912
FXUS02 KWBC 260700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 AM EST THU DEC 26 2024
VALID 12Z SUN DEC 29 2024 - 12Z THU JAN 02 2025
...HEAVY RAIN AND MOUNTAIN SNOW LIKELY FOR THE WEST COAST/NORTHWEST
EARLY NEXT WEEK...
..OVERVIEW
PROGRESSIVE FLOW WITH MULTIPLE SHORTWAVES ROTATING AROUND THE
SOUTHERN SIDE OF A SOUTH-CENTRAL CANADA UPPER LOW WILL BE IN PLACE
NEXT WEEK, PRODUCING MANY LOW PRESSURE/FRONTAL SYSTEMS ACROSS THE
LOWER 48. IN THE WEST ON SUNDAY, AMPLE MOISTURE AHEAD OF AN EASTERN
PACIFIC UPPER TROUGH SHOULD PRODUCE POSSIBLY HEAVY RAIN CONTINUING
ALONG THE WEST COAST AND SNOW OVER THE NORTHERN HALF OR SO OF THE
ROCKIES, ALONG WITH SOME AREAS OF STRONG WINDS. THE NORTHWEST LOOKS
TO QUIET SOMEWHAT INTO THE WORKWEEK BUT MODEST RAIN AND SNOW MAY
STILL BE EXPECTED, ESPECIALLY INTO MIDWEEK. FARTHER EAST, RAIN THAT
COULD BE HEAVY IS LIKELY OVER THE APPALACHIANS TO EASTERN SEABOARD
EARLY NEXT WEEK, WITH FREEZING RAIN POSSIBLE IN NORTHERN NEW
ENGLAND. ANOTHER ROUND OF PRECIPITATION (INCLUDING POSSIBLE SNOW IN
THE NORTH-CENTRAL PLAINS) IS FORECAST TO PUSH FROM THE NORTH-
CENTRAL TO EASTERN U.S. MONDAY-WEDNESDAY. WARMER THAN NORMAL
TEMPERATURES FOR MOST OF THE LOWER 48 EARLY NEXT WEEK WILL
MODERATE CLOSER TO NORMAL FROM WEST TO EAST DURING THE PERIOD.
..GUIDANCE/PREDICTABILITY ASSESSMENT
RECENT MODEL GUIDANCE MAINTAINS THE THEME OF REASONABLE AGREEMENT
FOR THE OVERALL PATTERN BUT WITH IMPORTANT DETAIL DIFFERENCES THAT
AFFECT SENSIBLE WEATHER. AS THE MEDIUM RANGE PERIOD BEGINS SUNDAY,
ONE SUCH ISSUE IS WITH THE DEPTH AND ORIENTATION OF A TROUGH MOVING
THROUGH THE EAST-CENTRAL TO EASTERN U.S. ECMWF RUNS HAVE SHOWN
THIS TROUGH TO BE QUITE SHARP/NARROW, ENDING UP MORE NEGATIVELY
TILTED THAN OTHER GUIDANCE AND LEADING TO A STRONGER DEEP-LAYER
GRADIENT FEEDING MOISTURE INTO THE MID-ATLANTIC AND NORTHEAST THAT
PROGRESSES RAIN/CONVECTION MORE SLOWLY. THE 00Z ECMWF HAS NOW
TRENDED BROADER AND NOT AS DEEP WITH THE TROUGH, WHICH MATCHES
OTHER MODELS BETTER, BUT THE FLOW PATTERN WITH MULTIPLE SHORTWAVES
DURING THE SHORT RANGE PERIOD INTERACTING DOES LEND LOW CONFIDENCE
THAT THIS IS A SOLID TREND. AT LEAST FOR NOW, THE 00Z EC QPF
TIMING AND PLACEMENT FORTUNATELY ALIGNS BETTER THAN THE 12Z EC DID.
THE CMC/UKMET WERE MEANWHILE ON THE FASTER SIDE WITH LOW TRACK/QPF
PROGRESSION WHILE GFS RUNS SEEM LIKE A FAIR MIDDLE GROUND.
THE NEXT SIGNIFICANT SYSTEM IN THE SERIES REACHES THE WEST COAST
BY EARLY SUNDAY, WITH MODELS REASONABLY AGREEABLE ON THE TROUGH AND
A SOMEWHAT ELONGATED AREA OF SURFACE LOW PRESSURE. GUIDANCE
PRESSES THE SHORTWAVE QUICKLY ACROSS THE CENTRAL AND EASTERN U.S.
MONDAY-TUESDAY WHILE A COUPLE OF SURFACE LOWS SPIN UP IN THE HIGH
PLAINS AND MOVE EAST. MODELS SHOWED MOSTLY MINOR DIFFERENCES UNTIL
TUESDAY-WEDNESDAY, WHEN THE 12/18Z GFS INDICATED A SHARPER TROUGH
TRACKING ACROSS THE NORTHEAST AND LEADING TO A QUICKER/FARTHER
NORTHEAST SURFACE LOW IN THE WESTERN ATLANTIC BY WEDNESDAY. WITH
THE CONTROL RUN OF THE EC, EC MEAN, AND AI/ML MODELS AS WELL AS THE
CMC SHOWING GOOD AGREEMENT IN THE SURFACE LOW POSITION NEAR NEW
JERSEY OR SO 12Z WEDNESDAY, FAVORED THESE RATHER THAN THE GFS. THE
NEW 00Z GFS SEEMS LESS SHARP WITH THE TROUGH ALOFT BUT STILL FASTER
WITH THE EASTERN SIDE OF THE TROUGH AND THUS THE LOW POSITION.
MORE SHORTWAVES COMING IN FROM THE WEST INTO THE CENTRAL U.S. SHOW
TYPICAL DETAIL/TIMING DIFFERENCES. THE PATTERN BY WEDNESDAY-
THURSDAY MAY START TO BE MORE DOMINATED BY LONGWAVE FEATURES AS AN
UPPER RIDGE AXIS FORMS OVER THE WEST IN RESPONSE TO PACIFIC
TROUGHING AND BEHIND THE CENTRAL-EASTERN U.S. TROUGH, HOPEFULLY
INCREASING PREDICTABILITY IN FUTURE FORECASTS.
THE WPC FORECAST USED A MULTI-MODEL DETERMINISTIC BLEND EARLY IN
THE PERIOD. ELIMINATED THE GFS RUNS FROM THE BLEND BY DAY 6 GIVEN
THE AFOREMENTIONED ISSUES, AND GRADUALLY REDUCED THE PROPORTION OF
OTHER DETERMINISTIC MODELS SO THAT ENSEMBLE MEANS MADE UP HALF THE
FORECAST BLEND BY DAY 6 AND MORE DAY 7.
..WEATHER/HAZARDS HIGHLIGHTS
AN ATMOSPHERIC RIVER WILL TAKE AIM AT THE WEST COAST OVER THE
WEEKEND. THE HEAVIEST RAIN IS LIKELY ON SATURDAY (NOW INTO THE
SHORT RANGE), BUT RAIN TOTALS STILL APPEAR SUFFICIENT TO WARRANT A
MARGINAL RISK FOR COASTAL OREGON TO NORTHERN CALIFORNIA IN THE DAY
4/SUNDAY ERO, ESPECIALLY CONSIDERING THE INCREASED SENSITIVITY DUE
TO PRIOR RAINFALL. MOISTURE SPREADING FARTHER INLAND INTO THE
HIGHER TERRAIN WILL ALLOW FOR HEAVY SNOW TOTALS ON SUNDAY IN THE
CASCADES AND NORTHERN ROCKIES, WITH LESSER AMOUNTS IN THE SIERRA
NEVADA AND CENTRAL ROCKIES. SNOW LEVELS SHOULD BE LOWERED BY SUNDAY
COMPARED TO EARLIER DAYS. PRECIPITATION SHOULD LESSEN ACROSS THE
NORTHWEST MONDAY-TUESDAY WITH LIGHTER AMOUNTS EXPECTED.
PRECIPITATION COULD INCREASE AGAIN TUESDAY NIGHT INTO WEDNESDAY BUT
CURRENTLY THIS ROUND DOES NOT LOOK TO BE AS HEAVY AS THE WEEKEND
SYSTEM.
DOWNSTREAM, THE EASTERN U.S. CAN EXPECT MODERATE TO HEAVY RAIN ON
SUNDAY IN THE VICINITY OF A COUPLE OF FRONTAL SYSTEMS. WHILE THERE
IS A FAIR AMOUNT OF SPREAD IN THE TIMING AND EVOLUTION OF HEAVIEST
RAIN, HIGH PROBABILITIES FOR RAINFALL EXCEEDING AN INCH ON SUNDAY
ARE IN GOOD AGREEMENT IN THE GEFS AND EC ENSEMBLES ACROSS
PENNSYLVANIA GIVEN LIKELY GULF AND ATLANTIC MOISTURE INFLOW.
FARTHER NORTH INTO NEW YORK STATE, FROZEN GROUND/SNOW COVER WILL
LEAD TO ADDED SENSITIVITY. MEANWHILE FARTHER SOUTH ACROSS THE
APPALACHIANS TO PIEDMONT, RELATIVELY MORE INSTABILITY MAY OVERCOME
RELATIVELY DRIER TO AVERAGE ANTECEDENT CONDITIONS TO PRODUCE
CONVECTION WITH HEAVY RAIN RATES ON SUNDAY, CONTINUING FROM
SATURDAY-SATURDAY NIGHT'S HEAVY RAIN IN THE MID-SOUTH. THUS THE
MARGINAL RISK IN THE DAY 4/SUNDAY ERO WAS EXPANDED SOUTH IN THIS
FORECAST PACKAGE TO ENCOMPASS THESE AREAS AS WELL. ANOTHER THREAT
WILL BE THE POTENTIAL FOR FREEZING RAIN ACROSS MAINE IN PARTICULAR
WITH THIS SYSTEM SUNDAY-MONDAY.
A LOW PRESSURE SYSTEM WILL DEVELOP IN THE PLAINS BY MONDAY WITH A
SHORTWAVE EJECTING FROM THE WEST. GUSTY WINDS ARE LIKELY BEHIND
THIS LOW ACROSS PARTS OF THE ROCKIES AND HIGH PLAINS. MEANWHILE
NORTH OF THE LOW TRACK, SNOW IS POSSIBLE ACROSS THE NORTH-CENTRAL
PLAINS MONDAY AND THEN WILL SPREAD ANOTHER AREA OF PRECIPITATION
ACROSS THE EAST TUESDAY-WEDNESDAY. AS WITH THE LEADING SYSTEM,
WINTRY WEATHER SHOULD BE CONFINED FAIRLY FAR NORTHWARD. HOWEVER THE
EXTENT OF SNOW/ICE OVER THE NORTHEAST AROUND MIDWEEK WILL BE
SENSITIVE TO SURFACE LOW TRACK DETAILS THAT WILL TAKE MORE TIME TO
BE RESOLVED.
MUCH OF THE COUNTRY WILL BE WELL ABOVE AVERAGE TEMPERATURE-WISE
EARLY NEXT WEEK. LOW TEMPERATURES OF 20-30 DEGREES ABOVE NORMAL
WILL BE COMMON IN THE MIDWEST TO EAST ON SUNDAY, WITH 50S FOR LOWS
REACHING ALL THE WAY INTO THE OHIO VALLEY. HIGH TEMPERATURES SHOULD
BE MAXIMIZED ACROSS THE SOUTHERN PLAINS ON SUNDAY AND ESPECIALLY
MONDAY, WITH HIGHS IN THE 70S AND 80S ACROSS TEXAS THAT COULD BREAK
DAILY RECORD HIGHS. AS MULTIPLE COLD FRONTS TRACK ACROSS THE
NATION, MORE SEASONABLE COOL AIR SHOULD GRADUALLY TAKE OVER AS THE
WEEK PROGRESSES. THE NORTHWESTERN TO CENTRAL U.S. IS FORECAST TO
SEE NEAR NORMAL TEMPERATURES MONDAY-TUESDAY, AND COOLER
TEMPERATURES WILL SPREAD ACROSS THE EASTERN U.S. WEDNESDAY-
THURSDAY. BY MIDWEEK AND BEYOND THE MAIN AREA OF ABOVE NORMAL
TEMPERATURES IS FORECAST TO BE THE SOUTHWESTERN U.S., BUT ONLY BY
ABOUT 5-10 DEGREES.
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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