144  
FXUS02 KWBC 221601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT SUN OCT 22 2017  
 
VALID 12Z WED OCT 25 2017 - 12Z SUN OCT 29 2017  
   
..OVERVIEW/GUIDANCE EVALUATION/PREFERENCES
 
 
AT THE START OF THE PERIOD...A BROAD LONGWAVE TROUGH WILL EXTEND  
OVER THE EASTERN U.S. WHILE A FLATTENING UPPER RIDGE IS NOTED FROM  
THE HIGH PLAINS WESTWARD. AMPLIFYING FLOW ATOP THIS RIDGE ACROSS  
WESTERN CANADA IS FORECAST TO ACCELERATE SOUTH-SOUTHEASTWARD WHICH  
HELPS CARVE OUT A MEAN TROUGH WHICH WILL TRANSLATE FROM THE  
ROCKIES TOWARD THE APPALACHIANS FROM DAY 4 THROUGH 7...OCTOBER  
26-29. THIS ULTIMATELY WILL HELP RE-ESTABLISH UPPER RIDGING ALONG  
THE WEST COAST AND MAINTAIN THE AMPLIFIED MID-LATITUDE PATTERN.  
ALONG THE EASTERN SEABOARD...THE MENTIONED UPPER TROUGH SHOULD BE  
SLOW TO EXIT THE COAST AS IT ATTAINS A NEGATIVE-TILT AND FORMS A  
POTENT AREA OF LOW PRESSURE LIFTING INTO NEW ENGLAND BY LATE  
WEDNESDAY/EARLY THURSDAY. MODELS CONTINUE TO CARRY THIS FEATURE  
TOWARD THE HIGHER LATITUDES OF CANADA AS HEIGHTS RAPIDLY BUILD  
THEREAFTER ACROSS THE EASTERN STATES.  
 
REGARDING THE INITIAL UPPER TROUGH ACROSS THE EASTERN U.S...THE  
00Z CMC/UKMET CONTINUE TO BE SLOWER TO EJECT THE PARENT SURFACE  
LOW TOWARD THE ONTARIO/QUEBEC BORDER EARLY ON. CONSIDERING OTHER  
SOLUTIONS...GENERAL AGREEMENT EXISTS IN ALLOWING FOR A  
NEGATIVELY-TILTED TROUGH TO EXIT THE MID-ATLANTIC EARLY THURSDAY  
WHICH MAY BE ACCOMPANIED BY A STRONG CLOSED UPPER LOW AS NOTED IN  
THE 00Z GFS/ECMWF. BOTH SOLUTIONS FAVOR A SUB 1000-MB LOW MOVING  
TOWARD NOVA SCOTIA WITH SOME NORTH/SOUTH SPREAD IN THE ENSEMBLE  
LOW PLOTS. LOOKING FARTHER UPSTREAM...THE SPATIAL SPREAD IN THE  
GUIDANCE HAS DIMINISHED SOME RELATIVE TO YESTERDAY. HOWEVER...THE  
GLOBAL MODEL CAMPS DIVERGE BY DAY 4/THURSDAY AS SEEN IN RECENT  
ENSEMBLE SPAGHETTI PLOTS. MULTI-DAY COMPARISONS SHOW A GRADUAL  
EASTWARD SHIFT IN THE ECMWF ENSEMBLE MEMBERS WHICH TIES IT A BIT  
MORE CLOSELY WITH THE GEFS MEMBERS AS WELL AS THE 00Z UKMET. THE  
00Z ECMWF AND ITS ENSEMBLE MEMBERS CONTINUE TO BE OFF TO THE WEST  
AS THEY DIG THE TROUGH MORE SIGNIFICANTLY TOWARD THE FOUR CORNERS  
REGION WHILE THE GEFS FAVORS MORE EASTWARD PROGRESSION IN TIME.  
THEY HAVE REMAINED ON THESE OPPOSING SIDES THE PAST COUPLE OF DAYS  
ALTHOUGH THE GAP HAS BEEN BRIDGED SOMEWHAT MOST RECENTLY. THIS  
DOES LEAD TO TIMING DIFFERENCES WITH THE NORTH/SOUTH ORIENTED  
FRONTAL ZONE WHICH MARCHES TOWARD THE EAST. ONE SET OF SOLUTIONS  
NOT CONSIDERED HERE IS THE 00Z CMC/CMC ENSEMBLE MEAN WHICH ALSO  
AFFECTS THE 00Z NAEFS ENSEMBLE MEAN AS THEY ARE MUCH MORE  
PROGRESSIVE WITH THE ENTIRE SYNOPTIC-SCALE PATTERN. FURTHER...THE  
FORMER CONJURES UP A TROPICAL CYCLONE LATER IN THE PERIOD WHICH  
WOULD MOVE TOWARD CUBA AND THE BAHAMAS.  
 
AS THE OVERALL FORECAST SPREAD HAS DECREASED...WAS ABLE TO  
MAINTAIN MORE OF AN OPERATIONAL INFLUENCE DURING MUCH OF THE  
PERIOD. TOOK A DIRECT SPLIT BETWEEN THE 00Z GFS/ECMWF FOR THE  
FIRST FEW DAYS. DID SELECT THE 00Z GFS OVER THE 06Z RUN AS THE  
LATTER WAS A BIT QUICKER WHICH LED TO LARGER SPREAD BETWEEN IT AND  
THE 00Z ECMWF. BY DAY 5/FRIDAY ONWARD...ADDED MORE ENSEMBLE  
CONTRIBUTIONS TO THE PICTURE WHILE STILL KEEPING AROUND 30 PERCENT  
OF THE OPERATIONAL MODELS INTO DAY 7/OCTOBER 29.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
WITH AMPLIFIED FLOW IN PLAY...SOME DAILY TEMPERATURE RECORDS MAY  
BE BROKEN GIVEN THE VERY MILD OVERNIGHT LOWS EXPECTED ON  
WEDNESDAY/THURSDAY. IN PARTICULAR...THE UPPER RIDGE CENTERED OVER  
THE DESERT SOUTHWEST WILL BRING VERY WARM CONDITIONS TO THE REGION  
AS LOWS STAY WELL INTO THE MID/UPPER 60S ACROSS SOUTHERN CA. AND  
FROM THE MID-ATLANTIC NORTHWARD...STRONG WARM ADVECTION AHEAD OF  
THE CLOSED LOW WILL ALLOW MILD TROPICAL AIR TO INFILTRATE THE  
EASTERN SEABOARD. EVENTUALLY THE PATTERN SHIFTS TOWARD COOLER  
READINGS AS THE UPPER TROUGH DIGS THROUGH THE CENTRAL U.S. LATER  
NEXT WEEK. DEPARTURES FROM NORMAL ON THE ORDER OF 10 TO 15 DEGREES  
WILL BE LIKELY AS HIGHS REMAIN ONLY IN THE 40S AS FAR SOUTH AS THE  
TX PANHANDLE. THE AIR MASS SHOULD MODIFY SOME ON ITS EASTWARD  
APPROACH ALTHOUGH IT WILL FEEL MUCH MORE LIKE AUTUMN AS THIS  
SYSTEM SPREADS ACROSS THE CENTRAL TO EASTERN U.S.  
 
THERE WILL BE AN INITIAL HEAVY RAINFALL THREAT ACROSS UPPER NEW  
ENGLAND AS THE SURFACE LOW PIVOTS THROUGH THE REGION. SOME FAIRLY  
HEFTY TOTALS MAY BE POSSIBLE GIVEN THE 850-700 MB MOISTURE FLUXES  
BEING AROUND 4 TO 5 STANDARD DEVIATIONS ABOVE CLIMATOLOGY.  
GRADUALLY THE FOCUS OF THIS BAND WILL MOVE INTO QUEBEC WITH  
SCATTERED SHOWERS IN ITS WAKE. FARTHER WEST...WITH THE NEXT SYSTEM  
MOVING THE CENTRAL U.S...SOME MOUNTAIN SNOWS WILL BE POSSIBLE  
ACROSS THE NORTHERN/CENTRAL ROCKIES WITH A GRADUAL HEAVY RAINFALL  
THREAT BY PERHAPS FRIDAY ACROSS THE RED RIVER NORTHEASTWARD ALONG  
THE FRONT. THE DEGREE OF RETURN FLOW AND HOW MUCH MOISTURE CAN  
MOVE BACK TOWARD THE GREAT PLAINS IS STILL AN UNKNOWN.  
ADDITIONALLY...DEPENDING ON THE STRENGTH OF THE PARENT SURFACE LOW  
AND AMOUNT OF COLD AIR PUSHING ACROSS THE CANADIAN BORDER...SOME  
SNOW MAY BE POSSIBLE OVER NORTHERN MN INTO THE UPPER GREAT LAKES.  
 
RUBIN-OSTER  
 

 
 
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