527
FXUS02 KWBC 050703
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
303 AM EDT FRI APR 5 2024
VALID 12Z MON APR 08 2024 - 12Z FRI APR 12 2024
...HEAVY RAIN AND FLASH FLOODING CONCERNS WILL INCREASE IN THE
SOUTHERN U.S. NEXT WEEK...
..OVERVIEW
BEHIND A WEAKENING SYSTEM TRACKING SLOWLY NORTHEASTWARD FROM THE
NORTHERN PLAINS/UPPER MIDWEST, THE DOMINANT FOCUS OF THE FORECAST
WILL TURN TO DIGGING WESTERN ENERGY THAT MAY ALREADY FORM A CLOSED
UPPER LOW OVER THE SOUTHWEST BY MONDAY AND THEN GRADUALLY TRACK
EASTWARD INTO THE PLAINS BY MIDWEEK OR SO. AMPLIFYING NORTHERN
STREAM ENERGY DOWNSTREAM FROM AN UPPER RIDGE MOVING INTO THE WEST
SHOULD ULTIMATELY PHASE WITH THE SOUTHERN FEATURE TO YIELD A LARGER
SCALE MEAN TROUGH CENTERED OVER THE EAST-CENTRAL U.S. BY LATE IN
THE WEEK. HOWEVER THERE ARE A LOT OF DETAIL UNCERTAINTIES WITH THIS
PHASING PROCESS AND THE RESULTING SURFACE EVOLUTION AFTER MIDWEEK.
IN THE MEANTIME THERE IS GREATER CONFIDENCE IN THE THREAT OF A
MULTI- DAY HEAVY/EXCESSIVE RAINFALL EVENT FROM PARTS OF THE
SOUTHERN PLAINS INTO LOWER MISSISSIPPI VALLEY EARLY-MID WEEK, AND
EXTENDING EASTWARD THEREAFTER, AS THE SOUTHERN UPPER LOW INTERACTS
WITH A LEADING FRONT AND PERSISTENT LOW LEVEL GULF INFLOW. THIS
SYSTEM MAY ALSO PRODUCE SIGNIFICANT SNOWFALL OVER PORTIONS OF THE
COLORADO/NEW MEXICO ROCKIES.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE DYNAMICAL MODELS/ENSEMBLES AND 12Z ECMWF-INITIALIZED MACHINE
LEARNING MODELS (MLS) OFFER GOOD CLUSTERING IN PRINCIPLE FOR
SIGNIFICANT FEATURES FROM MONDAY INTO EARLY WEDNESDAY. THE PRIMARY
STRAY SOLUTION IS THE NEW 00Z CMC WHICH TRACKS THE SOUTHWESTERN
UPPER LOW FARTHER SOUTHWEST THAN OTHER GUIDANCE FROM MONDAY ONWARD,
LEADING TO DELAYED PHASING TO THE EAST. THE 12Z CMC WAS A BIT ON
THE SOUTHWEST SIDE AS WELL BUT FIT CLOSER TO THE WESTERN SIDE OF
THE GUIDANCE ENVELOPE.
BY THE LATTER HALF OF THE WEEK, THE DYNAMICAL MODELS/ENSEMBLES AND
MLS SIGNIFICANTLY DIVERGE WITH RESPECT TO EXACTLY HOW THE
AMPLIFYING NORTHERN STREAM FLOW INTERACTS WITH THE SOUTHERN SYSTEM.
THIS LEADS TO WIDENING SPREAD FOR WHERE THE BEST-DEFINED SURFACE
LOW PRESSURE WILL TRACK BY LATE IN THE WEEK, WITH SOLUTIONS BY
FRIDAY RANGING ANYWHERE FROM THE TENNESSEE VALLEY INTO SOUTHEASTERN
CANADA. A RELATIVELY HIGHER PERCENTAGE OF SOLUTIONS WOULD SUGGEST
A SURFACE LOW IN THE VICINITY OF THE EASTERN GREAT LAKES OR
SOUTHEASTERN CANADA AS OF EARLY FRIDAY. ANY MEANINGFUL DELAY IN
STREAM PHASING WOULD LEAD TO A FARTHER SOUTH SURFACE LOW EMPHASIS.
CURRENTLY A MODEL/ENSEMBLE MEAN BLEND PROVIDES A REASONABLE
INTERMEDIATE REPRESENTATION OF THE MOST LIKELY EVOLUTION.
FARTHER WEST, GUIDANCE BECOMES RATHER CHAOTIC REGARDING DETAILS OF
PACIFIC ENERGY THAT MAY TRY TO PUSH INTO THE WESTERN NORTH AMERICA
RIDGE BY NEXT FRIDAY. AT THE VERY LEAST THE DEEP TROUGH REACHING
THE PACIFIC NORTHWEST COAST IN THE NEW 00Z CMC IS FAIRLY EXTREME
VERSUS OTHER GUIDANCE, WITH EITHER A MORE SUBTLE SHORTWAVE OR A
FARTHER OFFSHORE TROUGH BEING THE MORE LIKELY OPTIONS.
GUIDANCE COMPARISONS AMONG THE 12Z/18Z GUIDANCE LED TO STARTING
WITH AN OPERATIONAL MODEL COMPOSITE FOR THE FIRST HALF OF THE
PERIOD, FOLLOWED BY INCORPORATING SOME OF THE 18Z GEFS/12Z ECENS
MEANS WHILE ALSO SPLITTING GFS INPUT BETWEEN THE 12Z/18Z RUNS AS
DETAIL DIFFERENCES EMERGE. THIS LED TO AN EVEN WEIGHT BETWEEN THE
ENSEMBLE MEANS AND GFS/ECMWF/CMC BY NEXT FRIDAY.
..WEATHER/HAZARDS HIGHLIGHTS
THE DOMINANT FOCUS OF NEXT WEEK'S FORECAST FROM A HAZARD
PERSPECTIVE WILL BE A MULTI-DAY HEAVY RAINFALL THREAT FROM PARTS
OF THE SOUTHERN PLAINS EASTWARD AS THE UPPER LOW TRACKING OUT OF
THE SOUTHWEST INTERACTS WITH A PERSISTENT FLOW OF MOISTURE FROM THE
GULF OF MEXICO AND ONE OR MORE LEADING SURFACE FRONTS. THE DAYS
4-5 EXCESSIVE RAINFALL OUTLOOKS COVERING THE MONDAY THROUGH TUESDAY
NIGHT TIME FRAME DEPICT SLIGHT RISK AREAS CENTERED OVER NORTHERN
LOUISIANA AND SOUTHERN ARKANSAS, WITH THE DAY 5 AREA (PLUS THE
SURROUNDING MARGINAL RISK) EXPANDING RELATIVE TO DAY 4 AS GUIDANCE
SHOWS INCREASING MOISTURE ANOMALIES AIDING THE POTENTIAL FOR
HEAVIER RAINFALL. DEPENDING ON HOW CONSISTENT GUIDANCE IS IN
SHOWING TUESDAY'S ACTIVITY OVERLAPPING WITH MONDAY'S, THE CURRENT
DAY 5 TIME FRAME HAS POTENTIAL FOR A MODERATE RISK UPGRADE AT SOME
POINT IN THE COMING CYCLES. BEYOND TUESDAY NIGHT, EXPECT THE HEAVY
RAINFALL TO CONTINUE EASTWARD ACROSS THE SOUTHERN TIER WHILE LESS
EXTREME BUT STILL POTENTIALLY SIGNIFICANT RAIN EXPANDS ACROSS THE
EAST LATER IN THE WEEK. SOUTHERN TIER RAINFALL SHOULD GRADUALLY
BECOME A LITTLE MORE PROGRESSIVE AFTER MIDWEEK ASSUMING SURFACE LOW
PRESSURE TRACK NORTHWARD WITH ITS TRAILING COLD FRONT BECOMING THE
PRIMARY RAINFALL FOCUS. THE STORM PREDICTION CENTER IS ALSO
MONITORING THE POTENTIAL FOR SEVERE WEATHER WITH THIS SOUTHERN TIER
EVENT. CHECK THEIR LATEST OUTLOOKS FOR MORE INFORMATION.
OTHERWISE, THE SYSTEM TRACKING NORTHEASTWARD FROM THE UPPER MIDWEST
MAY PRODUCE SOME LIGHT PRECIPITATION IN ITS VICINITY AND FARTHER
EAST ACROSS THE GREAT LAKES ALONG ITS TRAILING FRONT. TO THE WEST
OF THE SOUTHERN TIER HEAVY RAIN AREA, THE SYSTEM TRACKING THROUGH
THE SOUTHWEST SHOULD BRING A PERIOD OF SIGNIFICANT SNOWFALL TO
PORTIONS OF THE ROCKIES IN COLORADO AND NEW MEXICO DURING THE FIRST
HALF OF THE WEEK. GREATEST ACCUMULATIONS SHOULD BE DURING TUESDAY-
TUESDAY NIGHT. THE NORTHERN PACIFIC NORTHWEST/ROCKIES SHOULD SEE
SOME LIGHT TO MODERATE PRECIPITATION EARLY IN THE WEEK AS A FRONTAL
SYSTEM BRUSHES THE REGION. A LITTLE MOISTURE COULD RETURN TO THE
PACIFIC NORTHWEST BY NEXT FRIDAY WITH ANOTHER FRONT, BUT CONFIDENCE
IN DETAILS IS FAIRLY LOW AT THIS TIME.
MODERATELY BELOW NORMAL HIGH TEMPERATURES WILL COVER MOST OF THE
WEST ON MONDAY AND THEN CONCENTRATE MORE STRONGLY OVER THE SOUTHERN
ROCKIES/HIGH PLAINS TUESDAY-WEDNESDAY (UP TO 10-20F BELOW AVERAGE)
AS THE SOUTHWEST UPPER LOW PASSES OVERHEAD. THE WEST WILL SEE A
WARMING TREND FROM WEST TO EAST NEXT WEEK AS UPPER RIDGING BUILDS
IN, WITH AN EXPANDING AREA OF TEMPERATURES AT LEAST 10F ABOVE
NORMAL. THIS WARMTH WILL EXTEND INTO THE NORTHERN PLAINS AS WELL.
THE EASTERN HALF OF THE COUNTRY WILL GENERALLY BE ABOVE NORMAL DUE
TO BEING IN THE WARM SECTOR OF AN INITIAL UPPER MIDWEST SYSTEM AND
THEN A TRAILING FRONT AND EJECTING SOUTHWEST SYSTEM. FRONTAL
PASSAGE LATE IN THE WEEK WILL BRING A SOMEWHAT COOLER TREND TO THE
EAST.
RAUSCH
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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