654 FXUS02 KWBC 191554 PMDEPD EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1153 AM EDT THU APR 19 2018 VALID 12Z SUN APR 22 2018 - 12Z THU APR 26 2018 15Z UPDATE: THERE WILL BE TWO MAIN FEATURES OF INTEREST IN THE MEDIUM RANGE PERIOD. THE FIRST WILL BE THE GULF COAST LOW THAT TRACKS TOWARDS THE EAST COAST IN CONJUNCTION WITH AN UPPER LEVEL LOW OVER THE INTERIOR SOUTHEAST, AND THE SECOND WILL BE A CLOSED LOW APPROACHING CALIFORNIA AND ENSUING LOW PRESSURE OVER THE INTERMOUNTAIN WEST. THE MODELS AND ENSEMBLE MEANS ARE INDICATING GOOD SYNOPTIC SCALE AGREEMENT THROUGH MONDAY BOTH SURFACE AND ALOFT, SO A DETERMINISTIC MODEL BLEND WAS INCORPORATED INTO THE FORECAST PROCESS FOR THAT TIME. BY TUESDAY, THE 6Z GFS AND 00Z CMC ARE FASTER WITH THE UPPER TROUGH AND LOW OVER THE EASTERN PACIFIC COMPARED TO THE SLOWER 00Z ECMWF AND UKMET, SO A POSITION CLOSE TO THE EC MEAN WAS USED FOR THIS FORECAST. THE CMC DEVELOPS MORE UPPER LEVEL RIDGING OVER SOUTHERN CANADA BY THE END OF THE FORECAST COMPARED TO THE REMAINING GUIDANCE, SO IT WAS NOT USED AFTER DAY 6. REGARDING THE EAST COAST SYSTEM NEXT WEEK, THE 6Z GFS BECOMES MUCH FASTER WITH THE SOUTHEASTWARD PROGRESSION OF THE NORTHERN STREAM TROUGH COMPARED TO BOTH ITS 00Z RUN AND THE MODEL CONSENSUS BY WEDNESDAY MORNING, SO THE 00Z RUN WAS SUBSTITUTED IN THE FORECAST BLEND FOR DAYS 6 AND 7 IN LIEU OF THE 6Z RUN. THERE HAS ALSO BEEN POOR MODEL CONTINUITY OVER THE PAST 36 HOURS WITH BOTH THE ECMWF AND GFS CONCERNING THE ORIENTATION OF THE TROUGH AXIS, SO THE GEFS AND EC MEAN PROVIDE A BETTER IDEA ON THE LOCATION OF THE LOW AND THIS SUGGESTS A SURFACE LOW REACHING SOUTHERN NEW ENGLAND BY THURSDAY MORNING, PERHAPS AS A NOR'EASTER. THE PREVIOUS MEDIUM RANGE DISCUSSION IS APPENDED BELOW. D. HAMRICK ---------- ...OVERVIEW... THE SYSTEM EXPECTED TO BRING THE THREAT OF HEAVY RAINFALL TO SOME LOCATIONS ACROSS THE SOUTHEAST AND POSSIBLY INTO THE SOUTHERN MID ATLANTIC SUN-TUE WILL MOST LIKELY LIFT NORTHWARD ALONG THE EAST COAST THEREAFTER IN RESPONSE TO THE APPROACH OF SHORTWAVE ENERGY THAT SHOULD BE REACHING WESTERN NORTH AMERICA AROUND THE START OF THE PERIOD. THIS ENERGY TRAILS A LEADING VIGOROUS SHORTWAVE THAT INITIALLY PUSHES A COLD FRONT INTO THE NORTHERN PLAINS/NORTH-CENTRAL WESTERN STATES. THERE ARE SOME LOW-PREDICTABILITY ASPECTS TO THE EASTERN U.S. EVOLUTION BY THE LATTER HALF OF THE PERIOD BUT AGREEMENT IS STILL MUCH BETTER IN PRINCIPLE THAN FOR THE EASTERN PACIFIC/WESTERN U.S. FORECAST BY NEXT TUE-THU, AS GUIDANCE DIVERGES IN A WIDE VARIETY OF WAYS FOR ENERGY WITHIN A TROUGH THAT SHOULD BE NEAR 150W LONGITUDE AS OF EARLY SUN. ...GUIDANCE/PREDICTABILITY ASSESSMENT... THE BROADENING ENVELOPE OF ENSEMBLE MEMBERS MORE THAN ACCOUNTS FOR THE WIDE SPECTRUM OF CURRENT AND RECENT OPERATIONAL MODEL SOLUTIONS FOR THE PACIFIC TROUGH ENERGY THAT MAY OR MAY NOT REACH THE WESTERN U.S. CONSISTENT WITH THEIR HISTORICAL TIMING BIASES, RECENT ECMWF RUNS HAVE BEEN PULLING OFF A CLOSED LOW THAT STAYS WELL OFFSHORE WHILE THE MAJORITY OF GFS RUNS BRING MUCH OF THE TROUGH ENERGY AT LEAST TO THE COAST IF NOT WELL INLAND. THE 00Z GFS FROM 24 HOURS AGO (00Z/18 CYCLE) WAS THE CLOSEST TO THE ECMWF SCENARIO. ON THE OTHER HAND THE 18Z AND NEW 00Z GFS RUNS ARE QUITE PROGRESSIVE WITH A FAIRLY STRONG TROUGH REACHING THE FOUR CORNERS/ROCKIES BY DAY 7 THU. UKMET RUNS SO FAR HAVE BEEN GRAVITATING TOWARD THE ECMWF AND THE PAST COUPLE CMC RUNS ARE BETWEEN THE TWO EXTREMES. WHILE THEY GET THERE IN DIFFERENT WAYS, GEFS/ECMWF/CMC ENSEMBLE MEANS PROVIDE A MORE SIMILAR AND INTERMEDIATE SOLUTION WITH GREATER RUN TO RUN STABILITY THAN SOME OPERATIONAL GUIDANCE. THUS FOR THE PURPOSES OF PROVIDING A SINGLE DETERMINISTIC FORECAST THE PREFERENCE GOES TO THE MEANS ACROSS THE EASTERN PACIFIC/WESTERN U.S. LATE TUE ONWARD UNTIL GUIDANCE CLUSTERING PROVIDES ENOUGH CONFIDENCE TO SUPPORT AN ALTERNATIVE SCENARIO. FARTHER EAST THERE IS GOOD AGREEMENT WITH A VIGOROUS SOUTHERN CANADA TROUGH/UPPER LOW THAT INITIALLY PUSHES A COLD FRONT INTO THE NORTHERN PLAINS AND BACK INTO THE NORTH-CENTRAL WESTERN U.S. HOWEVER DETAIL DIFFERENCES ARISE WITH IMMEDIATELY TRAILING ENERGY THAT ORIGINATES FROM THE NORTH PACIFIC, WITH SOME RECENT MODEL RUNS SHOWING AMPLIFICATION/SHARPENING OF SOUTHERN CANADA ENERGY TO INTERACT WITH A SEPARATE NORTHERN U.S. SHORTWAVE. THESE LOWER-PREDICTABILITY DETAILS AFFECT SPECIFICS OF WAVINESS ALONG THE INITIAL PLAINS/WESTERN U.S. FRONT WITH A COMPROMISE APPROACH PREFERRED TO RESOLVE DIFFERENCES. EVEN WITH THE UNCERTAINTY INVOLVED, THERE IS A FAIR CONSENSUS SUGGESTING THAT THE OVERALL SHORTWAVE SHOULD REACH THE EAST-CENTRAL U.S. BY WED-THU. IT REMAINS TO BE SEEN WHETHER AN EMBEDDED UPPER LOW CLOSES OFF AND WHERE IT TRACKS (RECENT GFS/CMC RUNS, 00Z/18 ECMWF), OR THE TROUGH REMAINS OPEN (12Z ECMWF, ENSEMBLE MEANS). THE SHAPE AND TIMING OF THIS APPROACHING ENERGY WILL BECOME INCREASINGLY IMPORTANT IN DETERMINING THE EVOLUTION/TRACK OF LOW PRESSURE INITIALLY MOVING ALONG OR JUST NORTH OF THE GULF COAST IN ASSOCIATION WITH A SOUTHERN STREAM UPPER LOW TRACKING FROM THE EASTERN PLAINS INTO THE SOUTHEAST/TN VALLEY DURING THE FIRST HALF OF THE PERIOD. FOR MOST OF THE PERIOD THE UPDATED FORECAST BLEND GAVE ONLY MINORITY WEIGHT TO RECENT GFS RUNS AS THEY HAVE BEEN ON THE FASTER SIDE OF THE ENVELOPE. THE NEW 00Z GFS SEEMS TO BE TRENDING SLOWER FOR AT LEAST PORTIONS OF THE FORECAST. BY WED-THU THERE IS DECENT CONFIDENCE IN LOW PRESSURE TRACKING NORTHWARD NEAR THE EAST COAST BUT POSSIBLE SCENARIOS RANGE BETWEEN A STRONGER/WESTWARD SYSTEM IF GREAT LAKES ENERGY CLOSES OFF AND A WEAKER COASTAL WAVE IN THE EVENT OF AN OPEN TROUGH ALOFT. A BLEND OF MOSTLY OPERATIONAL GUIDANCE EARLY TRANSITIONING TO THE ENSEMBLE MEANS (MORE 12Z ECMWF MEAN THAN 18Z GEFS/12Z NAEFS) BY DAYS 6-7 WED-THU PROVIDED GOOD CONTINUITY FOR SYSTEM DEPICTION OVER THE SOUTH AND EAST THROUGH THE PERIOD, EVEN IF CONFIDENCE IN THE EXACT FORECAST IS NOT PARTICULARLY HIGH BY WED-THU. ...WEATHER/THREATS HIGHLIGHTS... THE SYSTEM TRACKING ALONG THE SOUTHERN TIER/GULF COAST EARLY IN THE WEEK WILL BRING AN EPISODE OF HEAVY RAINFALL TO LOCATIONS FROM THE LOWER MS VALLEY INTO THE SOUTHEAST DURING THE LATE WEEKEND/EARLY NEXT WEEK TIME FRAME. THEN THE MOISTURE SHIELD WILL LIKELY SPREAD NORTHWARD WITH TIME AS LOW PRESSURE BEGINS TRACKING ALONG/NEAR THE EAST COAST. RAINFALL AMOUNTS OVER THE CENTRAL/NORTHERN PARTS OF THE EAST SHOULD TEND TO BE LOWER THAN OVER THE SOUTH, BUT THERE ARE POSSIBLE SYSTEM EVOLUTIONS AT THE SURFACE AND ALOFT THAT COULD LEAD TO SOME AREAS OF ENHANCED RAINFALL. THERE MAY ALSO BE A PERIOD OF STRONG WINDS ALONG THE EAST COAST GIVEN THE GRADIENT BETWEEN THE SURFACE LOW AND STRONG RIDGE TO THE NORTH. THE COMBINATION OF SHORTWAVE ENERGY ALOFT AND WAVY FRONT PUSHING SOUTHEAST FROM THE NORTHERN PLAINS/NORTH-CENTRAL WEST WILL BRING AREAS OF PRECIP TO PORTIONS OF THE ROCKIES/PLAINS AND POSSIBLY INTO THE MIDWEST/GREAT LAKES BEFORE POSSIBLY MERGING WITH THE EASTERN SYSTEM. THE NORTH-CENTRAL ROCKIES/HIGH PLAINS APPEAR TO HAVE BEST POTENTIAL FOR HIGHEST AMOUNTS WITHIN THIS AREA OF MOISTURE. SOME SNOW IS POSSIBLE OVER HIGH ELEVATIONS. UNCERTAINTY WITH THE LARGE SCALE EVOLUTION BY THE LATTER HALF OF THE PERIOD LOWERS CONFIDENCE IN COVERAGE/INTENSITY OF ANY PRECIP THAT MAY OCCUR OVER THE WEST BY MIDWEEK. THE PREFERRED ENSEMBLE MEAN SCENARIO ALOFT WOULD AT LEAST SUPPORT SOME LIGHT/SCATTERED ACTIVITY. ASIDE FROM SOME NEAR TO SLIGHTLY BELOW NORMAL READINGS OVER THE NORTHWEST EARLY IN THE PERIOD, EXPECT MUCH OF THE WESTERN U.S. TO SEE ABOVE NORMAL TEMPS DURING THE PERIOD WITH ANOMALIES REACHING PLUS 10-15F AT SOME LOCATIONS. ISOLATED DAILY RECORDS FOR HIGHS/WARM MINS MAY BE POSSIBLE. CLOUDS/PRECIP WILL KEEP HIGHS WELL BELOW NORMAL OVER THE SOUTH AND THEN EAST AND THE FRONT PROGRESSING SOUTHEAST FROM THE NORTHERN PLAINS WILL BRING REINFORCING COOL AIR TO THE CENTRAL/EAST BY TUE-THU. 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 $$