426  
FXUS02 KWBC 130446  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1146 PM EST MON FEB 12 2018  
 
VALID 12Z FRI FEB 16 2018 - 12Z TUE FEB 20 2018  
   
..SIGNIFICANT TROUGH POSSIBLE FOR THE WEST NEXT WEEK
 
 
   
..OVERVIEW
 
 
MAIN DRIVERS OF THE MEDIUM RANGE FORECAST WILL BE UPPER RIDGING  
NEAR THE GULF OF ALASKA AND OVER THE SOUTHEAST US AND A PERSISTENT  
UPPER LOW OVER NORTHERN HUDSON BAY. THAT WILL FAVOR A RELATIVELY  
STABLE LONGWAVE PATTERN WITH SYSTEMS MOVING THROUGH THE FLOW --  
OUT OF THE NORTHEASTERN PACIFIC AND THROUGH THE WEST. BY NEXT  
WEEK, ENSEMBLES WERE STRONGLY SHOWING INCREASING HEIGHTS IN THE  
EAST AS THE UPPER HIGH EXPANDS NORTHWARD AND THE POTENTIAL FOR THE  
UPSTREAM TROUGH TO DIG SMARTLY INTO CALIFORNIA AND THE INTERIOR  
SOUTHWEST.  
 
   
..GUIDANCE PREFERENCES
 
 
LATEST MODELS/ENSEMBLES WERE GENERALLY CLUSTERED ENOUGH TO ALLOW  
FOR A PRIMARILY DETERMINISTIC BLEND TO START (FRI/SAT FORECAST)  
WITH A TREND TOWARD A MAJORITY ENSEMBLE MEAN BLEND BY NEXT TUESDAY  
GIVEN THE UNCERTAINTY IN THE STRENGTH OF THE WESTERN TROUGH AND  
EMBEDDED SHORTWAVES COMING OUT OF THE NORTHEASTERN PACIFIC.  
NOTEWORTHY IS THE VERY GOOD AGREEMENT ON THE EASTERN RIDGING THAT  
CONTINUES TO TREND STRONGER IN THE ENSEMBLES. TYPICAL VERIFICATION  
SHOWS BETTER RELIABILITY FOR STRONG RIDGES RATHER THAN DEEP  
TROUGHS IN THE ENSEMBLES AND THE UPPER PATTERN CONFIGURATION  
CERTAINLY SUPPORTS IT.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
TEMPERATURES...  
 
SURGE OF MILD TEMPERATURES AHEAD OF THE LEAD FRONTAL BOUNDARY  
PUSHING THROUGH THE EAST ON FRIDAY WILL BE REPLACED BY NEAR TO  
BELOW AVERAGE TEMPERATURES IN ITS WAKE FROM THE PLAINS EASTWARD.  
TEMPERATURES WILL AGAIN RISE SUN/MON IN THE EAST AHEAD OF THE  
WESTERN SYSTEM. TEMPERATURES MAY AGAIN RISE TO 10-20 DEGREES ABOVE  
AVERAGE. FOR THE WEST, TEMPERATURES WILL GENERALLY TREND DOWNWARDS  
FROM THE WEEKEND INTO NEXT MON/TUE AS THE TROUGH SINKS IN AND THE  
ARCTIC FRONT TO THE NORTH SLIPS SOUTHWARD AS WELL. TEMPERATURES  
WILL BE ANYWHERE FROM 5-30 DEGREES BELOW AVERAGE.  
 
PRECIPITATION...  
 
EXPECT A FOCUS OF PRECIPITATION ALONG THE FRONT FROM THE SOUTHERN  
PLAINS EAST-NORTHEASTWARD TO THE CENTRAL APPALACHIANS LATER THIS  
WEEK INTO THE WEEKEND. BY NEXT SUN-TUE, PRECIPITATION WILL EXPAND  
THROUGH THE WEST, FIRST THROUGH OR/WA (WITH SNOW FOR THE COASTAL  
RANGES AND CASCADES), BUT THEN ESPECIALLY THROUGH THE INTERIOR AND  
PERHAPS THROUGH MUCH OF CALIFORNIA. SNOW LEVELS WILL LOWER AS  
COLDER AIR WORKS IN (POSSIBLY 1500-2000 FT) ALL THE WAY TO SOME  
VALLEY LOCATIONS IN NORTHERN CALIFORNIA AND OREGON/NEVADA.  
PRECIPITATION COVERAGE WILL DEPEND ON THE DEPTH OF THE TROUGHING  
(12Z/12 ECMWF FARTHER WEST THAN MOST MODELS/ENSEMBLES) BUT FOCUS  
AS OF NOW WILL BE FROM OREGON THROUGH NEVADA INTO  
UTAH/ARIZONA/COLORADO. THIS WILL INCLUDE THE COASTAL RANGES OF  
WA/OR LATER THIS WEEKEND. FARTHER EAST, WITH EXPANDING RIDGING  
OVER THE SOUTHEAST, THE SLOWLY MOVING COLD FRONT MAY ACT AS A  
FOCUS FOR MODEST RAINFALL FROM THE LOWER MISSISSIPPI VALLEY INTO  
THE OHIO AND TENNESSEE VALLEYS MON/TUE WITH SOME WINTRY WEATHER  
FOR THE NORTHERN FRINGE.  
 
FRACASSO  
 

 
 
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