337  
FXUS02 KWBC 131556  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1056 AM EST TUE FEB 13 2018  
 
VALID 12Z FRI FEB 16 2018 - 12Z TUE FEB 20 2018  
 
...A SIGNIFICANT WRN US WINTER TROUGH AND EAST-CENTRAL  
US/MID-SOUTH HEAVY RAIN THREAT...  
 
   
..OVERVIEW
 
 
IT REMAINS THE CASE THAT THE MAIN DRIVERS OF THE MEDIUM RANGE  
FORECAST WILL BE UPPER RIDGING NEAR THE EASTERN PACIFIC/GULF OF  
ALASKA AND ALSO OVER THE SOUTHEASTERN 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  
AND THE 00 UTC ECMWF IN PARTICULAR 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 INTERIOR WEST.  
 
   
..GUIDANCE PREFERENCES
 
 
LATEST MODELS/ENSEMBLES REMAIN CLUSTERED WELL INTO SATURDAY, BUT  
MODEL SPREAD STEADILY INCREASES INTO EARLY NEXT WEEK. EVEN  
SO...PREDICTABILITY SEEMS AT LEAST AVERAGE DURING THIS PERIOD  
GIVEN COMMON FORECAST EVOLUTION THEMES. THIS MOST UNCERTAINTY  
SEEMS TO BE ASSOCIATED WITH THE STRENGTH OF THE WESTERN TROUGH AS  
EMBEDDED SHORTWAVES DIG FROM THE NORTHEASTERN PACIFIC AND THE  
EXTENT OF HEIGHT RISES DOWNSTREAM OVER THE EAST.  
 
THE WPC PRODUCT SUITE DAYS 4-7 ARE MAINLY DERIVED FROM A BLEND  
FEATURING THE 00 UTC ECMWF ENSEMBLE MEAN ALONG WITH INPUTS FROM  
THE 00 UTC NAEFS MEAN AND 00 UTC ECMWF. THIS BLEND PRODUCES A  
RESLUTANT FLOW OVER THE CONUS AND VICINITY THAT IS SLIGHTLY MORE  
AMPLIFIED THAN A PURE GUIDANCE COMPOSITE BLEND AS THE LATEST 06  
UTC GFS/GEFS TRENDED LESS AMPLIFIED.  
TELECONNECTIONS ON THE ERN PACIFIC/GULF OF AK AND SERN US RIDGE  
SUPPORT AMPLIFED FLOW IN BETWEEN THAT WOULD OFFER REPEATED THREATS  
OF WINTERY PCPN SWATHS DIGGING INTO THE WEST IN A PERIOD WITH  
COOLING VERTICAL TEMPERATURE PROFILES/LOWERING SNOW LEVELS AND  
ALSO DEEP DOWNSTREAM GULF MOISTURE RETURN/HEAVY RAIN POTENTIAL  
OVER THE EAST-CENTRAL US/MID-SOUTH.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
TEMPERATURES...  
 
A 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...  
 
STILL 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  
(00 UTC ECMWF MORE AMPLIFIED OVER THE WEST THAN MOST GUIDANCE) BUT  
FOCUS WILL BE AT LEAST AS FAR WEST AS 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 MODERATE TO POTENTIALLY LOCALLY HEAVY RAINFALL FROM THE  
LOWER MISSISSIPPI VALLEY INTO THE OHIO/TENNESSEE VALLEYS MON/TUE  
WITH WINTRY WEATHER ON THE NORTHERN PERIPHERY OF THE PRECIPITATION  
SHIELD.  
 
SCHICHTEL  
 

 
 
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