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000
FXUS63 KLSX 171112
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
612 AM CDT Wed Apr 17 2024

.KEY MESSAGES...

- Abnormal warmth will persist across the region today,
  particularly along and south of I-70. A strong west wind,
  coupled with low relative humidity and the warm temperatures,
  will introduce elevated fire danger in southeast Missouri and
  southwest Illinois this afternoon.

- Another round of strong to potentially-severe thunderstorms
  will threaten the region tomorrow, particularly along and
  southeast of a line extending roughly from Jefferson City, MO to
  Quincy, IL. While the potential is conditional, damaging wind
  and large hail would be the dominant threats if strong
  thunderstorms materialize.

- Temperatures cool noticeably tomorrow night and remain
  seasonable at best through the weekend.

&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 325 AM CDT Wed Apr 17 2024

Mostly clear skies prevail as gusty west winds and drier air
behind a weak dryline advect into the region from the Great
Plains. The occluded surface low that currently dominates our
sensible weather is inching east across northern Iowa. Persistent
westerlies aloft exist across the bi-state region, with a cutoff
low to our north. Skies will remain clear today, and in absence of
any cold air advection, the unfiltered sunshine will send
temperatures back into the mid-70s (north) to mid-80s (south).
Strong surface heating will cause lapse rates to jump considerably
into the afternoon and promote efficient vertical mixing. This
will act with the weakening (yet still sufficient) pressure
gradient in place to keep winds gusty generally out of the west.
The strong mixing and dry west winds will also send relative
humidity values lower along and south of the I-70 corridor in
Missouri and Illinois. With RH values dropping well-below 30%, and
sufficiently-strong winds persisting, elevated fire danger will
exist across the region mentioned above. Caution must be taken
with any open flames from roughly noon to 7pm, despite greenup
being well underway.

Attention then turns to a few developing areas of convection
overnight in the central Great Plains. By this point, the flow
pattern aloft becomes nearly zonal with an anomalous longwave trough
developing over the north-central CONUS and southern Canada. At the
surface, the weak dryline/cold front will stall to our south
across the Ozarks and Ohio Valley. During the late evening, mid-
level ascent becomes more prominent across the central Plains. A
weak surface low develops amidst this upper-level ascent and
quickly ejects into the Plains, drawing moisture and instability
northward. All of these features converge to spark convection on
the nose of an 850mb low-level jet in the Plains. CAMs uniformly
depict an MCS developing and moving east across northern Missouri
during the morning on Thursday. Given the well-capped environment,
a threat for damaging wind is marginal at best as most of this
convection won`t be rooted in the boundary layer.

Previous guidance suggested that the boundary that stalls over the
Ozarks tonight would remain to the south, limiting instability on
Thursday. A dramatic shift in the guidance now has a slightly
stronger low that will pull the front north and expose at least part
of the region to a warm and moist airmass, increasing a conditional
threat for severe weather. While convective-allowing models are
rather bullish regarding a surge of surface-based instability
northward, I`m not convinced that this surge is realistic as-
depicted. The Ozarks will likely act to block low-level moisture and
limit the redevelopment of instability to some degree, which is
always poorly-resolved by guidance of most resolution. Another
potential wrench in our instability is the morning convection: if it
lays a cold pool across the region it may quash instability to a
certain extent, depending on its timing and strength. Conversely,
it may also serve as another focus for redevelopment if
instability can persist south of it.

A surface cold front attending the low, mid-level ascent, and upper-
level jet divergence will coincide Thursday afternoon and grant some
degree of forcing for convective initiation. It`s worth noting that
if we do not become sufficiently unstable and the relatively-weak
ascent cannot compensate, a more marginal severe weather threat
will exist. Assuming at least ample instability, with strong mid-
level westerlies and weak 0-6km bulk shear values somewhat
parallel to the front, I`d expect discrete convection to be short-
lived before growing upscale into line segments that would exhibit
a damaging wind threat. The conditional threat will most likely
begin in an area from central Missouri to west-central Illinois
and advance southeast with time. Large hail would also be a
concern, but that potential wanes as more linear structures
develop. 0-3km shear is borderline (around 30kts) for any QLCS
tornado threat, but if line segments orient properly amidst that
shear I cannot rule out a tornado or two from I-44 and southeast.
While some degree of variance exists, this convection will likely
(70-80% chance) exit the forecast area by roughly 9pm.

MRB

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 325 AM CDT Wed Apr 17 2024

Longwave troughing will become more pronounced over the central
CONUS Thursday night, which will further promote cooler air
filtering into the mid-Mississippi Valley behind the cold front..
NBM and global ensemble probabilities of below-normal
temperatures increase across the region on Friday as cooler air
settles in, reaching nearly 100% areawide over the weekend. The
cooler air does introduce a low (10-20%) threat for frost. Given
winds won`t ever calm during the colder stretch and probabilities
of reaching 34F overnight this weekend are 20% at most, frost is
an outside possibility at best. The greatest threat, again capped
around 20%, would be early Sunday morning in northeast Missouri.

The flow pattern will also promote a stretch of dry conditions
through at least the weekend, potentially into Monday. The next
appreciable chance for rain would be Tuesday along a weak surface
cold front when a northern stream trough dives into the region.
While considerable differences exist in the ensemble solutions, the
front appears to have trouble exiting the region and may meander
into mid-week. At the same time, the longwave trough will erode
and give way to some degree of ridging in the central CONUS.
Temperatures will gradually rise as a result, and we may (30%
chance) return to a wetter pattern from mid-week onward. This is
reflected well in the CPC 6-10 and 8-14 Day Outlooks, which show
increasing probabilities for abnormal warmth and a weak signal for
higher-than-normal precipitation.

MRB

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 604 AM CDT Wed Apr 17 2024

Other than low-end VFR stratus moving into KUIN at this hour
extending from a low in Iowa, clear skies exist across the region.
The main threat to aviation interests will be the persistent,
strong west wind today, gusting as high as 30kts late this morning
into early this afternoon. Winds will gradually weaken this
afternoon as the low to our north drifts away, and calmer winds
will settle in tonight.

There is a potential for some thunderstorms across central
Missouri near the end of the current TAF period, but that threat
is too low to mention in the TAF at this time. The more impressive
thunderstorm threat will be with a line that impacts the terminals
later tomorrow morning into the afternoon and evening.

MRB

&&

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

$$

WFO LSX