119  
FXUS02 KWBC 181906  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
204 PM EST THU NOV 18 2021  
 
VALID 12Z SUN NOV 21 2021 - 12Z THU NOV 25 2021  
 
...POTENTIAL CONTINUES FOR A STRONG COLD FRONT AND SIGNIFICANT  
STORM TO AFFECT THE EAST NEXT WEEK...  
   
..OVERVIEW
 
 
FOR THE EARLY PART OF NEXT WEEK, A COMPLEX LOW PRESSURE SYSTEM IS  
FORECAST TO DEVELOP NEAR THE GREAT LAKES REGION AND TRACK EASTWARD  
TOWARD THE NORTHEAST U.S./SOUTHEAST CANADA WHILE THE UPPER-LEVEL  
PATTERN AMPLIFIES ALOFT. THE COLD FRONTS ASSOCIATED WITH THIS  
STORM SYSTEM WILL BRING COLDER TEMPERATURES AND POSSIBLY GUSTY  
WINDS TO THE EAST, WHILE HEAVY RAIN IS A POSSIBILITY IN THE WARM  
SECTOR IN NORTHERN NEW ENGLAND, WITH SNOW POSSIBLE ON THE BACKSIDE  
OF THE LOW IN THE INTERIOR NORTHEAST, ESPECIALLY IN LAKE  
EFFECT/TERRAIN ENHANCED AREAS. CONTINUE TO MONITOR FORECASTS AS  
DETAILS MAY CHANGE. FARTHER WEST, A SOUTHERN STREAM UPPER LOW  
INITIALLY SOUTHWEST OF CALIFORNIA SHOULD COMBINE WITH A PACIFIC  
SHORTWAVE AND YIELD AN UPPER TROUGH ALIGNED FROM THE NORTHERN  
PLAINS INTO NORTHWESTERN MEXICO BY THANKSGIVING, BRINGING SOME  
LIGHT TO MODERATE PRECIPITATION TO THE PACIFIC NORTHWEST AND SOME  
HIGHER ELEVATION SNOW IN THE ROCKIES AROUND MONDAY-WEDNESDAY, AND  
DEVELOPING INTO A POSSIBLE HEAVY RAIN THREAT ON THANKSGIVING  
ACROSS PARTS OF TEXAS AND THE LOWER MISSISSIPPI VALLEY AS GULF  
MOISTURE INTERACTS WITH A PLAINS FRONT.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE FIRST NOTABLE FEATURE TO WATCH IS AN UPPER TROUGH WITH ITS  
AXIS INITIALLY OVER CENTRAL NORTH AMERICA ON SUNDAY THAT PUSHES  
EASTWARD AND DEEPENS CONSIDERABLY AS ADDITIONAL ENERGY STREAMS IN  
AND A RIDGE INITIALLY OVER THE THE WEST COAST TRACKS EAST AS WELL.  
THE VORTICITY/ENERGY COMING IN THAT CAUSES THE TROUGH TO DIG IS  
STILL ALOFT OVER OR NEAR ALASKA AT THIS TIME, SO THE DETAILS OF  
THE TROUGH/EVENTUALLY DEVELOPING CLOSED LOW AS WELL AS THE  
ASSOCIATED SURFACE LOW STILL HAVE LOWER PREDICTABILITY SEVERAL  
DAYS OUT IN TIME. MODELS HAVE BEEN WAFFLING ON IF/WHERE AN  
EMBEDDED UPPER LOW MAY FORM BETWEEN SUNDAY AND EARLY TUESDAY, BUT  
THERE IS A DECENT CONSENSUS THAT AN UPPER LOW SHOULD REACH ALONG  
OR JUST OFFSHORE THE MID-ATLANTIC OR NEW ENGLAND COAST BY LATER  
TUESDAY OR WEDNESDAY. AT THE SURFACE, THERE IS REASONABLE  
CONTINUITY WITH LOW PRESSURE CONSOLIDATING OVER THE UPPER GREAT  
LAKES/SOUTHEASTERN CANADA SUNDAY-MONDAY. AFTER THAT THE FORECAST  
GETS MORE COMPLICATED, AS MULTIPLE SURFACE LOW CENTERS MAY WRAP  
UNDERNEATH THE SPINNING UPPER LOW THAT MAY GET BLOCKED FROM MOVING  
AWAY FROM THE NORTHEAST DUE TO A NORTH ATLANTIC RIDGE. FOR THE  
00/06Z MODEL CYCLE, GFS RUNS AND THE ECMWF ARE QUITE SIMILAR TO  
EACH OTHER IN TERMS OF UPPER AND SURFACE LOW PLACEMENT THROUGH AT  
LEAST TUESDAY. HOWEVER, CLUSTER ANALYSIS AND SPAGHETTI PLOTS OF  
ENSEMBLE MEMBERS FROM THE ECMWF, GEFS, AND CMC ENSEMBLES SHOW THAT  
THESE DETERMINISTIC MODELS ARE ON THE SLOWER/DEEPER END OF THE  
MODEL SPREAD DESPITE THEIR GOOD AGREEMENT WITH EACH OTHER. THE  
GEFS AND EC ENSEMBLE MEANS ARE FAIRLY WELL ALIGNED THROUGH THE  
PERIOD, AND APPEAR THE BEST PROXY FOR A MIDDLE GROUND SOLUTION IN  
TERMS OF TIMING AND POSITION, SINCE THE GFS AND EC ARE SLOWER AND  
THE 00Z CMC TAKES A BIT FASTER TRACK. THE 12Z GFS AND ECMWF APPEAR  
A BIT FASTER AND THUS CLOSER TO CONSENSUS.  
 
FOR THE WEST, MODEL GUIDANCE IS REASONABLY AGREEABLE WITH BOTH THE  
SOUTHERN STREAM LOW IN THE PACIFIC WEST OF CALIFORNIA AND IT  
OPENING UP AND POSSIBLY COMBINING WITH THE NORTHERN STREAM AROUND  
TUESDAY AS A SHORTWAVE COMES INTO THE PACIFIC NORTHWEST, THOUGH  
WITH THE 00Z UKMET REMAINS ON THE SLIGHTLY SLOWER SIDE. AFTER  
THAT, MODELS ARE STILL VARYING FROM RUN TO RUN WITH TIMING OF THE  
TROUGH'S MOVEMENT, AND NEWER 12Z GUIDANCE COMING IN SEEMS TO  
MAINTAIN OR REPRODUCE MORE FLOW SEPARATION WITH A SLOWER TRACK OF  
THE TROUGH/POSSIBLE CLOSED LOW IN THE SOUTHEAST, WHICH WAS FIRST  
SEEM IN THE 00Z CMC.  
 
FOR THE UPDATED MEDIUM RANGE FORECAST, A DETERMINISTIC MODEL BLEND  
WAS USED INITIALLY, FAVORING THE GFS RUNS AND THE 00Z ECMWF. AS  
THE PERIOD PROGRESSED, PHASED IN MORE ENSEMBLE MEAN GUIDANCE AND  
ENDED UP WITH A FAIRLY EVEN BLEND OF THE 00Z ECMWF AND ITS  
ENSEMBLE MEAN AND GFS RUNS AND THE 06Z ENSEMBLE MEAN. THIS  
MAINTAINED GOOD CONTINUITY WITH THE PREVIOUS FORECAST.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
THE CONSOLIDATING NORTHERN TIER U.S./SOUTHERN CANADA LOW PRESSURE  
WILL BRING A LEADING FRONT AND STRONGER TRAILING FRONT INTO AND  
THROUGH THE EAST EARLY IN THE CALENDAR WEEK. PRECIPITATION (MOSTLY  
RAIN) WILL DEVELOP ALONG AND EAST OF THE MISSISSIPPI VALLEY ON  
SUNDAY. MOST RAIN SHOULD BE LIGHT TO MODERATE BUT LOCALLY MORE  
INTENSE ACTIVITY MAY BE POSSIBLE OVER PARTS OF THE OHIO/TENNESSEE  
VALLEYS. AS THE FRONT AND POSSIBLE WAVES OF LOW PRESSURE HEAD INTO  
NEW ENGLAND, THE TREND CONTINUES TO BE FOR SOMEWHAT LOWER  
PRECIPITATION TOTALS OVER WESTERN/SOUTHERN AREAS BUT MAINTAIN THE  
POTENTIAL FOR SOME HEAVY RAINFALL OVER MAINE, AIDED BY STRONG  
ATLANTIC INFLOW. SNOW IS STILL MOST LIKELY IN THE COLD AIR BEHIND  
THE SYSTEM, TO THE LEE OF THE GREAT LAKES AND ALONG  
WESTWARD-FACING TERRAIN FROM THE CENTRAL APPALACHIANS  
NORTHEASTWARD. THERE IS STILL ENOUGH UNCERTAINTY IN SYSTEM  
EVOLUTION TO ALLOW FOR OTHER POSSIBILITIES REGARDING PRECIPITATION  
TYPES AND AMOUNTS. MEANWHILE CONFIDENCE IS HIGHER IN A PERIOD OF  
BREEZY/GUSTY WINDS FROM THE UPPER MIDWEST THROUGH THE EAST SUNDAY  
INTO TUESDAY AND THEN POSSIBLY LINGERING OVER THE NORTHEAST  
THEREAFTER. ACROSS THE NORTHERN TIER THE COMBINATION OF LOW  
TEMPERATURES AND WIND WOULD PRODUCE COLD WIND CHILLS.  
 
THE PACIFIC NORTHWEST INTO NORTHERN ROCKIES SHOULD SEE MOSTLY  
LIGHT TO PERHAPS LOCALLY MODERATE PRECIPITATION DURING THE FIRST  
HALF OF NEXT WEEK WITH A SHORTWAVE/COLD FRONT ARRIVING FROM THE  
PACIFIC. THE COMBINATION OF THIS ENERGY ALOFT AND THE  
OPENING/PROGRESSION OF THE UPPER LOW SOUTHWEST OF CALIFORNIA MAY  
SPREAD SOME MOISTURE INTO OTHER PARTS OF THE WEST BY  
TUESDAY/WEDNESDAY, BUT STILL WITH MODEST CONFIDENCE FOR  
PRECIPITATION COVERAGE/AMOUNTS. AROUND THANKSGIVING THE WESTERN  
FRONT IS CURRENTLY EXPECTED TO REACH FAR ENOUGH INTO THE PLAINS TO  
BEGIN INTERACTING WITH GULF MOISTURE TO PRODUCE AN EXPANDING AREA  
OF RAIN ALONG AND NORTH OF THE WESTERN GULF COAST. SOME OF THIS  
RAIN MAY BECOME HEAVY.  
 
THE STRONG COLD FRONT CROSSING THE EAST EARLY IN THE WEEK WILL  
BRING WELL BELOW NORMAL TEMPERATURES THAT SHOULD BE MOST  
PRONOUNCED BETWEEN MONDAY AND EARLY WEDNESDAY WHEN THERE SHOULD BE  
DECENT COVERAGE OF HIGHS/MORNING LOWS 10-15F BELOW NORMAL, WITH  
SOME POCKETS OF HIGHS POSSIBLY 15-20F BELOW NORMAL OVER SOUTHERN  
AREAS. TEMPERATURES MAY FALL BELOW FREEZING RATHER FAR INTO THE  
SOUTH. READINGS SHOULD REBOUND TOWARD NORMAL BY THANKSGIVING.  
PORTIONS OF THE ROCKIES/PLAINS WILL SEE A BRIEF WARM PERIOD EARLY  
IN THE WEEK WITH SOME PLUS 10-15F OR SO ANOMALIES. A COLD FRONT  
MOVING FROM THE WEST INTO THE PLAINS DURING TUESDAY THROUGH  
THANKSGIVING WILL BRING MODERATELY BELOW NORMAL HIGHS IN ITS WAKE.  
 
TATE/RAUSCH  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDICES ARE 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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