EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1059 AM EST TUE FEB 20 2018 VALID 12Z FRI FEB 23 2018 - 12Z TUE FEB 27 2018 ...OVERVIEW... EXPECT A FAIRLY PERSISTENT LARGE SCALE PATTERN FEATURING A MEAN TROUGH OVER THE WEST AND A RIDGE OFF THE GULF/SOUTHEAST COASTS. MOST GUIDANCE EXPECTS THE RIDGE TO FLATTEN GRADUALLY WITH ITS CENTER DRIFTING FROM JUST NORTH OF THE BAHAMAS INTO THE GULF OF MEXICO. THIS FLOW REGIME WILL CORRESPOND TO BELOW NORMAL TEMPERATURES OVER MUCH OF THE WESTERN U.S. INTO THE NORTHERN PLAINS AND VERY WARM CONDITIONS--PERHAPS SETTING DAILY RECORDS ONE OR MORE DAYS--OVER THE THE EAST. FOCUSED BY INDIVIDUAL SHORTWAVES, THERE WILL BE PERIODS OF UNSETTLED WEATHER WITH LOW SNOW LEVELS OVER PORTIONS OF THE WEST WHILE A HEAVY RAINFALL THREAT WILL PERSIST APPROXIMATELY FROM THE SOUTHERN PLAINS NORTHEASTWARD THROUGH THE OHIO VALLEY. ...MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES... AS HAS GENERALLY BEEN THE CASE IN THIS PATTERN RECENTLY, THERE IS REASONABLE AGREEMENT FROM THE MULTI-DAY MEAN PERSPECTIVE BUT GREATER UNCERTAINTY FOR EMBEDDED DETAILS. THE MOST AGREEABLE FEATURE IN PRINCIPLE IS THE SHORTWAVE EJECTING FROM THE WEST FRI ONWARD, SUPPORTING SURFACE LOW PRESSURE THAT SHOULD REACH THE GREAT LAKES BY AROUND EARLY DAY 5 SUN. THE PARENT LOW SHOULD CONTINUE INTO CANADA THEREAFTER WHILE ANOTHER WAVE TRACKS OFF THE NEW ENGLAND COAST. THERE IS STILL A FAIR AMOUNT OF MODEL/ENSEMBLE SPREAD FOR DEPTH AND TRACK THOUGH. 00Z/06Z GFS RUNS ARE SIGNIFICANTLY MORE WRAPPED UP AT THE SURFACE AND ALOFT VERSUS OTHER GUIDANCE THOUGH THERE ARE SOME INCONSISTENCIES AMONG OTHER SOLUTIONS IN TERMS OF HEIGHTS ALOFT AND CORRESPONDING SURFACE LOW DEPTH. ECMWF CONTINUITY AND SOMEWHAT GREATER TENDENCY OF GFS RUNS TO DEEPEN MIDWEST SYSTEMS A LITTLE PREMATURELY SEEM TO FAVOR LEANING CLOSEST TO THE 00Z ECMWF DEPTH AT THIS TIME. GUIDANCE SUGGESTS THERE WILL BE A MORE DIFFUSE SHORTWAVE APPROACHING THE NORTHWEST ON DAY 4 SAT AND PROGRESSING THROUGH THE WEST AND BEYOND FROM SUN ONWARD. 00Z GEFS MEMBERS DIFFERED FROM MOST OTHER SOLUTIONS IN HOLDING BACK TROUGH ENERGY FARTHER WESTWARD BUT THE 06Z GEFS MEAN HAS TRENDED FASTER TO COMPARE MORE FAVORABLY TO OTHER GUIDANCE. BY EARLY NEXT WEEK MODELS/ENSEMBLES DIVERGE MORE WITH RESPECT TO EAST PACIFIC/WESTERN U.S. DETAILS ALOFT, AS SOLUTIONS RANGE BETWEEN AN INLAND OPEN TROUGH TO SEPARATED FLOW WITH AN UPPER LOW DRIFTING SOUTHWARD JUST OFF THE WEST COAST. RECENT ECMWF/GFS RUNS DO NOT APPEAR TO PROVIDE ADDED CLUES AS THEY HAVE WAFFLED BETWEEN THESE DIFFERING SOLUTIONS IN RECENT DAYS. ENSEMBLE MEANS HAVE BEEN FAIRLY STABLE IN DEPICTING AN OPEN TROUGH ALONG OR JUST INLAND FROM THE WEST COAST BY DAY 7 TUE, WITH THE 06Z GFS/00Z ECMWF SIMILAR TO THE MEANS. THUS PREFER THIS SCENARIO AS THE MOST LIKELY TO OCCUR FOR THE PURPOSES OF A SINGLE DETERMINISTIC FORECAST. HOWEVER NOTE THAT THE LAST COUPLE DAYS OF THE PERIOD FEATURE CONSIDERABLE ENSEMBLE SPREAD BOTH FOR THE WESTERN U.S. EVOLUTION AS WELL AS THE RESULTING SHORTWAVE DETAILS DOWNSTREAM. FOR DAY 3 FRI INTO THE WEEKEND THE FORECAST STARTED WITH AN OPERATIONAL MODEL BLEND INCORPORATING THE 00Z ECMWF/06Z GFS/00Z UKMET/00Z CMC IN ORDER FROM MOST TO LEAST WEIGHT, TO REPRESENT PREFERENCES FOR THE SYSTEM TRACKING INTO THE GREAT LAKES AND ACCOUNT FOR CURRENT GUIDANCE SPREAD. THE FORECAST BEGAN TO INCORPORATE A LITTLE 06Z GEFS/00Z ECMWF MEAN INPUT BY DAY 5 SUN WITH THE MEANS BECOMING 70 PERCENT OF THE FORECAST BY DAY 7 TUE WHILE HOLDING ONTO MINORITY WEIGHT OF 06Z GFS/00Z ECMWF RUNS. ...WEATHER HIGHLIGHTS/THREATS... THE HEAVY RAINFALL THREAT FROM THE ARKLATEX REGION THROUGH THE OH VALLEY AND POSSIBLY INTO THE CENTRAL APPALACHIANS WILL BE A MAJOR ISSUE DURING THE LATE WEEK/WEEKEND TIME FRAME. WITH SIGNIFICANT RAINFALL LIKELY TO OCCUR ALREADY BEFORE THE START OF THIS PERIOD, ADDITIONAL HEAVY RAIN MAY EXACERBATE FLOODING CONCERNS WITH RIVERS LIKELY TO BE AT VARIOUS FLOOD STAGES. SYSTEM EXPECTED TO TRACK THROUGH THE GREAT LAKES BY MID-LATE WEEKEND WILL PUSH A COLD FRONT THROUGH THE EAST, WHICH SHOULD SUPPORT A DRIER TREND BY EARLY NEXT WEEK. HOWEVER THE FRONT MAY STALL NEAR THE GULF COAST AND PROMOTE LINGERING RAINFALL ACROSS PARTS OF THE SOUTHERN TIER WITH SOME POCKETS OF MODERATE TO HEAVY ACTIVITY POSSIBLE. NORTHWEST/NORTH OF THE HEAVY RAINFALL AREA, LOCATIONS FROM THE NORTH-CENTRAL PLAINS THROUGH THE UPPER MIDWEST INTO INTERIOR NEW ENGLAND WILL SEE THE BEST POTENTIAL FOR WINTRY WEATHER. OVER THE WEST, EXPECT PERIODS OF COASTAL/LOW ELEVATION RAIN AND SNOW OTHERWISE OVER THE PACIFIC NORTHWEST INTO THE UPPER INTERMOUNTAIN WEST. SNOW LEVELS SHOULD CONTINUE TO FALL WITH THE LOWERING THICKNESS VALUES. UNCERTAINTY OVER THE DETAILS OF FLOW ALOFT DURING THE LATTER HALF OF THE PERIOD KEEPS CONFIDENCE FAIRLY LOW WITH RESPECT TO HOW MUCH PRECIP EXTENDS SOUTHWARD INTO CALIFORNIA AND CORRESPONDING SNOW LEVELS. SPRING-LIKE MOISTURE RETURN LIFTING UP FROM THE LOWER MS VALLEY INTO THE EASTERN U.S. WILL SUPPORT UNSEASONABLY MILD OVERNIGHT TEMPERATURES WITH FORECAST ANOMALIES AS HIGH AS 20-30 DEGREES ABOVE CLIMATOLOGY. SUCH ANOMALIES ARE MOST LIKELY OVER THE MID-SOUTH/TN VALLEY INTO PARTS OF THE MID ATLANTIC. MANY DAILY TEMPERATURE RECORDS COULD BE BROKEN GIVEN SUCH WARM OVERNIGHT LOWS EXPECTED. CONDITIONS WILL REMAIN MILD DURING THE DAYS AS WELL ALTHOUGH RECORDS SHOULD BE BROKEN ON A MORE ISOLATED BASIS. ON THE CONTRARY, COLD WEATHER WILL PERSIST OVER THE WESTERN STATES INTO THE NORTHERN PLAINS. EXPECT THE SHARPEST DEPARTURES FROM AVERAGE TO BE OVER THE NORTHERN ROCKIES/HIGH PLAINS, GENERALLY IN THE 20 TO 30 DEGREES BELOW NORMAL RANGE, BUT THE ENTIRE REGION WILL BE QUITE CHILLY. NEAR FREEZING TEMPERATURES SHOULD CONTINUE TO BE A DAILY OCCURRENCE EACH MORNING OVER THE PACIFIC NORTHWEST AND EXTENDING INTO VAST SECTIONS OF CA. WHILE THE MAJOR CITY CENTERS SHOULD STAY ABOVE FREEZING, MANY OF THE COOLER VALLEY LOCATIONS CAN EXPECT A FREEZE. RAUSCH/RUBIN-OSTER