EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1110 AM EDT TUE APR 17 2018 VALID 12Z FRI APR 20 2018 - 12Z TUE APR 24 2018 ...FOUR CORNERS/SOUTHERN ROCKIES-PLAINS TO SOUTHEAST U.S. SPRING STORM THREAT... 15 UTC UPDATE... FORECAST UPDATE THIS MORNING REMAINED FAIRLY CLOSE TO CONTINUITY FOR MOST AREAS. A COUPLE TRENDS WERE SHOWN AMONG THE GUIDANCE WITH RESPECT TO THE UPPER LOW TRACKING FROM THE FOUR CORNERS REGION ON FRI TO THE SOUTHEAST BY MON-TUE. ENSEMBLE GUIDANCE OVER THE PAST 24 HOUR HAS SHOWN A SMALL BUT UNIFORM TREND TOWARD A SLIGHTLY SLOWER EASTWARD PROGRESSION OF THIS FEATURE, AND THE WPC FORECAST WAS ADJUSTED ACCORDINGLY. ADDITIONALLY, AS NOTED OVERNIGHT, A TREND TOWARD A FARTHER SOUTH SOLUTION WITH THE SURFACE LOW PRESSURE SYSTEM BY MON-TUE WAS NOTED, AND THE UPDATED WPC FORECAST KEEPS THE SURFACE LOW SUPPRESSED INTO THE NORTHERN GULF OF MEXICO DURING THAT TIME FRAME, A SOUTHWARD SHIFT FROM THE OVERNIGHT ISSUANCE. ELSEWHERE, MODELS REMAINED PRETTY CONSISTENT WITH SHORTWAVE ENERGY SKIRTING THE PAC NW AND THEN CROSSING THE CANADIAN PRAIRIES SAT-MON, AND THE ASSOCIATED SURFACE COLD FRONT ENTERING CROSSING THE U.S. NORTHERN ROCKIES/NORTHERN PLAINS. BY DAY 7 (TUE) MODELS/ENSEMBLES ARE IN GENERAL AGREEMENT ON A TROUGH REACHING THE WEST COAST, BUT SOLUTIONS DIFFER WITH RESPECT TO TIMING/AMPLITUDE (GFS WAS THE MOST AMPLIFIED AND CMC WAS THE WEAKEST). THE UPDATED FORECAST WAS BASED ON A HEAVILY DETERMINISTIC BLEND INCLUDING THE 00Z ECMWF/GFS/CMC DURING DAYS 3-5, WITH INCREASING WEIGHT PLACED ON THE 00Z ECENS/NAEFS ENSEMBLE MEANS ON DAY 6 AND ESPECIALLY BY DAY 7. RYAN PREVIOUS DISCUSSION (ISSUED AT 0646 UTC)... ...OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT... THE MOST PROMINENT FOCUS OF THE FORECAST WILL CONTINUE TO BE ON A CLOSED UPPER LOW FORECAST TO APPROACH THE FOUR CORNERS AS OF EARLY FRI. THIS FEATURE WILL LIKELY MAKE STEADY PROGRESS ACROSS THE SOUTHERN HALF OF THE LOWER 48 INTO THE FIRST HALF OF NEXT WEEK, WITH ASSOCIATED WESTERN U.S. LOW PRESSURE EMERGING INTO THE SOUTHERN PLAINS BY SAT AND REACHING THE EXTREME SOUTHEAST BY TUE. ELSEWHERE A DEEP UPPER TROUGH INITIALLY NEAR THE EAST COAST WILL LIFT AWAY BUT SOME DEGREE OF CYCLONIC FLOW SHOULD PERSIST THROUGH AT LEAST SUN. FARTHER UPSTREAM A NORTH PACIFIC SHORTWAVE ENTERING THE NORTHWEST U.S./WESTERN CANADA ON SAT WILL PUSH A COLD FRONT ACROSS THE NORTHWESTERN STATES AND NORTHERN PLAINS. FOR THESE COMPONENTS OF THE FORECAST THE 18Z GFS/GEFS MEAN AND 12Z ECMWF/ECMWF MEAN (MORE OPERATIONAL WEIGHT EARLY FOLLOWED BY A TREND TOWARD MORE EVEN MODEL/ENSEMBLE WEIGHT LATE) PROVIDED GOOD CLUSTERING AND CONTINUITY ASIDE FROM MODEST NUDGES REFLECTING RECENT TRENDS. IN PARTICULAR SLOWER TRENDS IN THE GFS OVER THE PAST COUPLE DAYS HAVE TIGHTENED THE TIMING SPREAD. MEANWHILE THERE IS ALSO A TENDENCY FOR THE OVERALL SYSTEM TO TRACK FARTHER SOUTH--SUPPRESSED ALOFT BY A NORTHERN STREAM RIDGE THAT MOVES ALONG IN TANDEM FOR A PERIOD OF TIME AND AT THE SURFACE BY STRONG GREAT LAKES/NORTHEAST HIGH PRESSURE. LATEST CMC/UKMET RUNS SUGGEST A FURTHER EXTENSION OF THIS SOUTHWARD TREND COULD BE POSSIBLE. THE AFOREMENTIONED MODEL/MEAN SOLUTION CLUSTER ALSO YIELDED A REASONABLE INTERMEDIATE STARTING POINT FOR EASTERN PACIFIC/WESTERN U.S. FLOW LATE IN THE PERIOD. OVER THE PAST COUPLE DAYS MODEL/ENSEMBLE SOLUTIONS HAVE BEEN QUITE VARIED FOR SPECIFICS OVER THIS AREA EARLY NEXT WEEK AS THE NEARLY OPPOSITE 12Z AND 00Z/16 ECMWF RUNS PROVIDE A SIMPLE EXAMPLE OF THE GUIDANCE VARIABILITY/SPREAD THAT EXISTS. THUS CONFIDENCE IS IN THE LOWER SIDE OF THE SPECTRUM FOR THIS PART OF THE FORECAST. ENSEMBLE MEANS AND GFS RUNS HAVE BEEN SOMEWHAT MORE STABLE IN DEPICTING SOME FORM OF TROUGHING JUST OFF THE WEST COAST AS OF EARLY TUE. THE 12Z ECMWF RUN WAS MORE IN LINE WITH THIS IDEA. ...WEATHER/THREATS HIGHLIGHTS... FROM FRI INTO SAT THE ROCKIES/PLAINS SYSTEM WILL GENERATE AREAS OF HEAVY RAINFALL OVER THE PLAINS AND HIGHER ELEVATION SNOW OVER THE ROCKIES. SOME CONVECTION OVER THE PLAINS MAY BE STRONG TO SEVERE, WITH SPC OUTLOOKS OVER RECENT DAYS HIGHLIGHTING POTENTIAL OVER PARTS OF THE SOUTHERN PLAINS. A SLIGHTLY WARMER TREND FOR LOW LEVEL TEMPS OVER PORTIONS OF THE CENTRAL/NORTH-CENTRAL HIGH PLAINS COMPARED TO PREVIOUS DAYS HAS REDUCED SOMEWHAT THE POTENTIAL DURATION OF ANY SNOWFALL AWAY FROM THE HIGHER ELEVATIONS. AFTER SAT THE STEADY PROGRESSION OF LOW PRESSURE ACROSS THE GULF COAST STATES AND LEADING INFLOW OF GULF MOISTURE SHOULD PUSH THE HEAVY RAIN THREAT EASTWARD INTO THE LOWER MS VALLEY AND THEN INTO THE SOUTHEAST. BEHIND CANADIAN MARITIMES LOW PRESSURE, THE NORTHEAST WILL SEE BREEZY CONDITIONS AND AREAS OF RAIN/SNOW ON FRI FOLLOWED BY A TREND TOWARD DRIER WEATHER AND LIGHTER WINDS AS HIGH PRESSURE BUILDS IN FROM THE WEST. MOSTLY LIGHT RAIN/SNOW WILL ACCOMPANY THE FRONT PUSHING INTO AND THROUGH THE NORTHWEST DURING THE WEEKEND. BY EARLY NEXT WEEK SOME TRAILING SHORTWAVE ENERGY AND FRONTAL WAVINESS MAY PRODUCE AREAS OF PRECIP OVER THE NORTHERN ROCKIES/PLAINS. AFTER DEPARTURE OF THE FOUR CORNERS STORM THE SOUTHERN TWO THIRDS OF THE WEST SHOULD SEE MULTIPLE DAYS OF WARM TEMPS WITH MANY AREAS 5-15F ABOVE NORMAL BY SUN-TUE. COLDEST ANOMALIES OVER THE CENTRAL U.S., UP TO 15-25F BELOW NORMAL, WILL LIKELY EXIST IN THE FRI-SUN TIME FRAME FOLLOWED BY A WARMING TREND. EASTERN STATES WILL BE CHILLY DURING THE FIRST HALF OF THE PERIOD AS WELL. RAINFALL ACROSS THE SOUTHERN TIER WILL KEEP DAYTIME HIGHS WELL BELOW NORMAL THROUGH TUE WHILE TEMPS FARTHER NORTH RETURN CLOSER TO NORMAL EARLY NEXT WEEK. ASSOCIATED WPC MEDIUM RANGE FORECASTS OF 500 MB HEIGHTS...SURFACE SYSTEMS...WEATHER GRIDS...QUANTITATIVE PRECIPITATION FORECASTS (QPF)...AND WINTER WEATHER OUTLOOK SNOW/SLEET PROBABILITIES CAN BE FOUND AT... http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 RAUSCH