EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 231 AM EDT TUE SEP 05 2017 VALID 12Z FRI SEP 08 2017 - 12Z TUE SEP 12 2017 ...INCREASING CHANCE OF SOME IMPACTS FROM HURRICANE IRMA IN THE FLORIDA PENINSULA AND KEYS LATER THIS WEEK AND THIS WEEKEND... ...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... THE LARGE SCALE FLOW PATTERN ACROSS NORTH AMERICA APPEARS POISED TO BECOME INCREASINGLY CHAOTIC DURING THE MEDIUM RANGE FORECAST PERIOD, WITH PREDICTABILITY DECREASING RATHER SIGNIFICANTLY THROUGH TIME. AT LEAST INITIALLY ON DAY 3 (FRI), MODELS SHOW GOOD AGREEMENT ON A LONG WAVE TROUGH ACROSS THE GREAT LAKES/NORTHEAST BEGINNING TO LIFT OUT, WITH UPPER RIDGING BUILDING IN QUICKLY IN ITS WAKE. BY DAYS 4-5 (SAT-SUN), HOWEVER, MODELS CONTINUE TO SHOW SIGNIFICANT DISCREPANCIES ON JUST HOW QUICKLY THE TROUGH WILL LIFT OUT. THE 12Z ECMWF/00Z CMC ARE SIGNIFICANTLY SLOWER THAN THE GFS WITH LIFTING THE TROUGH OUT, AND THESE SLOWER SOLUTIONS HAVE THE SUPPORT OF THE ECENS/GEFS/NAEFS ENSEMBLE MEANS. ON THOSE GROUNDS, WPC IS FAVORING THE LESS PROGRESSIVE 12Z ECMWF/00Z CMC SOLUTIONS ACROSS THE NORTHEASTERN U.S. ON DAYS 4-6. FURTHER UNCERTAINTY ARISES, WITH RESPECT TO NORTHERN STREAM ENERGY ORIGINATING IN THE NORTH PACIFIC/GULF OF ALASKA EARLY IN THE MEDIUM RANGE. THIS ENERGY IS FORECAST TO ROUND THE NORTHERN EDGE OF THE FLATTENING WESTERN U.S. RIDGE BEFORE ENTERING THE CANADIAN PRAIRIES BY SUN-MON. THE LAST FEW RUNS OF THE GFS HAVE QUICKLY AMPLIFIED THIS ENERGY AS IT ENTERS THE UPPER MIDWEST BY MON-TUE, WHILE THE ECMWF HAS BEEN CONSISTENT WITH KEEPING THE SHORTWAVE VERY LOW IN AMPLITUDE AND CONFINED TO THE HIGHER LATITUDES OF CANADA. ENSEMBLES DO NOT PROVIDE SUBSTANTIAL SUPPORT FOR THE GFS SOLUTION HERE, WITH THE 12Z ECENS AND NAEFS RESEMBLING THE DETERMINISTIC ECMWF. THUS, THE WPC PREFERENCE IS FOR THE LESS AMPLIFIED/MORE PROGRESSIVE SOLUTION SHOWN BY THE 12Z ECMWF. ADDITIONAL LOW AMPLITUDE SHORTWAVE ENERGY, ALSO ORIGINATING IN THE NORTH PACIFIC, WILL MOVE SOUTHEASTWARD ACROSS THE CENTRAL PLAINS BY DAYS 4-5, REACHING THE LOWER MS VALLEY BY DAY 6. WHILE ALL SOLUTIONS AGREE ON THE PRESENCE AND GENERAL LOCATION OF THIS FEATURE BY DAY 6-7, THEY DIFFER ON THE EXACT CHARACTER OF THE ENERGY, WHETHER IT IS A RELATIVELY CONSOLIDATED VORT MAX OR MORE ELONGATED. NORMALLY, SUCH A SEEMINGLY BENIGN UPPER-LEVEL FEATURE WOULD NOT WARRANT MENTION, HOWEVER THIS FEATURE WILL LIKELY BE CLOSE ENOUGH TO THE EVENTUAL TRACK OF IRMA TO HAVE SOME INFLUENCE. THESE THREE FEATURES AND THEIR COMBINED INTERACTION WILL DETERMINE THE ULTIMATE TRACK THAT HURRICANE IRMA FOLLOWS BY DAYS 6-7. THUS, THE FORECAST FOR IRMA FOR LATE IN THE MEDIUM RANGE CONTINUES TO BE FRAUGHT WITH UNCERTAINTY. THE WEST COAST WILL NOT BE IMMUNE TO LOW FORECAST CONFIDENCE BY LATE IN THE PERIOD. CONFIDENCE IS RELATIVELY HIGH WITH RESPECT TO A TROUGH/UPPER LOW JUST OFF THE WEST COAST ON DAY 3, WHICH IS FORECAST BY THE MAJORITY OF GUIDANCE TO LINGER AND/OR DRIFT SLOWLY SOUTHWARD THROUGH DAY 7. ANY EASTERN PROGRESS INHIBITED BY PERSISTENT RIDGING ACROSS THE SOUTHWESTERN STATES. UNCERTAINTY INCREASES SUBSTANTIALLY BY LATE IN THE PERIOD, HOWEVER, PARTICULARLY FOR THE PAC NW. MODELS SHOW WIDELY VARYING SOLUTIONS FOR TIMING OF NORTHERN STREAM PACIFIC ENERGY APPROACHING/ARRIVING BY DAY 7. GIVEN A WIDE RANGE OF SOLUTIONS AMONG DETERMINISTIC GUIDANCE, AN ENSEMBLE APPROACH WAS PREFERRED, PARTICULARLY THE 12Z ECENS/NAEFS MEANS. ...WEATHER HIGHLIGHTS... RIDGING SHIFTING EAST FROM THE WESTERN U.S. INTO THE MIDWEST WILL SUPPORT ABOVE AVERAGE TEMPERATURES. HIGH TEMPERATURES MAY REACH 5 TO 20 DEG F ABOVE AVERAGE FROM THE NORTHERN ROCKIES TO THE UPPER MIDWEST THROUGH MUCH OF THE MEDIUM RANGE. MEANWHILE, THE EASTERN U.S. TROUGH ALONG WITH STRENGTHENING LOW-LEVEL EASTERLY/NORTHEASTERLY FLOW ALONG THE EAST COAST WILL RESULT IN BELOW AVERAGE TEMPERATURES, 5 TO 15 DEG F BELOW AVERAGE FOR MANY AREAS. THE TROUGH/UPPER LOW WILL ALSO PRODUCE AREAS OF SHOWERS ACROSS THE NORTHEASTERN U.S., PARTICULARLY DURING DAYS 3-5, BEFORE IT LIFTS OUT. FARTHER WEST, SCATTERED MAINLY DIURNAL SHOWERS AND THUNDERSTORMS ARE EXPECTED ACROSS PORTIONS OF THE GREAT BASIN/CENTRAL ROCKIES AS RELATIVELY MODEST LEVELS OF MONSOONAL MOISTURE REMAIN IN PLACE BENEATH THE UPPER RIDGE. HURRICANE IRMA IS FORECAST BY THE NATIONAL HURRICANE CENTER TO MOVE APPROACH THE FLORIDA STRAITS, NORTH OF CUBA, BY LATE SATURDAY. IRMA MAY BEGIN TO IMPACT PORTIONS OF THE FLORIDA PENINSULA OR KEYS WITH RAIN AND WIND BY LATE THIS WEEK OR THIS WEEKEND. THE EVENTUAL TRACK OF IRMA DURING DAYS 6-7 WILL DETERMINE WHERE AND TO WHAT DEGREE THE SYSTEM WILL HAVE IMPACTS FARTHER NORTH ACROSS THE SOUTHEASTERN U.S. RYAN