362  
FXUS02 KWBC 181600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST THU JAN 18 2018  
 
VALID 12Z SUN JAN 21 2018 - 12Z THU JAN 25 2018  
   
..OVERVIEW
 
 
WITHIN A MEAN PATTERN THAT AVERAGES OUT TO BROAD CYCLONIC FLOW  
OVER THE LOWER 48, THERE WILL BE TWO PRIMARY AREAS OF SIGNIFICANT  
WEATHER DURING THE PERIOD. THE FIRST WILL BE WITH A STRENGTHENING  
STORM SYSTEM TRACKING NORTHEAST FROM THE CENTRAL PLAINS SUN  
ONWARD, BRINGING AN AXIS OF HEAVY SNOW AND STRONG WINDS MOST  
LIKELY ALONG AN AXIS FROM THE NORTH-CENTRAL PLAINS INTO THE UPPER  
GREAT LAKES WITH SOME SNOW POSSIBLY REACHING NORTHERN NEW ENGLAND  
AS WELL. BEHIND THIS SYSTEM AT LEAST A COUPLE OF FEATURES WILL  
BRING EPISODES OF HEAVY RAIN/HIGHER ELEVATION SNOW TO LOCATIONS  
ALONG THE CENTRAL/NORTHERN WEST COAST WITH SOME OF THIS ACTIVITY  
EXTENDING INTO THE ROCKIES. THUS FAR THE SIGNAL FOR THE LEADING  
PLAINS STORM HAS BEEN MORE STABLE THAN FOR SOME DETAILS OF PACIFIC  
FLOW REACHING THE WEST.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES
 
 
FOR THE DEEPENING STORM SYSTEM TRACKING NORTHEAST FROM THE CENTRAL  
PLAINS, MULTI-DAY TRENDS AND THE OVERALL MAJORITY OF GUIDANCE  
(INCLUDING THE 00Z CMC/CMC MEAN) CONTINUE TO FAVOR TILTING THE  
FORECAST ABOUT 2/3 TO 3/4 TOWARD THE ECMWF/ECMWF MEAN VERSUS THE  
FASTER GFS/GEFS MEAN. THIS MAJORITY STILL INDICATES LESS PHASING  
WITH CANADIAN FLOW THAN DEPICTED IN GFS/GEFS RUNS AS THE UPPER  
TROUGH REACHES THE UPPER MS VALLEY/UPPER GREAT LAKES. THERE  
CONTINUES TO BE THE STRAY SOLUTION THAT IS EVEN A BIT SLOWER THAN  
THE ECMWF, SUCH AS THE 12Z/17 CMC AND CURRENT 00Z UKMET. HOWEVER  
THE RELATIVELY LOW AMPLITUDE OF UPSTREAM PACIFIC FLOW AS IT MOVES  
INTO THE WEST PROVIDES A LINGERING POTENTIAL THAT THE SYSTEM OF  
INTEREST COULD END UP PROGRESSING A LITTLE FASTER THAN THE SLOW  
SIDE OF THE SPREAD, HENCE PRECLUDING FULL EXCLUSION OF THE GFS  
SCENARIO. THERE IS STILL SOME MODEL/ENSEMBLE SPREAD FOR LATITUDE  
OF THE SURFACE LOW TRACK BUT A MULTI-RUN CONSENSUS HAS PROVEN TO  
BE FAIRLY STABLE OVER THE PAST COUPLE DAYS.  
 
IN GENERAL GUIDANCE HAS BEEN MORE DIVERSE AND VARIABLE WITH  
DETAILS OF UPSTREAM FLOW. TO THE SOUTH OF AN UPPER LOW NEARING  
THE BC COAST AS OF EARLY DAY 3 SUN, ENERGY STREAMING INTO THE WEST  
HAS GENERALLY BEEN TRENDING FASTER AND LESS AMPLIFIED OVER RECENT  
DAYS. HOWEVER THERE IS STILL A LOT OF MODEL/ENSEMBLE SPREAD FOR  
THE DETAILS OF INDIVIDUAL IMPULSES PROGRESSING ACROSS THE  
CONTIGUOUS U.S. WITH THE BROAD CYCLONIC NATURE OF THE MEAN PATTERN  
OFFERING LITTLE IN THE WAY OF A SIGNAL TO ASSIST IN DETERMINING  
THE BEST SOLUTION. THESE DIFFERENCES END UP PLAYING A SIGNIFICANT  
ROLE IN DETERMINING HOW MUCH GULF MOISTURE MAY EXTEND TO THE  
EAST/NORTHEAST ACROSS FLORIDA/EXTREME SOUTHEAST TOWARD DAYS 6-7  
WED-THU. FOR EXAMPLE THE SLOWER/SHARPER GFS DEPICTIONS OF ENERGY  
ALOFT LEAD TO VERY WET SOLUTIONS IN CONTRAST TO THE FASTER 00Z  
ECMWF LEADING TO A FAIRLY DRY FORECAST. ON THE OTHER HAND THE  
12Z/17 ECMWF RUN HAD LOOKED MORE LIKE THE GFS RUNS. RELATIVELY  
HIGH UNCERTAINTY ARGUES FOR A CONSERVATIVE BLENDED APPROACH AT  
THIS TIME.  
 
MEANWHILE GUIDANCE HAS BEEN WAFFLING WITH RESPECT TO SHORTWAVE  
ENERGY NEARING THE PACIFIC NORTHWEST BY DAY 5 TUE AND TRAILING  
LARGER SCALE TROUGH EXPECTED TO MOVE INTO THE WEST THEREAFTER. AS  
A WHOLE, OVER THE PAST DAY MOST OPERATIONAL MODEL RUNS HAVE  
TRENDED BACK SOUTHWARD WITH A POTENTIAL WAVE TRACKING TOWARD THE  
BC OR PACIFIC NORTHWEST COAST ON TUE. DURING THE DAY TUE 00Z  
ENSEMBLE SURFACE LOW PLOTS STILL SHOW A VERY BROAD NORTH-SOUTH  
ENVELOPE WITH LITTLE INDICATION OF ANY TRUE CLUSTERING. BY EARLY  
WED THE BEST DENSITY OF ENSEMBLES EXTENDS FROM JUST OFF THE WA  
COAST NORTHWESTWARD. BY DAY 7 THU THE 00Z ECMWF/ECMWF MEAN HAVE  
TRENDED NOTICEABLY MORE AMPLIFIED/FASTER THAN PREVIOUS RUNS WITH  
OVERALL TROUGHING REACHING THE WEST. TO VARYING DEGREES THE  
CMC/CMC MEAN ALSO BRING SOMEWHAT MORE TROUGHING INTO THE WEST.  
FOR BOTH ASPECTS OF THE EASTERN PACIFIC/WESTERN U.S. FORECAST, AN  
INTERMEDIATE SOLUTION THAT INCORPORATES TRAITS OF BOTH MODEL AND  
ENSEMBLE GUIDANCE APPEARS BEST WHILE AWAITING BETTER GUIDANCE  
AGREEMENT AND RUN TO RUN STABILITY.  
 
COMBINING PREFERENCES FOR SYSTEMS OF INTEREST, THE UPDATED BLEND  
STARTED WITH A 50/30/20 BLEND OF THE 00Z ECMWF/06Z GEFS/00Z CMC  
INTO EARLY DAY 5 TUE. AFTER THAT TIME THE BLEND BEGAN TO  
INCORPORATE 06Z GEFS/00Z ECMWF MEAN INPUT WITH ENSEMBLE INPUT  
ACCOUNTING FOR 60 PERCENT OF THE FORECAST BY DAY 7 THU.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
IN SPITE OF LINGERING UNCERTAINTY ON TIMING AND TO SOME DEGREE  
TRACK OF DEEPENING LOW PRESSURE EMERGING FROM THE CENTRAL PLAINS  
ON SUN, THERE HAS BEEN A FAIRLY CONSISTENT SIGNAL FOR THE THREAT  
OF A BAND OF HEAVY SNOW EXTENDING FROM THE NORTH-CENTRAL PLAINS  
INTO THE UPPER GREAT LAKES. TIGHT PRESSURE GRADIENT AROUND THE  
STORM MAY PRODUCE WINDS STRONG ENOUGH TO PROMOTE A PERIOD OF  
BLIZZARD CONDITIONS. AHEAD OF THE SYSTEM, NORTHERN NEW ENGLAND  
MAY BE COLD ENOUGH TO SEE SOME SNOW FROM THIS SYSTEM AS WELL. IN  
THE WARM SECTOR, EXPECT SOME AREAS OF LOCALLY MODERATE/HEAVY  
RAINFALL OVER/NEAR THE LOWER HALF OF THE MS VALLEY LATE SUN INTO  
MON. AMOUNTS MAY TAPER OFF FOR A TIME FARTHER EAST BUT ATLANTIC  
INFLOW AHEAD OF THE COLD FRONT MAY BRING SOMEWHAT HEAVIER ACTIVITY  
INTO PARTS OF THE NORTHEAST BY LATE MON-TUE.  
 
THE PRECIP FORECAST FOR AREAS ALONG THE GULF COAST AND  
FLORIDA/EXTREME SOUTHEAST BECOMES VERY UNCERTAIN TOWARD MIDWEEK,  
WITH SOLUTIONS SPANNING THE FULL RANGE OF DRY TO WET SCENARIOS.  
THE RELATIVELY SMALLER SCALE OF IMPORTANT DETAILS ALOFT WITHIN A  
MORE AGREEABLE MEAN PATTERN LEADS TO LOW  
PREDICTABILITY/CONFIDENCE, THUS FAVORING A COMPROMISE FOR ANY  
DETERMINISTIC FORECAST.  
 
IN SPITE OF SOME DETAIL DIFFERENCES AT THE SURFACE AND ALOFT FROM  
DAY TO DAY, THERE HAS BEEN A CONSISTENT SIGNAL IN PRINCIPLE FOR  
HEAVY RAIN/HIGHER ELEVATION SNOW FROM THE PACIFIC NORTHWEST INTO  
NORTHERN CALIFORNIA WITH LESSER TOTALS EXTENDING INTO THE NORTHERN  
ROCKIES. DETAIL UNCERTAINTIES LATE IN THE PERIOD AFFECT THE  
SOUTHWARD EXTENT OF PRECIP. THERE SHOULD BE ONE PRONOUNCED WAVE  
OF PRECIP AROUND SUN AND THEN ANOTHER AROUND TUE-WED. PERIODS OF  
STRONGER WINDS ARE POSSIBLE AS WELL.  
 
IN THE WARM SECTOR AHEAD OF THE SYSTEM TRACKING OUT OF THE PLAINS,  
EXPECT THE EASTERN HALF OF THE COUNTRY TO SEE HIGHS UP TO 10-20F  
ABOVE NORMAL AND MINS AS WARM AS 15-25F ABOVE NORMAL INTO EARLY  
NEXT WEEK. TRAILING COLDER AIR OVER THE WEST SHOULD BE 5-15F  
BELOW NORMAL. TEMPERATURE ANOMALIES SHOULD BE MODEST OVER MOST OF  
THE LOWER 48 FOR A DAY OR TWO AROUND MIDWEEK, FOLLOWED BY A  
COOLING TREND OVER THE WEST AND WARMING OVER THE PLAINS/MS VALLEY  
AS THE UPPER PATTERN BEGINS TO AMPLIFY.  
 
RAUSCH  
 

 
 
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