EXTENDED FORECAST DISCUSSION NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD 226 PM EDT FRI OCT 26 2012
VALID 12Z MON OCT 29 2012 - 12Z FRI NOV 02 2012
...DANGEROUS HURRICANE/POST-TROPICAL CYCLONE SANDY IS LIKELY TO SPREAD HIGH WINDS/HEAVY RAINS AND INTERIOR HEAVY SNOWS FROM THE CAROLINAS AND CENTRAL APPALACHIANS NORTHWARD INTO CANADA...
GENERAL FLOW PATTERN ==================== A RETROGRADING POSITIVE ANOMALY MOVING FROM SOUTHERN GREENLAND INTO NORTHERN QUEBEC FAVORS A QUASI-STATIONARY DEEP CYCLONE NEAR THE NORTHEAST, IN THIS CASE HURRICANE SANDY AND ITS POST-TROPICAL REMAINS. OTHERWISE...RIDGING IS GENERAL EXPECTED OUT WEST...THOUGH FLOW ACROSS THE PACIFIC IS EXPECTED TO SEND AMPLE ENERGY/HEAVY PRECIPITATION ACROSS THE NORTHWEST. GUIDANCE IS IN EXCELLENT AGREEMENT ON THE GENERAL FLOW PATTERN, THOUGH ISSUES REMAIN WITH SANDY'S FUTURE COURSE AND STRENGTH.
MODEL PREFERENCE ================ THE NATIONAL HURRICANE CENTER (NHC) TRACK NOW BEST RESEMBLES THE 00 UTC ECMWF ENSEMBLE MEAN. THE EXTRATROPICAL TRANSITION OF SANDY MAY HAVE ALREADY BEGUN BASED UPON RECENT SATELLITE IMAGERY...A PROCESS EXPECTED TO COMPLETE TUESDAY PER NHC. THE DETERMINISTIC GUIDANCE (PARTICULARLY THE 00 UTC ECMWF) SHOW PRESSURE SOLUTIONS WELL BEYOND WHAT HAS EVER BEEN OBSERVED NEAR THE NEW JERSEY/NEW YORK COAST (EVEN EXCEEDING THE 1938 LONG ISLAND EXPRESS HURRICANE) EARLY IN THE MEDIUM RANGE PERIOD. THE DETERMINISTIC GUIDANCE (CANADIAN, GFS, AND ECMWF INCLUDED) HAS SHOWN A VERY STRONG BIAS WITH TROPICAL CYCLONES GAINING LATITUDE AND/OR TRANSITIONING INTO NON-TROPICAL STORMS OVER THE PAST COUPLE YEARS... INCLUDING (BUT NOT EXCLUSIVE TO) LESLIE 2012...ISAAC 2012...DEBBY 2012 IN THE WESTERN SUBTROPICAL ATLANTIC IRENE 2011 AND IGOR 2010. PLEASE REFER TO THE NHC FOR UPDATED INFORMATION ON SANDY.
THE 00 UTC ECMWF ENSEMBLE MEAN ALSO DEPICTS AN OVERALL FLOW PATTERN UPSTREAM ACROSS THE W-CENTRAL CONUS AND ERN PACIFIC THAT IS ON THE MORE AMPLIFIED AND LESS PROGRESSIVE SIDE OF THE FULL ENVELOPE OF GUIDANCE SOLUTIONS OVER THE NEXT WEEK. THIS SEEMS QUITE REASONABLE CONSIDERING DOWNSTREAM BLOCKING.
WEATHER IMPACTS =============== PER THIS SOLUTION FROM SUNDAY THROUGH WEDNESDAY...WINDS AS HIGH AS HURRICANE-FORCE ARE EXPECTED TO LASH EXPOSED AREAS OF THE NORTHEAST/MID-ATLANTIC STATES (THE COAST AND TOPOGRAPHY)...LEADING TO POTENTIALLY SERIOUS COASTAL EROSION AND COASTAL FLOODING. THE TIMING OF THE FULL MOON AND THE BUILD-UP OF TIDES OVER MULTIPLE TIDAL CYCLES SHOULD EXACERBATE THE SITUATION ALONG THE COAST...PARTICULARLY IN CORNERS SUCH AS THE NEW YORK BIGHT. HEAVY RAINS ARE EXPECTED TO FOCUS WITHIN SANDY'S WARM CONVEYOR BELT CIRCULATION AS WELL AS ALONG ITS DRAPING WARM FRONT FROM THE NORTHERN MID-ATLANTIC STATES AND INTO NEW ENG AND ERN CANADA...WITH LOCAL AMOUNTS EXCEEDING 10 INCHES POSSIBLE. THIS SOLUTION WOULD MEANWHILE ALLOW SIGNIFICANT SNOWS WITHIN ITS COMMA HEAD PATTERN UNDER THE SUPPORTING/COOLING MID-UPPER LEVEL TROUGH TO FALL AS CENTERED/FOCUSING OVER THE CENTRAL THEN NRN APPALACHIANS AND VICINITY. LAKE EFFECT SNOWS ACROSS THE EASTERN GREAT LAKES ARE POSSIBLE THROUGH THE PERIOD WITHIN THE CYCLONIC FLOW IN THE SYSTEM'S COLD SECTOR. AS THE CYCLONE GRADUALLY WEAKENS AS IT PULLS NORTHWARD THURSDAY INTO NEXT FRIDAY...WINDS SHOULD SLACKEN AS SHOULD NERN US PRECIPITATION AMOUNTS/COVERAGE...BUT EXPECT THAT AMOUNTS WILL REMAIN SIGNIFICANT CONSIDERING TROPICAL ORIGIN OF THE LOW.
UPSTREAM...A SOLUTION CLOSEST TO THE 00 UTC ECMWF ENSEMBLE MEAN SLOWS FROM HPC CONTINUITY SLIGHTLY THE INLAND PROGRESSION OF ERN PACIFIC SYSTEM ENERGY ACROSS THE WRN US MID-LATE NEXT WEEK...MORE SLUGGISH TO BUMP INTO A LEAD AMBIENT MEAN MID-UPPER RIDGE POSITION. THIS SOLUTION OFFERS STRONG POTENTIAL FOR HEAVY PRECIPIATION/MOUNTAIN SNOWS ACROSS MUCH OF THE NWRN US NEXT WEEK...INCLUDING THE THREAT FOR MEDIUM RANGE DAYS 4-7 LIQUID AMOUNTS UPWARDS TO 5 INCHES OVER MOST FAVORED PAC NW COASTAL TERRAIN AND THE CASCADES...WITH DECREASING BUT STILL WIDESPREAD AMOUNTS SPREADING FURTHER INLAND.
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