145
FXUS02 KWBC 150708
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
207 AM EST FRI DEC 15 2023
VALID 12Z MON DEC 18 2023 - 12Z FRI DEC 22 2023
...COASTAL STORM TO LIFT HEAVY RAIN/WIND AND MARITIME THREATS FROM
THE MID-ATLANTIC TO NEW ENGLAND EARLY NEXT WEEK...
...EMERGING HEAVY PRECIPITATION THREAT TO FOCUS INTO CALIFORNIA
AND THE SOUTHWEST NEXT WEEK...
..OVERVIEW
A MAIN AND DEEPENED COASTAL STORM IS FORECAST TO CONTINUE A TRACK
UP ALONG THE EAST COAST FROM THE COASTAL MID-ATLANTIC TO OFF NEW
ENGLAND EARLY NEXT WEEK WITH WIDESPREAD HEAVY WRAPPING
RAINFALL/FAR INTERIOR SNOWS ALONG WITH HIGH WIND AND MARITIME
THREATS. SOME UNCERTAINTY REMAINS IN THE AMOUNTS AND PLACEMENT OF
THE HEAVY RAIN DUE TO MODEL DIFFERENCES IN THE UPPER LEVELS AND
COMPLEX SURFACE LOW TRACK. AFTER THIS SYSTEM PUSHES OFFSHORE, A
DRIER PATTERN WILL DEVELOP OVER THE EASTERN TWO-THIRDS OF THE
NATION AS AN UPPER LOW MOVES SOUTH ALONG THE WEST COAST TOWARD THE
SOUTHWEST, WITH PERIODS OF INCREASINGLY ORGANIZED PRECIPITATION
WITH TRACK AND SLOW INLAND APPROACH.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE MODELS GENERALLY AGREE THAT THERE WILL BE A LEADING SOUTHERN
STREAM LOW SWINGING UP THE EAST COAST AND A SECOND NORTHERN STREAM
LOW DIVING SOUTH FROM THE UPPER MIDWEST TOWARDS THE MID-ATLANTIC
THAT WILL INTERACT ALONG THE EAST COAST EARLY-MID NEXT WEEK.
HOWEVER, THERE ARE STILL SOME MODEL DIFFERENCES WITH HOW THEY
HANDLE THE INTERACTIONS BETWEEN UPPER LEVEL LOWS. 12/18Z GUIDANCE
FORECAST SPREAD NOW SEEMS TO PORTEND NEAR AVERAGE CONFIDENCE IN
THE SENSIBLE WEATHER FORECAST DETAILS LIKE QPF GIVEN LINGERING
UNCERTAINTIES, BUT THE OVERALL PATTERN AND THREAT POTENTIAL
REMAINS MORE UNIVERSALLY NOTED, SO A COMPOSITE GUIDANCE SOLUTION
WAS USED TO MITIGATE SMALLER VARIANCES.
AGREEMENT AMONG MODELS REMAINS MUCH BETTER IN THE WEST WITH
RIDGING OVER THE INTERIOR WEST EARLY NEXT WEEK AND AN UPPER LEVEL
TROUGH/LOW MOVING SOUTH ALONG THE WEST COAST MID-NEXT WEEK PRIOR
TO WORKING ALONG WITH ORGANIZED PRECIPITATION GRADUALLY TO THE
SOUTHWEST LATER NEXT WEEK. THERE ARE EXPECTED DIFFERENCES IN THE
POSITION AND TIMING OF THE SYSTEM, AND A GENERAL MIDDLE OF THE
ROAD SOLUTION WAS USED FOR THIS FORECAST.
THE WPC FORECAST WAS BASED ON THE 12/18Z GUIDANCE THAT WAS
AVAILABLE AT FORECAST PRODUCTION TIME. A BLEND OF THE 12Z
ECMWF/UKMET/CMC AND 18Z GFS DETERMINISTIC GUIDANCE ALONG WITH THE
GEFS AND ECMWF ENSEMBLE MEANS WAS USED ALONG WITH THE 01Z NBM AND
WPC CONTINUITY. A COMPOSITE OF THE LATEST 00Z GUIDANCE OVERALL
MAINTAINS GOOD PRODUCT CONTINUITY.
..WEATHER/HAZARDS HIGHLIGHTS
A DEEPENED COASTAL LOW AND DEEP TROUGHING ALOFT WILL SUPPORT HEAVY
RAINFALL THAT COULD BE WIDESPREAD INTO EARLY NEXT WEEK. THERE ARE
SOME LINGERING DIFFERENCES IN THE PLACEMENT AND AMOUNTS OF
RAINFALL, BUT FORECAST SPREAD IS DECREASING. BY MONDAY, THE LOW IS
FORECAST TO LIFT QUICKLY NORTHWARD AND RAIN (AND SOME PLOWABLE
SNOW ON THE BACKSIDE OF THE SYSTEM) WILL SPREAD/FOCUS INTO THE
NORTHEAST. A DAY 4/MONDAY MARGINAL RISK OF EXCESSIVE RAINFALL IS
IN PLACE FOR THE POTENTIALLY HEAVY RAINFALL AND PLAN AN UPGRADE TO
A SLIGHT RISK FOR NORTHERN NEW ENGLAND GIVEN THE WET ANTECEDENT
CONDITIONS AND GIVEN MODEL AND ENSEMBLE PROBABILITIES FOR ENHANCED
WRAP-BACK AXIS AMOUNTS. PRECIPITATION SHOULD PULL AWAY ON TUESDAY,
BUT LINGERING COASTAL SHOWERS COULD CONTINUE. THE DEEPENED SYSTEM
SHOULD ALSO FAVOR ENHANCED WINDS.
PRECIPITATION CHANCES WILL BE INCREASING NEXT WEEK FOR CALIFORNIA
AS SHORTWAVE ENERGY SUPPORTING A POSSIBLE ATMOSPHERIC RIVER
APPROACHES. RAIN AMOUNTS AND RATES INTO MONDAY COULD BE SUFFICIENT
TO CAUSE SOME FLOODING CONCERNS, ESPECIALLY GIVEN SOME INSTABILITY
COULD REACH COASTAL AREAS. MARGINAL RISKS OF EXCESSIVE RAINFALL
ARE IN PLACE FOR MONDAY AND TUESDAY WHERE FLOODING MAY BE A
CONCERN. MODERATE PRECIPITATION IS FORECAST TO ALSO PICK UP IN THE
PACIFIC NORTHWEST MONDAY INTO TUESDAY AS A AN UPPER LOW MOVES DOWN
THE COAST. SOME HIGHER ELEVATION SNOW WILL ALSO BE POSSIBLE. THE
SLOW APPROACH OF THE CLOSED SYSTEM TOWARD SOUTHERN CALIFORNIA
MID-LATER NEXT WEEK WOULD ACT TO SPREAD PRECIPITATION GRADUALLY
INLAND, WITH ORGANIZED LEAD ACTIVITY WORKING INTO THE SOUTHWEST
INTO LATER NEXT WEEK. THERE MAY BE AN EMERGING WET PATTERN INTO
THE SOUTH-CENTRAL U.S. WITH SOME DOWNSTREAM ENERGY FLOW AND RETURN
GULF INFLOW.
SCHICHTEL
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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