501  
FXUS02 KWBC 200700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST MON NOV 20 2017  
 
VALID 12Z THU NOV 23 2017 - 12Z MON NOV 27 2017  
 
...RECORDS HIGHS ACROSS THE SW/WRN US AND PROLONGED HEAVY  
RAIN/SNOW OVER THE NWRN US...  
 
   
..OVERVIEW
 
 
IT REMAINS THE CASE THAT STRONG UPPER HIGHS NEAR SAN DIEGO AND THE  
DAVIS STRAIT BETWEEN FAR NERN CANADA AND GREENLAND CONTINUES TO  
FAVOR ERN US TROUGHING AND COOLER THAN AVERAGE TEMPERATURES.  
RECORD WARM TEMPERATURES ARE LIKELY FOR PARTS OF THE SOUTHWEST AND  
SRN CA INTO THE INTERMOUNTAIN WEST/ROCKY STATES...BUT WEST TO  
SOUTHWESTERLY FLOW INTO THE PACIFIC NORTHWEST WILL MAINTAIN A  
PROLONGED WET PATTERN. ADDITIONAL WET WEATHER IS LIKELY OVER  
FLORIDA ASTRIDE A SLOWLY MOVING AND WAVY FRONTAL BOUNDARY. THE  
MIDDLE PART OF THE COUNTRY IS EXPECTED TO BE RATHER DRY AND MILD  
TO START BUT TRENDING COOLER BY THE WEEKEND.  
 
   
..GUIDANCE/UNCERTAINTY PREFERENCES...
 
 
MODELS/ENSEMBLES ARE IN GOOD AGREEMENT ON THE LARGE SCALE PATTERN  
BUT DIFFER ON SOME TIMING/AMPLITUDE DETAILS. FORECAST SPREAD AND  
UNCERTAINTY IS SLOWLY DECREASING...BUT REMAINS HIGHER THAN  
AVERAGE. THE WPC PRODUCT SUITE WAS MAINLY DERIVED FROM THE 12 UTC  
ECMWF ENSEMBLE MEAN TO MAINTAIN MAX WPC CONTINUITY...BUT INCLUDED  
SMALLER INPUTS FROM THE LARGER SCALE ALOFT COMPATABLE BUT HIGHLY  
RUN TO RUN VARIABLE WITH LOWER ATMOSPHERIC PARAMETERS 18 UTC  
GFS/GEFS AND 12 UTC ECMWF. THIS ADDED A BIT MORE DETAIL CONSISTENT  
WITH BELOW NORMAL PREDICTABILITY.  
 
   
..WEATHER HIGHLIGHTS
 
 
HEAVY DAILY RAINS AND LOCALLY HEAVY ELEVATION SNOWS WILL PERSIST  
OVER THE NWRN US THIS WEEK. UPPER RIDGING WILL SUPPORT WELL ABOVE  
AVERAGE TEMPERATURES IN THE SOUTHWEST AND SRN CA TO THE  
INTERMOUNTAIN WEST/ROCKY STATES. SOME RECORD HIGHS/LOWS ARE  
LIKELY. MILD TEMPERATURES WILL EXTEND EASTWARD THROUGH THE PLAINS  
AHEAD OF A COLD FRONT THU/FRI WITH COOLER TEMPERATURE TO FOLLOW  
HEADING INTO THE WEEKEND. THERE IS A GOOD CHANCE FOR APPRECIABLE  
PRECIPITATION ALONG THE GULF/SERN US COAST...ESPECIALLY FL...AS  
LOW PRESSURE CENTERS MEANDER ALONG A SLOW MOVING FRONT.  
MEANWHILE...EXPECT SOME ORGANIZED RAIN/SNOW AROUND THE GREAT LAKES  
WITH POTENT NRN STREAM CLIPPER PASSAGES AND POST-SYSTEM LAKE  
EFFECT.  
 
   
..THANKSGIVING FORECAST
 
 
PRECIPITATION WILL CONCENTRATE FROM THE PACIFIC NORTHWEST INTO NRN  
CA AS WELL AS NRN ID/WRN MT/NW WY...INCLUDING HIGHER ELEVATION  
SNOWS. MODEST PRECIPITATION IS ALSO POSSIBLE ACROSS THE GREAT  
LAKES WITH LEAD CLIPPER PASSAGE. ORGANIZED RAIN IS POSSIBLE OVER  
CENTRAL AND SRN FL AND THE COASTAL SOUTHEAST DEPENDING ON HOW THE  
SURFACE SYSTEMS EVOLVE. HIGHEST RECORD HIGH TEMPS ARE LIKELY IN  
THE SOUTHWEST WITH 80S AND SOME 90S. PHOENIX HAS THE CHANCE OF  
RECORDING THEIR LATEST 90 DEGREE DAY SO LATE IN THE YEAR (RECORD  
IS NOV 15 1999) INCLUDING THEIR WARMEST THANKSGIVING DAY (RECORD  
IS 87 ON NOV 23 1950 AND NOV 27 2014 - THANKS TO THE PHOENIX NWS  
OFFICE FOR THE CLIMATE/RECORD INFORMATION). COOLER 30S/40S IN THE  
UPPER MIDWEST/GREAT LAKES/NORTHEAST/MID-ATLANTIC ARE LIKELY BY  
NEXT WEEKEND BEHIND THE FRONT.  
 
SCHICHTEL/FRACASSO  
 

 
 
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