803  
FXUS02 KWBC 180649  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
148 AM EST SUN FEB 18 2018  
 
VALID 12Z WED FEB 21 2018 - 12Z SUN FEB 25 2018  
 
...ARCTIC AIR DROPS THROUGH THE WEST-CENTRAL U.S. WITH RECORD  
WARMTH ACROSS THE EAST...  
...CLASHING AIR MASSES SET STAGE FOR ARKLATEX TO TN/OH VALLEYS  
HEAVY RAINFALL THREAT...  
 
   
..PATTERN OVERVIEW
 
 
A SUBTROPICAL RIDGE ACROSS THE SOUTHWESTERN ATLANTIC WILL CONTINUE  
TO EXERT ITS DOMINANCE ON THE MID-LATITUDE PATTERN. CONSIDERING  
TELECONNECTIONS WITH THIS PERSISTENT POSITIVE HEIGHT ANOMALY...THE  
UPSTREAM PATTERN WILL FEATURE BROAD UPPER TROUGHING OVER THE  
WESTERN/CENTRAL U.S. THIS SUPPORTS THE MOST ANOMALOUS COLD WEATHER  
OVER THE NORTHERN/CENTRAL ROCKIES AND ADJACENT GREAT PLAINS. THE  
PARTICULAR SET UP IN PLACE ALSO SUPPORTS A HEAVY RAINFALL SIGNAL  
AS UNIDIRECTIONAL SOUTH-SOUTHWESTERLY FLOW PREVAILS FROM THE  
ARKLATEX UP TOWARD THE OH/TN VALLEYS. THIS SHOULD BE AN ISSUE FOR  
MULTIPLE DAYS DURING THE PERIOD AS THE WESTERN U.S. TROUGH  
CONTINUALLY RE-LOADS. NOT A WHOLE LOT OF CHANGE IS EXPECTED IN THE  
PATTERN GIVEN PERSISTENT RIDGING OVER THE SOUTHWESTERN ATLANTIC  
AND OUT OVER THE EASTERN PACIFIC.  
 
   
..MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES
 
 
STARTING ON WEDNESDAY MORNING...A 594-DM MID-LEVEL RIDGE WILL BE  
CENTERED JUST SOUTHWEST OF BERMUDA WHICH IS ANYWHERE FROM 2.5 TO  
POSSIBLY 3 STANDARD DEVIATIONS ABOVE CLIMATOLOGY. GRADUAL NORTHERN  
EROSION OF THIS ANTICYCLONE CENTER WILL TAKE PLACE AS UPSTREAM  
SHORTWAVES FLATTEN THE FLOW OFFSHORE OF THE NORTHEASTERN U.S.  
ULTIMATELY THIS SHOULD AFFORD A FRONTAL PASSAGE ACROSS THE I-95  
CORRIDOR BY LATE WEDNESDAY WITH THE GFS DEPICTING A SLOWING TREND  
OVER THE PAST SEVERAL RUNS. THIS BRINGS ITS SOLUTION MORE IN  
ALIGNMENT WITH THE 12Z ECMWF/CMC WHICH HAVE BEEN A TAD MORE STABLE  
IN RECENT CYCLES. BY THURSDAY...HEIGHTS BEGIN TO RISE EAST OF THE  
MS RIVER AGAIN WHICH WILL CAUSE THE FRONTAL ZONE TO STALL AND  
MEANDER FROM THE MID-ATLANTIC BACK THROUGH THE TN VALLEY AND INTO  
THE OZARKS. THIS PARTICULAR BOUNDARY SHOULD REMAIN A FOCUS FOR  
MULTIPLE ROUNDS OF HEAVY RAINFALL WITH A PLACEMENT LIKELY BEING  
DRIVEN BY MESOSCALE PROCESSES AND WHERE DIFFERENTIAL HEATING ZONES  
SET UP.  
 
WHILE GENERAL MID-LEVEL SOUTH-SOUTHWESTERLY FLOW SHOULD BE  
ESTABLISHED ACROSS THE CENTRAL/EASTERN U.S...HOW THIS OCCURS IS  
UNCERTAIN GIVEN MANY UPSTREAM DIFFERENCES. INITIALLY...A LONGWAVE  
TROUGH WILL BE POSITIONED OVER THE SOUTHWESTERN U.S. WHILE THE  
00Z/18Z GFS CONTINUE TO FAVOR A SOUTHWARD PROPAGATING IMPULSE  
PARALLELING THE CA COAST. ENSEMBLE SPAGHETTI PLOTS SHOW THE 12Z  
GEFS MEMBERS TAKE THIS MORE ELONGATED PATH AROUND THE MEAN TROUGH  
AS OTHER SOLUTIONS LOOM FARTHER DOWNSTREAM. AS THIS SYSTEM PUSHES  
TOWARD THE PERSISTENT REGION OF MID-LEVEL CONFLUENCE OVER THE  
CENTRAL U.S...ADDITIONAL SHORTWAVE ENERGY DIGS THROUGH THE WESTERN  
U.S. ON FRIDAY. THERE ARE SOME TIMING DIFFERENCES EVIDENT AMONG  
THE OPERATIONAL/ENSEMBLE MODELS WITH A VAST MAJORITY OF THE 12Z  
ECMWF ENSEMBLES FAVORING A SLOWER/MORE WESTWARD DISPLACED  
SOLUTION. LIKE PRECEDING SYSTEMS...THIS FEATURE TOO WILL MARCH  
TOWARD THE CENTER OF THE COUNTRY WITH ADDITIONAL ENERGY DROPPING  
DOWN INTO THE PACIFIC NORTHWEST BY NEXT WEEKEND. GIVEN THE  
MULTITUDE OF FEATURES TO ACCOUNT FOR DURING THE  
PERIOD...UNCERTAINTY BUILDS LATE IN THE PERIOD WITH DETAILS WITHIN  
THIS ACTIVE TROUGH.  
 
GIVEN THE CONSISTENCY OF THE 12Z ECMWF...KEPT A SIGNIFICANT CHUNK  
OF ITS SOLUTION IN THE BLEND THROUGH DAY 5/FRIDAY WITH SMALLER  
CONTRIBUTIONS FROM THE 12Z GFS. MAINTAINED ROUGHLY 75 PERCENT OF  
THE 12Z ECMWF/ECMWF ENSEMBLE MEAN THROUGHOUT THE FORECAST PERIOD  
WITH NONE OF THE OPERATION RUN UTILIZED BEYOND DAY 5/FRIDAY. THE  
CONFIDENCE IN THE FORECAST DOES DETERIORATE INTO NEXT WEEKEND  
GIVEN SO MANY UNKNOWNS WITHIN THE ACTIVE PATTERN ACROSS THE  
WESTERN/CENTRAL STATES.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
WITH HIGH-AMPLITUDE FLOW AND ANOMALOUS MOISTURE IN THE  
PICTURE...IT SHOULD BE A RATHER ACTIVE PERIOD WITH BOTH DAILY  
RECORD TEMPERATURES AND POSSIBLE FLOODING ISSUES. BEGINNING WITH  
THE FORMER...WELL ABOVE AVERAGE READINGS ARE TO PREVAIL FROM THE  
LOWER MS VALLEY UP INTO THE NORTHEASTERN U.S. DEPARTURES FROM  
CLIMATOLOGY SHOULD BE ON THE ORDER OF 20 TO 30 DEGREES ON  
WEDNESDAY AS HIGHS SURGE INTO THE 70S AS FAR NORTH AS THE  
MID-ATLANTIC. WHILE SOME REDUCTION IN THESE ANOMALIES IS EXPECTED  
AS A BACK DOOR FRONT STALLS OVER THE MID-ATLANTIC/TN VALLEY  
REGION...OVERNIGHT WARMTH SHOULD CONTINUE THROUGHOUT NEXT WEEK.  
WELL ABOVE AVERAGE TEMPERATURES ARE LIKELY WITH READINGS ABOVE  
FREEZING UP INTO THE LOWER GREAT LAKES AND NEW ENGLAND AT TIMES.  
OVERALL...DAILY TEMPERATURE RECORDS SHOULD BE BROKEN DURING THE  
PERIOD...PARTICULARLY ON WEDNESDAY AND INTO THE FOLLOWING MORNING.  
 
ON THE OPPOSITE SIDE OF THE SPECTRUM...FRIGID TEMPERATURES SHOULD  
CONTINUE ACROSS MUCH OF THE WESTERN/CENTRAL U.S. EXPECTED LOWS AND  
HIGHS MAY DEPART ANYWHERE FROM 20 TO 30 DEGREES FROM MID/LATE  
FEBRUARY CLIMATOLOGY ACROSS THE CENTRAL/NORTHERN ROCKIES AND  
ADJACENT GREAT PLAINS. EVEN THE WEST COAST CAN EXPECT COLDER  
TEMPERATURES AS BELOW FREEZING LOWS MOVE ALONG INTERSTATE 5 FROM  
THE PACIFIC NORTHWEST DOWN INTO NORTHERN/CENTRAL CA. WHILE WELL  
BELOW AVERAGE TEMPERATURES SHOULD CONTINUE OVER THE NORTH-CENTRAL  
U.S. INTO NEXT WEEKEND...THE ANOMALIES DO WEAKEN OVER A MAJORITY  
OF THE WESTERN/CENTRAL STATES.  
 
HEAVY RAINFALL SHOULD BE AN ONGOING ISSUE ACROSS THE ARKLATEX WITH  
THE DAY 3 QPF SUGGESTING AREAL AVERAGES OF 2 TO 4 INCHES OVER THIS  
REGION THROUGH 21/1200Z WITH SLIGHTLY LIGHTER AMOUNTS EXTENDING  
INTO THE OZARKS AND MO VALLEY. WHILE THE FIRST BATCH OF HEAVY  
RAINS WILL FALL FROM THE ARKLATEX INTO THE OZARKS AND TN/OH  
VALLEYS...THERE ARE SUGGESTIONS ANOTHER ROUND SHOULD OCCUR ON  
FRIDAY/SATURDAY. PLACEMENT IS UNCERTAIN AT THIS POINT WITH THE 12Z  
ECMWF FAVORING SIMILAR LOCATIONS WHILE THE 00Z GFS PUSHES THE NEW  
AXIS WELL TO THE NORTH. IT WILL PROVE TO BE A PERIOD FILLED WITH A  
DISTINCT POSSIBILITY FOR FLOODING ISSUES GIVEN THE MULTI-DAY  
NATURE OF THE EVENT. ADDITIONALLY...SPRINGTIME MOISTURE RETURN  
COUPLED WITH PERSISTENT TRAINING WILL LIKELY CAUSE FLASH FLOODING  
CONCERNS TO INCREASE AS WELL. MEANWHILE...TO THE NORTH OF THIS  
SHIELD OF RAINFALL...A SLIVER OF WINTRY PRECIPITATION COULD FALL  
OVER THE GREAT LAKES. OUT WEST...LIGHT TO MODERATE RAINFALL IS  
EXPECTED ALONG THE WEST COAST WITH HIGHER ELEVATIONS SNOWS.  
AMOUNTS DO NOT APPEAR SUBSTANTIAL GIVEN THE MORE NORTH/SOUTH TRACK  
OF THESE DISTURBANCES.  
 
RUBIN-OSTER  
 

 
 
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