FXUS63 KLSX 260427

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1127 PM CDT Tue Sep 25 2018

Issued at 840 PM CDT Tue Sep 25 2018

At 8pm, the surface cold front extended from just east of
Springfield, IL (KSPI) to southern Lincoln County Missouri to near
Jefferson City, MO (KJEF). Surprisingly very little convection has
developed along this front in our forecast area from earlier and
the front is currently precipitation-free. Examination of
soundings showed a bit stronger mid level cap that led to much of
the frontal scale forcing being suppressed. A slight chance token
PoP will be retained until 06z as this cold front continues to
push southeast through our region.

Of a more important forecast change is the HRRR/RAP have really
picked up on post-frontal development of showers with isolated
thunderstorms currently over western Missouri due to
frontogenetical forcing (welcome to the COOL season!) and
broadscale forcing aloft. Have decided to run with this depiction
for overnight PoPs, which places many areas in likely category.
Instability will be quite limited when this does enter our
forecast area late this evening and have showers going instead.
These showers are expected to exit the forecast area to the east by
mid-morning Wednesday.

Otherwise, the rest of the forecast is on track with only minor
cosmetic changes made.



.SHORT TERM... (Through Late Wednesday Afternoon)
Issued at 330 PM CDT Tue Sep 25 2018

Isolated showers and a lone thunderstorm have developed over the
area this afternoon. Am surprised there hasn't been more lighting
as of yet given the 1500-2000 J/Kg MLCAPE on indicated by SPC
Mesoanalysis over eastern Missouri and southern Illinois. RAP
soundings are showing some low level CAPE with a mid level wedge of
warmer air around 10,000 feet. While the soundings are uncapped by
the warm mid levels, it's possible the RAP is a little too cool
aloft and updrafts aren't able to punch up high enough to cause
glaciation and lightning production. I expect that between the
instability and the approaching shortwave the convection should
eventually be able to grow tall enough to produce lightning later
this afternoon. 0-6km shear is in the 40-50kt range, with the
higher end of those values over northeast Missouri and west central
Illinois. With the aforementioned MLCAPE values and shear, will
have to remain vigilant through the early evening for severe
thunderstorms. The greatest potential should be over northeast
Missouri and west central Illinois where the highest shear and CAPE
values are superimposed. CAMS have been too strong/too widespread
with convection so far this afternoon...so I can't say I have a lot
of faith in them for the evening. That being said, the operational
HRRR seems to finally be figuring out that there may not be as much
convection this afternoon and evening as originally thought, so am
leaning toward it for guidance on coverage tonight.

Instability drops off very quickly behind the cold front tonight
which should pass from central into east central Missouri between 00-
04Z, and from there through our southeast Missouri and south central
Illinois counties by 07-08Z according to the RAP. This should put
an end to the threat for thunderstorms, but short-range models are
showing some moderate low-mid level frontogenesis behind the front
into early Wednesday morning. Will keep low chance/slight chance
PoPs in the forecast behind the front tonight, ending from northwest
to southeast through 12Z Wednesday to cover this. This looks like a
nice strong cold front, with temperatures dropping 20 degrees or
more in the cold sector. Expect lows Wednesday to be 15-20 degrees
below this morning's readings in the upper 40s to mid 50s, and highs
similarly around 15 degrees cooler in the upper 60s to low 70s.


.LONG TERM... (Wednesday Night through Next Tuesday)
Issued at 330 PM CDT Tue Sep 25 2018

By early Wed night, the ridge axis of a broad surface high
pressure center will stretch from southwest to northeast across the
region. Looking aloft, one PV anomaly is forecast to remain in TX/OK
near the base of an upper trough axis while a much stronger one
sinks southward into ND/MN and eventually develops into a low
pressure system. The surface ridge axis will be nearly overhead for
most of the night, but mid/high cloudiness associated with the TX/OK
disturbance will interfere with what would otherwise be good
radiational cooling conditions. Overnight lows are forecast to fall
into the mid-40s to low/mid-50s across the area with the coldest
temperatures in the northwestern CWA where the least cloudiness is
expected. This will likely be the coolest night out of the next
several, especially if less cloud cover occurs and leads to better
radiational cooling conditions.

The aforementioned strong PV anomaly over ND/MN continues moving
southeastward into the western Great Lakes on Thu/Thu night and
develops into a low pressure system before lifting northeastward
into Canada. This low pressure system will send a cold front into
MO/IL on Friday, but the upper forcing across our area appears
limited due to the distance from the surface and upper lows. Any
rain showers associated with this front would be limited to the
northern CWA and likely sparse to widely scattered in coverage.
Models then depict a 1025+ hPa surface high settling into the region
behind this front which will ensure quiet and mostly dry conditions
over the weekend.

Models diverge late this weekend regarding their handling of a
closed upper low which approaches the northern California coast.
Despite the timing and evolution differences, models are in vague
agreement that lee cyclogenesis associated with this system produces
a developing surface low near the Rockies which lifts the effective
frontal boundary northeastward through the CWA on Sunday/Monday. The
combination of the warm frontal passage and rising heights aloft
downstream of the upper trough will lead to warming temperatures for
Sunday through next Tuesday. Moisture convergence associated with a
low-level jet could produce isolated to scattered SHRA/TSRA early
next week.



.AVIATION... (For the 06z TAFs through 06z Wednesday Night)
Issued at 1127 PM CDT Tue Sep 25 2018

Rather solid band of showers and some thunderstorms is sliding
through our region and will affect the TAF sites, especially the
STL metro ones where it is approaching. This will take much of the
overnight to move through. Look for a period of MVFR ceilings as
well, with all sites becoming VFR by sunrise Wednesday. NW surface
winds, gusty at times to 20-25kts, will diminish Wednesday

SPECIFICS FOR KSTL: A few hours of showers are now expected
overnight, possibly a thunderstorm. This will be coincident with
with MVFR ceilings for the most part. Otherwise, VFR conditions
will settle in by sunrise Wednesday and continue through the
remainder of the valid period with gusty NW surface winds
diminishing during Wednesday morning.





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