392  
FXUS02 KWBC 050627  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
227 AM EDT FRI OCT 05 2018  
 
VALID 12Z MON OCT 08 2018 - 12Z FRI OCT 12 2018  
 
...CENTRAL U.S. EXCESSIVE RAINFALL THREAT/ACTIVE WESTERN U.S.  
PATTERN...  
...SEPARATE AREAS OF TROPICAL MOISTURE POSSIBLY ARRIVING INTO THE  
GULF COAST AND SOUTHWEST/SOUTHERN ROCKIES...  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE FORECAST PERIOD WILL START ON MON WITH A DEEP NEUTRALLY-TILTED  
WESTERN U.S. TROUGH BETWEEN TWO VERY STRONG RIDGES (MAX HEIGHTS  
594+ DM) OVER THE EASTERN PACIFIC AND MID-ATLANTIC REGION/WESTERN  
ATLANTIC OCEAN. THE MAJORITY OF GUIDANCE AGREES IN PRINCIPLE THAT  
WITH THE PASSAGE OF TIME THE MEAN TROUGH WILL TAKE ON MORE OF A  
POSITIVE TILT AS THE EASTERN PACIFIC RIDGE SLOWLY WEAKENS AND THE  
EAST COAST RIDGE FLATTENS.  
 
FOR SPECIFICS WITHIN THE OVERALL TROUGH, ONE UNCERTAINTY WILL BE  
WITH HOW ENERGY OVER THE GULF OF ALASKA/NORTHWESTERN NORTH AMERICA  
AT THE START OF THE PERIOD DIGS INTO THE MEAN TROUGH AND IN THE  
PROCESS EJECTS INITIAL TROUGH ENERGY INTO THE PLAINS AND BEYOND.  
THERE ARE ALSO QUESTION MARKS OVER HOW THIS EJECTING TROUGH ENERGY  
WILL INTERACT WITH ENERGY THAT MAY BE LINGERING OVER SOUTHERN  
CANADA. MODELS/ENSEMBLE MEANS PROVIDE DECENT CLUSTERING REGARDING  
CONSOLIDATION OF LOW PRESSURE OVER OR NEAR THE UPPER MS VALLEY BY  
AROUND EARLY DAY 5 WED IN ASSOCIATION WITH THE EJECTING WESTERN  
ENERGY BUT WITH A FAIR AMOUNT OF SPREAD ON EXACT CHARACTER OF THE  
LOW DUE TO DETAIL DIFFERENCES ALOFT. THIS SYSTEM SHOULD PROVIDE A  
PRONOUNCED EASTWARD PUSH FOR THE WAVY MOISTURE-FOCUSING FRONT OVER  
THE PLAINS DURING THE FIRST HALF OF THE PERIOD. UPSTREAM PACIFIC  
ENERGY WILL INTRUDE UPON THE RIDGE OFF THE WEST COAST LATER IN THE  
PERIOD WITH SOLUTIONS PROVIDING MEANINGFUL DIFFERENCES.  
 
THERE WILL BE TWO FEATURES OF INTEREST IN THE TROPICS. GUIDANCE  
INDICATES THAT THE EVOLUTION OF FLOW ALOFT WILL ENCOURAGE  
HURRICANE SERGIO IN THE EASTERN PACIFIC TO TRACK NORTHEASTWARD AND  
POTENTIALLY BRING ANOTHER EPISODE OF SIGNIFICANT RAINFALL TO PARTS  
OF THE SOUTHWEST/SOUTHERN ROCKIES IN THE LATTER PART OF THE  
PERIOD. GFS RUNS ARE ON THE FASTER SIDE OF THE SPREAD BUT RECENT  
TRENDS AND OCCASIONALLY SLOW BIASES OF THE ECMWF SUGGEST THAT THE  
LATTER MODEL AND ITS MEAN MAY BE TOO SLOW TO BRING THE SYSTEM AND  
ITS MOISTURE NORTHEASTWARD. MEANWHILE NHC'S TROPICAL WEATHER  
OUTLOOK SHOWS A MEDIUM/50 PERCENT CHANCE OF TROPICAL CYCLONE  
FORMATION OVER THE NEXT 5 DAYS FOR A DISTURBANCE THAT IS OVER THE  
EXTREME WESTERN CARIBBEAN AND MAY ULTIMATELY TRACK NORTHWARD  
THROUGH THE GULF. TO VARYING DEGREES OTHER OPERATIONAL MODELS  
SUGGEST A BETTER DEFINED SYSTEM THAN THE GFS. GUIDANCE SPREAD IS  
VERY WIDE FOR THIS FEATURE SO CONFIDENCE IS BELOW AVERAGE AT THIS  
TIME.  
 
THE UPDATED FORECAST REFLECTS A COMBINATION OF CONTINUITY AND  
MODEST ADJUSTMENTS REFLECTED IN THE MAJORITY OF GUIDANCE. THE  
FIRST HALF OF THE FORECAST PLACED MORE EMPHASIS ON THE OPERATIONAL  
SOLUTIONS (18Z GFS AND 12Z ECMWF AND LESS 12Z CMC) WHILE THE  
LATTER PART OF THE FORECAST LEANED ALMOST EXCLUSIVELY TO THE 18Z  
GEFS/12Z ECMWF MEANS GIVEN THE INCREASING DETAIL UNCERTAINTY.  
MANUAL ADJUSTMENTS WERE MADE FOR HURRICANE SERGIO AND YESTERDAY'S  
COORDINATED TRACK FOR THE POSSIBLE TROPICAL FEATURE IN THE GULF OF  
MEXICO.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
DURING THE FIRST HALF OF THE PERIOD THE COMBINATION OF LOW LEVEL  
GULF INFLOW AND VERY SLOW MOTION OF A WAVY FRONT WILL LIKELY  
RESULT IN AN EXCESSIVE RAINFALL THREAT OVER PORTIONS OF THE  
CENTRAL U.S., WITH EPISODES OF HEAVY RAINFALL/CONVECTION AND  
POTENTIAL TRAINING. DURING THE LATTER HALF OF THE WEEK THE FRONT  
SHOULD PROGRESS INTO THE EAST WITH THE FASTER MOTION PROMOTING  
LOWER RAINFALL TOTALS THAN EXPECTED OVER THE PLAINS AND VICINITY.  
HOWEVER THE POTENTIAL TROPICAL FEATURE TRACKING THROUGH THE  
GULF--WHICH MAY BRING ENHANCED MOISTURE TO PARTS OF THE  
CENTRAL/EASTERN GULF COAST REGION--COULD CONTRIBUTE ADDITIONAL  
MOISTURE TO THE EASTERN FRONT DEPENDING ON THE RELATIVE TIMING OF  
THE TWO FEATURES. MEANWHILE DEEP-LAYER ATLANTIC FLOW MAY BRING  
MOISTURE INTO THE SOUTHEAST COAST.  
 
THE AMPLIFIED WESTERN TROUGH ALOFT WILL GENERATE PERIODS OF RAIN  
AND SNOW WITH COLD AIR EXTENDING FAR ENOUGH SOUTH TO BRING SNOW  
POTENTIAL INTO THE CENTRAL ROCKIES FOR AT LEAST A PART OF THE  
PERIOD. EXPECT SOME PRECIPITATION ENHANCEMENT FROM  
TERRAIN/UPSLOPE FLOW OVER PORTIONS OF THE GREAT BASIN/ROCKIES/HIGH  
PLAINS. HIGHEST 5-DAY TOTALS SHOULD BE OVER THE NORTHERN-CENTRAL  
ROCKIES AND EASTERN GREAT BASIN. FARTHER SOUTH THERE IS  
INCREASING POTENTIAL THAT MOISTURE FROM HURRICANE SERGIO WILL  
SPREAD NORTHEASTWARD AND REACH ARIZONA AND THE SOUTHERN ROCKIES  
LATE IN THE WEEK. THIS MOISTURE WILL HAVE TO BE MONITORED CLOSELY  
AS IT MAY AFFECT SOME LOCATIONS THAT RECENTLY RECEIVED SIGNIFICANT  
RAINFALL FROM ROSA.  
 
THERE WILL BE A PERSISTENT CORE OF MUCH BELOW NORMAL TEMPERATURES  
FROM THE NORTHERN PLAINS/ROCKIES INTO THE SOUTHWEST WITH THE  
NORTHERN HALF OF THIS AREA LIKELY TO SEE HIGH TEMPERATURES 20-30F  
BELOW NORMAL ON MULTIPLE DAYS. ON THE OTHER HAND MOST OF THE  
EASTERN HALF OF THE COUNTRY WILL SEE MUCH ABOVE NORMAL  
TEMPERATURES. HIGHEST ANOMALIES SHOULD BE FOR MORNING LOWS WITH  
PLUS 20F OR GREATER ANOMALIES POSSIBLE OVER THE OHIO VALLEY/LOWER  
GREAT LAKES/CENTRAL-NORTHERN APPALACHIANS. SOME OF THESE READINGS  
MAY EXCEED DAILY RECORD WARM LOW VALUES.  
 
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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