599  
FXUS02 KWBC 041532  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1031 AM EST THU JAN 04 2018  
 
VALID 12Z SUN JAN 07 2018 - 12Z THU JAN 11 2018  
 
15 UTC UPDATE...  
 
FORECAST UPDATE WAS KEPT GENERALLY CLOSE TO CONTINUITY EARLY IN  
THE MEDIUM RANGE. MODELS SHOWED RELATIVELY GOOD CONSENSUS WITH  
RESPECT TO SHORTWAVE CROSSING THE SOUTHERN PLAINS/MS VALLEY DAYS  
3-4 (SUN-MON), WITH DIFFERENCES PRIMARILY RELATED TO THE EXACT  
AMPLITUDE OF THE WAVE. A BLEND OF THE 00Z ECMWF/UKMET AND 06Z GFS  
SEEMED TO REPRESENT CONSENSUS WELL, AND THIS SERVED AS A FORECAST  
STARTING POINT FOR DAYS 3-4. UNCERTAINTY INCREASES SIGNIFICANTLY  
FROM DAY 5 ONWARD, WITH SOME MODEL GUIDANCE MAKING A RATHER  
SIGNIFICANT CHANGE WITH RESPECT TO ENERGY OFF THE WEST COAST ON  
DAY 5 (TUE) AND WHETHER A CUTOFF UPPER LOW DEVELOPS, WHICH IN TURN  
AFFECTS HOW QUICKLY THE SYSTEM PROGRESSES EASTWARD ACROSS THE  
ROCKIES AND INTO THE CENTRAL U.S. THE 00Z ECMWF MADE A RATHER  
DRAMATIC CHANGE FROM ITS PREVIOUS RUN, NO LONGER DEVELOPING A  
CUTOFF LOW OFF THE WEST COAST, AND QUICKLY MOVING THE TROUGH EAST  
ACROSS THE ROCKIES BY WED AND INTO THE PLAINS/MS VALLEY BY THU. A  
SUBSTANTIAL QUANTITY OF ECENS MEMBERS ALSO TRENDED FASTER WITH THE  
SYSTEM, BUT THE DETERMINISTIC 00Z ECMWF IS NOW AMONG THE FASTEST  
SOLUTIONS EVEN ACCOUNTING FOR ALL 50 ENSEMBLE MEMBERS. THE GFS  
ALSO TRENDED FASTER WITH THE 06Z RUN, ALTHOUGH NOT TO THE SAME  
DEGREE AS THE ECMWF. THE GFS CONTINUES TO DEVELOP A CUTOFF UPPER  
LOW OFF THE CA COAST LATE TUE (ALTHOUGH MUCH FARTHER SOUTHEAST  
COMPARED TO PREVIOUS RUNS), WHICH THEN CROSSES THE ROCKIES WED  
NIGHT AND MOVES INTO THE CENTRAL U.S. THU. CONVERSELY, THE 00Z  
UKMET MADE VERY LITTLE ADJUSTMENT, AND STILL KEEPS A CUTOFF LOW  
(WELL NORTHWEST OF THE 06Z GFS) OFF THE CA COAST THROUGH TUE. THE  
00Z ECENS/06Z GEFS SOLUTIONS BOTH TRENDED A BIT FASTER, ALTHOUGH  
BOTH REMAIN SLOWER THAN THEIR RESPECTIVE DETERMINISTIC SOLUTIONS.  
GIVEN THE DRAMATIC CHANGE IN SOME OF THE DETERMINISTIC SOLUTIONS,  
THE FORECAST FROM DAY 5 ONWARD WAS BASED MOSTLY ON A BLEND OF THE  
00Z ECENS/06Z GEFS MEANS, WITH 20 PERCENT OF WPC CONTINUITY  
INCLUDED AS WELL. THIS BLEND RESULTS IN A SHIFT TO ACCOUNT FOR THE  
NOTED TREND AMONG THE GUIDANCE, BUT KEEPS THE CHANGE SOMEWHAT  
TEMPERED AND NOT AS DRAMATIC AS THE 00Z ECMWF WOULD INDICATE. WILL  
HAVE TO WAIT AND SEE HOW FUTURE MODEL RUNS HANDLE THIS SYSTEM, AND  
SEE SOME RUN-TO-RUN CONSISTENCY BEFORE CONFIDENCE INCREASES IN THE  
FORECAST FROM DAY 5 ONWARD.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0656 UTC)...  
   
..OVERVIEW
 
 
THE GREATEST FOCUS OF THE MEDIUM RANGE PERIOD WILL CONTINUE TO BE  
ON TWO SYSTEMS. THE FIRST WILL PROGRESS EAST FROM THE PLAINS SUN  
INTO EARLY NEXT WEEK AND BRING A FAIRLY BROAD AREA OF VARIOUS  
PRECIP TYPES TO THE EASTERN HALF OF THE LOWER 48. THE SECOND HAS  
POTENTIAL TO BE A FAIRLY STRONG SYSTEM THAT DEVELOPS OFFSHORE THE  
CENTRAL WEST COAST EARLY IN THE WEEK AND THEN WEAKENS SOMEWHAT AS  
IT REACHES THE CALIFORNIA COAST. THIS SYSTEM MAY BRING  
SIGNIFICANT PRECIP INTO CALIFORNIA WITH SOME MOISTURE SPREADING  
INLAND LATE IN THE PERIOD. FOR BOTH FEATURES THERE ARE SOME  
COMMON THEMES IN THE GUIDANCE BUT ALSO SOME MEANINGFUL DIFFERENCES  
IN CURRENT/RECENT SOLUTIONS.  
 
   
..GUIDANCE ASSESSMENT AND PREFERENCES
 
 
FOR THE SYSTEM INITIALLY EMERGING FROM THE PLAINS, THE COMPLEX  
NATURE OF THE ORIGINATING EASTERN PACIFIC SHORTWAVE ENERGY  
CONTINUES TO PRODUCE GREATER THAN DESIRED SPREAD FOR  
CENTRAL/EASTERN NORTH AMERICA DETAILS ALOFT AND CORRESPONDING  
SURFACE EVOLUTION. THROUGH THE 12Z/18Z CYCLES THE MAJORITY OF  
OPERATIONAL SOLUTIONS WOULD HAVE A CENTRAL ROCKIES/PLAINS  
SHORTWAVE TENDING TO TRACK FAR ENOUGH SOUTH TO REMAIN SEPARATE  
FROM CANADIAN/NORTHERN TIER FLOW OVER MOST OF THE CENTRAL/EASTERN  
U.S., LEADING TO A FRONTAL WAVE THAT TRACKS FAIRLY FAR SOUTH.  
SOME ENSEMBLE MEMBERS SHOW THIS SOUTHERN WAVE BUT NOT A HIGH  
ENOUGH PERCENTAGE TO HAVE GREAT CONFIDENCE, LEADING TO A MORE  
PHASED SOLUTION AND AT BEST A HINT AT A SOUTHERN WAVE. ULTIMATELY  
PREFERRED TO USE THE 12Z GFS/ECMWF AS THE OPERATIONAL COMPONENT OF  
THE FORECAST TO NUDGE CONTINUITY A LITTLE SOUTH OVER THE MS VALLEY  
BUT END UP NEAR CONTINUITY THEREAFTER. THIS IS A MIDDLE GROUND  
BETWEEN THE 12Z CMC AND FARTHER SOUTH 18Z GFS/12Z UKMET. NEW 00Z  
GUIDANCE DOES NOT IMPROVE CONFIDENCE IN ANY PARTICULAR SCENARIO,  
WITH THE CMC JUMPING TO DEEPER/SHARPER NORTHERN STREAM PHASING AND  
STRONG MID ATLANTIC SYSTEM-- REMINISCENT OF YESTERDAY'S 00Z  
ECMWF-- AND THE UKMET/CMC KEEPING THE TWO STREAMS WELL SEPARATED.  
FARTHER NORTH THERE ARE ALSO MEANINGFUL DETAIL DIFFERENCES ALOFT  
AND THUS FOR SPECIFICS OF THE OVERALL AREA OF LOW PRESSURE  
FORECAST TO TRACK ACROSS SOUTHERN CANADA AND PERHAPS BRUSH THE  
UPPER GREAT LAKES.  
 
MINUS THE NEW 00Z CMC THERE IS SOMEWHAT IMPROVED CLUSTERING OF  
GUIDANCE FOR THE UPPER LOW THAT MOST OPERATIONAL MODELS EXPECT TO  
CLOSE OFF OVER THE EASTERN PACIFIC MON-TUE WITH A SURFACE SYSTEM  
REACHING A CENTRAL PRESSURE AT LEAST AS LOW AS THE 990'S MB,  
FOLLOWED BY GRADUAL WEAKENING AS IT TRACKS TOWARD THE CALIFORNIA  
COAST. TRENDS OVER THE PAST DAY HAVE PRIMARILY INVOLVED THE GFS  
TRENDING FASTER TOWARD THE ECMWF AND THE NEW 00Z GFS/UKMET ARE  
FAIRLY CLOSE TO THAT SCENARIO AS WELL THROUGH 00Z WED. TYPICAL  
VERIFICATION FOR THE TIME FRAME INVOLVED AS WELL AS THE CURRENT  
ENSEMBLE SPREAD SUGGEST THERE IS STILL POTENTIAL FOR SIGNIFICANT  
CHANGES, BUT MOST OPERATIONAL MODELS HAVE BUILT UP ENOUGH  
CONTINUITY TO SUGGEST THAT THE MODELS ARE LEADING THE MEANS FOR  
DEFINITION OF THE SYSTEM. THUS WOULD PREFER TO MAINTAIN AT LEAST  
A SLIGHT MAJORITY OF OPERATIONAL GUIDANCE VERSUS THE MEANS THROUGH  
THE PERIOD. A COMPROMISE APPROACH HELPS TO MITIGATE CONTINUED  
DIFFERENCES IN TIMING AS WELL AS DAY 7 THU DIFFERENCES IN  
AMPLITUDE OF THE REMAINING TROUGH THAT CROSSES THE SOUTHERN  
PORTION OF THE WEST WITH THE GFS FLATTER THAN 12Z ECMWF/CMC.  
 
DAYS 3-5 SUN-TUE STARTED WITH A 70 MODEL/30 ENSEMBLE BLEND OF THE  
12Z GFS/ECMWF AND THEIR ENSEMBLE MEANS TO REPRESENT PREFERENCES  
FOR THE WAVY FRONTAL SYSTEM AFFECTING THE EASTERN HALF OF THE  
COUNTRY AND SYSTEM DEVELOPING OFF THE WEST COAST. DAYS 6-7  
WED-THU KEPT A TOTAL OF 60 PERCENT MODEL WEIGHT BUT WITH THE  
ADDITION OF THE 00Z/03 ECMWF AND 18Z GFS DUE TO SIGNIFICANT BUT  
FAIRLY LOW CONFIDENCE DIFFERENCES THAT DEVELOP WITH NORTHERN  
TIER/SOUTHERN CANADA FLOW ALOFT AND THE RESULTING SURFACE PATTERN.  
THIS HELPED TO GUIDE THE FORECAST MORE TOWARD THE MEANS WHERE THE  
MOST CONTENTIOUS ISSUES EXISTED WHILE MAINTAINING DEFINITION OF  
THE SOUTHWESTERN U.S. SYSTEM.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE WAVY FRONTAL SYSTEM AFFECTING THE EASTERN HALF OF THE COUNTRY  
DURING THE FIRST HALF OF THE PERIOD WILL INITIALLY DEVELOP PRECIP  
OVER THE EASTERN PLAINS/MS VALLEY ON SUN WITH AN EXPANDING  
MOISTURE SHIELD THAT CONTINUES EASTWARD THEREAFTER. THE BEST  
SIGNAL FOR LOCALLY HEAVY AMOUNTS CONTINUES TO BE FROM THE LOWER  
HALF OF THE MS VALLEY EASTWARD TOWARD THE APPALACHIANS/FLORIDA  
PANHANDLE. OTHER AREAS MAY HAVE POTENTIAL FOR ENHANCED ACTIVITY  
IF A LESS PROBABLE SURFACE WAVE EVOLUTION OCCURS. EXPECT RAIN  
ACROSS AT LEAST THE SOUTHERN HALF OF THE PRECIP SHIELD WITH MOSTLY  
SNOW LIKELY FROM THE GREAT LAKES INTO NEW ENGLAND. A TRANSITION  
ZONE SHOULD EXIST TO THE SOUTH OF THE SNOW AREA, WITH THE RISK  
THAT COLD CONDITIONS LEADING INTO THIS EVENT COULD RESULT IN A  
DELAY OF CHANGEOVER TO RAIN COMPARED TO WHAT SOME GUIDANCE  
SUGGESTS.  
 
EXPECT TWO FAVORED AREAS OF ENHANCED RAIN/HIGH ELEVATION SNOW OVER  
THE WEST COAST STATES. ONE WILL BE OVER THE NORTHERN PACIFIC  
NORTHWEST WITH A COUPLE SYSTEMS THAT WILL STAY OFFSHORE BUT PULL  
MOISTURE INTO THE REGION STARTING THIS WEEKEND. CALIFORNIA WILL  
BE THE OTHER AREA OF FOCUS AHEAD OF THE FAIRLY STRONG SYSTEM  
LIKELY TO DEVELOP OFFSHORE AND THEN WEAKEN AS IT MOVES EASTWARD.  
SOUTHWESTERLY FLOW ALOFT MAY PROMOTE SIGNIFICANT TOTALS OVER SOME  
COASTAL LOCATIONS AND THE SIERRA NEVADA RANGE. RAINFALL WITH THIS  
SYSTEM MAY ALSO LEAD TO RUNOFF ISSUES OVER RECENT BURN AREAS.  
MOISTURE FROM THIS SYSTEM SHOULD SPREAD ACROSS THE INTERIOR WEST  
AND ROCKIES BY WED-THU. AT THAT SAME TIME SOME SNOW MAY SPREAD  
ACROSS PARTS OF THE NORTHERN TIER WITH GENERALLY LIGHT/MODERATE  
AMOUNTS BUT LOW CONFIDENCE IN TIMING/COVERAGE.  
 
LINGERING EFFECTS OF THE CONTINUING EASTERN U.S. COLD EVENT WILL  
BE IN THE FORM OF POTENTIAL RECORD LOWS UNDER EAST COAST HIGH  
PRESSURE EARLY SUN, THOUGH AN ISOLATED RECORD COLD HIGH MAY ALSO  
BE POSSIBLE. AFTER A BRIEF WARMUP AND PASSAGE OF THE  
CENTRAL/EASTERN U.S. SYSTEM THERE WILL BE A RETURN OF CHILLY TEMPS  
TO AREAS FROM THE NORTHERN PLAINS TO NEW ENGLAND, BUT LESS EXTREME  
WITH MOST READINGS NOT EXCEEDING 10-20F BELOW NORMAL. EXPECT A  
MAJORITY OF THE WEST TO SEE ABOVE NORMAL TEMPS WITH UP TO PLUS  
10-20F ANOMALIES ACROSS SOME INTERIOR AREAS.  
 
RAUSCH  
 

 
 
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