963  
FXUS02 KWBC 231854  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
153 PM EST FRI FEB 23 2018  
 
VALID 12Z MON FEB 26 2018 - 12Z FRI MAR 02 2018  
 
...HEAVY RAIN/FLOOD THREAT FOR SATURATED SOILS/SWELLING RIVERS  
ACROSS PORTIONS OF THE MID-MISSISSIPPI VALLEY AND MID-SOUTH...  
 
PATTERN OVERVIEW  
~~~~~~~~~~~~~~~~  
THE FLOW PATTERN DRAMATICALLY TRANSFORMS THIS PERIOD AS A LONGWAVE  
TROUGH ACROSS NORTH AMERICA BECOMES MORE ALIGNED WITH LITTLE TILT  
BETWEEN A STRONG MID-LEVEL VORTEX NORTH OF ALASKA AND TROUGHING  
ACROSS THE WEST, WHICH DEVELOPS DOWNSTREAM OF AN INCREASINGLY  
AMPLIFIED RIDGE MOVING INTO WESTERN AK. DOWNSTREAM, THIS  
DISLODGES THE VORTEX NEAR HUDSON BAY AND LEADS TO RETROGRADING AND  
STRONG RIDGING MOVING WESTWARD OUT OF EUROPE INTO SOUTHERN  
GREENLAND. AS ENERGY IN THE FORM OF DEEP LAYER CYCLONES UNDERCUTS  
THE GREENLAND BLOCK, THE PATTERN ASSUMES A REX BLOCK CONFIGURATION  
ON TUESDAY WHICH EVOLVES INTO AN OMEGA BLOCK OVER THE NORTH  
ATLANTIC/NORTHEAST UNITED STATES BY NEXT FRIDAY.  
 
MODEL ASSESSMENT & PREFERENCES  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
IN GENERAL, THE MODELS/ENSEMBLES SHOWED DECENT AGREEMENT WITH  
RESPECT TO THE LARGE SCALE PATTERN THROUGHOUT THE PERIOD.  
SIGNIFICANT DIFFERENCES AMONG THE GUIDANCE WERE CONSTRAINED TO THE  
NORTHEAST PACIFIC EARLY AND NEAR THE EAST COAST LATE. ENSEMBLE  
SPREAD IS RATHER HIGH AND COVERS THE ENTIRE RANGE OF  
POSSIBILITIES. THE BIGGEST ISSUE LIES ACROSS THE NORTHEAST, WHERE  
THE 00Z ECMWF SHOWS A SOLUTION WITH A CLOSED LOW NEAR THE GREAT  
LAKES IN MULTIPLE PIECES AND WEAKER THAN ITS OWN ENSEMBLE MEAN.  
 
THE WPC PRESSURES, WINDS, AND FRONTS WERE BASED HEAVILY ON  
DETERMINISTIC GUIDANCE DURING DAYS 3-5, WITH A 00Z ECMWF/06Z  
GFS/00Z UKMET/00Z CANADIAN BLEND. AFTER THAT TIME, DUE TO  
INCREASING UNCERTAINTY, ENSEMBLE MEAN (00Z ECENS/NAEFS) WEIGHTING  
WAS GRADUALLY BOOSTED THROUGH TIME, EVENTUALLY MAKING UP 50%+ OF  
THE DISTRIBUTION BY NEXT FRIDAY. THE TEMPERATURES/RAIN  
CHANCES/CLOUDS/DEW POINTS/WEATHER GRIDS WERE BASED ON AN EVEN  
BLEND OF THE ENSEMBLE AND DETERMINISTIC GUIDANCE, WITH A  
PREFERENCE SLIGHTLY MORE IN THE DIRECTION OF THE 00Z ECMWF  
ENSEMBLE MEAN THAN THE 00Z NAEFS MEAN DUE TO DIFFERENCES IN THE  
SOUTHEAST. THE QPF CHOICE WAS A COMPROMISE OF THE 00Z ECMWF/06Z  
GFS/12Z GFS SOLUTIONS.  
 
WEATHER HIGHLIGHTS/THREATS  
~~~~~~~~~~~~~~~~~~~~~~~~~~  
FOR THE WEST...  
PERSISTENT TROUGHING ALONG THE WEST COAST WILL KEEP CONDITIONS  
UNSETTLED THERE, WITH THE THREAT FOR HEAVY PRECIPITATION  
INCREASING BY MID TO LATE NEXT WEEK AS THE NEXT TROUGH/UPPER LOW  
DIGS ALONG THE WEST COAST. HEAVY PRECIPITATION IS POSSIBLE FROM  
THE CASCADES SOUTHWARD INTO NORTHERN & CENTRAL CA IN THE WAKE OF A  
COLD FRONT THAT PUSHES INLAND. PERSISTENT TROUGHING AND  
PRECIPITATION WILL KEEP TEMPERATURES BELOW AVERAGE ACROSS MUCH OF  
THE WEST THROUGH NEXT WEEK.  
 
FROM THE PLAINS EASTWARD...  
WIDESPREAD CONVECTION AND POTENTIALLY HEAVY RAINS WILL ONCE AGAIN  
POSE A THREAT ACROSS PORTIONS OF THE LOWER MS VALLEY, TN VALLEY,  
AND PERHAPS THE SOUTHEAST/OHIO VALLEY FROM LATE TUE THROUGH THU  
NIGHT AS MOIST LOW-LEVEL SOUTHERLY FLOW WITHIN A CYCLONE'S WARM  
CONVEYOR BELT SPREADS AMPLE MOISTURE NORTHWARD OVER A RETURNING  
WARM FRONT. FARTHER NORTH, A LOW PRESSURE SYSTEM ON  
WEDNESDAY/THURSDAY TRACKING ACROSS THE CENTRAL  
PLAINS/MIDWEST/GREAT LAKES WOULD BRING CHANCES OF SNOWFALL TO  
PORTIONS OF THOSE REGIONS, ALTHOUGH CONFIDENCE IS LOW IN THE  
SPECIFICS AT THIS POINT. BROAD POSITIVE HEIGHT ANOMALIES WILL KEEP  
TEMPERATURES ABOVE AVERAGE FOR MUCH OF THE CENTRAL/EASTERN U.S.,  
WITH HIGHS REACHING DOUBLE DIGITS ABOVE AVERAGE ACROSS PORTIONS OF  
THE MID-MS VALLEY/MIDWEST BY TUESDAY.  
 
ROTH/RYAN  

 
 
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