707  
FXUS02 KWBC 060659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
258 AM EDT SAT OCT 06 2018  
 
VALID 12Z TUE OCT 09 2018 - 12Z SAT OCT 13 2018  
   
..CENTRAL U.S. EXCESSIVE RAINFALL THREAT EXTENDS INTO MIDWEEK
 
 
...SEPARATE AREAS OF TROPICAL MOISTURE MAY REACH INTO THE GULF  
COAST-SOUTHEAST AND SOUTHWEST-SOUTHERN ROCKIES...  
...ACTIVE PATTERN CONTINUES OVER THE WEST THROUGH AT LEAST  
MIDWEEK...  
 
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
DURING DAYS 3-5 TUE-THU MOST GUIDANCE AGREES THAT THE PATTERN WILL  
COMMENCE ITS ADVERTISED TRANSITION TOWARD A MORE POSITIVELY TILTED  
WESTERN U.S. TROUGH WHILE ANOMALOUSLY STRONG RIDGES OVER THE  
EASTERN PACIFIC AND WESTERN ATLANTIC GRADUALLY WEAKEN.  
MODELS/ENSEMBLES SHOW A SIMILAR SHAPE TO THE FLOW BUT IMPORTANT  
DIFFERENCES EXIST FOR IMPORTANT DETAILS. THE MAJORITY CLUSTER  
SUGGESTS THAT ENERGY ALOFT DROPPING INTO THE NORTHWEST AS OF THE  
START OF THE PERIOD TUE WILL LIKELY BE DEEPER, MORE CLOSED, AND  
REACH FARTHER SOUTHWARD THAN DEPICTED IN RECENT GFS RUNS. THIS  
FEATURE WILL PROMOTE EJECTION OF THE LEADING TROUGH INITIALLY OVER  
THE SOUTHERN ROCKIES. AROUND WED-THU THERE IS A DECENT SIGNAL FOR  
SOME INTERACTION AMONG THE TWO FEATURES, BUT POOR AGREEMENT ON  
EXACTLY HOW THIS OCCURS LOWERS CONFIDENCE IN SPECIFICS OF LOW  
PRESSURE EXPECTED TO TRACK INTO THE UPPER MS VALLEY AND UPPER  
GREAT LAKES. ALREADY BY DAYS 4-5 THERE IS ENOUGH DETAIL SPREAD TO  
SUPPORT INCLUDING SOME ENSEMBLE MEAN GUIDANCE. ALSO OF NOTE THERE  
HAS BEEN A NOTABLE SLOWER TREND FOR THIS SYSTEM COMPARED TO THE  
PAST 12-24 HOURS OF GUIDANCE.  
 
THE GREATEST UNCERTAINTY IN THE LARGE SCALE FORECAST BY DAYS 6-7  
FRI-SAT REGARDS HOW MUCH PROGRESSION OCCURS FOR EASTERN  
PACIFIC/WESTERN U.S. FEATURES. THE 18Z AND NEW 00Z GFS RUNS  
BECOME QUITE FAST COMPARED TO OTHER GUIDANCE INCLUDING RECENT GEFS  
MEANS. ON THE OTHER HAND THE 12Z ECMWF DIGS AN UPPER LOW TO THE  
CALIFORNIA COAST AND THEN BRINGS IT INTO THE SOUTHWEST. ENSEMBLES  
CAPTURE BOTH EXTREMES AND THEN SOME. TELECONNECTIONS RELATIVE TO  
A CORE OF NEGATIVE HEIGHT ANOMALIES OVER THE ALEUTIANS BY THE D+8  
TIME FRAME HINTS A THE POTENTIAL FOR AT LEAST A MODEST DEGREE OF  
SPLIT FLOW NEAR 120W LONGITUDE. THIS SEEMS TO RECOMMEND TILTING  
THE FORECAST A LITTLE MORE TOWARD THE 12Z ECMWF/ECMWF MEAN  
RELATIVE TO THE GEFS MEAN IN PRINCIPLE BUT DOWNPLAYING THE MORE  
EXTREME SPECIFICS OF THE 12Z ECMWF, WHILE REMOVING THE  
LEAST-SUPPORTED GFS FROM THE FORECAST.  
 
REGARDING HURRICANE SERGIO, THE FASTER JUMP BY YESTERDAY'S 00Z  
ECMWF HAS YIELDED IMPROVED CLUSTERING AMONG THE GFS/ECMWF/UKMET  
OVER THE PAST DAY. THE CMC AS WELL AS MULTIPLE ENSEMBLE MEMBERS  
STILL SUGGEST MEANINGFUL TIMING UNCERTAINTY THOUGH. PREFERENCES  
THAT LEAN AWAY FROM THE GFS EVOLUTION ALOFT BY THE LATTER HALF OF  
THE PERIOD WOULD LEAD TO A MUCH LESS PRONOUNCED SURFACE FEATURE  
REACHING THE PLAINS FRI-SAT.  
 
THERE IS INCREASING CONFIDENCE THAT A DISTURBANCE OVER THE EXTREME  
WESTERN CARIBBEAN WILL BECOME BETTER ORGANIZED OVER THE COMING  
DAYS AS IT TRACKS NORTHWARD INTO THE GULF OF MEXICO. HOWEVER  
MODEL/ENSEMBLE SPREAD REMAINS QUITE BROAD--BOTH OVER THE GULF AND  
FARTHER NORTH WHERE THE CHARACTER OF MID-LATITUDE FLOW COMES INTO  
PLAY AS WELL--SO CONFIDENCE IN A PARTICULAR TRACK AND TIMING IS  
MUCH LOWER THAN THAT FOR THE SYSTEM'S EXISTENCE. ESPECIALLY AFTER  
EARLY WED THE CENTROID OF ENSEMBLE MEMBERS HAS SHOWN A SOMEWHAT  
FASTER/EASTWARD SHIFT COMPARE TO 24 HOURS AGO. THE MANUAL  
FORECAST NUDGES CONTINUITY A BIT FASTER AS A RESULT BUT ONLY  
TOWARD THE SLOWER SIDE OF THE OPERATIONAL MODEL ENVELOPE (12Z  
ECMWF).  
 
BASED ON THE ABOVE CONSIDERATIONS THE UPDATED FORECAST STARTED  
WITH AN OPERATIONAL MODEL BLEND FOR DAY 3 TUE FOLLOWED BY  
GRADUALLY INCREASING ENSEMBLE MEAN WEIGHT AND ELIMINATION OF THE  
18Z GFS AFTER DAY 5 THU. DAYS 6-7 FRI-SAT EMPHASIZED THE 18Z  
GEFS/12Z ECMWF MEANS WITH A SMALL LINGERING 12Z ECMWF COMPONENT.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
HEAVY RAINFALL OVER THE PLAINS ON TUE ALONG A WAVY FRONT WILL  
BEGIN TO MOVE EASTWARD FROM WED ONWARD, AS DEVELOPING LOW PRESSURE  
TRACKING INTO THE UPPER MS VALLEY/GREAT LAKES PUSHES THE FRONTAL  
BOUNDARY TOWARD/INTO THE EAST. SOME ACTIVITY MAY STILL BE FAIRLY  
INTENSE TO THE EAST OF THE PLAINS BUT AMOUNTS SHOULD BE LOWER  
GIVEN FASTER PROGRESSION. LOCATIONS ALONG THE EASTERN HALF OF THE  
GULF COAST AND INTO THE SOUTHEAST SHOULD MONITOR FORECASTS FOR THE  
POSSIBLE TROPICAL FEATURE THAT COULD BRING SIGNIFICANT MOISTURE  
INTO THIS AREA. DEPENDING ON THE RELATIVE TIMING OF THIS SYSTEM  
AND COLD FRONT REACHING THE EAST, POTENTIAL EXISTS FOR SOME OF THE  
TROPICAL MOISTURE/HEAVY RAINFALL TO EXTEND WELL NORTHWARD. A  
SEPARATE MOISTURE FEED INTO THE SOUTHEAST/SOUTHERN APPALACHIANS  
AHEAD OF THE TROPICAL FEATURE MAY ALSO YIELD AREAS OF ENHANCED  
RAINFALL. FARTHER WEST IN THE COLD SECTOR, SNOW WILL BE POSSIBLE  
OVER HIGHER ELEVATIONS OF THE NORTHERN-CENTRAL ROCKIES AND INTO  
THE NORTHERN PLAINS, WHILE RAIN PREVAILS AT LOWER ELEVATIONS.  
NORTHERN PARTS OF THE WEST SHOULD TREND GRADUALLY DRIER AS UPPER  
TROUGHING BECOMES MORE POSITIVELY TILTED. ON THE OTHER HAND THE  
ARRIVAL OF SERGIO'S MOISTURE SHOULD BRING INCREASING RAINFALL INTO  
ARIZONA AND THE SOUTHERN ROCKIES LATE THIS WEEK. SOME OF THIS  
MOISTURE MAY EXTEND INTO THE PLAINS BUT WITH DECREASING CONFIDENCE  
FOR SPECIFICS GIVEN DIVERGENCE OF SOLUTIONS FOR LARGER SCALE FLOW.  
PRECIPITATION COULD LINGER OVER PARTS OF THE SOUTHWEST/SOUTHERN  
ROCKIES DEPENDING ON HOW MUCH UPPER TROUGHING PERSISTS NEAR THIS  
AREA LATE IN THE PERIOD.  
 
EXPECT CONTINUED BELOW TO MUCH BELOW NORMAL TEMPERATURES FROM THE  
NORTHERN PLAINS/ROCKIES INTO THE SOUTHWEST. ONE OR MORE DAYS MAY  
FEATURE HIGHS 20-30F BELOW NORMAL OVER AND NEAR THE NORTHERN HIGH  
PLAINS. SOME OF THIS CHILLY AIR WILL GRADUALLY PUSH SOUTHEASTWARD  
ACROSS THE CENTRAL U.S. MUCH ABOVE NORMAL TEMPERATURES WILL  
PREVAIL OVER MOST OF THE EAST DURING THE FIRST HALF OF THE PERIOD  
WITH MIN TEMPS IN PARTICULAR (SOME PLUS 20F OR GREATER ANOMALIES)  
POSSIBLY REACHING DAILY RECORD VALUES. THE COLD FRONT CROSSING  
THE EAST LATER IN THE WEEK WILL FINALLY BRING TEMPERATURES CLOSER  
TO NORMAL.  
 
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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