus of guidance also suggests ceilings lowering into MVFR
overnight tonight and lasting into early Tuesday morning before
lifting.

Complicated scenario for tomorrow. Not sure how much convective activity
will be still ongoing at daybreak but frontal boundary will remain
draped across the CWA on Tuesday. Believe there likely will be a
lull however in activity from late morning to early afternoon.
Additional activity may develop by mid afternoon with the best
chances right along the frontal boundary.


Specifics for KSTL:

Cold front will stall near the terminal late tonight and bring a
chance of showers/storms. Latest HRRR also suggests some scattered
activity a bit earlier along a prefrontal trough though not sure
if this actually will develop. Guidance suggests MVFR ceilings
also coming in around daybreak and lasting into late Tuesday
morning. Convective trends looking into Tuesday are highly
uncertain but with frontal boundary remaining near the
terminal...storms could develop at any time.

Gosselin

&&

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

$$

WFO LSX



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