EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1103 AM EST MON FEB 23 2015 VALID 12Z THU FEB 26 2015 - 12Z MON MAR 02 2015 ...PATTERN CHANGE FOR THE LOWER 48... ...GENERAL OVERVIEW... BROAD TROUGHING WILL REMAIN CARVED OUT OVER MUCH OF THE NATION THROUGH THE END OF THE WORK WEEK...BUT A PATTERN CHANGE IS IN STORE FOR THE LOWER 48 THIS WEEKEND. SHORTWAVE ENERGY CROSSING THE GULF OF ALASKA ON THURSDAY WILL AMPLIFY WHILE DIVING SOUTH DOWN THE WEST COAST...EVENTUALLY LEADING TO AN ELONGATED MEAN TROUGH CENTERED OVER THE WESTERN U.S. BY SUNDAY. DOWNSTREAM...THE RETURN OF A SOUTHEAST RIDGE WILL RAISE HEIGHTS EAST OF THE MISSISSIPPI. ...MODEL EVALUATION AND PREFERENCES... ALTHOUGH MODEL GUIDANCE IS IN FAIRLY GOOD AGREEMENT WITH THE MEAN FLOW OVER THE LOWER 48...THERE IS STILL SOME UNCERTAINTY WITH THE SMALLER SCALE DETAILS. DETERMINISTIC RUNS APPEAR LOCKED INTO A SOLUTION FOR A SURFACE LOW STREAKING OFF THE SOUTHEAST COAST ON THURSDAY...BUT THE GFS STILL HINTS AT THE POSSIBILITY OF ANOTHER LOW DEVELOPING ALONG THE TRAILING COLD FRONT ON FRIDAY. MODELS ARE STILL STRUGGLING WITH THE EXACT EVOLUTION OF THE TROUGH DIGGING IN OVER THE WEST THIS WEEKEND...AND THERE IS STILL SOME UNCERTAINTY DOWNSTREAM WITH HEIGHTS BUILDING IN EAST OF THE MISSISSIPPI EARLY NEXT WEEK. DIFFERENCES ARE ESPECIALLY DRASTIC ON DAY 6...WHEN THE GFS HOLDS A COLD HIGH IN OVER THE MID-ATLANTIC STATES WHILE THE ECMWF MOVES IT WELL OFFSHORE. TO ACCOUNT FOR ALL THE DETAIL DIFFERENCES...THE WPC DAY 3-7 FORECAST STARTED WITH A BLEND OF THE 00Z GEFS MEAN/ECMWF ENSEMBLE MEAN. ...SENSIBLE WEATHER HIGHLIGHTS... IN THE WEST... DIGGING TROUGHING SHOULD BRING LOWER TEMPERATURES AND A WIDESPREAD CHANCE OF RAIN/SNOW TO NEARLY THE ENTIRE REGION BUT FOCUSED FROM THE PAC NW THROUGH THE GREAT BASIN TOWARD THE FOUR CORNERS. A SFC FRONT EAST OF THE DIVIDE WILL ENHANCE UPSLOPE PRECIP IN NM/CO/WY. AS THE MEAN LONGWAVE TROUGH LIFTS NORTHEASTWARD LATE THIS WEEKEND PRECIPITATION SHOULD BLOSSOM OVER AND THEN EAST OF THE PLAINS. ENSEMBLES HAVE A STRONG SIGNAL FOR WIDESPREAD RAIN ALONG/AHEAD OF THE SFC FRONT WITH SNOW TO THE NORTH AND NORTHWEST OF THE BOUNDARY...WHICH WILL PUSH TOWARD THE OH/TN VALLEYS AND INTO THE MID-ATLANTIC AND NORTHEAST EARLY NEXT WEEK...HOWEVER...THERE IS STILL A GREAT AMOUNT OF UNCERTAINTY WITH THE TIMING OF PRECIPITATION SPREADING EASTWARD. IN THE EAST... PESKY SYSTEM OFF THE SOUTHEAST COAST ON THURSDAY WILL EXIT INTO THE ATLANTIC AND ALLOW COLD CANADIAN HIGH PRESSURE TO SLIDE INTO THE MIDWEST AND OHIO VALLEY ON FRIDAY... SETTING UP ANOTHER COLD NIGHT. SOME RECORD LOWS AND LOW MAXES ARE POSSIBLE. THE HIGH WILL LIFT THROUGH NEW ENGLAND AND INTO ATLANTIC CANADA SETTING UP A RETURN FLOW AND MUCH MILDER AIR -- NEAR TO JUST ABOVE AVERAGE -- TO STREAM NORTHWARD EARLY NEXT WEEK. GERHARDT