840  
FXUS02 KWBC 190700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST SAT JAN 19 2019  
 
VALID 12Z TUE JAN 22 2019 - 12Z SAT JAN 26 2019  
   
..OVERVIEW
 
 
LATEST GUIDANCE IS PERSISTENT AND AGREEABLE REGARDING THE EXPECTED  
PATTERN EVOLUTION TOWARD A VERY AMPLIFIED EASTERN PACIFIC/WEST  
COAST RIDGE AND EAST-CENTRAL NORTH AMERICAN TROUGH. ESTABLISHMENT  
OF THIS PATTERN WILL PROMOTE A SOUTHWARD PUSH OF COLD CANADIAN AIR  
INTO THE NORTHERN U.S. BY LATE THIS WEEK WITH THE UPPER HALF OF  
THE MISSISSIPPI VALLEY AND VICINITY THE MOST LIKELY TO SEE THE  
LOWEST TEMPERATURES. TELECONNECTIONS RELATIVE TO CORRESPONDING  
HEIGHT ANOMALY CENTERS SUPPORT THE EXPECTED AXIS OF COLD AIR OVER  
THE EAST-CENTRAL U.S. ALONG WITH SOME PRECIPITATION CHANCES ALONG  
THE EAST COAST, WHILE THE WEST COAST SHOULD BE DRY NEAR THE UPPER  
RIDGE AXIS. WITHIN THE CONSENSUS MEAN PATTERN ALOFT THERE WILL  
CONTINUE TO BE SIGNIFICANT MEDIUM TO SMALLER SCALE DIFFERENCES  
THAT WILL AFFECT SENSIBLE WEATHER AND TAKE ADDITIONAL TIME TO  
RESOLVE.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS/ENSEMBLES CONTINUE TO SHOW SIGNIFICANT DIFFERENCES AND  
TRENDS FOR VARIOUS ASPECTS OF THE FORECAST, HIGHLIGHTING THE  
SENSITIVITY OF SHORTWAVE SPECIFICS AND STREAM INTERACTION. EARLY  
IN THE PERIOD MODELS/MEANS ARE IMPROVING THEIR CLUSTERING FOR LOW  
PRESSURE TRACKING FROM THE CENTRAL PLAINS THROUGH THE GREAT LAKES  
AND INTO EASTERN CANADA. SOLUTIONS APPEAR TO BE CONVERGING TOWARD  
AN INTERMEDIATE TIMING SOMEWHAT CLOSER TO EARLIER GFS RUNS VERSUS  
THE ECMWF THAT HAD BEEN MUCH SLOWER. MEANWHILE THE PAST DAY HAS  
SEEN AN INCREASING SIGNAL TOWARD TRAILING SHORTWAVE ENERGY HANGING  
BACK ENOUGH TO PRODUCE WHAT COULD BE A STRONG FRONTAL WAVE  
AFFECTING THE EASTERN STATES MID-LATE WEEK. IN SPITE OF THIS  
IMPROVING SIGNAL FOR WAVE EXISTENCE, MODEL/ENSEMBLE SPREAD IS  
STILL DRAMATIC AND SHOWS LITTLE CLUSTERING. FOR EXAMPLE MODEL  
FORECASTS VALID 12Z THU RANGE BETWEEN THE EAST-CENTRAL GULF COAST  
(12Z ECMWF) TO THE NORTHERN MID-ATLANTIC (NEW 00Z GFS/CMC).  
LONGITUDE OF THIS WAVE IS ANOTHER ISSUE BEYOND TIMING. OVERALL AN  
INTERMEDIATE SOLUTION APPEARED BEST IN LIGHT OF THE FULL ARRAY OF  
GUIDANCE THROUGH THE 12Z/18Z CYCLES.  
 
BEHIND THIS SYSTEM THERE WILL LIKELY BE MULTIPLE SHORTWAVES  
PASSING THROUGH/AROUND THE NORTHERN PART OF THE EASTERN PACIFIC  
AND PROGRESSING INTO NORTH AMERICA WHILE FLOW AROUND THE MEAN  
UPPER LOW OVER/NEAR HUDSON BAY SHOULD EXTEND SOUTHWARD INTO THE  
NORTHERN U.S.. FOR THE MOST PART SHORTWAVE DETAILS ARE  
SUFFICIENTLY SMALL IN SCALE TO YIELD LOW CONFIDENCE IN SPECIFICS,  
AND WILL BE PRONE TO RUN-RUN/DAY-DAY VARIABILITY. THUS EXPECT  
WORSE THAN AVERAGE CONTINUITY FOR ASSOCIATED WAVES/FRONTAL  
SYSTEMS. IN THE THU-SAT TIME FRAME THE PRIMARY MODEL ISSUE OF  
NOTE IS THAT THE 12Z ECMWF SHOWS GREATER WESTWARD EXTENT OF UPPER  
TROUGHING OVER THE WEST THAN MOST GUIDANCE--ACHIEVING EXTREME TO  
OUTLIER STATUS DEPENDING ON HEIGHT CONTOUR AND LOCATION.  
 
GUIDANCE EVALUATION LED TO A BLEND PREFERENCE INCORPORATING THE  
18Z GFS AND 12Z ECMWF/UKMET/CMC EARLY IN THE PERIOD. RAPIDLY  
LOWERING CONFIDENCE IN SPECIFICS AFTER WED FAVORED A CORRESPONDING  
INCREASE OF 18Z GEFS/12Z ECMWF MEAN INPUT SO THAT THE MEANS  
PROVIDED 80 PERCENT OF THE MASS FIELD INGREDIENTS BY DAYS 6-7  
FRI-SAT.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
DURING TUE-WED PLAINS LOW PRESSURE TRACKING THROUGH THE GREAT  
LAKES AND INTO EASTERN CANADA WILL SPREAD SNOW FROM THE  
NORTH-CENTRAL PLAINS AND UPPER MISSISSIPPI VALLEY THROUGH THE  
CENTRAL-NORTHERN GREAT LAKES INTO NEW ENGLAND. THE TRAILING FRONT  
SHOULD BEGIN TO INTERACT WITH GULF MOISTURE BY TUE NIGHT-WED WITH  
EXPANDING AREAS OF LOCALLY MODERATE/HEAVY WARM SECTOR RAINFALL.  
CONFIDENCE IS INCREASING THAT A SIGNIFICANT WAVE MAY FORM ALONG  
THE TRAILING FRONT AND SPREAD A BROAD SHIELD OF  
PRECIPITATION--SOME HEAVY--ACROSS THE EASTERN STATES AT LEAST  
THROUGH THU. HOWEVER TRACK AND TIMING OF THIS WAVE REMAIN VERY  
UNCERTAIN AND THIS TEMPERS CONFIDENCE IN PINPOINTING LOCATION OF  
HEAVIEST RAIN/SNOW AS WELL AS PRECIPITATION TYPES. CURRENTLY THE  
BEST POTENTIAL FOR HIGHEST TOTAL PRECIPITATION IS WITHIN AN AREA  
FROM THE EAST-CENTRAL GULF COAST/TENNESSEE VALLEY/SOUTHERN  
APPALACHIANS NORTHEASTWARD INTO SOUTHERN NEW ENGLAND. BEST  
SNOW/WINTRY MIX POTENTIAL WILL LIE TO THE NORTHWEST OF THAT AREA.  
AFTER PASSAGE OF THIS SYSTEM, ONE OR MORE WAVES/FRONTS MAY PRODUCE  
AREAS OF LIGHT-MODERATE SNOW OVER SOME AREAS EAST OF THE ROCKIES  
AND COLD FLOW OVER THE MOSTLY ICE-FREE GREAT LAKES SHOULD PROMOTE  
EPISODES OF LAKE EFFECT SNOW.  
 
THE FORECAST LOOKS FAIRLY CONSISTENT FOR THE FOCUSED BAND OF  
SHORTWAVE ENERGY AND MOISTURE THAT WILL BRING A PERIOD OF ENHANCED  
SNOW AND LOW ELEVATION RAIN TO LOCATIONS FROM THE PACIFIC  
NORTHWEST INTO THE NORTHERN/CENTRAL ROCKIES TUE-WED NIGHT. LATE  
WEEK INTO THE WEEKEND ADDITIONAL SHORTWAVE ENERGY WITHIN  
NORTHWESTERLY FLOW ALOFT ALONG WITH A PERSISTENT STATIONARY FRONT  
AT THE SURFACE MAY PROMOTE SOME SNOW ALONG THE NORTHERN HALF OF  
THE ROCKIES.  
 
PORTIONS OF THE EAST WILL SEE A VERY COLD START TO THE PERIOD ON  
TUE (SOME LOWS 20F OR MORE BELOW NORMAL OVER THE SOUTHEASTERN  
GREAT LAKES/NORTHERN APPALACHIANS) BEFORE A BRIEF WARM-UP AHEAD OF  
THE APPROACHING FRONT/WAVE, WITH SOME READINGS UP TO 10-20F OR SO  
ABOVE NORMAL. COLD AIR DROPPING INTO THE EAST-CENTRAL U.S. LATER  
IN THE PERIOD MAY BRING TEMPERATURES DOWN TO AT LEAST 10-20F BELOW  
NORMAL NEAR THE MISSISSIPPI VALLEY. CONTINUE TO EXPECT MODERATELY  
ABOVE NORMAL TEMPERATURES OVER THE WEST COAST STATES AFTER MIDWEEK  
AS THE UPPER RIDGE AXIS SETTLES NEARBY.  
 
RAUSCH  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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