976
FXUS63 KEAX 121059
AFDEAX

Area Forecast Discussion
National Weather Service Kansas City/Pleasant Hill MO
559 AM CDT Wed Mar 12 2025

...Updated 12z Aviation...

.KEY MESSAGES...

- Hot and dry Today and Thursday

- Strong storm system Friday and Saturday; Gusty winds, fire Weather
concerns, rain showers, and possible thunderstorms

- If thunderstorms storms develop, storms could become severe if
there is enough moisture when forcing moves through

&&

.DISCUSSION...
Issued at 358 AM CDT Wed Mar 12 2025

Thermal boundary has stalled just north of the Interstate 70
corridor during the overnight hours, and have seen thin upper-level
cloud cover develop where we had H5 height falls during the evening
on Tuesday. PV anomaly over the Pacific continues to send short-wave
pulses and vorticity maxima into the CONUS in this progressive
Spring-time flow regime we have been experiencing. A notable mid-
level short-wave trough can be seen on GOES-16 Water Vapor imagery
with its axis moving across the Four Corners Region. Stronger height
falls over far desert southwest indicate that the trough has
deepened but jet streak rounding the base indicates this will start
to progress eastward into the Southern Plains today. Stronger dCVA
extends into the TX Panhandle Region, and seeing some signal for
dCVA in portions of the Front Range in some mid-level objective
analysis that will continue to result in surface pressure falls
across much of the Plains. For our area, this is turning flow south-
southwesterly, providing strong WAA. As a result, we will actually
start to see a period of H5 height rises as a thermal ridge builds
back in across the area, and this will send the stalled boundary
back northward. Weak AVA should promote weaker subsidence to clear
some of this cloud cover the area has had overnight. Temperatures
today are forecast to rise back into the 70s across the entire area,
and it is possible for our southern counties to hit the upper 70s
and maybe an hour or two right at the 80F mark, though should note
that hitting 80F would mean needing to almost achieve a 90th
percentile value amongst NBM members. Thursday, PV anomaly over the
Pacific moves eastward toward the coast with a deep trough working
toward the southern California coast. This forces a ridge axis into
the desert southwest, and causes the trough axis currently in the
Four Corners to propagate eastward across the southern Plains into the
lower Mississippi River Valley. For our area, this pushes the
thermal ridge axis eastward, but keeps 850mb flow and levels below
that southwesterly, maintaining WAA. Subtle H5 height falls occur
across our area Thursday morning into the early afternoon as the
main feature moves well south of the area. The strongest vorticity
and moisture does not travel much further north of Interstate 44,
therefore, no precipitation activity is expected to make it the area
with this first system. Temperatures once again will be in the mid
to upper 70s for most of the area. NBM inner-quartile spread for
high temperatures is narrow, lending to confidence forecast in these
temperatures the remain above normal. H5 ridge axis this moves in
behind the exiting system, with an AVA regime providing weak
subsidence that works to erode the weak surface low pressure system
that has been trying to develop across the Plains with the preceding
short-wave troughs. Therefore, pressure gradient will not be strong,
thus winds not expected to be overly strong Thursday. So although
relative humidity values may drop quite a bit through the afternoon,
not greatly concerned about fire weather related issues on Thursday.
H5 height falls come to an end Thursday evening, and the focus turns
to the strong system we have not been tracking for nearly a week.

Friday morning, robust H5 trough axis quickly propagates across the
Intermountain West toward the Front Range, with the H5 jet streak
progged at 100+ kts. Deterministic guidance continues to depict this
system acquiring a negative tilt by Friday morning, placing favorable
dCVA into the Front Range for robust surface cyclogenesis. This setup
remains favorable for the system to achieve some the extreme
parameter spaces relative to mid-March that NAEFS and other related
systems were providing a signal for last week. By Friday late
morning, expecting surface pressure falls to expand across the
Plains into the middle Mississippi River Valley. Surface flow is
favored to turn more southerly, and should start to transport better
moisture into the region compared to the preceding short-waves
during the early part of the week. It will take a lot of work though
to attempt to saturate the boundary layer, which is being largely
influenced by fairly dry soils. Stronger H5 height falls should
arrive by late morning as well, as broad scale synoptic lift picks
up in intensity. The enhanced southerly flow should help maintain
temperatures in the upper 70s for much of area, and would not be
surprised to see a few spots hit the lower 80s. While these values
are just above the NBM 75th percentile, this system should have the
strength to get it done, especially with stronger winds promoting
more turbulent mixing, resulting in a deep boundary layer ahead of
this system in an expanding warm-sector. Overall, would expect
isentropic upglide to start Friday in the morning. Normally would
expect an expansive stratiform precipitation shield, but showers in
the morning may be more spotty in nature given the very dry boundary
layer. Could perhaps see some virga develop if any mid-level
moisture can be advected in ahead of the main forcing. This could
result in wind gusts above 40 MPH reaching the surface prior to the
strongest forcing moving through. With respect to QPF, there is
still a large degree of uncertainty due to the dry boundary layer.
Deterministic GFS has come down on QPF across the southwestern third
of our forecast area in recent runs, while respective ensemble means
still have a few tenths of an inch. Overall probabilities for
measurable QPF are still above 60 percent for areas east of
Interstate 35, and above 30 percent for areas westward. For a
threshold of 0.10 inches of QPF, higher probabilities are
concentrated in the eastern half of the forecast area (essentially
Hwy. 65 and east). Ensemble members that are producing more robust
QPF are achieving most of that through convection, concentrated
mainly east of Interstate 35. This is where the forecast for our
area gets more complex. Thunderstorm confidence is higher for
eastern Missouri, as moisture transport and warm-sector
destabilization should last for a longer period of time there. As
for our counties from the Hwy. 63 corridor and westward,
probabilities for some form of instability have increased slightly,
with about a 40-50 percent chance for seeing some CAPE values above
500 J/kg. However, at 1000 J/kg, the probabilities drastically drop
off. Deterministic GFS and ECMWF continue to develop about 750 J/kg
MLCAPE in our eastern counties, with around 450-500 J/kg of SBCAPE.
However, these also depict a stout EML, which largely caps the CBL.
Overall forcing could be enough to break this cap though, as the
lift with the vorticity maxima moving through will be extremely
strong. The next question though, is how much moisture will be
available as the cold front starts to push across Kansas into
Missouri. As the cold front starts to move through, there remains a
notable signal for a Pacific Cold Front or pseudo dryline like
feature to eject well ahead of the the main thermal boundary,
promoting robust mixing that drastically lowers dewpoints, creating
dewpoint depressions of nearly 30F to 40F. If this moisture
discontinuity boundary surges ahead across the warm sector before
the stronger lifting sources come in, the previously mentioned CAPE
values will be completely eroded, with a very dry boundary layer.
This feature is the reason why the probabilities for CAPE above 1000
J/kg remain low in current ensemble output, along with some members
having a different point in which the system wraps up and slows down
its propagation speed. If this dry air surge happens before the main
cold front and stronger surface convergence arrives, thunderstorm
chances could be completely eliminated for area, then starts to
increase in eastern Missouri once the main thermal boundary catches
up to it. Therefore, the severe thunderstorm threat remains
conditional for our area. Depending on how much moisture transport
we realize overnight and Friday morning, perhaps a few isolated
storms could develop in the warm-sector, especially if any cloud
cover moves through and creates some kind of differential heating
boundary for these to latch onto. Depends on if broad scale ascent
is strong enough to get a robust updraft (it will be strong enough
for general showers). If a thunderstorm updraft develops, deep layer
shear will already be pushing 50 kts on the leading edge of this
system, which could help it to organize. A discrete storm would
mostly like present a wind threat. Strong theta-e deficits between
the surface and the top of the boundary layer would result in strong
evaporational cooling, providing strong wind gusts. Some model
soundings do show higher CAPE within the hail growth zone, that
could allow for dry hail growth, though if storm motions are above
40 kts, storm-relative flow could become too disruptive for that.
LCLs above 1500m will limit any kind of tornado threat early, even
though lower-level hodographs will be quite curved with back surface
winds ahead of the deepening cyclone. If the cold front catches up
to the pseudo-dry line, deep layer shear vectors remain parallel to
it that should result in any multi-cell development quickly
congealing, but shear is strong enough to perhaps see the onset of a
QLCS (though right now most of the probability space favors that
east of Hwy. 63 and into the Ohio River Valley). Therefore, with
background winds and storm motions already at 50kts, it will not
take much to see wind gusts, if we are able to realize convection
initiation at all in our area.

Again, all this discussion about severe thunderstorm hazards is
conditional on having moisture in the warm-sector to get these
storms. Right now, would still consider this low-confidence in
occurrence, but higher impact if does occur. The CAMs starting at
00z tonight may give us an idea of how the cold front and pseudo
dryline like feature behave, and perhaps HREF probabilities may be
able to better pinpoint how instability and moisture return to our
area end up materializing. If thunderstorms are unable to develop,
expect very windy conditions to still occur, with light showers
possible once the main forcing and thermal front comes through. The
main cold front may actually bring in slightly higher dewpoints late
Friday evening through the overnight hours. Finally, colder air may
filter into Northern Missouri Saturday morning, which could result
in light wintry precipitation. However, QPF probabilities are very
low at the time of the temperature drop across the north, therefore
still am not overly concerned about winter weather with this system,
as the bulk of favorable wintry precipitation parameters are still
in the Northern Plains and Upper Midwest.

After this system passes, expecting a few days of colder
temperatures, but stronger ridge axis is progged to move through
early next week that sends temperatures back into the 70s. There are
some ensemble signals for precipitation again through the middle of
next week.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 558 AM CDT Wed Mar 12 2025

High-level clouds will pass through this morning providing VFR
ceilings. Winds will slowly shift from eastern to more southerly
through the day but will be light.

&&

.EAX WATCHES/WARNINGS/ADVISORIES...
MO...None.
KS...None.

&&

$$

DISCUSSION...Krull
AVIATION...Krull