EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1059 AM EST SAT FEB 17 2018 VALID 12Z TUE FEB 20 2018 - 12Z SAT FEB 24 2018 ...ARCTIC AIR DROPS THROUGH THE WEST-CENTRAL US AND RECORD WARMTH FOR THE EAST... ...CLASHING AIRMASSES SET STAGE FOR ARKLATEX TO TN/OH VALLEYS HEAVY RAINFALL THREAT... ...PATTERN OVERVIEW... EXPECT LITTLE CHANGE IN THE SYNOPTIC-SCALE PATTERN OVER THE NEXT WEEK AS A SUBTROPICAL RIDGE HOLDS STRONG OVER THE SOUTHWESTERN ATLANTIC. A MID-LEVEL RIDGE WITH AN INTENSITY POSSIBLY REACHING 594-DM MAY APPROACH OR EVEN EXCEED 2.5 STANDARD DEVIATIONS ABOVE CLIMATOLOGY FOR MID-FEBRUARY. THE RESULTANT UPSTREAM PATTERN WILL FAVOR BELOW AVERAGE HEIGHTS WITH AN UPPER TROUGH THAT CONTINUALLY RE-LOADS ACROSS THE SOUTHWESTERN STATES. A STEADY PERIOD OF SOUTHWARD PROGRESSING SYSTEMS ARE NOTED ALONG THE EASTERN PACIFIC GIVEN THE STOUT UPPER RIDGE REMAINING IN PLACE FARTHER TO THE WEST. THIS OVERALL SETUP OVER NORTH AMERICA WILL FAVOR A RATHER SHARP AXIS OF MID-LEVEL CONFLUENCE EXTENDING FROM THE ARKLATEX UP TO THE TN/OH VALLEYS WHERE A HEAVY RAINFALL THREAT REMAINS IN PLAY. ...GUIDANCE/UNCERTAINTY ASSESSMENT... OVER THE WEST...MODELS ARE STRUGGLING MOST WITH AN ACTIVE LONGWAVE TROUGH EXTENDING OVER THE WRN TO CENTRAL STATES. UPSTREAM FLOW OVER THE ERN PACIFIC/WRN U.S. REMAINS SOMEWHAT COMPLEX WITH UNCERTAIN JET SEPARATION. A COMPOSITE GUIDANCE SOLUTION IN LINE WITH ENSEMBLE MEANS SEEMS REASONABLE INTO MIDWEEK THAT MITIGATES SMALL SCALE INDIVIDUAL MODEL VARIANCE. LATER...AMPLITUDE OF THE UPSTREAM ERN PACIFIC MID-UPPER LEVEL RIDGE MAY SUGGEST PREFERENCE FOR MORE AMPLIFIED WRN US TROUGHING DAYS 5-7 MORE IN LINE WITH ECMWF ENSEMBLES. DOWNSTREAM...AMPLE HEIGHT FALLS EJECT FROM THE SRN/CENTRAL ROCKIES WITH MODEL DIFFERENCES STARTING AS EARLY AS TUE. RECENT GFS/GEFS MEMBERS TEND TO FAVOR QUICKER PROGRESSION MID-LATE WEEK AND SPATIAL DIFFERENCES IN FRONTAL PLACEMENTS MAINLY OCCUR FROM MORE RAPID EXPANSION OF LOW LEVEL HIGH PRESSURE OUT FROM THE PLAINS. THIS SEEMS MORE LIKELY CONSIDERING EARLY PERIOD HIGH STRENGTH. THESE SOLUTIONS DOWNSTREAM SUPPORT A COLD FRONT THROUGH THE NERN US/MID-ATLANTIC WED AS OTHER MODELS KEEP THE FRONT BACK TOWARD THE APPALACHIANS WED THEN OFF THE ERN SEABOARD THU. THIS AFFORDS A COOL DOWN FROM RECORD WARMTH. THE SWRN FLANK OF THIS BAROCLINIC ZONE THOUGH SHOULD BE SLOWER MOVING ACCOMPANIED BY A SERIES OF WEAK PERTURBATIONS ALONG THE FRONT. THIS SUPPORTS AN EXTENDED PERIOD OF MODERATE/HEAVY RAINFALL FROM THE ARKLATEX INTO THE OZARKS/LOWER MS VALLEY AS THE MID-LEVEL FLOW GENERALLY ALIGNS WITH THE BOUNDARY. ...WEATHER HIGHLIGHTS/THREATS... PERSISTENT HIGH-AMPLITUDE FLOW MAKES AN ACTIVE PERIOD FOR PRECIPITATION ALONG WITH SOME VERY IMPRESSIVE TEMPERATURE ANOMALIES. IN SOME CASES...DEPARTURES FROM CLIMATOLOGY MAY BE NEAR 30 TO 40 DEGREES...PARTICULARLY IN THE ARCTIC AIR DOME. FRIGID TEMPERATURES FOCUS FROM THE NORTHERN/CENTRAL ROCKIES TO THE GREAT PLAINS TUE/WED. COLDEST AIR IS EXPECTED ACROSS THE NORTHERN ROCKIES WITH SINGLE DIGIT HIGHS AND NEGATIVE TEENS LOWS. SOME MODERATION IS EXPECTED STARTING MIDWEEK ALTHOUGH COOLER THAN AVERAGE TEMPERATURES WILL PREVAIL. ARCTIC AIR EVEN SPILLS ACROSS THE CONTINENTAL DIVIDE INTO THE PACIFIC NORTHWEST. THE EXPANSIVE WARM SECTOR STRETCHING FROM THE SOUTH-CENTRAL U.S. UP TOWARD NEW ENGLAND WILL FEATURE SOME EXTREME ANOMALIES AS WELL...MAXIMIZING IN THE 30 TO 35 DEGREE RANGE FOR OVERNIGHT MINIMA. WIDESPREAD RECORDS MAY BE BROKEN ON TUESDAY/WEDNESDAY MORNINGS AS MORNING LOWS STAY IN THE 50S AS FAR NORTH AS THE OH VALLEY/LOWER GREAT LAKES. CONSIDERING FORECAST HIGHS...70S ARE EXPECTED UP INTO THE OH VALLEY...CENTRAL APPALACHIANS...AND MID-ATLANTIC. A FEW 80 DEGREE READINGS COULD POP UP IN THIS REGION THROUGH MID-WEEK. SOME COOLING IS EXPECTED LATER IN THE WEEK ALTHOUGH THIS COULD BE BRIEF AS A WARM FRONT LIFTS NORTHWARD UP TOWARD 40N LONGITUDE BY EARLY NEXT WEEKEND. WITH UNIDIRECTIONAL FLOW LIKELY SETTING UP ACROSS THE ARKLATEX UP THROUGH THE OZARKS AND INTO THE OH VALLEY...MULTIPLE ROUNDS OF HEAVY RAINFALL IS LIKELY...PARTICULARLY IN THE TUESDAY THROUGH THURSDAY TIME SPAN. THEREAFTER...UNCERTAINTIES IN THE PATTERN MAKE HEAVY RAINFALL PLACEMENT DIFFICULT. HOWEVER...WITH THIS BOUNDARY STILL LOOMING OVER THE CENTRAL/EASTERN U.S...ADDITIONAL PRECIPITATION IS LIKELY GIVEN LITTLE CHANGE IN THE SYNOPTIC SETUP. GENERALLY FLOODING CONCERNS MAY BE AN ISSUE WITH THIS MULTI-DAY HEAVY RAINFALL THREAT IN PLACE. THE DURATION OF THIS EVENT COULD LEAD TO GENERAL FLOODING CONCERNS BUT SPRINGTIME WARMTH/MOISTURE RETURN COULD SUPPORT SOME FLASH FLOODING ISSUES GIVEN SUFFICIENT INSTABILITY AND FAVORABLE BACKBUILDING ACTIVITY. ELSEWHERE...UNSETTLED CONDITIONS ARE TO RETURN TO THE PACIFIC NORTHWEST GIVEN THE SERIES OF SHORTWAVES MOVING THROUGH. MODIFIED ARCTIC AIR WILL LOWER SNOW LEVELS APPRECIABLY TO BRING SOME ACCUMULATING SNOWS TO VALLEY LOCALES IN ADDITION TO THE USUAL HIGHER TERRAIN. SCHICHTEL