EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1112 AM EST SAT DEC 20 2014 VALID 12Z TUE DEC 23 2014 - 12Z SAT DEC 27 2014 ---WET AND WINDY FOR THE HOLIDAYS--- EAST OF THE MISSISSIPPI...PACIFIC NORTHWEST AND GREAT BASIN. ...OVERVIEW... THE EMERGENCE OF A NEGATIVE-TILT TROUGH OVER THE NORTHEAST PORTION OF THE CONTINENT IS EXPECTED TO PRODUCE AN INTENSE SURFACE CYCLONE OVER SOUTHEAST CANADA AND WINDY CONDITIONS ACROSS THE NORTHEAST QUARTER OF THE CONUS. THE LARGE EXPANSE OF TROUGHING IN EASTERN CANADA WILL GENERATE A NORTHWEST FLOW PATTERN IN THE WEST...AND A PACIFIC STORM TRACK THAT SLOWLY CARVES OUT ACROSS THE GREAT BASIN AND SOUTH CENTRAL ROCKIES DURING THE LATTER HALF OF THE MEDIUM RANGE PERIOD. ...UNCERTAINTY ASSESSMENT... HONESTLY...THE UNCERTAINTY IS BEYOND DAY 7---AND HOW THE DEEP SURFACE CYCLONE 'RESHUFFLES' THE PATTERN OVER THE NORTHEAST QUARTER OF CANADA AND ACROSS THE HIGH-LATITUDE WESTERN HEMISPHERIC CIRCULATION. MIDWEST/GREAT LAKES-- THE 20/00Z MODEL GUIDANCE---GFS/ECMWF/CANADIAN ARE VERY REASONABLY CLUSTERED ON CHRISTMAS EVE MORNING (24/12Z) WITH THE SURFACE REFLECTION AND ITS DEPTH...INCLUDING THE INITIAL SEQUENCING OF TRIPLE-POINT LOW FORMATION MOVING NORTHWARD THROUGH THE OHIO VALLEY. GIVEN THE DYNAMICAL MODEL SCENARIOS...THE CONSISTENT SIGNAL FOR MULTIPLE TRIPLE-POINT REFLECTION(S) DURING THE STRONG SYSTEM'S "MORPHOLOGY" AND A RAPID DEEPENING OF THE CYCLONE WITHIN A 24-HOUR PERIOD---BETWEEN LOUISVILLE AND DETROIT---ANTICIPATE A SUB 976MB LOW CENTER INVOF LAKE HURON (AT 25/00Z) AND A WELL-DEFINED 'TROWAL' INVOF LAKE SUPERIOR. THE 'TROWAL' AND AXIS OF BEST MID-LEVEL FRONTOGENESIS IS 'ANOTHER UNCERTAINTY'---AND PROBABLY NOT A MAJOR MEDIUM RANGE ISSUE---EXCEPT FOR CENTRAL CANADA AND POST-FRONTAL CLOUD COVER ISSUE OVER THE UPPER LAKES AFTER THE DAY 4-5 TIME FRAME. SOUTHEAST/EASTERN SEABOARD-- THE FRONTAL PASSAGE 'TIMING' IS SHOWING LITTLE RUN-TO-RUN VARIANCE. WHAT DIFFERENCES REMAIN---ARE TIED TO THE DEPTH/BREATH OF THE INDIVIDUAL DETERMINISTIC SOLUTIONS AND THEIR 'LOCALIZED' INFLOW SCENARIOS FROM ATLANTIC OCEAN TO EASTERN GREAT LAKES. IT IS SOMEWHAT OF AN UNDERSTATEMENT HERE---DESCRIBING THE VOLATILITY DEVELOPING OVER THE GULF STREAM AND ITS IMPACTS ACROSS SOUTHERN NEW ENGLAND AND THE INTERIOR MOUNTAINS. TO STILL HAVE ABOVE AVERAGE CONFIDENCE IN A SUB-970MB CYCLONE CENTER NEAR/INVOF OTTAWA ONTARIO AT 25/12Z AND COMBINE THIS WITH THE UPPER-LEVEL SUPPORT TO GENERATE THE INTENSE LOW-LEVEL GRADIENTS AND ATLANTIC MOISTURE 'INFLOW' WITHIN THE SYSTEM'S WARM SECTOR---IS...EQUALLY MIRRORED AND SUPPORTED BY THE DRY SLOT AND DRAMATIC WIND SHIFT AND WESTERLY FLOW COMPONENT ON THE BACKSIDE OF THE FRONT. THE CONSISTENT SIGNAL IS ILLUSTRATED BY THE 925MB-850MB WIND FORECASTS---PROJECTED NOW FOR SEVERAL DAYS WORTH OF FORECAST CYCLES. THE POST-FRONTAL SHEAR IS JUST AS IMPRESSIVE ALONG THE ENTIRE STRETCH OF THE APPALACHIAN CHAIN. PACIFIC NORTHWEST/NORTHERN CONTINENTAL DIVIDE/GREAT BASIN-- THE MODEL SPREAD IS TIED TO THE DEPTH OF A CANADIAN AIRMASS 'PUSH' INTO THE NORTHWEST QUARTER OF THE NATION. THE INDIVIDUAL RUN-TO-RUN SOLUTIONS SEEM TO STRUGGLE WITH HOW/WHEN/WHERE TO EJECT THE ENERGY ACROSS THE DIVIDE---AND WHERE TO DELINEATE THE WESTWARD EXTENT/INFLUENCE OF BROAD-SCALE CYCLONIC FLOW ESTABLISHED OVER THE MIDWEST---WITH RESPECT TO THE DOWNSTREAM NEGATIVE-TILTED TROUGH. ALOFT...THERE IS FAR LESS DISAGREEMENT WITH THE SOUTHEASTWARD-MOVING MID/UPPER-LEVEL PACIFIC TROUGH AXIS ...MODEL PREFERENCES... THE MEDIUM RANGE FORECAST IS WEIGHTED WITH THE 20/00Z ECENS/ECMWF SOLUTION IN MIND. OVERALL...THE 20/00Z ECENS MEAN WILL BE SUFFICIENT TO HANDLE THE EVOLVING NORTH AMERICA FLOW PATTERN ALOFT. AT THE SURFACE...IT CARRIES THE DEEPEST SURFACE SOLUTION IN SOUTHEAST CANADA---AND FOR ALL KEY FEATURES AT THE SYNOPTIC-SCALE BETWEEN DAY4-7. THE CONCERN IS THE ECENS MEAN---OR ANY ENSEMBLE MEAN FOR THAT MATTER--- WILL NOT PROVIDE THE GOOD DETAIL---THE WIND FORECAST DESERVES. OF THE DETERMINISTIC GUIDANCE... THE 20/00Z ECMWF WOULD BE MY RECOMMENDATION FOR THE NEGATIVE-TILT TROUGH AND THE EVOLUTION OF THE SURFACE FEATURES IN THE SOUTHERN HIGH PLAINS THROUGH THE LATTER HALF OF DAY 5(26/00). BUT THE ISSUE WITH THE DETERMINISTIC ECMWF IS ITS BIAS--AND INCORRECTLY DEPICTING THE SYNOPTIC PATTERN ACROSS THE CENTRAL PLAINS. IN THAT...ITS DYNAMICAL SOLUTION IS TOO DEEP AND GENERATES A MATURING SURFACE CYCLONE THAT MIGRATES ACROSS THE SOUTHERN HIGH PLAINS AND MID-MS VALLEY FOR DAYS 5-7. THE DETERMINISTIC SEEMS TO 'LIKE' CRASHING A DEEP SURFACE CYCLONE INTO DOWNSTREAM PATTERNS THAT ARE OBVIOUSLY 'BLOCKED'. IN THE DAY 5-6 SCENARIO---THIS 'BLOCK' IS THE DEEP CYCLONIC FLOW OVER CANADA---NOT A STABLE RIDGE. ITS MEAN SOLUTION AND MORE OF AN INVERTED TROUGH SOLUTION WITH A WEAK/FLAT SURFACE WAVE IS MORE LIKELY. A GFS/GEFS-LIKE SOLUTION WOULD 'HEAP' A TREMENDOUS AMOUNT OF WARM ADVECTION AND ATLANTIC MOISTURE ACROSS SOUTH CENTRAL ONTARIO---INTO THE DEVELOPING 'TROWAL'---AND IF ONE FELT COMFORTABLE USING THAT SOLUTION---THEN EXPECT MUCH LESS WINTER P-TYPE CONDITIONS IN THE POST-FRONTAL ENVIRONMENT OVER THE GREAT LAKES. THE GEFS SOLUTION BEYOND DAY 5 IS MORE WESTERN CANADA-PACIFIC INFLUENCED THAN THE ECENS/ECMWF SCENARIO. ... SENSIBLE WEATHER HIGHLIGHTS/THREATS... THIS PARTICULAR CHRISTMAS HOLIDAY FORECAST CONTINUES TO BE A WARM SECTOR-TYPE STORM SYSTEM FOR THE EASTERN SEABOARD. AND VERY WET...LOCALLY 'CONVECTIVE' AND WINDY AT TIMES. POST-FRONTAL COLD ADVECTION AND 'TROWAL' INFLUENCES REMAIN POSITIONED OVER THE GREAT LAKES...WITH SOME UPSLOPE-ASSISTED---BUT 'LOCALIZED' SNOW SHOWERS OVER THE APPALACHIANS. LOCAL LAKE-EFFECT CONDITIONS APPLY WITHIN THE SOUTHWEST QUADRANT OF THE EXITING SURFACE CIRCULATION. THE PACIFIC MID-LEVEL TROUGH AND LEADING EDGE OF THE HEIGHT FALLS WILL BE THE FOCAL POINTS FOR PRECIPITATION IN THE WEST...AND A CONSOLIDATION OF THE DIGGING PACIFIC TROUGH AXIS WILL BE INFLUENTIAL IN THE DEVELOPMENT OF PRECIPITATION ALONG THE WINDWARD SLOES OF THE GREAT BASIN AND CONTINENTAL DIVIDE THROUGH DAY 5. EXPECT A SLOW-EVOLVING WINTER WEATHER P-TYPE SCENARIO FOR THE CENTRAL AND SOUTHERN PLAINS STATES AS THE PACIFIC ENERGY EJECTS A SERIES OF OVER-RUNNING TYPE PRECIPITATION EVENTS. VOJTESAK