520
FXUS02 KWNH 210700
PREEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 AM EST SAT DEC 21 2024
VALID 12Z TUE DEC 24 2024 - 12Z SAT DEC 28 2024
..GENERAL OVERVIEW
A PROGRESSIVE UPPER LEVEL FLOW PATTERN IS EXPECTED TO BE IN PLACE
ACROSS THE CONTINENTAL U.S. FOR THE EARLY TO MIDDLE PORTION OF THE
WEEK. AN ENERGETIC STORM TRACK ACROSS THE EASTERN PACIFIC WILL
RESULT IN MULTIPLE FAST MOVING SHORTWAVES CROSSING THE NATION
THROUGH THE END OF THE WEEK AND INTO NEXT WEEKEND, AND THIS WILL
TEND TO KEEP A SEPARATE NORTHERN AND SOUTHERN STREAM FLOW. ONE OF
THESE SHORTWAVES CROSSING TEXAS WILL HAVE ENOUGH LIFT AND DYNAMICS
TO SUPPORT A HEAVY RAINFALL EVENT ACROSS PORTIONS OF THE CENTRAL
GULF COAST AND DEEP SOUTH ON TUESDAY, AND HEAVY RAIN AND MOUNTAIN
SNOW IS LIKELY TO CONTINUE ACROSS PORTIONS OF THE WEST COAST WITH
SOME ATMOSPHERIC RIVER EVENTS LIKELY. MOST OF THE NATION SHOULD
HAVE ABOVE NORMAL TEMPERATURES WITH NO ARCTIC AIR MASSES EXPECTED.
..GUIDANCE/PREDICTABILITY ASSESSMENT
GIVEN THE COMPLEXITY OF THE OVERALL WEATHER PATTERN, THE 00Z MODEL
GUIDANCE SUITE IS IN REASONABLY GOOD AGREEMENT FOR THE BEGINNING OF
THE FORECAST PERIOD TUESDAY. THE GFS BECOMES STRONGER WITH THE
SHORTWAVE EXITING THE EAST COAST CHRISTMAS MORNING, AND THE ECMWF
IS SLOWER WITH THE SOUTHERN STREAM SHORTWAVE ACROSS EASTERN TEXAS.
THERE HAS BEEN A TREND FOR A STRONGER TROUGH TO DEVELOP OVER THE
WESTERN HIGH PLAINS BY THURSDAY, WITH THE ALSO A LITTLE SLOWER WITH
THAT. THINGS BECOME MORE UNCERTAIN FOR THE WEST COAST WITH THE
TIMING OF FUTURE PACIFIC SHORTWAVES TOWARDS THE END OF THE WEEK,
WITH THE GFS BECOMING MORE PROGRESSIVE. THIS ALSO HOLDS TRUE GOING
INTO NEXT SATURDAY AS WELL. UPON EXAMINATION OF THE MACHINE
LEARNING GUIDANCE, THERE IS BETTER SUPPORT FOR THE CMC AND ECMWF
SOLUTIONS BY FRIDAY/SATURDAY, SO THE GFS WAS REMOVED FROM THE
FORECAST BLEND FOR DAY 7. THE ENSEMBLE MEANS WERE GRADUALLY
INCREASED TO ABOUT HALF BY THAT TIME.
..WEATHER/HAZARDS HIGHLIGHTS
IT STILL REMAINS THE CASE THAT MODERATE TO HEAVY RAINFALL IS
EXPECTED TO PERSIST ACROSS PORTIONS OF NORTHERN CALIFORNIA AND INTO
WESTERN OREGON EARLY ON TUESDAY, WITH THE HEAVIEST RAIN EXPECTED
FOR THE WESTERN SLOPES OF THE SIERRA NEVADA IN CENTRAL/NORTHERN
CALIFORNIA, WHERE A MARGINAL RISK OF EXCESSIVE RAINFALL IS VALID
FOR THE DAY 4 OUTLOOK, AND SUBSEQUENT ROUNDS OF RAIN AND MOUNTAIN
SNOW APPEAR LIKELY THROUGH CHRISTMAS WITH A DAY 5 MARGINAL RISK IN
EFFECT FOR WESTERN OREGON AND WASHINGTON, AND PROBABLY CONTINUING
INTO THE END OF THE WEEK. HEAVY SNOW IS EXPECTED FOR THE HIGHER
TERRAIN OF THE CASCADES AND THE SIERRA, THEN WORKING INLAND ACROSS
THE NORTHWEST AND NORTH-CENTRAL INTERMOUNTAIN WEST/ROCKIES.
THE SOUTHERN STREAM TROUGH IS EXPECTED TO DEVELOP FURTHER WITH
POTENTIAL CLOSED LOW DEVELOPMENT ON TUESDAY AS DEEPER MOISTURE
SURGES NORTHWARD FROM THE GULF OF MEXICO. GIVEN THE POTENTIAL FOR
HIGHER INSTABILITY AND RAINFALL RATES OF AN INCH PER HOUR OR
GREATER WITH THE CONVECTION THAT DEVELOPS, A MARGINAL RISK REMAINS
VALID FOR THE WPC DAY 4/TUESDAY EXCESSIVE RAINFALL OUTLOOK FROM
EASTERN TEXAS THROUGH ARKANSAS. THERE IS THE POTENTIAL THAT SOME OF
THIS OUTLOOK AREA COULD EVENTUALLY NEED A SLIGHT RISK AS THE EVENT
ENTERS THE SHORT RANGE PERIOD, BUT FOR NOW WILL KEEP IT AS A
MARGINAL RISK GIVEN SOME QPF PLACEMENT DIFFERENCES IN THE MODEL
GUIDANCE. SOME STRONG THUNDERSTORMS MAY ALSO DEVELOP IN THIS SAME
GENERAL AREA.
AFTER A COLD START TO THE WEEK, THE EASTERN U.S. SHOULD SEE A
MODERATION TREND IN TEMPERATURES GOING INTO THE CHRISTMAS HOLIDAY
AS HIGHS RETURN CLOSER TO CLIMATOLOGICAL AVERAGES. WIDESPREAD ABOVE
NORMAL TEMPERATURES WILL EXPAND FROM THE WEST INTO THE PLAINS AND
MIDWEST THROUGH THE MAJORITY OF THE PERIOD, WITH NO SIGNS OF ANY
ARCTIC AIR MASS INCURSIONS WITH FLOW ORIGINATING FROM THE PACIFIC.
DAYTIME HIGHS IN SOME LOCATIONS COULD BE 15 TO 25 DEGREES ABOVE
NORMAL.
HAMRICK
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
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page