EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 150 AM EDT SUN OCT 08 2017 VALID 12Z WED OCT 11 2017 - 12Z SUN OCT 15 2017 ...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... TELECONNECTIONS FROM PERSISTENT UPPER RIDGE ACROSS THE NORTH CENTRAL PACIFIC THROUGH THE MEDIUM RANGE WILL SUPPORT CONTINUED TROUGHING ACROSS THE WESTERN U.S. AND DOWNSTREAM EXPANSION OF THE SUBTROPICAL RIDGE FROM THE SOUTHEAST INTO THE SOUTHERN PLAINS. MODELS AND ENSEMBLE SHOW ABOVE AVERAGE AGREEMENT ON THE LARGE SCALE PATTERN, BUT DIFFERENCES ARISE SURROUNDING THE CHARACTER OF THE WESTERN U.S. TROUGH AND TIMING/AMPLITUDE OF SHORTWAVE EJECTING FROM THE TROUGH INTO THE CNTRL/NRN PLAINS. ONE SHORTWAVE IS FORECAST TO BE CROSSING THE MIDWEST/GREAT LAKES ON DAY 3 (WED). THE ECMWF AND NOW THE GFS HAVE TRENDED A BIT MORE AMPLIFIED WITH THIS WAVE OVER THE PAST DAY, WHICH SHOULD SUPPORT A MORE ROBUST FRONTAL WAVE AT THE SFC ACROSS THE OH VALLEY. FARTHER WEST, SHORTWAVE REACHING THE WEST COAST ON DAY 3 SHOWS GOOD AGREEMENT AMONG DETERMINISTIC/ENSEMBLE SOLUTIONS, BUT SOME DIFFERENCES ARISE AS THE WAVE AMPLIFIES ACROSS THE INTERIOR WEST, AND ENERGY EJECTS EASTWARD INTO THE NORTHERN PLAINS BY DAY 4 (THU). AS ANOTHER SHORTWAVE MOVES SOUTH AROUND THE NORTH PACIFIC RIDGE BY THU-FRI, THE GFS CONTINUES TO DIG ENERGY FARTHER SOUTH ALONG THE WEST COAST, LOWERING HEIGHTS MORE QUICKLY FROM THE COAST EAST INTO THE GREAT BASIN. THE 12Z ECMWF DIRECTS THE ENERGY MORE QUICKLY EASTWARD, WITH ANOTHER SHORTWAVE REACHING THE NRN/CNTRL PLAINS BY DAY 7 (SUN). ENSEMBLE MEANS SUGGEST THAT SOMETHING IN BETWEEN THESE TWO SOLUTIONS IS A GOOD POSSIBILITY, AND THE 00Z GFS TRENDED FASTER, TOWARD THE 12Z ECMWF. GIVEN THE ORIGIN OF THIS SHORTWAVE ACROSS THE HIGH LATITUDES OF RUSSIA DURING THE SHORT RANGE, WOULD EXPECT TO SEE CONTINUED RUN-TO-RUN VARIABILITY FOR A FEW DAYS. AS SUCH, PREFER TO LEAN TOWARD ENSEMBLE MEANS BY LATE IN THE FORECAST PERIOD. GIVEN THE FAST SOLUTION SHOWN FOR THIS WAVE BY THE 12Z ECMWF, THE MODEL DEVELOPS A DEEP SFC CYCLONE ACROSS THE MIDWEST BY SUN. ENSEMBLE SCATTER PLOTS SUGGEST THAT A SFC LOW IS LIKELY SOMEWHERE ACROSS THE CNTRL/NRN PLAINS OR MIDWEST BY SUN, BUT SPREAD IS QUITE LARGE AS TO THE SPECIFIC LOCATION. AT THIS TIME, THE WPC FORECAST WILL SHOW A WEAK SFC LOW ALONG THE FRONT ACROSS THE UPPER MIDWEST ON DAY 7, CLOSE TO THE ECMWF ENSEMBLE MEAN POSITION. THE WPC FORECAST WAS BASED HEAVILY ON A BLEND OF THE DETERMINISTIC 12Z ECMWF/18Z ECMWF DURING DAYS 3-5, GIVEN THE ABOVE AVERAGE LARGE SCALE AGREEMENT. FOR DAYS 6-7, GIVEN THE INCREASING SPREAD SURROUNDING THE WESTERN TROUGH AND ENERGY EJECTING EAST, WEIGHTING OF THE GEFS/NAEFS/ECENS MEANS WAS INCREASED SUBSTANTIALLY. ...WEATHER HIGHLIGHTS/THREATS... THE SHORTWAVE CROSSING THE OH VALLEY/GREAT LAKES DAYS 3-4 ALONG THE ASSOCIATED SFC FRONTAL WAVE WILL SUPPORT RAINFALL/CONVECTION FROM THE GREAT LAKES SOUTH ACROSS THE CNTRL APPALACHIANS/MID-ATLANTIC TO THE SOUTHEAST. LOCALLY HEAVY RAINFALL IS POSSIBLE, BUT WIDESPREAD HEAVY RAIN IS NOT ANTICIPATED. A COLD FRONT MOVING FROM THE ROCKIES INTO THE CENTRAL U.S. DAYS 3-5 WILL STRUGGLE TO PRODUCE MUCH IN THE WAY OF PRECIPITATION, AS MODELS SHOW VERY POOR MOISTURE RETURN AHEAD OF THE SYSTEM EARLY ON. BY DAYS 6-7, MOISTURE RETURN INCREASES AS THE FRONT STALLS ACROSS THE CNTRL PLAINS/MIDWEST, AND MODELS SHOW THE POTENTIAL FOR AT LEAST LOCALLY HEAVY PRECIPITATION BY THIS TIME GENERALLY ALONG THE FRONT FROM THE CNTRL PLAINS TO THE UPPER MIDWEST/GREAT LAKES. MOISTURE AND PRECIPITATION WILL GRADUALLY INCREASE ACROSS FLORIDA BY MID T0 LATE WEEK AS A BROAD EASTERLY WAVE APPROACHES. THE PACIFIC NORTHWEST WILL REMAIN WET THROUGH MUCH OF THE MEDIUM RANGE WITH THE AMPLIFYING WESTERN U.S. TROUGH AND MULTIPLE SHORTWAVES MOVING SOUTHWARD FROM BRITISH COLUMBIA INTO THE NORTHWEST. THE HEAVIEST PRECIP WILL FOCUS ALONG AREAS OF TERRAIN, WHERE HIGHER ELEVATION SNOWS WILL ALSO BE POSSIBLE. THE EASTERN HALF OF THE CONUS WILL SEE ABOVE AVERAGE TEMPERATURES THROUGH THE MEDIUM RANGE AS THE SFC FRONT REMAINS ACROSS THE CENTRAL U.S. AND THE UPPER RIDGE INITIALLY ACROSS THE SOUTHEAST EXPANDS WEST AND NORTH. HIGH TEMPERATURES RANGING FROM 5 TO 15 DEGREES ABOVE AVERAGE WILL BE COMMON THROUGH MUCH OF THE MEDIUM RANGE. FARTHER WEST, BELOW AVERAGE TEMPERATURES WILL PREVAIL AS THE TROUGH AMPLIFIES. BY LATE IN THE WEEK, HIGHS ACROSS MUCH OF THE GREAT BASIN/NORTHERN ROCKIES ARE FORECAST TO BE 10 TO 20 DEGREES BELOW AVERAGE. RYAN