671  
FXUS02 KWBC 260649  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
248 AM EDT THU OCT 26 2017  
 
VALID 12Z SUN OCT 29 2017 - 12Z THU NOV 02 2017  
 
...POTENTIAL HEAVY RAINFALL EVENT FOR THE NORTHEASTERN U.S. SUNDAY  
AND MONDAY...  
 
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
WITH THE DEPARTURE OF AN INITIALLY SHARP/AMPLIFIED EASTERN U.S.  
TROUGH, GUIDANCE CONTINUES TO SHOW A GENERAL BROADENING TREND FOR  
CYCLONIC MEAN FLOW ACROSS THE LOWER 48 AS THE MEAN RIDGE OVER THE  
WEST COAST/EASTERN PACIFIC RETROGRADES WITH TIME. THIS  
RETROGRESSION IS REPRESENTED IN DAILY FORECASTS BY THE COLLAPSE OF  
A WESTERN RIDGE ALREADY IN PROGRESS AS OF EARLY SUN FOLLOWED BY  
THE BUILDING OF A STRONG NORTHEAST PACIFIC RIDGE AND THEN FINALLY  
INCREASING DOMINANCE OF AN UPSTREAM RIDGE OVER/NEAR ALASKA. THE  
UNCERTAIN DETAILS OF THIS LATTER RIDGE PRODUCE A CONSIDERABLE  
PORTION OF MODEL/ENSEMBLE SPREAD BY DAYS 6-7 WED-THU. BY LATE IN  
THE PERIOD THIS PATTERN EVOLUTION MAY INCREASE POTENTIAL FOR ANY  
SURGES OF COLD AIR REACHING THE NORTHERN TIER STATES TO HAVE SOME  
CONNECTION TO ALASKAN/NORTHERN CANADA HIGH PRESSURE VERSUS  
PREVIOUS HIGHS MIGRATING ACROSS THE NORTH PACIFIC.  
TELECONNECTION-FAVORED FLOW AT THAT TIME ALSO HIGHLIGHTS THE  
NORTHERN TIER AS A LIKELY FOCUS FOR BELOW NORMAL TEMPS.  
 
APPROXIMATELY THE FIRST HALF OF THE FORECAST PERIOD INCORPORATES  
IDEAS FROM THE VARIOUS 12Z/18Z OPERATIONAL MODELS. THE FINER  
DETAILS WILL TAKE MORE TIME TO RESOLVE BUT GUIDANCE IS QUITE  
AGREEABLE IN PRINCIPLE ON THE IDEA OF EASTERN U.S. TROUGH ENERGY  
BECOMING NEGATIVELY TILTED AS IT LIFTS NORTHEASTWARD AHEAD OF  
ANOTHER VIGOROUS SYSTEM DROPPING INTO THE NORTHERN TIER. THERE IS  
STILL A DECENT SIGNAL THAT THE EAST COAST SYSTEM WILL INCORPORATE  
LOW PRESSURE/MOISTURE EMERGING FROM THE NORTHWEST CARIBBEAN  
ULTIMATELY LEADING TO A STRONG/DEEPENING SURFACE LOW TRACKING  
TOWARD/INTO THE NORTHEAST. AS OF 12Z MON MOST MODELS SHOW A DEPTH  
IN THE UPPER 970'S/LOW 980'S MB WHICH COULD CHALLENGE OCTOBER SEA  
LEVEL PRESSURE RECORDS AT SOME LOCATIONS OVER THE NORTHEAST.  
DEEPENING TRENDS IN THE ENSEMBLE MEANS SUPPORT AN OPERATIONAL  
MODEL COMPROMISE AT THIS TIME. RECENT FASTER TRENDS IN MOST  
GUIDANCE FOR THE UPSTREAM SYSTEM WOULD SUGGEST ANY FUTURE  
ADJUSTMENTS NEAR THE EAST COAST MAY BE TOWARD THE FASTER SIDE OF  
THE SPREAD. TRENDS WITH THE TRAILING SYSTEM ALSO LEAD TO A FASTER  
SOUTHWARD PROGRESSION OF THE COLD FRONT OVER THE WEST/PLAINS.  
AFTER EARLY MON THE ANCHORING SURFACE LOW REACHING THE UPPER GREAT  
LAKES MAY BECOME ABSORBED INTO THE SOUTHERN PERIPHERY OF THE  
LEADING SYSTEM'S BROAD CIRCULATION. ALONG THE WEST COAST THERE  
ARE A COUPLE SHORTWAVE FEATURES FOR WHICH LATEST GUIDANCE SEEMS TO  
HAVE ACQUIRED GREATER DEFINITION ALBEIT WITH SOME SPREAD FOR  
SPECIFICS. CONFIDENCE WITH THESE FEATURES IS ON THE LOWER SIDE OF  
THE SPECTRUM GIVEN THEIR SMALL SCALE.  
 
THE LATTER HALF OF THE FORECAST BECOMES DOMINATED BY HOW TROUGH  
ENERGY EVOLVES DOWNSTREAM FROM THE STRENGTHENING ALASKA RIDGE.  
THUS FAR THE ECMWF MEAN HAS BEEN THE MOST STABLE AND PERSISTENT  
GUIDANCE SOURCE IN SHOWING THE DEVELOPMENT OF A TROUGH EXTENDING  
FROM CANADA INTO THE NORTHEAST PACIFIC, JUST OFF THE PACIFIC  
NORTHWEST COAST. GEFS MEANS CAUGHT ONTO THIS EVOLUTION IN THE  
00Z/25 RUN. OPERATIONAL MODELS HAVE VARIED DETAILS DEPENDING ON  
CHARACTER OF THE UPSTREAM RIDGE. TELECONNECTIONS RELATIVE TO THE  
STRONG POSITIVE HEIGHT ANOMALY CENTER NEAR THE SOUTHWEST CORNER OF  
MAINLAND ALASKA IN LATEST D+8 MULTI-DAY MEANS WOULD FAVOR A  
PATTERN CLOSEST TO THE 18Z GEFS/12Z ECMWF ENSEMBLE MEANS. THE 18Z  
GFS APPEARS TO BE ONE OF THE LESS LIKELY SOLUTIONS AS IT HAS  
UPSTREAM FLOW CUTTING THROUGH THE ALASKA RIDGE. DEEPER NORTHEAST  
PACIFIC TROUGHING IN 12Z GFS/ECMWF RUNS AND NEW 00Z GFS ALONG WITH  
SOME SHORTWAVE RIDGING OVER THE WEST-CENTRAL U.S. IS WITHIN THE  
REALM OF POSSIBILITY BUT LOW ENOUGH IN CONFIDENCE THAT FAR OUT IN  
TIME TO FAVOR STAYING CLOSER TO THE MEANS. AS FOR WESTERN CANADA  
LOW PRESSURE TRACKING ALONG/NORTH OF THE U.S.-CANADIAN BORDER  
TUE-THU, A COMPROMISE AMONG THE 18Z GEFS MEAN/12Z ECMWF AND  
FARTHER SOUTH 12Z ECMWF MEAN-00Z/25 ECMWF LOOKS REASONABLE.  
COMBINING THESE CONSIDERATIONS LEADS TO TRENDING THE FORECAST  
TOWARD A BLEND OF THOSE SOLUTIONS FROM LATE TUE ONWARD.  
 
   
..WEATHER/THREATS HIGHLIGHTS
 
 
GUIDANCE CONTINUES TO TARGET PARTS OF THE NORTHEAST FOR A PERIOD  
OF VERY INTENSE RAINFALL ALONG WITH STRONG WINDS SUN-MON AS  
TROPICAL MOISTURE/LOW PRESSURE INTERACT WITH VIGOROUS DYNAMICS  
ALOFT TO BRING A DEEP LOW PRESSURE SYSTEM ACROSS THE REGION. THIS  
SYSTEM WILL LIKELY AFFECT SOME AREAS THAT RECEIVED SIGNIFICANT  
RAINFALL IN THE PRECEDING EVENT. MEANWHILE THE GREAT LAKES WILL  
SEE PERIODS OF LAKE EFFECT/LAKE ENHANCED RAIN AND SNOW WITH CHILLY  
CYCLONIC MEAN FLOW IN PLACE AND SUPPORT FROM ONE OR TWO SYNOPTIC  
SYSTEMS. EXPECT COVERAGE OF RAINFALL TO INCREASE FROM THE  
SOUTHERN PLAINS NORTHEASTWARD BY TUE-WED AS LOW LEVEL FLOW FROM  
THE GULF INTERSECTS THE FRONT THAT SHOULD PUSH SOUTHEAST THROUGH  
THE PLAINS AND THEN STALL/LIFT BACK AS A WARM FRONT. PARTS OF THE  
ROCKIES AND NEARBY AREAS MAY SEE ONE OR MORE PERIODS OF MOSTLY  
LIGHT RAIN OR SNOW DEPENDING ON ELEVATION.  
 
MINUS 10-20F ANOMALIES FOR TEMPS OVER THE SOUTH AND EAST TO START  
THE PERIOD SUN SHOULD MODERATE THEREAFTER. THE COLD FRONT  
DROPPING SOUTH FROM THE NORTHERN PLAINS SUN ONWARD WILL BRING  
SIMILARLY COOL ANOMALIES ACROSS THE PLAINS DURING MON-TUE. THE  
LEADING EDGE OF POTENTIALLY COLDER AIR MAY REACH THE NORTHERN  
PLAINS BY NEXT THU. OVER THE WEST, HIGHS OF 10-20F ABOVE NORMAL  
SHOULD BE MOST COMMON ON SUN THEN DECREASE IN COVERAGE MON ON THE  
WAY TO READINGS GENERALLY WITHIN A FEW DEGREES OF NORMAL FOR THE  
REST OF THE PERIOD.  
 
RAUSCH  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page