EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 120 AM EST TUE NOV 17 2015 VALID 12Z FRI NOV 20 2015 - 12Z TUE NOV 24 2015 ...OVERVIEW... ACTIVE BUT EXITING WEATHER SYSTEM ---CURRENTLY IN THE CENTRAL PLAINS --- WILL USHER IN A COOLER CANADIAN AIRMASS AND DRY WEATHER PATTERN ACROSS MUCH OF THE LOWER 48 --- EAST OF THE CONTINENTAL DIVIDE. THE EXCEPTION WILL BE A LIGHT PRECIPITATION PATTERN --- ACROSS THE NORTHERN PLAINS FRIDAY AND THE GREAT LAKES/MIDWEST ON SATURDAY. ...MODEL PREFERENCES... EAST OF THE CONTINENTAL DIVIDE --- THE 16/12Z ECENS/NAEFS AND GEFS SERVE AS A DECENT BASELINE SOLUTION WITH THE BROAD TROUGH THAT HAS CARVED OUT --- THE SOUTHEASTWARD RELEASE OF ARCTIC AIR FROM NORTHWEST CANADA AND ALASKA. THE SCENARIO PLAYING OUT THROUGH DAY 5 AND MEDIUM RANGE FORECAST MAINTAINS THE COLDER TROUGH IN EASTERN NORTH AMERICA AND THE PACIFIC JET AXIS STREAKING WEAK SHORTWAVE ENERGY ACROSS THE PLAINS...MIDWEST AND THE NORTHEAST. ACROSS THE EAST --- FORECAST REMAINS UNCHANGED THROUGH DAY 5. ACTUALLY THOUGHT THE DETERMINISTIC 15/12Z & 16/00Z ECMWF LOOKED BETTER THAN ITS 16/12Z RUN ... WITH THE ENERGY TRANSITING THE GREAT LAKES ON SATURDAY ALONG THE WEST COAST...DESERT SOUTHWEST AND ACROSS THE GREAT BASIN --- BETTER CONTINUITY AND FIT WITHIN THE ENTIRE ENSEMBLE SPREAD? --- USING THE DETERMINISTIC 16/00Z ECMWF...16/12Z UKMET UNTIL DAY 5 FOR THE SMALLER-SCALE DETAILS. UNFORTUNATELY --- THE 16/12Z GFS WAS A BIG OUTLIER --- WITH NOT ONLY THE OTHER DETERMINISTIC GUIDANCE --- BUT EVEN WITH ITS OWN GEFS MEAN. A 16/12Z ECENS/NAEFS BLEND WOULD SERVE AS A DECENT PLACEHOLDER ALONG THE WEST COAST AT DAY 7. ...UNCERTAINTY ASSESSMENT... THE PREVIOUS SHIFT PRETTY MUCH DESCRIBES THE SCENARIO UNFOLDING WITHIN THE BROAD TROUGH. "THE MAIN CONSIDERATION IS THE CHOICE OR POSSIBLY SIMULTANEOUS OCCURRENCE OF A SEMI-PERMANENT EASTERN CONUS TROUGH ALONG WITH DEVELOPMENT OF A WESTERN CONUS TROUGH LATE IN THE PERIOD. THE ECMWF ENSEMBLES SUPPORT THE WEST WHILE THE GEFS ENSEMBLES SUPPORT THE EAST." AND LOOKS LIKE A LITTLE OF BOTH WILL BE 'THE WAY TO GO' DURING THIS MEDIUM RANGE PERIOD. THE LOWER 48 UNDERGOES A 2-3 DAY TRANSITION PERIOD BETWEEN PREVAILING UPPER-LEVEL TROUGHS. IN OTHER WORDS ... CAN EASILY SEE A LOBE OF THE ARCTIC-MODIFIED AIR TO RUN ITS COURSE ACROSS THE GREAT LAKES...NORTHEAST AND EASTERN CANADA --- THEN ALLOW THE WEST-NORTHWESTERLY PACIFIC JET AXIS TO SWEEP THROUGH THE NORTHERN PLAINS FRIDAY...THE MIDWEST ON SATURDAY. AND NEW ENGLAND AND THE NORTHERN MID-ATLANTIC REGION ON SUNDAY. IN EFFECT --- LIFTING OUT THE MAJORITY OF THE COOLER CANADIAN AIR. THAT SAID... THEN HOW LONG OF A TRANSITION IN THE ROCKIES? --- BEFORE ANOTHER TEMPORARY 'SPLIT-FLOW' SCENARIO EMERGES AND THE SOUTHERN JET MOISTURE "RE-LOAD" BEGINS ANEW. ANOTHER CUTOFF-LIKE LOW SOLUTION ALONG THE WEST COAST MAY BE A STRETCH FOR DAY 7. TWO CHALLENGES HERE --- THE SUBTLE SENSIBLE WEATHER SCENARIO (DURING THE FRIDAY THROUGH SUNDAY TRANSITION PERIOD) ACROSS THE MIDWEST ---WITH THE SHALLOW SHORTWAVE ENERGY TRACKING THROUGH THE CONTINENTAL DIVIDE (IN THE WEST-NORTHWEST 250MB FLOW ALOFT) ---AND SPREADING DOWNSTREAM "BEHIND" THE MODIFIED-ARCTIC AIRMASS. THIS COLDER AIRMASS --- A VERY DRY ONE --- WILL BE IN PLACE OVER NEW ENGLAND AHEAD OF THE EXITING SHORTWAVE ON SUNDAY. AND THE SECOND FORECAST CHALLENGE --- HOW LONG WILL THE TRANSITION ACTUALLY LAST --- BEFORE THE INTERMOUNTAIN WEST AND 'TRANQUIL' PATTERN DOMINATED BY CANADIAN HIGH PRESSURE EAST OF THE ROCKIES BREAKS DOWN IN EARNEST? IE... A RELATIVELY DRY PATTERN FOR MOST OF THE LOWER 48 --- IS THE 'TRANSITION' --- REPLACED BY A NEW UPPER-LEVEL TROUGH AND SOUTHERN JET STREAM BEGINNING TO WORK ACROSS THE WEST COAST AND GREAT BASIN. NOT SURE THIS 'NEW' DAY 7 WEST COAST TROUGH AND EMBEDDED SUB-SYNOPTIC-SCALE FEATURES WITHIN IT --- WILL LOOK MUCH AT ALL LIKE THIS LATEST ONE --- THE ENERGETIC 'FOUR-CORNERS LOW' AND MIDWEST STORM TRACK (WITH DUAL-JET PHASED ENERGY. RIGHT NOW ... THIS DAY 7 TROUGH LOOKS TO BE A SLOWER-MOVING GULF OF ALASKA SYSTEM WITH A MORE DISTINCT (BUT ZONAL SOUTHERN STREAM MOISTURE SET UP) RACING OUT AHEAD AND CLOUDIER...WETTER? PATTERN FOR THE RIO GRANDE VALLEY ... SOUTHEAST AND WESTERN GULF COAST. ...SENSIBLE WEATHER HIGHLIGHTS... WIDESPREAD RAINFALL...SOME HEAVY...SHOULD BE EXITING THE EAST COAST EARLY IN THE PERIOD BUT STILL GENERATING SHOWERS ALONG THE FRONT ACROSS MAINE AND THE FLORIDA PENINSULA. BEHIND THIS SYSTEM .... MAINLY LAKE-EFFECT SHOWERS FOR THE GREAT LAKES REGION. LIGHT PRECIPITATION SHOULD ACCOMPANY THE PACIFIC SHORTWAVE ENERGY STREAKING ACROSS THE NORTHERN PLAINS FRIDAY AND MIDWEST/NORTHEAST NEXT WEEKEND. THIS WEAK SYSTEM COULD GENERATE SOME PERIODS OF LIGHT SNOW --- ACROSS THE GREAT LAKES AND NORTHERN NEW ENGLAND. VOJTESAK/JAMES