EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 150 AM EST FRI FEB 16 2018 VALID 12Z MON FEB 19 2018 - 12Z FRI FEB 23 2018 ...EXTREMELY COLD TEMPERATURES EXPECTED OVER THE NORTHERN ROCKIES WHILE RECORD WARMTH STRETCHES FROM THE SOUTHERN PLAINS TO NEW ENGLAND... ...HEAVY RAINFALL THREAT LOOMS FROM THE ARKLATEX INTO THE TN/OH RIVER VALLEYS... ...PATTERN OVERVIEW... A RATHER STAGNANT SYNOPTIC-SCALE PATTERN IS FAVORED NEXT WEEK AS A SUBTROPICAL RIDGE HOLDS STRONG FROM THE CENTRAL/EASTERN CARIBBEAN UP TOWARD BERMUDA. WELL ABOVE AVERAGE MID-LEVEL HEIGHTS ARE EXPECTED ACROSS THE EASTERN U.S. AS VALUES IN THE 582-588 DM RANGE ARE SUGGESTED BY ALL OF THE GUIDANCE. RELATIVE TO CLIMATOLOGY...THESE COULD EASILY REACH THE 2 TO 2.5 STANDARD DEVIATION RANGE. MEANWHILE...ACROSS THE WESTERN/CENTRAL U.S...A MEAN UPPER TROUGH WILL SITUATE ITSELF OVER THE REGION WITH A TENDENCY FOR UPPER TROUGHS TO PEEL BACK OVER THE SOUTHWESTERN U.S. AND INTO THE OFFSHORE PACIFIC WATERS. AN EVENTUAL SPLIT IN THE FLOW IS ANTICIPATED AS NORTHERN STREAM AMPLIFICATION TAKES SHAPE ACROSS THE MIDDLE OF THE COUNTRY BY DAY 6/7...FEBRUARY 22/23. ...GUIDANCE EVALUATION/FORECAST TRENDS/PREFERENCES... THE LATEST GUIDANCE REMAINS IN DECENT LARGE-SCALE AGREEMENT WITH THE HIGH AMPLITUDE PATTERN IN PLACE. A DEEP UPPER TROUGH WILL INITIALLY SITUATE ITSELF OVER THE DESERT SOUTHWEST WITH GRADUAL EASTWARD PROGRESSION INTO THE FOUR CORNERS BY THE FOLLOWING DAY. SHORTWAVE ENERGY ASSOCIATED WITH THIS FEATURE SHOULD BECOME MORE STRETCHED OUT AS IT TRAVERSES INTO A HIGHLY CONFLUENT FLOW ACROSS THE MIDDLE OF THE COUNTRY. WEAK WAVES OF LOW PRESSURE ARE EXPECTED TO MOVE ALONG THE ADVANCING BOUNDARY WITH THE BAROCLINIC ZONE LIKELY REACHING THE I-95 CORRIDOR BY WEDNESDAY AFTERNOON/EVENING. WHILE 500-MB HEIGHTS REMAIN RATHER HIGH INTO THE MIDDLE OF NEXT WEEK...SURFACE TEMPERATURES SHOULD DROP CONSIDERABLY AS A STRONG ARCTIC RIDGE SETS UP ALONG THE EASTERN SEABOARD BY THURSDAY. THIS ANTICYCLONE IS FORECAST TO WEAKEN AND MOVE OFFSHORE ALLOWING FOR RETURN FLOW TO ENSUE IN EARNEST. OVERALL...TIMING/PLACEMENT DIFFERENCES ARE EVIDENT FROM MID-WEEK ONWARD AS THE 00Z/12Z CMC LAG THE CONSENSUS WITH THE EVOLUTION OF THE MENTIONED COLD FRONT. ADDITIONALLY...THE ENSEMBLE MEANS DISPLAY SOME UNCERTAINTY WITH THE POSITION OF THE ARCTIC HIGH AS THE QUICKER 12Z GEFS/NAEFS ENSEMBLE MEANS SITS DOWNSTREAM OF THE 12Z ECMWF ENSEMBLE MEAN. BACK TO THE WESTERN U.S...THE RE-LOADING OF THE UPPER TROUGH APPEARS TO BE ANOTHER REGION OF CONTENTION IN THE GUIDANCE. RECENT RUNS OF THE GFS/ECMWF FAVOR A SEPARATION OF THE STREAMS AS A SLUG OF SHORTWAVE ENERGY SINKS SOUTHWARD ALONG THE EASTERN PACIFIC...GENERALLY SETTLING OFFSHORE OF CA BY WEDNESDAY/THURSDAY. ON THE CONTRARY...THE 00Z/12Z CMC FAVOR KEEPING MORE OF A LONGWAVE TROUGH PATTERN. ENSEMBLE SPAGHETTI PLOTS DO SHOW PLENTY OF MEMBERS WITH THIS MORE SEPARATED SOLUTION. MANY OF THE OPERATIONAL MODELS DEPICT THIS SLOWER SCENARIO UNFOLDING BUT THERE IS PLENTY OF NOISE AMONGST THEIR FORECASTS. ENSEMBLE MEANS CLUSTER WELL WITH THE 12Z ECMWF ENSEMBLE MEAN A HAIR AHEAD OF THE OTHER ENSEMBLE SOLUTIONS WHILE EVEN HIGHER CONFIDENCE EXISTS WITH NORTHERN STREAM AMPLIFICATION OVER THE DAKOTAS/UPPER MIDWEST. GIVEN SOLID OPERATIONAL MODEL AGREEMENT BETWEEN THE 12Z GFS/ECMWF/UKMET...FAVORED A THREE-WAY BLEND OF THESE FORECASTS THROUGH DAY 4/TUESDAY BEFORE REMOVING THE 12Z UKMET. GRADUALLY INCORPORATED EQUAL CONTRIBUTIONS OF THE 12Z GEFS/ECMWF ENSEMBLE MEANS WHILE HOLDING ONTO A SMALLER FRACTION OF THE 12Z GFS/ECMWF THROUGH THE REMAINDER OF THE PERIOD. WHILE DIFFERENCES IN THE DETAILS EXIST...THE LARGE-SCALE PATTERN APPEARS TO BE RESOLVED WELL ENOUGH TO ADD SOME CONFIDENCE IN THE FORECAST. ...WEATHER HIGHLIGHTS/THREATS... AS IS COMMON WITH PERSISTENT HIGH-AMPLITUDE PATTERNS...THERE WILL BE IMPRESSIVE TEMPERATURE ANOMALIES COUPLED WITH HEAVY PRECIPITATION THREATS. STARTING WITH THE TEMPERATURE FORECAST...RATHER EXTREME DEPARTURES FROM CLIMATOLOGY EXIST ON MONDAY/TUESDAY ACROSS THE NORTHERN ROCKIES INTO THE ADJACENT HIGH PLAINS. OVERALL TEMPERATURES MAY RANGE FROM 35 TO 40 DEGREES BELOW AVERAGE WHICH SUPPORTS HIGHS HOVERING AROUND 0 DEGREES WHILE OVERNIGHT LOWS PLUNGE INTO THE NEGATIVE TEENS/POSSIBLY LOWER -20S. IN SPITE OF THESE EXTREME READINGS...WIDESPREAD DAILY TEMPERATURE RECORDS WILL LIKELY NOT BE BROKEN. UNLIKE PREVIOUS ARCTIC SURGES...COLDER WEATHER WILL SPILL WEST OF THE CONTINENTAL DIVIDE INTO THE PACIFIC NORTHWEST INCLUDING SEATTLE WA AND PORTLAND OR. MEANWHILE...IN THE EXPANSIVE WARM SECTOR...PERSISTENT MID-LEVEL SOUTHWESTERLY FLOW COUPLED WITH WELL ABOVE AVERAGE HEIGHTS WILL KEEP TEMPERATURES AROUND 20 TO 30 DEGREES ABOVE AVERAGE. THIS WILL ULTIMATELY SUPPORT A MULTI-DAY PERIOD OF BROKEN DAILY TEMPERATURE RECORDS...PARTICULARLY EAST OF THE MS RIVER. ON TUESDAY...READINGS IN THE 70S CAN BE FOUND AS FAR NORTH AS SOUTHERN OH INTO THE CENTRAL MID-ATLANTIC STATES WHILE LOWS HOLD IN THE 50S UP TO THE LOWER GREAT LAKES. WHILE TEMPERATURES SHOULD COOL AS A FRONTAL ZONE STALLS OVER THE CAROLINAS BACK TOWARD THE MID-SOUTH...READINGS SHOULD STILL STAY SOMEWHAT ABOVE CLIMATOLOGY. THE HIGHLY CONFLUENT MID-LEVEL FLOW OVER THE CENTRAL U.S. WILL BE THE SITE OF A FAIRLY EXTENSIVE PERIOD OF HEAVY RAINFALL. INITIALLY...THE UPPER LOW BARRELING OUT OF THE SOUTHWESTERN U.S. WILL BE THE FOCUS FOR PRECIPITATION EXITING THE FOUR CORNERS WITH SNOW LINGERING OVER THE CENTRAL ROCKIES. EVENTUALLY THE DOWNSTREAM FRONTAL ZONE SHOULD LIGHT UP WITH MODERATE TO HEAVY RAINFALL ANYWHERE FROM THE ARKLATEX THROUGH THE OZARKS AND INTO THE TN/OH VALLEYS. THE PERIOD OF INTEREST GENERALLY SPANS FROM TUESDAY THROUGH THURSDAY ALTHOUGH THE TIMING IS VARIABLE BASED ON GUIDANCE. SOME WINTRY PRECIPITATION THREAT WOULD BE LIKELY ALONG THE NORTHERN EXTENT WHICH WOULD INCLUDE THE UPPER MIDWEST INTO THE GREAT LAKES. EVENTUALLY THE ACTIVITY WILL SLIDE EASTWARD AND WEAKEN IN INTENSITY WHILE UNSETTLED CONDITIONS RETURN TO THE PACIFIC NORTHWEST. RUBIN-OSTER