HPC - As of now, polar westerlies will have little, if any, influence on 97L (future Irene)
Posted by DTB_2009 on 8/20/2011, 3:57 pm
In case you are not a regular on this board, the HPC produces the official NWS medium range forecast and coordinates with the NHC on tropical cyclone forecast positions. Of course, a 6 or 7 day forecast is HIGHLY UNCERTAIN and subject to change. http://www.hpc.ncep.noaa.gov/basicwx/day0-7loop.html EXTENDED FORECAST DISCUSSION NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD 330 PM EDT SAT AUG 20 2011 VALID 12Z TUE AUG 23 2011 - 12Z SAT AUG 27 2011 MODELS DEPICT A SLOWLY EVOLVING LONGWAVE PATTERN OVER NORTH AMERICA DURING THE UPCOMING MEDIUM RANGE PERIOD. AN UNUSUALLY STRONG MID LEVEL HEIGHT ANOMALY WILL REMAIN NEARLY STATIONARY OVER THE DAVIS STRAIT AND GREENLAND...FAVORING BROAD UPPER TROUGHS FROM HUDSON BAY SOUTHWARD TO THE GREAT LAKES...AND FROM ALASKA ACROSS THE NORTHEAST PACIFIC. IN BETWEEN...A FLAT RIDGE WILL DRIFT EWD FROM THE PACIFIC NW TO MT BY THE END OF THE PERIOD. THE LONGWAVE PATTERN IS SLIGHTLY PROGRESSIVE ABOVE LATITUDE 40N BUT STATIONARY S OF THAT LATITUDE. DAY-TO-DAY DIFFERENCES ARE AVERAGE STARTING DAY 3/TUE...BUT QUICKLY BECOME LARGE VICINITY OF THE STRONG AND COMPLEX UPPER TROUGH OVER THE GULF OF ALASKA. THEY ALSO BECOME LARGE IN THE CARIBBEAN SEA/GULF OF MEXICO DUE TO INCREASINGLY DIFFERENT TRACK FORECASTS FOR THE TROPICAL WAVE NOW NEARING THE LEEWARD ISLANDS. TO ADDRESS THESE INCREASINGLY LARGE DIFFERENCES...WE USED A BLEND OF ECMWF/GEFS MEAN/ECMWF MEAN FOR DAYS 3-7. THIS GAVE ONLY SLIGHTLY DIFFERENT RESULTS THAN THE BLEND USED IN THE PRELIMS. FINAL GRAPHICS WERE LEFT UNCHANGED FROM THE UPDATED PRELIM. 00Z/20 MODELS HAD TRENDED SLIGHTLY TO THE RIGHT OF PRIOR FORECASTS WITH THE TROPICAL WAVE NEARING THE LEEWARD ISLANDS WITH THE 00Z OPERATIONAL ECMWF CLOSEST TO THE DETERMINISTIC CONSENSUS AND GLOBAL ENSEMBLE MEANS. 12Z MODELS DETERMINISTIC MODELS RANGE FROM TRACKING THE SYS ON THE LEFT SIDE OF THE ENVELOPE INTO THE GULF OF MEXICO AS PER THE CANADIAN/UKMET...TO THE RIGHT SIDE OF THE ENVELOPE ALONG THE E COAST OF FL AS PER THE ECMWF/UKMET. FOR NOW...WE HAVE USED THE LATEST COORDINATION POINTS AGREED UPON FROM THE NHC AND EXTRAPOLATE ONE DAY FORWARD FOR THE NEW DAY 7 POINT. THIS WILL PLACE IT OFF THE W COAST OF FL IN THE EASTERN GULF ON SATURDAY....TO THE LEFT OF THE 12Z ECMWF/GFS POSITIONS.NEW 00Z/20 ENSEMBLE SPAGHETTI PLOTS SHOWED THE DETERMINISTIC GFS GETTING ON THE FAR RIGHT OF THE ENVELOPE OF SOLUTIONS WITH THE TROPICAL SYS VICINITY OF FL BY NEXT WEEKEND. THE 06Z/20 GFS BACKED OFF FOR NEXT SAT ON BEING S FAR N WITH IT. THE DETERMINISTIC ECMWF CLUSTERS WELL WITH ITS OWN ENSEMBLE MEMBERS AND EVEN SOME GEFS ENSEMBLE MEMBERS. THE NEW 12Z GFS FOLLOWS ITS 00Z CONTINUITY REASONABLY WELL THRU DAY 5 ON ALL THE MAIN FEATURES. WE SEE BIGGER CHANGES FOR DAYS 6-7 AS TIMING/AMPLITUDE DIFFERENCES CROP UP ACROSS CANADA. THESE CHANGES WOULD REMOVE THE POLAR WESTERLIES ACROSS SRN CANADA EVEN FARTHER AWAY FROM HAVING ANY INFLUENCE ON THE TRACK OF THE TROPICAL SYS THREATENING THE SERN CONUS. IN FACT ALL THE 12Z DETERMINISTIC MODELS KEEP THE POLAR WESTERLIES FAR ENOUGH N ACROSS SRN CANADA THAT AFTER A PINCH OF ENERGY DROPS S INTO THE OH VLY THE FIRST PART OF THE MEDIUM RANGE PERIOD...THERE SHOULD BE MINIMAL...IF ANY...INTERACTION OF ANY SE CONUS TROPICAL SYS LATE NEXT WEEK WITH THE POLAR WESTERLIES. FLOOD |
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