597  
FXUS02 KWBC 181600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 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 AN ARKLATEX TO TN/OH VALLEYS  
HEAVY RAINFALL THREAT...  
 
   
..PATTERN OVERVIEW
 
 
A SUBTROPICAL RIDGE FROM THE SOUTHWESTERN ATLANTIC INTO THE SERN  
US AND AN AMPLIFIED EASTERN PACIFIC RIDGE WILL CONTINUE TO ANCHOR  
THE MID-LATITUDE PATTERN. CONSIDERING TELECONNECTIONS WITH THESE  
PERSISTENT POSITIVE HEIGHT ANOMALIES...THE PATTERN IN BETWEEN  
FEATURES MID-UPPER LEVEL TROUGHING OVER THE WESTERN/CENTRAL U.S.  
AND CENTRAL CANADA. THIS SUPPORTS THE MOST ANOMALOUS COLD WEATHER  
FOR THE NATION OVER THE NORTHERN/CENTRAL ROCKIES AND ADJACENT  
GREAT PLAINS. THE PARTICULAR SET UP 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
 
 
A 594-DM RIDGE ALOFT WILL BE CENTERED SOUTHWEST OF BERMUDA BY  
MIDWEEK. THIS IS 2.5 TO 3 STANDARD DEVIATIONS ABOVE CLIMATOLOGY.  
NORTHERN EROSION OF THIS ANTICYCLONE TAKES PLACE AS UPSTREAM  
SHORTWAVES FLATTEN THE FLOW OFFSHORE OF THE NORTHEASTERN U.S.  
ULTIMATELY THIS AFFORDS BACKDOOR FRONTAL PASSAGE SOUTHWARD INTO  
THE MID-ATLANTIC. HEIGHTS THEN 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 SHORTWAVE DIFFERENCES.  
 
ACCORDINGLY TO MITIGATE SHORTWAVE TIMING VARIANCE AND ASSOCIATED  
UNCERTAIN SURFACE SYSTEM GENESIS...THE WPC MEDIUM RANGE PRODUCT  
SUITE WAS MAINLY DERIVED FROM A COMPOSITE OF THE 06 UTC GEFS MEAN  
AND YESTERDAYS 12 UTC ECMWF ENSEMBLE MEAN THAT OFFERS BETTER  
CLUSTERING WITH THE 06 UTC GEFS MEAN THAN THE NEWER/MORE  
PROGRESSIVE 00 UTC ECMWF ENSEMBLE MEAN.  
 
   
..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.  
 
FRIGID TEMPERATURES LINGER BUT GRADUALLY MODERATE ACROSS THE  
WESTERN/CENTRAL U.S. WITH EXPECTED EARLY PERIOD LOWS AND HIGHS  
DEPARTING 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 FROM  
THE PACIFIC NORTHWEST INTO CA.  
 
HEAVY RAINFALL INTO MEDIUM RANGE TIME SCALES WILL BE AN ONGOING  
ISSUE ACROSS THE ARKLATEX EXTENDING A DAY 2/3 QPF THREAT. WHILE  
THE FIRST BATCH OF HEAVY RAINS WILL FALL FROM THE ARKLATEX INTO  
THE OZARKS AND TN/OH VALLEYS...THERE IS SOME SIGNAL FOR ANOTHER  
ROUND INTO FRIDAY/SATURDAY...BUT PLACEMENT IS UNCERTAIN AT THIS  
POINT. 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 MAY FALL OVER THE  
GREAT LAKES. OUT WEST...LIGHT TO MODERATE RAINFALL IS EXPECTED FOR  
THE WEST COAST WITH HIGHER ELEVATIONS SNOWS. AMOUNTS DO NOT APPEAR  
SUBSTANTIAL GIVEN NORTH/SOUTH DISTURBANCE TRACK.  
 
SCHICHTEL  
 

 
 
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