374
FXUS63 KLSX 201720
AFDLSX
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
1120 AM CST MON JAN 20 2025
KEY MESSAGES
- THE COMBINATION OF LOWS BETWEEN -5 AND +5 DEGREES AND NORTHWEST WINDS
AROUND 10 MPH ARE EXPECTED TO YIELD WIND CHILL VALUES OF -5 TO
-20 DEGREES LATE TONIGHT INTO TUESDAY MORNING. A COLD WEATHER
ADVISORY HAS BEEN ISSUED FOR MUCH OF THE REGION, WITH
EXCEPTIONS INCLUDING PARTS OF METRO ST. LOUIS AND ADJACENT
SOUTHEAST MISSOURI AND SOUTHWEST ILLINOIS.
- TEMPERATURES WILL MODERATE QUICKLY WEDNESDAY DURING THE DAY, AND
MORE SEASONABLE TEMPERATURES ARE EXPECTED THROUGH THE END OF THE
WEEK.
SHORT TERM
(THROUGH LATE TUESDAY AFTERNOON)
ISSUED AT 321 AM CST MON JAN 20 2025
ALL THE FOCUS REMAINS ON THE DANGEROUS COLD OVER THE NEXT 24-36
HOURS. A SURFACE RIDGE AXIS IS CURRENTLY MOVING ACROSS THE CWA,
WITH MOSTLY LIGHT/VARIABLE WINDS. SURFACE TEMPERATURES HAVE
DROPPED TO NEAR ZERO DEGREES FROM CENTRAL MISSOURI NORTHEASTWARD
INTO WEST- CENTRAL ILLINOIS. VALUES SHOULD DROP AT LEAST ANOTHER
FEW DEGREES THROUGH 7 AM. THERE ISN'T TOO MUCH WIND AS ALLUDED TO
EARLIER, BUT WE STILL EXPECT MINIMUM WIND CHILLS DROPPING TO
BETWEEN -10 AND -15F BY EARLY THIS MORNING IN NORTHEAST MISSOURI
AS WELL AS WEST CENTRAL AND SOUTH CENTRAL ILLINOIS. A COLD WEATHER
ADVISORY IS IN EFFECT UNTIL 10 AM FOR PARTS OF SOUTH-CENTRAL
ILLINOIS. HIGHS TODAY ARE EXPECTED TO "REBOUND" AHEAD OF THE
APPROACHING ARCTIC COLD FRONT, WITH PLENTY OF SUNSHINE AND EVEN
SOME WEAK DOWNSLOPING OFF OF THE OZARK PLATEAU. THIS SHOULD HELP
BOOST TEMPERATURES MAYBE AN EXTRA COUPLE OF DEGREES IN PARTS OF
CENTRAL AND EAST-CENTRAL MISSOURI. THE TERM "REBOUND" IS USED IN
QUOTES THOUGH FOR A REASON. IT STILL WILL BE A VERY COLD DAY, EVEN
FOR MID/LATE JANUARY STANDARDS. HIGHS ARE FORECAST TO RANGE FROM
THE MID TEENS TO LOW 20S FROM NORTH TO SOUTH.
WEAK LOW-LEVEL ASCENT ASSOCIATED WITH THE ARCTIC COLD FRONT COMBINED
WITH WEAK MIDLEVEL FORCING DOWNSTREAM OF AN APPROACHING SHORTWAVE
TROUGH SUGGESTS AT LEAST THE POSSIBILITY OF LIGHT SNOW OR SNOW
FLURRIES. THERE IS EVEN SOME WEAK, TRANSIENT LOW/MID LEVEL
FRONTOGENESIS. HOWEVER, THERE IS PLENTY OF DRY AIR AND THESE FORCING
MECHANISMS LISTED ABOVE LIKELY WILL DO JUST ENOUGH TO SATURATE THE
LOWER TROPOSPHERE VERY BRIEFLY (IF AT ALL) BEFORE IT SHIFTS OFF TO
THE SOUTHEAST. PROBABILITIES FOR MEASURABLE SNOW ON THE HREF TOP OUT
AT ABOUT 20%, WITH 10% OR LESS OF LREF MEMBERS. FLURRIES WERE ADDED
TO THE FORECAST FOR THIS EVENING FOR ABOUT A ~3 HOUR PERIOD FOR MOST
OF THE AREA, BUT THE THREAT FOR EVEN THE LIGHTEST OF DUSTINGS
APPEARS FAIRLY LOW AT THIS TIME.
MODEL GUIDANCE HAS TRENDED COLDER AND COLDER OVER THE PAST COUPLE OF
DAYS WITH THE UPCOMING AIR MASS BEHIND THE ARCTIC FRONTAL BOUNDARY.
GIVEN SURFACE OBSERVATIONS AT 0900 UTC SHOWING TEMPERATURES IN THE -
20S/-30S, THESE TRENDS LIKELY HAVE MERIT. BY 1200 UTC TUESDAY, 850-
HPA TEMPERATURES ARE FORECAST TO RANGE FROM -22 TO -30C.
STANDARDIZED ANOMALIES RANGE FROM -3 TO -4 SIGMA. THE INCOMING
SURFACE HIGH (~1044 HPA) IS ALSO NEAR RECORD STRENGTH FOR THE DATE.
YET ANOTHER PIECE OF EVIDENCE IS SURFACE DEWPOINTS DROPPING DOWN
BETWEEN -10 AND -20F. THOSE VALUES WOULD THREATEN DAILY RECORD LOW
DEWPOINT VALUES AT KCOU AND KUIN. LONG STORY SHORT, THIS IS
CERTAINLY A VERY IMPRESSIVE ARCTIC AIR MASS WITH BACKWARD
TRAJECTORIES SHOWING THE AIR MASS ORIGINATED IN SIBERIA - TYPICALLY
THE COLDEST SOURCE REGION IN THE NORTHERN HEMISPHERE FOR ARCTIC AIR
OUTBREAKS. WITH THIS FORECAST PACKAGE, LOW TEMPERATURES FOR TONIGHT
WERE DROPPED ABOUT A DEGREE FROM THE PREVIOUS SHIFT, AND THERE ARE
SOME CONCERNS THIS STILL MAY NOT BE COLD ENOUGH. THE LACK OF DEEPER,
FRESH SNOW COVER WILL BE THE MAIN MISSING INGREDIENT TO SEEING
TEMPERATURES THAT WOULD THREATEN DAILY RECORDS. LOWS TONIGHT ARE
FORECAST TO RANGE FROM ABOUT -5F IN PARTS OF NORTHEAST MISSOURI AND
WEST-CENTRAL ILLINOIS TO ABOUT +5F IN PORTIONS OF SOUTHEAST MISSOURI
AND SOUTHWEST ILLINOIS. NORTHWEST WINDS LOOK TO STAY UP ENOUGH ON
THE NORTHEASTERN PERIPHERY OF THE HIGH, WITH SPEEDS AROUND 10 MPH.
THESE SUBZERO OR NEAR ZERO TEMPERATURES COMBINED WITH THE
AFOREMENTIONED NORTHWEST WINDS ARE EXPECTED TO PRODUCE MINIMUM WIND
CHILL VALUES RANGING FROM -5F IN SOUTHEAST MISSOURI TO -20F IN PARTS
OF NORTHEAST MISSOURI AND WEST-CENTRAL ILLINOIS. A NEW COLD WEATHER
ADVISORY HAS BEEN ISSUED FOR TONIGHT THROUGH TUESDAY MORNING FOR
MOST OF THE REGION, WITH THE EXCEPTIONS OF PARTS OF THE ST. LOUIS
METROPOLITAN AREA AND ADJACENT SOUTHEAST MISSOURI AND SOUTHWEST
ILLINOIS.
TUESDAY'S HIGHS ARE ONLY EXPECTED TO BE IN THE TEENS FOR MOST OF THE
AREA, WITH PARTS OF WEST CENTRAL AND SOUTH CENTRAL ILLINOIS LIKELY
STAYING IN THE SINGLE DIGITS. THESE READINGS WOULD BE SOME 25-30+
DEGREES BELOW NORMAL FOR THE DATE.
GOSSELIN
LONG TERM
(TUESDAY NIGHT THROUGH SUNDAY)
ISSUED AT 321 AM CST MON JAN 20 2025
(TUESDAY NIGHT)
A RAPID WARMUP WILL BE UNDERWAY TUESDAY NIGHT AS THE SURFACE RIDGE
SLIDES OFF INTO THE OHIO VALLEY, WITH INCREASING SOUTHERLY RETURN
FLOW IN ITS WAKE. IN ADDITION, INCREASING CLOUDS ARE EXPECTED FROM
THE NORTHWEST OVERNIGHT. THE NET IMPACT WILL BE FOR LOWS SOME 5-10
DEGREES WARMER THAN TONIGHT, WITH TEMPERATURES ACTUALLY RISING
FROM THE LATE EVENING HOURS ONWARD.
(WEDNESDAY - FRIDAY)
STRONG LOW-LEVEL WARM AIR ADVECTION CONTINUES DURING THE DAY ON
WEDNESDAY AHEAD OF AN APPROACHING COLD FRONT. HIGHS ARE EXPECTED TO
BE IN THE MAINLY IN THE 30S, THOUGH THE MOSTLY CLOUDY SKY AND GUSTY
SOUTHERLY WINDS WILL MAKE IT LOOK AND FEEL COLDER THAN THAT.
THERE CONTINUES TO BE AT LEAST A LOW CHANCE FOR SOME LIGHT SNOW
WEDNESDAY NIGHT ACROSS PARTS OF NORTHEAST MISSOURI AND WEST-CENTRAL
ILLINOIS WITH THIS FRONTAL PASSAGE. INCREASING MIDLEVEL ASCENT IS
DENOTED DOWNSTREAM OF AN AREA OF VORTICITY MOVING ACROSS THE REGION
ALONG WITH ENHANCED DIVERGENCE BENEATH THE LEFT-EXIT REGION OF AN
UPPER-LEVEL JET STREAK. PROBABILITIES ON THE LREF FOR MEASURABLE
SNOWFALL HAVE CLIMBED CONSIDERABLY OVER THE PAST 24-48 HOURS.
CHANCES AS HIGH AS 40-60% ARE LOCATED FROM CENTRAL MISSOURI
NORTHEASTWARD INTO WEST-CENTRAL ILLINOIS. THERE ARE STILL ENOUGH
SUBTLE TIMING DIFFERENCES HOWEVER, AND ANY SNOW WOULD BE VERY LIGHT.
DUE TO THESE FACTORS, THE CURRENT FORECAST STILL DOES NOT HAVE MUCH
IN THE WAY OF POPS DURING THIS PERIOD, BUT IF THIS SIGNAL MAINTAINS
ITSELF OR STRENGTHENS FURTHER, POPS WILL NEED TO BE ADDED DOWN THE
ROAD. AS WAS ALLUDED TO EARLIER, ANY SNOW WOULD BE VERY LIGHT.
PROBABILITIES FOR EVEN A HALF INCH OR MORE OF SNOW ON THE LREF ARE
AT OR BELOW 10%.
THE PATTERN FOR THE REMAINDER OF THE WEEK REMAINS QUASI-STATIONARY,
WITH CYCLONIC FLOW ALOFT AND AT LEAST ONE MORE DISTINCT MIDLEVEL
SHORTWAVE TROUGH SWINGING THROUGH THE MID-MISSISSIPPI VALLEY
THURSDAY NIGHT/EARLY FRIDAY. THIS FEATURE ALSO MAY SUPPORT SOME
LIGHT SNOW, WITH LREF PROBABILITIES FOR MEASURABLE SNOW RANGING FROM
15-30%. THE HIGHEST CHANCES AT THIS TIME ARE IN EASTERN MISSOURI
OVER INTO OUR ILLINOIS COUNTIES. AT THE VERY LEAST, I WOULD EXPECT
SOME SNOW FLURRIES/SCATTERED VERY LIGHT SNOW SHOWERS ASSOCIATED WITH
THIS FEATURE.
TEMPERATURES ARE EXPECTED TO CONTINUE TO BE BELOW NORMAL THROUGH THE
END OF THE WORK WEEK, BUT NOTHING LIKE EARLIER IN THE WEEK. HIGHS IN
THE 30S AND LOWS IN THE 10S/20S ARE MOST LIKELY. THE SPREAD BETWEEN
THE 25TH AND 75TH PERCENTILE OF THE NBM DO CLIMB BY FRIDAY HOWEVER,
DUE TO UNCERTAINTIES WITH RESPECT TO THE STRENGTH OF THE INCOMING
AIR MASS AND HOW FAR WEST SOME OF THE COLDEST AIR WILL REACH.
(FRIDAY NIGHT - SUNDAY)
THERE IS PRETTY HIGH AGREEMENT IN THE GENERAL SYNOPTIC PATTERN
ACROSS THE CONUS THROUGH NEXT WEEKEND. THE CONSENSUS IS FOR
ANOMALOUS MID/UPPER LEVEL TROUGHING TO CARVE OUT IN THE DESERT
SOUTHWEST/BAJA REGION, WITH ZONAL FLOW ACROSS THE MIDSECTION OF THE
COUNTRY. SOME ENERGY MAY TRY AND EJECT OUT OF THE DESERT SOUTHWEST
AND INTERACT WITH AN APPROACHING FRONTAL BOUNDARY TO PRODUCE SOME
LIGHT PRECIPITATION. HOWEVER, STRONG CONFLUENCE
SHOULD REALLY SHEAR OUT ANY OF THIS ENERGY THAT EJECTS OUT OF THE
DESERT SOUTHWEST. THE CURRENT FORECAST MAINTAINS SLIGHT
CHANCE/CHANCE POPS FOR SATURDAY NIGHT/SUNDAY FOR SOUTHEASTERN
SECTIONS OF THE FORECAST AREA. ANY PRECIPITATION LOOKS LIGHT AT THIS
STAGE, AND WOULD MOST LIKELY FALL AS A CHILLY RAIN.
GOSSELIN
AVIATION
(FOR THE 18Z TAFS THROUGH 18Z TUESDAY AFTERNOON)
ISSUED AT 1120 AM CST MON JAN 20 2025
DRY AND VFR FLIGHT CONDITIONS ARE EXPECTED THROUGH THE TAF PERIOD. A
COLD FRONT ACCOMPANIED BY LOWER VFR CLOUDS WILL PASS THROUGH THE
AREA THIS EVENING, VEERING WESTERLY WINDS TO THE NORTHWEST THAT
COULD GUST 15 TO 20 KT AT TIMES THROUGH TUESDAY MORNING. WINDS WILL
GRADUALLY SLACKEN AND BACK LATE TUESDAY MORNING THROUGH AFTERNOON AS
A SURFACE PRESSURE RIDGE ARRIVES.
PFAHLER
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
WFO LSX
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