EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 140 AM EDT THU AUG 06 2015 VALID 12Z SUN AUG 09 2015 - 12Z THU AUG 13 2015 ...OVERVIEW... AN ACTIVE WEST COAST TROUGH---ANCHORED IN PLACE BY A DOWNSTREAM UPPER-LEVEL RIDGE ALONG 100W LONGITUDE AND THE ANTICYCLONE CENTER---OVER NORTH CENTRAL TEXAS---SHOULD PRODUCE A PERSISTENCE-TYPE FORECAST. DOWNSTREAM OF THE RIDGE AXIS---THE UPPER TROUGH IN PLACE OVER EASTERN CANADA SENDS ANOTHER PROGRESSIVE MID-LEVEL SYSTEM AND REINFORCING SHOT OF CANADIAN AIR ACROSS THE EASTERN THIRD OF THE NATION---DAYS 4 THROUGH 6. THIS DAY 4-6 CANADIAN FRONT APPEARS TO BE A VERY ACTIVE ONE FOR THE EAST---AND DOES OFFER SOME RELIEF TO THE SOUTHERN PLAINS AND THE TRIPLE-DIGIT HEAT. ...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT... THE 5/12Z DETERMINISTIC MODEL GUIDANCE MAINTAINS BETTER THAN AVERAGE AGREEMENT ALONG THE WEST COAST THROUGH DAY 6...BUT CONTINUES TO STRUGGLE FOR CONSISTENCY ACROSS THE NORTHEAST---WITH RESPECT TO THE EXITING SHORTWAVE TROUGH ACROSS SOUTHERN NEW ENGLAND DAY 3. THE 'KICKER' SYSTEM AND SURFACE FRONT MIGRATING ACROSS THE EAST COAST DAY 4-6 CONTINUES TO BE A 'HEAD-SCRATCHER'---IN THAT THE 5/12Z DETERMINISTIC ECMWF CONTINUES TO FLIP-FLOP WITH A 'PHASED' SOLUTION IN THE NORTHEAST. OVERALL... THOUGHT THE MEANS SERVE THE PATTERN PRETTY WELL. AND IN THE PROCESS...THE DAY4-6 FRONTAL PROGRESSION 'BRIEFLY' LIFTS A PORTION OF THE UPPER-LEVEL LOW FROM JAMES BAY---INTO ATLANTIC CANADA (LABRADOR). THIS HAS BEEN A CONSISTENT SIGNAL ACROSS CANADA FOR CLOSE TO A WEEK NOW. I MENTIONED 'BRIEFLY LIFTS'...WITH RESPECT TO THE UPPER-LEVEL TROUGH IN EASTERN CANADA---IT'S MORE LIKE 'RELAXES'. THAT TROUGH WILL ATTEMPT TO RELOAD AND ANCHOR BACK INVOF JAMES BAY FOR DAY 7 AND BEYOND AS ENERGY FROM THE PACIFIC NORTHWEST BEGINS TO EXIT THE NORTH CENTRAL ROCKIES AND MIGRATES TO THE GREAT LAKES. UNLIKE PREVIOUS DAYS...THE 5/12Z DETERMINISTIC GUIDANCE ACTUALLY HAS MORE UTILITY (ECMWF...GFS AND CANADIAN) TO DAY 6...AND COULD USE SOME OF THE DETAILS TO NARROW DOWN THE SERIES OF ONSHORE FLOW SURGES AND SW-TO-NE MOVING SHORTWAVES EJECTING ACROSS THE PACIFIC NORTHWEST...NORTHERN GREAT BASIN AND NORTHERN DIVIDE REGION. THE PATTERN OUT WEST DOES GET COMPLICATED BY THE PERSISTENCE OF A MONSOONAL MOISTURE CHANNEL---BETWEEN SOUTHERN NEVADA AND SOUTH CENTRAL MONTANA--IE MOISTURE MIGRATING ALONG THE WESTERN PERIPHERY OF THE ANTICYCLONE. THE NORTHERN PACIFIC...GULF OF ALASKA AND PACIFIC NORTHWEST DO BEGIN TO A SEE A PATTERN 'CHANGE' AROUND DAY 7...AS THE SOUTHWEST PACIFIC MONSOON TROUGH AND DEPRESSION CURRENTLY EMBEDDED WITHIN IT (INVOF THE MARIANAS) ATTEMPTS TO ENTRAIN INTO THE MID-LATITUDE WESTERLIES. BY THEN...BELIEVE THE UPPER-LEVEL TROUGH INVOF HUDSON BAY WILL BECOME AN IMPORTANT SYNOPTIC-SCALE FEATURE OF THE FLOW FOR EASTERN NORTH AMERICA. IN THE EAST...THE DETERMINISTIC RUN-TO-RUN SURFACE SOLUTION DIFFERENCES 'TILT ME' MORE TO AN 5/12Z ECENS/NAEFS MEAN AND PREVIOUS WPC FORECAST BLEND. MEDIUM RANGE FORECAST PHILOSOPHY REMAINS UNCHANGED. THE DEEPER SURFACE SOLUTIONS (FOR DAY 3-4) APPEAR TO BE A FUNCTION OF CONVECTIVE PROCESSES---AND ALL THE INTENSE SURFACE LOW SOLUTIONS GENERATED---SEEM TO APPEAR AND 'DISAPPEAR' VERY NEAR WHERE THE GULF STREAM WOULD BE PRESENT. THOUGHT THE MEANS HAVE BEEN A VERY GOOD PREDICTOR OF THE MASS FIELDS...IE THE 500MB HEIGHTS/PATTERN---AND TRUE DEPTH OF THE SERIES OF CANADIAN AIRMASS INTRUSIONS --- WHICH HAVE BEEN FORECASTING TWO DISTINCT CANADIAN BOUNDARY SURGES INTO THE LOWER 48. AND HAVE EQUALLY PROJECTED THEIR 'DECAYING' NATURE BETWEEN 35N-45N LATITUDE OVER THE OPEN WATERS OF THE WESTERN ATLANTIC. WHAT LOW-LEVEL BAROCLINICITY EXISTS...REMAINS LOCKED IN ACROSS THE ST LAWRENCE VALLEY AND WESTERN SIDE OF THE APPALACHIAN TRAIL (THE EASTERN DIVIDE) WITH BRIEF 'SPILL-OVER' ACROSS NORTHERN NEW ENGLAND. THIS IS THE LAST OF 7 FOR ME...AND THE MODEL SOLUTIONS THAT HAVE PERIODICALLY WANTED TO PHASE ENERGY BETWEEN TORONTO AND BOSTON JUST NEVER 'GAIN MOMENTUM'. BETTER SUITED IN AUTUMN AND EARLY WINTER WHEN BAROCLINICITY AND STRONG HEIGHT FALLS ARE MORE COMMON. FOR SOME REASON...THE MODELS --- ESPECIALLY THE ECMWF --- REALLY 'LIKES' THIS PARTICULAR SYNOPTIC PATTERN OVER THE GULF OF MAINE AND OUTER BANKS. FOR DEEP MID-AUGUST CYCLOGENESIS? IT SEEMS A STRETCH TO GET SUCH STRONG LOW-LEVEL BAROCLINICITY WHEN THE 500MB HEIGHTS IN THE TORONTO-BOSTON CORRIDOR CANNOT GET 'LOWER' THAN MAYBE 576DM WITHIN THE BASE OF THIS PARTICULAR JAMES BAY UPPER-LEVEL SYSTEM. THAT SAID...THERE IS A VERY PRONOUNCED WEST TO NORTHWEST MID-LEVEL FLOW---AND ABOVE AVERAGE 700MB WIND SPEEDS---ESPECIALLY AROUND DAY 5 (BEHIND THE 2ND FRONT)---WHERE I CAN SEE BROAD-SCALE OROGRAPHIC LIFT (IF THE TIMING AND INSTABILITY ARE FAVORABLE ALONG THE COLD FRONT)---TO GENERATE SOME LOCALLY DEEP CONVECTIVE ELEMENTS DOWNWIND OF THE APPALACHIANS. IT SHOULD BE INTERESTING TO WATCH THIS ALL UNFOLD. THE 700MB FLOW PATTERN ALMOST LOOKS TOO PROGRESSIVE---FOR A SURFACE CYCLONE TO TAKE SHAPE ---EMBEDDED WITH SUCH A BROAD 700MB WESTERLY WIND FIELD. ...SENSIBLE WEATHER HIGHLIGHTS... MUCH LIKE THE CURRENT PATTERN...THE FOCUS FOR PRECIPITATION INTO THE MEDIUM RANGE---EAST OF THE ROCKIES WILL BE FOCUSED ALONG THE TWO CANADIAN FRONTS---WITH SOME MCS-TYPE CONVECTIVE INITIATION IN THE NORTHERN PLAINS MIGRATING DOWNWIND AND ENTRAIN INTO THE MID-LEVEL NORTHWEST FLOW PATTERN. THE DAY 4-6 FRONT SHOULD DEVELOP A BROAD ARC OF CONVECTIVE ELEMENTS ALONG ITS LEADING EDGE AND MOVE SOUTHWARD INTO THE SOUTHERN PLAINS...SOUTHEAST AND NORTHWARD INTO THE NORTHEAST. THIS FRONT HAS POTENTIAL TO BE A BROAD-SCALE SENSIBLE WEATHER-MAKER. IN THE WEST...TWO...MAYBE THREE MOISTURE PLUMES ATTEMPT TO BECOME ENTRAINED INTO THE BASE OF THE UPPER-LEVEL TROUGH ANCHORING ACROSS THE BC COAST. TWO OF THESE PLUMES ORIGINATE IN THE SUBTROPICAL PACIFIC---A THIRD...OVER THE DESERTS/MOUNTAINS OF NORTHERN MEXICO (SUMMER MONSOON). THIS TROUGH DOES USHER IN A MARITIME PACIFIC AIRMASS---SUFFICIENTLY DEEP TO ASCEND THE CASCADES AND BRIEFLY REACH THE FAR NORTHERN SIERRA...BUT TOO SHALLOW TO REACH THE CONTINENTAL DIVIDE. BY DAY 5...SHOULD SEE THE CANADIAN FRONT WORKING SOUTH ACROSS THE SOUTH CENTRAL PLAINS---AND SOME LOWER TEMPERATURES ACROSS OKLAHOMA AND NORTH TEXAS. THE FORECAST CHALLENGE HERE WILL BE TO DETERMINE IF PRECIPITATION WILL BE WIDESPREAD ALONG THE LEADING EDGE OF THE FRONT...AND WHETHER AN UPSLOPE COMPONENT TO THE LOW-LEVEL FLOW CAN HAVE SUFFICIENT MOMENTUM TO MOVE INTO THE TEXAS PANHANDLE/PERMIAN BASIN. VOJTESAK