535  
FXUS02 KWBC 191551  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1150 AM EDT THU OCT 19 2017  
 
VALID 12Z SUN OCT 22 2017 - 12Z THU OCT 26 2017  
 
...HEAVY RAINFALL POSSIBLE ACROSS THE EASTERN U.S. WHILE RECORD  
HEAT RETURNS TO THE DESERT SOUTHWEST NEXT WEEK...  
 
   
..PATTERN OVERVIEW/MODEL GUIDANCE/WPC PREFERENCES
 
 
AN ACTIVE PERIOD AHEAD ACROSS THE NATION WITH AMPLIFIED FLOW  
EXPECTED AS WELL BELOW AVERAGE HEIGHTS CONCENTRATE TO THE EAST OF  
THE GREAT PLAINS. AS THE UPPER TROUGH FURTHER DIGS INTO THE  
SOUTHEASTERN U.S...THE 00Z GFS SUGGESTS 500-MB HEIGHT ANOMALIES  
MAY EXCEED 4 STANDARD DEVIATIONS BELOW CLIMATOLOGY. A COMPLEX  
SPLIT-FLOW SET UP EXISTS NEXT WEEK AS A POTENT SOUTHERN STREAM  
SYSTEM BARRELS ACROSS THE GULF COAST WITH A NUMBER OF MODEL  
UNCERTAINTIES AT HAND. THIS IS FURTHER COMPLICATED BY THE NORTHERN  
STREAM CARRYING A BROAD LONGWAVE TROUGH INTO THE EASTERN U.S. FROM  
DAY 5-7...OCTOBER 24-26. POTENTIAL INTERACTION WITH THESE TWO  
STREAMS IN CONJUNCTION WITH EVENTUAL PLACEMENT OF SUCH FEATURES  
RAISES A NUMBER OF QUESTIONS WITHIN THE FORECAST. ACROSS THE  
WESTERN STATES...STRONG UPPER RIDGING WILL BE THE NORM FOR THE  
MOST PART WITH 500-MB HEIGHT DEPARTURES ON THE ORDER OF 2 STANDARD  
DEVIATIONS BY THE EARLY/MIDDLE PART OF NEXT WEEK. GRADUAL EROSION  
OF THE RIDGE IS EXPECTED AS HEIGHT FALLS DIP THROUGH THE NORTHERN  
TIER BY LATE IN THE PERIOD.  
 
MUCH OF THE FORECAST WAS DRIVEN BY THE HANDLING OF THE SPLIT FLOW  
ACROSS THE CENTRAL/EASTERN U.S. WHICH ROUGHLY COVERS THE SUNDAY  
THROUGH TUESDAY TIMEFRAME. OPERATIONAL MODELS CONTINUE TO WAVER  
BACK AND FORTH BETWEEN A MORE WOUND UP CLOSED LOW DRIFTING  
EASTWARD ACROSS THE CENTRAL GULF COAST VERSUS A LESS SUBSTANTIAL  
BUT MORE PROGRESSIVE SYSTEM. THE 00Z UKMET AND MANY OF ITS PRIOR  
RUNS HAVE FAVORED A MORE ROBUST UPPER LOW WHILE BEING THE SLOWEST  
IN THE PACK. THE 00Z CMC HAS BEEN IN A SIMILAR POSITION AS THE  
UKMET...BUT THE LATEST RUN IS DISPLACED NORTHWARD AND BECOMES  
ABSORBED MORE READILY INTO THE NORTHERN STREAM. CONSIDERING THE  
GFS/ECMWF...RUN-TO-RUN CONTINUITY IS NOT GREAT ALTHOUGH THE  
06Z/00Z GFS STICK WITH THE WEAKER MID-LEVEL REFLECTION AS  
SUPPORTED BY THE PREVIOUS 12Z ECMWF. THE 00Z ECMWF WENT WITH THE  
MORE AGGRESSIVE SOLUTION AS IT HAS SHOWN FROM TIME TO TIME.  
EVENTUALLY THIS SYSTEM LIFTS NORTHWARD ALONG THE EASTERN SEABOARD  
IN ADVANCE OF THE NORTHERN STREAM TROUGH. TOWARD THE DAY  
6/7...OCTOBER 25/26 WINDOW...AN EXPANSIVE CYCLONIC CIRCULATION IS  
EVIDENT AMONG ALL SOLUTIONS BUT WITH VARYING DEGREES OF STRENGTH  
ACCOMPANIED BY A MYRIAD OF POSITIONS. SOME FEATURE A NEGATIVE TILT  
TO THE TROUGH LEADING TO ADDITIONAL WAVES OF LOW PRESSURE.  
 
ACROSS THE WESTERN/CENTRAL U.S...THE UPPER RIDGE APPEARS TO BE A  
WELL ESTABLISHED FEATURE WHICH IS AGREED UPON...ALBEIT WITH MINOR  
DIFFERENCES. THE CENTER OF THE UPPER RIDGE WILL BE LOCATED OVER  
THE SOUTHWESTERN U.S. WHERE SOME OF THE MOST ANOMALOUS HEAT WILL  
BE SITUATED. GRADUAL AMPLIFICATION ACROSS THE NORTHEASTERN PACIFIC  
SHOULD USHER IN JET ENERGY INTO WESTERN CANADA WHICH WILL  
ULTIMATELY DENT THE NORTHERN EXTENT OF THE RIDGE. THIS LEADS TO  
LOWERING HEIGHTS DOWNSTREAM ACROSS THE NORTH-CENTRAL U.S. WITH THE  
STRONGEST HEIGHT FALLS APPROACHING THE REGION BY NEXT THURSDAY. AT  
THIS POINT...LONGWAVE TROUGHING WILL LIKELY EXTEND OVER THE  
CENTRAL/EASTERN U.S. WITH A SHORTWAVE RIDGE SEPARATING THE TWO  
MAJOR FEATURES DISCUSSED.  
 
WITH THE OVERNIGHT CONTINUITY FAVORING A LOWER AMPLITUDE/MORE  
PROGRESSIVE SOUTHERN STREAM...DECIDED TO STICK CLOSER TO THAT  
BEFORE A CONSENSUS DEVELOPS. THUS...THE FORECAST WAS LED BY THE  
00Z GFS/GEFS MEAN AND PREVIOUS 12Z ECMWF/ECMWF ENSEMBLE MEAN IN  
CONJUNCTION WITH ABOUT 20 PERCENT OF THE INHERITED CONTINUITY.  
GRADUALLY INCREASED ENSEMBLE INFLUENCES IN THE FORECAST BEFORE  
DEFAULTING TO ALL OPERATIONAL SOLUTIONS BY MID-WEEK.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
GIVEN THE AMPLIFIED FLOW IN PLACE...SOME FAIRLY IMPRESSIVE  
TEMPERATURE ANOMALIES ARE NOTED DURING THE PERIOD. WITH A DEARTH  
OF ANY APPRECIABLE ARCTIC AIR TO WORK WITH...ALL OF THE POTENTIAL  
DAILY TEMPERATURE RECORDS WILL BE ON THE WARM SIDE OF THE  
EQUATION. SOME OF THE WARMEST TEMPERATURE ANOMALIES SHOULD BE OVER  
THE SOUTHWESTERN STATES WITH DEPARTURES POSSIBLY REACHING THE 15  
TO 20 DEGREE RANGE BY MONDAY. A NUMBER OF DAILY RECORDS MAY BE  
BROKEN AS HIGHS SOAR ABOVE THE CENTURY MARK ACROSS SOUTHERN CA. ON  
TUESDAY...SOME COULD BE SHATTERED WITH BURBANK AND LONG BEACH CA  
EXPECTING HIGHS 5 DEGREES ABOVE THEIR CURRENT RECORDS.  
UNFORTUNATELY THIS HEAT WILL ALSO ENHANCE THE WILDFIRE DANGER  
GIVEN EXPECTED OFFSHORE GRADIENTS LEADING TO DRY/GUSTY WINDS.  
ELSEWHERE...IN ADVANCE OF THE SLOW-MOVING UPPER TROUGH...SOME  
RECORD WARMTH MAY BE EXPERIENCED OVERNIGHT WITH LOWS IN THE 60S  
FROM THE LOWER MS VALLEY UP INTO MO/IL ON SUNDAY.  
 
WHILE CONDITIONS BEGIN TO IMPROVE ACROSS THE PACIFIC NORTHWEST AS  
HEIGHTS BEGIN TO RISE...THE FOCUS SHIFTS TO THE EASTERN THIRD OF  
THE NATION. IT SHOULD BE A RATHER WET PERIOD OVER THE REGION ALONG  
AND AHEAD OF THE ADVANCING FRONTAL ZONE. THE SOUTHERN STREAM UPPER  
LOW WILL ALSO PROVIDE AN ADDITIONAL FOCUS FOR HEAVY  
RAINFALL...PARTICULARLY IF IT ENDS UP MOVING AS SLOW AS SOME  
MODELS SUGGEST. BY TUESDAY THROUGH THURSDAY...THE FOCUS SHOULD  
SHIFT INTO NEW ENGLAND AS A SERIES OF WAVES OF LOW PRESSURE LIFT  
THROUGH THE REGION. WHILE SOME MODELS SHOW SOME RATHER COOL  
1000-500 MB THICKNESSES...THERE DO NOT APPEAR TO BE ANY MAJOR  
WINTRY PRECIPITATION IMPLICATIONS AT THIS TIME.  
 
RUBIN-OSTER  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page