| PRELIMINARY EXTENDED FORECAST DISCUSSION NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD 231 AM EST THU FEB 11 2010 VALID 12Z MON FEB 15 2010 - 12Z THU FEB 18 2010 A COMPLICATED LONGWAVE BLOCK...MOST INDICATIVE OF A REX-BLOCK...CONSISTING OF AN ELONGATED UPPER HIGH EXTENDING FROM GREENLAND INTO CENTRAL CANADA AND AN ELONGATED UPPER LOW OVER THE CANADIAN MARITIMES/GREAT LAKES/NEW ENGLAND...IS EXPECTED TO PREVAIL DURING THE MEDIUM RANGE PERIOD. THUS... THE DAY-TO-DAY DETAILS WILL LIKELY ORIGINATE WITHIN AND NEAR THE PERIPHERY OF THE BROAD UPPER LOW...WITH THE NEWEST GUIDANCE SHOWING GOOD AGREEMENT THAT A SMALL UPPER LOW WILL EJECT OUT OF THE CENTRAL CANADA BLOCK AND ROTATE SOUTHEASTWARD FROM THE UPPER MIDWEST TO THE MID-ATLANTIC REGION DAYS 3-5...WHILE PERIODIC SHORTWAVE TROUGHS CONTINUE TO ARRIVE INTO THE PACIFIC NORTHWEST. BY DAYS 6/7...THE SMALL UPPER LOW REACHING THE EAST COAST IS EXPECTED TO BECOME RE-ABSORBED BY THE BROADER UPPER LOW TO ITS NORTH...WHILE IN THE WEST THE LONGWAVE RIDGE JUST INLAND FROM THE WEST COAST IS EXPECTED TO RETROGRADE SLIGHTLY WHICH SHOULD SUPPORT DECREASING PRECIPITATION CHANCES OVER THE PACIFIC NORTHWEST. GIVEN RECENT TRENDS WITH THE SYSTEM REACHING THE DAKOTAS BY 72 HRS AND EXISTENCE OF THE LARGE SCALE BLOCK...RECOMMEND SOLUTIONS WHICH ARE GENERALLY NEAR THE SLOWER SIDE OF THE GUIDANCE. THE 00Z ECMWF DEPICTS THIS APPROACH BEST THROUGH THE ENTIRE PERIOD. HOWEVER... SOLUTION SPREAD...ESPECIALLY OVER THE PACIFIC...BECOMES TOO GREAT AFTER DAY 5 TO WARRANT A TRANSITION TO A 50/50 BLEND OF THE OPERATIONAL ECMWF AND THE GEFS MEAN TO BEST DEPICT THE DAY 6/7 DETAILS. JAMES EXTENDED FORECAST DISCUSSION NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD 201 PM EST WED FEB 10 2010 VALID 12Z SAT FEB 13 2010 - 12Z WED FEB 17 2010 AN UPPER HIGH NEAR GREENLAND AND A MEAN UPPER LOW OVER THE CANADIAN MARITIMES... COMBINED WITH ABOVE NORMAL HGTS EXTENDING WWD ACROSS MUCH OF THE REMAINDER OF CANADA... SHOULD CAUSE UPSTREAM SHRTWVS ENTERING WESTERN CANADA/PACIFIC NORTHWEST TO SEPARATE INTO TWO OR MORE STREAMS. THIS SPLITTING OF FLOW IS FCST TO OCCUR MUCH FARTHER NORTH THEN SEEN IN RECENT WEEKS. MODELS SHOW WELL ABOVE AVERAGE AGREEMENT WITH THE LONGWAVE PATTERN. HOWEVER THERE ARE MEANINGFUL DIFFS IN THE TIMING/STRENGTH OF THE SHRTWVS REACHING WESTERN CANADA AND TO WHAT EXTENT THEY SEPARATE OVER CANADA. THERE ARE ALSO POTENTIAL ISSUES WITH HOW PIECES OF ENERGY MAY INTERACT WITH THE CANADIAN MARITIMES MEAN UPPER LOW. |