274   
FXUS02 KWBC 171832  
PMDEPD  
  
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
132 PM EST TUE DEC 17 2024  
  
VALID 12Z FRI DEC 20 2024 - 12Z TUE DEC 24 2024  
  
...HEAVY PRECIPITATION/WINDY PATTERN FOCUS FOR THE PACIFIC   
NORTHWEST/NORTHERN CALIFORNIA THIS WEEKEND INTO THE HOLIDAYS...  
  
   
..OVERVIEW  
  
IT REMAINS THE CASE THAT THE UPPER PATTERN TO BECOME MORE   
AMPLIFIED DURING THE LATE WEEK THROUGH WEEKEND TIME FRAME AS MEAN   
TROUGHING BECOMES ESTABLISHED OVER THE EAST AND A STRONG RIDGE   
BUILDS OVER THE WEST. A SHORTWAVE AND ITS SURFACE SYSTEM SHOULD   
BRING SNOW POTENTIAL ACROSS THE GREAT LAKES AND A FRESH BLAST OF   
ARCTIC AIR TO THE EAST FRIDAY INTO THE WEEKEND. TRAILING ENERGY   
WILL LIKELY REINFORCE THE TROUGH AGAIN DURING THE WEEKEND. IN   
CONTRAST, THE WEST WILL BE QUITE WARM DUE TO THE MEAN RIDGE ALOFT.   
A SERIES OF SHORTWAVES WILL PUSH INTO/THROUGH THE MEAN RIDGE, WITH   
A COUPLE OF ATMOSPHERIC RIVERS LIKELY BRINGING A PERIOD OF HEAVY   
RAIN/MOUNTAIN SNOWS TO PARTS OF THE WEST COAST. CURRENT FORECASTS   
SHOW CONTINUED EASTWARD PROGRESS OF THE LEADING PACIFIC SHORTWAVE   
INTO THE PLAINS BY EARLY NEXT WEEK HELPING TO PUSH THE EASTERN U.S.  
UPPER TROUGH EASTWARD.   
   
..GUIDANCE/PREDICTABILITY ASSESSMENT  
  
MODEL FORECAST SPREAD HAS IMPROVED THROUGH MEDIUM RANGE TIME  
SCALES, BOLSTERING FORECAST CONFIDENCE. FORECAST CLUSTERING AND   
PREDICTABILITY SEEM MUCH BETTER THAN AVERAGE FOR BOTH THE LARGER   
SCALE FLOW EVOLUTION AND MANY EMBEDDED SYSTEMS DAYS 3-7. A   
PREFERRED COMPOSITE OF THE 06 UTC GFS AND 00 UTC ECMWF/UKMET/CANADIAN  
MODELS WAS USED FOR WPC PRODUCT CREATION FOR DAYS 3-7. THIS   
GENERALLY SEEMED CONSISTENT WITH CONTINUITY AND THE 13 UTC NATIONAL  
BLEND OF MODELS. THE BLEND TENDED TO MITIGATE LINGERING SMALLER   
SCALE MODEL VARIANCES AS CONSISTENT WITH INDIVIDUAL SYSTEM   
UNCERTAINTY. THE NEWER 12 UTC MODELS SUITE ALSO REMAINS IN LINE.  
   
..WEATHER/HAZARDS HIGHLIGHTS  
   
  
GENERALLY NO CHANGE THAT FOLLOWING A DEPARTING INITIAL SHORTWAVE   
OUT OF THE EAST, AND SUBSEQUENT SYSTEMS, EXPECT LAKE EFFECT SNOWS   
TO CONTINUE IN FAVORABLE DOWNWIND LOCATIONS OF LAKES MICHIGAN,   
ERIE, AND ONTARIO. WITH LIGHT SNOW POSSIBLE ACROSS PORTIONS OF THE   
INTERIOR NORTHEAST AS WELL. BY NEXT WEEKEND THE TRAILING ARCTIC   
AIRMASS SHOULD BRING DAYTIME HIGHS DOWN TO AS MUCH AS 15 TO 20   
DEGREES BELOW NORMAL FOR ESPECIALLY PARTS OF THE NORTHEAST AND   
CLOSE TO THAT IN THE MID- ATLANTIC.  
  
OVER THE WEST, UPPER DYNAMICS AND WINDY SURFACE SYSTEMS WILL START  
TO PUSH FARTHER EASTWARD BY NEXT WEEKEND, WITH THE FIRST OF A   
SERIES OF ATMOSPHERIC RIVERS ON SATURDAY AND INCREASING RAINFALL   
POTENTIAL FOR PORTIONS OF NORTHERN CALIFORNIA AND MOUNTAIN SNOW IN   
THE HIGHER TERRAIN OF THE CASCADES AND CALIFORNIA. THIS SYSTEM   
SHOULD BE FAIRLY PROGRESSIVE, BUT MODEST MOISTURE VALUES AND SOME   
HIGHER RAIN RATES WERE ENOUGH TO WARRANT A MARGINAL RISK ON THE DAY  
5 EXCESSIVE RAINFALL OUTLOOK ACROSS COASTAL NORTHERN CALIFORNIA.   
THIS ENHANCED ACTIVITY WILL LIKELY CONTINUE THROUGH NEXT MONDAY AS   
ANOTHER COUPLE OF ARS MOVE THROUGH, WITH THE LAST ONE POSSIBLY   
STRONGER THAN THE PREVIOUS TWO WITH HIGHER POTENTIAL FOR IMPACTFUL   
RAINFALL. THIS LEADS INTO A CLIMATE PREDICTION CENTER FORECAST WET   
PERIOD INTO WEEK 2 TO MONITOR. AS THE LEADING SHORTWAVE REACHES THE  
CENTRAL U.S. BY NEXT MONDAY, PRECIPITATION MAY DEVELOP OVER PARTS   
OF THE PLAINS/MIDWEST BUT WITH LOW CONFIDENCE IN SPECIFICS AT THIS   
TIME. MEAN RIDGING ALOFT WILL GENERALLY KEEP TEMPERATURES OVER THE   
WEST INTO HIGH PLAINS ABOVE AVERAGE THROUGH THE PERIOD WITH BROAD   
COVERAGE OF PLUS 10 TO 20 DEGREE ANOMALIES. A FEW DAILY RECORDS   
(MAINLY FOR DAYTIME HIGHS) ARE POSSIBLE FOR THE WEST LATE WEEK.  
  
SANTORELLI/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 (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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