592
FXUS63 KLSX 271107
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
Issued by National Weather Service Kansas City/Pleasant Hill MO
601 AM CDT Thu Jun 27 2024

...Updated 12z Aviation...

.KEY MESSAGES...

- Several rounds of showers and thunderstorms are expected Friday
through Saturday. A few strong to severe storms are possible with
damaging winds Friday night. Saturday`s storms could be   stronger
ahead of a cold front, although potentially less wide-   spread than
Friday night`s storms. Damaging winds will be the   primary threat
on Saturday. Large hail is also a possibility.

- Below normal temperatures are expected on Sunday and Monday behind
the front. Temperatures rise back above normal by the  middle of
next week.


&&

.SHORT TERM... (Through Late Friday Afternoon)
Issued at 252 AM CDT Thu Jun 27 2024

High pressure centered over the Upper Midwest will drift southeast
over the Great Lakes today.  The wind will veer to the northeast and
then east as the high moves today.  While the flow won`t be
particularly strong, this will funnel cooler and drier air into the
Mid Mississippi Valley.  While it won`t be a "cool" day, it will be
cooler and much less humid than earlier in the week. Guidance is in
good agreement that highs will max out in the mid to upper 80s which
is right around normal for late June.

The high will continue east tonight into Friday, moving into New
England, and then off the East Coast by Friday evening.  Meanwhile,
a mid-upper level short wave will move across the Rockies into the
northern Plains tonight and Friday resulting in low level
cyclogenesis.  The low level jet ramps up over the Plains ahead of
the system tonight which forces convection to develop.  The showers
and storms move east overnight into Missouri on Friday morning.
Short range guidance is in good agreement that this broad area of
showers and thunderstorms will move across Missouri into Illinois on
Friday. Clouds and precip should suppress temperatures particularly
across northeast Missouri and west central Illinois where the most
rain is forecast to fall. A couple of days ago, we were expecting
highs on Friday to rise back into the 90s across much of the area.
With the inclusion of clouds and precip, our latest guidance keeps
highs in the 80s. The cooler temps limit how unstable the atmosphere
can get over our area Friday afternoon, and now most of the high
instability stays over eastern Kansas and western Missouri. The
models develop convection on the nose of a low level jet in these
areas during the late afternoon or early evening, and move the area
of storms east through the night. While precip does look likely for
our area on Friday night, the strongest storms should stick to where
the highest instability will be, over western Missouri. Several CAMs
bear this out, showing that the highest simulated reflectivity will
remain outside of our area. With all that said, Friday`s rain
doesn`t materialize, we could get hot and unstable Friday afternoon
which would produce stronger storms Friday evening.

Saturday`s forecast will depend on a number of factors including,
the position of the cold front, where the effective boundary from
Friday night`s storms is located, and how much lingering clouds and
showers are left over from Friday night`s convection to affect
temperatures.  The deterministic GFS and ECMWF show similar
solutions with the cold front pushing south-southeast through
Missouri and Illinois during the day.  The GFS pools dew points
ahead of the front showing a broad area of 75-78 degrees.  This
produces MLCAPE values in excess of 3000 J/Kg ahead of the front
during the afternoon and early evening.  The EC is more believable
at 72-75 dew point, producing between 2000-2500 J/Kg.  This agrees
well with the LREF mean SBCAPE values for Saturday afternoon and
evening so I`m tending to lean in that direction.  Regardless, there
will be more than enough instability to produce strong updrafts,
however the shear will be marginal at best with most guidance
members showing between 15-25 kts of deep layer shear. Even the
stronger GFS only gets to around 30kts. Guidance is also showing
some CINH above the front which could limit the coverage of storms,
particularly since low level convergence along the front doesn`t
look very strong. Overall, I think the severe weather threat for
Saturday is limited, but probably higher than Friday night`s threat.
If we realize that much instability, think the primary threat will
be damaging winds. Can`t rule out hail entirely either, but it`s
less likely than wind due to the warm temperatures forecast ahead of
the front.

Carney

&&

.LONG TERM...  (Sunday through Wednesday)
Issued at 252 AM CDT Thu Jun 27 2024

Sunday and Monday look dry and cooler behind the front and high
pressure crossing the Mississippi Valley into the Great Lakes.  Bias
corrected guidance looks like it`s affecting the NBM as it`s highs
both days were slightly above the 75th percentile.  With that
said...it`s almost July...and the sun angle doesn`t get any higher.
It`s also 4-5 days out, so there`s plenty that can still go wrong
with the forecast.  Winds turn back to the south Tuesday and persist
into Wednesday bringing warmer and more humid air back to the Mid-
Mississippi Valley, as well as a chance for more Thunderstorms.


Carney

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 0601 AM CDT Thu Jun 27 2024

Patchy fog has been ongoing this morning at KSUS and KCPS with on
and off period of IFR visibility. Expect this to mixout and diminish
with sunrise through the morning hours. The start of the TAF period
will feature clear skies and generally light winds at the terminals.
May see increasing cumulus cloud development through the afternoon
introduce few clouds or perhaps periods of scattered cloud deck with
VFR bases. Increasing upper-level cloud cover expected to bring in
broken ceilings later this evening and continue into Friday. Rain
showers may start to move across Central Missouri after 06z this
evening and then closer to the St. Louis metro terminals after 12z.
However confidence is not high enough at this time to mention in the
TAFs. Greater precipitation potential will be early Friday afternoon
and into the evening, especially for KCOU and KJEF.

WFOEAX

&&

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

$$

WFO LSX