252  
FXUS02 KWBC 261607  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1106 AM EST TUE DEC 26 2017  
 
VALID 12Z FRI DEC 29 2017 - 12Z TUE JAN 02 2018  
 
ARCTIC AIR WILL CONTINUE ITS DOMINANCE EAST OF THE ROCKIES INTO  
THE NEW YEAR...  
 
...PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY  
ASSESSMENT/PREFERENCES.....  
 
ALTHOUGH THE MODELS ARE WILDLY VARIABLE WHEN DOING SOME  
COMPARISONS...THE IDEA OF A NEGATIVE HEIGHT ANOMALY REMAINS A  
FIXTURE IN THE FORECAST EAST OF THE ROCKIES. DEPENDING ON THE  
PIECE OF GUIDANCE...500-MB HEIGHT DEPARTURES ARE LIKELY TO BE  
AROUND 2 TO 3 STANDARD DEVIATIONS BELOW AVERAGE AT POINTS DURING  
THE PERIOD. THIS WILL ULTIMATELY ENSURE A COLD FINISH TO THE YEAR  
2017 AND CONTINUING THROUGH AT LEAST DAY 7...JANUARY 2.  
MEANWHILE...THE WESTERN U.S. SHOULD REMAIN MILD GIVEN GENERAL  
RIDGING WITH OCCASIONAL EASTERN PACIFIC INFLUENCES ON THE WEATHER.  
 
UNFORTUNATELY THE GFS/ECMWF ARE AT COMPLETE ODDS WITH ONE ANOTHER  
WHICH LEADS TO A RATHER LOW CONFIDENCE FORECAST. THE LARGE ISSUE  
REVOLVES AROUND TWO PRIMARY REGIONS OF THE NORTHERN HEMISPHERE. IN  
THE VICINITY OF THE NORTH POLE...A LARGE LOBE OF VORTICITY IS  
FORECAST TO DESCEND TOWARD CENTRAL CANADA DURING THE NEXT FEW  
DAYS. GIVEN THE DEARTH OF UPPER AIR AND SURFACE OBSERVATIONS...IT  
MAY INITIALLY BE DIFFICULT TO RESOLVE THIS SYSTEM. DAY 4  
DIFFERENCES BETWEEN THE GFS/ECMWF APPEAR MORE LIKE THOSE SEEN AT  
DAY 6/7. ELSEWHERE...MODELS ARE STRUGGLING WITH THE HANDLING OF  
SPLIT FLOW ACROSS THE EASTERN PACIFIC. IT APPEARS A STRONG  
LONGWAVE TROUGH WILL BE MOVING THROUGH THE NORTHEAST PACIFIC WHILE  
A SEPARATE ENTITY SHOULD BE EXITING HAWAII. AS NOTED IN THE  
ENSEMBLE SPAGHETTI PLOTS...THE GUIDANCE ARE STRUGGLING WITH THE  
PROGRESSION OF THESE SYSTEMS. THE 06Z/00Z GFS SHIFT THE SOUTHERN  
STREAM SYSTEM BACK TOWARD THE SUBTROPICS WHILE THE 00Z ECMWF/CMC  
SHOW MORE EASTWARD PROGRESSION. ON THE OTHER SIDE...THE 00Z UKMET  
PHASES THE TWO FEATURES LEADING TO A 973-MB LOW MOVING INTO  
COASTAL BRITISH COLUMBIA ON 30/1200Z. SUFFICE TO SAY...THE MODELS  
DISPLAY A LOT OF UNCERTAINTY WHICH HAS COMPANY ACROSS THE  
CENTRAL/EASTERN U.S. BASED ON EVALUATION OF THE PAST FEW RUNS OF  
THE GFS/ECMWF...THE TWO DISPLAY COMPLETELY OPPOSITE SYNOPTIC  
EVOLUTIONS. LOOKING AT THE GFS...THESE RUNS FAVOR LIKELY NEAR  
RECORD HIGH PRESSURES ACROSS THE GREAT PLAINS AS MSLP FORECASTS  
RISE TO ABOVE 1060-MB ON SUNDAY ACROSS THE DAKOTAS. AT THE SAME  
TIME...THE 00Z ECMWF/CMC/UKMET SHOW A MUCH WEAKER ANTICYCLONE TO  
THE EAST ACROSS THE UPPER GREAT LAKES/MIDDLE MS VALLEY. ALONG THE  
WESTERN ATLANTIC...THE 06Z/00Z GFS SPAWN A COASTAL LOW WHICH COULD  
AFFECT NOVA SCOTIA ON NEW YEARS DAY IF THE 06Z CYCLE IS TO  
BELIEVED. ON THE CONTRARY...THE 00Z ECMWF/CMC/UKMET SUPPORT AN  
EARLIER WAVE OF LOW PRESSURE A COUPLE OF DAYS PREVIOUS.  
OVERALL...THESE MARKED DIFFERENCES LEAD TO ADDITIONAL  
UNCERTAINTIES MOVING INTO THE NEW YEAR. BUT IT DOES APPEAR LOWER  
HEIGHTS SHOULD REMAIN EAST OF THE ROCKIES WHILE QUASI-ZONAL FLOW  
WITH EMBEDDED SHORTWAVES ARE LIKELY OVER THE WEST.  
 
DECIDED TO DISMISS ANY GFS/GEFS BASED SOLUTION GIVEN SOME OF THE  
EXTREMES EARLIER NOTED. THIS ALSO DISCOUNTED THE 00Z NAEFS  
ENSEMBLE MEAN HERE. THE FORECAST WAS INITIALLY BASED ON A  
COMBINATION OF THE 00Z ECMWF/CMC/UKMET/ECMWF ENSEMBLE MEAN BEFORE  
REMOVING THE 00Z UKMET BY DAY 4/SATURDAY GIVEN THE DEEP LOW  
APPROACHING BRITISH COLUMBIA. EVENTUALLY REMOVED CONTRIBUTIONS  
FROM THE 00Z CMC BEYOND DAY 5/SUNDAY AS IT BEGAN TO DIVERGE FROM  
THE 00Z ECMWF/ECMWF ENSEMBLE MEAN. FOR THE NEW YEAR...WENT WITH A  
COMPLETELY 00Z ECMWF/ECMWF ENSEMBLE MEAN BASED FORECAST WITH 80  
PERCENT DIRECTED TOWARD THE ENSEMBLES.  
 
   
..WEATHER/THREATS HIGHLIGHTS
 
 
MULTIPLE SHOTS OF ARCTIC AIR WILL ENSURE VERY COLD TEMPERATURES TO  
PREVAIL GENERALLY FROM THE NORTHERN ROCKIES EASTWARD. A  
QUASI-STATIONARY FRONTAL BOUNDARY SHOULD BANK UP AGAINST THE  
ROCKIES WHICH WILL KEEP LOCATIONS TO THE WEST ON THE MILDER SIDE.  
MEANWHILE...READINGS DURING THE PERIOD SHOULD RANGE ANYWHERE FROM  
20 TO 30 DEGREES BELOW CLIMATOLOGY WITH THE COLDEST AIR SETTLING  
OVER THE NORTH-CENTRAL U.S. ON SATURDAY...HIGH TEMPERATURES OVER  
THE UPPER MIDWEST MAY SIT AROUND -10F TO -15F WHILE LOWS PLUNGE  
WELL INTO THE -20S. AS THE ARCTIC AIR TRAVELS SOUTH AND  
EAST...SOME DAILY TEMPERATURE RECORDS MAY BE BROKEN ACROSS  
SECTIONS OF THE GREAT LAKES ON SUNDAY. COLD DAYTIME HIGHS IN THE  
LOWER TEENS MAY BREAK SUCH RECORDS OVER AREAS OF LOWER MI INTO  
NORTHERN IN/OH. OUT WEST...CONTINUED RIDGING OR PACIFIC INFLUENCES  
SHOULD KEEP TEMPERATURES AROUND 10 TO 15 DEGREES ABOVE AVERAGE.  
SLIGHTLY HIGHER ANOMALIES WILL BE POSSIBLE OVER THE DESERT  
SOUTHWEST INTO THE FOUR CORNERS REGION.  
 
IN TERMS OF PRECIPITATION...GUIDANCE IS VARIABLE GIVEN THE EXTREME  
MODEL DIFFERENCES AT HAND. ANY COASTAL LOW WHICH MAY FORM OVER THE  
WESTERN ATLANTIC SHOULD HAVE POSSIBLE EASTERN SEABOARD IMPACTS.  
HOWEVER...ENSEMBLE LOW PLOTS SHOW TOO MUCH SPREAD TO HONE IN ON  
ANY ONE SOLUTION. CYCLONIC FLOW OVER THE GREAT LAKES SHOULD KEEP  
LAKE EFFECT SNOW PRODUCTION IN PLACE ALTHOUGH PRECIPITABLE WATER  
VALUES ARE EXPECTED TO BE QUITE LOW DURING THE PERIOD. ACROSS THE  
GULF COAST...ANY MOISTURE RETURN COULD BRING PRECIPITATION CLOSER  
TO THE COAST BUT IT IS UNCERTAIN HOW FAR NORTH IT WILL PROGRESS.  
AND OVER THE WEST COAST...SOME THREATS FOR PRECIPITATION WILL BE  
POSSIBLE BUT THIS IS HIGHLY MODEL DEPENDENT. THE 00Z  
CMC/UKMET/ECMWF FAVOR MORE AMPLIFICATION INTO EARLY NEXT WEEK  
WHICH INCREASES PRECIPITATION CHANCES OVER THE WEST VERSUS THE  
06Z/00Z GFS WHICH SUPPORTS BROADER RIDGING. WOULD EXPECT THE  
FORMER TO BE MORE LIKELY BASED ON THE LATEST FORECAST PACKAGE.  
 
RUBIN-OSTER  
 

 
 
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