760
FXUS63 KLSX 061727
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1227 PM CDT Fri Jun 6 2025

.KEY MESSAGES...

- Showers and thunderstorms are likely (60-80%) late this morning
  and afternoon, mainly across parts of southeast Missouri and southwest
  Illinois. Some of these storms may be strong to severe, with
  damaging winds and a few weak tornadoes the main threats.

- Another round of showers and thunderstorms is likely (60-80%) on
  Saturday. Some of these storms also may be strong to severe,
  though there is a lot of uncertainty on how much instability
  (fuel) there will be available for thunderstorms.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 347 AM CDT Fri Jun 6 2025

Scattered showers and thunderstorms have developed along a west-
east axis across parts of southeast Missouri over the past couple
of hours. This activity is on the nose of a 20-25 knot low-level
jet where moisture convergence is maximized. Further west, there
is a line or organized convection in far northern Oklahoma. This
line is moving nearly due east. While this line of convection will
stay south/southwest of the CWA, an MCV is expected to develop on
the northeastern fringe of this activity. Flow at 700 hPa is
southwesterly, which means this MCV should track toward southeast
Missouri very late this morning. This feature should help lead to
additional convective initiation downstream. How organized the
convection will get by early afternoon is a question mark, due to
uncertainty with how much instability there will be ahead of the
approaching MCV. The latest HREF probabilities for at least 1000
J/kg of CAPE peak out at 40-60% in far southeast Missouri. The
timing of the MCV also isn`t ideal, coming through about 2-3 hours
earlier than peak heating. That being said, if there are more
breaks in the clouds and instability is higher, that would likely
increase the severe threat across the area. The convective mode
should be a mix of multicellular clusters and line segments.
Damaging winds will be the primary threat, though isolated weak
tornadoes are also possible. The 0-3km shear vector would be
perpendicular to any segments oriented northwest to southeast,
which some CAMs suggest may occur. The magnitude of the 0-3km
shear vector however is only forecast to be around 25 to maybe 30
knots, or just under what is normally sufficient for the
generation of mesovortices. The low-level environment does not
appear overly favorable for tornadogenesis with weak 0-1km shear
and helicity forecast. However, LCLs will be on the low side, and
MCVs have been known to produce multiple weak/brief tornadoes in
the past so this is definitely something to monitor for later
today.

The threat for any strong to severe convection should quickly wane
by late afternoon into the early evening, as the MCV moves on into
the lower Ohio Valley. Rising mid/upper level heights and subsidence
should set in behind this feature, with rapidly decreasing chances
of any shower or thunderstorm activity. Much of the rest of the
night should be dry with a mostly cloudy sky and temperatures in the
upper 50s to mid 60s.

Another complex of thunderstorms is forecast overnight tonight into
Saturday morning across northern Oklahoma and northern Arkansas.
Another MCV may emanate out of this convection. The flow at 700 hPa
though is more westerly, with very little if any meridional
component. This suggests any influence from this MCV is more likely
to be felt south of the CWA Saturday afternoon. However, there is
another midlevel shortwave that will be moving out of the mid-
Missouri Valley early Saturday afternoon. This feature is expected
to develop widespread showers and thunderstorms ahead of it and move
west to east across much of the region. Uncertainty with how much
instability will be available again is a primary concern, with the
HREF showing chances for at least 1000 J/kg of SBCAPE above 40% only
in a small part of central and southeast Missouri. A large part of
the limited CAPE is due to thick, low stratus that likely will be
across the area. The best chances of this stratus lingering into the
afternoon is across northeast and east-central Missouri as well as
western Illinois. Deep-layer shear however will be stronger on
Saturday than this afternoon, with midlevel flow more on the order
of 50 knots ahead of the approaching shortwave trough. Therefore,
while the severe threat Saturday is highly conditional, there may be
more potential for widespread severe weather. This means that
supercells would be possible if enough instability materializes. All
severe hazards would be on the table, with the tornadic threat
possibly enhanced near the track of a weak surface low. This is
where low-level shear and helicity will be enhanced compared to the
rest of the region. Where exactly this tracks is unknown, but most
likely it will be from west to east roughly near the I-70 corridor.

Saturday`s temperatures also shed some uncertainty, with the NBM
showing an interquartile range of 5-7 degrees for most locations.
This is due in part to how thick and prevalent the low stratus will
be along with the track of the surface low. Highs are most likely to
range from the mid 70s to low 80s, but could be at least a few
degrees higher or lower than forecast due to the uncertainties
mentioned above.


Gosselin

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 347 AM CDT Fri Jun 6 2025

(Saturday Night)

Similar to tonight, mostly dry weather is forecast on Saturday night
as chances of showers and thunderstorms rapidly decrease behind the
departing midlevel shortwave trough. Behind this feature, there is
some weak mid/upper level height rises (implying subsidence aloft)
and increasing low-level cold/dry air advection.


(Sunday)

A strong mid/upper level trough is forecast to move southward across
the US-Canadian border late Sunday, with a secondary cold front
moving through the mid-Mississippi Valley. At least some convective
development is expected along this cold front, though the coverage
may be limited by weak surface convergence in our area along the
front and the stronger mid/upper level forcing for ascent staying to
our north. Unlike the previous 2 days, it does appear more likely
that more instability will be available Sunday afternoon ahead of
the front. The latest LREF probabilities for 1500+ J/kg of SBCAPE
are in the 50-70% range. Given the anomalously strong northwesterly
flow aloft, multicellular clusters as well as supercells may occur
Sunday afternoon. Large hail may tend to be the primary threat due
to steepening midlevel lapse rates and the potential for rotating
updrafts.


(Sunday Night - Thursday)

A transition to a drier, quieter pattern looks to be in the offing
for early/mid next week. Sunday`s cold front is expected to
penetrate well south of the area toward the Gulf Coast. Temperature
anomalies on both the GEFS/EPS are expected to be in the -4 to -6C
range, pretty respectable for mid June. Along with the dry weather,
at least slightly below normal temperatures are forecast during this
time period. Highs in the upper 70s to low 80s are forecast each
afternoon, with lows dropping back into the 50s areawide.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1221 PM CDT Fri Jun 6 2025

Showers are moving through central Missouri and the St Louis metro
this afternoon. Couldn`t rule out a few rumbles of thunder, but
confidence is too low to include that in any TAF at this time.
Quincy should remain dry, but some minor visibility reductions
have continued up there all morning. This may improve briefly
before coming back down this evening.

Biggest uncertainty is with ceilings and visibility overnight
tonight. Areas of rain this afternoon are causing some IFR
ceilings and are likely to leave some fairly extensive clouds in
their wake even after the rain stops. But if clouds clear out,
then it won`t take much cooling at all to cause fog or very low
ceilings to develop again as the near-surface is able to cool in a
very moist low level environment. With this TAF issuance, we have
more confidence in some clearing in central Missouri so fog was
included there. However, clouds are more likely to linger in the
St Louis area and prevent the overnight fog development. However,
a lot of model guidance does show this fog development, and trends
in cloud cover this evening will go a long way in helping gain
some confidence on whether this will occur or not.

Kimble

&&

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

$$

WFO LSX