183
FXUS63 KLSX 152027
AFDLSX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
227 PM CST SUN DEC 15 2024
KEY MESSAGES
- A ROUND OF SHOWERS IS EXPECTED OVERNIGHT THROUGH TOMORROW
MORNING, WITH BEST CHANCES (50 TO 90%) ROUGHLY ALONG AND SOUTH
OF I-44. A FEW THUNDERSTORMS ARE ALSO POSSIBLE.
- ANOTHER ROUND OF SHOWERS IS LIKELY (50 TO 70%) IN ROUGHLY THE
SAME AREAS TUESDAY NIGHT THROUGH EARLY WEDNESDAY MORNING AS
WELL.
- COOLER, DRIER CONDITIONS ARE LIKELY WEDNESDAY THROUGH THE END OF
THE WORK WEEK.
SHORT TERM
(THROUGH LATE MONDAY NIGHT)
ISSUED AT 222 PM CST SUN DEC 15 2024
WHILE LOW CLOUDS HAVE HUNG ON FOR QUITE A WHILE TODAY, THE SOUTHERN
EDGE OF THIS STRATUS SHIELD IS SLOWLY ADVANCING NORTH AND MUCH OF
THE AREA IS FINALLY EMERGING INTO THE SUNLIGHT, INCLUDING FINALLY
THE ST. LOUIS METRO AREA. HOWEVER, MANY AREAS ACROSS NORTHERN
MISSOURI AND WEST-CENTRAL MISSOURI ARE NOT QUITE AS FORTUNATE, AND
REMAIN SOCKED IN WITH GLOOMY CLOUD COVER AND EVEN A BIT OF LINGERING
FOG AS OF 1 PM. THIS IS LIKELY TO PERSIST ANOTHER FEW HOURS, LEAVING
ONLY A NARROW WINDOW OF POSSIBLE SUNLIGHT BEFORE IT SINKS BELOW THE
HORIZON.
MEANWHILE, WEAK SOUTHERLY FLOW HAS RESUMED ACROSS THE AREA AS A WARM
FRONT SLOWLY BEGINS TO ADVANCE NORTHWARD. MOST OF THE LOWER MISSOURI
BASIN CURRENTLY RESIDES UNDERNEATH A TRANSIENT UPPER LEVEL RIDGE,
BUT A BROAD, PROGRESSIVE TROUGH IS QUICKLY APPROACHING FROM THE
WEST, ALONG WITH STEADILY INCREASING SOUTHWESTERLY LOW-LEVEL FLOW
AND WARM AIR/MOISTURE ADVECTION. SPC MESOANALYSIS REVEALS A POOL OF
RICH MOISTURE LOCATED TO OUR SOUTH AND SOUTHWEST ACROSS OKLAHOMA AND
ARKANSAS, AND THIS MOISTURE WILL QUICKLY ADVANCE NORTHWARD AND INTO
THE AREA BY LATER THIS EVENING. AS THIS OCCURS, LOW LEVEL ISENTROPIC
ASCENT IS EXPECTED TO GENERATE AN EXPANSIVE FIELD OF LIGHT SHOWERS
AND/OR DRIZZLE, LIKELY BY A FEW HOURS AFTER SUNSET. THIS INITIAL
ROUND OF PRECIPITATION IS NOT EXPECTED TO PRODUCE MUCH IN THE WAY OF
ACCUMULATIONS, BUT MAY BE ENOUGH TO GET THE GROUND WET, AND MAY ALSO
BE ACCOMPANIED BY SOME PATCHY FOG.
HOWEVER, MORE SUBSTANTIAL SHOWERS ARE LIKELY TO DEVELOP LARGELY
AFTER MIDNIGHT ROUGHLY ALONG AND SOUTH OF I-44 AS A COLD FRONT
SLOWLY APPROACHES AND LIFT INCREASES. MODELS ALSO CONTINUE TO DEPICT
SOME MODEST INSTABILITY, BUT THIS GENERALLY REMAINS BELOW 250 J/KG
LOCALLY IN THE VAST MAJORITY OF ENSEMBLE MEMBERS. MEANWHILE, PWAT
VALUES ARE LIKELY TO CLIMB TO BETWEEN 1 AND 1.5 INCHES IN THE WARM
SECTOR, WHICH NEAR OR EVEN SLIGHTLY HIGHER THAN CLIMATOLOGICAL
MAXIMUMS PER THE SPC SOUNDING CLIMATOLOGY ANALYSIS. THUS, HIGH
CONFIDENCE EXISTS THAT SUBSTANTIAL SHOWERS, AND LIKELY A FEW
THUNDERSTORMS WILL DEVELOP ALONG A RELATIVELY NARROW CORRIDOR
OVERNIGHT THROUGH TOMORROW MORNING, WITH STORM MOTIONS NOT QUITE
PARALLEL TO THE ADVANCING BOUNDARY BUT NOT FAR OFF, EITHER.
AS SUCH, THERE IS THE POTENTIAL TO SEE SOME FAIRLY SUBSTANTIAL RAIN
WITHIN A NARROW CORRIDOR DURING THIS EVENT, BUT CONFIDENCE IS LOW
THAT THE MOST SIGNIFICANT RAIN WILL OCCUR WITHIN OUR FORECAST AREA.
AS OF NOW, LATEST HREF GUIDANCE AND LREF CLUSTER ANALYSIS SUGGESTS
THAT IT IS MUCH MORE LIKELY FOR THE HEAVIEST RAIN TO FALL NEAR THE
OHIO RIVER, WITH LOWER PROBABILITIES FOR RAIN EXCEEDING 1 INCH (30%
OR LOWER) ACROSS OUR COUNTIES. STILL, A DECENT SOAKING RAIN WILL BE
POSSIBLE ACROSS PARTS OF SOUTHEAST MISSOURI AND SOUTHWEST ILLINOIS,
POTENTIALLY AS FAR NORTH AS THE ST. LOUIS METRO AREA. AGAIN,
THUNDERSTORMS REMAIN POSSIBLE, BUT INSTABILITY PROJECTIONS HAVE
TRENDED LOWER OVERALL, AND THE POTENTIAL FOR STRONG/SEVERE STORMS
WILL BE LIMITED AND LIKELY FAVORED SLIGHTLY TO OUR SOUTH.
SHOWERS ARE EXPECTED TO MOVE EAST OF THE AREA BEFORE ROUGHLY NOON
TOMORROW, AND A COLD FRONT WILL USHER IN DRIER AIR ALONG WITH GUSTY
WESTERLY WINDS. THERE IS NOT LIKELY TO BE A HUGE TEMPERATURE DROP
BEHIND THIS COLD FRONT, BUT DAILY MAXIMUMS MAY BE REACHED IN THE
MORNING AS OPPOSED TO THE TYPICAL TIME IN THE AFTERNOON AS A RESULT.
ON THAT NOTE, WE MAY EVEN REACH OR EXCEED 60 DEGREES IN SOME PLACES
MONDAY MORNING THANKS TO THE STRONG WARM AIR ADVECTION AHEAD OF THIS
COLD FRONT, ESPECIALLY ACROSS THE OZARKS AND SOUTHWEST ILLINOIS.
BRC
LONG TERM
(TUESDAY THROUGH NEXT SUNDAY)
ISSUED AT 222 PM CST SUN DEC 15 2024
ZONAL FLOW ALOFT WILL REMAIN IN PLACE FOLLOWING MONDAY'S DEPARTING
SHORTWAVE, KEEPING TRAFFIC OPEN FOR ANOTHER SHORTWAVE TO FOLLOW
QUICKLY BEHIND IT. TUESDAY IS LIKELY TO BE ANOTHER DRY TRANSITION
DAY, AS GUSTY LOW LEVEL WESTERLIES QUICKLY SWITCH BACK TO THE SOUTH
AND SOUTHWEST AHEAD OF THE NEXT APPROACHING SYSTEM. THIS WILL SET UP
A VERY SIMILAR SITUATION AS TONIGHT/MONDAY, AT LEAST SUPERFICIALLY.
AN INCREASING SOUTHWESTERLY LOW LEVEL JET WILL ONCE AGAIN TRANSPORT
A WARM AND HUMID AIRMASS BACK INTO THE AREA TUESDAY NIGHT AHEAD OF
ANOTHER ADVANCING COLD FRONT, AGAIN WITH THE RICHEST MOISTURE
CONTENT REACHING ROUGHLY ALONG AND SOUTH OF THE I-44 CORRIDOR.
ISENTROPIC LIFT IN THE WARM SECTOR WILL GENERATE ANOTHER ROUND OF
SHOWERS AS A RESULT, WHICH WILL SLOWLY MOVE SOUTHEASTWARD AND END
SOMETIME ON WEDNESDAY. SOUND FAMILIAR?
NO TWO WEATHER EVENTS ARE EXACTLY ALIKE, AND THERE ARE A FEW
DIFFERENCES WORTH DISCUSSING. FIRST, WHILE PRECIPITATION
PROBABILITIES HAVE SLOWLY INCREASED FOR THIS EVENT OVER TIME,
ENSEMBLE QPF CONTINUES TO BE MOSTLY LIGHT ACROSS OUR AREA, WITH
PROBABILITIES OF MORE THAN 1/2" REMAINING BELOW 30% ACROSS OUR
AREA. IT SHOULD ALSO BE NOTED THAT 3 OUT OF 4 LREF CLUSTERS ARE
RELATIVELY CONSISTENT IN KEEPING THE MORE SIGNIFICANT RAIN TOTALS
TO OUR SOUTH AS WELL. HOWEVER, THERE IS ONE CLUSTER IN PARTICULAR
THAT PLACES THE HIGHER PRECIPITATION AMOUNTS FARTHER NORTH, WITH
ROUGHLY EQUAL MEMBER REPRESENTATION AMONG THE GEFS/GEPS/ENS AND A
NON-TRIVIAL MEMBER SHARE (~25% OF ALL MEMBERS). THIS PARTICULAR
CLUSTER FEATURES A SLOWER AND DEEPER TROUGH SOLUTION, WHICH LIKELY
ALLOWS FOR A LONGER PERIOD OF WARM AIR/MOISTURE ADVECTION ACROSS
THE AREA. WHILE THIS IS STILL IN THE MINORITY, IT HAS ENOUGH
REPRESENTATION THAT IT SHOULD NOT BE DISCOUNTED.
FINALLY, TEMPERATURES REMAIN LIKELY TO BE WELL ABOVE FREEZING WITHIN
THE WARM SECTOR, AND THIS STRONGLY FAVORS EXCLUSIVELY RAIN FOR
THE VAST MAJORITY OF THE PRECIPITATION. HOWEVER, TEMPERATURES
BEHIND WEDNESDAY'S COLD FRONT ARE EXPECTED TO BE MUCH COLDER THAN
TOMORROW'S COLD FRONT, AND AS A RESULT, NBM AND LREF GUIDANCE DOES
PAINT A 10-20% CHANCE OF WINTRY PRECIPITATION ACROSS MAINLY
NORTHEAST MO AND WEST-CENTRAL ILLINOIS. IT'S NOT SOMETHING TO GET
TOO EXCITED ABOUT JUST YET, BUT SOMETHING TO KEEP AN EYE ON IN
CASE WEDNESDAY'S COLD FRONT TRENDS A BIT STRONGER IN FUTURE RUNS.
WEDNESDAY ONWARD, THE AFOREMENTIONED COLD FRONT WILL USHER IN A MORE
SEASONABLY COOL AND DRIER AIRMASS, AND NORTHWEST FLOW ALOFT IS
LIKELY TO DEVELOP AS A LONGWAVE TROUGH DEVELOPS OVER THE EASTERN
CONUS. ANOTHER CLIPPER-TYPE SHORTWAVE IS LIKELY TO DIG INTO THE AREA
LATE IN THE WEEK OR OVER THE WEEKEND WITHIN THIS NORTHWEST FLOW,
WHICH PRESENTS A LOW (10-30% CHANCE) FOR PRECIPITATION IN SOME AREAS
AS WELL. THERE REMAINS SIGNIFICANT VARIABILITY IN TIMING AND
STRENGTH WITH THIS FEATURE, BUT THERE IS AT LEAST ONE LREF CLUSTER
THAT PRODUCES NON-TRIVIAL WINTRY PRECIPITATION AS THIS WAVE MOVES
THROUGH. IT'S SOMETHING TO KEEP AN EYE ON, BUT VERY LITTLE CAN BE
SAID ABOUT ITS POTENTIAL IMPACTS JUST YET.
BRC
AVIATION
(FOR THE 18Z TAFS THROUGH 18Z MONDAY AFTERNOON)
ISSUED AT 1130 AM CST SUN DEC 15 2024
LOW STRATUS, FOG, AND ANOTHER ROUND OF MOSTLY LIGHT RAIN CAN BE
EXPECTED TO IMPACT TERMINALS AT VARIOUS TIMES THROUGHOUT THE 18Z
TAF PERIOD.
STRATUS HAS ERODED AT CENTRAL MISSOURI TERMINALS (OR
WILL VERY SHORTLY IN THE CASE OF COU), BUT CONTINUES TO LINGER AT
ST. LOUIS AREA TERMINALS AND UIN. MVFR CEILINGS AROUND ST. LOUIS
ARE LIKELY TO SLOWLY DISSIPATE TO VFR COVERAGE WITHIN THE FIRST
COUPLE HOURS OF THE PERIOD, BUT IFR AND EVEN SOME LIFR CEILINGS
MAY PERSIST FOR SEVERAL MORE HOURS AT UIN. IN FACT, IT'S POSSIBLE
THAT CEILINGS NEVER IMPROVE BEYOND MVFR LEVELS AT UIN BEFORE
ANOTHER ROUND OF LOW CEILINGS AND LIGHT RAIN ARRIVES LATER THIS
EVENING.
THIS LATTER ROUND OF LOW CEILINGS AND LIGHT RAIN WILL
IMPACT ALL OTHER TERMINALS AS WELL, WITH IFR CEILINGS LIKELY AND
POTENTIALLY SOME VISIBILITY REDUCTIONS AS WELL. THIS MAY RESEMBLE
MORE OF A HEAVY DRIZZLE THAN TRUE STRATIFORM RAIN, SO WHILE RAIN
ACCUMULATIONS ARE NOT LIKELY TO BE SIGNIFICANT, VISIBILITIES MAY
BE IMPACTED ANYWAY. SOME HEAVIER SHOWERS WILL BE POSSIBLE AROUND
ST. LOUIS TERMINALS VERY LATE TONIGHT UNTIL NEAR SUNRISE, BUT
CONFIDENCE IS SLIGHTLY LOWER REGARDING THIS POSSIBILITY. PATCHY
FOG WILL ALSO BE POSSIBLE AT ALL TERMINALS THROUGH TOMORROW
MORNING.
A COLD FRONT IS EXPECTED TO SCOUR OUT CLOUDS AND LIGHT RAIN NEAR
THE END OF THE PERIOD, FOLLOWED BY GUSTY WEST WINDS.
BRC
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
WFO LSX
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