514
FXUS63 KLSX 040356
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1056 PM CDT Fri May 3 2024

.UPDATE...
Issued at 1036 PM CDT Fri May 3 2024

Regional satellite shows skies are clearing over small envelope
of the mid-Mississippi Valley, as surface ridging extends from
the Great Lakes Region through southeast Missouri. Surface
observations show winds range from calm to light and variable
through much of the area. These conditions, along with recent
rainfall, have provided ample surface moisture resulting in
dewpoint depressions that range of 0 to 3 degrees over far
southern sections of the CWA. Hi-res soundings plot shallow
inversion developing through the early morning hours, which should
support the development of patchy fog as far north as I-70. Fog
could have greater spread and become dense for a few hours through
early Saturday morning over southeast Missouri through sections of
southwest Illinois. This has prompted the issuance of a dense fog
advisory through 14z.

Maples

&&

.KEY MESSAGES...

- Isolated strong to severe thunderstorms are possible from 2pm to 7pm
  Saturday. Hazards include up to 60 mph winds and up to quarter
  sized hail.

- Confidence is increasing in a multi-day severe event taking
  place in the central U.S early to mid-next week. However, there
  is still uncertainty regarding details like timing, exact
  location, and how one day`s thunderstorms will impact the next
  day`s severe potential.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 249 PM CDT Fri May 3 2024

Thursday night/Friday`s cold front will continue to slowly buckle
northward through the bi-state tonight as a quasi-stationary
boundary. Meanwhile, a mid-level shortwave and surface cold front
will push east through the Plains, arriving in Mid-Missouri early
Saturday afternoon. This system will push the boundary to the
northeast and leave us within its warm sector. Because of this,
temperatures Saturday will be much warmer than they were today,
reaching the mid-70s to mid-80s. Moisture will also pool into the
area, mostly along and just ahead of the incoming cold front.
Instability will be maximized here with SBCAPE values of 1500-2000
J/kg. Deep layer shear will be lacking, though, ranging from 20-30
kts. Modest CAPE and bulk shear values are expected to result in a
severe weather threat much like yesterday - disorganized clusters of
outflow dominant cells. Although we`re confident in thunderstorm
development, strong to severe thunderstorms are expected to be
isolated in nature due to them being outflow dominant and likely
quick to intensify and even quicker to die. The greatest potential
for severe weather will be between 2pm and 7pm as the front is
passing through the CWA. In any thunderstorms that become severe,
damaging winds of up to 60 mph and up to quarter sized hail are
possible. We`re confident in the end time of the severe potential, as
the front will be close to exiting to the southeast, and that
coincides with the downfall of daytime instability. With the lack of
appreciable shear, diminishment in instability will be detrimental
to the life of any thunderstorm.

Saturday night will be relatively chilly for portions of the areas
surface high pressure passes to our north. Areas in northeast
Missouri and west-central Illinois are forecast to fall into the
upper 40s. To the south, further from the influence of the high and
under more dense cloud cover, low temperatures may not fall under 60
degrees.

Jaja

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 249 PM CDT Fri May 3 2024

Sunday will be a relatively quiet opening to the week, almost
literally the calm before the storm as we face a potential multi-day
severe event between Monday and Wednesday. Despite the relative
quiet, wet weather is expected beginning Sunday afternoon and
continuing into overnight. This is due to a shortwave passing
through the Mid-Mississippi Valley from the southwest and driving a
weak surface low through our CWA. This surface low will cause
Saturday`s remnant cold/stationary front to buckle back north as a
warm front with showers with embedded thunderstorms along it and
within the system`s warm sector. Severe weather is not expected,
even earlier in the afternoon, due to weak lift, weak moisture
convergence along the front, mid-level subsidence, and minimal
instability (<250 J/kg MUCAPE). High temperatures Sunday will be
near normal, landing in the 70s areawide.

Monday kicks off the potential for multiple days of severe weather
extending from the central and southern Plains into the southwestern
Great Lakes and western Ohio Valley. A deep mid-level closed low
will be responsible for this as it ejects northeast into the Plains
on Monday. At the surface, two low pressure systems will develop lee
of the Rockies, advecting warm air and moisture into the region
ahead of a dryline and developing cold front. The dryline will act
as a trigger for convective initiation in the Plains on Monday. On
Tuesday, the system as a whole will be further east, and a shortwave
and vorticity lobe will swing through our region. The cold front
will be further east as well, shifting the severe threat into our
area. Guidance across the board is in consensus that instability,
deep layer shear, moisture, and synoptic level dynamics will all be
favorable for severe weather on Tuesday and again on Wednesday as
the cold front continues its eastward advancement. With the
environment that may be in place, no hazard can be discounted at
this point.

Of course, this multi-day event begins four days out from now, so
there`s uncertainty to discuss. Firstly, although WPC cluster
analysis shows consistency amidst guidance in the mid-level
trough/closed low early next week (positioning, strength, size),
there are differences in the phasing that impact the mid-level flow
and thus impact the timing of the overall system. For a multi-day
system like this, a faster or slower passage of the system could
increase or decrease our potential for severe and/or lengthen or
shorten our window for it. A faster frontal passage may shift
Wednesday`s severe threat further east and out of our CWA, whereas a
slower frontal passage may extend our severe threat into Thursday.
There`s also the question of how/if severe weather to our west
impacts our severe weather threat for the following day (Monday into
Tuesday and Tuesday into Wednesday). Overnight low-level jets may
cause ongoing convection to continue into the next morning,
potentially dampening the potential for severe that day. The
opposite scenario is also equally as likely - thunderstorms dying in
the evening could lay down outflow boundaries and open up the
potential for rapid destabilization the next day, increasing our
chances for severe. All of this to say despite the very favorable
synoptic set-up for severe, there`s still plenty of variability that
could sway our severe potential and its timing.

Jaja

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1036 PM CDT Fri May 3 2024

The only change since the last TAF update is the increasing
potential for fog development early Saturday morning. The
potential for dense fog should remain over southeast Missouri and
far southwest Illinois. However, patchy fog could develop as far
north as I-70, including the metro terminals. The sites to monitor
will be KSUS/KCPS, as recent observations show winds have gone
calm at KSUS. TAFs do not include the mention of fog, but may
later be amended if trends continue in this direction.

The main impact will stem from thunderstorm potential Saturday,
progressing west to east along and ahead of an advancing cold
front. Thunderstorm potential still look highest after 18z through
about 00z. Cloud bases remain at VFR level through much of the
period until the cold front pushed through Saturday evening, at
which time a brief period of MVFR ceilings are likely to
accompany the front. Outside of this, any conditions lower than
VFR will depend on direct impacts from thunderstorms.

Maples

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CDT Saturday for Iron MO-Madison
     MO-Reynolds MO-Saint Francois MO-Sainte Genevieve MO.

IL...Dense Fog Advisory until 9 AM CDT Saturday for Randolph IL-
     Washington IL.

&&

$$

WFO LSX