EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 207 AM EDT SUN SEP 04 2016 VALID 12Z WED SEP 07 2016 - 12Z SUN SEP 11 2016 ...POTENTIAL FOR SIGNIFICANT SURGE OF TROPICAL MOISTURE INTO THE DESERT SOUTHWEST LATER THIS WEEK... ...OVERVIEW... THE AMPLIFIED PATTERN IN PLACE TO START THE MEDIUM RANGE PERIOD ON WEDNESDAY IS FORECAST TO BREAK DOWN INTO A MORE ZONAL FLOW ACROSS THE CONUS BY NEXT WEEKEND. THIS WILL HAVE THE DUAL EFFECT OF HELPING TO KICK OUT POST-TROPICAL CYCLONE HERMINE FROM OFF THE EAST COAST AS WELL AS HELP DRAW IN TROPICAL MOISTURE, POTENTIALLY ASSOCIATED WITH AN ORGANIZED SYSTEM, INTO THE SW CONUS. ...GUIDANCE ASSESSMENT AND PREFERENCES... PATTERN BREAKDOWNS ARE USUALLY ENSCONCED IN UNCERTAINTY. ALTHOUGH THE ENSEMBLES REMAIN IN GOOD AGREEMENT OVERALL, THEY HAVE BEEN SHIFTING IN SUCCESSIVE RUNS AND IT HAS BEEN HARD TO TELL IF THE DETERMINISTIC RUNS ARE LEADING OR FOLLOWING THE ENSEMBLES, ESPECIALLY BY NEXT WEEKEND. TREND HAS BEEN AWAY FROM A DEEPER WESTERN TROUGH IN FAVOR OF MULTIPLE NORTHERN STREAM SHORTWAVES MOVING ACROSS THE NORTHERN TIER. THIS MAY BE COMPLICATED BY THE INFUSION OF A TROPICAL SYSTEM INTO THE SW, WHICH HAS BEEN ADVERTISED BY THE ECMWF AND ITS ENSEMBLES MUCH MORE STRONGLY THAN THE GFS/GEFS. FOR NOW, A MULTI-MODEL/ENSEMBLE CONSENSUS APPROACH IS SUFFICIENT FOR THE CENTRAL CONUS, BUT PREFER TO GIVE THE ECMWF ENSEMBLES MORE WEIGHT IN THE DESERT SW THAN THE GEFS (THOUGH THE GEFS HAVE BEEN TRENDING WETTER; I.E., MORE MEMBERS BRINGING A SYSTEM OF SOME SORT NORTHWESTWARD THEN NORTHWARD ACROSS THE BAJA). 17Z NHC/WPC TROPICAL COORDINATION CALL FAVORED A MORE ROBUST SYSTEM MOVING INTO THE NORTHERN BAJA ON THURSDAY. IN THE EAST, THE NHC SIDED MOST CLOSELY TO THE 18Z GFS RATHER THAN THE QUICKER 12Z ECMWF. THIS OFFERED A GOOD COMPROMISE BETWEEN THE LARGER CLUSTER OF SLOWER GUIDANCE, CONTINUITY, AND THE QUICKER ECMWF ENSEMBLE MEAN. BY NEXT WEEKEND, WOULD NOT TRUST ANY MODEL SOLUTION VERBATIM. SPLIT THE DIFFERENCE BETWEEN THE 18Z GEFS MEAN AND 12Z ECMWF ENSEMBLE MEAN AS THERE IS CONSIDERABLE SPREAD UPSTREAM IN THE NORTH PACIFIC. ENSEMBLES DO AT LEAST AGREE ON MAINTAINING UPPER RIDGING ALONG 14OW AND 50W WITH TROUGHING IN BETWEEN ACROSS NORTH AMERICA. ...WEATHER HIGHLIGHTS/THREATS... ENSEMBLE QPF SPREAD FOR THE SOUTHWEST REMAINS LARGE, AND ANY RAIN AT ALL WOULD BE A FIRST IN SEVERAL MONTHS FOR SOME LOCATIONS (IN AN OTHERWISE TYPICALLY DRY MONTH). DETERMINISTIC OUTPUT OF RECENT ECMWF/UKMET RUNS HAS BEEN RATHER EXTREME -- ONE TO THREE INCHES OF RAIN FOR PORTIONS OF AZ AND SOUTHERN CALIFORNIA INCLUDING THE I-5 CORRIDOR (LA/SAN DIEGO). WITH THE PREFERRED TRACK THROUGH THE NORTHERN BAJA, PRECIP SHOULD FOCUS MOSTLY OR MAYBE ENTIRELY EAST OF THE SAN BERNARDINO/JACINTO MOUNTAINS. PLEASE SEE THE LATEST NHC OUTLOOKS FOR MORE INFORMATION. ELSEWHERE, PRECIP FOCUS WILL BE ALONG THE LEAD FRONTAL BOUNDARY MOVING THROUGH THE GREAT LAKES WED/THU AND THEN INTO THE NORTHEAST. QPF AXIS SHOULD SINK SOUTHWARD AS THE NEXT FRONT PUSHES THROUGH THE PLAINS ON FRIDAY AND THEN SOUTHEASTWARD THEREAFTER. TEMPERATURES WILL BE WELL ABOVE AVERAGE IN THE EAST AHEAD OF THE FRONT WITH WIDESPREAD 80S AND 90S, POSSIBLY NEAR RECORD HIGHS. FRACASSO