EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1204 PM EDT THU SEP 22 2016 VALID 12Z SUN SEP 25 2016 - 12Z THU SEP 29 2016 ...HEAVY RAIN LIKELY FOR TEXAS EARLY NEXT WEEK BUT AN UNCERTAIN FORECAST THEREAFTER... ...OVERVIEW... THE MEDIUM RANGE FORECAST CONTINUES TO BE MORE UNCERTAIN THE USUAL. THE AMPLIFIED PATTERN MAY TURN INTO BLOCKED/STUCK FLOW EARLY NEXT WEEK BUT AFTER THIS...THE MODELS AND ENSEMBLES CONTINUE TO SHOW STRONG DISAGREEMENT. THIS TYPE OF FLOW WOULD MEAN A SURFACE FRONT MOVING SLOWLY FROM THE PLAINS AND BECOMING EITHER STATIONARY OR QUASI-STATIONARY BY MID-WEEK ACROSS THE MIDWEST. AFTER THAT...ITS STILL UNCERTAIN WHETHER THAT FRONT EVEN MAKES IT TO THE EAST COAST. OUT WEST...THERE IS RELATIVELY GOOD AGREEMENT THAT A COLD FRONT WILL PUSH ACROSS THE PACIFIC NORTHWEST/INTERMOUNTAIN WEST ON TUESDAY AND WEDNESDAY. ...GUIDANCE EVALUATION AND PREFERENCES... MODELS ARE IN PRETTY GOOD AGREEMENT THAT THE CORE OF LOWER HEIGHTS WILL EXTEND FROM THE FOUR CORNERS REGION NORTHEASTWARD TO THE HIGH PLAINS ON SUNDAY...INCLUDING AN UPPER LOW NEAR THE NORTH DAKOTA/CANADA BORDER. INCLUDING THE PRESENT MODEL RUNS...THIS IS THE POINT AT WHICH THE MODEL AGREEMENT DISINTEGRATES AND SOLUTIONS ARE LITERALLY ALL OVER THE PLACE FROM LATE DAY 4 THROUGH DAY 7. BUT LOOKING BEYOND THE MEDIUM RANGE PERIOD (DAY 9 AND 10)...THERE IS REMARKABLY GOOD AGREEMENT. SO ITS OUR FORECAST TIME WHICH PRESENTS A TRANSITION PERIOD AND A HUGE RANGE OF SOLUTIONS. WE CONTINUE TO BE RELUCTANT TO BITE ONTO ANY ONE SOLUTION SINCE THE RUN TO RUN CONTINUITY OVER THE PAST FEW DAYS HAS BEEN SO DIFFERENT BETWEEN EVEN THE DETERMINISTIC MODELS AND THE ENSEMBLES. THE FOCUS STILL SEEMS TO BE ON THE NORTHERN STREAM (NEAR THE GREAT LAKES) BUT THE IDEA OF A SOUTHERN STREAM SYSTEM (FOUR CORNERS REGION) CANNOT BE RULED OUT YET. THE DETERMINISTIC GFS SHOWS RELATIVELY GOOD RUN TO RUN CONTINUITY WHICH WOULD SUPPORT THE NORTHERN STREAM UPPER LOW AND A MORE BLOCKY PATTERN. YESTERDAYS 00Z AND 12Z RUNS OF THE DETERMINISTIC ECMWF AGREES WITH THIS AS WELL...BUT TODAYS 00Z RUN PICKED BACK UP ON THE FOUR CORNERS LOW...WHICH WOULD AGREE WITH THE LATEST UKMET AND CANADIAN DETERMINISTIC RUNS. AS FAR AS THE ENSEMBLES...RUN TO RUN CONTINUITY BEYOND DAY 4 IS NOT GREAT...BUT THE LATEST RUNS SUPPORT A WEAKER NORTHERN STREAM LOW AND ARE MORE PROGRESSIVE WITH IT. FOR THIS REASON...DECIDED TO NOT STRAY FAR FROM WPC CONTINUITY. DAYS 3 AND 4 INCLUDE AN EQUAL BLEND OF THE 00Z GFS/NAEFS/EC ENSEMBLE MEAN AND YESTERDAYS 12Z ECMWF. AFTER THAT...IT WAS AN EQUAL BLEND OF THE 00Z NAEFS/ECMEAN. THIS BLEND SUPPORTS A SLOWLY MOVING FRONT FROM THE PLAINS ON DAY 3 TO THE MIDWEST/OHIO VALLEY AND LOWER MISSISSIPPI VALLEY BY DAY 7. TO THE WEST, ENSEMBLES STILL SHOW LOWERING HEIGHTS INTO THE PAC NW BY TUESDAY, BRINGING A COLD FRONT TOWARD THE NORTHERN DIVIDE/ID/NORTHERN CALIFORNIA BY WED/THU. FARTHER SOUTH, THE NATIONAL HURRICANE CENTER IS MONITORING AN AREA OFF CENTRAL MEXICO FOR POSSIBLE TROPICAL DEVELOPMENT. THIS MAY LURK OFF BAJA CALIFORNIA NEXT WEEK. ...WEATHER HIGHLIGHTS/THREATS... DESPITE THE DISAGREEMENT IN THE ORIENTATION/EVOLUTION OF THE FLOW THROUGH TUESDAY, THE LONGWAVE SETUP REMAINS MOSTLY IN AGREEMENT WHICH STILL SUPPORTS A HEAVY RAINFALL THREAT TO TEXAS AND PERHAPS NORTHWARD ALONG THE FRONTAL BOUNDARY. A STALLED FRONT WOULD EXACERBATE THE EFFECT. AFTER TUESDAY, UPPER SUPPORT FOR HEAVY RAIN SHOULD EASE EITHER IF THE NORTHERN STREAM UPPER LOW FILLS OR MOVES EAST OR IF IT MEANDERS IN PLACE AS A VERTICALLY STACKED SYSTEM DUE TO THE FACT THAT HEIGHTS SHOULD BUILD WESTWARD THROUGH THE GULF NEXT WEEK PER THE ENSEMBLES. SANTORELLI/FRACASSO