EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1059 AM EST SUN JAN 21 2018 VALID 12Z WED JAN 24 2018 - 12Z SUN JAN 28 2018 ...OVERVIEW... EXPECT FLOW ALOFT TO MAINTAIN MODERATE PROGRESSION BUT BECOME AMPLIFIED WITH TIME. A VIGOROUS TROUGH OVER THE EASTERN PACIFIC AS OF EARLY WED WILL PROCEED INTO THE CENTRAL U.S. BY NEXT WEEKEND WHILE THE DOWNSTREAM RIDGE WILL REACH THE WESTERN ATLANTIC AND JOIN WITH RIDGING ALREADY PRESENT EAST OF THE BAHAMAS. BEHIND THE UPPER TROUGH, A STRONG RIDGE WILL BUILD INTO THE WEST WITH SOME AID FROM A DEEP TROUGH/CLOSED LOW BECOMING ANCHORED OVER THE NORTHEAST PACIFIC/GULF OF ALASKA BY THE LATTER HALF OF THE PERIOD. THE TROUGH CROSSING THE LOWER 48 WILL SPREAD A BROAD AREA OF ACTIVE WEATHER ACROSS THE LOWER 48 AND SUPPORT A WAVY FRONTAL SYSTEM SEPARATING VERY CONTRASTING AIR MASSES, WHILE EVOLUTION OF EASTERN PACIFIC FLOW MAY BRING INCREASING MOISTURE INTO PARTS OF THE PACIFIC NORTHWEST NEXT WEEKEND. ...GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES... BROADLY SPEAKING GUIDANCE CONTINUES TO SHOW BETTER THAN AVERAGE AGREEMENT WITH FAIRLY GOOD CONTINUITY IN MOST RESPECTS, WITH SOME EMBEDDED DETAIL/TIMING QUESTIONS EARLY-MID PERIOD AND LINGERING UNCERTAINTIES ON A SOMEWHAT LARGER SCALE BY NEXT WEEKEND. OF NOTE OFF THE PACIFIC NORTHWEST ON DAY 3 WED, THE 00Z ECMWF IS ON THE SOUTHERN FRINGE OF GUIDANCE INCLUDING NEARLY ALL 00Z GEFS/ECMWF/CMC ENSEMBLE MEMBERS FOR SURFACE LOW PRESSURE NEARING THE COAST. FARTHER EAST DIFFERENCES/ADJUSTMENTS ARE GENERALLY DUE TO SMALLER SCALE SHORTWAVE DETAILS THAT ARE TYPICALLY NOT WELL RESOLVED BEYOND THE SHORT RANGE TIME FRAME. THERE IS GOOD CONTINUITY WITH THE CONCEPT OF LOW PRESSURE TRACKING ACROSS THE EXTREME NORTHERN TIER BUT SOME ENSEMBLES AND THE UKMET ARE HINTING AT THE POTENTIAL FOR SLIGHTLY FASTER TIMING. THE FULL CONSENSUS OF MODELS/MEANS IS STILL FAIRLY CLOSE TO CONTINUITY BUT THE TRAILING FRONT HAS ADJUSTED A BIT FASTER DUE TO THE LOW BEING SOMEWHAT STRONGER THAN FORECAST YESTERDAY. AS THE WAVY FRONT HEADS INTO THE EAST DURING THE WEEKEND THERE IS DECENT AGREEMENT THAT EVOLUTION OF UPPER TROUGH ENERGY SHOULD SUPPORT CONSOLIDATION OF LOW PRESSURE BETWEEN THE EASTERN GREAT LAKES AND CENTRAL APPALACHIANS BY THE END OF DAY 7 SUN. BY NEXT WEEKEND SOME UNCERTAINTY ARISES WITH RESPECT TO THE PRECISE AMPLITUDE OF EASTERN PACIFIC/WESTERN U.S. FLOW ALOFT. GEFS/ECMWF MEANS HAVE BEEN FAIRLY STABLE OVER THE PAST DAY BUT INDIVIDUAL ENSEMBLE MEMBERS AND GFS/ECMWF RUNS DISPLAY SUFFICIENT DIFFERENCES TO HAVE AN IMPACT ON DETAILS OF THE STRONG FLOW OF MOISTURE (WITH SOME CONNECTION TO LOWER LATITUDES) EXPECTED TO REACH THE PACIFIC NORTHWEST/VANCOUVER ISLAND AREA. DETAILS HERE WILL ULTIMATELY AFFECT THE CHARACTER OF THE UPPER TROUGH REACHING THE PLAINS/MS VALLEY LATE IN THE PERIOD. MANY CANADIAN ENSEMBLES ARE STILL FLATTER THAN MOST OTHER GUIDANCE BUT NOT TO THE DEGREE SEEN YESTERDAY, WHILE THE OPERATIONAL 00Z CMC HAS ADJUSTED TOWARD ITS MEAN VERSUS YESTERDAY'S 00Z RUN THAT WAS MORE SIMILAR TO THE GFS/ECMWF AND THEIR MEANS. FOR DAYS 3-5 WED-FRI A BLEND OF THE 00Z/06Z GFS AND 00Z ECMWF ALONG WITH LESSER WEIGHT OF THE 00Z CMC AND UKMET REPRESENTED CONSENSUS WELL FOR THAT PART OF THE FORECAST, WITH INPUT FROM REMAINING MODELS HELPING TO DOWNPLAY THE LESS CONFIDENT 00Z ECMWF DETAILS OFF THE PACIFIC NORTHWEST ON WED. DAYS 6-7 SAT-SUN BEGAN TO INCLUDE 06Z GEFS/00Z ECMWF ENSEMBLE MEANS ALONG WITH THE GFS/ECMWF RUNS (WHILE PHASING OUT THE CMC) TO RESOLVE UNCERTAINTY OF SPECIFICS WITHIN THE FAIRLY AGREEABLE LARGE SCALE PATTERN. ...SENSIBLE WEATHER HIGHLIGHTS/THREATS... THE STRONG SYSTEM HEADING INTO THE WEST WILL INITIALLY FOCUS HEAVIEST RAIN/HIGHER ELEVATION SNOW OVER/NEAR NORTHERN CALIFORNIA ON WED WITH LESSER AMOUNTS TO THE NORTH/NORTHEAST. PRECIP SHOULD LESSEN IN INTENSITY SOMEWHAT, BUT STILL WITH POTENTIAL FOR LOCALIZED ENHANCEMENT, AS MOISTURE EXTENDING SOUTH AND EAST EVENTUALLY CROSSES MUCH OF THE NORTHERN/CENTRAL WESTERN STATES THROUGH LATE WEEK. EMERGENCE OF THE LEADING COLD FRONT OVER THE PLAINS BY FRI AND FLOW AROUND EASTERN U.S. INTO WESTERN ATLANTIC HIGH PRESSURE WILL BRING GULF OF MEXICO MOISTURE NORTHWARD, EVENTUALLY GENERATING A LARGE AREA OF PRECIP OVER THE EASTERN HALF OF THE COUNTRY FROM FRI NIGHT THROUGH THE WEEKEND. THE FULL ARRAY OF MODEL/ENSEMBLE GUIDANCE CURRENTLY SIGNALS TWO AREAS THAT SHOULD BE MOST FAVORED FOR HEAVY RAINFALL POTENTIAL, ONE FROM THE SOUTH-CENTRAL MS VALLEY INTO THE OH VALLEY/LOWER GREAT LAKES AND A SECOND FROM THE CENTRAL/EASTERN GULF COAST INTO SOUTHERN APPALACHIANS/MID ATLANTIC. EXTREME NORTHERN TIER LOCATIONS MAY SEE SNOW IN THE VICINITY OF ONE OR MORE SURFACE WAVES. A MAJORITY OF THE WEST WILL SEE A DRIER TREND BY NEXT WEEKEND AS THE ABOVE SYSTEM DEPARTS. THE ONE EXCEPTION WILL BE OVER PORTIONS OF THE PACIFIC NORTHWEST INTO VANCOUVER ISLAND AND POSSIBLY TO SOME DEGREE INTO THE EXTREME NORTHERN ROCKIES AS EVOLUTION OF FLOW ALOFT SUPPORTS A POTENTIAL ATMOSPHERIC RIVER EVENT. CURRENTLY THE FAVORED TERRAIN OVER WA AND VANCOUVER ISLAND HAVE THE HIGHEST PROBABILITY OF SEEING HEAVY PRECIP NEXT WEEKEND BUT THERE IS ENOUGH SPREAD IN GUIDANCE TO ALLOW FOR THE POTENTIAL THAT SOME OF THIS MOISTURE COULD EXTEND SOMEWHAT FARTHER SOUTH. AROUND MIDWEEK EXPECT MOSTLY LIGHT LAKE EFFECT SNOW IN THE WAKE OF THE STRONG/WINDY SHORT RANGE SYSTEM TRACKING AWAY FROM NEW ENGLAND, WHILE A FRONTAL PASSAGE MAY BRING A PERIOD OF RAIN TO SOUTHERN FLORIDA. WARM FLOW AHEAD OF THE SYSTEM PROGRESSING INLAND FROM THE WEST COAST WILL INITIALLY BRING MODERATELY ABOVE NORMAL READINGS TO THE WESTERN STATES AND THEN MUCH ABOVE NORMAL TEMPS TO THE CENTRAL-EASTERN STATES THU ONWARD, WITH SOME PLUS 15-30F ANOMALIES. TRAILING COLDER AIR CROSSING THE WEST LATE WEEK AND REACHING THE CENTRAL U.S. NEXT WEEKEND SHOULD BE LESS EXTREME, ONLY 5-15F BELOW NORMAL AT MOST LOCATIONS. RAUSCH