534
FXUS02 KWBC 241859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 PM EDT SUN MAR 24 2024
VALID 12Z WED MAR 27 2024 - 12Z SUN MAR 31 2024
...HEAVY RAIN THREAT ACROSS THE SOUTHEAST MAY CONTINUE INTO MIDWEEK...
...ROUNDS OF HEAVY RAIN AND SNOW POSSIBLE ACROSS MUCH OF THE WEST
THROUGH THE PERIOD...
..OVERVIEW
A COLD FRONT OR TWO PUSHING AHEAD OF A BROAD UPPER TROUGH IN THE
CENTRAL-EASTERN U.S. AND EVENTUAL ATLANTIC SURFACE LOW DEVELOPMENT
WILL HELP FOCUS RAINFALL FOR PORTIONS OF THE EASTERN SEABOARD FOR
THE LATTER HALF OF THE WORKWEEK. THIS ACTIVITY COULD BE
PARTICULARLY HEAVY FOR PORTIONS OF FLORIDA AND GEORGIA INTO THE
CAROLINAS, WITH EFFECTS FROM THE MID-ATLANTIC THROUGH NEW ENGLAND
VERY SENSITIVE TO STRENGTH/TRACK OF THE LOW. MEANWHILE, ONE OR
MORE PACIFIC SYSTEMS SHOULD BRING INCREASING TOTALS OF RAIN AND
MOUNTAIN SNOW TO THE WEST COAST STATES AND THEN INTO THE ROCKIES
MID-LATE WEEK INTO NEXT WEEKEND. INCREASING MOISTURE MAY COMBINE
WITH SHORTWAVES TO PRODUCE SOME PRECIPITATION FROM THE NORTH-
CENTRAL TO EAST-CENTRAL U.S. FRIDAY-SUNDAY, THOUGH WITH AMPLE
UNCERTAINTY IN THE DETAILS OF COVERAGE/TOTALS AND TYPE.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MOST GUIDANCE IS SIMILAR IN TERMS OF THE BIG PICTURE, WITH A
WESTERN U.S. UPPER RIDGE BROADENING AND SETTLING OVER THE CENTRAL
U.S. AS CENTRAL-EASTERN U.S. UPPER TROUGHING TAKES ON A MORE
NEGATIVE TILT BY FRIDAY-SATURDAY AS IT REACHES THE EAST
COAST/WESTERN ATLANTIC WHILE DEEPENING MEAN TROUGHING BECOMES
ESTABLISHED NEAR THE WEST COAST. THE MOST PROMINENT FORECAST ISSUES
CONTINUE TO INVOLVE THE EAST COAST/WESTERN ATLANTIC EVOLUTION LATE
THIS WEEK, ALONG WITH INDIVIDUAL FEATURES WITHIN THE WEST COAST
TROUGH PLUS EJECTING ENERGY AND ITS INFLUENCE ON CENTRAL U.S.
SURFACE WAVINESS.
GUIDANCE CONTINUES TO HAVE A DIFFICULT TIME RESOLVING THE FINER BUT
EXTREMELY IMPORTANT DETAILS OF EAST COAST/WESTERN ATLANTIC
EVOLUTION DURING THE LATTER HALF OF THIS WEEK. SURFACE DETAILS WILL
BE VERY SENSITIVE TO EVOLUTION ALOFT, WITH SHARPENING/NEGATIVE TILT
OF SOUTHEAST ENERGY AND UNCERTAIN INTERACTION WITH NORTHERN STREAM
FLOW SUGGESTING BELOW AVERAGE PREDICTABILITY. THROUGH THE 00Z/06Z
CYCLES, THE FULL ARRAY OF OTHER MODELS/MEANS AND 00Z ECMWF-
INITIALIZED MACHINE LEARNING MODELS (MLS) WOULD SAY THAT THE 00Z
ECMWF IS TOO SLOW WITH ITS ATLANTIC SURFACE SYSTEM. INDEED, THE NEW
12Z ECMWF HAS TRENDED FASTER. MEANWHILE THE 12Z UKMET/CMC CLOSED
LOW SOLUTIONS ULTIMATELY TRACK THE DEEP SURFACE LOW CLOSER TO THE
COAST THAN MOST OTHER MODEL/ENSEMBLE GUIDANCE, WITH THE MLS NOT
SUPPORTING SUCH A WESTERN TRACK EITHER.
SOME NORTH-SOUTH SPREAD DEVELOPS FOR THE STRONG STORM FORECAST TO
BE OFFSHORE THE PACIFIC NORTHWEST BY THURSDAY WITH NO CLEAR
CLUSTERING BETWEEN THE SOUTHERN GFS AND NORTHERN ECMWF (BUT THE NEW
12Z RUN ADJUSTING A BIT SOUTH). GUIDANCE REMAINS SPLIT BY FRIDAY-
SATURDAY REGARDING WHETHER A SEPARATE WELL-DEFINED SYSTEM ROUNDING
THE BASE OF THE UPPER LOW ALREADY IN PLACE WILL TRACK TOWARD
CALIFORNIA (ECMWF/CMC), WHILE THE NORTHERN SYSTEM WEAKENS, OR A
MORE AMBIGUOUS ENERGY TRANSFER TAKES PLACE (WITH A WEAKER SOUTHERN
REFLECTION INITIALLY) TO YIELD THE SOUTHERN SYSTEM EVENTUALLY (GFS
AND ENSEMBLE MEANS). THE 00Z UKMET WAS BETWEEN THESE TWO IDEAS
WHILE THE 12Z RUN JUMPED TO THE ECMWF CLUSTER. MLS WERE MIXED WITH
RESPECT TO HOW THIS PATTERN WOULD EVOLVE. CONFLICTING SIGNALS
FAVORED AN INTERMEDIATE APPROACH CLOSEST TO CONTINUITY THAT WAS
SOMEWHAT CLOSER TO THE ECMWF CLUSTER IN PRINCIPLE. AS A HIGHER-
CONFIDENCE UPPER TROUGH/LOW REACHES THE VICINITY OF CALIFORNIA NEXT
WEEKEND, THE GUIDANCE MAJORITY INCLUDING THE MLS PLUS RECENT
TRENDS FAVOR TIMING SLOWER THAN THE 00Z AND ESPECIALLY 06Z GFS
(AND THE GEFS TO SOME DEGREE). THE 12Z GFS HAS ADJUSTED SLOWER.
SHORTWAVES FARTHER EAST ARE VARIABLE BUT IN GENERAL HAVE BEEN
TRENDING WEAKER, LEADING TO A MORE SUPPRESSED SURFACE PATTERN. GFS
RUNS HAVE BEEN SOMEWHAT AT ODDS WITH THIS THEME.
FORECAST CONSIDERATIONS LED TO AN EQUAL WEIGHT OF 06Z GFS, 12Z
ECMWF/UKMET/CMC, AND REMAINING 20 PERCENT INPUT OF THE 06Z GEFS/00Z
ECENS MEANS EARLY IN THE PERIOD TO BALANCE CONSIDERATIONS FOR THE
EAST COAST/ATLANTIC SYSTEM AND EAST PACIFIC EVOLUTION. THEN THE
FORECAST SHIFTED 70 PERCENT TOWARD THE ECMWF/CMC AND THEIR MEANS
GIVEN TIMING PREFERENCES FOR THE UPPER TROUGH/LOW NEARING
CALIFORNIA.
..WEATHER/HAZARDS HIGHLIGHTS
A COLD FRONT OR TWO AND DEVELOPING LOW PRESSURE AT THE SURFACE
COMBINING WITH A STRONG JET AND ITS RIGHT ENTRANCE REGION TO THE
EAST OF AN UPPER TROUGH WILL SUPPORT POTENTIALLY HEAVY RAIN ACROSS
PORTIONS OF THE SOUTHEAST/CAROLINAS AND PERHAPS AS FAR NORTH AS
SOUTHERN VIRGINIA ON WEDNESDAY. RECENT MODEL GUIDANCE HAS CONTINUED
TO OSCILLATE FOR TIMING AND EXTENT OF SIGNIFICANT RAINFALL, WITH
YESTERDAY'S FASTER/LIGHTER TRENDS NOW COUNTERED BY A SLOWER/HEAVIER
TREND IN THE 12Z RUNS THUS FAR. REGARDLESS OF SPECIFICS,
DECELERATION OF MOISTURE ANOMALIES AS A SURFACE WAVE REACHES NEAR
THE SOUTHEAST COAST BY EARLY THURSDAY ALONG WITH SOME INSTABILITY
MAY PROMOTE SOME LOCALLY HEAVY RAIN THAT COULD CAUSE FLOODING
ISSUES. TRENDS FAVORED SOME NORTH/NORTHWEST EXPANSION OF THE
EXISTING MARGINAL RISK AREA IN THE DAY 4 EXCESSIVE RAINFALL
OUTLOOK. ERRATIC GUIDANCE BEHAVIOR THUS FAR AND LESS THAN DESIRED
CLUSTERING FOR PLACEMENT OF HIGHEST QPF PRECLUDES ADDING AN
EMBEDDED SLIGHT RISK AREA, BUT STABILIZATION OF CURRENT GUIDANCE
THEMES WOULD MAKE SUCH AN UPGRADE MORE LIKELY. BY THURSDAY,
GUIDANCE INCREASINGLY DIVERGES REGARDING COVERAGE OF POTENTIALLY
HEAVY RAINFALL FROM THE MID-ATLANTIC NORTHEASTWARD. OVERALL THE LATEST
TRENDS SUGGEST GREATER COVERAGE OF SIGNIFICANT RAINFALL. THE
EXPANDED DAY 5 MARGINAL RISK AREA FROM THE EASTERN CAROLINAS AND
MID-ATLANTIC COAST INTO FAR SOUTHERN NEW ENGLAND MADE USE OF MID-
HIGHER END ENSEMBLE QPF PROBABILITIES AS A STARTING POINT GIVEN THE
OPERATIONAL MODEL SPREAD. THE PRECIPITATION UNCERTAINTY CONTINUES
INTO FRIDAY OVER THE NORTHEAST, WITH SOME POTENTIAL FOR SIGNIFICANT
TOTALS OF RAIN--OR EVEN HEAVY SNOW ON THE WESTERN SIDE OF THE
MOISTURE SHIELD. THE SURFACE LOW MAY PRODUCE A PERIOD OF BRISK TO
STRONG WINDS OVER NEW ENGLAND AS WELL.
DEEPER MOISTURE STARTS TO STREAM INTO THE WEST COAST BY WEDNESDAY
AS UPPER TROUGHING AND LOW PRESSURE APPROACH. RAIN RATES COULD BE
HIGH ENOUGH TO BE CONCERNING FOR FLOODING ACROSS PORTIONS OF
CALIFORNIA INTO WESTERN OREGON (WITH CURRENT SOIL MOISTURE FAIRLY
HIGH), SO THE DAY 4 ERO SHOWS A MARGINAL RISK THERE FOR WEDNESDAY.
HEAVY SNOW IS LIKELY INTO THE CASCADES/SIERRA NEVADA AND TOWARD
THE NORTHERN ROCKIES BEGINNING WEDNESDAY. COASTAL/LOWER ELEVATION
RAIN TOTALS LOOK TO LESSEN BY THURSDAY BUT SNOW IS FORECAST TO
EXPAND INTO THE INTERMOUNTAIN WEST THURSDAY AND BEYOND. THEN MODEL
TRENDS HAVE CONTINUED TO INDICATE THAT CALIFORNIA MAY SEE
INCREASING HEAVY PRECIPITATION BY FRIDAY INTO THE WEEKEND BASED ON
WHERE AN EASTERN PACIFIC UPPER LOW/TROUGH SETS UP AND DIRECTS A
POSSIBLE ATMOSPHERIC RIVER INTO THE STATE. THIS SOUTHERN SHIFT
WOULD ALSO BRING PRECIPITATION INTO THE CENTRAL GREAT BASIN AND
SOUTHWEST FOR THE WEEKEND. FARTHER EAST, SHORTWAVES GENERALLY
RIDING ALONG THE NORTHERN SIDE OF A CENTRAL U.S. RIDGE COULD
COMBINE WITH MOISTURE TO PRODUCE SOME LIGHT TO MODERATE
PRECIPITATION FRIDAY INTO THE WEEKEND ACROSS THE NORTH-CENTRAL
U.S. AND GREAT LAKES REGION INTO OHIO VALLEY. CONFIDENCE REMAINS
LOW FOR WHERE PRECIPITATION WILL OCCUR AS WELL AS THE PRECIPITATION
TYPES.
COLDER THAN AVERAGE TEMPERATURES BY 15-20 DEGREES ARE LIKELY TO
REMAIN INTO MIDWEEK FOR THE NORTH-CENTRAL PLAINS, WITH SOME GRADUAL
MODERATION BUT REMAINING ON THE COOLER SIDE INTO THE WEEKEND. COOL
TEMPERATURES BUT WITH LESSER ANOMALIES EXTEND FARTHER SOUTH IN THE
CENTRAL U.S. WEDNESDAY, BUT THEN A NOTABLE WARMING TREND IS
EXPECTED THERE AS UPPER RIDGING BUILDS. THE SOUTH-CENTRAL PLAINS
INTO MISSISSIPPI VALLEY CAN EXPECT TEMPERATURES AROUND 10-15
DEGREES ABOVE AVERAGE BY FRIDAY-SUNDAY, BRINGING HIGHS IN THE 70S INTO
KANSAS AND MISSOURI. MEANWHILE IN THE WEST, NEAR TO ABOVE NORMAL
LOWS BUT NEAR TO BELOW NORMAL HIGHS ARE FORECAST, CONSISTENT WITH
THE CLOUDS AND PRECIPITATION THERE. ONE EXCEPTION MAY BE OVER THE
PACIFIC NORTHWEST WHERE ENOUGH UPPER RIDGING MAY BUILD IN TO PUSH
HIGHS A LITTLE ABOVE NORMAL NEXT WEEKEND.
RAUSCH/TATE
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 (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
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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