032  
FXUS02 KWNH 260620  
PREEPD  
 
PRELIMINARY EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
219 AM EDT WED JUL 26 2017  
 
VALID 12Z SAT JUL 29 2017 - 12Z WED AUG 02 2017  
   
..PATTERN OVERVIEW/GUIDANCE EVALUATION/PREFERENCES
 
 
MODEL GUIDANCE REMAINED FAIRLY CONSISTENT AND STABLE WITH THE  
AMPLIFICATION OF FLOW OVER THE LOWER 48 THIS COMING WEEKEND AND  
INTO EARLY NEXT WEEK. UPPER RIDGING SHOULD AGAIN BUILD IN OVER  
THE WESTERN U.S. (WITH THE MEAN RIDGE AXIS EXPECTED TO ALIGN ALONG  
THE GREAT BASIN/INTERMOUNTAIN WEST BY THE MIDDLE OF NEXT WEEK),  
WHILE ANOTHER LONGWAVE TROUGH CARVES OUT DOWNSTREAM OVER THE  
EASTERN U.S..  
 
IN-BETWEEN THE RIDGE IN THE WEST AND TROUGH IN THE EAST, SOME  
UNCERTAINTY DEVELOPS WITH FLOW OVER THE UPPER MISSISSIPPI  
VALLEY/GREAT LAKES NEXT MONDAY AND TUESDAY. RECENT RUNS OF THE  
GFS SUGGEST A DEEP VORTEX WILL CENTER ITSELF NEAR THE HUDSON BAY  
AND ALLOW MORE TROUGHING OVER THE NORTH CENTRAL U.S., COMPARED TO  
THE 12Z ECMWF/ECMWF ENSEMBLE MEAN WHICH EXPAND THE WESTERN U.S.  
RIDGE OVER THE REGION EARLY NEXT WEEK. SINCE WIDELY VARYING  
ENSEMBLE MEMBERS SHOW SUPPORT FOR EITHER SOLUTION (EVEN THE 18Z  
GFS WHICH DROPS THE DEEP VORTEX SOUTH OF THE HUDSON BAY), THE WPC  
FORECAST STARTED WITH A GENERAL BLEND OF MODELS, WITH MORE WEIGHT  
TOWARDS ENSEMBLE MEAN GUIDANCE IN ORDER TO ACCOUNT FOR THE  
UNCERTAINTY.  
 
A GENERAL MODEL COMPROMISE ALSO SMOOTHED OUT THE LINGERING  
DIFFERENCES WITH THE EVOLUTION OF A SHORTWAVE AND SURFACE LOW  
LIFTING FROM THE MID-ATLANTIC TO NORTHEAST COAST SATURDAY INTO  
SUNDAY, AND THE GENERAL MODEL BLEND ACCOUNTED FOR THE UNCERTAINTY  
WITH HEIGHTS OVER THE PACIFIC NORTHWEST THIS WEEKEND (WHERE THE  
ECMWF STAYED CONSISTENT WITH ADVERTISING LOWER HEIGHTS OVER THE  
REGION THAN THE GFS).  
 
   
..WEATHER HIGHLIGHTS
 
 
THE PRIMARY CONCERNS FOR HEAVY RAINFALL THIS WEEKEND AND INTO  
EARLY NEXT WEEK WILL BE FROM THE SOUTHEAST TO THE CENTRAL GULF  
COAST, WHERE THE TAIL END OF A FRONTAL BOUNDARY PUSHING OFF THE  
EAST COAST ON SATURDAY WILL STALL AND SERVE AS A FOCUS FOR  
CONVECTION, AS WELL AS OVER THE FOUR CORNERS STATES, WHERE  
PERSISTENT MONSOONAL MOISTURE SHOULD LEAD TO MULTIPLE DAYS OF  
LOCALLY HEAVY SHOWERS AND THUNDERSTORMS (ESPECIALLY IF THERE IS  
ANY CONNECTION WITH MOISTURE FROM EASTERN PACIFIC TROPICAL  
SYSTEMS). HEAVY RAINFALL COULD ALSO BECOME A THREAT OVER THE  
CENTRAL/SOUTHERN HIGH PLAINS DURING THE FIRST HALF OF NEXT WEEK AS  
ENERGY DROPPING SOUTHWARD AROUND A WESTERN U.S. RIDGE INTERACTS  
WITH ANOMALOUS MOISTURE ACROSS THE REGION, BUT THE DETAILS OF THIS  
FEATURE REMAIN VERY UNCERTAIN.  
 
TEMPERATURES SHOULD GENERALLY BE BELOW NORMAL FROM THE MISSISSIPPI  
TO THE EAST COAST BENEATH ANOTHER UPPER TROUGH SETTING UP OVER THE  
EASTERN HALF OF THE NATION THIS WEEKEND AND EARLY NEXT WEEK.  
MEANWHILE, RIDGING BUILDING BACK INTO THE WESTERN U.S. WILL BRING  
ABOVE NORMAL TEMPERATURES TO MUCH OF THE WEST, WITH THE EXCEPTION  
OF THE FOUR CORNERS REGION, WHERE MONSOONAL CONVECTION SHOULD KEEP  
DAYTIME TEMPERATURES CLOSER TO SUMMERTIME AVERAGES.  
 
GERHARDT  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page