897  
FXUS02 KWBC 131558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1057 AM EST WED FEB 13 2019  
 
VALID 12Z SAT FEB 16 2019 - 12Z WED FEB 20 2019  
   
..16Z UPDATE
 
 
THE OVERALL PATTERN AND THINKING FROM WPC REMAINED SIMILAR TO THE  
PREVIOUS SHIFT. THE LATEST SUITE OF MODEL GUIDANCE CONTINUES TO  
HONE IN ON A PATTERN OF MEAN TROUGHING IN THE WESTERN U.S., WHILE  
DETAILS OF THE SMALLER SCALE SHORTWAVES REMAIN ELUSIVE FOR NOW.  
THROUGH DAYS 3-4, A BLEND OF THE DETERMINISTIC 00Z UKMET, 00Z  
ECMWF, AND THE 00Z AND 06Z GFS WERE USED ALONG WITH SMALL  
COMPONENTS OF THE GEFS AND ECMWF MEANS. BY DAY 5-7, THE 00Z AND  
06Z OPERATIONAL GFS RUNS DEVELOP DIFFERENTLY FROM OTHER  
MODELS--COMBINING ENERGY NEAR ALASKA IN THOSE MODEL RUNS LEADS TO  
AN UPPER-LEVEL LOW THAT MOVES MORE SLOWLY THROUGH THE PACIFIC. THE  
ECMWF ON THE OTHER HAND KEEPS THE VORTICITY MAXES SEPARATE, FASTER  
MOVING, AND ONE SERVES TO REINFORCE THE MAIN TROUGH. THE GFS  
SOLUTIONS SEEMED UNLIKELY AS THE REX BLOCK HAS TAKEN HOLD IN THE  
PACIFIC, AND AN UPPER LOW MAY NOT BE ABLE TO ENCROACH ON THAT TOO  
EASILY. THUS, EXCLUDED THE OPERATIONAL GFS RUNS FROM THAT  
TIMEFRAME IN FAVOR OF THE EC AND GEFS MEANS AND THE OPERATIONAL  
ECMWF. FOR MORE DETAILS AND FOR SENSIBLE WEATHER IMPACTS, PLEASE  
SEE THE PREVIOUS DISCUSSION BELOW.  
 
TATE  
 
   
..PREVIOUS DISCUSSION
 
 
   
..OVERVIEW
 
 
MODELS/ENSEMBLES CONTINUE TO SHOW STRONG CONSENSUS THAT A REX  
BLOCK PATTERN ACROSS THE CENTRAL/NORTH PACIFIC, EXTENDING FROM  
NEAR HAWAII TO ALASKA, SHOULD PERSIST THROUGH THE NEXT WEEK. THE  
STRONGEST POSITIVE HEIGHT ANOMALY CENTER ACROSS THE NORTH PACIFIC  
IS FORECAST TO SHIFT A BIT SOUTH OVER THE NEXT SEVERAL DAYS, WITH  
HEIGHTS FALLING AT LEAST SOMEWHAT ACROSS ALASKA. THE DOWNSTREAM  
RESULT OF THIS SLIGHT MODULATION IN PATTERN ACROSS THE PACIFIC  
WILL ALLOW THE MEAN UPPER TROUGH WHICH HAS BEEN PERSISTENT NEAR  
THE PACIFIC NORTHWEST TO SHIFT INLAND, TO A POSITION ACROSS THE  
ROCKIES BY THE MIDDLE OF NEXT WEEK. NUMEROUS SMALLER SCALE  
SHORTWAVES WILL CONTINUE TO TRAVERSE THIS FLOW ACROSS THE CONUS,  
ORIGINATING IN THE HIGHER LATITUDES OF THE NORTH PACIFIC. MODEL  
GUIDANCE CONTINUES TO SUGGEST THAT THE DOMINANT STORM TRACK ACROSS  
THE CONUS SHOULD SHIFT SOUTHWARD OVER THE NEXT WEEK, WITH A MUCH  
MORE ACTIVE SOUTHERN STREAM TAKING SHAPE. ADDITIONALLY, THE  
SUBTROPICAL RIDGE WILL INTENSIFY OFF THE SOUTHEAST U.S. COAST  
EARLY NEXT WEEK, WHICH WILL AID IN THE TRANSPORT OF GULF OF MEXICO  
MOISTURE NORTHWARD AHEAD OF A POTENTIALLY SIGNIFICANT LOW PRESSURE  
SYSTEM FOR THE EASTERN THIRD OF THE CONUS NEXT TUE-WED.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
FREQUENT SMALLER-SCALE SHORTWAVES CROSSING THE SOUTHERN TIER AND  
THE LARGE WESTERN TROUGH AXIS SHIFTING INLAND WILL BE THE MOST  
SIGNIFICANT FEATURES THIS FORECAST. AN ACTIVE SOUTHERN STREAM WILL  
BE QUICKLY TAKING SHAPE BY DAY 3 (SAT) WITH A FRONTAL WAVE LIKELY  
CROSSING THE SOUTHEAST U.S. THE PAST FEW MODEL CYCLES HAVE  
REVEALED A MUCH CLEARER PICTURE AND MODEL/ENSEMBLE SOLUTIONS ARE  
NOW DECENTLY CLUSTERED FOR THIS FEATURE AS IT QUICKLY MOVES  
NORTHEAST INTO THE ATLANTIC BY LATE SAT. NEXT SHORTWAVE-INDUCED  
FRONTAL WAVE SHOULD GET GOING LATE FRI ACROSS THE SOUTHERN  
PLAINS/LOWER MISSISSIPPI VALLEY AND QUICKLY LIFT NORTHEAST TOWARD  
THE TENNESSEE VALLEY/SOUTHERN APPALACHIANS SAT NIGHT/SUN. SPREAD  
IS A BIT HIGHER ON THIS FEATURE, WITH SOME SOLUTIONS TRYING TO  
TAKE THE LOW FARTHER NORTH INTO THE OHIO VALLEY, BUT SPREAD HAS  
ALSO REDUCED SIGNIFICANTLY OVER THE PAST 24 HOURS WITH RESPECT TO  
THIS SYSTEM. AS THE LARGE UPPER TROUGH SHIFTS EAST INTO THE  
SOUTHWEST SUN-MON, MODELS SHOW GOOD CONSENSUS ON THE LARGE SCALE  
ASPECTS OF THIS FEATURE. GIVEN THESE CONSIDERATIONS, THE WPC  
FORECAST DURING DAYS 3-5 WAS BASED ON A MULTI-MODEL BLEND OF  
DETERMINISTIC GUIDANCE INCLUDING THE 12Z ECMWF/UKMET AND 18Z GFS.  
 
EVEN IN THE LATTER PORTION OF THE EXTENDED FORECAST PERIOD, MODEL  
CONSENSUS HAS IMPROVED OVER THE PAST FEW RUNS. THE SPEED WITH  
WHICH AN AMPLIFIED LEADING SHORTWAVE AHEAD OF THE MEAN TROUGH  
MOVES EAST INTO THE CENTRAL U.S. TUE-WED CONTINUES TO SHOW SOME  
VARIABILITY IN SOLUTIONS, BUT THE TREND HAS BEEN TOWARD A SLOWER  
EJECTION EASTWARD, AND THIS IDEA SEEMS REASONABLE GIVEN THE  
AMPLIFYING LARGE SCALE FLOW AND EVOLVING TROUGH/RIDGE PATTERN  
ACROSS THE CONUS. SOME CONSENSUS IS EVIDENT AMONG  
DETERMINISTIC/ENSEMBLE SOLUTIONS THAT AN AREA OF LOW PRESSURE  
SHOULD DEVELOP ALONG A STRONG FRONTAL BOUNDARY DRAPED ALONG THE  
GULF COAST TUE AS THE UPPER WAVE REACHES THE CENTRAL U.S., WITH  
THE WAVE LIFTING NORTHEASTWARD TOWARD THE APPALACHIANS TUE  
NIGHT-WED, AND PERHAPS A SECONDARY COASTAL LOW TAKING SHAPE NEAR  
THE MID-ATLANTIC COAST. SOME MODEST CLUSTERING IS EVIDENT AMONG  
ENSEMBLE LOWS EVEN ON DAY 7, SO FORECAST CONFIDENCE IS AT LEAST A  
BIT HIGHER THAN IT HAS BEEN IN RECENT DAYS. FINALLY, FARTHER WEST,  
SHORTWAVE ENERGY WILL CONTINUE TO SKIRT THE PACIFIC NORTHWEST EVEN  
INTO THE MIDDLE OF NEXT WEEK - WITH SOME CONSENSUS AMONG THE  
GUIDANCE THAT ONE WEAKENING WAVE SHOULD ARRIVE ON TUE WITH ANOTHER  
STRONGER ONE BY WED. KEPT 40 PERCENT OF THE GFS/ECMWF  
DETERMINISTIC SOLUTIONS IN THE BLEND THROUGH DAYS 6-7, WITH THE  
SLIGHT MAJORITY OF THE FORECAST BASED ON THEIR RESPECTIVE ENSEMBLE  
MEANS.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE ACTIVE SOUTHERN STREAM STORM TRACK SETTING UP WILL MEAN A  
PERIOD OF GENERALLY WET CONDITIONS FOR AREAS FROM THE SOUTHERN  
PLAINS TO THE EASTERN SEABOARD. LOW PRESSURE TRAVERSING THE  
SOUTHEAST/SOUTHERN MID-ATLANTIC ON SAT COULD PRODUCE AN AREA OF  
RELATIVELY LIGHT SNOWS NORTH OF THE SYSTEM ACROSS THE  
MID-ATLANTIC, WITH LIGHT TO MODERATE RAINS FOR PORTIONS OF THE  
SOUTHEAST. THE NEXT FRONTAL WAVE ON SUN MAY BRING ANOTHER ROUND OF  
LIGHT TO MODERATE RAIN/SNOW TO PORTIONS OF THE MID-ATLANTIC AND  
SOUTHEAST. THE MORE SIGNIFICANT LOW PRESSURE SYSTEM EXPECTED TO  
DEVELOP ALONG THE WESTERN GULF COAST EARLY NEXT WEEK WILL HAVE  
ACCESS TO MUCH DEEPER MOISTURE COURTESY OF THE AMPLIFYING TROUGH  
TO THE WEST AND SUBTROPICAL RIDGE TO THE EAST, AND THIS SYSTEM  
LOOKS TO BE A MAJOR PRECIPITATION PRODUCER. ISENTROPIC LIFT ACROSS  
A LINGERING POLAR FRONT AHEAD OF THE DEVELOPING LOW PRESSURE  
SYSTEM IS EXPECTED TO PRODUCE WIDESPREAD MODERATE TO HEAVY RAIN  
FROM PORTIONS OF THE LOWER MISSISSIPPI VALLEY TO THE SOUTHEAST AND  
TENNESSEE VALLEYS LATE MON INTO TUE/WED. COLDER AIR TO THE NORTH  
OF THE SYSTEM MAY RESULT IN AN AREA OF WINTRY PRECIPITATION FROM  
PORTIONS OF THE MID-MISSISSIPPI VALLEY TO THE MID-ATLANTIC.  
CURRENT MODEL GUIDANCE SUGGESTS SOME POTENTIAL FOR A SIGNIFICANT  
WINTER WEATHER EVENT ACROSS PORTIONS OF THE MID-ATLANTIC ON TUE,  
WITH SNOW, SLEET, AND FREEZING RAIN ALL POSSIBLE. PLEASE REFER TO  
THE WINTER WEATHER OUTLOOK PROBABILITIES FOR DAYS 4-7 FOR FURTHER  
DETAILS ON THE WINTER WEATHER THREATS IN THE MEDIUM RANGE.  
MEANWHILE, WHILE A SIGNIFICANT SHORTWAVE SAT-SUN WILL ENSURE THAT  
RAIN/MOUNTAIN SNOW CONTINUE ALONG THE WEST COAST, THIS REGION WILL  
SEE SOME RESPITE BY MON-TUE AS THE MAIN TROUGH AXIS SHIFTS EAST  
AND HEIGHTS RISE A BIT. ARRIVAL OF ANOTHER SHORTWAVE BY LATE TUE  
INTO WED MAY BRING ANOTHER ROUND OF PRECIPITATION TO THE NORTHWEST.  
 
WELL BELOW AVERAGE TEMPERATURES WILL PERSIST AND BECOME MORE  
WIDESPREAD ACROSS THE WESTERN/CENTRAL U.S. THROUGH THE MEDIUM  
RANGE. THE CORE OF THE COLD AIR LATE THIS WEEK WILL INITIALLY BE  
FROM THE NORTHERN PLAINS TO THE UPPER MIDWEST, WITH TEMPERATURES  
15 TO 25 DEG F BELOW AVERAGE. BY SUN-MON AS THE WESTERN TROUGH  
PUSHES INLAND EXPECT HIGH TEMPERATURES OF 15 TO 30 DEG BELOW  
AVERAGE ACROSS MUCH OF THE WESTERN AND CENTRAL U.S., CONTINUING  
THROUGH THE END OF THE FORECAST PERIOD (WED).  
 
RYAN  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION AND WINTER WEATHER OUTLOOK  
PROBABILITIES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
 

 
 
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