875
FXUS02 KWBC 271859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 PM EDT SAT APR 27 2024
VALID 12Z TUE APR 30 2024 - 12Z SAT MAY 04 2024
...ROUNDS OF CONVECTION ARE LIKELY IN THE CENTRAL U.S. FOR MUCH OF
NEXT WEEK...
..OVERVIEW
AS THE MEDIUM RANGE PERIOD BEGINS TUESDAY, QUASI-ZONAL UPPER FLOW
WILL BE IN PLACE OVER MUCH OF THE LOWER 48, BUT EMBEDDED SHORTWAVES
COULD PUSH FRONTAL SYSTEMS THROUGH THE CENTRAL AND EASTERN U.S. AND
CAUSE SOME SHOWERS AND THUNDERSTORMS. MEAN TROUGHING SHOULD AMPLIFY
OVER THE INTERMOUNTAIN WEST WEDNESDAY-THURSDAY AND PROVIDE SUPPORT
FOR INCREASING CONVECTION OVER MUCH OF THE CENTRAL U.S., WITH
HEAVY RAIN AND FLASH FLOODING AMONG THE POSSIBLE HAZARDS. THIS
CONVECTION WILL LIKELY SHIFT GRADUALLY EASTWARD INTO LATE WEEK.
PRECIPITATION INCLUDING SOME HIGHER ELEVATION SNOW IS POSSIBLE
ACROSS THE NORTHWEST INTO THE NORTHERN/CENTRAL ROCKIES, BUT WITH
RATHER LOW CONFIDENCE IN THE DETAILS AT THIS TIME.
..GUIDANCE/PREDICTABILITY ASSESSMENT
WHILE GUIDANCE RUNS SINCE THE 12Z/26 RUN APPEAR TO HAVE RESOLVED
MOST OF THE PRIOR DIVERGENCE FOR THE INITIAL UPPER LOW ENERGY (MOST
LIKELY COMPRISING TWO SEPARATE CENTERS) ALONG THE WESTERN
U.S.-CANADIAN BORDER, THERE IS STILL A LOT OF SPREAD FOR WHAT
BECOMES OF THE WESTERN HALF OF THE FEATURE AS WELL AS UPSTREAM
DETAILS.
RECENT TRENDS, INCLUDING FROM ECMWF-INITIALIZED MACHINE LEARNING
MODELS (MLS), SUPPORT FAIRLY AMPLIFIED UPPER TROUGHING OVER THE
WEST BY WEDNESDAY-THURSDAY. HOWEVER FROM THAT POINT BOTH DYNAMICAL
GUIDANCE AND THE MLS DIVERGE FOR TIMING AND SHAPE AS THE FEATURE
EJECTS FROM THE WEST, AFFECTING THE SURFACE EVOLUTION. THE 06Z GFS
APPEARED TO BE SOMEWHAT ON THE FAST SIDE OF THE SPREAD BUT
OTHERWISE THE ENVELOPE WAS BROAD ENOUGH TO PRECLUDE DISCOUNTING
MANY SOLUTIONS. 12Z MODEL RUNS CONTINUE TO DIFFER BY AT LEAST A DAY
FOR TIMING OF THE UPPER SYSTEM BY NEXT FRIDAY, WITH CORRESPONDING
EFFECTS ON THE PLAINS INTO GREAT LAKES SURFACE DEVELOPMENT THAT
MOST GUIDANCE IS SHOWING. FARTHER EAST THE ONE ITEM OF NOTE IS THAT
UKMET RUNS ARE NOTABLY AGGRESSIVE WITH SHORTWAVE ENERGY CROSSING
NEW ENGLAND AROUND THURSDAY WHEN CONSENSUS SHOWS MORE UPPER
RIDGING.
THERE IS STILL CONSIDERABLE SPREAD FOR A COMPACT UPPER LOW EXPECTED
TO REACH WESTERN NORTH AMERICA BY THURSDAY. THE NEW 12Z GFS/CMC
RUNS MAINTAIN THEIR THEME FROM PRIOR RUNS IN SHOWING A TRACK INTO
WASHINGTON STATE WHILE THE ECMWF IS WELL NORTH OVER BRITISH
COLUMBIA. THE UKMET FLIPPED FROM THE GFS CLUSTER IN THE 00Z RUN TO
SHEARING IT OUT NORTH OF THE ECMWF IN THE 12Z RUN. THE GEFS/ECENS
MEANS NOT SURPRISINGLY WEAKEN THE FEATURE AS IT ARRIVES BUT WOULD
IMPLY A SOMEWHAT FARTHER NORTH TRACK THAN THE GFS CLUSTER. THE 00Z
MLS WERE UNANIMOUS WITH A BRITISH COLUMBIA TRACK. PREFERENCE LEANS
TO THE ECMWF/ML CLUSTER. INTERESTINGLY THE GUIDANCE GETS A LITTLE
CLOSER TOGETHER BY FRIDAY AS THE ECMWF/ML CLUSTER DROPS THE UPPER
LOW SOUTHEAST TO AROUND THE CANADIAN BORDER. WHAT HAPPENS WITH THIS
FEATURE MAY ULTIMATELY HAVE SOME INFLUENCE ON DETAILS OF FLOW TO
THE EAST.
SIGNIFICANT DIFFERENCES CONTINUE AS ANOTHER UPPER TROUGH (WITH
POSSIBLE LOW) APPROACHES THE NORTHWEST. THE 06Z GFS WAS ON THE
EASTERN SIDE OF THE SPREAD IN BRINGING A DEEP UPPER LOW INTO
WASHINGTON STATE. MEANWHILE THE MLS ON AVERAGE SHOW AN INTERMEDIATE
IDEA BETWEEN THE DEEP AND SOMETIMES EASTWARD GFS RUNS VERSUS THE
SLOWER/FLATTER ECMWF AND ECENS MEAN RUNS. THE MLS ARE ACTUALLY
REMARKABLY AGREEABLE IN SHOWING THE ASSOCIATED SURFACE FRONT
SOMEWHAT OFFSHORE THE PACIFIC NORTHWEST AS OF EARLY SATURDAY. AS
EXPECTED, GEFS/EC/CMC ENSEMBLE MEMBERS SHOW CONSIDERABLE SPREAD FOR
THIS AND AFOREMENTIONED FEATURES.
IN ORDER TO ACCOMMODATE CONSIDERATIONS FOR THE VARIOUS FEATURES OF
INTEREST, THE UPDATED FORECAST STARTED WITH A 00Z/06Z OPERATIONAL
MODEL COMPOSITE EARLY IN THE PERIOD. THEN THE BLEND ELIMINATED THE
UKMET BY THURSDAY, FOLLOWED BY SPLITTING GFS/ECMWF INPUT BETWEEN
CONSECUTIVE 6-/12-HOURLY RUNS AND INCORPORATING 20-40 PERCENT TOTAL
OF THE 06Z GEFS/00Z ECENS MEANS.
..WEATHER/HAZARDS HIGHLIGHTS
ON TUESDAY, LESSENING UPPER-LEVEL SUPPORT SHOULD LEAD TO LOWER
RAINFALL AMOUNTS AND RAINFALL RATES IN THE CENTRAL U.S. COMPARED
EARLIER AND LATER IN THE WEEK. HOWEVER, SOME CONVECTION IS STILL
LIKELY TO OCCUR IN A MOIST AND UNSTABLE ENVIRONMENT. CURRENTLY THE
BEST CHANCE FOR RAIN WITH ISOLATED FLOODING POTENTIAL IS ACROSS THE
SOUTH-CENTRAL PLAINS TO MID-MISSISSIPPI VALLEY AS A FRONT STALLS.
WITHIN THAT REGION THERE IS SOME SPREAD IN LOCATION OF LOCALLY
HEAVY CONVECTIVE POTENTIAL--GFS RUNS (INCLUDING THE NEW 12Z
VERSION) HAVE BEEN PERSISTENT IN SHOWING CONVECTION NEAR THE RED
RIVER WHILE OTHER MODELS INDICATE LOCAL MAXIMA FARTHER NORTH. A DAY
4 MARGINAL RISK IS IN PLACE IN THE ERO TO COVER THESE AREAS AND
WILL BE REFINED WITH TIME. SOME ISOLATED MODERATE TO LOCALLY HEAVY
RAIN MAY BE POSSIBLE ALONG/NORTH OF THE TEXAS AND LOUISIANA COAST
BUT WHAT SIGNAL EXISTS REMAINS DIFFUSE ENOUGH NOT TO MERIT A RISK
AREA (MAINTAINING THE PRIOR CYCLE'S REDUCTION THAT FAVORED AN
EARLIER MARGINAL RISK AREA). BY WEDNESDAY, THE UPPER FLOW (WITH
TROUGHING TO THE WEST AND A SOUTHERN STREAM SHORTWAVE) WILL BECOME
MORE SUPPORTIVE OF MORE WIDESPREAD HEAVY RAIN AND THUNDERSTORMS FOR
THE CENTRAL/SOUTHERN PLAINS AND MID/LOWER MISSISSIPPI VALLEY AS
MOISTURE SURGES AHEAD OF A WAVY FRONTAL SYSTEM. ONCE AGAIN THERE IS
SPREAD IN THE EXACT POSITION OF THE HEAVIEST TOTALS, BUT RAIN
AMOUNTS OF 3-5 INCHES ARE LIKELY, AND SOME AREAS OF HEAVY RAIN ARE
LIKELY TO OVERLAP WITH CURRENT/SHORT RANGE HEAVY RAINFALL
LOCATIONS. THE DAY 5/WEDNESDAY ERO UPDATE INCORPORATED LATEST
GUIDANCE TO YIELD ONLY MILD ADJUSTMENTS FOR THE ESTABLISHED SLIGHT
RISK AREA EXTENDING FROM PARTS OF TEXAS INTO SOUTHERN IOWA. WITHIN
THIS RISK AREA, THERE MAY BE SOMEWHAT GREATER POTENTIAL BETWEEN
KANSAS AND IOWA GIVEN PROXIMITY TO THE STALLING SURFACE FRONT.
THERE IS A LITTLE MORE SPREAD FARTHER SOUTH. THIS OVERALL REGION
WILL BE SENSITIVE DUE TO WET ANTECEDENT CONDITIONS AND POTENTIAL
FOR EMBEDDED UPGRADES WILL BE POSSIBLE DEPENDING ON DETAILS OF
PRIOR CONVECTION AND HOW GUIDANCE CONSOLIDATES FOR AMOUNTS/LOCATION
OF HEAVIEST RAINFALL. EXPECT A SLOW TREND OF THE
RAIN/THUNDERSTORMS EASTWARD THROUGH THE REST OF THE WEEK,
CORRESPONDING TO PROGRESSION OF THE SURFACE FRONT.
ELSEWHERE, ONE OR MORE SHORTWAVES AND UPPER LOWS WILL LEAD TO
ROUNDS OF PRECIPITATION ACROSS THE NORTHERN TIER. ON TUESDAY, AMPLE
MOISTURE WITH PRECIPITABLE WATER VALUES OVER THE 90TH OR EVEN 95TH
PERCENTILE WILL BE IN PLACE ACROSS THE NORTHEASTERN U.S., WITH
RAIN AND THUNDERSTORMS LIKELY. A SHORTWAVE ALOFT MOVING THROUGH
(WITH ASSOCIATED WAVY SURFACE REFLECTION) AND SOME INSTABILITY IN
PLACE COULD LEAD TO HEAVY RAINFALL CAUSING ISOLATED FLOODING
CONCERNS, SO THE DAY 4/THURSDAY ERO MAINTAINS THE PREVIOUSLY
INTRODUCED MARGINAL RISK AREA CENTERED IN UPSTATE NEW YORK WITH
ITS FOOTPRINT STRETCHING A BIT INTO THE NORTHERN MID-ATLANTIC AND
WESTERN NEW ENGLAND. MEANWHILE THE NORTHWEST AND INTO THE NORTHERN
AND CENTRAL ROCKIES COULD GET ROUNDS OF PRECIPITATION THROUGH THE
WEEK, INCLUDING HIGHER ELEVATION SNOW. THERE IS STILL SOME
POTENTIAL FOR ONE OR MORE AREAS OF LOCALLY ENHANCED TOTALS BUT
RESOLVING SPECIFICS FOR IMPORTANT DETAILS OF THE SUPPORTING UPPER
AND SURFACE PATTERN CONTINUES TO BE A CHALLENGE. A FRONT NEARING
THE PACIFIC NORTHWEST COAST MAY BRING SOME PRECIPITATION TO THE
REGION BY NEXT SATURDAY.
TEMPERATURES ACROSS THE SOUTH-CENTRAL U.S. ALL THE WAY TO THE
EASTERN SEABOARD WILL BE WARMER THAN AVERAGE BY GENERALLY 10-15F
ON TUESDAY AND WEDNESDAY, WHILE COOLER THAN NORMAL TEMPERATURES ARE
LIKELY FOR THE NORTHWESTERN QUADRANT OF THE LOWER 48. THEN UPPER
TROUGHING AND A COLD FRONT PUSHING FROM THE NORTHWEST SHOULD COOL
TEMPERATURES TO NEAR OR BELOW NORMAL ACROSS THE CENTRAL U.S. BY
THURSDAY AND REACHING THE EAST-CENTRAL U.S. FRIDAY AND SATURDAY.
TEMPERATURES OVER THE WEST MAY REBOUND TO NEAR NORMAL OR MODERATELY
ABOVE NORMAL LEVELS BY LATE WEEK, BUT CONFIDENCE REMAINS LOWER
THAN AVERAGE GIVEN CONTINUED GUIDANCE SPREAD FOR UPPER FLOW
DETAILS.
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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