558  
FXUS02 KWBC 151558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1057 AM EST MON JAN 15 2018  
 
VALID 12Z THU JAN 18 2018 - 12Z MON JAN 22 2018  
   
..ACTIVE PATTERN FOR MUCH OF THE WEST
 
 
   
..OVERVIEW
 
 
UPPER PATTERN WILL SUPPORT WESTERN TROUGHING AND EASTERN RIDGING  
LATER THIS WEEK INTO EARLY NEXT WEEK. A COUPLE PACIFIC SYSTEMS  
WILL PUSH THROUGH THE WESTERN STATES AND MAY HELP DEVELOP A MORE  
ROBUST STORM IN THE CENTRAL PLAINS/GREAT LAKES NEXT SUNDAY/MONDAY.  
 
   
..GUIDANCE ASSESSMENT/PREFERENCES
 
 
NOTICEABLE DIFFERENCES WERE SEEN AT THE SHORT/MEDIUM RANGE  
HAND-OFF IN THE MID-ATLANTIC/NORTHEAST AND ADJACENT W ATLANTIC AS  
A SYSTEM DEPARTS WELL OFFSHORE. MID-LEVEL TROUGH (OR PERHAPS  
CLOSED LOW) SHOULD EXIT THE DELMARVA EARLY THURSDAY PER THE 00Z  
ECMWF/UKMET/CANADIAN (AND THEIR ENSEMBLES) BUT THE 00Z/06Z GFS  
RUNS (AND GEFS ENSEMBLES) WERE QUICKER/WEAKER WITH IT. GIVEN THE  
TENDENCY FOR THE GFS TO BE TOO QUICK OPTED TO PUT MORE WEIGHT ON  
THE SLOWER CAMP BUT STILL PREFERRED A QUICKER SOLUTION THAN THE  
ECMWF-LED CLUSTER VERBATIM. PLEASE SEE THE LATEST PMDHMD (SHORT  
RANGE DISCUSSION) FOR DETAILS ON THIS FEATURE (AND WITH THE LATEST  
12Z GUIDANCE) AS IT TRANSLATES INTO THAT TIME RANGE.  
 
IN THE WEST, FIRST PACIFIC FRONT WILL MOVE THROUGH THE INTERIOR  
NORTHWEST AND THROUGH THE HIGH PLAINS/INTERMOUNTAIN WEST FRIDAY.  
ENSEMBLES SHOW THEIR EXPECTED TIMING/AMPLITUDE DIFFERENCES WITH  
THE FRONT THROUGH THE FOUR CORNERS AND THE GFS (AGAIN) ENDED UP  
QUICKER THAN THE ECMWF/CANADIAN BY LATE FRIDAY/EARLY SATURDAY  
(THOUGH THE UKMET WAS AS QUICK AS THE GFS). ENSEMBLE TREND HAS  
BEEN A BIT SLOWER OVER THE PAST COUPLE OF DAYS BUT NEXT UPSTREAM  
PACIFIC SYSTEM JUST OFF COASTAL WASHINGTON BY THEN MEANS THAT THE  
WAVELENGTH/SYSTEM SPACING CAN ONLY BE SO SMALL. AGAIN LEANED ON  
THE SLOWER SET OF GUIDANCE BUT DID NOT COMPLETELY WRITE OFF THE  
QUICKER SOLUTIONS. THIS KEPT GOOD CONTINUITY AS WELL.  
 
BY NEXT SUN/MON, ENSEMBLES CONTINUE TO TREND TOWARD A MORE ROBUST  
SYSTEM, AS THE ROCKIES SYSTEM PROGRESSES EASTWARD. BOTH THE  
GEFS/ECMWF ENSEMBLE MEANS HAVE SHOWN A DEVELOPED LOW PRESSURE  
MOVING NORTHEASTWARD FROM SE COLORADO TO SE KS TO N MI SAT/SUN/MON  
FOR A FEW RUNS (GENERALLY TRENDING DEEPER) AND THE DETERMINISTIC  
SOLUTIONS HAVE BEEN BOUNDING AROUND THIS IDEA AS WELL. CONSENSUS  
APPROACH WAS PREFERRED BETWEEN THE RECENT GFS/ECMWF RUNS WHICH  
BOUNDED THE ENSEMBLE MEANS.  
 
   
..WEATHER HIGHLIGHTS
 
 
TEMPERATURES WILL MODERATE IN THE EAST/SOUTH AS HEIGHTS RISE  
THROUGH THE PERIOD. THE ABOVE AVERAGE TEMPERATURES (+10 TO +20  
ANOMALIES) LATER THIS WEEK IN MUCH OF THE WEST INTO THE HIGH  
PLAINS WILL SHIFT EASTWARD IN TIME. THIS TRANSLATES INTO HIGH  
TEMPERATURES ABOVE FREEZING FOR EVERYWHERE EAST OF THE FRONT NEXT  
SUN/MON (FROM MICHIGAN EASTWARD). THE WEST WILL SEE BELOW AVERAGE  
TEMPERATURES AFTER THE INITIAL FRONT PUSHES THROUGH LATER THIS  
WEEK (ANOMALIES -5 TO -15).  
 
PRECIPITATION WILL SPREAD THROUGH MOST OF THE WEST EXCEPT FOR  
PERHAPS SOUTHERN CALIFORNIA INTO THE DESERTS IN ARIZONA. SURFACE  
COLD FRONT WILL BRING WINDY CONDITIONS AND LOWERING SNOW LEVELS TO  
THE INTERMOUNTAIN WEST WITH SIGNIFICANT SNOW FOR THE CASCADES INTO  
THE SIERRAS (SEVERAL FEET) EASTWARD THROUGH THE  
SAWTOOTH/BITTERROOTS DOWN THROUGH THE UINTAS/WASATCH THEN INTO THE  
ABSAROKA RANGE INTO COLORADO AS THE SFC LOW REDEVELOPS TO THE LEE  
OF THE ROCKIES. EXPECT LESSER AMOUNTS OF RAIN/SNOW INTO NORTHERN  
ARIZONA/NEW MEXICO AS THE 700MB TROUGH OR BARELY CLOSED LOW TRACK  
SHOULD BE THROUGH S UT/CO.  
 
BY NEXT SUN/MON, SCENARIOS EXPAND FOR THE PLAINS INTO THE UPPER  
GREAT LAKES. POTENTIAL FOR A SIGNIFICANT WINTER STORM CERTAINLY  
EXISTS FOR THESES AREAS CONTINGENT UPON A DEVELOPED SYSTEM. THE  
TYPICAL WINTER HAZARDS WOULD ACCOMPANY THIS SCENARIO (SNOW AND  
BLOWING SNOW TO THE NORTH AND NORTHWEST OF THE LOW) BUT ALSO RAIN  
AND PERHAPS CONVECTION IN THE WARM SECTOR FROM THE LOWER  
MISSISSIPPI VALLEY NORTHWARD.  
 
LASTLY, SECOND PACIFIC SYSTEM IS FORECAST TO PUSH INLAND NEXT WEEK  
WITH PERHAPS MORE RAIN/SNOW THAN THE LEAD SYSTEM (ENSEMBLE SIGNAL  
IS JUST AS STRONG BUT THE LONGER LEAD TIME SUGGESTS HIGHER  
POTENTIAL). THIS WOULD MEAN ANOTHER ROUND OF MODEST RAIN AND HEAVY  
SNOW FOR THE COASTAL RANGES AND CASCADES INTO NORTHERN  
CALIFORNIA/SIERRAS.  
 
FRACASSO  
 

 
 
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