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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