869
FXUS02 KWBC 111913
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
213 PM EST WED DEC 11 2024
VALID 12Z SAT DEC 14 2024 - 12Z WED DEC 18 2024
...ATMOSPHERIC RIVER COULD CAUSE HEAVY RAIN AND SNOW IN THE WEST
COAST/NORTHWEST BY THIS WEEKEND...
..OVERVIEW
MOST GUIDANCE CONTINUES TO SHOW A SEQUENCE OF SHORTWAVES
APPROXIMATELY 2-3 DAYS APART FROM THE PACIFIC THROUGH THE LOWER 48
FROM THE WEEKEND INTO THE FIRST HALF OF NEXT WEEK. THE CHARACTER OF
THE PATTERN MAY BEGIN TO CHANGE TOWARD THE MIDDLE OF NEXT WEEK AS
THE LAST SHORTWAVE OF THE SERIES REACHES THE WESTERN-CENTRAL U.S.,
WITH LARGER SCALE RIDGING POTENTIALLY BUILDING INTO THE WEST COAST.
EACH SHORTWAVE'S ASSOCIATED SURFACE SYSTEM SHOULD PRODUCE EPISODES
OF RAIN AND HIGHER ELEVATION SNOW OVER PORTIONS OF THE WEST AND
THEN DEVELOPMENT OF PRECIPITATION OVER THE SOUTHERN PLAINS AND
EXPANDING ACROSS THE EAST-CENTRAL U.S. THEN CONTINUING INTO
PORTIONS OF THE EAST. GUIDANCE FINALLY AGREES BETTER FOR THE
SYSTEM APPROACHING THE PACIFIC NORTHWEST BY EARLY SATURDAY BUT
THERE ARE OTHER ASPECTS OF THE FORECAST THAT STILL SHOW A FAIR
AMOUNT OF SPREAD. THE FORECAST PATTERN WILL LEAD TO MOSTLY NEAR OR
ABOVE NORMAL TEMPERATURES DURING THE PERIOD, WITH ONLY LOCALIZED
AREAS OF BELOW NORMAL READINGS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
AT THE START OF THE FORECAST PERIOD SATURDAY, TROUGHING WITH
EMBEDDED VORT MAXES OVER THE EASTERN PACIFIC AND A
SHORTWAVE/EMBEDDED UPPER LOW ATOP THE CENTRAL PLAINS ARE BOTH
REASONABLY AGREEABLE IN THE MODEL GUIDANCE. THE FORMER WILL SUPPORT
A DEEP SURFACE LOW IN THE EASTERN PACIFIC TOWARD VANCOUVER ISLAND,
GENERALLY WITHIN THE 970S-980S MB EARLY SATURDAY AND THE NEWER 12Z
MODELS ARE IN EVEN BETTER AGREEMENT WITH PLACEMENT. AS THAT
WESTERN TROUGH CONTINUES EASTWARD/INLAND, THE 00Z ECMWF BECOMES A
SLOW OUTLIER EVEN BY SUNDAY COMPARED TO THE PREPONDERANCE OF OTHER
GUIDANCE INCLUDING THE EC-BASED AI/ML MODELS. THE 12Z EC IS NOW
MORE CONSISTENT WITH THE CONSENSUS. TROUGHING IS FORECAST TO
CONTINUE INTO THE CENTRAL U.S. MONDAY AND LIFT AS IT PUSHES INTO
THE NORTHEAST. THERE IS SOME TIMING SPREAD WITH THIS (00Z GFS
FASTER, 00Z ECMWF SLOWER, AND THE 06Z GFS IN BETWEEN), AND
DIFFERENCES IN THE ORIENTATION OF THE TROUGH/POSSIBLE UPPER LOW AND
HOW THAT AFFECTS THE ASSOCIATED SURFACE LOW(S) IN THE GREAT LAKES
AND/OR FARTHER NORTH IN CANADA.
MEANWHILE, THE AFOREMENTIONED UPPER LOW OVER THE CENTRAL U.S. IS
MOSTLY AGREEABLE THOUGH THE 00Z ECMWF REMAINED FARTHER SOUTH THAN
OTHER GUIDANCE. THEN THERE IS ANOTHER ROUND OF TROUGHING PUSHING
ACROSS THE EASTERN PACIFIC EARLY NEXT WEEK AND INTO THE
WESTERN/CENTRAL U.S. TUESDAY-WEDNESDAY. THE 00Z CMC IS SLOW WITH
THIS EVEN IN THE PACIFIC, AND THE 00Z EC ENDS UP SEPARATING A
SOUTHERN STREAM LOW INTO THE SOUTHWEST UNLIKE GUIDANCE CONSENSUS.
THE GFS RUNS AND AI MODELS ARE MORE AGREEABLE WITH THE ENSEMBLE
MEANS IN A MORE PHASED TROUGH MAKING IT TO THE PLAINS BY WEDNESDAY.
DOWNSTREAM, THIS LEADS TO THE ECMWF SHOWING TROUGHING IN THE EAST
BY MIDWEEK WHEREAS RIDGING TO THE EAST OF THE TROUGH IS MORE
FAVORED.
THE WPC FORECAST FAVORED THE 00Z/06Z GFS IN TERMS OF DETERMINISTIC
RUNS GIVEN THE AFOREMENTIONED ISSUES WITH THE ECMWF AND CMC.
INCREASED THE PROPORTION OF THE GEFS AND EC ENSEMBLE MEANS AS THE
PERIOD PROGRESSED TO OVER HALF BY DAY 6-7 THOUGH AS MODEL SPREAD
INCREASED IN GENERAL.
..WEATHER/HAZARDS HIGHLIGHTS
THE STORM LIKELY NEARING THE PACIFIC NORTHWEST BY EARLY SATURDAY
WILL BRING THE POTENTIAL FOR AREAS OF SIGNIFICANT RAIN AND HIGHER
ELEVATION SNOW TO LOCATIONS ALONG THE CENTRAL AND NORTHERN WEST
COAST. BRISK TO STRONG WINDS MAY BE POSSIBLE AT LEAST OFFSHORE.
GUIDANCE CONTINUES TO INDICATE A FAIRLY PROGRESSIVE TRACK OF THE
ATMOSPHERIC RIVER, BUT RAIN RATES AND TOTALS COULD BE HIGH ENOUGH
TO SUPPORT LOCALIZED FLOODING. A MARGINAL RISK REMAINS IN PLACE FOR
DAY 4/SATURDAY. MINOR ADJUSTMENTS FROM CONTINUITY WERE TO MODIFY
THE RISK AREA OUT OF SOUTHWESTERN OREGON WHILE EXPANDING TO THE
SOUTH TOWARD THE BAY AREA DUE TO THE FASTER PROGRESSION OF THE AR.
SOME MOISTURE SHOULD CONTINUE ACROSS THE WEST OVER THE WEEKEND AND
HEAVY SNOW IS POSSIBLE IN THE HIGHER ELEVATIONS OF THE CASCADES,
SIERRA NEVADA, AND NORTHERN ROCKIES. ANOTHER ROUND OF PRECIPITATION
IS LIKELY TO COME INTO THE WEST EARLY NEXT WEEK.
A SHORTWAVE WITH AN EMBEDDED UPPER LOW SHOULD TRACK THROUGH THE
MIDWEST AND GREAT LAKES/OHIO VALLEY DURING THE WEEKEND. THIS
FEATURE AND ASSOCIATED SURFACE WAVE/FRONTAL SYSTEM SHOULD SPREAD
PRECIPITATION FROM THE MISSISSIPPI VALLEY EASTWARD, POSSIBLY
INCLUDING SOME AREAS BACK INTO EASTERN TEXAS AS WELL. MULTI-DAY
TRENDS HAVE BEEN FAVORING SOMEWHAT HIGHER RAINFALL TOTALS WITHIN AN
AXIS FROM EASTERN TEXAS INTO THE OHIO VALLEY ON SATURDAY, BUT
CONSENSUS OF GUIDANCE STILL INDICATES THAT SYSTEM PROGRESSION AND
RELATIVE LACK OF INSTABILITY SHOULD KEEP RAIN RATES BELOW EXCESSIVE
THRESHOLDS. MOST OF THIS REGION HAS NEUTRAL TO DRY GROUND
CONDITIONS AS WELL. MODEST TO PERHAPS LOCALLY HEAVY RAIN PROGRESSES
INTO THE MID-ATLANTIC SUNDAY. PARTS OF THE UPPER MIDWEST INTO
NORTHEAST COULD SEE SOME WINTRY PRECIPITATION AT TIMES AS THE
MOISTURE SHIELD PROGRESSES EASTWARD. PROBABILITIES FOR 0.25 INCH
LIQUID IN THE FORM OF SNOW REMAIN BELOW 30 PERCENT IN MOST AREAS,
EXCEPT WITH HIGHER PROBABILITIES IN THE ADIRONDACKS. THEN INTO THE
WORKWEEK, ANOTHER ROUND OF PRECIPITATION IS LIKELY TO DEVELOP IN
THE CENTRAL AND EASTERN U.S. AS ANOTHER LOW PRESSURE/FRONTAL SYSTEM
PULLS IN MOISTURE. THIS CURRENTLY LOOKS TO BE A WARMER SYSTEM
PRODUCING MOSTLY RAIN, THOUGH WINTRY WEATHER MAY BE POSSIBLE IN THE
FAR NORTHERN TIER. THERE MAY BE MORE INSTABILITY WITH THIS SYSTEM
AS WELL, FOR HIGHER RAINFALL RATES AND TOTALS COMPARED TO
SATURDAY'S SYSTEM. THOUGH THIS IS CURRENTLY OUT OF THE ERO
TIMEFRAME, FLOODING POTENTIAL WILL HAVE TO BE MONITORED IN FUTURE
FORECASTS.
MUCH OF THE LOWER 48 SHOULD SEE NEAR TO ABOVE NORMAL TEMPERATURES
DURING THE PERIOD, WITH THE WARMEST ANOMALIES FOR HIGHS (10-15F
ABOVE NORMAL) FORECAST TO BE OVER THE CENTRAL/EAST-CENTRAL U.S.
SUNDAY-MONDAY AND NEAR THE EAST COAST BY TUESDAY AHEAD OF A COUPLE
COLD FRONTS. SOME DAYS COULD FEATURE SOME POCKETS OF MORNING LOWS
UP TO 15-20F ABOVE NORMAL. LOCALIZED AREAS WITH SLIGHTLY BELOW
NORMAL HIGHS SHOULD INCLUDE SOME AREAS NEAR THE EAST COAST ON
SATURDAY, THE CENTRAL WEST COAST FROM THE WEEKEND INTO MONDAY, AND
THE FAR NORTHERN TIER TUESDAY-WEDNESDAY.
TATE/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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