015
FXUS63 KLSX 170521
AFDLSX
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
1221 AM CDT SAT APR 17 2021
SHORT TERM
(THROUGH SUNDAY AFTERNOON NIGHT)
ISSUED AT 255 PM CDT FRI APR 16 2021
KEY MESSAGES THROUGH THE SHORT-TERM PERIOD:
1) WIDESPREAD LIGHT RAIN IS STILL EXPECTED TO DEVELOP, STARTING IN
CENTRAL MO AND SPREADING EAST THROUGH THE EVENING. TIMING WAS SLOWED
SLIGHTLY IN LIGHT OF LATEST MODEL GUIDANCE.
2) RAIN WILL BECOME LESS WIDESPREAD TOMORROW MORNING, BECOMING
SHOWERY AND INTERMITTENT.
3) ANOTHER ROUND OF RAIN IS EXPECTED SUNDAY, WITH A CHANCE FOR
ISOLATED THUNDERSTORMS.
HIGH CLOUDS CONTINUE TO OVERSPREAD THE MID-MISSISSIPPI VALLEY IN
ADVANCE OF AN INVERTED SURFACE TROUGH, CURRENTLY ALONG THE KS/MO
BORDER, SPARKING SHOWERS UP TO THE MO/IL BORDER. ALOFT, BROAD
LONGWAVE TROUGHING EXISTS ACROSS MOST OF THE CONUS, WITH THE
EXCEPTION OF WEAK RIDGING IN THE NORTH-CENTRAL U.S. IN BETWEEN TWO
AMPLIFIED ULLS. THE PASSAGE OF TWO CLOSED ULLS WILL BE THE CATALYST
FOR OUR SHORT-TERM PRECIPITATION CHANCES AND AN OVERALL CRUMMY
WEEKEND.
THE FIRST ROUND OF LIGHT PRECIPITATION WILL ACCOMPANY A WEAK
SHORTWAVE PASSAGE LATER THIS AFTERNOON. THE LATEST HIGH-RESOLUTION
ENSEMBLE GUIDANCE POINTS TO A SLOWER PRECIPITATION ONSET ACROSS THE
AREA TONIGHT THAN IN PREVIOUS FORECASTS, AND DETERMINISTIC
MODELS/CAMS ARE ALSO SUPPORTING THIS SCENARIO. THE LIMITING FACTORS
FOR PRECIPITATION WILL BE A COMBINATION OF WEAK FORCING AND POOR
MOISTURE AVAILABILITY. PVA ASSOCIATED WITH THE SHORTWAVE IS NOT VERY
ROBUST ACCORDING TO THE DETERMINISTIC GUIDANCE, AND LOW-LEVEL
FORCING IS EQUALLY UNIMPRESSIVE. ALSO, MODEL SOUNDINGS ARE
CONSISTENTLY SHOWING A RELATIVELY DRY BOUNDARY LAYER THAT THE
PRECIPITATION WILL NEED TO OVERCOME BEFORE REACHING THE GROUND. ALL
THAT BEING SAID, CAMS AND DETERMINISTIC GUIDANCE DOES EVENTUALLY
HAVE WIDESPREAD PRECIPITATION BEGINNING LATER THIS AFTERNOON IN
CENTRAL MO. BY LATE THIS EVENING, LIGHT STRATIFORM RAIN IS EXPECTED
ACROSS MOST OF MO AND COULD VERY WELL BE INTO WESTERN IL.
BY TOMORROW MORNING, AS THE INVERTED TROUGH/LEADING SHORTWAVE PASS
TO OUR EAST, PRECIPITATION SHOULD BECOME LESS WIDESPREAD. AMIDST THE
BASE OF THE SHORTWAVE, LOW-LEVEL LAPSE RATES SHOULD START TO
INCREASE WITH DIURNAL HEATING SATURDAY AFTERNOON. MARGINAL
INSTABILITY, MAINLY SHALLOW IN NATURE, SHOULD REDEVELOP SHOWERS
ACROSS THE AREA LATE SATURDAY MORNING THROUGH THE AFTERNOON. POPS
WERE INCREASED WHERE AN AXIS OF NON-ZERO MUCAPE IS RESOLVED IN THE
MODELS (MAINLY ALONG/NORTH OF I-70), BUT PRECIPITATION CHANCES EXIST
AREAWIDE THROUGH THE EVENING SATURDAY. ANY PRECIPITATION WILL REMAIN
SHOWERY AND INTERMITTENT IN THE ABSENCE OF ORGANIZATION OR STRONG
DYNAMIC FORCING. WITH THE SHALLOW NATURE OF THE CONVECTION,
THUNDERSTORMS ARE NOT EXPECTED TO DEVELOP. THE WIDESPREAD CLOUD
COVER AND SHOWERS WILL HELP KEEP HIGH TEMPERATURES IN THE UPPER 50S
AREAWIDE TOMORROW, WHICH IS AROUND 10F BELOW CLIMATOLOGY.
THE SECOND ROUND OF PRECIPITATION WILL ACCOMPANY A MORE ROBUST
CLOSED ULL THAT DROPS SOUTH INTO THE REGION LATE SATURDAY NIGHT.
GUIDANCE CONTINUES TO STRUGGLE WITH PRODUCING RAIN WITH THIS
SYSTEM, GIVEN THE LACK OF APPRECIABLE MOISTURE ASSOCIATED WITH THE
SYSTEM. THE MORE ROBUST FORCING SHOULD PRODUCE ANOTHER ROUND OF
LIGHT RAIN, DESPITE THE MOISTURE-STARVED SYSTEM. POPS WERE
INCREASED AREAWIDE ON SUNDAY, BUT ANY RAIN THAT FALLS WILL AGAIN
BE SHOWERY IN NATURE. LAPSE RATES IN THE LOW AND MID LEVELS ARE
MORE IMPRESSIVE ON SUNDAY, AND MUCAPE VALUES AMONG THE
DETERMINISTIC AND SHORT-RANGE ENSEMBLE GUIDANCE ARE SLIGHTLY
HIGHER THAN SATURDAY'S SYSTEM. WHILE ISOLATED, THUNDERSTORMS
CANNOT BE RULED OUT THROUGH THE DAY ON SUNDAY. AS THE SYSTEM
DEPARTS TO THE EAST LATER SUNDAY AFTERNOON, PRECIPITATION CHANCES
DIMINISH AS WELL.
MRB
LONG TERM
(SUNDAY NIGHT THROUGH NEXT FRIDAY)
ISSUED AT 255 PM CDT FRI APR 16 2021
KEY MESSAGES THROUGH THE EXTENDED PERIOD:
1) PRECIPITATION RETURNS TO THE AREA TUESDAY WITH THE PASSAGE OF A
COLD FRONT. EXACT TIMING OF THE FRONT IS STILL UNCERTAIN, WHICH WILL
IMPACT PRECIPITATION TIMING, HIGH TEMPERATURES, AND PRECIPITATION
TYPE.
2) BEHIND THE COLD FRONT, TEMPERATURES NEAR 15F BELOW NORMAL ARE
FORECAST. SUB-FREEZING TEMPERATURES WEDNESDAY MORNING INTRODUCE THE
POTENTIAL FOR WIDESPREAD FROST.
MONDAY WILL REMAIN DRY AMIDST ZONAL FLOW AND AN ABSENCE OF ANY
FORCING, GIVING US A REPRIEVE FROM THE DREARY WEEKEND WEATHER.
CLEARING SKIES AND SOUTHERLY WINDS AHEAD OF A COLD FRONT WILL BRING
TEMPERATURES BACK TO THE NORMAL MID/UPPER 60S WE'D EXPECT FOR MID
APRIL. HOWEVER, THE NICE WEATHER LOOKS TO BE SHORT-LIVED: A STRONG
COLD FRONT LOOKS TO SAG SOUTH ACROSS THE REGION AND SPARK WIDESPREAD
PRECIPITATION. WPC CLUSTER ANALYSIS SHOWS EXCELLENT AGREEMENT
REGARDING THE OCCURRENCE OF PRECIPITATION ON TUESDAY, BUT DIFFERS ON
THE TIMING. ONE CLUSTER, COMPRISED ALMOST EXCLUSIVELY OF EPS
MEMBERS, IS SLOWER TO BRING THE FRONT THROUGH THAN THE OTHER THREE.
POPS RIGHT NOW DO NOT EXCEED 55% TO ACCOUNT FOR THIS TIMING
UNCERTAINTY, BUT THE CHANCE FOR MEASURABLE PRECIPITATION ON TUESDAY
LOOKS HIGH. IF THE SLOWER SOLUTION IS REALIZED, HIGH TEMPERATURES
WOULD BE WARMER ON TUESDAY THAN CURRENTLY FORECAST. RIGHT NOW,
GUIDANCE POINTS TO A QUICKER FRONTAL PASSAGE, THUS TUESDAY'S HIGH
TEMPERATURES WERE LOWERED USING OUR CURRENT FORECAST AND THE 25TH
PERCENTILE OF THE NATIONAL BLEND OF MODELS.
ANOTHER AREA OF UNCERTAINTY SURROUNDING THE FRONTAL PASSAGE IS THE
POTENTIAL FOR WINTRY PRECIPITATION. DETERMINISTIC GUIDANCE IS
SUGGESTING THAT THE PRECIPITATION WITH THIS FRONT WILL BE LARGELY
ANAFRONTAL, WHICH INTRODUCES A NARROW WINDOW OF TIME FOR
PRECIPITATION TO INTERACT WITH MUCH COLDER AIR. SAY IT ISN'T SO, BUT
THE POTENTIAL DOES EXIST FOR SNOWFLAKES TO MIX WITH RAIN EARLY
TUESDAY MORNING. WHILE DETERMINISTIC GUIDANCE, PARTICULARLY THE GFS,
IS BULLISH IN BRINGING MEASURABLE SNOW AS FAR SOUTH AS ST. LOUIS,
THAT WILL BE QUITE DIFFICULT AND ENSEMBLE GUIDANCE DOES NOT SUPPORT
IT. THE BEST CHANCES FOR ANY FROZEN PRECIPITATION EXIST IN FAR
NORTHEAST AND CENTRAL MO, WHERE PRECIPITATION WILL HAVE LONGER
RESIDENCE TIME IN SUFFICIENTLY-COLD AIR. AS OF NOW, NO SNOW
ACCUMULATIONS ARE FORECAST.
BEHIND THE FRONT TUESDAY, VERY COLD AIR WILL ENTER THE MID-
MISSISSIPPI VALLEY. CIPS ANALOG GUIDANCE FROM SAINT LOUIS
UNIVERSITY, WHICH HAS BEEN SIGNALING THIS COOLDOWN FOR A WEEK,
CONTINUES TO SHOW 850MB TEMPERATURES LESS OVER 1.5 SIGMA BELOW
CLIMATOLOGY FOR WEDNESDAY. IT ALSO SHOWS THAT OVER 70% OF THE TOP 15
HISTORICAL ANALOGS FOR WEDNESDAY FEATURE MINIMUM TEMPERATURES BELOW
FREEZING ACROSS MOST OF THE REGION. WITH THIS MAGNITUDE OF COLD, THE
POTENTIAL FOR WIDESPREAD FROST IS BECOMING HIGHER WEDNESDAY MORNING.
VEGETATION ACROSS THE AREA REMAINS SUSCEPTIBLE AFTER WE FLIRTED WITH
FROST EARLIER THIS WEEK. IF CLOUDS ARE ABLE TO CLEAR OUT ON TUESDAY
EVENING, FORECAST LOW TEMPERATURES COULD STILL BE TOO HIGH, WHICH
INTRODUCES A FREEZE POTENTIAL.
AFTER TUESDAY'S PRECIPITATION, THE COLD AIR WILL KEEP US DRY THROUGH
THE MAJORITY OF THE WEEK. WHILE MORE PRECIPITATION COULD BE ON TAP
FOR US THIS WEEKEND, THERE IS STILL CONSIDERABLE TIMING AND LOCATION
UNCERTAINTY WITH THE SYSTEM.
MRB
AVIATION
(FOR THE 06Z TAFS THROUGH 06Z SATURDAY NIGHT)
ISSUED AT 1220 AM CDT SAT APR 17 2021
SHOWERS HAVE FINALLY SPREAD INTO THE ST. LOUIS REGION TERMINALS AND
THE MAIN CONCENTRATION OF SHOWERS OVERNIGHT SHOULD BE FROM CENTRAL
THROUGH SOUTHEAST AND EAST CENTRAL MO INTO SOUTHWEST IL, IMPACTING
KCOU AND KSTL/KSUS/KCPS. IT'S UNCLEAR HOW MUCH RAIN WILL MAKE IT
INTO KUIN BUT I THINK THEY WILL SEE AT LEAST SOME LIGHT RAIN
OVERNIGHT. COVERAGE OF SHOWERS IS EXPECTED TO WANE BETWEEN 10-12Z
WITH SPOTTY/SCATTERED COVERAGE SATURDAY MORNING AND THEN SCATTERED
SHOWERS AGAIN IN THE AFTERNOON. FLIGHT CONDITIONS WILL GRADUALLY
DETERIORATE OVERNIGHT INTO EARLY SATURDAY MORNING AND THEN IMPROVE
AGAIN LATE MORNING AND INTO THE AFTERNOON.
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
WFO LSX
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