570  
FXUS02 KWBC 180656  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
156 AM EST THU JAN 18 2018  
 
VALID 12Z SUN JAN 21 2018 - 12Z THU JAN 25 2018  
 
...POTENTIAL BLIZZARD CONDITIONS ACROSS THE UPPER MIDWEST/GREAT  
LAKES ON SUNDAY/MONDAY...  
 
   
..PATTERN OVERVIEW
 
 
WHILE A BROAD AND STRETCHED OUT BAND OF VORTICITY SLIDES ACROSS  
THE FL PENINSULA ON SUNDAY MORNING...A WELL DEVELOPED SHORTWAVE  
WILL BE IN THE PROCESS OF DIGGING THROUGH THE SOUTHERN HIGH  
PLAINS. RAPID STRENGTHENING OF THE ATTENDANT SURFACE CYCLONE WILL  
ENSUE ON SUNDAY NIGHT/MONDAY MORNING ACROSS THE MIDDLE MS  
VALLEY/UPPER GREAT LAKES REGION. AFTER LIKELY DUMPING HEAVY SNOW  
OVER SECTIONS OF THE CENTRAL PLAINS THROUGH THE UPPER MIDWEST AND  
INTO THE GREAT LAKES...THE DEEP SURFACE LOW WILL PUSH INTO  
SOUTHERN QUEBEC BY TUESDAY WITH A COLD FRONTAL PASSAGE EXPECTED  
ALONG THE I-95 CORRIDOR BY 23/1200Z. ACROSS THE WESTERN U.S...A  
WEAKENING SYSTEM IS FORECAST TO PUSH INTO THE PACIFIC NORTHWEST ON  
SUNDAY AFTERNOON WITH ADDITIONAL LONGWAVE ENERGY CARVING OUT A  
TROUGH ACROSS THE FOUR CORNERS REGION BY TUESDAY. MID-LEVEL  
HEIGHTS SHOULD BRIEFLY BUILD OVER THE WEST COAST BEFORE RENEWED  
TROUGHING RETURNS TO THE REGION BY MID-WEEK WITH GRADUAL  
ACCELERATION INLAND BY DAY 7/JANUARY 25.  
 
   
..GUIDANCE ASSESSMENT...MODEL UNCERTAINTIES...AND PREFERENCES
 
 
CONSIDERING THE INITIAL SHORTWAVE CROSSING INTO THE FL PENINSULA  
ON SUNDAY...THE PREVIOUSLY QUICKER 12Z CMC/UKMET HAVE MADE SLOWER  
ADJUSTMENTS IN THEIR LATEST RUNS. THIS BRINGS THE 00Z UKMET  
SOLUTION IN LINE WITH THE 00Z/18Z GFS RUNS BUT STILL REMAIN  
QUICKER THAN THE MORE WESTERN 12Z ECMWF. FARTHER UPSTREAM...MODEL  
DIFFERENCES REMAIN AN ISSUE WITH THE DEVELOPING CYCLONE ACROSS THE  
CENTRAL PLAINS ON SUNDAY AFTERNOON WITH THE 00Z/18Z GFS OVER  
SOUTHERN IA WHILE THE REMAINING CONSENSUS IS BACK NEAR THE  
VICINITY OF SOUTHERN KS. SUCH UNCERTAINTIES PROJECT IN THE FUTURE  
AS THE LOW CONTINUES DEEPENING INTO THE UPPER MIDWEST/GREAT LAKES  
EARLY NEXT WEEK. ENSEMBLE LOW PLOTS THE PAST FEW MODEL CYCLES HAVE  
SHOWN THE SPREAD HAS DECREASED A BIT BUT A DIVIDE REMAINS BETWEEN  
THE GEFS/ECMWF ENSEMBLE CAMPS. IN TERMS OF SUPPORT FROM OTHER  
MODELS...THE 00Z/12Z CMC REMAIN CLOSER TO THE ECMWF WITH THE  
SOUTHWESTWARD TRENDING 00Z UKMET JOINING THIS SLOWER GROUP.  
THUS...THE OVERNIGHT WPC MEDIUM RANGE PACKAGE COULD BE A TAD FAST.  
MULTI-DAY ENSEMBLE SPAGHETTI TRENDS HAVE SHOWN A SLOWING WITH THE  
ECMWF RUNS BEING MOST EMPHATIC ABOUT THIS SCENARIO. EVENTUALLY THE  
MID-LEVEL PATTERN RELAXES AS LOWER HEIGHTS LIFT UP INTO EASTERN  
CANADA BY THE MIDDLE OF NEXT WEEK.  
 
ACROSS THE EASTERN PACIFIC AND ADJACENT WEST COAST STATES...AN  
UPPER LOW WILL BE SPINNING OFF THE COAST OF BRITISH COLUMBIA ON  
21/1200Z. AS THIS SYSTEM MOVES INLAND AND WEAKENS...LONGWAVE  
ENERGY SHOULD PUSH INTO THE PACIFIC NORTHWEST WHILE BEING  
STRETCHED AND SHEARED UPON MIGRATION DOWNSTREAM. GENERAL TROUGHING  
TAKES SHAPE FARTHER INLAND ACROSS MUCH OF THE INTERIOR WEST WITH A  
MEAN TROUGH POSITION ALONG THE HIGH PLAINS BY 24/0000Z AS SHOWN BY  
THE 00Z GFS/12Z ECMWF AND 12Z ECMWF ENSEMBLE MEAN. THE CMC  
SOLUTIONS HAVE BEEN FLIP-FLOPPING A BIT WHILE NOT BEING ALIGNED  
WITH THE ENSEMBLE MEANS. MULTI-DAY COMPARISONS OF RELEVANT  
ENSEMBLE SPAGHETTI PLOTS SHOW A LOT OF NOISE WITH LITTLE REDUCTION  
IN FORECAST SPREAD. MOST ENSEMBLE MEMBERS HAVE TRENDED QUICKER  
ALTHOUGH A VAST MAJORITY OF THE ECMWF MEMBERS HAVE STUCK TO A  
SLOWER PROGRESSION.  
 
WHILE MID-LEVEL RIDGING WILL REBUILD OVER THE WEST COAST BY  
MID-WEEK...A DEEP UPPER TROUGH LOOMS OFFSHORE WITH A PROGRESSION  
INLAND BY PERHAPS WEDNESDAY AFTERNOON. BASED ON THE ENSEMBLE  
MEANS...THIS LONGWAVE PATTERN SHOULD ACCELERATE INLAND WITH THE  
00Z GFS/12Z ECMWF BOTH SHOWING A POTENT SYSTEM DIGGING INTO THE  
SOUTHWESTERN U.S. BY 26/0000Z.  
 
REGARDING PREFERENCES...TOOK A THREE-WAY CONSENSUS OF THE 18Z GFS  
AND 12Z ECMWF/CMC FOR THE FIRST COUPLE OF DAYS OF THE PERIOD. AS  
MENTIONED EARLIER...THE SLOWING TREND IN THE 00Z GUIDANCE SUGGESTS  
THE FORECAST MAY BE TOO QUICK GIVEN SUFFICIENT USE OF THE 18Z  
GFS/GEFS MEAN SOLUTIONS. BY DAY 5/TUESDAY...STARTED TO INCORPORATE  
MORE OF THE 18Z GEFS MEAN AND 12Z ECMWF/NAEFS ENSEMBLE MEANS  
ALTHOUGH DID MAINTAIN APPROXIMATELY 20 PERCENT OF THE OPERATIONAL  
COMPONENT TO THE BLEND OUT TO DAY 7/JANUARY 25.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
IMPRESSIVE WARMTH IS EXPECTED AHEAD OF THE DEEPENING CYCLONE OVER  
THE CENTRAL U.S. AFTERNOON TEMPERATURES SHOULD BE AROUND 1O TO  
POSSIBLY 20 DEGREES ABOVE CLIMATOLOGY FROM THE MO RIVER VALLEY OUT  
TOWARD THE LOWER GREAT LAKES ON SUNDAY/MONDAY. THIS WILL RAISE  
HIGHS INTO THE 50S ACROSS THIS STRETCH OF THE COUNTRY WITH SUCH  
READINGS STRETCHING EASTWARD TOWARD MUCH OF THE I-95 CORRIDOR BY  
TUESDAY. OVERNIGHT LOWS SHOULD BE EVEN MORE ANOMALOUS WITH  
EXPECTED VALUES AROUND 20 TO 25 DEGREES ABOVE AVERAGE WITHIN THE  
EXPANSIVE WARM SECTOR. MEANWHILE...A RATHER ACTIVE PATTERN ACROSS  
THE WESTERN U.S. WILL LIKELY KEEP TEMPERATURES AROUND TO SLIGHTLY  
BELOW CLIMATOLOGY THROUGHOUT THE PERIOD.  
 
ON SUNDAY...MODERATE TO LOCALLY HEAVY RAINFALL IS POSSIBLE UP AND  
DOWN THE COLD FRONT STRETCHING FROM THE LOWER MS VALLEY UP INTO  
THE MIDDLE MS VALLEY. WITHIN THE COMMA-HEAD OF PRECIPITATION  
AROUND THE DEEPENING LOW CENTER...EXPECT HEAVY SNOW TO FALL  
ANYWHERE FROM NEB UP INTO THE UPPER MIDWEST/GREAT LAKES WITH  
BLIZZARD CONDITIONS POSSIBLE AT TIMES DUE TO THE TIGHT PRESSURE  
GRADIENT. SOME RESIDUAL SNOW SHOWERS ARE LIKELY OVER THE GREAT  
LAKES GIVEN CYCLONIC FLOW ALOFT REMAINING WHILE FRONTAL RAINFALL  
REMAINS IN THE FORECAST OUT TOWARD THE EASTERN SEABOARD ON  
MONDAY/TUESDAY. BASED ON THE 00Z GFS/12Z ECMWF...AMPLE RETURN FLOW  
FROM THE GULF OF MX BY WEDNESDAY/THURSDAY MAY BRING HEAVY RAINFALL  
BACK IN THE PICTURE ACROSS AREAS OF THE CENTRAL/EASTERN GULF  
COAST. THE SIGNAL IS STRONG ENOUGH TO SUPPORT ADDING SOME HEAVIER  
AMOUNTS INTO THE MANUAL DAY 6/7 QPF.  
 
ACROSS THE WESTERN U.S...MULTIPLE ROUNDS OF PRECIPITATION ARE  
LIKELY WITH LOWER ELEVATION/VALLEY FLOOR RAIN AND MOUNTAIN SNOW.  
THE FIRST WAVE PRIMARILY AFFECTS NORTHERN CA UP TOWARD THE  
OLYMPICS/CASCADES WITH PRECIPITATION SPREADING INTO THE UPPER  
INTERMOUNTAIN WEST/CENTRAL GREAT BASIN THEREAFTER. MODELS SHOW THE  
NEXT SURGE MOVING INTO SIMILAR REGIONS ON TUESDAY/WEDNESDAY WHICH  
WILL KEEP CONDITIONS ON THE WET SIDE FOR THE PERIOD. MULTI-DAY  
PRECIPITATION TOTALS MAY BE QUITE IMPRESSIVE...PARTICULARLY OVER  
AREAS OF CONTINUED UPSLOPE EFFECTS.  
 
RUBIN-OSTER  
 

 
 
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