936  
FXUS02 KWBC 240700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 AM EST SUN FEB 24 2019  
 
VALID 12Z WED FEB 27 2019 - 12Z SUN MAR 03 2019  
   
..PATTERN OVERVIEW
 
 
THE PERSISTENT REX BLOCK PATTERN OVER THE EAST-CENTRAL  
PACIFIC--AND IN PARTICULAR THE RIDGE EXTENDING THROUGH  
NORTHWESTERN NORTH AMERICA INTO THE ARCTIC--WILL CONTINUE TO  
PROVIDE SOME CHALLENGES FOR DETERMINING FORECAST SPECIFICS.  
HOWEVER IN SPITE OF THE ONGOING UNCERTAINTY FOR SOME ASPECTS OF  
THE FORECAST, THERE ARE THE FOLLOWING COMMON THEMES: SIGNIFICANT  
PRECIPITATION FROM CALIFORNIA EASTWARD INTO THE ROCKIES DUE TO  
SYSTEMS EARLY AND LATE IN THE PERIOD, A CONTINUATION OF VERY COLD  
TEMPERATURES OVER AND NEAR THE NORTHERN HALF OF THE PLAINS WITH  
REINFORCEMENT FROM A DEEP UPPER LOW THAT MAY TRACK NEAR THE  
CANADIAN BORDER FOR A TIME, AND POTENTIAL FOR A LATE WEEK/WEEKEND  
STORM SYSTEM TO AFFECT THE EASTERN U.S. WITH VARIOUS WEATHER  
HAZARDS.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
TWO DOMINANT FEATURES OF INTEREST DURING THE PERIOD WILL BE AN  
UPPER LOW INITIALLY OFF THE PACIFIC NORTHWEST COAST AND AN UPPER  
LOW OVER SOUTHERN CANADA. THE COMBINED EVOLUTION OF THESE  
FEATURES WILL DETERMINE THE CHARACTER OF LOW PRESSURE DEVELOPMENT  
FOR WHICH GUIDANCE IS SHOWING A STRONGER SIGNAL OVER THE EASTERN  
STATES INTO SOUTHEASTERN CANADA BY FRI INTO THE WEEKEND. THROUGH  
THE 18Z MODEL CYCLE, GUIDANCE STILL SHOWED SOME SPREAD FOR HOW THE  
PACIFIC NORTHWEST UPPER LOW ENERGY WILL EJECT INLAND BUT WITH SOME  
APPARENT TRENDS TOWARD A COMMON SOLUTION. GFS RUNS HAVE BEEN ON  
THE FAST SIDE OF THE SPECTRUM--BUT WITH THE 12Z UKMET ADDING SOME  
SUPPORT TO THE 12-18Z GFS WHICH WERE NOT QUITE AS FAST/AMPLIFIED  
WITH EJECTING ENERGY AS SOME EARLIER RUNS. ON THE OTHER HAND  
ECMWF-BASED GUIDANCE STILL WANTS TO KEEP THE ENERGY FARTHEST WEST  
BUT NOT TO THE EXTENT SEEN 24 HOURS AGO. MEANWHILE GUIDANCE  
SUGGESTS THAT STRONG JET ENERGY DROPPING DOWN THE EAST SIDE OF THE  
UPPER RIDGE MAY PULL THE INITIAL SOUTHERN CANADA UPPER LOW CENTER  
A BIT TO THE WEST/SOUTHWEST MID-LATE WEEK WITH A SUBSEQUENT TRACK  
VERY CLOSE TO THE U.S.-CANADIAN BORDER. GFS RUNS HAD BEEN ON THE  
SOUTHERN SIDE OF THE SOLUTION ENVELOPE FOR THIS FEATURE BUT THERE  
IS SOME INCREASING MODEL SUPPORT FOR A FAIRLY FAR SOUTHWARD TRACK.  
TRENDS WITH THE CANADIAN UPPER LOW ON THEIR OWN ARE RAISING THE  
PROBABILITY OF SURFACE DEVELOPMENT REACHING THE GREAT LAKES REGION  
BY DAY 6 SAT. STRENGTH OF THIS DEVELOPMENT APPEARS MORE UNCERTAIN  
DUE TO LOWER CONFIDENCE IN THE TIMING/STRENGTH OF EJECTING PACIFIC  
NORTHWEST ENERGY AND HOW IT MAY INTERACT WITH THE CANADIAN UPPER  
LOW. THROUGH THE 18Z GFS RUN THE MODEL HAD BEEN BACKING OFF THE  
DEPTH OF ITS GREAT LAKES SYSTEM DUE TO ITS SLOWER TREND FOR THE  
PACIFIC NORTHWEST ENERGY, BUT THE 00Z RUN HAS REVERTED BACK TO A  
DEEP SCENARIO MORE SIMILAR FROM 24 HOURS AGO. 12Z ECMWF AND PAST  
COUPLE CMC RUNS OFFER A LESS EXTREME EVOLUTION. A COMPROMISE  
AMONG THE DAYTIME MODEL RUNS AND ENSEMBLE MEANS SHOULD PROVIDE A  
REASONABLE STARTING POINT TO REFLECT INCREASING LIKELIHOOD OF  
SYSTEM EXISTENCE BUT UNCERTAINTY FOR DEPTH.  
 
FOR THE SYSTEM APPROACH CALIFORNIA DURING THE LATTER HALF OF THE  
PERIOD, THERE ARE ONGOING TIMING DIFFERENCES THAT HAVE YET TO BE  
WORKED OUT. THE ECMWF/ECMWF MEAN HAVE ADJUSTED BACK SOMEWHAT FROM  
EARLIER RUNS THAT HAD BEEN ON THE FASTER SIDE OF THE ENVELOPE. IN  
CONTRAST THE 18Z GFS AND THE NEW 00Z RUN ARE ON THE SLOW SIDE. AN  
INTERMEDIATE SOLUTION STILL APPEARS BEST UNTIL THERE IS A  
PRONOUNCED SHIFT TOWARD EITHER SIDE OF THE SPREAD.  
 
EARLY IN THE PERIOD OVER THE EASTERN HALF OF THE CONTINENT, THERE  
ARE STILL DISCREPANCIES OVER SMALL-MEDIUM SCALE SHORTWAVE DETAILS  
OVER SOUTHERN CANADA/NORTHERN U.S. WITH CORRESPONDING SURFACE  
DIFFERENCES. AS WITH THE PREVIOUS COUPLE DAYS THE GFS IS  
STRONGEST AND ECMWF WEAKEST WHILE THE CMC/UKMET GENERALLY OFFER A  
COMPROMISE. A BLEND OFFERS THE MOST REASONABLE STARTING POINT  
GIVEN LOW PREDICTABILITY DUE TO RELATIVELY SMALL SCALE, AND  
INCOMING 00Z GUIDANCE SEEMS TO SUPPORT THIS IDEA.  
 
BASED ON ABOVE CONSIDERATIONS THE UPDATED FORECAST STARTED WITH AN  
OPERATIONAL MODEL BLEND (12-18Z GFS AND 12Z ECMWF/UKMET/CMC) DAYS  
3-4 WED-THU AND THEN GRADUALLY TRENDED TOWARD ALMOST EVEN WEIGHT  
AMONG OPERATIONAL MODELS AND 18Z GEFS/12Z ECMWF MEANS.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
OVER THE WEST EXPECT AREAS OF ENHANCED PRECIPITATION (RAIN AND  
HIGHER ELEVATION SNOW) TO EXTEND FROM CALIFORNIA THROUGH THE GREAT  
BASIN AND INTO THE NORTH-CENTRAL/CENTRAL ROCKIES. THERE WILL BE  
TWO MAIN SYSTEMS OF NOTE, ONE ARRIVING AROUND THE START OF THE  
PERIOD AND BRINGING RAIN/SNOW DURING WED-THU AND THE NEXT MOST  
LIKELY SPREADING MOISTURE ACROSS THE REGION DURING THE WEEKEND.  
THE SECOND SYSTEM'S MOISTURE AXIS SHOULD BE SOMEWHAT SOUTHWARD OF  
THE FIRST. HIGHEST TOTALS DURING THE PERIOD SHOULD BE OVER THE  
SIERRA NEVADA RANGE.  
 
AROUND MIDWEEK MODEST SHORTWAVE ENERGY/SURFACE WAVINESS MAY BRING  
AN AREA OF MOSTLY LIGHT SNOW ACROSS THE NORTHEASTERN QUADRANT OF  
THE LOWER 48. ANOTHER SMALL AREA OF RAIN/SNOW MAY CROSS PARTS OF  
THE OHIO VALLEY/MID-ATLANTIC LATER IN THE WEEK. THEN GUIDANCE IS  
STARTING TO SHOW MORE SIMILAR IDEAS FOR POTENTIALLY STRONG SURFACE  
DEVELOPMENT WITH LOW PRESSURE REACHING THE GREAT LAKES AROUND SAT  
AND CONTINUING NORTHEASTWARD THEREAFTER. HIGHEST SNOW POTENTIAL  
WOULD EXTEND ACROSS PARTS OF THE MIDWEST INTO THE NORTHWESTERN  
GREAT LAKES WITH MEANINGFUL SNOWFALL ALSO POSSIBLE OVER NEW  
ENGLAND. EXPECT RAIN FARTHER SOUTH AND SOME POCKETS OF LOCALLY  
MODERATE-HEAVY ACTIVITY MAY BE POSSIBLE. SLIGHTLY BETTER  
POTENTIAL FOR HIGHEST TOTALS WILL BE OVER THE GULF COAST STATES  
BUT RELATIVE MAXIMA MAY BE POSSIBLE OVER OTHER PARTS OF THE EAST  
AS WELL.  
 
CONTINUE TO EXPECT VERY COLD TEMPERATURES CENTERED OVER AND NEAR  
THE NORTHERN HALF OF THE PLAINS. WED-FRI READINGS WILL BE EXTREME  
ENOUGH AT 10-30F BELOW NORMAL BUT A STRONGER SURGE OF COLD AIR FRI  
INTO THE WEEKEND COULD BRING SOME AREAS TO 30-40F BELOW NORMAL  
WHILE LESS EXTREME COLD ANOMALIES SPREAD FARTHER SOUTH AND EAST.  
MEANWHILE PACIFIC FLOW WILL KEEP MIN TEMPERATURES IN PARTICULAR  
ABOVE NORMAL OVER THE SOUTHERN 2/3 OF THE WEST. THE SOUTHERN TIER  
STATES SHOULD SEE ABOVE NORMAL TEMPERATURES ON AVERAGE BUT WITH A  
COOLING TREND NEXT WEEKEND.  
 
RAUSCH  
 
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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