642
FXUS63 KLSX 030345
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1045 PM CDT Sun Jun 2 2024

.KEY MESSAGES...

- A more active pattern will ensue Monday and continue into
  Tuesday night with multiple periods with showers and
  thunderstorms. A few of those storms could be strong to severe
  on Monday afternoon and evening. Temperatures during this period
  will be above normal.

- There is increased concern for locally heavy rainfall on Monday
  night into early Tuesday morning that could pose a flash flood
  threat but the exact location is uncertain.

- A pattern change late week lasting into next weekend will bring
  cooler temperatures although the magnitude remains uncertain.
  While overall less active, it may not be totally void of shower
  and thunderstorm chances.


&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 208 PM CDT Sun Jun 2 2024

Quasi-zonal mid-upper flow has developed across the Nation`s
midsection with retreating high pressure at the surface. Weather
late this afternoon and evening will be relatively benign but
there could be a brief light shower or sprinkles impacting parts
of central and northeast MO associated with some mid level
moisture and WAA early on. The more active weather pattern we have
been discussing will begin to unfold later tonight, impacting the
area Monday-Tuesday. During this time frame, there is a lack of
well-defined larger scale forcing mechanisms, and sensible weather
will be largely controlled by mesoscale features which will be
dependent on weather/convection the previous 6-12+ hours.

Precipitation chances late tonight and into early Monday morning
will be tied to the development of an upstream MCS across the
Plains tonight, its organization, and longevity or rate of decay
as it moves into central/northeast MO. There are both signals of a
decaying MCS or remnant boundary moving into central and
northeast MO on Monday morning. This boundary and potential for an
attendant MCV should be the catalyst for convection across our
area on Monday as it interacts with an increasingly unstable
environment in the afternoon, however the trends/evolution are
unclear. The plethora of short-range guidance forecasts abundant
heating on Monday with temperatures well into the 80s which will
result in significant instability and SBCAPE of 3000+ J/KG. While
instability will be large, deep layer shear is quite weak,
generally well below 20 kts. Storm modes could be messy with pulse
storms and storm interactions leading to multicell clusters. A
few strong to severe storms would be possible given the amount of
CAPE, with damaging winds the greatest threat and marginal hail to
a lesser extent.

Of growing concern is the potential for locally heavy and possibly
excessive rainfall on Monday night into early Tuesday. The short
range model guidance has an MCV/mid level vortex moving SW-NE
across the area within weak flow aloft. The interaction of this
MCV and a modest (20-30 kt) LLJ along with any remnant boundaries
would be the foci for showers and thunderstorms. Slow storm
motion, persistent cells and training potential, and an
environment characterized by PWs of 1.5+ inches would support
locally heavy rainfall. Somewhere across eastern MO possibly
extending into western IL would be the zone that seems most
favored, however the footprint may be small, only 2-4 counties.
The latest HREF LPPM supports this scenario showing several small
pockets of rainfall amounts of 3-7+ inches.

Glass
&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 208 PM CDT Sun Jun 2 2024

A notable pattern change is still forecast to occur beginning on
Tuesday night as an upper low moves across Saskatchewan to Lake
Winnipeg and the accompanying short wave trof digs east-
southeastward through the mid-upper MS Valley and into the Great
Lakes. This trof will drive a cold front through the area on
Tuesday night into Wednesday morning. Greater large scale and
low-level forcing attendant with the advancing cold front and
plentiful moisture/instability will lead to widespread showers and
thunderstorms along and ahead of it, centered on Tuesday night.

Amplification in the large scale pattern across the CONUS
proceeds into the later part of the week and into the weekend with
the upper low and trof digging southeastward through the Great
Lakes and into the eastern U.S.. This trend has been apparent for
several days in the guidance although the details of the upper low
position and trof depth/orientation across SE Canada and the
eastern U.S. seem to change with each consecutive model cycle. The
latest ensemble guidance and cluster analysis from the 100 member
LREF continues to show diverse solutions with this regard in the
Friday-Sunday period with either WNW flow aloft or NNW flow aloft
and 500 mb height differences as great as 100 meters. The
accompanying 850 mb temperature differences are as great as 3-5
degC and these differences lead to low confidence in forecast
details. In terms of sensible weather, this translates to the
difference in below normal or near normal temperatures and
possibility of any showers or thunderstorms. The IQR for high
temperatures in the LREF is as much as 10+ degrees and the median
temperatures and overall distribution would favor below normal
values. The more NNW flow solution would support predominately dry
weather, while a WNW flow solution suggests potential for several
short waves traversing the area with an accompanying chance of
showers and thunderstorms sometime over the weekend.

Glass
&&

.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1043 PM CDT Sun Jun 2 2024

VFR flight conditions expected through midday on Monday. Still
expecting increasing chances of showers and storms on Monday,
depending on when possible remnants of overnight MCS activity to
our west approaches the region. For now have vicinity storms
moving into KCOU, KJEF and KUIN between 16z-17z Monday, while for
the St. Louis metro area TAF sites it will be a bit later, between
20z-21z Monday. Widespread rain will be between 18z Monday and 01z
Tuesday for KCOU, KJEF and KUIN, but after 03z Tuesday for St.
Louis metro area.

As for winds, they will remain light and variable through early
Monday morning, then pickup from the southeast to south by mid
morning.

Byrd
&&

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

$$
WFO LSX