439
FXUS02 KWBC 011900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 PM EDT MON MAY 01 2023
VALID 12Z THU MAY 04 2023 - 12Z MON MAY 08 2023
...PROGRESSIVELY COOLER AND WETTER PATTERN FOR THE WEST
COAST/GREAT BASIN/ROCKIES THIS WEEK...
..OVERVIEW
THE UPPER LEVEL PATTERN SHOULD EVENTUALLY TRANSITION AWAY FROM THE
STUBBORN OMEGA BLOCK CONFIGURATION IN PLACE THROUGH THE EARLY PART
OF THE FORECAST PERIOD. HOWEVER RIDGING WILL LIKELY PERSIST OVER
CENTRAL AND NORTHWESTERN CANADA INTO EARLY NEXT WEEK, FAVORING
ANOTHER UPPER TROUGH SETTLING OVER THE NORTHEASTERN PACIFIC/WEST
COAST IN THE WAKE OF AN EJECTING UPPER LOW/TROUGH WHILE SOME
DEGREE OF NORTHWESTERLY MEAN FLOW ALOFT MAY PERSIST OVER NEW
ENGLAND. THE REST OF THE LOWER 48 SHOULD SEE A GRADUAL TREND
TOWARD FLATTER MEAN FLOW, WHICH MAY PROMOTE A SURFACE SYSTEM
EMERGING FROM THE PLAINS BY SUNDAY-MONDAY DEPENDING ON THE DETAILS
OF EJECTING WESTERN ENERGY. THE FORECAST PATTERN EVOLUTION WILL
FAVOR ONE OR MORE DAYS OF PRECIPITATION ACROSS A BROAD AREA FROM
THE NORTHERN HALF OR SO OF THE WEST THROUGH PARTS OF THE
PLAINS/MISSISSIPPI VALLEY AND INTO THE SOUTH. VERY COOL CONDITIONS
OVER THE SOUTHWEST AND NORTHEAST, PLUS VERY WARM WEATHER OVER THE
NORTHERN ROCKIES/HIGH PLAINS, ON THURSDAY SHOULD GRADUALLY
MODERATE BY THE WEEKEND AND EARLY NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
LATEST GUIDANCE DISPLAYS SOME SIMILAR IDEAS IN THE LARGER SCALE
BUT THERE ARE ALSO SIGNIFICANT DETAIL DIFFERENCES THAT HAVE MUCH
LOWER PREDICTABILITY DUE TO AN UNKNOWN DEGREE OF INTERACTION AMONG
SEPARATE FEATURES. THE PRIMARY FOCUS FOR THESE ISSUES INVOLVE THE
RELATIVE INFLUENCE OF A SOUTHERN CANADA UPPER LOW RETROGRADING
UNDER A CENTRAL CANADA RIDGE, THE UPPER LOW FORECAST TO TRACK
NORTHEASTWARD FROM CALIFORNIA, ENERGY ENTERING THE OVERALL TROUGH
IMMEDIATELY TO THE WEST, AND A LARGER SCALE NORTHEASTERN PACIFIC
TROUGH THAT SHOULD DESCEND TOWARD THE WEST COAST BY THE WEEKEND.
FLOW OVER THE NORTHEAST ALSO BECOMES A QUESTION MARK TOWARD DAYS
6-7 SUNDAY-MONDAY. BROADLY SPEAKING, AFTER AN AVERAGE OF LATEST
GUIDANCE (WITH MORE EMPHASIS ON THE 06Z GFS/00Z ECMWF) REFLECTED
THE PREVAILING THEMES OF GUIDANCE FOR ABOUT THE FIRST HALF OF THE
PERIOD, THE GFS/ECMWF AND THEIR ENSEMBLE MEANS PROVIDED THE BEST
TEMPLATE BY DAYS 6-7 SUNDAY-MONDAY. THIS SOLUTION SHOWED A STEADY
EASTWARD PROGRESSION OF THE SHORTWAVE EMERGING FROM THE WEST, AND
BY DAY 7 MAINTAINED A SURFACE LOW TRACK SOMEWHAT SOUTH OF THE 06Z
GFS ON ITS OWN (AS RECOMMENDED BY THE ECMWF MEAN AND LATEST GEFS
MEANS). THE 00Z CMC/CMCENS HELD ONTO MEAN RIDGING ALOFT OVER THE
EAST-CENTRAL U.S. INTO DAY 7, THUS HOLDING THE SURFACE REFLECTION
WELL WESTWARD, THOUGH THE NEW 12Z CMC IS TRENDING SOME IN THE GFS
DIRECTION. THERE IS STILL LOW CONFIDENCE IN HOW THIS SHORTWAVE
ENERGY WILL LOOK THAT FAR OUT IN TIME. MEANWHILE THE ENSEMBLES
HELP TO PROVIDE AN ANCHORING INFLUENCE ON THE FORECAST WITH THE
LATE PERIOD WEST COAST UPPER TROUGH, AS INDIVIDUAL OPERATIONAL
MODEL RUNS VARY WITH SHORTWAVE DETAILS. GFS RUNS ARE GENERALLY ON
THEIR OWN WITH HOW MUCH RIDGING THEY BUILD INTO THE NORTHEAST
AROUND THE START OF NEXT WEEK, WITH THE ENSEMBLE MEANS MAINTAINING
NORTHWESTERLY OR CYCLONIC FLOW.
..WEATHER/HAZARDS HIGHLIGHTS
PRECIPITATION CHANCES WILL STEADILY INCREASE FROM THE WEST COAST
THROUGH THE GREAT BASIN AND NORTHERN ROCKIES MID- TO LATE WEEK AS
A CLOSED LOW OVER THE WEST EJECTS NORTHEASTWARD OVER THE GREAT
BASIN AND EVENTUALLY NORTHWARD INTO CANADA. THE PRECIPITATION MAY
BE LOCALLY MODERATE TO HEAVY AT TIMES, WITH SOME SCATTERED
THUNDERSTORM CHANCES. SNOW LEVELS WILL BEGIN HIGH BUT ARE FORECAST
TO FALL THROUGH THE LATTER PART OF THE WEEK AS THE
UPPER-LOW/TROUGH PROGRESSES INLAND AND PROVIDES COOL AIR ALOFT.
THE HIGHEST SNOW CHANCES WILL BE FOR THE SIERRA NEVADA ON
THURSDAY. THE RAINFALL ON TOP OF ONGOING SNOW MELT MAY LEAD TO
SOME FLOODING CONCERNS IN TERRAIN SENSITIVE AREAS AROUND THE
FOOTHILLS OF LOCAL MOUNTAIN RANGES AS WELL AS ALONG LOCAL
STREAMS/RIVERS THURSDAY-FRIDAY, BUT THIS WILL LIKELY BE DRIVEN
MORE BY LOCAL HYDROLOGICAL CONDITIONS HARDER TO ANTICIPATE AT THIS
RANGE. HOWEVER THERE ARE GRADUALLY IMPROVING GUIDANCE SIGNALS FOR
LOCALLY HEAVY RAINFALL POTENTIAL OVER PARTS OF THE GREAT
BASIN/NORTHERN ROCKIES, SUFFICIENT TO PROPOSE ADDING MARGINAL RISK
AREAS IN THE DAYS 4-5 (12Z THURSDAY-12Z FRIDAY, 12Z FRIDAY-12Z
SATURDAY) EXCESSIVE RAINFALL OUTLOOKS OVER PORTIONS OF THIS
REGION. MOIST, UPSLOPE FLOW SETTLING IN ALONG PORTIONS OF THE
NORTHERN/CENTRAL HIGH PLAINS AND POSSIBLY A SEPARATE NORTH-SOUTH
CONVERGENCE AXIS FARTHER EAST MAY PROMOTE LOCALLY MODERATE TO
HEAVY RAINFALL LATE THIS WEEK INTO THE WEEKEND. THE DAY 5 OUTLOOK
PROPOSES A MARGINAL RISK AREA OVER THE NORTH-CENTRAL PLAINS TO
REFLECT THE BEST OVERLAP OF GUIDANCE PARAMETERS FOR THIS POTENTIAL
ACTIVITY.
TO THE EAST, GULF MOISTURE FLOWING NORTHWARD AS WELL AS ADDITIONAL
MOISTURE SOURCED FROM THE PACIFIC WILL SPREAD NORTHEASTWARD ACROSS
THE CENTRAL/SOUTHERN PLAINS AND MIDDLE/LOWER MISSISSIPPI VALLEY
INTO THE SOUTHEAST MID- TO LATE WEEK LEADING TO A BROAD REGION OF
SHOWERS AND THUNDERSTORMS. THE EVOLUTION OF THE PATTERN HAS
REMAINED RATHER NEBULOUS THE LAST FEW DAYS AS THE GUIDANCE HAS
VARIED ON THE TIMING AND EVOLUTION OF A 'BACKDOOR' COLD FRONT
SINKING SOUTHWESTWARD AS WELL AS EMBEDDED SHORTWAVE ENERGY ALOFT,
LESSENING CONFIDENCE ON THE SPECIFICS OF REGIONS OF HEAVIER
RAINFALL AND POTENTIAL FLASH FLOODING CONCERNS. HOWEVER, THE
GUIDANCE HAS SHOWN THE HIGHEST CONFIDENCE IN WIDESPREAD CONVECTION
AND HIGH QPF AMOUNTS OVER THE CENTRAL PLAINS/MIDDLE MISSISSIPPI
VALLEY ON THURSDAY, WHERE A DAY 4 MARGINAL RISK AREA CONTINUES IN
THE EXCESSIVE RAINFALL OUTLOOK. THIS CONVECTION MAY TREND
DOWNSTREAM FRIDAY INTO THE TENNESSEE VALLEY, WITH THE POTENTIAL
FOR TRAINING APPEARING SUFFICIENT TO SUPPORT A MARGINAL RISK AREA
ON THE DAY 5 OUTLOOK. ADDITIONAL HEAVY RAINFALL CHANCES LOOK
POSSIBLE OVER THE WEEKEND MORE BROADLY ACROSS THE
CENTRAL/SOUTHEASTERN U.S., BUT CONFIDENCE REMAINS LOW GIVEN THE
GUIDANCE SPREAD IN HOW THE SURFACE PATTERN AND SUPPORTING
SHORTWAVE DYNAMICS WILL EVOLVE.
THE PERSISTENT COOL/WET PATTERN OVER THE GREAT LAKES AND NORTHEAST
DUE TO THE STUBBORN UPPER-LEVEL LOW WILL FINALLY START TO MODERATE
LATER THIS WEEK AS THE LOW DEPARTS OVER THE ATLANTIC, WITH EASTERN
U.S. HIGHS FINALLY REACHING NEAR TO SLIGHTLY ABOVE NORMAL LEVELS
BY THE WEEKEND AND EARLY NEXT WEEK AFTER STARTING THE PERIOD 5-15F
BELOW NORMAL ON THURSDAY. THURSDAY WILL ALSO SEE SOME TEMPERATURE
EXTREMES FARTHER WEST, WITH HIGHS AT LEAST 15-20F ABOVE NORMAL
OVER THE NORTHERN HALF OF THE HIGH PLAINS/ROCKIES AND 10-20F BELOW
NORMAL OVER THE SOUTHWESTERN STATES. THEREAFTER, BELOW NORMAL HIGH
TEMPERATURES SHOULD SPREAD ACROSS MORE OF THE WEST BUT WITH LESS
EXTREME ANOMALIES, WHILE MODERATELY ABOVE NORMAL TEMPERATURES WILL
SETTLE OVER THE CENTRAL U.S.
RAUSCH/PUTNAM
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, EXPERIMENTAL EXCESSIVE RAINFALL
OUTLOOK, WINTER WEATHER OUTLOOK PROBABILITIES AND HEAT INDICES 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
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