608
FXUS63 KLSX 250843
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
343 AM CDT Tue Jun 25 2024

.KEY MESSAGES...

- The heat continues for one more day. Heat index values of 105+ are
  likely across parts of central, east central, and southeast
  Missouri as well as southwest and south central Illinois. The
  Heat Advisory remains in effect with no changes needed.

- Thunderstorms are expected to form this afternoon/evening along
  a slow moving cold front over Iowa, and move into our area
  during the late evening or overnight. Some storms could produce
  damaging winds of 60 mph or greater. A few could also produce
  some 1 inch hail. The highest threat of severe storms will be
  across northeast Missouri and west central Illinois, with a
  diminishing threat as the storms move farther south.

- The cold front passes more slowly than expected in previous forecasts.
  While We`ll see relief from the heat on Wednesday, it will be
  more from clouds, and showers and thunderstorms, particularly in
  the afternoon south of I-70 where a few strong and possibly
  severe storms may develop.

&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 341 AM CDT Tue Jun 25 2024

The Heat Advisory should be in good shape with the latest
temperature guidance giving us upper 90s and even some 100-101 highs
in the St. Louis Metro Area.  The one fly in the ointment is the
potential for mixing this afternoon.  The RAP and to a lesser extent
the GFS forecast soundings show deep mixing (over 700 mb on the
RAP!) which drops surface dew point temperatures into the 60s across
parts of southeast and east central Missouri into southwest
Illinois.  In this scenario the heat index would be closer to the
air temperature.  Though we technically wouldn`t make it to advisory
criteria, the air temperature would likely be hotter than forecast,
and the potential health effects would be the same, so I`ve made no
changes to the advisory at this time.

The heat and humidity will produce strong instability over northern
Missouri into Iowa this afternoon ahead of the cold front which will
ultimately bring us relief from the heat.  The GFS and RAP show 4500
to 5000+ J/Kg MLCAPE late this afternoon across Iowa.  They both
develop convection and push it south slowly ahead of the front. Most
CAMs have not been handling the ongoing convection across the Upper
Midwest well, but the ARW WRF does seem to have a clue, which is
making me lean on its solution.  It develops several clusters of
convection along the front which grow upscale into a QLCS that races
south-southeast through the forecast area.  Timing may not be
perfect, but the evolution makes sense.  The RAP and GFS never show
more than about 2000 J/Kg MUCAPE Tuesday night as the line moves
through, and deep layer shear is only about 15-25kts.  That being
said, we could see some damaging wind, particularly across northern
portions of the area as the QLCS moves out of Iowa.  Not sure how
far south the wind threat will continue, but the storms should
weaken as they move south away from the greatest instability.

The latest guidance continues to show a slower solution for the
FROPA which will bring us relief from our hot spell.  This looks
like it`s primarily due to a slower and sharper upstream short wave
moving across the northern Plains and Upper Midwest late tonight
into early Wednesday morning.  The stronger wave forces low level
cyclogenesis over the eastern Plains to along the Missouri/Iowa
border.  The surface-850 low closes off and takes its time
meandering southeast through Missouri and southern Illinois
Wednesday.  While the front doesn`t make it all the way through the
area until some time during the late afternoon/early evening, clouds
and convection should keep temperatures in the 80s.  Speaking of
convection, both the RAP and to a lesser extent the GFS show
instability increasing again in the afternoon across southern
Missouri and Illinois ahead of the front.  The RAP is showing 3000+
J/Kg of MLCAPE with deep-layer shear of 30+ kts, so there will be
another threat for severe storms Wednesday afternoon ahead of the
front.

Carney

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 341 AM CDT Tue Jun 25 2024

Not much has changed for the remainder of the forecast. After a
cooler Thursday under the influence of high pressure centered over
the Great Lakes, southerly flow returns on Friday and we warm back
into the 90 on Friday.  Another cold front develops over northern
Plains as a short wave aloft moves across the US/Canada border late
Friday and the cold front associated with this system dives through
the Mid Mississippi Valley Saturday.  Temperatures ahead of the
front on Saturday are forecast to reach into the 90s again, although
ensemble guidance is showing a pretty wide spread across the area
due to differences with the speed of the front.  The 25th to 75th
percentile high temperature spread on the LREF is 9 degrees at
Lambert on Saturday, and the NBM forecast is warmer than the 75th
percentile, so the current forecast may be a little hot.  I did not
adjust it this morning due to the uncertainty in the speed of the
front at this time.  One way or another, Sunday should be much
cooler with another strong high pressure system moving into the
Midwest and continuing to the east over the Great Lakes on Monday.

Carney

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1043 PM CDT Mon Jun 24 2024

VFR conditions are expected through much of the period as we await
the potential for thunderstorms develop late Tuesday.

Focus continues to be on fine tuning thunderstorm potential
Tuesday afternoon, evening, and into the overnight period. A
number of the hi-res guidance has either backed away from or
delayed afternoon convection. Trends do not provide any higher
confidence (actually less) in where isolated to scattered
thunderstorms could develop during the afternoon, if any do.
Thunderstorms over the Upper Midwest tonight will need to be
watched to determine where, or if, resulting outflow will make it
far enough south to assist in development. Any daytime
thunderstorms would be after 18z through 00z. Otherwise, the
primary impacts will hold off until later in the evening into the
overnight hours.

Another complex of thunderstorms is expected originate somewhere
over southwest Iowa, which the pushes southeast into northern
Missouri and west-central Illinois. The breadth of the complex
will be such that all TAF site will eventually be affected with
KUIN first in line, followed by central MO and metro terminals.
Trends seems to be delaying it`s arrival by an hour or two at any
given site, placing the complex just northwest of KUIN by 06z
tomorrow night, KCOU/KJEF around 09z and metro terminal 10z and
afterward. The other question is just how long in maintains
intensity this far south. This complex, especially while mature,
could result in high winds of 60-70 mph, heavy rain, and frequent
lightning. This potential is highest around KUIN/KCOU/KJEF, but
become questionable further south and east. Any direct impact
could result in MVFR/IFR visibility and localize IFR cigs.

Maples

&&

.CLIMATE...
Issued at 353 AM CDT Mon Jun 24 2024

With hot temperatures forecast, here are some of the daily high
temperature records:

            MON JUN 24     TUE JUN 25
ST LOUIS    102 in 1988    102 in 1954
COLUMBIA    103 in 1988    102 in 1988
QUINCY       99 in 1988    102 in 1931


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from noon today to 8 PM CDT this evening for
     Audrain MO-Callaway MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Lincoln MO-Madison MO-Montgomery MO-
     Osage MO-Reynolds MO-Saint Charles MO-Saint Francois MO-
     Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-
     Warren MO-Washington MO.

     Heat Advisory until 8 PM CDT this evening for Boone MO-Cole MO-
     Moniteau MO.

IL...Heat Advisory from noon today to 8 PM CDT this evening for Bond
     IL-Clinton IL-Fayette IL-Madison IL-Marion IL-Monroe IL-
     Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX