EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 303 AM EDT TUE MAY 16 2017 VALID 12Z FRI MAY 19 2017 - 12Z TUE MAY 23 2017 ...PATTERN OVERVIEW/GUIDANCE EVALUATION/PREFERENCES... A WELL ADVERTISED UPPER LOW EJECTING OUT OF THE CENTRAL ROCKIES HAS CONTINUED TO BE PLAGUED BY SOME RUN-TO-RUN MODEL INCONSISTENCIES. IN PREVIOUS DAYS...MODEL TRENDS WERE MOVING TOWARD A SLOWER SOLUTION. HOWEVER...ANALYZING THE PAST FEW RUNS DURING THIS SHIFT HAS SHOWN A SLIGHT EASTWARD JUMP IN THE SOLUTION ENVELOPE. AT THE TIME OF FORECAST ISSUANCE...THE 12Z CMC/UKMET STOOD OUT AS BEING QUICKER OUTLIERS WITH THEIR MORE RECENT CYCLES JOINING THE STRONGER WESTWARD CONSENSUS. THE GUIDANCE SHOW THE UPPER FEATURE TRANSLATING EASTWARD IN TWO PARTS AS THE LEAD CLOSED LOW/OPENING WAVE LIFTS INTO THE MIDDLE MS VALLEY ON SATURDAY MORNING. A MODEST SURFACE CYCLONE WILL BE MOVING INTO NORTHERN MO/SOUTHERN IA BY 20/1200Z WITH PLENTY OF NORTH/SOUTH SCATTER STILL NOTED AMONG THE INDIVIDUAL ENSEMBLE MEMBERS. WHAT OCCURS AFTERWARD REMAINS UP FOR DEBATE AS THE ECMWF ENSEMBLE SYSTEM FAVORS MORE EASTWARD PROGRESSION RELATIVE TO THE SLOWER GEFS/NAEFS MEANS. THIS WILL ULTIMATELY HELP BREAK DOWN A MEAN UPPER RIDGE WHICH LOOMS OVER THE EASTERN U.S. INTO THE WEEKEND. AVAILABLE GUIDANCE SHOW THE 588-DM HEIGHT CONTOUR PUNCHING AS FAR NORTH AS THE CENTRAL APPALACHIANS INDICATING JUST HOW STRONG THIS SUBTROPICAL RIDGE WILL BE. WHILE THE STRONGER BAND OF HEIGHT FALLS TRAVERSE TOWARD THE LOWER GREAT LAKES/NEW ENGLAND EARLY NEXT WEEK...THERE IS GENERAL AGREEMENT ON LARGE-SCALE CYCLONIC FLOW OVER MUCH OF THE CENTRAL/EASTERN STATES. MULTIPLE PERTURBATIONS IN THE NORTHWESTERLY FLOW SHOULD SWING THROUGH THE INTERMOUNTAIN WEST/ROCKIES WITH VERY LOW PREDICTABILITY IN PLACEMENT/STRENGTH GIVEN THE TIME-SCALES INVOLVED. FARTHER UPSTREAM...THERE IS A GROWING CONSENSUS FOR A REX BLOCK OF SORTS TO FORM ACROSS THE EASTERN PACIFIC EARLY NEXT WEEK. SOME VARIANCE AMONG THE OPERATIONAL/ENSEMBLE GUIDANCE IS NOTED WITH THE ECMWF ENSEMBLES BEING CLOSER TO THE SOUTHERN CA COAST BY DAY 7/MAY 23. THE GEFS/NAEFS MEANS FAVOR THE SYSTEM BEING PINCHED OFF FARTHER TO THE WEST INTO THE OFFSHORE WATERS. GIVEN MORE CONFIDENCE IN THE DETAILS OF THE PATTERN EARLY IN THE FORECAST...WAS ABLE TO UTILIZE A COMBINATION OF THE 12Z GFS/ECMWF THROUGH DAY 4/SATURDAY BEFORE ADDING ENSEMBLE MEANS TO THE MIX. THIS IS PRIMARILY GIVEN THE UNCERTAINTIES IN HOW QUICKLY TO MOVE THE NEGATIVE HEIGHT ANOMALY EASTWARD INTO THE GREAT LAKES/NORTHEASTERN U.S. GIVEN MORE STABILITY IN THE GEFS SOLUTION...ALLOWED ITS OUTPUT TO DOMINATE THE LATTER HALF OF THE PERIOD...UP TO 50 PERCENT BY DAYS 6/7. ADDED A MIX OF OTHER ENSEMBLE MEANS...ECMWF/NAEFS...AS WELL TO ACCOUNT FOR GENERAL UNCERTAINTIES BUT THE FORECAST PLAYED A SLOWER PROGRESSION AS NOTED BY THE GEFS/NAEFS MEANS. ...SENSIBLE WEATHER HIGHLIGHTS... UNDERNEATH THE UPPER LOW ACROSS THE CENTRAL ROCKIES...SOME LINGERING HIGHER ELEVATION SNOW IS POSSIBLE ALTHOUGH THIS MOSTLY OCCURS BEFORE THE MEDIUM RANGE PERIOD COMMENCES...19/1200Z. ADDITIONALLY...WELL BELOW NORMAL TEMPERATURES WILL PREVAIL AS THE HIGHS STRUGGLE TO GET OUT OF THE 40S ON FRIDAY FROM THE CENTRAL ROCKIES EASTWARD INTO AREAS OF NEB/SD. FORECAST DEPARTURES FROM CLIMATOLOGY ARE IN THE 20 TO 30 DEGREE RANGE. WHILE THE ANOMALIES DIMINISH SOME AS THE COLDER AIR ADVANCES NORTH AND EAST...IT SHOULD STILL BRING SOMEWHAT CHILLIER WEATHER TO THE NORTHERN TIER OF THE COUNTRY THIS WEEKEND. FARTHER DOWNSTREAM...SOME DAILY TEMPERATURE RECORDS MAY BE BROKEN IN RESPONSE TO THE LARGE POSITIVE HEIGHT ANOMALY OVER THE EASTERN U.S. THIS WILL PARTICULARLY BE THE CASE FOR THE MILD OVERNIGHT LOWS ACROSS THE NORTHEASTERN U.S. ON FRIDAY MORNING. GRADUALLY TEMPERATURES SHOULD COME DOWN A BIT AS A BACK DOOR FRONT SWEEPS SOUTHWARD BEFORE RETURNING NORTHWARD AS A WARM FRONT OVER THE WEEKEND. LOOKING BACK WEST...ABUNDANT WARMTH IS EXPECTED ALONG THE WEST COAST AS HEIGHTS BEGIN TO BUILD. WIDESPREAD HIGHS IN THE 90S ARE IN THE FORECAST ACROSS THE CA VALLEY LOCALES WITH SOME LOW 100S POSSIBLE IN THE DESERT SOUTHWEST. THERE WILL INITIALLY BE A HEAVY RAINFALL THREAT ALONG WITH ACCOMPANYING BOUTS OF SEVERE WEATHER EARLY ON ACROSS THE SOUTHERN/CENTRAL PLAINS. IN PARTICULAR...MODELS CONTINUE TO ADVERTISE A DECENT HEAVY PRECIPITATION SIGNAL ON FRIDAY INTO SATURDAY MORNING FROM THE RED RIVER NORTHEASTWARD INTO MO. EVENTUALLY THE WET WEATHER WILL ADVANCE EASTWARD ALONG AND AHEAD OF THE COLD FRONT. THE COMBINATION OF AMPLE GULF MOISTURE LIFTING NORTHWARD COUPLED WITH FAIRLY UNSTABLE THERMODYNAMIC PROFILES SUGGESTS SOME AREAS MAY SEE SOMEWHAT HEFTY PRECIPITATION EAST OF THE MS RIVER. HOW QUICKLY THIS UPPER TROUGH IS ABLE TO ADVANCE EASTWARD WILL ALSO PLAY A KEY ROLE AS WELL. EVENTUALLY THE ACTION SHIFTS BACK TOWARD THE HIGHER TERRAIN AS MULTIPLE LOW-AMPLITUDE WAVES PUSH THROUGH THE INTERMOUNTAIN WEST/ROCKIES. MUCH OF THE ACTIVITY WILL BE ELEVATED IN NATURE WITH THE SURFACE BOUNDARY HANGING CLOSER TO THE GULF COAST. RUBIN-OSTER