829
FXUS63 KLSX 192305
AFDLSX
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
605 PM CDT THU JUN 19 2025
KEY MESSAGES
- FRIDAY KICKS OFF THE FIRST OF SEVERAL DAYS OF IMPACTFUL HEAT AND
HUMIDITY IN PARTS OF CENTRAL AND NORTHEAST MISSOURI. A HEAT
ADVISORY TAKES EFFECT AT NOON TOMORROW AND LASTS THROUGH
TUESDAY.
- PROLONGED HEAT AND HUMIDITY WILL EXPAND ACROSS THE REGION
SATURDAY THROUGH AT LEAST TUESDAY. THE MOST DANGEROUS CONDITIONS
WILL EXIST IN THE ST. LOUIS METROPOLITAN AREA, WHERE AN EXTREME
HEAT WATCH TAKES EFFECT SATURDAY AFTERNOON.
SHORT TERM
(THROUGH LATE FRIDAY NIGHT)
ISSUED AT 231 PM CDT THU JUN 19 2025
BEHIND A COLD FRONT THAT EXITED THE REGION THIS MORNING AND AMIDST A
BUILDING RIDGE ALOFT, WE ARE ENJOYING COMPARATIVELY DRIER AND MORE
PLEASANT CONDITIONS TODAY. THE SURFACE RIDGE WILL SCOOT EAST THIS
EVENING, GRADUALLY ESTABLISHING SOUTHERLY FLOW THAT MARKS THE
BEGINNING OF A DRAMATIC WARMUP GOING INTO THE WEEKEND. WHILE A FEW
LOW-LYING AREAS IN THE OZARKS MAY SEE SOME PATCHY FOG TONIGHT, MORE
UNSHELTERED PARTS OF THE REGION WILL EXPERIENCE JUST ENOUGH WIND TO
PRECLUDE ANY THREAT.
CONTINUED MID-LEVEL HEIGHT RISES AND DEEPER, MORE PRONOUNCED
SOUTHWEST FLOW THROUGH THE BOUNDARY LAYER EVIDENT ON ALL AVAILABLE
GUIDANCE WILL INTENSIFY THE WARM AIR ADVECTION REGIME DURING THE DAY
ON FRIDAY. WHILE NOT PARTICULARLY DANGEROUS FOR MOST OF THE REGION,
TEMPERATURES ARE FORECAST TO JUMP BACK INTO THE UPPER 80S TO LOW
90S, COUPLING WITH INCREASED HUMIDITY TO SEND HEAT INDEX VALUES INTO
THE UPPER 90S TO LOW 100S. THE HIGHEST VALUES WILL BE ACROSS CENTRAL
MISSOURI, WHICH TYPICALLY WARMS MORE THAN SURROUNDING AREAS IN THIS
FLOW REGIME. AS THIS IS VERY LIKELY THE BEGINNING OF AT LEAST 4
CONSECUTIVE TRIPLE-DIGIT HEAT INDEX VALUE DAYS, A HEAT ADVISORY WAS
ISSUED BEGINNING AT NOON FRIDAY AND LASTING THROUGH TUESDAY. THERE
IS INCREASING CONFIDENCE THAT THIS ADVISORY WILL NEED TO BE EXPANDED
IN SUBSEQUENT UPDATES FOR THE REST OF THE FORECAST AREA (EXCEPT FOR
THE ST. LOUIS METRO AREA, WHICH IS CURRENTLY UNDER AN EXTREME HEAT
WATCH). THE BUILDING UPPER-LEVEL RIDGE WILL PROMOTE LARGE-SCALE
SUBSIDENCE THAT KEEPS RAIN CHANCES FAR FROM THE MID-MISSISSIPPI
VALLEY THROUGH AT LEAST LATE MONDAY.
MRB
LONG TERM
(SATURDAY THROUGH NEXT THURSDAY)
ISSUED AT 231 PM CDT THU JUN 19 2025
DANGEROUS HEAT AND HUMIDITY BECOME MORE ESTABLISHED ACROSS THE
REGION BEGINNING SATURDAY WITH THE INTENSIFYING (99TH CLIMATOLOGICAL
PERCENTILE) UPPER RIDGE AXIS PARKED SQUARELY OVER THE OHIO RIVER
VALLEY. A PREPONDERANCE OF THE GLOBAL-SCALE ENSEMBLE MEMBERS ALSO
DRAW A 20-22C 850MB THERMAL RIDGE INTO THE MID-MISSISSIPPI VALLEY.
WHILE NOT OVERLY IMPRESSIVE FROM A CLIMATOLOGICAL STANDPOINT, IT
FITS THE LOCAL CONCEPTUAL MODEL FOR 100-104F HEAT INDEX VALUES WHEN
COUPLED WITH THE STRENGTH AND POSITION OF THE RIDGE ALOFT. THE
THERMAL RIDGE WILL ALSO ACT TO STUNT BOUNDARY LAYER MIXING AND
OVERALL HEIGHT, WHICH WILL KEEP SURFACE DEWPOINT VALUES RELATIVELY
HIGHER THAN IF MIXING WERE MORE ROBUST.
THE RESULTANT COMBINATION OF HEAT AND HUMIDITY WILL SEND HEAT INDEX
VALUES OVER 100F VIRTUALLY AREAWIDE SATURDAY, WITH THE URBAN HEAT
ISLANDS OF COLUMBIA AND ST. LOUIS PEAKING IN THE MID-100S. THE ONE
FLY IN THE OINTMENT HERE WOULD BE IF DOWNSLOPING FROM THE OZARKS
LOCALLY DRIES THE AIR NEAR THE MISSOURI RIVER VALLEY. IN THIS
SITUATION, HOWEVER, IT'S PLAUSIBLE THAT THE DOWNSLOPING WOULD ALSO
WARM TEMPERATURES MORE THAN CURRENTLY FORECAST. IN EITHER CASE,
DANGEROUS HEAT AND HUMIDITY WOULD RESULT. THE EXTREME HEAT WATCH
CURRENTLY SCHEDULED TO TAKE EFFECT IN THE ST. LOUIS METROPOLITAN
AREA LOOKS ON TRACK, ESPECIALLY GIVEN THE VULNERABILITY OF DISPLACED
ST. LOUISANS FOLLOWING LAST MONTH'S TORNADO. AS GREATER CLARITY
COMES REGARDING THE REST OF THE REGION, THE HEAT ADVISORY IN EFFECT
IN CENTRAL/NORTHEAST MISSOURI MAY NEED TO BE EXPANDED IN SUBSEQUENT
UPDATES.
AT FIRST GLANCE, THE PATTERN REMAINS VIRTUALLY UNCHANGED THROUGH
MONDAY. HOWEVER, AN UPPER-LEVEL TROUGH DIVING THROUGH SOUTH-CENTRAL
CANADA LATE SUNDAY (AFTER ANOTHER DAY OF DANGEROUS HEAT AND
HUMIDITY) WILL BEGIN TO IMPINGE ON THE AMPLIFIED RIDGE AND INFLUENCE
THE QUALITY OF THE THERMAL RIDGE AND WARM AIR ADVECTION. SUBTLE
DIFFERENCES IN THE GLOBAL-SCALE ENSEMBLES' DEPICTION OF THIS PATTERN
ARE EVIDENT: SOME KEEPING THE RIDGE RESOLUTE OVER THE EASTERN CONUS
AND OTHERS ERODING ITS NORTHWEST PERIPHERY. THE FORMER WOULD KEEP
RIDGE-RUNNING CONVECTION AT BAY AND PROMOTE SUFFICIENT 850MB
TEMPERATURES FOR CONTINUED DANGEROUS HEAT. THE LATTER INTRODUCES THE
POTENTIAL FOR AT THE LEAST BUILDING CLOUD COVER FROM CONVECTION TO
OUR NORTH, AND AT THE MOST SOME THREAT FOR THUNDERSTORMS FROM
TUESDAY ONWARD. SHOULD THE RIDGE WIN OUT, THE DANGEROUS HEAT AND
HUMIDITY MAY CONTINUE INTO MID-WEEK AT LEAST. IF THE RIDGE ERODES,
IT WOULD PAVE THE WAY FOR A TEMPORARY REPRIEVE FROM THE MORE
DANGEROUS CONDITIONS. EXACTLY HOW LONG THAT REPRIEVE WOULD LAST
DEPENDS ON HOW MUCH THE RIDGE WEAKENS, WHICH IS FAR FROM CERTAIN.
THE CURRENT FORECAST DOES REPRESENT A GRADUAL "COOLDOWN" INTO THE
LOW/MID-90S BY WEDNESDAY, NODDING TO THE MAJORITY OF GUIDANCE
FAVORING A WEAKER RIDGE ALOFT.
MRB
AVIATION
(FOR THE 00Z TAFS THROUGH 00Z FRIDAY EVENING)
ISSUED AT 604 PM CDT THU JUN 19 2025
THERE IS HIGH CONFIDENCE IN DRY AND VFR FLIGHT CONDITIONS AT ALL
LOCAL TERMINALS THROUGH THE TAF PERIOD. WINDS WILL GRADUALLY
BECOME SOUTHERLY TONIGHT AND TOMORROW MORNING AND STRENGTHEN.
ELMORE
CLIMATE
ISSUED AT 1225 PM CDT THU JUN 19 2025
RECORD MAXIMUM TEMPERATURES AT ST. LOUIS, MO (STL), COLUMBIA, MO (COU),
AND QUINCY, IL (UIN) ALONG WITH THE YEAR OF THE RECORD:
| 6/21 | 6/22 | 6/23 | 6/24 | 6/25
--------------------------------------------------------------------
STL | 99 (1988) | 102 (1930) | 101 (1930) | 102 (1988) | 103 (2024)
| | | | |
COU | 98 (1988) | 100 (1988) | 100 (1988) | 103 (1988) | 102 (1988)
| | | | |
UIN | 100 (1988) | 100 (1930) | 100 (1934) | 99 (1988) | 102 (1931)
LSX WATCHES/WARNINGS/ADVISORIES
MO...EXTREME HEAT WATCH FROM SATURDAY AFTERNOON THROUGH TUESDAY
EVENING FOR JEFFERSON MO-SAINT CHARLES MO-SAINT LOUIS CITY
MO-SAINT LOUIS MO.
HEAT ADVISORY FROM NOON FRIDAY TO 9 PM CDT TUESDAY FOR AUDRAIN
MO-BOONE MO-CALLAWAY MO-COLE MO-KNOX MO-MONITEAU MO-MONROE
MO-OSAGE MO-SHELBY MO.
IL...EXTREME HEAT WATCH FROM SATURDAY AFTERNOON THROUGH TUESDAY
EVENING FOR JERSEY IL-MADISON IL-MONROE IL-SAINT CLAIR IL.
WFO LSX
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