500
FXUS02 KWBC 221901
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM EST FRI DEC 22 2023
VALID 12Z MON DEC 25 2023 - 12Z FRI DEC 29 2023
...HEAVY SNOW THREAT OVER THE NORTH-CENTRAL PLAINS ON CHRISTMAS
DAY...
...HEAVY RAIN THREAT FROM THE SOUTHEAST CHRISTMAS DAY THEN INTO
THE MID-ATLANTIC BY TUESDAY...
..OVERVIEW
MOST GUIDANCE SUGGESTS THAT AN INITIAL CENTRAL PLAINS UPPER LOW
WILL DRIFT NORTHEASTWARD EARLY NEXT WEEK AND THEN NORTHERN STREAM
ENERGY MAY DIG INTO WHAT SHOULD REMAIN A FAIRLY AMPLIFIED UPPER
TROUGH REACHING THE EASTERN U.S. BY LATE WEEK. THE SYSTEM
ASSOCIATED WITH THE PLAINS UPPER LOW WILL SPREAD A BROAD AREA OF
RAINFALL OVER THE EAST EARLY TO MID-WEEK WITH SOME LOCALLY HEAVY,
AND PORTIONS OF THE NORTH-CENTRAL PLAINS COULD SEE SIGNIFICANT
SNOWFALL ON THE BACK SIDE OF THE SYSTEM ON CHRISTMAS DAY. PACIFIC
SHORTWAVES PUSHING INTO A PERSISTENT WESTERN U.S. MEAN RIDGE ALOFT
WILL BRING SOME EPISODES OF PRECIPITATION TO THE CENTRAL AND
NORTHERN WEST COAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE GUIDANCE CONTINUES TO SHOW FAIRLY GOOD AGREEMENT ON THE
CLUSTERING OF THE INITIAL PLAINS/MIDWEST UPPER LOW AND SURFACE
SYSTEM EARLY IN THE WEEK. THE 00Z CMC WAS A EASTERN OUTLIER WITH
THE GFS, ECMWF, AND UKMET ALL SHOWING BETTER AGREEMENT WITH THE
ENSEMBLE MEANS ON UPPER AND SURFACE LOW PLACEMENTS MONDAY-TUESDAY
ACROSS THE UPPER MIDWEST REGION. FOR THIS REASON, THE CMC WAS NOT
USED IN TODAYS BLEND AT ALL. BY TUESDAY AND BEYOND THOUGH, THE
FORECAST CONTINUES TO BECOME MORE AND MORE UNCERTAIN WITH REGARDS
TO HOW DIGGING NORTHERN STREAM ENERGY INTERACTS WITH THE INITIAL
LOW. PREVIOUS RUNS HAS SUGGESTED IT MAY ACT MORE TO KICK THE
ORIGINAL ENERGY INTO THE NORTHEAST, BUT NOW SEEM TO BE TRENDING
TOWARDS MORE INTERACTION AND SOME GENERAL COMBINATION OF THE TWO
ENERGIES. THE ENSEMBLE MEANS PROVIDE DECENT CONFIDENCE IN AN
EASTERN U.S. UPPER TROUGH LATE WEEK BUT THE FULL ARRAY OF GUIDANCE
CONTINUES TO OFFER LITTLE AGREEMENT ON HOW ENERGY MAY BE
DISTRIBUTED WITHIN THE TROUGH. FARTHER WEST, THERE IS REASONABLE
AGREEMENT ON A SHALLOW PACIFIC SHORTWAVE BRUSHING THE PACIFIC
NORTHWEST BY EARLY TUESDAY AND THEN A SOMEWHAT MORE AMPLIFIED ONE
CROSSING THE WEST COAST BY THURSDAY. AGAIN, THE 00Z CMC IS THE
OUTLIER HERE WITH THE THURSDAY TROUGH DEEPER AND SLOWER TO MOVE
THROUGH THE WEST, WHILE THE REST OF GUIDANCE SHOWS A BUILDING
RIDGE BY FRIDAY AHEAD OF ANOTHER TROUGH IN THE EASTERN PACIFIC.
THE WPC BLEND FOR TODAY FAVORED THE GFS, ECMWF, AND UKMET EARLY IN
THE PERIOD, TRENDING MORE TOWARDS THE ENSEMBLE MEANS DAY 6 AND 7.
WAS ABLE TO USE SOME MODEST AMOUNTS OF THE DETERMINISTIC GFS/ECMWF
AS THEY WERE MOST CONSISTENT WITH THE ENSEMBLE MEANS. OVERALL,
THIS APPROACH MAINTAINED GOOD CONTINUITY WITH THE PREVIOUS WPC
FORECAST.
..WEATHER/HAZARDS HIGHLIGHTS
THE SYSTEM AFFECTING THE CENTRAL AND EASTERN U.S. EARLY-MID WEEK
WILL SPREAD PRECIPITATION ACROSS THE NORTH-CENTRAL PLAINS AND MOST
LOCATIONS FROM THE MISSISSIPPI VALLEY TO EAST COAST. THE
NORTH-CENTRAL PLAINS WILL BE ON THE COLD SIDE OF THE SYSTEM AND
GUIDANCE DOES SUGGEST SOME DECENT SNOWFALL (POTENTIALLY SEVERAL
INCHES OR MORE) ACROSS EASTERN SOUTH DAKOTA AND NEBRASKA ON
CHRISTMAS DAY. SOME AREAS ACROSS THE FAR NORTHERN TIER MAY ALSO
SEE SOME WINTRY WEATHER BUT WITH MORE UNCERTAINTY IN SPECIFICS.
MEANWHILE EXPECT THE POTENTIAL FOR LOCALLY HEAVY RAINFALL AHEAD OF
THE LOW PRESSURE/FRONTAL SYSTEM IN THE EAST THAT SHOULD DRAW UPON
MOISTURE FROM THE GULF AND THEN THE ATLANTIC. THE DAY 4 EXCESSIVE
RAINFALL OUTLOOK VALID MONDAY/CHRISTMAS DAY MAINTAINS A MARGINAL
RISK AREA OVER THE SOUTHEAST WITH AN EMBEDDED SLIGHT RISK AREA
CENTERED OVER AND NEAR THE FLORIDA PANHANDLE WHERE THE BEST
COMBINATION OF MOISTURE AND INSTABILITY SHOULD EXIST. THE DAY 5
OUTLOOK VALID ON TUESDAY HAS A MARGINAL RISK AREA OVER THE
MID-ATLANTIC AS THE MOISTURE AXIS PROGRESSES NORTHEASTWARD. ON
BOTH DAYS, SOUTHEASTERLY LOW LEVEL FLOW MAY HELP TO ENHANCE TOTALS
OVER FAVORED TERRAIN ALONG THE APPALACHIANS. PRECIPITATION WILL
CONTINUE INTO THE NORTHEAST BY MIDWEEK. SOME LAKE EFFECT
PRECIPITATION WILL BE POSSIBLE AFTER SYSTEM PASSAGE AS WELL.
ALONG THE WEST COAST, THE DAY 4 MONDAY/CHRISTMAS DAY EXCESSIVE
RAINFALL OUTLOOK MAINTAINS A MARGINAL RISK AREA OVER THE NORTHERN
PACIFIC NORTHWEST COAST WITH A CONSISTENT SIGNAL FOR A DEFINED
ATMOSPHERIC RIVER, ALBEIT WITH SOME CONTINUED SPREAD FOR EXACT
RAINFALL TOTALS. SOME POTENTIAL FOR MIXED PRECIPITATION OR
FREEZING RAIN IN SOME OF THE LOWER ELEVATIONS/VALLEYS TOO. THERE
IS MORE GUIDANCE SPREAD BY TUESDAY SO NO AREA IS DEPICTED IN THE
DAY 5 OUTLOOK AT THIS TIME. A SOMEWHAT MORE AMPLIFIED SHORTWAVE
APPROACHING/REACHING THE WEST COAST SHOULD HELP TO FOCUS MODERATE
TO HEAVY PRECIPITATION ALONG THE CENTRAL WEST COAST BY WEDNESDAY.
THEN CONFIDENCE DECREASES REGARDING COVERAGE/AMOUNTS OF ANY
ADDITIONAL PRECIPITATION AROUND THE END OF THE WEEK AS GUIDANCE
SOLUTIONS DIVERGE FOR FLOW ALOFT.
TEMPERATURES WILL BE QUITE WARM AHEAD OF THE SYSTEM AFFECTING THE
CENTRAL/EASTERN U.S. WITH THE GREATEST ANOMALIES FROM THE MIDWEST
INTO THE NORTHEAST. MORNING LOWS SHOULD BE UP TO 20-30F OR SO
ABOVE NORMAL WHILE HIGHS WILL TEND TO BE 10-25F ABOVE NORMAL.
MOST DAILY RECORDS WILL LIKELY BE FOR WARM LOWS OVER THE
MISSISSIPPI VALLEY INTO GREAT LAKES/OHIO VALLEY EARLY IN THE WEEK,
WITH A FEW STILL POSSIBLE FARTHER TO THE EAST THEREAFTER. COOLER
AIR BEHIND THE SYSTEM WILL STILL BE ON THE MILD SIDE, WITH ONLY
SLIGHTLY BELOW NORMAL READINGS OVER THE CENTRAL-SOUTHERN
ROCKIES/HIGH PLAINS ON MONDAY AND THEN THE GULF COAST/FLORIDA BY
LATER IN THE WEEK AS THE WEST AND HIGH PLAINS TREND GRADUALLY
WARMER.
SANTORELLI/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, 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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