430  
FXUS02 KWBC 050634  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
133 AM EST FRI JAN 05 2018  
 
VALID 12Z MON JAN 08 2018 - 12Z FRI JAN 12 2018  
   
..RAIN FINALLY RETURNS TO SOUTHERN CALIFORNIA
 
 
   
..OVERVIEW
 
 
A PACIFIC SYSTEM WILL PUSH THROUGH CALIFORNIA ON TUESDAY WITH RAIN  
AND HIGH MOUNTAIN SNOW FOR MUCH OF THE GOLDEN STATE (INCLUDING  
DOWN TO SAN DIEGO) AS ANOTHER IN THE EAST SLOWLY EXITS OFF THE  
SOUTHEAST/FLORIDA COASTS. ANOTHER ARCTIC AIRMASS WILL DIVE INTO  
THE HIGH PLAINS MIDWEEK AND MERGE WITH THE FORMER CALIFORNIA  
SYSTEM TO SPREAD MORE RAIN AND NORTHERN SNOW TO THE  
CENTRAL/EASTERN STATES THU/FRI.  
 
   
..GUIDANCE ASSESSMENT AND PREFERENCES
 
 
FOR THE WESTERN SYSTEM, THE 18Z/12Z GFS AND 12Z ECMWF CLUSTERED  
WELL WITH THEIR ENSEMBLE MEANS (AND 12Z CANADIAN ENSEMBLE MEAN)  
WHILE THE 12Z UKMET/CANADIAN LAGGED BEHIND. A BLEND OF THE  
NON-UKMET/CANADIAN MODELS SERVED AS A GOOD STARTING POINT AS THE  
COLD FRONT WILL PUSH INTO CALIFORNIA AND THE SOUTHWEST TUE/WED  
THEN INTO THE SOUTHERN PLAINS BY THURSDAY. NORTHERN PORTION OF THE  
SYSTEM REMAINS MORE UNCERTAIN AS THE MODELS DO NOT SHOW A  
CONSENSUS AS THAT SFC LOW HEADS INTO SW CANADA AND DRAGS ITS  
WEAKENING COLD FRONT THROUGH WA/OR EARLY TUESDAY. BEST ENSEMBLE  
CONSENSUS APPEARS TO BE NEAREST THE 12Z GFS.  
 
IN THE EAST, ENSEMBLES HAVE TRENDED TOWARD A SLOWER AND PERHAPS  
CLOSED OFF COMPACT UPPER LOW SLIDING THROUGH THE SOUTHEAST/FLORIDA  
TUE/WED, THOUGH NO MODEL HAS BEEN ALL THAT CONSISTENT. GIVEN THE  
TREND, OPTED TO FAVOR A SLOWER/DEEPER SOLUTION NEAR THE 12Z GFS  
(BUT NOT AS STRONG/SLOW AS THE 12Z ECMWF).  
 
FOR NEXT THU/FRI, ARCTIC AIRMASS SPILLING SOUTHEASTWARD EAST OF  
THE DIVIDE WILL LIKELY CATCH UP TO THE OLD PACIFIC FRONT MOVING  
THROUGH THE SOUTHERN PLAINS ON THURSDAY. 12Z/18Z GFS WERE LIKELY A  
BIT TOO QUICK (PERHAPS ITS TYPICAL FAST BIAS AFTER ~5 DAYS) WITH  
THE OLD PACIFIC UPPER LOW BUT MANAGES TO BECOME SLOWER AS IT  
INFUSED TRAILING VORTICITY INTO THE TROUGH. TRENDED TOWARD THE  
ECMWF ENSEMBLE MEAN AND GEFS MEAN BY NEXT FRIDAY AS THE DETAILS OF  
THE GFS/ECMWF BECAME UNLIKELY VERBATIM (ALSO IN SW BRITISH  
COLUMBIA AND OFF THE SOUTHEAST COAST).  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
AFTER MANY MONTHS WITHOUT APPRECIABLE RAIN, SOUTHERN CALIFORNIA  
AND THE SAN JOAQUIN VALLEY SHOULD SEE AT LEAST MODEST TO LOCALLY  
HEAVY AMOUNTS OF RAIN AND HIGH ELEVATION SNOW (RATHER WARM SYSTEM)  
ESPECIALLY TUESDAY. S/SW SLOPES WILL BE FAVORED AND SEVERAL FEET  
OF SNOW ARE EXPECTED IN THE SIERRAS. IN THE SOUTHEAST, A  
SLOWER/DEEPER CLOSED LOW AND SFC LOW WITH ATTENDANT COLD FRONT  
WOULD YIELD MODEST RAINS TO AL/GA/FL EARLY IN THE WEEK WITH  
LIGHTER AMOUNTS TO THE NORTH AND NORTHEAST INTO NEW ENGLAND  
(GENERALLY COLD ENOUGH FOR SNOW IN THE INTERIOR). MORE RAIN AND  
NORTHERN SNOW WILL BE LIKELY LATER IN THE WEEK ALONG/AHEAD OF THE  
PACIFIC AND ARCTIC COLD FRONTS IN THE CENTRAL US.  
 
TEMPERATURES WILL BE A BIT VARIABLE OVER THE CONUS, WITH A  
MODERATING TREND IN THE EAST BUT A COOLING TREND OVER MONTANA INTO  
THE PLAINS STARTING WEDNESDAY BEHIND THE ARCTIC FRONT.  
 
FRACASSO  
 

 
 
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