EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1059 AM EST SAT FEB 24 2018 VALID 12Z TUE FEB 27 2018 - 12Z SAT MAR 03 2018 ...ANOTHER HEAVY RAIN EVENT FOR PARTS OF THE MID-MISSISSIPPI VALLEY/MID-SOUTH AND ONGOING FLOOD CONCERNS FROM SATURATED SOILS/SWELLING RIVERS... ...PATTERN OVERVIEW... EXPECT SOME ADJUSTMENT IN THE NORTH AMERICAN FLOW PATTERN DURING THE MEDIUM RANGE PERIOD. A BUILDING UPPER RIDGE ACROSS THE HIGH LATITUDES OF THE NORTH ATLANTIC WILL PROMOTE DEVELOPMENT OF A REX BLOCK ALONG APPROXIMATELY 30 DEG W LONGITUDE AT THE START OF THE FORECAST DAY 3 TUE. THE POSITIVE HEIGHT ANOMALY CENTER WILL GRADUALLY SHIFT WESTWARD AND LIKELY REACH THE LABRADOR SEA BY NEXT FRI-SAT, WITH THE AXIS OF THE REX BLOCK DIPOLE SHIFTING WEST AS WELL. AT THE SAME TIME POSITIVE HEIGHT ANOMALIES WILL ALSO BUILD NORTH FROM THE EAST-CENTRAL PACIFIC ACROSS ALASKA AND INTO THE ARCTIC OCEAN. THE IMPLICATIONS OF THIS EVOLUTION FOR WEATHER ACROSS THE CONUS ARE SOMEWHAT UNCLEAR. TELECONNECTIONS WITH RESPECT TO TEMP/PRECIP ANOMALIES ACROSS THE CONUS ARE HIGHLY SENSITIVE TO THE EXACT POSITION OF THE NORTH ATLANTIC RIDGE. IF THIS NEGATIVE NAO/AO PATTERN CAN PERSIST FOR LONG ENOUGH, IT WOULD FOSTER A COLDER AND DRIER PERIOD FOR THE EASTERN SEABOARD THOUGH LIKELY BEYOND THE END OF THIS FORECAST PERIOD. THERE IS AT LEAST LESS SENSITIVITY TOWARD THE IDEA OF THE CORE OF NEGATIVE HEIGHT ANOMALIES REMAINING WELL OFF THE EAST COAST TOWARD 60W LONGITUDE. OUT WEST, THE IMPLICATIONS SEEM CLEARER FOR A TIME WITH STRENGTHENING RIDGING ACROSS THE NORTH PACIFIC/ALASKA SUPPORTING PERSISTENT TROUGHING FROM THE WEST COAST TO THE NORTHERN ROCKIES. HOWEVER A GRADUAL WESTWARD SHIFT IN EMPHASIS OF STRONGEST POSITIVE HEIGHT ANOMALIES OVER THE NORTH PACIFIC COULD LEAD TO SOME FLATTENING OF EASTERN PACIFIC/WESTERN U.S. FLOW (IN MULTI-DAY MEAN TERMS) AROUND OR AFTER THE END OF THE PERIOD. MEANWHILE RISING HEIGHTS APPEAR LIKELY ACROSS THE SOUTH CENTRAL U.S. AS THE SUBTROPICAL RIDGE SHIFTS WEST ACROSS THE GULF OF MEXICO. ...GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES... CONFIDENCE HAS ACTUALLY DECREASED A BIT FOR SPECIFICS OF CENTRAL-EASTERN U.S. EVOLUTION DAY 4 WED ONWARD. THE 00Z ECMWF HAS FURTHERED ITS TREND INTRODUCED IN ITS 12Z/23 RUN, FEATURING NORTHERN TIER U.S. FLOW THAT IS SUFFICIENTLY PROGRESSIVE TO RESULT IN SLOWER TIMING OF THE UPPER LOW ENERGY EJECTING FROM THE EXTREME SOUTHWEST TUE-WED. THE ECMWF SCENARIO ALOFT LEADS TO SLOWER/SOUTHWARD LOW PRESSURE FROM THE PLAINS EASTWARD AND THEN INTERACTION WITH NORTHERN STREAM ENERGY PRODUCES A VERY DEEP/WESTWARD SYSTEM JUST OFF THE EAST COAST BY EARLY DAY 7 SAT. THE 00Z UKMET ADJUSTED TO THE SLOWER TIMING THROUGH THE DAY THU. THE ENSEMBLE MEANS, AND THE ECMWF MEAN IN PARTICULAR, HAVE PROVIDED THE MOST STABLE FORECAST THUS FAR IN MAINTAINING A PHASED UPPER TROUGH WHICH SUPPORTS A SURFACE LOW TRACK FROM THE SOUTH-CENTRAL PLAINS THROUGH MID-MS VALLEY AND LOWER GREAT LAKES FOLLOWED BY SECONDARY DEVELOPMENT OFF THE EAST COAST. THE GFS/CMC HAVE LOOSELY FOLLOWED THIS IDEA IN PRINCIPLE ALBEIT WITH SOME SIGNIFICANT DIFFERENCES IN TIMING/STRENGTH. SOME GFS RUNS MAY BE A LITTLE FAR NORTHWARD WITH THE LOW PRESSURE TRACK OVER THE NORTHEAST QUADRANT OF THE U.S. IN LIGHT OF THE RIDGE RETROGRADING ACROSS SOUTHERN GREENLAND. THE 00Z GFS COMPARES BETTER TO CONSENSUS IN THIS REGARD THAN THE 06Z RUN. OVER THE PAST FEW DAYS ECMWF RUNS HAVE BEEN VERY INCONSISTENT WITH EJECTION OF THE SOUTHWEST U.S. UPPER LOW/TROUGH AND THE MEAN PATTERN APPEARS TO FAVOR REASONABLE PROGRESSION. THUS FOR THE PURPOSES OF A SINGLE DETERMINISTIC FORECAST, PREFER TO MAINTAIN A SOLUTION CLOSE TO OR SLIGHTLY DEEPER THAN CONTINUITY BY WAY OF THE 06Z GEFS MEAN (MUCH BETTER DEFINED THAN THE 00Z RUN) AND 00Z ECMWF MEAN ALONG WITH SOME 00Z GFS/CMC INPUT. SOLUTIONS ARE CLOSE ENOUGH TO ALLOW FOR SOME ECMWF INCLUSION TUE INTO WED. ALONG THE WEST COAST THE 06Z GFS COMPARES LESS FAVORABLY TO MOST GUIDANCE WITH A FARTHER SOUTH/SOUTHEAST TRACK OF UPPER ENERGY/SURFACE LOW PRESSURE NEARING THE PACIFIC NORTHWEST BY LATE DAY 4 WED. SO AS WITH THE FORECAST BLEND FARTHER EAST, PREFERENCE GOES TO THE 00Z RUN FOR THE GFS COMPONENT. THEN THERE IS REASONABLE AGREEMENT THAT AMPLIFYING FLOW ALOFT WILL SUPPORT A FAIRLY DEEP UPPER LOW SETTLING JUST OFF THE PACIFIC NORTHWEST COAST BY DAY 6 FRI. TOWARD THE LATTER HALF OF THE PERIOD MODEL/ENSEMBLE SOLUTIONS BEGIN TO DIVERGE WITH RESPECT TO HOW MUCH UPSTREAM ENERGY CUTS THROUGH THE EAST-CENTRAL PACIFIC RIDGE AND THE EFFECT OF THIS ENERGY ON WEST COAST TROUGH PROGRESSION BY EARLY NEXT WEEKEND. GFS RUNS ARE STILL A BIT ON THE STRONG/PROGRESSIVE SIDE WITH THE UPSTREAM ENERGY-- EVENTUALLY LEADING TO A FASTER WESTERN TROUGH ALOFT-- BUT THEY ARE MUCH IMPROVED OVER THEIR RUNS FROM 24 HOURS AGO WHICH HAD NEARLY FLAT FLOW OVER THE EAST PACIFIC/WESTERN U.S. BY FRI. EVEN IF THE GFS IS SOMEWHAT EXTREME, THERE ARE GROWING SIGNALS FROM MODEL/ENSEMBLE GUIDANCE AS A WHOLE THAT ENOUGH ENERGY MAY COME THROUGH THE RIDGE TO RESULT IN MORE WEST COAST TROUGH PROGRESSION BY NEXT SAT THAN ADVERTISED BY THE 00Z CMC MEAN. OVERALL PREFERRED A 00Z MODEL CONSENSUS TRANSITIONING TOWARD THE 06Z GEFS/00Z ECMWF MEANS. LATE IN THE PERIOD MINORITY INPUT FROM THE 00Z GFS/CMC USED FOR THE EASTERN SYSTEM OFFSET FAST/SLOW TRAITS RESPECTIVELY FOR THE WEST COAST TROUGH TO KEEP THE FORECAST ANCHORED NEAR THE MEANS. ...WEATHER HIGHLIGHTS/THREATS... MID-LATE WEEK UPPER TROUGH SETTLING ALONG THE WEST COAST AND POSSIBLE EMBEDDED CLOSED LOW NEARING THE PACIFIC NORTHWEST WILL BRING INCREASING POTENTIAL FOR AREAS OF HEAVY RAIN/HIGHER ELEVATION SNOW OVER PORTIONS OF THE WEST. EXPECT HIGHEST 5-DAY TOTALS TO EXTEND FROM THE COASTAL/CASCADE AREAS OF THE PACIFIC NORTHWEST DOWN INTO NORTHERN CALIFORNIA AND THE SIERRA NEVADA RANGE. SOME ENHANCED PRECIP IS ALSO POSSIBLE ALONG THE REST OF THE CA COAST AND FARTHER INLAND INTO THE ROCKIES. ARRIVAL OF UPPER TROUGHING WILL LEAD TO A DECREASE IN SNOW LEVELS. AHEAD OF THIS LARGE SCALE TROUGH, A SHORTWAVE/EMBEDDED CLOSED LOW TRACKING THROUGH THE EXTREME SOUTHWEST EARLY IN THE PERIOD WILL BE ACCOMPANIED BY SOME RAIN AND HIGHER ELEVATION SNOW. EXPECT BELOW NORMAL TEMPS TO PERSIST OVER MUCH OF THE WEST INTO THE NORTHERN HIGH PLAINS THROUGH THE PERIOD, WITH SOME LOCATIONS SEEING ONE OR MORE DAYS OF MINUS 10-20F ANOMALIES. WIDESPREAD CONVECTION AND POTENTIALLY HEAVY RAINS WILL ONCE AGAIN POSE A THREAT ACROSS PORTIONS OF THE LOWER MS VALLEY INTO THE TN VALLEY, AND PERHAPS SOMEWHAT NORTH OR SOUTH OF THIS REGION, FROM LATE TUE THROUGH THU NIGHT AS MOIST LOW-LEVEL SOUTHERLY FLOW WITHIN A CYCLONE'S WARM CONVEYOR BELT SPREADS AMPLE MOISTURE NORTHWARD OVER A RETURNING WARM FRONT. FARTHER NORTH LOW PRESSURE TRACKING ACROSS THE PLAINS/MIDWEST/GREAT LAKES WED-THU WOULD BRING CHANCES OF SNOWFALL TO LOCATIONS NORTH OF THE LOW TRACK. CONFIDENCE IS LOW IN THE SPECIFICS AT THIS POINT GIVEN SIGNIFICANT DIFFERENCES IN SOME GUIDANCE, BUT POTENTIAL EXISTS FOR HEAVY RAIN TO EXTEND INTO PORTIONS OF THE MID-ATLANTIC OR NORTHEAST THU-FRI WITH SNOW POSSIBLE FOR NEW ENGLAND. BROAD POSITIVE HEIGHT ANOMALIES/UPSTREAM SURFACE LOW PRESSURE WILL KEEP TEMPERATURES ABOVE AVERAGE FOR MUCH OF THE CENTRAL/EASTERN U.S. TUE-WED WITH MIN TEMPS IN PARTICULAR LIKELY TO BE MUCH ABOVE NORMAL (POSSIBLY EXCEEDING RECORD WARM VALUES AT SOME LOCATIONS). DEPARTURE OF LOW PRESSURE INTO THE WESTERN ATLANTIC SHOULD BRING TEMPS DOWN CLOSER TO NORMAL BY FRI-SAT. RAUSCH/RYAN