EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 939 AM EDT THU MAY 18 2017 VALID 12Z SUN MAY 21 2017 - 12Z THU MAY 25 2017 GENERAL FLOW PATTERN AND MODEL PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ MEAN RIDGING ALONG THE IMMEDIATE WEST COAST OF NORTH AMERICA WILL FAVOR BROAD CYCLONIC FLOW EXTENDING FROM THE ROCKIES EASTWARD TO THE WESTERN ATLANTIC. THE MOST UNCERTAIN PLAYER IN THE MEDIUM RANGE PERIOD WILL BE THE STRENGTH OF THE AFOREMENTIONED RIDGE. MODELS STRUGGLE WITH HOW TO HANDLE SHORTWAVE ENERGY TRAVERSING THE TOP OF THIS RIDGE, WITH THE 00Z ECMWF THE STRONGEST WITH SUCH ENERGY WHILE GFS/CANADIAN RUNS SHOW A STRONGER RIDGE. NORMALLY, THE GUIDANCE IS TOO QUICK TO BREAK DOWN RIDGES OF THIS AMPLITUDE, SO BELIEVE THE ECMWF IS AN OUTLIER HERE. A COMPROMISE OF THE 00Z ECMWF MEAN AND 00Z UKMET APPEARS BEST WITH THE EXPECTED RIDGING. ONE WOULD THINK THIS ISSUE WOULD NEGATIVELY IMPACT THE SYSTEM DOWNSTREAM ACROSS THE EAST, BUT THIS IS NOT THE CASE. MORE RIDGING OUT WEST WOULD SUPPORT A STRONGER TROUGH WITH A MORE SOUTHERN CENTER, WHICH FOR WHATEVER REASON IS ADVERTISED BY THE 00Z ECMWF. BELIEVE THE ECMWF MIGHT BE ON TO SOMETHING HERE, BUT SINCE ITS STANDARDIZED ANOMALIES AT 500 HPA JUST ECLIPSE FOUR SIGMAS FROM THE MEAN, IT MIGHT BE TOO STRONG/TOO FAR SOUTH. TO ACCOUNT FOR THE ABOVE, A BLEND OF THE 00Z ECMWF, 00Z UKMET, 00Z CANADIAN, AND 06Z GFS WAS USED FOR PRESSURES, FRONTS, AND WIND GRIDS SUNDAY INTO TUESDAY. THEREAFTER, ONCE THE UKMET RUN ENDS, USED INCREASING PERCENTAGES OF THE 00Z ECMWF/00Z NAEFS ENSEMBLE MEAN SOLUTIONS AND DIALED DOWN USE OF THE OPERATIONAL MODELS, PARTICULARLY THE 00Z ECMWF WHICH LOOKED WAY OFF NEAR THE WESTERN US/CANADIAN BORDER. THE TEMPERATURES, PRECIPITATION CHANCES, DEW POINTS, CLOUDS, AND WEATHER GRIDS WERE BASED ON AN EVEN BLEND OF THE DETERMINISTIC AND ENSEMBLE MEAN GUIDANCE, AS USUAL. THE PLAN FOR THE DAYS 4-7 QPF IS FOR A ROUGH COMPROMISE OF THE 00Z ECMWF/06Z GFS FOR DAYS 4-5. DAYS 6-7 WILL LIKELY BE A GFS/ECMWF COMPROMISE IN THE GREAT LAKES/NORTHEAST AND MORE TOWARDS THE 06Z GFS IN THE WEST, PLAINS, AND MIDWEST. WEATHER HIGHLIGHTS ~~~~~~~~~~~~~~~~~~ GIVEN THE PERSISTENCE OF UPPER TROUGHING ACROSS THE CENTRAL TO EASTERN U.S, TEMPERATURES IN THIS REGION SHOULD BE RESPLENDENT LYING 5-10F BELOW AVERAGE THROUGHOUT THE PERIOD. UPPER RIDGING ALONG THE WEST COAST WILL FEATURE SOME OF THE HOTTEST TEMPERATURES, WITH HIGHS 10-20F ABOVE CLIMATOLOGY APPROACHING 90F IN THE PACIFIC NORTHWEST WITH 100F+ READINGS ACROSS THE CA INTERIOR VALLEYS AND DESERT SOUTHWEST. SOME DAILY HIGH TEMPERATURE RECORDS COULD BE APPROACHED/BROKEN FROM NORTHERN CA INTO WA/OR EARLY NEXT WEEK. ON TUESDAY, THE FORECAST HIGH OF 90 DEGREES IN PORTLAND MAY MATCH THE RECORD SET IN 1940. A WET PERIOD IS IN STORE ALONG THE ADVANCING COLD FRONT WITH PRECIPITATION SPREADING ACROSS MOST REGIONS EAST OF THE MS RIVER. THE PARENT BOUNDARY SHOULD BE SOMEWHAT PROGRESSIVE BUT AMPLE MOISTURE AND INSTABILITY SHOULD BE AVAILABLE FOR CONVECTION FROM THE MID-SOUTH TOWARD THE GULF COAST. FARTHER NORTH, FORCING WILL BE MUCH STRONGER NEAR THE UPPER TROUGH AND RESULTANT DEEP-LAYERED CYCLONE. ANOTHER ROUND OF RAINFALL IS LIKELY WITH THE TRAILING SHORTWAVE CROSSING THE CENTRAL U.S. ON TUESDAY. MODELS SHOW DECENT MOISTURE RETURN AHEAD OF THIS WAVE BUT MUCH OF THE ACTIVITY MAY BE MORE ELEVATED IN NATURE AS THE LEAD BOUNDARY REMAINS CLOSER TO THE GULF COAST WHICH SHOULD BE THE MEAN ATTRACTION QPF-WISE. ROTH/RUBIN-OSTER