780  
FXUS02 KWBC 211556  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1156 AM EDT SAT OCT 21 2017  
 
VALID 12Z TUE OCT 24 2017 - 12Z SAT OCT 28 2017  
   
..OVERVIEW/GUIDANCE EVALUATION/PREFERENCES
 
 
THE LONGWAVE PATTERN ACROSS THE NATION WILL INITIALLY FEATURE AN  
AREA OF WELL BELOW NORMAL HEIGHTS ACROSS THE EASTERN U.S. AS THE  
CORE OF LOWER HEIGHTS TRAVERSE THROUGH THE SOUTHEASTERN U.S...THE  
00Z GFS SUGGESTS 500-MB HEIGHT ANOMALIES MAY APPROACH THE 3 TO 3.5  
STANDARD DEVIATION RANGE EARLY WEDNESDAY. EVENTUALLY THIS SYSTEM  
WILL ATTAIN A NEGATIVE TILT AS IT EXITS THE EASTERN SEABOARD ON  
THURSDAY BEFORE ACCELERATING TOWARD FAR EASTERN CANADA THEREAFTER.  
LOOKING BACK TOWARD THE WESTERN/CENTRAL U.S...A STRONG 594-DM  
RIDGE WILL BE IN PLACE OVER CA INTO THE CENTRAL GREAT BASIN ON  
TUESDAY. THIS VAST POSITIVE HEIGHT ANOMALY SHOULD RETROGRADE  
TOWARD THE EASTERN PACIFIC AS AMPLIFIED FLOW FROM BRITISH  
COLUMBIA/ALBERTA BRINGS HEIGHT FALLS INTO THE UPPER INTERMOUNTAIN  
WEST LATE WEDNESDAY. MODELS THEN TRACK THIS SYSTEM EITHER TOWARD  
THE GREAT LAKES BY EARLY NEXT WEEKEND OR POTENTIALLY ALLOW FOR  
MORE DIGGING TOWARD THE FOUR CORNERS REGION.  
 
THERE ARE A NUMBER OF MODEL DIFFERENCES TO ACKNOWLEDGE DURING THE  
PERIOD WITH SUCH UNCERTAINTIES NOTED BY AS EARLY AS DAY 3/TUESDAY.  
THE 06Z/00Z GFS NOTABLY LAG THE OTHER SOLUTIONS WITH HOW QUICKLY A  
SOUTHERN STREAM WAVE BECOMES ABSORBED INTO THE BROADER TROUGH  
ACROSS THE GREAT LAKES. THIS LEADS TO THESE GFS RUNS TO BE  
SOUTHWARD DISPLACED WITH THE SURFACE LOW OVER THE LOWER GREAT  
LAKES ON 24/1200Z. AS THE PARENT UPPER LOW LIFTS TOWARD SOUTHERN  
ONTARIO/QUEBEC...THE 00Z CMC/UKMET ARE SLOW TO ALLOW THIS TO OCCUR  
WHICH LEADS TO A NEARLY QUASI-STATIONARY MOTION ON  
TUESDAY/WEDNESDAY. EVENTUALLY A NEGATIVELY-TILTED UPPER TROUGH  
LIFTS UP ALONG THE WESTERN ATLANTIC WHICH IS WELL AGREED UPON  
OUTSIDE THE 00Z CMC WHICH IS FASTER WITH THE ENTIRE PATTERN ACROSS  
THE U.S.  
 
PROGRESSIVE FLOW ATOP A WESTERN U.S. RIDGE REMAINS A STRUGGLE FOR  
THE AVAILABLE GUIDANCE. MODEST PERTURBATIONS WITHIN ZONAL FLOW  
ACROSS WESTERN CANADA WILL LEAD TO A PAIR OF POTENTIALLY STRONG  
SURFACE WAVES. ONE SUCH SYSTEM MOVING TOWARD THE UPPER MIDWEST ON  
WEDNESDAY MORNING IS DEPICTED DIFFERENTLY IN THE 06Z/00Z GFS WHICH  
ARE 10-MB STRONGER THAN THE 00Z ECMWF. FARTHER UPSTREAM...THE 00Z  
ECMWF FAVOR A MUCH MORE INTENSE SURFACE SOLUTION ACROSS ALBERTA AT  
THE SAME TIME. DIFFERENCES EXCEED 15-MB ONLY FOUR DAYS INTO THE  
FORECAST PERIOD. THIS TRAILING SHORTWAVE IS THE HANDLED  
DIFFERENTLY BY MOST OF THE GUIDANCE WHICH LEADS TO A MESSY ARRAY  
OF ENSEMBLE SPAGHETTI PLOTS. ON THURSDAY EVENING...THE SPECTRUM OF  
SOLUTIONS ARE NUMEROUS WITH THE 00Z ECMWF AND THEIR ENSEMBLES  
BEING NEARLY ON THEIR OWN DIGGING THE TROUGH TOWARD THE FOUR  
CORNERS REGION. THE 00Z ECMWF STANDS OUT AS AN OUTLIER AT THIS  
POINT BEING WELL TO THE SOUTHWEST WITH POSSIBLY ONLY 3 GEFS  
MEMBERS SUGGESTING SUCH A FORECAST. MEANWHILE...THE 06Z/00Z GFS  
ARE MORE PROGRESSIVE USHERING LOWER HEIGHTS TOWARD THE GREAT LAKES  
LATE IN THE PERIOD. CONSIDERING OTHER MODELS...THE 00Z CMC AND ITS  
ENSEMBLES ARE WELL EAST OF THE CONSENSUS WHILE THE 00Z UKMET HAS  
MORE SIMILARITIES TO THE GFS CYCLES. THE DIFFERENCES PROPEL THEIR  
WAY THROUGH THE REMAINDER OF THE PERIOD WHICH SUGGESTS HEAVY USE  
OF ENSEMBLE MEANS DEEPER INTO THE FORECAST.  
 
GIVEN UNCERTAINTY IN THE PATTERN...DECIDED TO MAINTAIN 10 PERCENT  
OF THE OVERNIGHT SUITE IN THE MIX TO PRESERVE SOME LEVEL OF  
CONTINUITY. OTHERWISE...TOOK A DIRECT SPLIT BETWEEN THE 06Z  
GFS/00Z ECMWF EARLY ON WHILE GRADUALLY INCORPORATING THEIR  
RESPECTIVE ENSEMBLE MEANS BY DAY 4/WEDNESDAY ONWARD. AT THE DAY  
6/FRIDAY TIMEFRAME AND THEREAFTER...WENT WITH AN EVEN SPLIT OF THE  
06Z GEFS/00Z ECMWF ENSEMBLE MEANS WITH CONFIDENCE BECOMING BELOW  
AVERAGE AT THIS POINT.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
ON TUESDAY...WELL ABOVE NORMAL TEMPERATURES ARE EXPECTED BOTH  
ACROSS THE WEST COAST AS WELL AS THE NORTHEASTERN CORRIDOR OF THE  
U.S. A NUMBER OF DAILY TEMPERATURE RECORDS ARE LIKELY TO BE BROKEN  
ON TUESDAY/WEDNESDAY ACROSS SOUTHERN CA AS WELL AS ALONG THE  
EASTERN SEABOARD. AT THE FORMER...SUCH RECORDS MAY BE POSSIBLE  
BOTH FOR WARM MAXIMA AND MILD OVERNIGHT READINGS. TEMPERATURES  
JUST OVER THE 100 DEGREE MARK ARE EXPECTED ON TUESDAY ACROSS THE  
LOS ANGELES BASIN. UNFORTUNATELY...IDEAL CONDITIONS ARE IN PLACE  
TO SUPPORT AN ENHANCED WILDFIRE DANGER GIVEN DRY/GUSTY WINDS VIA  
OFFSHORE DOWNSLOPE EFFECTS. BACK TO THE EAST COAST...A HIGHLY  
MERIDIONAL FLOW REGIME AHEAD OF THE BARRELING UPPER LOW WILL  
TRANSPORT MILD TROPICAL AIR UP ALONG THE I-95 CORRIDOR LEADING TO  
SOME VERY WARM OVERNIGHT TEMPERATURES ON TUESDAY/WEDNESDAY.  
LOOKING FARTHER AHEAD...THE NEXT UPPER TROUGH MOVING THROUGH THE  
CENTRAL U.S. ON THURSDAY/FRIDAY WILL USHER IN TEMPERATURES 10 TO  
15 DEGREES BELOW CLIMATOLOGY AS HIGHS STAYS IN THE 40S ACROSS THE  
CENTRAL/NORTHERN ROCKIES AND GREAT PLAINS EASTWARD TO THE UPPER  
GREAT LAKES.  
 
HEAVY PRECIPITATION WILL BE LIKELY ALONG THE FRONTAL BOUNDARY  
EXTENDING BACK TOWARD NEW ENGLAND ON TUESDAY/WEDNESDAY. AS THE  
REGION OF LOWER HEIGHTS MOVES THROUGH THE GREAT LAKES/OH  
VALLEY...CONVECTIVELY-FORCED SHOWERS WILL BE LIKELY GIVEN THE  
STEEPENING LAPSE RATES. EVENTUALLY PRECIPITATION SHOULD BEGIN TO  
FILL THE CENTRAL U.S. AS THE NEXT UPPER TROUGH SWEEPS THROUGH THE  
REGION. UNCERTAINTIES IN THE FORECAST TRACK WILL LEAD TO DRAMATIC  
DIFFERENCES IN PRECIPITATION INTENSITY/TYPE. RETURN FLOW AHEAD OF  
THIS TROUGH SHOULD BRING AMPLE GULF MOISTURE BACK INTO THE PICTURE  
BY LATE NEXT WEEK.  
 
RUBIN-OSTER  
 

 
 
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