EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1059 AM EST TUE NOV 11 2014 VALID 12Z FRI NOV 14 2014 - 12Z TUE NOV 18 2014 ...WELL-BELOW AVERAGE TEMPERATURES---'COLD SNAP'---CONTINUES THROUGHOUT THE MEDIUM RANGE PERIOD... ...OVERVIEW... WELL-ADVERTISED AMPLIFIED AND VERY COLD FLOW PATTERN SUSTAINS UNSEASONABLE CONDITIONS FOR MUCH OF THE LOWER 48 THIS FORECAST PERIOD---WITH THE ONLY EXCEPTIONS BEING THE SOUTHWEST DESERTS AND SUBTROPICAL SOUTHERNMOST FRINGES OF FLORIDA DURING THE FIRST HALF OF THE FORECAST PERIOD. THE DAY 6-7 PERIOD DOES DISPLAY A TRANSITION FROM RIDGE TO TROUGH PATTERN EVOLVING ALONG THE WEST COAST. ...UNCERTAINTY ASSESSMENT... SEVERAL REGIONS OF THE COUNTRY WILL BE A TEMPERATURE AND P-TYPE CHALLENGE AS THE SHEETS OF LOW-LEVEL ARCTIC AIR SPREAD ACROSS THE LOWER 48 AND UPPER-LEVEL FLOW IN THE WEST 'DE-AMPLIFIES' THROUGH TIME. THE DE-AMPLIFICATION PROCESS OCCURRING OUT WEST CREATES A 'TANDEM' TRANSITIONAL FLOW DOWNSTREAM ACROSS THE SOUTHEAST AND EAST CENTRAL STATES...AS THE UPPER-LEVEL TROUGH OVER SOUTHEAST CANADA SLOWLY LIFTS NORTHEASTWARD. STARTING IN THE WEST...THOUGHT THE ENERGY WHICH UNDERCUTS THE RIDGE IN THE PACIFIC NORTHWEST DAY 4-6 AND BEYOND---IS MORE A BY-PRODUCT OF THE LOW-LEVEL FLOW TRAJECTORIES ALONG THE WEST COAST RATHER THAN A TOTAL COLLAPSE OF THE UPPER-LEVEL RIDGE AXIS BETWEEN 45N-50N ALONG 120W-125W. THE EFFECTS OF THE DEEP BERING SEA CYCLONE (REMNANTS OF 'NURI' CANNOT BE UNDERSTATED) AND HAVE TOTALLY DISPLACED THE LOW-LEVEL THERMAL PROFILES IN THIS REGION OF THE WESTERN HEMISPHERE . IN THAT---THE H85-H7 TEMPERATURE REGIME OVER INTERIOR ALASKA VERSUS THE SOUTHERN CANADIAN AND NORTHERN ROCKIES ARE ESSENTIALLY REVERSED---WITH ROUGHLY A M18C AIRMASS OVER MONTANA AND A +6C AIRMASS IN SOUTHEAST ALASKA AT MID-FORECAST PERIOD! WHAT DOES LOOK A TAD DIFFERENT MID-PERIOD FROM THE PROJECTED PATTERN AND SOLUTION FIVE DAYS AGO (NOV 6TH CYCLES) IS HOW DISPLACED THE AIRMASS HAS BECOME OVER ALASKA'S ARCTIC CIRCLE. MUCH OLDER RUNS AND MOMENTUM FROM 'NURI' AND ITS TROPICAL INTRUSION HAVE MADE MORE OF A NORTHWARD RATHER THAN EASTWARD ADJUSTMENT WITH THE MIGRATION OF TROPICAL MOISTURE INTO THE HIGH LATITUDE AND SLOWED THE OUTCOME OF TROUGHING IN THE GULF OF ALASKA. THE 11/00Z ECMWF/GFS DETERMINISTIC CONSENSUS DOES SUGGEST A NEGATIVE-TILT TROUGH BECOMES ESTABLISHED ALONG THE EASTERN GULF OF ALASKA SOUTHWARD TO 40N-45N ALONG THE WEST COAST OF THE LOWER 48 AROUND DAY 7. BUT THE UNCERTAINTY IS A MORE FUNCTION OF THE POLAR JET ALIGNMENT ACROSS THE BASE OF THE TROUGH IN THE EASTERN PACIFIC AND A GRADUAL MODIFICATION OF THE AIRMASS IN ALASKA (BACK TO A RELATIVELY COOLER ONE) THAN A COMPLETE 'EROSION' OF A STABLE 'BLOCKY' UPPER-LEVEL RIDGE. WHAT IT MAY SUGGEST IS THAT THE WEST COAST TRANSITIONS FROM---A CONTINENTAL INFLUENCE AND/OR TRAJECTORY TO AN UNSTABLE PACIFIC MARITIME INFLUENCE---IS JUST AS LIKELY A CONCLUSION VS BLOCKY RIDGING ALONG THE WEST COAST...AND A 'NO ONSHORE FLOW' AND TROUGH SOLUTION WITH RESPECT TO THE LONGWAVE PATTERN. THE MAIN REASON FOR THIS...THE DISPLACEMENT OF THE AIRMASS IS SO FAR NORTH THAT THE COLD AIR ACTUALLY UNFOLDED INTO THE PACIFIC NORTHWEST...VERSUS A SOLUTION THAT TOOK THE AIR INTO THE GREAT LAKES AND MID-ATLANTIC REGION. A VERY SUBTLE DIFFERENCE IN THE UPPER-LEVEL RIDGE CONFIGURATION MAKES ALL THE DIFFERENCE. THE PRECIPITATION DISTRIBUTION ALONG THE JET AXIS FROM WEST TO EAST REMAINS IN FLUX AND WILL LIKELY CHANGE THROUGH TIME. BUT THE THEME REMAINS THE SAME...THE DAY 4-5 SURFACE WAVE AND ITS LOW-TO MID-LEVEL WARM ADVECTION PATTERN TRACKING NORTHWARD ACROSS THE CASCADES WILL MEET UP WITH A COLD/DRY AIRMASS. AFTER ITS PASSAGE...A RETURN FLOW WITH MARITIME CHARACTERISTICS---AND A SOUTH TO NORTHWEST WIND SHIFT SHOULD GRADUALLY WORK INTO THE BC COAST AND THEN SOUTHWARD INTO NORTHERN CALIFORNIA AROUND DAY 7. THE 11/00Z GEFS...ECENS AND NAEFS PROJECT THIS TO TAKE PLACE WITH THE NORMAL DAY 6-7 DIFFERENCES IN THE DETERMINISTIC GUIDANCE IN THE CENTRAL PORTION OF THE COUNTRY AND GREAT LAKES...THE SCENARIO IS MORE STRAIGHT-FORWARD IN THE SENSE THAT ONE CANADIAN SURFACE HIGH IS ESSENTIALLY REPLACED BY A SECOND CANADIAN SURFACE HIGH DURING THIS MEDIUM RANGE PERIOD. THE DIFFERENCE BEING---THE DAY 4-5 SURFACE HIGH HUGS THE EASTERN SLOPES OF THE ROCKIES AND ITS SHALLOW SOUTHERN PERIPHERY MANAGES TO GENERATE A SUDDEN INCREASE IN UPSLOPE FLOW AND SATURATION OF THE LOW LEVELS SATURATION SOUTHWARD INTO THE SOUTHERN HIGH PLAINS OF OK/TX AND NM. THAT UPSLOPE FLOW AND ITS LIGHT BUT NEAR FREEZING P-TYPE CHALLENGE WILL LIKELY FOLLOW THE LEADING EDGE OF THE MODIFIED-ARCTIC BOUNDARY EAST-NORTHEASTWARD ACROSS THE SOUTHERN PLAINS THROUGH THE COURSE OF ITS EASTWARD RETREAT. EXACTLY WHERE AND WHEN THE SHALLOW OVER-RUNNING EVENT DEVELOPS WILL BE A DETAIL THAT ESCAPES THE USE OF A 'MEAN' MASS-FIELD BLEND. IN THE EAST...THE UNCERTAINTY IS IN THE 'TIMING' OF THE LEADING EDGES OF THE ARCTIC BOUNDARIES---AND RELATED TO THE DEPTH OF THE MODIFIED-ARCTIC AIR AS THEY REACH THE APPALACHIANS...IE THE COLD AIR 'PILING UP' AND/OR 'BREAKING THROUGH' TO THE COAST. THE DETERMINISTIC GUIDANCE DOES AGREE THAT THE INITIAL SURGE HAS EXITED THE EASTERN SEABOARD BY DAY 3...BUT DIFFER WITH THE SECONDARY SURGE AND RATE OF EASTWARD EXPANSION FOR DAY 5-6 ACROSS THE MID-ATLANTIC STATES AND SOUTHERN NEW ENGLAND. THAT DIFFERENCE WILL BE THE PRIMARY SENSIBLE WEATHER CHALLENGE ALONG THE I-95 CORRIDOR HEADING INTO THE NEXT WORK WEEK AS THE UPPER-LEVEL JET AND A SURFACE WAVE WORK IN TANDEM ACROSS THE SOUTHEAST AND GULF STREAM AHEAD OF THE SECONDARY SURGE OF ARCTIC AIR. ...MODEL PREFERENCES... THE 11/00Z GEFS...ECENS AND NAEFS HAVE A DECENT HANDLE ON THE OVERALL PATTERN ACROSS NORTH AMERICA...AND THE UNDERCUTTING ENERGY/FLOW IN THE WEST ONLY REINFORCES THE POLAR JET ALONG THE WEST COAST BY PERIOD'S END. COULD NOT FIND MUCH FAULT IN THE 11/00Z DETERMINISTIC RUNS---AND IN PARTICULAR---THE GFS/ECMWF DEPICTION OF A SECONDARY SURGE OF CANADIAN AIR DOWN ALONG THE SPINE OF THE CONTINENTAL DIVIDE BY NEXT WEEKEND (DAY 5-6). THE ONE OBVIOUS TREND OF THE LATEST GUIDANCE SUGGESTS THIS SECOND SURGE IS MUCH DEEPER...AND WILL SPILL OVER THE DIVIDE AND SEEP INTO THE PACIFIC NORTHWEST AND NORTHERN GREAT BASIN (NRN UT/SRN ID/NRN NV). THE FACT THAT THE 850MB TEMP FORECAST (AT/BELOW M18C) ARE IN GOOD 'SYNCH' OVER ERN MANITOBA-WESTERN ONTARIO 'DEEP' INTO THIS MEDIUM RANGE PERIOD (TO 18/12Z)---ADDS CONFIDENCE IN THE GENERAL DETERMINISTIC GUIDANCE SOLUTION FOR THE LATTER HALF OF THE PERIOD IN THE SOUTHERN PLAINS. THAT SCENARIO WOULD ALLOW A SHEET OF MODIFIED BUT SHALLOW--- ARCTIC AIR TO PLUNGE ACROSS THE SOUTHERN PLAINS BY SUNDAY NIGHT (AROUND 17/06Z)...SETTING UP A TURBULENT...BUT SHALLOW UPSLOPE FLOW COMPONENT ALONG THE FRONT RANGE OF COLORADO AND DEEP INTO THE FOOTHILLS OF THE SOUTHERN ROCKIES OF NEW MEXICO AND UPPER HALF OF THE RIO GRANDE VALLEY--TX/NM. THEN FOLLOWED BY A RATHER RAPID REBOUND OF TEMPERATURES ALONG THE NM/TX BORDER AND AN OVER-RUNNING WEST-SOUTHWEST FLOW FILLING IN BEHIND THE FRONT. ...SENSIBLE WEATHER HIGHLIGHTS... LAKE-EFFECT SNOWS SHOULD PREVAIL ACROSS THE UPPER MIDWEST AND DOWNWIND OF LAKES---ONTARIO AND ERIE---WITH PORTIONS OF THE MID OHIO VALLEY AND WESTERN NEW YORK RECEIVING MODEST SNOW IN THIS COLD PATTERN. A DEVELOPING WAVE EMBEDDED IN THE GULF STREAM APPEARS TO BE THE 'WILD CARD' DURING THE DAY 5-6 IN THE EAST AS THE FLOW ALOFT TRANSITIONS AWAY FROM DEEP TROUGH. SECONDARY AND REINFORCING POOL OF CANADIAN AIR SURGING INTO THE PACIFIC NORTHWEST NEXT WEEKEND SETS UP THE POTENTIAL FOR FREEZING RAIN EAST OF THE CASCADES AND ALONG THE COLUMBIA RIVER BASIN. VOJTESAK