312  
FXUS02 KWBC 080644  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
243 AM EDT MON OCT 08 2018  
 
VALID 12Z THU OCT 11 2018 - 12Z MON OCT 15 2018  
 
...MICHAEL EXPECTED TO CONTINUE LIFTING NORTHEAST INTO THE WESTERN  
ATLANTIC...   
..MOISTURE FROM SERGIO MAY REACH INTO ARIZONA/SOUTHERN ROCKIES
 
 
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
GUIDANCE HAS MAINTAINED GOOD CONTINUITY FOR THE EVOLUTION OF THE  
MEAN PATTERN, SHOWING A TRANSITION FROM A POSITIVELY TILTED  
SOUTHERN CANADA INTO WESTERN U.S. TROUGH AS OF DAY 3 THU TOWARD A  
RIDGE BUILDING NEAR THE WEST COAST AND DOWNSTREAM TROUGH SETTLING  
OVER EAST-CENTRAL NORTH AMERICA. MEANWHILE MOST SOLUTIONS SHOW  
SOME DEGREE OF SMALLER-SCALE TROUGHING LIKELY TO PERSIST OVER THE  
SOUTHWESTERN U.S. AND EXTREME NORTHWESTERN MEXICO.  
 
OVER THE WEST, THE GREATEST UNCERTAINTY WILL BE WITH THE SPECIFICS  
OF INITIAL WESTERN ENERGY FOR WHICH MODELS/ENSEMBLES ARE SHOWING  
AN INCREASING SIGNAL OF CLOSING OFF FOR A TIME NEAR OR OFFSHORE  
SOUTHERN CALIFORNIA--PLUS INTERACTION WITH SERGIO WHICH SHOULD BE  
TRACKING NORTHEASTWARD AHEAD OF THE AFOREMENTIONED ENERGY. THIS  
INTERACTION WILL BE VERY SENSITIVE TO THE EXACT TIMING OF SERGIO  
AND HOW THE WESTERN U.S. SHORTWAVE ENERGY IS EVOLVING AT A  
SPECIFIC POINT IN TIME (AND WHAT PROPORTION FEEDS SOUTHWESTWARD  
VERSUS CONTINUING ALONG IN THE WESTERLIES)--THUS PREDICTABILITY AT  
EXTENDED TIME FRAMES APPEARS WELL BELOW AVERAGE FAVORING AN  
INTERMEDIATE SOLUTION. THE 12Z ECMWF MEAN PROVIDES FURTHER  
SUPPORT FOR BRINGING MOISTURE FARTHER NORTHWARD THAN THE  
OPERATIONAL RUN EVEN IF NOT AS FAR AS THE GFS/GEFS MEAN. AT THE  
SAME TIME THE PAST COUPLE PARALLEL FV3 GFS RUNS HAVE KEPT MOISTURE  
MORE SUPPRESSED THAN OPERATIONAL RUNS.  
 
MEANWHILE THE FORECAST FOR MICHAEL, WHICH HAS REACHED TROPICAL  
STORM STATUS AT THIS TIME AND SHOULD AFFECT PARTS OF THE EAST  
PRIMARILY IN THE FIRST HALF OF THE PERIOD, IS ALSO QUITE UNCERTAIN  
IN LIGHT OF CONTINUED VERY BROAD MODEL/ENSEMBLE SPREAD FOR TRACK  
AND ESPECIALLY TIMING. AT THE VERY LEAST THE OVERALL PATTERN  
APPEARS SUPPORTIVE FOR FASTER PROGRESSION THAN WHAT HAS BEEN  
DEPICTED IN ECMWF/ECMWF MEAN RUNS THUS FAR. CONSULT NATIONAL  
HURRICANE CENTER PRODUCTS FOR THE LATEST INFORMATION ON MICHAEL.  
 
MINUS THE UNCERTAINTY FOR MICHAEL, THERE IS GOOD CONSENSUS FOR  
EASTERN CANADA LOW PRESSURE TO PUSH A TRAILING COLD FRONT THROUGH  
THE EASTERN U.S. LATE THIS WEEK WITH PLAINS HIGH PRESSURE BUILDING  
IN BEHIND IT. FARTHER UPSTREAM CONTINUITY HAS HELD UP FAIRLY WELL  
FOR A CLIPPER SYSTEM EXPECTED TO TRACK ACROSS OR NEAR THE NORTHERN  
PLAINS/GREAT LAKES/NEW ENGLAND SAT ONWARD. OPERATIONAL MODELS  
SHOW TYPICALLY BETTER DETAIL THAN THE MEANS WHILE THE MEANS  
PROVIDE A GOOD CENTERED TIMING RELATIVE TO THE FASTER 18Z GFS AND  
SLOWER 12Z ECMWF. THEREFORE A MODEL/ENSEMBLE MEAN BLEND PROVIDES  
A GOOD DEPICTION OF THIS SYSTEM AND ITS TRAILING FRONT. AS THIS  
FRONT REACHES THE EAST COAST BY EARLY NEXT MON MUCH OF THE LOWER  
48 SHOULD BE DOMINATED BY A BROAD AREA OF HIGH PRESSURE CENTERED  
OVER THE ROCKIES/PLAINS.  
 
THE FIRST HALF OF THE FORECAST USES MORE 18Z GFS/12Z UKMET INPUT  
VERSUS OTHER MODELS TO PROVIDE A BETTER STARTING POINT FOR THE  
OFFICIAL TRACK OF MICHAEL AND GIVEN DECENT AGREEMENT WITH  
CONSENSUS ELSEWHERE--INCLUDING THE FACT THAT THE ECMWF MEAN TILTS  
AWAY FROM SOME ASPECTS OF ITS OPERATIONAL RUN OVER THE SOUTHWEST.  
PREFERENCES FOR THE MID-LATE PERIOD CLIPPER SYSTEM LEADS TO AN  
EVENLY WEIGHTED BLEND OF THE 18Z GFS/12Z ECMWF AND THEIR MEANS FOR  
THE LATTER HALF OF THE PERIOD.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
PARTS OF THE EASTERN U.S. WILL LIKELY SEE AREAS OF HEAVY RAINFALL  
WITH THE COMBINATION OF T.S. MICHAEL AND INTERACTION WITH AN  
APPROACHING COLD FRONT. GUIDANCE CONTINUES TO VARY WIDELY FOR  
TRACK AND TIMING OF MICHAEL SO CONFIDENCE IS FAIRLY LOW WITH  
RESPECT TO PRECISE LOCATION OF HEAVIEST RAINFALL AND  
TIMING-DEPENDENT DURATION OF THIS RAINFALL. WHILE MICHAEL BEGINS  
TO TRACK AWAY FROM THE EAST COAST, SERGIO MAY BRING MOISTURE INTO  
PORTIONS OF ARIZONA AND THE SOUTHERN ROCKIES/PLAINS. HIGHLY  
SENSITIVE DETAILS FOR EVOLUTION ALOFT LEAD TO LOW CONFIDENCE IN  
EXACT COVERAGE/INTENSITY OF RAINFALL HERE AS WELL--ESPECIALLY THE  
LATITUDE OF THE AXIS OF HEAVIER RAINFALL. THIS LACK OF CONFIDENCE  
EXTENDS TO HOW MUCH MOISTURE MAY INTERACT WITH A FRONT THAT DROPS  
INTO THE PLAINS DURING THE WEEKEND--WITH SOLUTIONS SPANNING THE  
WHOLE SPECTRUM FROM A CONSIDERABLE AMOUNT TO VERY LITTLE.  
ELSEWHERE, THE INITIAL FRONT OVER THE EAST WILL LIKELY  
DECELERATE/STALL OVER THE FLORIDA PENINSULA LEADING TO PERIODS OF  
RAINFALL IN ITS VICINITY. THE CLIPPER SYSTEM BRUSHING THE EXTREME  
NORTHERN TIER WILL BE ACCOMPANIED BY AN AREA OF MOSTLY LIGHT RAIN  
AND TRAILING RAIN/SNOW FROM SAT ONWARD.  
 
THE ROCKIES AND HIGH PLAINS WILL GENERALLY BE THE FOCUS FOR THE  
GREATEST NEGATIVE TEMPERATURE ANOMALIES DURING THE PERIOD. THE  
NORTHERN HALF OF THE PLAINS/NORTH-CENTRAL ROCKIES MAY SEE SOME  
HIGHS 20-30F BELOW NORMAL LATE THIS WEEK. A BROAD AREA OF MINUS  
10-20F ANOMALIES WILL PERSIST AS REINFORCING HIGH PRESSURE BUILDS  
IN LATER IN THE PERIOD. BY THE START OF NEXT WEEK CLOUDS/RAINFALL  
WILL LIKELY PROMOTE COOLEST READINGS VERSUS NORMAL OVER PARTS OF  
THE SOUTHERN PLAINS. COLD FRONTAL PASSAGE OVER THE EAST EARLY IN  
THE PERIOD WILL BRING A SIGNIFICANT COOLING TREND TO  
NEAR/MODERATELY BELOW NORMAL READINGS. THE WEST COAST STATES WILL  
TREND GRADUALLY WARMER VERSUS NORMAL AS UPPER RIDGING BUILDS  
TOWARD/OVER THE REGION.  
 
RAUSCH  
 
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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