878  
FXUS02 KWBC 281606  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1106 AM EST THU DEC 28 2017  
 
VALID 12Z SUN DEC 31 2017 - 12Z THU JAN 04 2018  
 
...ARCTIC AIR WILL CONTINUE ITS DOMINANCE EAST OF THE ROCKIES WELL  
INTO THE NEW YEAR...  
 
...PATTERN OVERVIEW/GUIDANCE/PREDICTABILITY  
ASSESSMENT/PREFERENCES...  
 
LITTLE CHANGE IN THE MID/UPPER PATTERN IS EXPECTED MOVING INTO THE  
NEW YEAR AS AN EXPANSIVE NEGATIVE HEIGHT ANOMALY REMAINS A FIXTURE  
IN THE FORECAST FROM THE GREAT PLAINS EASTWARD. GENERALLY  
SPEAKING...MULTIPLE SHORTWAVES WILL PEEL OFF A REGION OF BROAD  
CYCLONIC FLOW ACROSS CENTRAL/EASTERN U.S. THIS SHOULD ALLOW FOR  
SEVERAL REINFORCING SURGES OF ARCTIC AIR WHICH WILL MAINTAIN WELL  
BELOW NORMAL TEMPERATURES FROM THE ROCKIES ALL THE WAY TO THE  
EASTERN SEABOARD. THE INITIAL SYSTEM WITHIN THE WAVE TRAIN WILL BE  
IN THE PROCESS OF EXITING NEW ENGLAND ON SUNDAY WHILE THE NEXT  
ROUND OF HEIGHT FALLS PUSH THROUGH THE NORTHERN ROCKIES EARLY NEW  
YEARS DAY. THIS PARTICULAR FEATURE IS OF INTEREST AS IT ROUNDS THE  
BASE OF THE DOWNSTREAM TROUGH AND POTENTIALLY STRENGTHENS BY  
ADDITIONAL AMPLIFICATION WITHIN THE NORTHERN STREAM. A COASTAL  
WAVE MAY DEVELOP ON WEDNESDAY MORNING OFFSHORE OF GA/SC WITH A  
NORTHWARD TRACK UP THE GULF STREAM INDICATED IN THE 00Z ENSEMBLES.  
PREVIOUS MODEL CYCLES DID NOT SHOW THIS SCENARIO SO WILL BE  
INTERESTING TO SEE WHAT FUTURE RUNS SHOW.  
 
ELSEWHERE...A MEAN RIDGE IS LIKELY TO STAY IN PLACE INTO EARLY  
NEXT WEEK ACROSS THE EASTERN PACIFIC/WEST COAST. SPLIT FLOW OVER  
THE OFFSHORE WATERS WILL ALLOW A SOUTHERN STREAM FEATURE TO SLOWLY  
PUSH TOWARD THE COASTLINE BY MID-WEEK BEFORE LIKELY  
WEAKENING/SHEARING THEREAFTER. IT DOES APPEAR THE UPSTREAM  
TROUGHING WILL RE-LOAD INTO DAY 7/JANUARY 4 WHILE HELPING  
REINFORCE THE PERSISTENT RIDGE OVER THE WESTERN U.S.  
 
WHILE GENERAL AGREEMENT EXISTS WITH THE INITIAL PATTERN ACROSS THE  
COUNTRY...THE GUIDANCE BREAKS DOWN WITH THE EVOLVING SHORTWAVE  
ENERGETICS MOVING THROUGH THE CENTRAL/EASTERN U.S. ON NEW YEARS  
DAY. THE 00Z UKMET WAS A SLOWER OUTLIER WITH THE MEAN TROUGH  
SETTLING OVER THE GREAT LAKES/NORTHEASTERN U.S. WHILE FURTHER  
DIFFERENCES ARE NOTED UPSTREAM WITH THE SHORTWAVE DIGGING INTO THE  
NORTHERN ROCKIES. ENSEMBLE SPAGHETTI PLOTS SHOW THIS SPREAD WELL  
AS 00Z GEFS MEMBERS ARE QUICKER TO ACCELERATE AND STRETCH THIS  
SYSTEM OUT ACROSS THE OZARKS/TN VALLEY ON TUESDAY. LIKEWISE...THIS  
SPEED DIFFERENCE IS EVIDENT WITHIN THE AMPLIFIED NORTHERN STREAM  
ACROSS THE GREAT LAKES. THE PAST COUPLE OF ECMWF RUNS AS WELL AS  
THE 00Z CMC FAVOR A SLOWER PROGRESSION OF BOTH ENTITIES WHICH  
LEADS TO SOMEWHAT SIMILAR FORECASTS MOVING INTO THE EARLY/MIDDLE  
PART OF NEXT WEEK. WHILE NOT UTILIZED...IT IS WORTH NOTING THE 00Z  
UKMET IS EVEN SLOWER HERE. BY DAY 6/7...JANUARY 3/4...THERE IS A  
GROWING CONSENSUS OF COASTAL WAVE DEVELOPMENT DURING THE PERIOD  
ALTHOUGH IT IS HEAVILY DRIVEN BY THE 00Z CMC/ECMWF ENSEMBLES. IT  
IS A WORTHWHILE TREND THOUGH WITH THE LATEST 00Z ECMWF ENSEMBLE  
MEAN SHOWING A 1004-MB ISOBAR NEAR THE 40N/70W BENCHMARK BY  
04/1200Z WHILE PRECEDING ECMWF ENSEMBLES SHOWED LITTLE TO NOTHING  
THERE. THE OPERATIONAL 00Z ECMWF IS A MORE EXTREME OUTLIER AS  
ENSEMBLE SPAGHETTI PLOTS SHOW BUT THERE IS A LOT OF SUPPORT FOR  
THIS MORE AMPLIFIED SCENARIO. IT IS DIFFICULT TO COMMIT TO THIS  
JUST YET BUT IT IS WORTH WATCHING AS FUTURE MODEL RUNS ARRIVE.  
ACROSS THE WEST...THE BIGGEST TREND IN THE GUIDANCE IS TO BE  
SLOWER BY A CONSIDERABLE AMOUNT RELATIVE TO MODELS A DAY OR TWO  
AGO. THIS ENSURES THE SOUTHERN STREAM FEATURE WOULD NOT REACH THE  
CA/OR COAST UNTIL THE MIDDLE OF NEXT WEEK. HOWEVER...THERE ARE A  
LOT OF ENSEMBLE SOLUTIONS WHICH ARE QUICKER THAN THE OPERATIONAL  
RUNS SO THERE COULD BE EASTWARD MOVEMENT IN THE NEXT MODEL RUNS.  
 
INITIALLY BASED ON THE FORECAST ON A COMBINATION OF THE 00Z  
GFS/ECMWF/CMC THROUGH DAY 4/MONDAY WITH A SMALL AMOUNT OF ENSEMBLE  
GUIDANCE INCLUDED IN THE MIX. BY DAY 5/TUESDAY ONWARD...STARTED TO  
MOVE TOWARD A 00Z GFS/GEFS MEAN AND 00Z ECMWF/ECMWF ENSEMBLE MEAN  
BLEND BEFORE MOVING TO AN 80 PERCENT ENSEMBLE MEAN BASED SOLUTION  
BY DAY 7/JANUARY 4 AS UNCERTAINTY GROWS ACROSS THE MAP. CONFIDENCE  
RESIDES AROUND AVERAGE ALTHOUGH WOULD LIKE TO SEE MORE GEFS  
MEMBERS SUPPORTING THE COASTAL LOW DEVELOPMENT OFF THE EAST COAST.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
MULTIPLE ARCTIC INTRUSIONS WILL ENSURE WELL BELOW NORMAL  
TEMPERATURES TO SPILL FROM THE ROCKIES EASTWARD WITH THE MOST  
ANOMALOUS READINGS EXPECTED TO REACH THE 30 TO 35 DEGREE BELOW  
AVERAGE RANGE. THIS WILL LIKELY BREAK A NUMBER OF DAILY  
TEMPERATURE RECORDS FOR COLD AFTERNOON HIGHS. HIGHS ACROSS SOUTH  
DAKOTA/NEBRASKA ON SUNDAY WILL STAY IN THE NEGATIVE SINGLE DIGITS  
WHILE THE FOLLOWING DAY NUMBERS IN THE SINGLE DIGITS AND TEENS ARE  
POSSIBLE ALONG THE I-95 CORRIDOR DOWN TO THE UPPER MID-ATLANTIC.  
EVEN SOME TEMPERATURES MAY STRUGGLE TO GET ABOVE FREEZING AS FAR  
SOUTH AS NORTHERN LOUISIANA ON MONDAY. THE TRUE COLD SPOT WILL BE  
ACROSS THE NORTHERN ROCKIES EASTWARD THROUGH THE NORTHERN  
PLAINS/UPPER MIDWEST AS LOW TEMPERATURES ON SUNDAY/MONDAY MAY STAY  
IN THE -20S TO POSSIBLY -30S IN SOME LOCATIONS.  
MEANWHILE...PERSISTENT RIDGING ALONG THE WEST COAST SHOULD KEEP  
TEMPERATURES AROUND 5 TO 10 DEGREES ABOVE CLIMATOLOGY DURING MUCH  
OF THE FORECAST PERIOD.  
 
REGARDING PRECIPITATION...CYCLONIC FLOW ACROSS THE GREAT LAKES  
WILL LEAD TO SOME LAKE EFFECT SNOWS DURING THE PERIOD. WITH 700-MB  
TEMPERATURES LIKELY IN THE -20C TO -30C RANGE...SOME VERY  
IMPRESSIVE LAPSE RATES ARE EXPECTED OVER THE ICE FREE LAKE  
SURFACES. ONE LIMITING FACTOR WILL BE THE EXTREMELY LOW  
PRECIPITABLE WATER VALUES WITHIN THE PERSISTENT ARCTIC AIR MASSES.  
ELSEWHERE...FRONTAL PRECIPITATION IS POSSIBLE FROM SOUTH TX  
EASTWARD INTO THE SOUTHEASTERN U.S...PARTICULARLY FLORIDA. MUCH  
HEAVIER AMOUNTS ARE DEPICTED BY TUESDAY/WEDNESDAY ALTHOUGH THIS IS  
PLACEMENT IS VERY MODEL DEPENDENT. WITH THAT IN MIND...ANY WAVE  
THAT DOES MOVE UP THE WESTERN ATLANTIC COULD BRING WINTRY  
IMPLICATIONS TO THE EASTERN SEABOARD DURING THE DAY 6/7...JANUARY  
3/4 WINDOW. OUT WEST...DEPENDING ON THE TIMING/STATE OF THE  
APPROACHING SHORTWAVE...SOME HEAVIER PRECIPITATION COULD FALL  
ACROSS NORTHERN CA AND INTO THE NORTHERN/CENTRAL SIERRA. THE GFS  
RUNS HAVE BEEN STRONGER ALOFT WHICH LEADS TO A MORE IMPRESSIVE  
PERIOD OF PRECIPITATION.  
 
RUBIN-OSTER  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page