729
FXUS63 KLSX 201758
AFDLSX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
1258 PM CDT THU AUG 20 2026
KEY MESSAGES
- PATCHY DENSE FOG IS EXPECTED THROUGH EARLY THIS MORNING,
MAINLY IN PARTS OF CENTRAL MISSOURI.
- NEAR TO SLIGHTLY BELOW AVERAGE TEMPERATURES ARE FORECAST TO
CONTINUE THROUGH THE WEEKEND INTO EARLY NEXT WEEK.
SHORT TERM /THROUGH FRIDAY/
ISSUED AT 322 AM CDT THU AUG 20 2026
ISOLATED TO SCATTERED SHOWERS AND WEAK THUNDERSTORMS ACROSS
SOUTHEAST MISSOURI AND SOUTH-CENTRAL ILLINOIS ARE EXPECTED TO
SHIFT OUT OF THE REGION OVER THE NEXT COUPLE OF HOURS. THE
LATEST RAP SHORT-TERM GUIDANCE SHOWS THE LOW-LEVEL JET
CONTINUING TO VEER WITH TIME, WITH THE STRONGER MOISTURE
CONVERGENCE SHIFTING INTO THE LOWER OHIO VALLEY AND OUT OF OUR
REGION. MOSTLY DRY WEATHER IS THEN EXPECTED FOR THE REST OF THE
DAY, THOUGH DID ADD ISOLATED SHOWERS FOR PORTIONS OF SOUTH-
CENTRAL ILLINOIS THIS AFTERNOON. THIS IS WHERE WEAK LOW-LEVEL
CONVERGENCE WILL REMAIN WITH A MIDLEVEL SHORTWAVE TROUGH MOVING
ACROSS THE MID-MISSISSIPPI VALLEY. IN ADDITION, GUIDANCE IS
SHOWING AT LEAST MODEST LOW-LEVEL INSTABILITY AIDED BY
STEEPENING LAPSE RATES. TEMPERATURES TODAY ARE EXPECTED TO BE A
BIT COOL FOR THE DATE ONCE AGAIN DUE TO WEAK TO MODERATE LOW-
LEVEL COLD AIR ADVECTION AND PLENTY OF STRATOCUMULUS CLOUDS
THROUGHOUT THE DAY. HIGHS ARE FORECAST TO TOP OUT IN THE LOW TO
MID 80S, WHICH WOULD BE ABOUT 3-5 DEGREES BELOW NORMAL.
THE OTHER CONCERN IS LOW STRATUS/FOG. THE GOES-19 NIGHTTIME
MICROPHYSICS RGB SHOWS A HODGE-PODGE OF BOTH STRATUS AND FOG. SOME
VERY PATCHY DENSE FOG WILL PROBABLY CONTINUE THROUGH EARLY THIS
MORNING, MAINLY IN CENTRAL MISSOURI. THIS FOG SHOULD DISSIPATE BY
MID MORNING, THOUGH THE FOG/LOW STRATUS LIKELY WILL TURN INTO MORE
OF A BROKEN STRATOCUMULUS DECK ACROSS MUCH OF THE AREA FOR THE
REMAINDER OF THE DAY.
TONIGHT LOOKS LIKE A REPEAT OF THIS PAST MONDAY NIGHT/TUESDAY
MORNING WITH A SURFACE RIDGE OF HIGH PRESSURE SLIDING INTO THE BI-
STATE REGION. LIGHT/VARIABLE WINDS ALONG WITH A MOSTLY CLEAR SKY IS
EXPECTED, ALLOWING TEMPERATURES TO DROP INTO THE LOW TO MID 60S. A
FEW SPOTS IN SOUTH-CENTRAL ILLINOIS AND/OR RIVER VALLEYS IN EAST
CENTRAL AND SOUTHEAST MISSOURI MAY ONCE AGAIN FLIRT WITH THE UPPER
50S. STEAM FOG IS POSSIBLE ON AREA RIVERS DUE TO WATER-AIR
TEMPERATURE DIFFERENCES NEARING 20 DEGREES IN SPOTS. A SLIGHT WARMUP
IS FORECAST ON FRIDAY DUE TO INCREASED SUNSHINE AND A SHIFT MORE
TOWARD WEAK LOW-LEVEL WARM AIR ADVECTION. LOOK FOR HIGHS TO BE
MAINLY IN THE MID TO UPPER 80S, OR RIGHT ABOUT AVERAGE FOR MID/LATE
AUGUST.
LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/
ISSUED AT 322 AM CDT THU AUG 20 2026
(FRIDAY NIGHT - SATURDAY)
ANOTHER NORTHWEST FLOW MIDLEVEL SHORTWAVE TROUGH IS EXPECTED TO
TRANSVERSE THE AREA FRIDAY NIGHT INTO EARLY SATURDAY MORNING. AT THE
SURFACE, A WEAK COLD FRONT WILL MOVE THROUGH. A FEW SHOWERS/WEAK
THUNDERSTORMS ARE POSSIBLE, BUT BY AND LARGE, THIS FRONT SHOULD PASS
THROUGH WITHOUT MUCH FANFARE. SURFACE CONVERGENCE ALONG THE FRONT IN
OUR AREA IS VERY WEAK, AND MORE CONCENTRATED ACROSS NORTHWEST
MISSOURI AS WELL AS THE SOUTHERN GREAT LAKES (ALSO WHERE THE BETTER
MID/UPPER LEVEL FORCING WILL RESIDE). FURTHERMORE, MUCH WEAKER
INSTABILITY IS EXPECTED TO RESIDE IN OUR AREA COMPARED TO THE MID-
MISSOURI VALLEY AND UPPER MIDWEST. MOST LOCATIONS PROBABLY WILL NOT
OBSERVE ANY MEASURABLE RAINFALL, AND THOSE THAT DO SHOULD NOT SEE
ANYTHING SIGNIFICANT (<0.25").
(SATURDAY NIGHT - WEDNESDAY)
THERE IS BETTER CONSENSUS WITH THE LONGWAVE TROUGH DIGGING INTO THE
GREAT LAKES THIS WEEKEND. THE GEFS HAS TRENDED MORE TOWARD THE EPS,
SHOWING A MORE ANOMALOUS (500-HPA HEIGHTS <10TH PERCENTILE OF
CLIMATOLOGY) TROUGH, BRINGING IN A MORE SUBSTANTIAL SHOT OF COOLER
AND DRIER AIR INTO THE MID-MISSISSIPPI VALLEY. FOR REFERENCE, THE
GEFS MEAN 850-HPA TEMPERATURES FOR SUNDAY EVENING ARE NOW ABOUT 4C
COOLER COMPARED TO 24 HOURS AGO. THE EPS REMAINS COOLER THAN THE
GEFS, SHOWING 850-HPA TEMPERATURE ANOMALIES OF -2 TO -5C. GIVEN THE
COOLER TRENDS, BOTH HIGH AND LOW TEMPERATURES ARE SEVERAL DEGREES
COOLER SUNDAY THROUGH MONDAY. HIGHS EACH DAY ARE MOST LIKELY TO BE
IN THE LOW 80S, WITH LOWS IN THE UPPER 50S TO LOW 60S. IF THE COOLER
EPS IS CORRECT, THESE TEMPERATURES STILL MAY BE A FEW DEGREES TOO
WARM.
MOSTLY DRY WEATHER IS FORECAST THROUGH THE END OF THE PERIOD. THE
ONLY CHANCE (20-30%) OF RAIN COMES SUNDAY NIGHT/MONDAY, MAINLY IN
CENTRAL/SOUTHEAST MISSOURI. THERE REMAINS A SIGNAL FOR AN MCS COMING
AROUND THE NORTHEAST PERIPHERY OF THE MIDLEVEL ANTICYCLONE ACROSS
THE SOUTHWEST/SOUTH CENTRAL PLAINS. THIS "RING OF FIRE" PATTERN
THOUGH LOOKS MORE LIKELY TO IMPACT AREAS TO OUR WEST, WITH COOLER
AND MORE STABLE AIR IN OUR REGION.
AVIATION /18Z TAFS THROUGH 00Z SATURDAY/
ISSUED AT 1257 PM CDT THU AUG 20 2026
FLIGHT CONDITIONS WILL CONTINUE TO IMPROVE TO VFR THROUGH EARLY
AFTERNOON AS CU/STRATOCU LIFTS AND SCATTERS. A NEARBY SURFACE HIGH
PRESSURE CENTER WILL LEAD TO LIGHT AND OFTEN VARIABLE WINDS AS WELL
AS CONTRIBUTE TO CONDITIONS FAVORABLE FOR ANOTHER ROUND OF FOG LATE
TONIGHT/FRIDAY MORNING. HOWEVER, RIVER VALLEYS ARE MOST LIKELY TO
SEE IFR FLIGHT CONDITIONS, WITH RELATIVELY LOWER CONFIDENCE IN FOG
OUTSIDE OF THOSE AREAS. FLIGHT CONDITIONS WILL IMPROVE AGAIN FRIDAY
MORNING AFTER SUNRISE, MORE QUICKLY THAN TODAY.
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
SHORT TERM...GOSSELIN
LONG TERM...GOSSELIN
AVIATION...PFAHLER
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