383  
FXUS02 KWBC 191554  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1053 AM EST TUE DEC 19 2017  
 
VALID 12Z FRI DEC 22 2017 - 12Z TUE DEC 26 2017  
   
..ARCTIC AIR TO PLUNGE INTO THE CENTRAL U.S. BY THE WEEKEND
 
 
...POTENTIAL FOR WINTRY WEATHER IN THE NORTHEAST ON CHRISTMAS  
DAY...  
 
   
..OVERVIEW
 
 
WINTER WILL MAKE ITS PRESENCE KNOWN STARTING LATER THIS WEEK IN  
THE WEST (NORTHERN ROCKIES) AND EVENTUALLY THE EAST BY NEXT WEEK  
AS THE COLDEST AIR IN NORTH AMERICA DESCENDS INTO THE UPPER  
MIDWEST/GREAT LAKES. A COUPLE OF LEAD SYSTEMS IN THE EAST FRI-SUN  
WILL BRING A LARGE AREA OF MODERATE TO LOCALLY HEAVY RAIN FROM THE  
LOWER MISSISSIPPI VALLEY TOWARD THE CENTRAL APPALACHIANS AND  
CAROLINA COASTAL PLAINS A COUPLE WAVES OF LOW PRESSURE RIDE OF A  
SLOWLY MOVING FRONTAL BOUNDARY. ARCTIC AIR (MODIFIED AS IT SURGES  
SOUTHWARD) WILL PUSH THROUGH THE ROCKIES/INTERMOUNTAIN WEST THIS  
WEEKEND, LEAVING ONLY CALIFORNIA WEST OF THE SIERRAS WITH NEAR  
AVERAGE TEMPERATURES. BY NEXT WEEK, SNOW IS POSSIBLE IN THE  
NORTHEAST ON CHRISTMAS DAY AS A WAVE OF LOW PRESSURE LIFTS  
NORTHEASTWARD NEAR OR JUST OFF THE MID-ATLANTIC AND NORTHEAST  
COAST, FOLLOWED BY ANOTHER SHOT OF REINFORCING ARCTIC AIR FOR THE  
UPPER MIDWEST AND GREAT LAKES.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE MAJOR PATTERN CHANGE WILL BE DRIVEN BY STRONG UPPER RIDGING  
PUNCHING NORTHWARD INTO EASTERN ALASKA AND NORTHWESTERN CANADA  
(500MB HEIGHTS +2 TO +3 STANDARD DEVIATIONS IN THE ENSEMBLE MEANS  
AND ABOVE THE 99TH PERCENTILE) SUPPORT BROAD CYCLONIC FLOW OVER  
CENTRAL/EASTERN CANADA INTO THE LOWER 48 WITH EMBEDDED IMPULSES  
ROTATING THROUGH THE FLOW. WELL TO THE SOUTHEAST, STRONG RIDGING  
(+2 SIGMA AND ALSO ABOVE THE 99TH PERCENTILE) ACROSS FLORIDA WILL  
ATTEMPT TO HOLD BUT ULTIMATELY GET SHUNTED EASTWARD INTO THE  
ATLANTIC, AIDING IN THE BAROCLINIC ZONE IN THE SOUTHEAST AND  
RESULTANT HEAVY PRECIP THREAT. THE 00Z MODELS AND ENSEMBLES WERE  
IN GOOD AGREEMENT FOR THE FIRST HALF OF THE PERIOD BUT WITH SOME  
CHANGES FROM JUST 12 HRS AGO. GIVEN THE SHORTER LEAD TIME, OPTED  
TO RELY ON THE DETERMINISTIC CONSENSUS (GFS/ECMWF/UKMET/CANADIAN)  
FOR FRI-SUN WITH THE LEAD FRONT AND TRAILING INITIAL ARCTIC FRONT  
AS THE MERGE IN THE NORTHEAST WITH A FIRST ROUND OF WINTRY WEATHER  
ALONG THE US/CANADIAN BORDER (FRI-SAT). NEXT WAVE ALONG THAT FRONT  
MAY HAVE MORE IMPACT AS IT MOVES ALONG THE COAST (ECMWF/CANADIAN  
WERE INLAND, GFS AND MOST ENSEMBLES JUST OFFSHORE) WITH COLDER AIR  
TO ITS WEST. SNOW LOOKS MORE LIKELY JUST INLAND FROM THE I-95  
CORRIDOR BUT THESE DETAILS WILL BE FURTHER RESOLVED OVER THE NEXT  
FEW DAYS. PLEASE CONSULT THE EXPERIMENTAL DAY 4-7 WINTER WEATHER  
OUTLOOK FOR A PROBABILISTIC FORECAST OF SNOW/SLEET IN THE MEDIUM  
RANGE.  
 
BY NEXT MON/TUE, THE UPPER RIDGE OVER NORTHWESTERN NORTH AMERICA  
SHOULD GET PINCHED OFF GIVEN ITS SKINNY RIDGE AXIS, ALLOWING A  
PACIFIC SYSTEM TO UNDERCUT THE RIDGE AND MOVE INTO THE PACIFIC  
NORTHWEST. HOW THIS UNFOLDS IS QUITE UNCERTAIN (MODELS HAVE  
TROUBLE ON THE TIMING OF THESE EVENTS) SO RELIED ON THE ENSEMBLE  
MEANS TO ROUND OUT THE FORECAST. THIS WAS ALSO PRUDENT IN THE EAST  
AS TIMING/AMPLITUDE DIFFERENCES EXPAND WITH NORTHERN STREAM  
SYSTEMS ROTATING AROUND THE UPPER LOW IN ONTARIO. NO DETERMINISTIC  
SOLUTION WAS PREFERRED (OR RULED OUT AS A PLAUSIBLE SOLUTION).  
 
FRACASSO  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE SURFACE LOW TRACKING ACROSS THE GREAT LAKES FRI-SAT WILL BE  
ACCOMPANIED BY A BAND OF SNOW ON THE NORTH SIDE OF THE SYSTEM.  
SNOW WILL ALSO BE POSSIBLE OVER NEW ENGLAND FRI/SAT WHERE WARM  
ADVECTION WILL BE THE LIKELY MECHANISM. THE AREA EXTENDING FROM  
FROM LOWER MS/TN VALLEYS INTO THE SOUTHERN APPALACHIANS HAS THE  
HIGHEST PROBABILITY OF SEEING HEAVY RAINFALL DURING THE MEDIUM  
RANGE. MODELS SHOW A GROWING CONSENSUS FOR AN AREA OF VERY  
HEAVY/EXCESSIVE RAINFALL POTENTIAL FRI-SAT ACROSS THIS AREA IN  
ASSOCIATION WITH A SLOW-MOVING FRONT WHICH IS FORECAST TO  
EVENTUALLY STALL OVER THE SOUTHEAST BY SUN. ADDITIONAL OVERRUNNING  
PRECIPITATION WILL BE POSSIBLE NORTH OF THE STALLED FRONT ON  
SUNDAY, PERHAPS LOCATED A BIT TO THE SOUTHEAST OF THE FRI-SAT  
HEAVY RAINFALL AREA, ALTHOUGH CONFIDENCE IN THE EXACT FRONTAL  
PLACEMENT REMAINS A BIT BELOW AVERAGE FOR THE REASONS DESCRIBED  
ABOVE. IF A FRONTAL WAVE DEVELOPS ALONG THE SOUTHEAST/MID-ATLANTIC  
COASTLINE AS FORECAST BY SUN-MON, THIS WOULD SPREAD AN ORGANIZED  
ROUND OF PRECIP ACROSS AREAS FROM THE CAROLINAS NORTHWARD,  
ALTHOUGH AT THIS TIME THERMAL FIELDS ARE TOO UNCERTAIN TO  
SPECULATE MUCH ON RAIN/SNOW AREAS. NONETHELESS, SOME POTENTIAL FOR  
WINTER WEATHER WOULD EXIST, ESPECIALLY FOR THE APPALACHIANS AND  
INTERIOR MID-ATLANTIC/NORTHEAST IN THIS SCENARIO. IF THE CURRENT  
FORECAST WERE TO VERIFY, THIS WOULD LIKELY MEAN A COLD RAIN FOR  
THE MAJOR CITIES ALONG THE I-95 CORRIDOR GIVEN THAT THE ARCTIC  
FRONT WOULD STILL BE TO THE WEST OF THE REGION AND NO OBVIOUS  
SOURCE OF LOW-LEVEL COLD AIR IS EVIDENT. THIS FORECAST IS STILL  
SUBJECT TO SIGNIFICANT CHANGES, HOWEVER, GIVEN THE DESCRIBED  
UNCERTAINTY.  
 
TEMPERATURES ACROSS THE CENTRAL U.S. WILL BE ON THE DECLINE  
THROUGHOUT THE MEDIUM RANGE AS A COUPLE ARCTIC FRONTAL BOUNDARIES  
IN QUICK SUCCESSION SPREAD INCREASINGLY COLD AIR SOUTHWARD. THE  
COLDEST TEMP ANOMALIES WILL INITIALLY BE JUST EAST OF THE ROCKIES  
SAT-SUN, WITH HIGH TEMPS 15 TO 30 DEG F BELOW AVERAGE. BY MON-TUE,  
THE COLDEST ANOMALIES, ALSO 15 TO 30 DEG F BELOW AVERAGE, WILL  
SHIFT TO THE NORTHERN PLAINS AND UPPER MIDWEST, WHERE HIGH  
TEMPERATURES FOR SOME AREAS WILL REMAIN BELOW ZERO, AND LOWS MAY  
REACH -20 DEG F. MEANWHILE, ABOVE AVERAGE TEMPS WILL PERSIST  
ACROSS THE EASTERN U.S. UNTIL THE FIRST COLD FRONT IS ABLE TO  
PASS. HIGH TEMPS FRI-SAT ARE FORECAST TO 10-20 DEG F ABOVE AVERAGE  
FOR THE SOUTHEAST, OH VALLEY, AND MID-ATLANTIC. RECORDS LOWS ARE  
POSSIBLE AT SOME LOCATIONS IN THE UPPER MIDWEST.  
 
RYAN  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page