559  
FXUS02 KWBC 241524  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1123 AM EDT TUE OCT 24 2017  
 
VALID 12Z FRI OCT 27 2017 - 12Z TUE OCT 31 2017  
 
15Z UPDATE...  
 
MODELS AND ENSEMBLES CONTINUE TO SHOW GOOD AGREEMENT ON THE LARGE  
SCALE FLOW PATTERN ACROSS THE CONUS THROUGH THE MEDIUM RANGE, AND  
THE FORECAST THINKING HAS NOT CHANGED SIGNIFICANTLY FROM  
OVERNIGHT. ABOVE AVERAGE CONSENSUS SUPPORTED USE OF A MULTI-MODEL  
DETERMINISTIC BLEND INCLUDING THE 00Z ECMWF/UKMET AND 06Z GFS,  
COMPRISING A MAJORITY OF THE BLEND FOR DAYS 3-5 (FRI-SUN).  
NON-ECMWF GUIDANCE HAS CONTINUED A TREND TOWARD A SLIGHTLY SLOWER  
PROGRESSION OF THE EASTERN TROUGH (AND SURFACE COLD FRONT), TOWARD  
A TIMING MORE SIMILAR TO THE ECMWF/ECMWF ENSEMBLE. THUS, THE 00Z  
ECMWF WAS WEIGHTED A BIT MORE HEAVILY RELATIVE TO THE OTHER  
DETERMINISTIC GUIDANCE FOR DAYS 3-5. SPREAD CONTINUES TO INCREASE  
BY DAYS 6-7 (MON-TUE) REGARDING THE EXACT EVOLUTION OF ENERGY  
MOVING AROUND THE EASTERN SIDE OF THE NORTH PACIFIC RIDGE AND  
AFFECTING THE WESTERN STATES. DETERMINISTIC GUIDANCE CONTINUE TO  
FLUCTUATE FROM RUN-TO-RUN ON THE EXACT NATURE OF THIS WAVE, WITH  
THE 00Z ECMWF ELONGATING THE ENERGY BY DAY 7 ALONG/OFF THE WEST  
COAST, AND THE 06Z GFS MOVING A MORE CONCENTRATED AREA OF  
VORTICITY WESTWARD ACROSS THE NORTH PACIFIC (THE 00Z GFS MORE  
CLOSELY RESEMBLED THE 00Z ECMWF). GIVEN THE UNCERTAIN EVOLUTION BY  
THIS TIME, THE WPC FORECAST WAS TRENDED TOWARD MUCH HEAVIER  
ENSEMBLE MEAN WEIGHTING (00Z ECENS/06Z GEFS) BY DAYS 6-7.  
 
MODELS CONTINUE TO SUGGEST THAT A TROPICAL DISTURBANCE WILL MOVE  
NORTH FROM THE WESTERN CARIBBEAN, NEAR/OFF THE SOUTHEASTERN U.S.  
COAST BY SAT/SUN. GIVEN A SLIGHTLY SLOWER PROGRESSION OF THE  
EASTERN TROUGH/COLD FRONT, POTENTIAL CONTINUES TO INCREASE FOR  
MOISTURE ASSOCIATED WITH THIS TROPICAL DISTURBANCE TO BE PULLED  
NORTHWARD INTO THE EASTERN SEABOARD AHEAD OF THE FRONTAL BOUNDARY,  
RESULTING IN THE POTENTIAL FOR HEAVY RAINFALL. THE FORECAST FOR  
THIS TROPICAL DISTURBANCE WAS NOT CHANGED SIGNIFICANTLY FROM THE  
PREVIOUS FORECAST, PENDING 17Z COORDINATION CALL WITH NHC.  
 
RYAN  
 
PREVIOUS DISCUSSION...  
 
...GUIDANCE/UNCERTAINTY ASSESSMENT AND WEATHER/THREATS  
HIGHLIGHTS...  
 
THE DOMINANT FOCUS OF THE FORECAST WILL BE ON THE EVOLUTION OF AN  
AMPLIFYING TROUGH ALOFT REACHING THE CENTRAL U.S. AS OF THE START  
OF THE PERIOD FRI. MODELS AND ENSEMBLES HAVE BEEN PROVIDING  
SIMILAR MESSAGES IN RECENT RUNS BUT WITH SOME SPREAD AND RUN TO  
RUN VARIABILITY FOR DETAILS/TIMING. THE MOST NOTABLE TREND  
REPRESENTED BY THE FAVORED 18Z GFS/GEFS MEAN AND 12Z ECMWF/ECMWF  
MEAN (PLUS THE 12Z CMC) IS AN INCREASING SIGNAL THAT THE CORE OF  
THE UPPER TROUGH MAY CLOSE OFF A LOW, LEADING TO A MORE PERSISTENT  
INITIAL GREAT LAKES SURFACE LOW AND SLOWER PROGRESSION OF THE  
TRAILING COLD FRONT. WHILE INCORPORATING THIS SLOWER TREND  
GUIDANCE AGREES FAIRLY WELL WITH THE IDEA OF A FRONTAL WAVE  
DEVELOPING NEAR THE EAST COAST AROUND DAY 5 SUN AS THE UPPER  
TROUGH TAKES ON A NEGATIVE TILT, AND THEN TRACKING NORTH-NORTHWEST  
INTO CANADA. 00Z GUIDANCE THUS FAR SEEMS TO BE CONSISTENT WITH  
THE ABOVE MAJORITY CLUSTER OF SOLUTIONS. THE RELATIVELY LARGE  
SCALE NATURE OF FLOW YIELDS DECENT CONFIDENCE IN THE OVERALL  
PATTERN BUT THE FINER DETAILS WILL LIKELY TAKE MULTIPLE DAYS TO  
RESOLVE. ONE SUCH DETAIL WILL BE THE EXTENT TO WHICH THIS SYSTEM  
INCORPORATES MOISTURE AND POTENTIALLY A TROPICAL SYSTEM TRACKING  
OUT OF THE NORTHWEST CARIBBEAN. CURRENT FORECAST REFLECTS  
YESTERDAY'S NHC/WPC COORDINATION FOR THE POTENTIAL TROPICAL  
FEATURE. OVERALL THE SLOWER TIMING OF THE UPPER TROUGH INCREASES  
THE POSSIBILITY FOR INCLUSION OF TROPICAL MOISTURE IN THIS SYSTEM  
NEAR THE EAST COAST.  
 
EARLY IN THE PERIOD THE INITIAL GREAT LAKES SURFACE LOW SHOULD  
SUPPORT A PERIOD OF FAIRLY STRONG WINDS ALONG WITH A CONCENTRATED  
AREA OF PRECIP WITH SOME IN THE FORM OF SNOW. RAIN ALONG THE  
TRAILING FRONT SHOULD DEVELOP NEAR THE MS VALLEY AROUND FRI AND  
THEN CONTINUE INTO THE EASTERN STATES WITH TIME. SOME POCKETS OF  
LOCALLY MODERATE-HEAVY RAIN MAY EXIST FROM THE MS VALLEY TO WEST  
OF THE APPALACHIANS. FROM NEAR THE APPALACHIANS TO THE EAST  
COAST, THE POTENTIAL FOR INCORPORATION OF TROPICAL MOISTURE ALONG  
WITH STRENGTHENING ATLANTIC INFLOW RAISE THE THREAT FOR A PERIOD  
OF MORE ORGANIZED HEAVY TO EXCESSIVE RAINFALL. ENSEMBLE GUIDANCE  
CURRENTLY SUGGESTS THE BEST PROBABILITY FOR HIGHEST TOTALS DURING  
THE PERIOD WILL EXTEND ACROSS THE NORTHERN MID ATLANTIC/EASTERN  
GREAT LAKES/NORTHEAST. EXPECT MOST PRECIP TO BE IN THE FORM OF  
RAIN THOUGH A LITTLE SNOW MAY NOT BE OUT OF THE QUESTION IF THE  
UPPER LOW CENTER TRACKS OVER HIGHER ELEVATIONS.  
 
AT LEAST TO ABOUT MID-PERIOD THERE IS GRADUALLY IMPROVING UPSTREAM  
SIMILARITY AMONG GUIDANCE REGARDING ANOTHER NORTHERN PACIFIC  
SHORTWAVE ROUNDING THE WESTERN RIDGE AND DESCENDING INTO THE LOWER  
48 BY EARLY NEXT WEEK. BY THAT TIME THE QUESTION POSED BY MODELS  
AND ENSEMBLE MEMBERS IS TO WHAT EXTENT THE ENERGY AMPLIFIES OVER  
THE WEST. TELECONNECTIONS RELATIVE TO THE CORE OF STRONG POSITIVE  
HEIGHT ANOMALIES OVER THE NORTHEAST PACIFIC IN LATEST D+8  
MULTI-DAY MEAN CHARTS OFFER POTENTIAL FOR SOME TROUGH ELONGATION  
OVER THE WEST SUCH AS IN THE 12Z ECMWF AND A MINORITY OF  
GEFS/ECMWF/CMC ENSEMBLE MEMBERS. HOWEVER THE RELATIVE LACK OF  
AGREEMENT/CONTINUITY THUS FAR RECOMMENDS LEANING CLOSER TO THE  
ENSEMBLE MEANS LATE IN THE PERIOD. THUS THE 18Z GFS/GEFS MEAN AND  
12Z ECMWF/ECMWF MEAN BLEND USED FOR THE FORECAST ADJUSTS FROM MORE  
OPERATIONAL WEIGHT EARLY TO MOSTLY ENSEMBLE MEAN WEIGHT LATE.  
THIS FEATURE AND ASSOCIATED FRONT MAY PRODUCE AN AREA OF MOSTLY  
LIGHT RAIN/SNOW OVER AND EAST OF THE ROCKIES.  
 
A BROAD AREA OF HIGHS 10-20F BELOW NORMAL OVER THE CENTRAL U.S. ON  
FRI WILL MAKE STEADY PROGRESS INTO THE EAST DURING THE  
WEEKEND/EARLY NEXT WEEK WITH MODERATION LIKELY BY TUE. GREATEST  
NEGATIVE ANOMALIES FOR MIN TEMPS SHOULD PROGRESS FROM THE SOUTHERN  
PLAINS INTO THE SOUTHEAST CORRESPONDING TO THE PATH OF SURFACE  
HIGH PRESSURE. ON THE OTHER HAND THE WEST SHOULD SEE ABOVE NORMAL  
TEMPS DURING THE PERIOD WITH HIGHEST ANOMALIES OF PLUS 10-20F FOR  
HIGHS LIKELY DURING FRI-SUN.  
 
RAUSCH  
 

 
 
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