718  
FXUS02 KWBC 101524  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1024 AM EST SAT FEB 10 2018  
 
VALID 12Z TUE FEB 13 2018 - 12Z SAT FEB 17 2018  
   
..WIDESPREAD PRECIPITATION EXPECTED IN THE SOUTHWEST NEXT WEEK
 
 
15Z UPDATE...  
 
MODELS SHOW GOOD LARGE SCALE AGREEMENT THROUGH THE MEDIUM RANGE  
WITH UPPER LOW PERSISTING ACROSS NORTHERN HUDSON BAY AND BROAD  
CYCLONIC FLOW ACROSS MUCH OF THE CONUS NORTHERN TIER. THE  
SUBTROPICAL RIDGE WILL ATTEMPT TO EXERT INFLUENCE THROUGH THE  
PERIOD, WITH POSITIVE HEIGHT ANOMALIES REMAINING IN PLACE FROM THE  
GULF OF MEXICO TO THE WESTERN ATLANTIC OCEAN. PRIMARY FORECAST  
CONCERNS ARE DIFFERENCES AMONG THE GUIDANCE WITH RESPECT TO HOW  
QUICKLY AN UPPER LOW OFF THE SW U.S. COAST MOVES EAST AND BEGINS  
TO BECOME ABSORBED INTO THE POLAR STREAM, AS WELL AS TIMING OF AN  
UPPER SHORTWAVE/SURFACE FRONT CROSSING THE CENTRAL/EASTERN U.S.  
WED-SAT (BEFORE THE FRONT POTENTIALLY STALLS ACROSS THE SOUTHERN  
PLAINS/LOWER MS VALLEY FRI-SAT. THE 00Z ECMWF APPEARS TO HOLD ONTO  
THE CLOSED UPPER LOW TOO LONG ACROSS THE EASTERN PACIFIC/U.S.  
SOUTHWEST, AND HAS ALSO SHOWN QUITE A BIT OF RUN-TO-RUN  
VARIABILITY. THE 06Z GFS WAS CLOSER TO CONSENSUS AS SHOWN BY THE  
ECENS/GEFS/NAEFS MEANS. FOR THE FRONTAL TIMING ACROSS THE NORTH  
CENTRAL U.S., A BLENDED APPROACH WAS PREFERRED, WITH NO SOLUTIONS  
CLEARLY AN OUTLIER. THUS, THE WPC FORECAST WAS INITIALLY BASED  
HEAVILY ON A MULTI-MODEL DETERMINISTIC BLEND, INCLUDING THE 00Z  
ECMWF/CMC/06Z GFS, WITH A TREND TOWARD HEAVIER GFS WEIGHTING BY  
DAYS 4-5 WHEN DIFFERENCES EMERGE SURROUNDING THE SOUTHWESTERN  
UPPER LOW. AFTER DAY 5, ENSEMBLE MEANS (00Z ECENS/NAEFS) COMPRISED  
A MAJORITY OF THE BLEND, WITH THE 06Z GFS ALSO INCLUDED. THIS  
RESULTED IN A FORECAST NOT CHANGED SIGNIFICANTLY FROM CONTINUITY.  
SEE BELOW FOR THE PREVIOUS DISCUSSION FROM OVERNIGHT.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0652 UTC)...  
   
..OVERVIEW
 
 
A CLOSED LOW WILL WOBBLE OFF THE COAST OF SOUTHERN CALIFORNIA NEXT  
WEEK BEFORE PUSHING INTO THE DESERT SOUTHWEST LATE IN THE WEEK  
BRINGING WIDESPREAD RAIN AND HIGH ELEVATION SNOW TO THE REGION.  
BROAD CYCLONIC FLOW OVER MOST OF THE REST OF THE LOWER 48 WILL  
BRING IN COLDER AIR TO MONTANA IN TO THE HIGH PLAINS WHILE MOST OF  
THE COUNTRY SEES ABOVE AVERAGE TEMPERATURES.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
LATEST GFS/ECMWF SOLUTIONS WERE IN GENERAL AGREEMENT WITH THEIR  
ENSEMBLE MEANS THROUGH THE PERIOD WITH THE OVERALL FLOW PATTERN.  
TIMING DIFFERENCES INCREASED BY LATER IN THE WEEK WITH THE  
WEAKENING UPPER LOW AND ALSO THE SOUTHWARD SURGE OF COOLER AIR  
FRI/SAT THROUGH THE PLAINS BUT A BLENDED SOLUTION SUFFICED GIVEN  
THE UPSTREAM DIFFERENCES OVER THE PACIFIC.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
PRECIPITATION WILL EXPAND THROUGH SOUTHERN CALIFORNIA INTO ARIZONA  
AND EVENTUALLY NEW MEXICO AFTER THE UPPER LOW NUDGES EASTWARD  
MIDWEEK. SOUTHWESTERLY FLOW WILL HELP RAISE PW VALUES TO +2 TO +4  
STANDARD DEVIATIONS WHICH SUGGESTS SOME LOCALLY HEAVY RAINFALL,  
DEPENDING ON OTHER FACTORS OF COURSE. TEMPERATURES WILL GENERALLY  
BE ABOVE AVERAGE OVERNIGHT BUT BELOW AVERAGE DURING THE DAY GIVEN  
CLOUD COVER AND PRECIPITATION.  
 
TO THE NORTH, GENERALLY BELOW AVERAGE TEMPERATURES ARE EXPECTED  
FROM MONTANA AND THE HIGH PLAINS INTO THE UPPER MIDWEST AS A  
COUPLE WEAKER SHORTWAVES GET SWEPT ALONG THE ZONAL TO  
WEST-SOUTHWEST UPPER-LEVEL FLOW FORECAST TO PERSIST OVER THE  
CENTRAL/EASTERN US. ABOVE AVERAGE TEMPERATURES ARE EXPECTED ACROSS  
MUCH OF THE EASTERN THIRD OF THE CONUS AS UPPER RIDGING MOVES INTO  
FLORIDA. 70S MAY REACH AS FAR NORTH AS CENTRAL VIRGINIA ON  
THURSDAY. COLD FRONT WILL KNOCK DOWN TEMPERATURES LATER IN THE  
WEEK, ESPECIALLY UP NORTH ALONG THE US/CANADIAN BORDER.  
 
FRACASSO  
 

 
 
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