352
FXUS02 KWBC 141859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 PM EST WED FEB 14 2024
VALID 12Z SAT FEB 17 2024 - 12Z WED FEB 21 2024
***MULTIPLE ATMOSPHERIC RIVER EVENTS POSSIBLE FOR PORTIONS OF
CALIFORNIA WITH HEAVY RAIN AND SIERRA SNOW***
..GENERAL OVERVIEW
UPPER TROUGHING TO THE EAST OF THE ROCKIES TO START THE WEEKEND
SHOULD STEADILY RETREAT NORTHWARD INTO CANADA WHILE AN INCOMING
PACIFIC SHORTWAVE PASSES THROUGH AN INTERMOUNTAIN WEST MEAN RIDGE
AND CONTINUES EASTWARD ACROSS THE LOWER 48. MEANWHILE A SOUTHERN
STREAM SHORTWAVE PROGRESSING FROM MEXICO INTO THE GULF WILL
SUPPORT ONE OR MORE WEAK FRONTAL WAVES THAT SHOULD TRACK ACROSS OR
JUST SOUTH OF FLORIDA AND THEN OFFSHORE. THE COMBINATION OF UPPER
TROUGHING EAST OF THE ROCKIES AND BROAD SURFACE HIGH PRESSURE
ORIGINALLY FROM CANADA SETTLING OVER THE SOUTHERN TIER THIS
WEEKEND WILL BRING CHILLY CONDITIONS, WITH A WARMER TREND NEXT
WEEK. THE WEST COAST WILL LIKELY BECOME QUITE UNSETTLED GOING
INTO THE WEEKEND AND ESPECIALLY INTO EARLY NEXT WEEK WITH
INCREASING CHANCES OF MULTIPLE ATMOSPHERIC RIVER EVENTS FOR
PORTIONS OF CENTRAL AND SOUTHERN CALIFORNIA. SOME AREAS OF
LIGHTER PRECIPITATION MAY SPREAD ACROSS OTHER PARTS OF THE WEST AS
WELL.
..GUIDANCE/PREDICTABILITY ASSESSMENT
ACROSS THE EASTERN PACIFIC/WESTERN U.S., GUIDANCE AGREES
REASONABLY WELL FOR THE LEADING SYSTEM AFFECTING THE REGION BUT
SOME DIFFERENCES ARISE FOR THE STRONGER TRAILING PACIFIC SYSTEM.
THE MOST PROMINENT DIFFERENCE INVOLVES LATEST GFS RUNS STRAYING
SLOWER WITH THE UPPER TROUGH AND FARTHER SOUTHWEST THAN CONSENSUS
WITH THE EMBEDDED UPPER LOW BY TUESDAY-WEDNESDAY. THIS REPRESENTS
A FLIP IN THE GFS/ECMWF COMPARISON VERSUS SOME EARLIER RUNS, BUT
THE ENSEMBLE MEANS STRONGLY SUPPORT FASTER PROGRESSION THAN THE
GFS BY THE MIDDLE OF NEXT WEEK AND IN FACT THE 00Z
ECMWF-INITIALIZED MACHINE LEARNING MODELS SUGGEST TIMING COULD
EVEN BE A LITTLE FASTER THAN THE MEANS. THUS PREFERENCES LEAN
MORE TOWARD THE ECMWF AND ENSEMBLE MEANS WHEN TIMING SPREAD
DEVELOPS. SOME CMC RUNS HAVE BEEN ON THE SLOW SIDE AS WELL BUT
WITH A QUESTIONABLE EVOLUTION OVER THE NORTHEASTERN
PACIFIC/NORTHWESTERN NORTH AMERICA. ALSO WORTH NOTING, THE ML
MODELS SUGGEST THAT THE UPPER LOW AND ASSOCIATED SURFACE SYSTEM
COULD ULTIMATELY TRACK FARTHER NORTH THAN EVEN THE ENSEMBLE MEANS
AND MOST 00Z/06Z MODELS. THE NEW 12Z ECMWF HAS IN FACT ADJUSTED
THAT WAY, BRINGING THE WEAKENING UPPER/SURFACE LOW TO THE PACIFIC
NORTHWEST INSTEAD OF CALIFORNIA, WHILE ALSO ACCELERATING THE UPPER
TROUGH.
ELSEWHERE, THERE IS DECENT AGREEMENT IN PRINCIPLE FOR AN UPPER
GREAT LAKES/NEW ENGLAND WAVE AROUND SUNDAY-MONDAY BUT CONTINUED
DETAIL DIFFERENCES. THE 06Z GFS DEVIATED THE MOST FROM OTHER
GUIDANCE BY EARLY MONDAY WITH A SLOW/DEEP SOLUTION. BEHIND THIS
SYSTEM THE GUIDANCE GENERALLY SHOWS A MEAN FRONTAL BOUNDARY DRAPED
ACROSS THE NORTHERN TIER BUT WITH POOR AGREEMENT/CONTINUITY FOR
SPECIFICS GIVEN SENSITIVITY OF SURFACE WAVES TO LOW-PREDICTABILITY
SHORTWAVE DETAILS. A MODEL COMPOSITE LOOKS REASONABLE FOR THE
SURFACE WAVE(S) CROSSING THE GULF OF MEXICO/FLORIDA DURING THE
WEEKEND.
BASED ON THE ABOVE CONSIDERATIONS, THE UPDATED FORECAST
INCORPORATED THE 00Z ECMWF/00Z-06Z GFS PLUS SMALLER WEIGHTS OF THE
00Z UKMET/CMC EARLY-MID PERIOD AND THEN QUICKLY TRANSITIONED TO
ABOUT HALF 06Z GEFS/00Z ECENS MEANS WITH REMAINING ECMWF/GFS INPUT
BY DAY 7 NEXT WEDNESDAY.
..WEATHER/HAZARDS HIGHLIGHTS
GUIDANCE CONTINUES TO SHOW ONE OR MORE FRONTAL WAVES DEVELOPING
OVER THE GULF BY SATURDAY MORNING AND THEN TRACKING ACROSS OR JUST
SOUTH OF THE SOUTHERN FLORIDA PENINSULA DURING THE REST OF THE
WEEKEND. THE MOST CONSEQUENTIAL RAINFALL SHOULD BE MAINLY
CONFINED TO NORTHERN FLORIDA ON SATURDAY AND THEN MOST LIKELY
SOUTH FLORIDA (BUT WITH CONSIDERABLE SPREAD FOR NORTHWARD EXTENT)
ON SUNDAY. ALTHOUGH RAINFALL TOTALS ARE GENERALLY EXPECTED TO BE
ON THE ORDER OF 1 TO 2.5 INCHES, THERE MAY BE POTENTIAL EPISODES
OF TRAINING CONVECTION ACROSS NORTHERN FLORIDA ON SATURDAY TO
JUSTIFY A MARGINAL RISK THERE IN THE DAY 4 EXCESSIVE RAINFALL
OUTLOOK. SOME SLOWER MOVING CELLS MAY DEVELOP ACROSS THE METRO
AREAS OF PALM BEACH, BROWARD, AND MIAMI-DADE COUNTIES, AND THIS
MAY LOCALLY PRODUCE ENOUGH HEAVY RAINFALL ON SUNDAY TO WARRANT A
MARGINAL RISK AREA FOR DAY 5 OUTLOOK. THE AFOREMENTIONED SPREAD
FOR SUNDAY'S RAINFALL COVERAGE TEMPERS CONFIDENCE IN THE DETAILS
BUT THERE IS BETTER MODEL/ENSEMBLE AGREEMENT THAT THE GREATEST
MOISTURE ANOMALIES WILL ALIGN OVER SOUTHERN FLORIDA, SUPPORTING
THE EXISTING MARGINAL RISK AREA.
A MORE SUBSTANTIAL AXIS OF HEAVIER RAINFALL IS BECOMING MORE
LIKELY ACROSS THE NORTHERN HALF OF CALIFORNIA GOING INTO SATURDAY
AND EXTENDING INLAND ACROSS UPSLOPE AREAS OF THE NORTHERN SIERRA,
WITH SOME RAINFALL TOTALS POTENTIALLY REACHING AT LEAST TWO
INCHES. THE AXIS OF HIGHEST PW ANOMALIES WILL LIKELY BE ORIENTED
TOWARDS THE HIGHER TERRAIN OF THE COASTAL MOUNTAIN RANGES BETWEEN
SAN FRANCISCO AND EUREKA, WHERE A SLIGHT RISK OF EXCESSIVE
RAINFALL IS DEPICTED FOR THE DAY 4 PERIOD. THERE ARE STILL DETAIL
DIFFERENCES THAT REQUIRE MONITORING, BUT FOR NOW THERE IS ENOUGH
SUPPORT TO MAINTAIN THE EXISTING SLIGHT RISK AND SURROUNDING
MARGINAL RISK AREA UNTIL MORE SPECIFIC TRENDS BECOME EVIDENT.
LOOKING AHEAD TO SUNDAY, EXPECT ANOTHER ATMOSPHERIC RIVER EVENT TO
ARRIVE AHEAD OF A SLOW MOVING PACIFIC LOW APPROACHING CALIFORNIA,
WITH ANOMALOUS MOISTURE FLUX DIRECTED TOWARDS THE COASTAL TERRAIN
OF CENTRAL CALIFORNIA. WHILE THE BEST MOISTURE ANOMALIES CLUSTER
OVER SOUTH-CENTRAL CALIFORNIA BY THE END OF THE DAY 5 PERIOD EARLY
MONDAY, MODEL/ENSEMBLE SIGNALS FOR HEAVIEST RAINFALL ARE LESS
DEFINED. THIS FAVORS KEEPING THE EXISTING MARGINAL RISK AREA
WHILE WAITING FOR A MORE COHERENT QPF FOCUS IN THE GUIDANCE.
BEYOND EARLY MONDAY, HIGHEST RAINFALL TOTALS OVER CALIFORNIA
SHOULD BE DURING THE FIRST HALF OF THE WEEK AND OVER THE
TRANSVERSE RANGES WHERE SEVERAL INCHES OF RAINFALL WILL BE
POSSIBLE, SO THIS WILL EVENT CONTINUE TO BE MONITORED CLOSELY.
HEAVY SNOW IS ALSO LIKELY FOR THE SIERRA AND NORTHERN CALIFORNIA
MOUNTAINS DURING THIS TIME PERIOD. THE TWO SEPARATE SYSTEMS
AFFECTING CALIFORNIA SHOULD SPREAD SOME MOISTURE WITH LIGHTER
PRECIPITATION TOTALS ACROSS PORTIONS OF THE INTERMOUNTAIN WEST AND
ROCKIES.
THE UPPER/EASTERN GREAT LAKES INTO NEW ENGLAND MAY SEE SOME AREAS
OF LAKE EFFECT/TERRAIN ENHANCED SNOWFALL DURING THE WEEKEND IN
ASSOCIATION WITH UPPER TROUGHING AND A SURFACE SYSTEM CROSSING THE
REGION AROUND SUNDAY-SUNDAY NIGHT. THEN A FRONT AND EMBEDDED
WAVES MAY PRODUCE SOME AREAS OF LIGHT RAIN/SNOW OVER THE GREAT
LAKES/NORTHEAST AND VICINITY BY TUESDAY OR WEDNESDAY, BUT WITH LOW
CONFIDENCE IN SPECIFICS AT THIS TIME.
THE SURFACE HIGH SETTLING OVER THE SOUTHERN TIER STATES THIS
WEEKEND BEHIND A COLD FRONT WILL LIKELY PROMOTE TEMPERATURES
RUNNING ABOUT 5-15 DEGREES BELOW MID-FEBRUARY AVERAGES FROM TEXAS
TO THE OHIO VALLEY ON SATURDAY WITH SOME BELOW NORMAL READINGS
PERSISTING OVER PARTS OF THE SOUTHERN TIER (ESPECIALLY NEAR THE
GULF COAST/FLORIDA) INTO SUNDAY OR MONDAY. THE PLAINS AND MIDWEST
WILL TREND WARMER FROM SUNDAY ONWARD AS MEAN FLOW ALOFT BECOME
MORE WESTERLY, WITH SOME LOCATIONS OVER THE CENTRAL PLAINS/MIDWEST
SEEING THE BEST POTENTIAL FOR HIGHS REACHING 20 DEGREES OR MORE
ABOVE NORMAL BY NEXT TUESDAY-WEDNESDAY. REMAINING AREAS EAST OF
THE ROCKIES SHOULD TREND ABOVE AVERAGE AS WELL. CLOUDS AND
PRECIPITATION WILL FAVOR ABOVE NORMAL LOWS AND NEAR TO SLIGHTLY
BELOW NORMAL HIGHS ACROSS THE WEST.
RAUSCH/HAMRICK
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,
QUANTITATIVE PRECIPITATION FORECAST, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES ARE AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=ERO
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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