820
FXUS63 KLSX 211723
AFDLSX
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
1123 AM CST SAT DEC 21 2024
KEY MESSAGES
- SEASONABLY-COLD WEATHER LINGERS THROUGH EARLY SUNDAY BEFORE
TEMPERATURES WARM ABOVE NORMAL AND REMAIN THERE THROUGH THE
UPCOMING WEEK.
- NEXT WEEK'S WARMUP IS ACCOMPANIED BY A MORE ACTIVE WEATHER
PATTERN THAT WILL BRING SEVERAL CHANCES FOR RAIN TO THE REGION,
INCLUDING ON CHRISTMAS EVE AND CHRISTMAS DAY.
SHORT TERM
(THROUGH LATE SUNDAY AFTERNOON)
ISSUED AT 214 AM CST SAT DEC 21 2024
THE AMPLIFIED RIDGE-TROUGH PATTERN EVIDENT OVER THE CONUS ON GOES-
EAST MID-LEVEL WATER VAPOR IMAGERY CONTINUES TO PROMOTE NORTHWEST
FLOW ALOFT AND SURFACE HIGH PRESSURE ACROSS THE UPPER MISSISSIPPI
VALLEY. A SLUG OF 925MB MOISTURE TRAPPED NEAR THE BASE OF THIS
LOW-LEVEL RIDGING OVER THE BI-STATE REGION IS PROMOTING STRATUS
OVER MOST OF THE FORECAST AREA. NIGHTTIME MICROPHYSICS IMAGERY
SHOWS THIS STRATUS ERODING FROM THE WEST AS 925MB WINDS VEER
NORTHEASTERLY AND STREAM RELATIVELY-DRIER AIR IN FROM ILLINOIS. CLOUD
COVER WILL GRADUALLY DIMINISH THIS MORNING FROM EAST TO WEST AS A
RESULT. TODAY'S HIGHS REMAIN NEAR YESTERDAY'S LOW/MID-30S OWING
TO THE ABSENCE OF STRONG TEMPERATURE ADVECTION, BUT WIND CHILLS
WILL BE MUCH MORE TOLERABLE WITH WEAKER WINDS.
THE SURFACE HIGH SLIDES EAST OVER THE GREAT LAKES TONIGHT, AND BY
SUNDAY MORNING WEAK RIDGING ALOFT TAKES HOLD OVER THE CENTRAL
CONUS. MEANWHILE, A NORTHERN-STREAM SHORTWAVE EVIDENT ON ALL
AVAILABLE DETERMINISTIC GUIDANCE (THOUGH VARYING IN STRENGTH)
DROPS INTO THE NORTHERN GREAT PLAINS AND DEVELOPS A SURFACE LOW
ACROSS NEBRASKA WITH A COLD FRONT DRAPED THROUGH THE CENTRAL
PLAINS. THE RESULTANT SOUTHERLY FLOW AHEAD OF THE FRONT WILL SEND
TEMPERATURES INTO THE 40S ACROSS MOST OF THE MID-MISSISSIPPI
VALLEY, HERALDING THE START OF A PROLONGED STRETCH OF ABNORMAL
WARMTH AND INCREASED RAIN CHANCES.
MRB
LONG TERM
(SUNDAY NIGHT THROUGH FRIDAY)
ISSUED AT 214 AM CST SAT DEC 21 2024
THE SURFACE LOW TRACKS EAST FROM NEBRASKA THROUGH THE UPPER
MISSISSIPPI VALLEY ON MONDAY, DRAWING ITS COLD FRONT THROUGH THE
FORECAST AREA. THE SYSTEM IS LARGELY MOISTURE-STARVED IN THE
NORTHERN STREAM, WHICH WILL INITIALLY INHIBIT ANY PRECIPITATION
ALONG THE FRONT. ONCE THE FRONT GETS GENERALLY TO THE I-44
(MO)/I-55 (IL) CORRIDOR MONDAY AFTERNOON, IT WILL ENCOUNTER PRECIPITABLE
WATER VALUES APPROACHING THE 90TH CLIMATOLOGICAL PERCENTILE. AS A
RESULT, RAIN CHANCES RISE ALONG THE FRONT STARTING MONDAY
AFTERNOON. THAT SAID, UPPER-LEVEL FORCING AND LIFT IS FAR REMOVED
FROM THIS PART OF THE FRONT, WHICH WILL WORK TO KEEP SHOWER
COVERAGE SCATTERED AND RAINFALL AMOUNTS RELATIVELY LIGHT (80%
CHANCE OF TOTALS BELOW 0.10 INCHES BY LATE MONDAY NIGHT). STRONGER
SOUTHERLY WINDS WILL CONTINUE TO BOLSTER TEMPERATURES AHEAD OF
THE FRONT, AND WHILE THERE'S SOME UNCERTAINTY AS TO HOW WARM ANY
ONE LOCATION WILL GET, THE NBM HIGH TEMPERATURE IQR REMAINS ABOVE
NORMAL.
THE FRONT STALLS GENERALLY ACROSS THE OZARKS BY EARLY CHRISTMAS EVE
AS A SOUTHERN-STREAM WAVE ALOFT DEEPENS OVER THE SOUTHERN GREAT
PLAINS. WITH COLDER 850MB TEMPERATURES LOCKED ACROSS THE GREAT
LAKES, TEMPERATURES ARE NOT EXPECTED TO DROP BY MORE THAN A FEW
DEGREES BEHIND THE FRONT AND WILL REMAIN AT LEAST NEAR-NORMAL
TUESDAY. THE WAVE WILL DEVELOP A SURFACE LOW ALONG OR JUST SOUTH
OF THE STALLED BAROCLINIC ZONE, AND WITH BETTER ACCESS TO GULF
MOISTURE, RAIN CHANCES INCREASE THROUGH THE DAY. THE BEST CHANCES
FOR RAIN (50-60%) ARE STILL ALONG THE SAME I-44/I-55 CORRIDOR
PREVIOUSLY MENTIONED, CENTERED ON TUESDAY AFTERNOON INTO THE
NIGHT. THAT SAID, RAIN CHANCES EXTEND FURTHER NORTH AND WEST OWING
TO UNCERTAINTY REGARDING WHERE THE SURFACE LOW AND FORCING ALOFT
WILL TRACK. AT LEAST A 30% CHANCE FOR RAIN WILL LINGER INTO
CHRISTMAS MORNING, PRIMARILY IN EASTERN MISSOURI AND MOST OF
ILLINOIS. BY THIS POINT, ALL AVAILABLE GUIDANCE SHOWS THE WAVE
ALOFT AND ITS ATTENDANT SURFACE LOW WEAKENING AND EXITING.
DEPENDING ON ITS SPEED AND AMPLITUDE, RAIN MAY LINGER LONGER INTO
THE EVENING ON CHRISTMAS DAY IN PARTS OF ILLINOIS. WHILE RAIN
CHANCES GRADUALLY DECREASE THROUGH THE DAY FROM WEST TO EAST,
LIGHT RAIN IS STILL NOT TOTALLY OUT OF THE QUESTION FOR SOME BY
SUNSET.
BEYOND THE HOLIDAY, SOUTHWEST FLOW ALOFT STAYS IN PLACE AND OPENS
THE REGION TO SEVERAL WAVES OF RAIN THROUGH THE WEEKEND. AT THIS
STAGE, NO ONE WAVE LOOKS PARTICULARLY IMPACTFUL: THERE IS LITTLE
TO NO INSTABILITY AVAILABLE FOR THUNDERSTORMS TO DEVELOP ACCORDING
TO ENSEMBLE GUIDANCE AND EACH SYSTEM LOOKS PROGRESSIVE ENOUGH TO
PRECLUDE ANY HEAVY RAIN THREAT. THE NBM AND LREF ARE ALSO NOT
POINTING TO A TANGIBLE HEAVY RAIN THREAT, WITH THE 90TH PERCENTILE
(NEAR WORST- CASE SCENARIO) RAINFALL AMOUNTS FROM TUESDAY TO
SATURDAY NIGHT FROM 1.50 TO 2.50 INCHES. OVER A SEVERAL-DAY SPAN,
THOSE VALUES ARE NOT CONCERNING. WHAT IS LESS CERTAIN IS EXACTLY
WHERE AND WHEN ANY ONE WAVE OF RAIN WILL OCCUR, LEADING TO BROAD
15-40% PRECIPITATION CHANCES THROUGH THE REST OF THE FORECAST
PERIOD. DESPITE THIS UNCERTAINTY, TEMPERATURES LIKELY REMAIN ABOVE
NORMAL IN THIS ACTIVE PATTERN. THE NBM HIGH TEMPERATURE IQR DOES
INCREASE BY THIS POINT TO 8-10 DEGREES, BUT EVEN THE LOW-END OF
THIS RANGE IS SEVERAL DEGREES ABOVE LATE- DECEMBER NORMALS.
MRB
AVIATION
(FOR THE 18Z TAFS THROUGH 18Z SUNDAY AFTERNOON)
ISSUED AT 1118 AM CST SAT DEC 21 2024
MVFR STRATUS OVER CENTRAL MO CONTINUES TO ERODE AND CLEAR
WESTWARD AND SHOULD EXIT KCOU AND KJEF BETWEEN 1800-1900 UTC.
OTHERWISE VFR FLIGHT CONDITIONS ARE EXPECTED AREA-WIDE. LIGHT
WINDS WILL BECOME SOUTHERLY ON SUNDAY MORNING AND GRADUALLY
INCREASE THROUGH THE MORNING INTO THE AFTERNOON.
GLASS
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
WFO LSX
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