EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 251 AM EDT WED MAY 24 2017 VALID 12Z SAT MAY 27 2017 - 12Z WED MAY 31 2017 PATTERN OVERVIEW AND MODEL PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ THE MEDIUM RANGE PERIOD THROUGH THE MIDDLE OF NEXT WEEK WILL GENERALLY BE FEATURED WITH A LARGE SCALE UPPER LEVEL TROUGH OVER THE CENTRAL U.S. AND A PERSISTENT UPPER LEVEL RIDGE ALONG THE WEST COAST. WEAK RIDGING ALOFT ACROSS THE EASTERN U.S. SHOULD GRADUALLY GIVE WAY TO LOWERING HEIGHTS AS THE CENTRAL U.S. TROUGH MOVES EASTWARD, WITH THE PARENT UPPER LOW TRACKING JUST NORTH OF THE GREAT LAKES. THIS WILL LEAD TO A NORTHWEST FLOW PATTERN ALOFT ACROSS THE EASTERN HALF OF THE NATION THAT WILL ALLOW FOR TWO SEPARATE FRONTAL BOUNDARIES TO PASS THROUGH AND MAINTAIN THE NEGATIVE TEMPERATURE ANOMALIES OVER THE NORTH-CENTRAL U.S. IN TERMS OF MODEL PERFORMANCE, THE 12Z UKMET HAS TRENDED CLOSER TO THE MODEL CONSENSUS REGARDING THE ORIENTATION OF THE UPPER LOW OVER SOUTHERN CANADA FOR THIS WEEKEND, ALTHOUGH IT IS STILL A LITTLE SLOWER THAN THE OTHER MODELS. THE 12Z CMC BECOMES QUICKER WITH THE LEAD SHORTWAVE TROUGH PASSING THROUGH THE MIDWEST STATES, AND THUS THE FASTER PROGRESSION OF ITS COLD FRONT DEPICTION ACROSS THE DEEP SOUTH EARLY NEXT WEEK. THE 18Z GFS INDICATES A SECONDARY REINFORCING SHORTWAVE TROUGH WITHIN THE BROADER CYCLONIC FLOW THAT IS MORE AMPLIFIED THAN THE OTHER GUIDANCE FOR TUESDAY AND WEDNESDAY, WITH THE OPERATIONAL ECMWF AND CMC SHOWING LESS OF THIS SHORTWAVE SIGNAL. THERE IS A SLIGHT HINT IN THE MODEL GUIDANCE FOR A WEAK CUT-OFF LOW DEVELOPING NEAR SOUTHERN CALIFORNIA BY THE MIDDLE OF NEXT WEEK, BUT GIVEN POOR MODEL CONTINUITY FROM YESTERDAY'S FORECAST AND HARDLY ANY SIGNAL IN THE ENSEMBLE MEANS, CONFIDENCE IN THIS MATERIALIZING REMAINS LIMITED FOR NOW. HOWEVER, FORECAST CONFIDENCE IS HIGH FOR THE BUILDING RIDGE OVER THE WESTERN U.S. THIS WEEKEND AND INTO EARLY NEXT WEEK. THROUGH THE UPCOMING WEEKEND, AN OPERATIONAL MODEL BLEND INCLUDING THE 12Z ECMWF/CMC AND THE 18Z GFS REFLECTED THE MOST AGREEABLE ASPECTS OF GUIDANCE WHILE DOWNPLAYING LESS CONFIDENT MEDIUM TO SMALLER SCALE DETAILS PRESENT IN EACH THOSE MODEL RUNS. GIVEN THAT THE OPERATIONAL ECMWF HAS SHOWN BETTER RUN-TO-RUN CONTINUITY WITH THE ORIENTATION OF THE UPPER LEVEL TROUGH OVER THE NORTH-CENTRAL U.S., A SLIGHTLY HIGHER PERCENTAGE OF THE ECMWF WAS USED. BY THE LATTER HALF OF THE FORECAST PERIOD, AN INCREASING PERCENTAGE OF THE 18Z GEFS MEAN AND 12Z EC MEAN WERE INCORPORATED TO ACCOUNT FOR THE INCREASING MODEL SPREAD. WEATHER HIGHLIGHTS ~~~~~~~~~~~~~~~~~~ THE BEGINNING OF THE MEDIUM RANGE PERIOD ON SATURDAY WILL FEATURE HOT AND DRY CONDITIONS ACROSS THE PACIFIC NORTHWEST. THE HEAT CONTINUES TO BUILD ACROSS THE GREAT BASIN AND THE DESERT SOUTHWEST BY THE BEGINNING OF NEXT WEEK, WITH WIDESPREAD 100+ DEGREE READINGS FOR THE LOWER ELEVATIONS OF CALIFORNIA, NEVADA, AND ARIZONA AS THE UPPER LEVEL RIDGE BUILDS IN. TEMPERATURES ARE EXPECTED TO BE BELOW NORMAL FROM THE THE CENTRAL PLAINS TO THE UPPER MIDWEST COURTESY OF THE UPPER LEVEL TROUGH IN PLACE. CONCERNING PRECIPITATION PROSPECTS DURING THIS TIME PERIOD, THE HEAVIEST RAINFALL AND GREATEST THUNDERSTORM ACTIVITY IS LIKELY FROM SOUTHERN TEXAS TO THE CENTRAL GULF COAST REGION AND ALSO FOR THE TENNESSEE RIVER VALLEY AS DEEP MOISTURE FROM THE GULF OF MEXICO ADVECTS NORTHWARD AND INTERACTS WITH A FRONTAL BOUNDARY. THERE MAY BE SOME INSTANCES OF RAINFALL HEAVY ENOUGH TO CAUSE FLASH FLOODING OVER THIS REGION, AND FUTURE EXCESSIVE RAINFALL OUTLOOKS WILL PROVIDE ADDITIONAL DETAILS AS WE GO FORWARD IN TIME. THE POTENTIAL EXISTS FOR SEVERAL INCHES OF RAIN OVER PARTS OF TEXAS NEXT WEEK AS A STATIONARY FRONTAL BOUNDARY PERSISTS ACROSS THE REGION. ANOTHER AREA OF ENHANCED RAINFALL IS EXPECTED ACROSS THE WESTERN HIGH PLAINS THIS WEEKEND IN ASSOCIATION WITH THE UPPER LEVEL TROUGH. WITH THE FRONT APPROACHING THE EAST COAST EARLY NEXT WEEK, AN AXIS OF ORGANIZED RAINFALL SEEMS MORE LIKELY FROM FROM THE CENTRAL APPALACHIANS TO NEW YORK STATE. HAMRICK