434  
FXUS02 KWBC 171558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1157 AM EDT MON SEP 17 2018  
 
VALID 12Z THU SEP 20 2018 - 12Z MON SEP 24 2018  
 
...TROPICAL MOISTURE FORECAST TO LIFT INTO PORTIONS OF THE  
SOUTHWEST LATER THIS WEEK...  
 
   
..PATTERN OVERVIEW
 
 
UPPER RIDGING NEAR THE ALEUTIANS AND IN THE SOUTHEAST CONUS WILL  
FAVOR POSITIVELY TILTED AND REINFORCED TROUGHING CENTERED NEAR THE  
PACIFIC NORTHWEST COAST THIS WEEKEND. A LEAD STORM SYSTEM WILL  
LIFT INTO SOUTHEASTERN CANADA LATE THIS WEEK AND SETTLE ITS COLD  
FRONT INTO THE MID-ATLANTIC AND OHIO VALLEY AS A WESTERN FRONT  
PUSHES THROUGH THE INTERIOR. THE NATIONAL HURRICANE CENTER IS  
MONITORING AN AREA OF DISTURBED WEATHER SOUTH OF BAJA CALIFORNIA  
THAT HAS THE POTENTIAL FOR TROPICAL DEVELOPMENT. THIS SYSTEM  
(NAMED OR NOT) WILL LIFT NORTHWARD AND SPREAD RAINFALL INTO  
PORTIONS OF AZ/NM LATER THIS WEEK.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE OVERALL PATTERN REMAINS SYNOPTICALLY STUCK AS THE ANOMALY  
CENTERS MEANDER IN THE SAME GENERAL AREA. AGREEMENT AMONG THE  
LATEST ENSEMBLES REMAIN AT LEAST AVERAGE WITH EXPECTED  
TIMING/AMPLITUDE DIFFERENCES. FOLLOWED A DETERMINISTIC BLEND TO  
START, WITH A PREFERENCE TOWARD THE 00Z PARALLEL GFS VS THE  
OPERATIONAL 00Z/06Z RUNS DUE TO A SEEMINGLY OVERDONE SFC LOW  
ACROSS SOUTHERN CANADA ON FRIDAY. BY LATE THIS WEEKEND INTO NEXT  
WEEK, THE ENSEMBLES CLUSTERED SLOWER TO LOWER HEIGHTS ACROSS THE  
INTERIOR WEST (EASTERN WA/OR INTO ID) VS THE DETERMINISTIC  
ECMWF/CANADIAN/GFS BUT THE PREDICTABILITY OF THE NORTHERN/SOUTHERN  
EMPHASIS IN HEIGHTS FALLS DUE TO THE POSITIVE TILT OF THE TROUGH  
IS LOW. THUS, OPTED TO SPLIT THE DIFFERENCE BETWEEN THE  
DETERMINISTIC MODELS AND THE ENSEMBLE MEANS BY NEXT MONDAY. TO THE  
SOUTH, ANY POTENTIAL TROPICAL DEVELOPMENT WILL BE SHORT-LIVED BUT  
THE MOISTURE ASSOCIATED WITH THE SYSTEM WILL TRAVERSE NW MEXICO  
AND PUSH THROUGH AZ/NM LATER THIS WEEK.  
 
FRACASSO  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
A PAIR OF HEAVY RAINFALL THREATS ARE IN THE PICTURE THROUGH  
PERHAPS SATURDAY. THE FIRST AREA SHOULD CONCENTRATE NEAR THE  
DYNAMIC AREA OF LOW PRESSURE SWEEPING THROUGH THE GREAT LAKES.  
SECONDLY, TROPICAL MOISTURE PUMPING OUT OF THE PACIFIC SHOULD  
ENHANCE THE THREAT FOR HEAVY PRECIPITATION OVER THE FOUR CORNERS  
REGION BUT PARTICULARLY INTO THE SOUTHERN HIGH PLAINS. THE LATTER  
AREA IS MORE MODEL DEPENDENT AS THERE ARE PLENTY OF SOLUTIONS  
SHOWING LOWER QPF VALUES. EVENTUALLY THE BOUNDARY STALLING OUT OF  
THE LOWER MID-ATLANTIC COULD PROVE TO KEEP CONDITIONS WET TOWARD  
THE LATTER HALF OF THE WEEKEND INTO THE FOLLOWING WEEK. ACROSS THE  
PACIFIC NORTHWEST, THE COMPACT UPPER LOW SHOULD SPREAD A BURST OF  
SHOWERS TO WESTERN WASHINGTON UP INTO BRITISH COLUMBIA ON FRIDAY  
WITH ADDITIONAL PRECIPITATION LIKELY DURING THE FOLLOWING DAYS IN  
RESPONSE TO THE NEXT TROUGH.  
 
THROUGHOUT THE FORECAST, THE MOST PERSISTENT AREA OF BELOW AVERAGE  
TEMPERATURES WILL RESIDE OVER THE UPPER INTERMOUNTAIN WEST  
EASTWARD ACROSS THE NORTHERN ROCKIES AND INTO THE DAKOTAS. ON  
THURSDAY, SUCH DEPARTURES COULD REACH THE 10 TO 15 DEGREE RANGE AS  
HIGHS STRUGGLE TO ESCAPE THE 50S. TO THE SOUTH OF THE MIGRATORY  
BOUNDARY, WARM CONDITIONS WILL PREVAIL ACROSS THE LOWER/MIDDLE  
MISSISSIPPI VALLEYS INTO THE OHIO/TENNESSEE VALLEYS. TOWARD THE  
WEEKEND, ANOMALIES SHOULD DECREASE A BIT BUT THAT IS ALSO AN  
ARTIFACT OF HEAVY ENSEMBLE USAGE. HOWEVER, IT SHOULD GENERALLY  
REMAIN ON THE COOL SIDE ACROSS THE NORTH-CENTRAL U.S. GIVEN THE  
INFLUENCE OF CANADIAN AIR MASSES.  
 
RUBIN-OSTER  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES 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  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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