286  
FXUS02 KWBC 141526  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1025 AM EST WED NOV 14 2018  
 
VALID 12Z SAT NOV 17 2018 - 12Z WED NOV 21 2018  
   
..OVERVIEW
 
 
UPPER-LEVEL RIDGING IS EXPECTED TO PERSIST ACROSS WESTERN CANADA  
THROUGH MUCH OF THE EXTENDED PERIOD, WITH THE POTENTIAL FOR  
PACIFIC SHORTWAVE ENERGY TO UNDERCUT THE RIDGE AND MOVE INTO THE  
WESTERN/SOUTHWESTERN U.S. FARTHER EAST, A MEAN TROUGH/UPPER LOW  
WILL PERSIST IN THE VICINITY OF HUDSON BAY, WITH PROGRESSIVE FLOW  
FROM NORTH CENTRAL CANADA INTO THE GREAT LAKES AND NORTHEASTERN  
U.S. THIS FLOW PATTERN WILL FAVOR CONTINUED COLD AIR INTRUSIONS  
INTO THE CENTRAL/EASTERN U.S. THROUGH MUCH OF THE EXTENDED PERIOD  
WITH THE POTENTIAL FOR FAST-MOVING, CLIPPER-LIKE SYSTEMS TO  
TRAVERSE THE U.S. NORTHERN TIER (WITH POTENTIAL CYCLOGENESIS  
ALONG/OFF THE NORTHEAST U.S./EASTERN CANADA COAST.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
FOR DAYS 3-5 (SAT-MON), MODELS SHOWED RELATIVELY GOOD CONSENSUS,  
ALLOWING USE OF A DETERMINISTIC MULTI-MODEL BLEND  
(ECMWF/GFS/UKMET). AN ELONGATED SHORTWAVE (AND ASSOCIATED SURFACE  
FRONTAL BOUNDARY) CROSSING THE ROCKIES/CENTRAL U.S., AND AN  
ADDITIONAL MORE VIGOROUS SHORTWAVE (ASSOCIATED WITH A SURFACE  
CLIPPER-LIKE SYSTEM) DIVING SOUTH ACROSS THE GREAT LAKES DURING  
THE TIME PERIOD ARE BOTH WELL REPRESENTED BY THIS MODEL BLEND. BY  
DAY 5, AS SOME DEGREE OF PHASING BEGINS TO OCCUR BETWEEN THESE TWO  
WAVES ACROSS THE APPALACHIANS, MODELS SHOW GENERAL AGREEMENT THAT  
A SURFACE LOW SHOULD BEGIN TO DEVELOP SOMEWHERE ALONG THE NORTHERN  
MID-ATLANTIC OR NORTHEAST COAST, AND THEN MOVE QUICKLY  
NORTHEASTWARD FROM THERE. BY DAYS 6-7 (TUE-WED) MODEL SPREAD  
BEGINS TO INCREASE ACROSS THE BOARD. TIMING AND INTENSITY  
DIFFERENCES EMERGE WITH RESPECT TO THE DEVELOPING LOW ALONG THE  
NORTHEAST COAST (WHICH SHOULD QUICKLY EXIT THE PICTURE AS IT MOVES  
INTO CANADA MON NIGHT-TUE). MORE SIGNIFICANT DIFFERENCES EMERGE  
ALONG THE WEST COAST, WITH THE ECMWF BREAKING DOWN THE RIDGING  
MUCH MORE QUICKLY THAN THE CONSENSUS OF MODELS/ENSEMBLES, AND  
MOVING A SIGNIFICANT UPPER-LEVEL TROUGH INTO THE GREAT BASIN BY  
WED MORNING (WHEN THE GFS STILL HAS THE TROUGH AXIS SEVERAL  
HUNDRED MILES OFF THE WEST COAST). THIS OBVIOUSLY HAS SIGNIFICANT  
IMPLICATIONS IN TERMS OF BOTH TEMPERATURES AND PRECIPITATION FOR  
THE WESTERN U.S. BY EARLY NEXT WEEK. ENSEMBLE MEANS (INCLUDING THE  
ECENS) HAVE BEEN FAIRLY PERSISTENT AND SUPPORT SOMETHING A GOOD  
BIT SLOWER THAN THE DETERMINISTIC ECMWF. GIVEN THESE  
CONSIDERATIONS, WEIGHTING OF ECENS/GEFS MEANS WAS INCREASED  
SUBSTANTIALLY DURING DAYS 6-7, WITH THE DETERMINISTIC ECMWF  
ELIMINATED FROM THE BLEND ENTIRELY BY DAY 7.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
EXPECT THE FORECAST PATTERN DURING THE SAT-WED PERIOD TO YIELD  
FAIRLY LOW COVERAGE OF MEANINGFUL PRECIPITATION ACROSS THE CONUS.  
SOME RAIN WILL LIKELY DEVELOP OVER PARTS OF SOUTHERN TEXAS BY LATE  
SUN IN ASSOCIATION WITH A WESTERN GULF COAST SURFACE TROUGH AND  
APPROACH OF THE COLD FRONT PUSHING SOUTHWARD THROUGH THE PLAINS.  
RAINFALL COVERAGE/INTENSITY MAY INCREASE OVER THE SOUTHERN PLAINS  
AND VICINITY TOWARD TUE-WED AS THE SOUTHERN STREAM UPPER SYSTEM  
APPROACHES. ALSO BY NEXT TUE-WED THE WEST COAST STATES SHOULD SEE  
SOME INCREASE IN MOISTURE BUT WITH CONSIDERABLE UNCERTAINTY  
REGARDING THE TIMING/COVERAGE/MAGNITUDE OF ASSOCIATED  
PRECIPITATION. FARTHER EASTWARD THE NORTH-CENTRAL ROCKIES/HIGH  
PLAINS WILL SEE SOME EARLY-PERIOD SNOW GENERATED BY SHORTWAVE  
ENERGY ALOFT AND LOW LEVEL UPSLOPE FLOW. SOME OF THE MOISTURE MAY  
EXTEND EASTWARD AS LIGHT RAIN/SNOW. THE GREAT LAKES REGION WILL  
SEE PERIODS OF LAKE EFFECT SNOW WITH THIS ACTIVITY TENDING TO  
FAVOR NORTHERN AND EASTERN AREAS. THE WAVY FRONT POTENTIALLY  
SETTLING OVER THE AREA NEXT WEEK MAY PROMOTE SOME SYNOPTIC-SCALE  
SNOW AS WELL.  
 
OVER THE COURSE OF THE PERIOD THE EMPHASIS FOR COLDEST ANOMALIES  
(SOME HIGHS AT LEAST 15-20F BELOW NORMAL) SHOULD SHIFT FROM THE  
CENTRAL U.S. DURING THE WEEKEND INTO THE NORTHEAST BY TUE-WED.  
BEHIND THE COLD AIR THERE WILL BE A MODERATING TREND THAT  
PROGRESSES FROM THE PLAINS INTO EAST-CENTRAL U.S. ANTICIPATE  
WARMEST HIGHS RELATIVE TO NORMAL OVER AND NEAR NORTHERN  
CALIFORNIA/SOUTHERN OREGON DURING THE FIRST HALF OF THE PERIOD  
WITH SOME PLUS 10-15F ANOMALIES.  
 
RYAN/RAUSCH  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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