| PRELIMINARY EXTENDED FORECAST DISCUSSION NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD 902 AM EST TUE DEC 21 2010 VALID 12Z SAT DEC 25 2010 - 12Z TUE DEC 28 2010 ...CONTINUED WET ACROSS THE PACIFIC NORTHWEST... ...BELOW NORMAL TEMPS FROM THE PLAINS EASTWARD... ...POTENTIAL EAST COAST WINTER STORM SAT-MON... THE NORTH ATLANTIC OSCILLATION INDEX REMAINS NEGATIVE THIS MORNING WITH A LARGE POSITIVE MID-LEVEL HEIGHT ANOMALY EXTENDING FROM GREENLAND WESTWARD TOWARD HUDSON BAY WHICH IS BLOCKING THE UPSTREAM FLOW...MUCH OF WHICH IS EXPECTED TO ORIGINATE FROM AN INTENSE POLAR JET STREAM TRAVERSING THE BASE OF A QUASI-STATIONARY UPPER CYCLONE OVER THE GULF OF ALASKA THROUGH THE MEDIUM RANGE PERIOD. MODEL AGREEMENT WITH THE LARGER-SCALE ASPECTS IS QUITE GOOD TO BEGIN THE PERIOD. HOWEVER...THIS IS NOT THE CASE OVER THE CENTRAL CONUS WHERE THE FLOW BEGINS TO SPLIT...WITH THE 00-06Z GFS/00Z CANADIAN FASTER AND MORE AGGRESSIVE THAN THE 00Z ECMWF/UKMET IN ALLOWING THE FLOW TO PHASE OVER THE MISSISSIPPI/TENNESSEE VALLEYS BY DAYS 4/5. SEVERAL ASPECTS...INCLUDING RECENT SATELLITE TRENDS OVER THE EASTERN PACIFIC SHOWING AN INTENSE POLAR JET TRAVERSING THE WESTERN PERIPHERY OF AN ACCOMPANYING UPPER TROUGH...HIGHER SPATIAL RESOLUTION OF THE ECMWF...AND MORE SOPHISTICATED INITIALIZATION OF THE ECMWF...FAVOR AN ECMWF-LIKE SOLUTION VERSUS THE 00-06Z GFS...IN ALL AREAS. HOWEVER...GIVEN THAT EVEN THE ECMWF CAN NOT ACCOUNT FOR UNCERTAINTY IN INITIAL CONDITIONS...FEEL IT IS MOST PRUDENT TO ADD INCREASING WEIGHT FROM ITS OWN ENSEMBLE MEAN THROUGH THE ENTIRE PERIOD. THUS...THE UPDATED PRESSURES/FRONTS WILL BEGIN WITH A 90/10 ECMWF/ECMWF ENSEMBLE MEAN BLEND RESPECTIVELY DAY 3...WHILE GRADUALLY TRANSITIONING TOWARD A 40/60 ECMWF/ECMWF ENSEMBLE MEAN BLEND RESPECTIVELY BY DAY 7. REGARDING SIGNIFICANT IMPACTS...THE PACIFIC NORTHWEST EXTENDING PERHAPS AS FAR SOUTH AS THE SIERRAS WILL REMAIN UNDER THE INFLUENCE OF AN UNUSUALLY STRONG POLAR JET STREAM EMBEDDED WITH PERIODS OF HEAVY PRECIPITATION. DAYS 4/5 AND DAYS 6/7 QPF ACROSS THE WEST ARE EXPECTED TO RANGE BETWEEN 2 AND 5 INCHES POSSIBLY HIGHER...WITH THE LAST 2 RUNS OF THE ECMWF AND ITS MOST RECENT ENSEMBLE MEAN USED AS A 1ST GUESS. MEANWHILE...THE EVOLVING TROUGH OVER THE GREAT LAKES/OHIO VALLEY EARLY IN THE PERIOD FAVOR BELOW NORMAL TEMPERATURES FOR ALL AREAS FROM THE PLAINS EASTWARD...WITH SUBFREEZING TEMPERATURES POSSIBLY RETURNING THE ENTIRE GULF COAST REGION AND MUCH OF FLORIDA BY DAYS 6/7. FINALLY...AN ECMWF-LIKE SOLUTION FAVORS A WINTER STORM BEGINNING TO DEVELOP NEAR THE SOUTHEAST COAST BY DAY 4 BEFORE ACCELERATING UP THE EAST COAST DAYS 5/6. IT IS STILL TOO EARLY TO BE DEFINITIVE WITH THE PRECISE DETAILS INCLUDING TRACK OF THIS SYSTEM AS MODEL SPREAD CONSIDERING THE SPLIT-FLOW UPSTREAM BECOMES QUITE LARGE. WHILE THE 00Z ECMWF IS MOST PREFERRED...MANY OF ITS OWN ENSEMBLE MEMBERS ARE BOTH FASTER AND SLOWER WITH THE TRACK OF THE LOW WHICH HAS HUGE IMPLICATIONS FOR THE ENTIRE EAST COAST. THUS...GIVEN THE UNCERTAINTIES AND SPREAD...THE UPDATED TRACK ALONG THE EAST COAST WAS MORE HEAVILY WEIGHTED TOWARD ITS OWN ENSEMBLE MEAN ESPECIALLY LATE IN THE PERIOD. JAMES EXTENDED FORECAST DISCUSSION...CORRECTION TO LAST SECTION NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD 215 PM EST MON DEC 20 2010 VALID 12Z THU DEC 23 2010 - 12Z MON DEC 27 2010 ...SIGNIFICANT PRECIPITATION EXPECTED TO CONTINUE ALONG AND JUST INLAND OF THE WEST COAST FOR ANOTHER 5 TO 6 DAYS... TELECONNECTIONS WITH A RIDGE MOVING SLOWLY ACROSS THE PRAIRIE PROVINCES OF CANADA FAVORS NEARLY CLOSED CYCLONES ALOFT MOVING UNDER ITS BASE THROUGH THE NORTHERN ROCKIES/NORTHERN PLAINS LATER THIS WEEK...A SOLUTION BEST INDICATED BY THE 00Z CANADIAN AND 00Z GEFS AND ECMWF ENSEMBLE MEAN GUIDANCE. OTHERWISE...ENERGY MOVING OUT INTO THE MIDWEST EARLY ON IS EXPECTED TO SCOOP UP SOME SHORTWAVE ENERGY MOVING OUT OF THE SOUTHWEST AND SOUTHERN PLAINS BEFORE LOSING LATITUDE AS IT MOVES THROUGH THE EAST UNDER THE BASE OF A PORTION OF THE POLAR VORTEX DESCENDING THROUGH EASTERN CANADA. THIS SOLUTION IS BEST REPRESENTED BY THE 06Z GFS/00Z CANADIAN/00Z ECMWF AND GEFS MEAN GUIDANCE. OVERALL...USED A COMPROMISE OF THE 00Z CANADIAN/00Z ECMWF ENSEMBLE MEAN...AS THE 06Z GFS MIGHT BE A TOUCH TOO QUICK PER ITS OCCASIONAL QUICK BIAS WITH SYSTEMS IN THE WESTERLIES...WHICH LED TO GOOD CONTINUITY WITH THE PRESSURES FROM SUNDAY AFTERNOON. THIS PATTERN SHOULD REMAIN MOIST ALONG THE WEST COAST...AND COLD ACROSS THE NORTHERN PLAINS/MIDWEST/MUCH OF THE EAST. WEST AND PLAINS THIS WEEKEND INTO EARLY NEXT WEEK... WITH STRONG RIDGING PERSISTING ACROSS WESTERN AND CENTRAL CANADA THROUGH THE PERIOD...A PACIFIC SYSTEM IS EXPECTED TO MOVE THROUGH THE REGION UNDERCUTTING THE RIDGE. TIMING ISSUES EXIST...WITH THE 06Z GFS THE QUICKEST. SINCE THE RIDGING ACROSS WESTERN AND CENTRAL CANADA IS SHOWING SOME PROGRESSION IN THE NORTHERN STREAM...A SLOWER PROGRESSION IS LIKELY IN THE SOUTHERN STREAM...SO WENT WITH THE NON-06Z GFS GUIDANCE HERE. OVERALL...THE 00Z CANADIAN HAD THE BEST ENSEMBLE MEAN SUPPORT...SO WENT WITH A 00Z CANADIAN/00Z ECMWF ENSEMBLE MEAN COMPROMISE WITH THIS SYSTEM. SIGNIFICANT PRECIPITATION ACROSS THE WEST IS EXPECTED FOR ANOTHER 5 TO 6 DAYS...WITH LOCAL LIQUID EQUIVALENT AMOUNTS OF 10 INCHES POSSIBLE ACROSS THE OLYMPIC PENINSULA...NORTHERN COASTAL RANGES OF CALIFORNIA AND SISKIYOU RANGE...AS WELL AS THE COASTAL RANGES OF SOUTHERN CALIFORNIA AND SOUTHERN WASATCH DURING THIS TIME FRAME. SOUTH AND EAST... THE 12Z ECMWF IS SLOW/MORE CLOSED THAN THE OTHER DETERMINISTIC GUIDANCE WITH ENERGY MOVING THROUGH THE SOUTHWEST AND SOUTHERN PLAINS...WITH LESS THAN ONE-QUARTER OF THE OVERALL 00Z GLOBAL ENSEMBLE GUIDANCE SUPPORTING ITS SOLUTION...IMPLYING IT IS TOO SLOW. HOWEVER...ALL OF THE 12Z GUIDANCE TRENDED SLOWER WITH THIS SYSTEMS PROGRESSION...SO SOME SLOWING WAS MADE TO THE MORNING PRESSURES. TELECONNECTIONS FARTHER NORTH IMPLY STRONGER ENERGY SHOULD BE MOVING THROUGH THE NORTHERN TIER OF THE COUNTRY RATHER THAN THE SOUTHERN TIER...WHICH SHOULD ABSORB THE SOUTHWESTERN ENERGY AS IT MOVES INTO THE PLAINS/MIDWEST. EITHER WAY...A SURFACE LOW IS EXPECTED TO MOVE THROUGH THE SOUTHERN PLAINS INTO/NEAR THE DEEP SOUTH BEFORE MOVING OFFSHORE MUCH OF THE EASTERN SEABOARD. THE BIGGER QUESTION IS HOW CLOSE TO THE NEW ENGLAND COAST THIS CYCLONE WILL TRACK. THE 00Z ECMWF IS THE MOST NORTHWESTERLY OF THE GUIDANCE AT THE SURFACE...AND IF THIS SURFACE LOW WERE MOVING UP THE EAST SIDE OF AN AMPLIFIED TROUGH RATHER THAN A PROGRESSIVE CLOSED CYCLONE...A MORE COASTAL TRACK FOR NEW ENGLAND WOULD MAKE SENSE. SINCE THIS DOES NOT APPEAR TO BE THE CASE...AND THE 12Z ECMWF BACKED OFF THIS IDEA SOMEWHAT...THE 00Z ECMWF CYCLONE TRACK ALONG THE EAST COAST IS PROBABLY TOO FAR NORTHWEST/TOO CLOSE TO THE COAST. OTHER GUIDANCE...INCLUDING THE ENSEMBLE MEAN SOLUTIONS...MOVE THIS SYSTEM CLOSER TO THE GULF STREAM...A NATURAL BAROCLINIC ZONE/AREA OF TEMPERATURE GRADIENT OFFSHORE THE EAST COAST...WHICH APPEARS MORE LIKELY CONSIDERING THE FLOW PATTERN ALOFT. THE 12Z CANADIAN MOVES IT DUE EAST AS IT EMERGES INTO THE ATLANTIC...TAKING NEW ENGLAND OUT OF THE PICTURE. DETAIL ISSUES WITH MID-LEVEL CYCLONE PLACEMENT ACROSS EASTERN CANADA HAVE LED TO INCREASED SPREAD OF THE LOW TRACK AS IT LEAVES THE EAST COAST. PER PREFERENCES ACROSS THE PLAINS AND WEST LATE IN THE PERIOD...STARTED WITH THE 00Z CANADIAN HERE...ALTHOUGH THE 00Z ECMWF ENSEMBLE MEAN/00Z GEFS MEAN/12Z GFS ARE REASONABLE ALTERNATIVES. EITHER WAY...WINTER WEATHER SHOULD SPREAD FROM THE CENTRAL PLAINS EASTWARD ACROSS THE TENNESSEE VALLEY THROUGH THE MID-ATLANTIC STATES...AS WELL AS SNOW SHOWERS UNDER THE BASE OF THE UPPER LEVEL LOW ACROSS THE GREAT LAKES AND NORTHEAST. THE IMPACT OF THIS SYSTEM ACROSS THE MID-ATLANTIC AND NORTHEAST REMAINS SOMEWHAT OF A QUESTION MARK...AS THE 12Z GUIDANCE HAS GENERALLY DRIED OUT OF THE MID-ATLANTIC STATES YET INCREASED AMOUNTS IN PORTIONS OF SOUTHEAST NEW ENGLAND. FOR THE TIME BEING THE MORE NORTHWESTERN 00Z ECMWF TRACK AND THE SLOW PACE OF THE 12Z ECMWF SOLUTION APPEAR TO BE LOWER PROBABILITY ALTERNATIVE SCENARIOS. ROTH |