EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1110 AM EST MON FEB 13 2017 VALID 12Z THU FEB 16 2017 - 12Z MON FEB 20 2017 ...VERY HEAVY PRECIPITATION EXPECTED ACROSS CA AND INTO THE DESERT SOUTHWEST... ...PATTERN OVERVIEW/MODEL EVALUATION/PREFERENCES... TO COMMENCE THE PERIOD ON THURSDAY MORNING...SPLIT FLOW WILL REMAIN A FIXTURE IN THE MID-LATITUDE PATTERN. WHILE ANOTHER BROAD CIRCULATION LINGERS ACROSS CENTRAL MX...HIGHLY AMPLIFIED FLOW WILL DOMINATE THE NORTHERN STREAM. THE AFOREMENTIONED SYSTEM ACROSS MX SHOULD BE A SLOW MOVER AS IT EVENTUALLY REACHES THE OZARKS BY 18/0000Z WITH EVENTUAL LOW PRESSURE DEVELOPING OFF THE SOUTHEASTERN U.S. COAST BY EARLY SUNDAY MORNING. CONSIDERING THE NORTHERN STREAM...THE MOST POTENT OF THE FEATURES IS A STRONG MID-LEVEL LOW WHICH MAY BE AROUND 510-DM ON 16/1200Z AS NOTED IN SOME OF THE MORE INTENSE MODELS. THE ATTENDANT COASTAL CYCLONE SHOULD BE EXITING TOWARD NOVA SCOTIA AT THIS TIME WITH CONSIDERABLE WINTER PRECIPITATION THREATS ACROSS NEW ENGLAND. TO THE WEST OF THIS LARGE HEIGHT ANOMALY...EXPANSIVE RIDGING WILL EXTEND ACROSS THE ROCKIES/GREAT PLAINS REGION. HOWEVER...THIS WILL FLATTEN AND ADVANCE EASTWARD IN RESPONSE TO A PERTURBATION IN THE FLOW WHICH LEADS TO AMPLIFICATION ACROSS SOUTH-CENTRAL CANADA BY THE CONCLUSION OF THE WORK WEEK. LOOKING FARTHER UPSTREAM...A SYSTEM WHICH LURKS WELL OFF THE WEST COAST WILL BECOME A MAIN PLAYER IN THE PATTERN BY FRIDAY/SATURDAY ACROSS THE SOUTHWESTERN U.S. A POWERFUL SURFACE LOW WHICH MANY PIECES OF GUIDANCE SUGGEST MAY DEEPEN TO BELOW 990-MB AS IT CROSSES NORTHERN CA LATE FRIDAY WILL BE A FOCUS FOR ANOTHER ROUND OF HEAVY PRECIPITATION ACROSS THE STATE OF CA. THIS BAND OF HEIGHT FALLS SHOULD NUDGE EASTWARD WHILE EVENTUALLY REACHING THE CENTER OF THE COUNTRY BY THE FEBRUARY 19/20 TIMEFRAME. IN TERMS OF THE GUIDANCE...THERE IS DECENT AGREEMENT WITH THE COASTAL LOW EXITING NEW ENGLAND ALTHOUGH ENSEMBLE SCATTER LOW PLOTS SHOW THE 00Z CMC ENSEMBLES LAGGING THE REST OF THE PACK. EVENTUALLY THIS MOVES OUT OF THE PICTURE AS MODEL DIFFERENCES DO INCREASE IN TIME. ELSEWHERE...THERE ARE ISSUES WITH HOW QUICKLY TO EJECT THE SYSTEM OUT OF CENTRAL MX WITH A STRONGER TREND NOTED AS NUMEROUS CLOSED 564-DM HEIGHT CONTOURS HAVE APPEARED IN THE GUIDANCE AT 500-MB. A MORE AGGRESSIVE SOLUTION HAS BEGUN SHOWING UP IN THE GUIDANCE WITH THE PAST FEW ECMWF MODELS LEADING THE WAY WITH THIS IDEA. ELSEWHERE...THERE ARE MARKED DIFFERENCES WITH HOW AGGRESSIVE TO BE WITH THE DEEP SURFACE LOW MOVING TOWARD NORTHERN CA. MANY OF THE RECENT SOLUTIONS...PARTICULARLY THE GFS HAVE DEPICTED SURFACE PRESSURES POSSIBLY NEARING 985-MB BY 18/0000Z. ON THE OUTSIDE LOOKING IN IS THE 00Z ECMWF ENSEMBLE MEAN WHICH IS DECIDEDLY WEAKER. GIVEN OTHER ENSEMBLES ARE STRONGER...THIS SEEMS TO BE MUCH TOO WEAK. EVENTUALLY THE ECMWF CAMP BECOMES MORE PROGRESSIVE EJECTING THESE HEIGHT FALLS TOWARD THE INTERMOUNTAIN WEST WITH SUCH DIFFERENCES CONTINUING THROUGH THE REMAINDER OF THE PERIOD. THIS ALSO APPLIES TO AN UPSTREAM PACIFIC SYSTEM WHICH IS EXPECTED TO AFFECT THE WEST COAST VERY LATE IN THE PERIOD OR JUST BEYOND THE MEDIUM RANGE. REGARDING PREFERENCES...INITIALLY TOOK AN EQUAL SPLIT OF THE 00Z GFS/ECMWF FOR THURSDAY BEFORE ADDING A BIT MORE OF THE 00Z GEFS/NAEFS ENSEMBLE MEANS TO ACCOUNT FOR INTENSITY DIFFERENCES WITH THE SURFACE LOW APPROACHING CA. MOVING TOWARD THE WEEKEND...BEGAN INCORPORATING ABOUT 60 PERCENT OF THE ENSEMBLE MEANS INTO THE MIX AS SPREAD MARKEDLY INCREASED...PARTICULARLY WITH THE SOUTHWESTERN U.S. SYSTEM. NOT COMPLETELY SOLD ON WHAT THE 00Z ECMWF ENSEMBLE MEAN WAS ADVERTISING SO KEPT ITS CONTRIBUTIONS AT NO MORE THAN 35 PERCENT THROUGHOUT THE FORECAST. DECIDED TO WEIGHT THE 00Z GEFS/NAEFS ENSEMBLE CAMPS A BIT MORE HEAVILY WHICH WOULD SLOW DOWN THE EASTERN PACIFIC/WESTERN U.S. SYSTEMS. ...WEATHER HIGHLIGHTS/THREATS... THE STRONG COASTAL LOW WILL BE EXITING NEW ENGLAND WHICH SHOULD ALLOW FOR MUCH OF THE HEAVY SNOWFALL TO END ACROSS THE REGION. COOL/BREEZY CONDITIONS ARE LIKELY IN ITS WAKE GIVEN THE TIGHT PRESSURE GRADIENT NOTED IN THE GUIDANCE. EVENTUALLY THE BIGGEST THREAT SHIFTS TO CA WHERE HEAVY RAINFALL IS LIKELY WITH SEVERAL INCHES POSSIBLE UP AND DOWN THE COAST AS WELL ACROSS THE SIERRA NEVADA RANGE. HIGHER ELEVATIONS CAN EXPECT WINTRY PRECIPITATION AS SNOW LEVELS BEGIN TO DROP AS THE UPPER TROUGH CROSSES THE REGION. GIVEN AN ALREADY WET WINTER ACROSS THE STATE...SOME HYDROLOGIC ISSUE CANNOT BE RULED OUT...ESPECIALLY IN AREAS MORE SUSCEPTIBLE TO SUCH PROBLEMS LIKE BURN SCARS AND URBANIZED CORRIDORS. GIVEN THE STRONG PRESSURE GRADIENT EXPECTED...GUSTY WINDS ARE LIKELY TO ACCOMPANY THIS INCOMING SYSTEM. ELSEWHERE...AN INCREASING THREAT FOR MODERATE/LOCALLY HEAVY RAINFALL IS POSSIBLE ACROSS THE SOUTHEASTERN U.S. GIVEN THE DEVELOPMENT OF LOW PRESSURE OFF THE COAST THIS WEEKEND. IN TERMS OF TEMPERATURES...THE PATTERN WILL FEATURE ABUNDANT WARMTH WHICH WILL EXPAND FROM WEST TO EAST THROUGHOUT THE PERIOD. INITIALLY...FORECAST HIGHS ON THURSDAY SHOULD BE 15 TO 25 DEGREES ABOVE CLIMATOLOGY ACROSS THE CENTER OF THE COUNTRY WITH SOME WARMTH EXPANDING EASTWARD INTO THE WEEKEND. THE ONLY TRUE COOL SPOT ACROSS THE U.S. WILL BE ALONG THE WEST COAST GIVEN THE PERSISTENCE OF CLOUDS AND PRECIPITATION. RUBIN-OSTER