EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1123 AM EDT THU JUN 15 2017 VALID 12Z SUN JUN 18 2017 - 12Z THU JUN 22 2017 ...EXTREME RECORD HEAT WAVE FOR PORTIONS OF THE SOUTHWEST NEXT WEEK... ...OVERVIEW... A WELL-ADVERTISED AND DANGEROUS HEAT WAVE WILL IMPACT THE SWRN QUAD OF THE COUNTRY FOR MUCH OF NEXT WEEK. A LARGE POSITIVE ANOMALY CENTERED ALONG THE WEST COAST NEAR NRN CA AND ACCOMPANIED SPRAWLING DOME OF UPPER HIGH PRESSURE WILL DOMINATE THE FORECAST FROM CA TO THE FOUR CORNERS REGION. IN FACT, MUCH OF THE OPERATIONAL AND EVEN MEANS SUGGEST A HEALTHY CLOSED 594DM CONTOUR. ALSO, A LARGE BERMUDA RIDGE OFF THE EASTERN SEABOARD AND STRETCHING INTO THE CANADIAN MARITIMES WILL BE ANOTHER LARGE SCALE RELATIVELY STATIONARY FEATURE DURING THE FORECAST PERIOD. MEANWHILE, RELATIVELY FAST FLOW OVER THE TOP OF THE WESTERN RIDGE WILL KEEP UPPER HEIGHTS BELOW AVG ACROSS THE NRN TIER OF THE COUNTRY FROM THE NRN PLAINS/UPPER MIDWEST TO GREAT LAKES/OH VALLEY. THIS WEAKNESS WILL GET PINCHED OFF BUT EXTEND INTO THE GULF OF MEXICO, AS MANY OF THE OPERATIONAL MODELS INDICATE A POSSIBLE ACTIVE TROPICAL WAVE MOVING MAINLY EAST TO WEST ACROSS THE SRN GULF/BAY OF CAMPECHE. ...GUIDANCE EVALUATION AND PREFERENCES... THE MODELS/ENSEMBLES REMAIN IN GENERALLY GOOD AGREEMENT WITH A COUPLE EXCEPTIONS -- THE INITIAL SHORT WAVE AND STRONG COLD FRONT STARING OVER THE MID SECTION OF THE COUNTRY ON SUN IS HANDLED WELL, AS IT ADVANCES DOWNSTREAM. HOWEVER, THE FOLLOWING SHORT WAVE/S WILL BE RIDING ALONG RELATIVELY FAST ZONAL FLOW AND THE GUIDANCE IS HANDLING DIFFICULTY WITH THESE FEATURE/S. MEANWHILE, CONCERNING THE TROPICS, LOTS OF SPREAD AND UNCERTAINTY WITH AN ACTIVE TROPICAL WAVE OR PERHAPS AN ORGANIZED SFC LOW MOVING WESTWARD ACROSS THE YUCATAN PENINSULA INTO THE SOUTHERN GULF OF MEXICO OR BAY OF CAMPECHE. THE 00Z CANADIAN WAS OVERLY AGGRESSIVE WITH THIS SYSTEM, WELL AWAY FROM THE SOMEWHAT CONSENSUS, AS CAN BE ITS BIAS IN THE SUMMER. THE 00Z UKMET TAKES A RELATIVELY WELL-DEVELOPED SYSTEM ACROSS THE YUCATAN INTO THE SWRN GULF. THE ECMWF AND GFS SUITE CARRY A PREVALENT WAVE AT THIS POINT AND THIS IS WHAT GENERALLY WAS COLLABORATED WITH NHC YESTERDAY. NO MATTER THE SPREAD AT THE MOMENT, SOUTHEAST FLOW AROUND THE HIGH IN THE ATLANTIC MAY CARRY MOISTURE MUCH FARTHER NORTHWARD. SEE THE TROPICAL WEATHER OUTLOOKS FROM THE NATIONAL HURRICANE CENTER ON THE POTENTIAL FOR TROPICAL CYCLONE DEVELOPMENT AND THE UPDATED FORECAST MAPS (ISSUED BY 2030Z) AFTER THE 17Z WPC/NHC CALL. OVERALL WPC PREFERRED STARTING OFF THE A SOLID BLEND OF THE 00Z ECMWF AND 00Z/06Z GFS SUN INTO EARLY NEXT WEEK BUT BY NEXT WED AND THURS, 70-75% OF THE BLEND WAS ECMWF, GEFS AND NAEFS MEANS. ...WEATHER HIGHLIGHTS... THE RATHER PERTINACIOUS HEATWAVE IN THE SOUTHWEST WILL LIKELY REACH ITS PEAK EARLY TO MID WEEK BUT TEMPERATURES ARE STILL FORECAST TO BE WELL ABOVE AVERAGE THROUGH NEXT THURSDAY. SOME LOCATIONS MAY CHALLENGE SEVERAL DAILY AND PERHAPS ALL-TIME RECORD HIGHS (LOWER PROBABILITY BUT STILL WITHIN REACH). ENSEMBLE SIGNAL FOR HEAT CONTINUES TO BE IMPRESSIVE FROM ALL ANGLES AND HAVE CONSISTENTLY FORECAST A FAIRLY RARE EVENT OVER A LARGE AREA. EVEN AN OVERREACH BY THE GUIDANCE STILL YIELDS A FORMIDABLE EVENT, EVEN FOR THE DESERT. PLEASE REFER TO THE LOCAL NWS STATEMENTS FOR SPECIFICS ON EXCESSIVE HEAT WATCHES/WARNINGS. THE WPC MEAN MAX TEMP ANOMALY FOR THE PERIOD SUGGEST BELOW AVG HIGHS FOR THE UPPER MS VALLEY/UPPER GREAT LAKES AND FOR PARTS OF THE SOUTHEAST, INCLUDING FL. PRECIPITATION ON SUN WILL BE MAINLY FOCUSED WEST OF THE APPALACHIANS FROM THE EASTERN GREAT LAKES DOWN INTO THE ERN OH VALLEY AND MUCH OF THE TN VALLEY, WHERE AREAL AVG OF 1 TO 2 INCH AMOUNTS WITH LOCALLY HIGHER VALUES WILL BE POSSIBLE. THEN ON MON, EXPECT SCATTERED SHOWERS/STORMS FOR MUCH OF THE EAST INTO THE SOUTH/SOUTHEAST. THEN ON TUES, THE SOUTHEAST WILL REMAIN IN THE PRECIP CHANCES, WHILE SCATTERED SHOWERS/STORMS RETURN FROM THE UPPER/MID MS VALLEY INTO GREAT LAKES. THEN NEXT WED AND THURS, EXPECT ISOLATED TO SCATTERED PRECIP ANYWHERE FROM THE UPPER MIDWEST TO NORTHEAST SECTION OF THE COUNTRY. THE BIGGEST QUESTION WITH PRECIPITATION AMOUNTS WILL BE ALONG THE IMMEDIATE GULF COAST DUE TO THE UNCERTAINTY REGARDING THE POTENTIAL TROPICAL WAVE/DISTURBANCE. MUSHER/FRACASSO