865
FXUS63 KLSX 212111
AFDLSX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
311 PM CST TUE JAN 21 2025
KEY MESSAGES
- BITTERLY COLD ARCTIC AIR WILL REMAIN IN PLACE UNTIL EARLY
WEDNESDAY MORNING WITH WIND CHILLS AS LOW AS -10 TO -17 F AGAIN
ACROSS PORTIONS OF IL THIS EVENING INTO EARLY WEDNESDAY
MORNING, WHERE A COLD WEATHER ADVISORY IS IN EFFECT.
- TEMPERATURES WILL BRIEFLY MODERATE CLOSER TO AVERAGE ON
WEDNESDAY, VARYING THEREAFTER BETWEEN BELOW AVERAGE (THURSDAY
AND FRIDAY) AND NEAR AVERAGE (WEEKEND ONWARD).
- THERE IS A 30 TO 50 PERCENT CHANCE OF VERY LIGHT SNOW AND SNOW
FLURRIES WEDNESDAY EVENING, MAINLY IN NORTHEASTERN MO AND WEST-
CENTRAL IL. IT IS UNCERTAIN IF/HOW MUCH OF THIS LIGHT SNOW WILL
BE MEASURABLE/ACCUMULATE.
SHORT TERM
(THROUGH LATE WEDNESDAY NIGHT)
ISSUED AT 309 PM CST TUE JAN 21 2025
AN ARCTIC HIGH PRESSURE CENTER, LOCATED ACROSS THE CWA THIS
AFTERNOON, WILL SHIFT EASTWARD INTO THE LOWER OHIO VALLEY TONIGHT
WITH LOW-LEVEL WAA INCREASING THROUGH THE NIGHT, BUT NOT BEFORE A
PERIOD OF EFFICIENT RADIATIONAL COOLING THIS EVENING FOLLOWING A DAY
OF HIGH TEMPERATURES IN THE MID-SINGLE DIGITS TO MID-TEENS F. AS A
RESULT, TEMPERATURES ARE EXPECTED TO FALL BACK INTO OR REMAIN IN THE
SINGLE DIGITS F, APPROACHING 0 F IN WEST-CENTRAL/SOUTH-CENTRAL IL.
SOUTHERLY SURFACE WINDS WILL INCREASE THROUGH THE NIGHT AND LEAD TO
WIND CHILLS REACHING -10 TO -15 F IN THESE COLDEST LOCATIONS WHERE A
COLD WEATHER ADVISORY IS IN EFFECT THROUGH EARLY WEDNESDAY MORNING.
TO THE WEST OF THESE AREAS, TEMPERATURES WILL RISE MORE QUICKLY INTO
THE TEENS F AS WAA DEEPENS AND INTERACTS WITH INCREASING UPPER/MID-
LEVEL MOISTURE TO PRODUCE A WAVE OF UPPER/MID-LEVEL CLOUDS LATE
TONIGHT INTO WEDNESDAY MORNING.
THE CLOUDS AND WAA WILL BE OCCURRING AHEAD OF AN UPPER-LEVEL
SHORTWAVE TROUGH TRACKING FROM THE CENTRAL PLAINS THROUGH THE MID-
MISSISSIPPI RIVER VALLEY ON WEDNESDAY. THE MORNING CLOUDS MAY
ACTUALLY BE PRECIPITATING BUT FORECAST SOUNDINGS AND ENSEMBLE MODEL
GUIDANCE UNANIMOUSLY SUPPORT NO PRECIPITATION REACHING THE GROUND.
NBM HIGH TEMPERATURE DISTRIBUTIONS ARE CONVERGING ON THE STRONG WAA
AND BREAKS IN CLOUDS RESULTING IN TEMPERATURES AROUND 30 TO THE
UPPER 30S F DURING THE AFTERNOON, MARKING THE FIRST TIME THAT
PORTIONS OF THE CWA RISE ABOVE 32 F SINCE LAST SATURDAY.
ANOTHER SHORTWAVE TROUGH WILL IMMEDIATELY FOLLOW THE FIRST SHORTWAVE
TROUGH WEDNESDAY EVENING, PROVIDING AN OPPORTUNITY FOR VERY LIGHT
SNOWFALL. BROAD, WEAK ASCENT IS INDICATED BY MODEL GUIDANCE DURING
THIS TIME, PRODUCED BY A COMBINATION OF A TRANSIENT UPPER-LEVEL JET
EXIT REGION AND WEAK LOW-LEVEL MOISTURE CONVERGENCE/FRONTOGENESIS.
LATEST HREF PROBABILITIES OF MEASURABLE SNOWFALL (0.01" OR MORE) ARE
30 TO 60 PERCENT ACROSS NORTHEASTERN MO AND WEST-CENTRAL IL, BUT THE
MAXIMUM OF THESE PROBABILITIES ARE ACROSS IA AND NORTHERN IL AND
PROBABILITIES OF MORE THAN 0.5" ARE GENERALLY 20 PERCENT OR LESS.
THEREFORE, CONFIDENCE IS HIGH THAT THERE WILL BE AT LEAST SNOW
FLURRIES, BUT IT IS UNCERTAIN IF LIGHT ACCUMULATING SNOWFALL CAN
OCCUR. WITH COLD TEMPERATURE PROFILES AND A DEEP DGZ, EVEN A SLIGHT
UPWARD TICK IN QPF CAN HAVE LARGE IMPLICATIONS IN SNOWFALL
ACCUMULATIONS AND IS WORTH CLOSELY MONITORING FOR AMOUNTS SUFFICIENT
FOR TRAVEL IMPACTS.
PFAHLER
LONG TERM
(THURSDAY THROUGH NEXT TUESDAY)
ISSUED AT 309 PM CST TUE JAN 21 2025
ALONG AND BEHIND A COLD FRONT ASSOCIATED WITH THE FIRST PAIR OF
SHORTWAVE TROUGHS, PERIODS OF SNOW FLURRIES ARE EXPECTED ACROSS MUCH
OF THE AREA LATE WEDNESDAY NIGHT THROUGH THURSDAY AS CAA STEEPENS
LOW-LEVEL LAPSE RATES AND DEEP CYCLONIC FLOW BECOMES ESTABLISHED
ACROSS THE MID-MISSISSIPPI RIVER VALLEY. MODEL GUIDANCE ARE COMING
INTO BETTER AGREEMENT THAT AN ADDITIONAL SHORTWAVE TROUGH WILL
FOLLOW A SLIGHTLY MORE SOUTHERLY TRACK THURSDAY EVENING AND BE
SLIGHTLY WEAKER THAN ITS PREDECESSORS, WITH PROBABILITIES OF
MEASURABLE SNOWFALL NOW 20 PERCENT OR LESS WHICH HAS BEEN STEADILY
LOWERING. A RELATIVELY COLDER AIRMASS WILL ALSO BE IN PLACE THURSDAY
AND FRIDAY WITH 850-HPA TEMPERATURES FALLING BELOW THE 10TH
CLIMATOLOGICAL PERCENTILE BUT 8 TO 10 C WARMER THAN THOSE CURRENTLY
IN PLACE TODAY. THIS AIRMASS WILL RESULT IN COOLER, BELOW AVERAGE
TEMPERATURES THAN WEDNESDAY, BUT NOT AS COLD AS THE TEMPERATURES IN
THE ARCTIC AIRMASS TODAY.
A PATTERN CHANGE WILL BE UNDERWAY LATE FRIDAY THROUGH SATURDAY WITH
LARGE-SCALE SUBSIDENCE FROM MID-LEVEL HEIGHT RISES AND STRONG WAA
ENSUING AHEAD OF A FRONT ARRIVING TO THE CWA, RESULTING IN DRY
CONDITIONS AND A RETURN TO NEAR OR SLIGHTLY ABOVE TEMPERATURES. WITH
THE PRESENCE OF THE FRONT LATER IN THE DAY AND INCREASING UPPER-
LEVEL CLOUDS, THERE IS SOME QUESTION IN EXACTLY HOW WARM IT WILL
GET, BUT NBM INTERQUARTILE RANGES ARE RELATIVELY SMALL (~5 F) FOR
THIS LEAD TIME, SUPPORTING HIGH TEMPERATURES IN THE UPPER 30S TO
TO NEAR 50 F.
ON SUNDAY INTO EARLY NEXT WEEK, AN UPPER-LEVEL TROUGH WILL BECOME
CUTOFF NEAR THE CALIFORNIA COAST AND/OR FOUR CORNERS REGION WITH A
PERIOD OF QUASI-ZONAL, CONFLUENT FLOW DOWNSTREAM ACROSS THE CENTRAL
CONUS. THERE HAS BEEN A TREND IN MODEL GUIDANCE FOR REGION OF
CONFLUENT FLOW TO BE POSITIONED FURTHER SOUTHWARD BY A NORTHERN
STREAM TROUGH, KEEPING THE TRACK OF WEAKENING, EASTWARD-EJECTING
SHORTWAVE TROUGHS ALSO LARGELY SOUTH OF THE MID-MISSISSIPPI RIVER
VALLEY. ACCORDINGLY, ROLLING 24-HOUR PROBABILITIES OF MEASURABLE
PRECIPITATION HAVE DECREASED ACROSS THE CWA TO 20 TO 35 PERCENT
SUNDAY INTO MONDAY AND ARE MAINLY ACROSS SOUTHEASTERN MO AND
SOUTHWESTERN IL, WITH THE AXIS OF HIGHEST PROBABILITIES MUCH FARTHER
TO THE SOUTH. AT THE SAME TIME, THE COLD FRONT THAT ARRIVES LATE
SATURDAY WILL ALSO PUSH TO THE SOUTH OF THE CWA BY SUNDAY, BUT MOST
INDICATIONS POINT TO POST-FRONTAL CAA BEING RELATIVELY WEAK WITH
TEMPERATURES STILL REMAINING APPROXIMATELY NEAR AVERAGE.
PFAHLER
AVIATION
(FOR THE 18Z TAFS THROUGH 18Z WEDNESDAY AFTERNOON)
ISSUED AT 1143 AM CST TUE JAN 21 2025
DRY AND VFR FLIGHT CONDITIONS WILL PREVAIL THROUGH THE TAF PERIOD AT
ALL TERMINALS WITH DEPARTING LOW-VFR STRATOCUMULUS IN THE VICINITY
OF ST. LOUIS METRO TERMINALS THIS AFTERNOON AND WAVES OF UPPER AND
MID-LEVEL CLOUDS OVERNIGHT INTO WEDNESDAY. WEST-NORTHWESTERLY WINDS
WILL BECOME SOUTHERLY TONIGHT UPON PASSAGE OF A SURFACE PRESSURE
RIDGE, OCCASIONALLY GUSTING 17 TO 22 KT BY WEDNESDAY MORNING.
PFAHLER
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
WFO LSX
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