925  
FXUS02 KWBC 200656  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
155 AM EST SAT JAN 20 2018  
 
VALID 12Z TUE JAN 23 2018 - 12Z SAT JAN 27 2018  
   
..OVERVIEW
 
 
A FAIRLY PROGRESSIVE FLOW REGIME WILL BE IN PLACE ACROSS THE CONUS  
DURING MUCH OF THE MEDIUM RANGE PERIOD. A BUILDING UPPER RIDGE  
EXTENDING FROM THE NORTH CENTRAL PACIFIC OCEAN TO THE BERING SEA  
THROUGH THE PERIOD WILL PROMOTE CONTINUED TROUGHING IN THE GULF OF  
ALASKA AND OFF THE NORTH AMERICAN WEST COAST THROUGH MIDWEEK. THIS  
PERSISTENT TROUGH WILL EJECT UPPER LEVEL SHORTWAVES EASTWARD  
ACROSS NORTH AMERICA, SOME OF WHICH WILL AMPLIFY AS THEY ENCOUNTER  
INCREASING BAROCLINICITY ACROSS THE CONUS. LARGER SCALE EAST  
PACIFIC TROUGHING WILL LIKELY MOVE INTO THE WEST LATER THIS WEEK.  
THE END RESULT WILL BE A PATTERN MORE REMINISCENT OF EARLY SPRING  
THAN ONE OF MID-WINTER, WITH FREQUENT AND PERHAPS RELATIVELY  
INTENSE SURFACE CYCLONES ACROSS THE CENTRAL U.S., CHARACTERIZED BY  
STRONG THERMAL GRADIENTS ACROSS THEM.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES
 
 
MODEL CONSENSUS HAS IMPROVED COMPARED TO THIS TIME LAST NIGHT  
THROUGHOUT THE EXTENDED FORECAST PERIOD. LOW PRESSURE SYSTEM  
CROSSING THE GREAT LAKES ON DAY 3 (TUE) SHOWS IMPROVED CONSENSUS  
AMONG THE GUIDANCE WITH THE ECMWF AND GFS BOTH CLUSTERED TIGHTLY  
WITHIN THEIR RESPECTIVE ENSEMBLE SPREADS. ALONG THE WEST COAST,  
SHORTWAVE EXPECTED TO REACH THE PAC NW TUE-TUE NIGHT ALSO SHOWS AN  
IMPROVEMENT IN CONSENSUS COMPARED TO LAST NIGHT. A SECOND  
SHORTWAVE WILL FOLLOW QUICKLY IN ITS HEELS, REACHING THE NW LATE  
WED. WHILE MODELS SHOWED SOME DIFFERENCES WITH RESPECT TO TIMING  
OF THESE FEATURES AND THEIR ASSOCIATED SURFACE LOWS, A CONSENSUS  
APPROACH IS PREFERRED AT THIS TIME. THUS, A MAJORITY DETERMINISTIC  
BLEND OF THE ECMWF/GFS WAS USED AS A FORECAST STARTING POINT  
DURING DAYS 3-4.  
 
FROM DAY 5 ONWARD, THE AFOREMENTIONED SHORTWAVE REACHING THE PAC  
NW EARLIER IN THE PERIOD WILL AMPLIFY AS IT CROSSES THE FOUR  
CORNERS/ROCKIES AND MOVES TOWARD THE CENTRAL U.S. BY LATE FRI INTO  
SAT. WHILE MODELS SHOW DIFFERENCES WITH RESPECT TO THE DETAILS  
SURROUNDING THE EVOLUTION OF THIS FEATURE, THEY SHOW ABOVE AVERAGE  
CONSENSUS ON THE LARGE SCALE, TAKING THE TIME RANGE INTO  
CONSIDERATION. SURFACE CYCLOGENESIS IS EXPECTED TO OCCUR BY LATE  
FRI SOMEWHERE ACROSS THE CENTRAL OR NORTHERN PLAINS. WHILE  
DETERMINISTIC SOLUTIONS DIFFER ON EXACTLY WHERE THIS WILL OCCUR,  
THE ECENS/GEFS MEANS SHOWED BETTER AGREEMENT. THUS, THE FORECAST  
WAS GRADUALLY TRENDED TOWARD HEAVIER ECENS/GEFS MEAN WEIGHTING  
FROM DAY 5 ONWARD, WITH CONTINUED MINORITY COMPONENTS OF THE ECMWF  
AND GFS INCLUDED.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
SNOW ALONG WITH GUSTY WINDS WILL BE POSSIBLE ACROSS PORTIONS OF  
THE GREAT LAKES TUE ON THE NORTHWEST SIDE OF THE SURFACE LOW  
PRESSURE SYSTEM MOVING THROUGH THE REGION. RAIN AND PERHAPS A FEW  
THUNDERSTORMS WILL BE POSSIBLE ALONG/AHEAD OF THE SURFACE COLD  
FRONT ON THU ALONG MUCH OF THE EAST COAST. AREAS OF HEAVY RAIN  
WILL BE POSSIBLE ACROSS SOUTHERN NEW ENGLAND WHILE A WINTRY MIX OF  
PRECIPITATION IS POSSIBLE FOR NORTHERN NEW ENGLAND. MEANWHILE, THE  
WESTERN U.S. WILL SEE A PROLONGED PERIOD OF ONSHORE FLOW WITH  
SEVERAL UPPER-LEVEL IMPULSES AND SURFACE FRONTAL SYSTEMS ENHANCING  
PRECIPITATION AT TIMES. LOW ELEVATION RAIN AND MOUNTAIN SNOWS ARE  
EXPECTED THROUGH MUCH OF THE PERIOD, WHICH COULD BE HEAVY. SNOW  
LEVELS SHOULD LOWER BY MID-WEEK AS HEIGHTS FALL IN RESPONSE TO THE  
AMPLIFYING UPPER TROUGH. THE SOUTH CENTRAL AND SOUTHEASTERN STATES  
WILL SEE INCREASING POTENTIAL FOR SHOWERS AND THUNDERSTORMS BY  
LATE NEXT WEEK AS SURFACE CYCLOGENESIS OCCURS ACROSS THE CENTRAL  
U.S. AND SOUTHERLY FLOW BEGINS TO SPREAD MOISTURE AND INSTABILITY  
FROM THE GULF OF MEXICO NORTHWARD. CONFIDENCE IN THE SPECIFICS OF  
WHERE/WHEN ANY POTENTIAL PRECIPITATION WOULD OCCUR REMAINS  
RELATIVELY LOW AT THIS TIME, WITH THE GFS REMAINING SLOWER TO  
SPREAD MOISTURE AND PRECIPITATION NORTHWARD RELATIVE TO THE ECMWF.  
 
TEMPERATURES ARE EXPECTED TO BE WELL ABOVE AVERAGE ON TUE ALONG  
THE EAST COAST IN THE WARM SECTOR AHEAD OF THE INCOMING COLD  
FRONT. MAX TEMPS MAY REACH 10-15 DEG F ABOVE AVERAGE. A BUILDING  
SUBTROPICAL RIDGE OFF THE SOUTHEAST COAST AND RETURNING WARM AIR  
ADVECTION AHEAD OF THE LATE WEEK WESTERN U.S. TROUGH WILL KEEP  
MUCH OF THE CENTRAL/EASTERN U.S. ABOVE AVERAGE THROUGH NEXT WEEK,  
WITH MAX TEMPS 10-20 DEG F ABOVE AVERAGE ACROSS MUCH OF THE  
CENTRAL/EASTERN U.S. BY LATE IN THE WEEK. MEANWHILE COLDER AIR  
WILL SPREAD INTO THE WEST AS A THE UPPER TROUGH AMPLIFIES  
OVERHEAD, AND MAX TEMPS OF 10-15 DEG F BELOW AVERAGE ARE EXPECTED  
BY LATE NEXT WEEK.  
 
RYAN  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page