183  
FXUS02 KWBC 281600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT FRI SEP 28 2018  
 
VALID 12Z MON OCT 01 2018 - 12Z FRI OCT 05 2018  
 
...HEAVY RAIN LIKELY EARLY NEXT WEEK FOR THE SOUTHWEST ASSOCIATED  
WITH HURRICANE ROSA...  
 
...WET PATTERN FOR THE UPPER MIDWEST AND WESTERN/CENTRAL GREAT  
LAKES...  
 
   
..HURRICANE ROSA
 
 
AS ALWAYS, PLEASE REFER TO THE LATEST NATIONAL HURRICANE CENTER  
ADVISORY FOR THE LATEST INFORMATION ON ROSA AND YOUR LOCAL NWS  
OFFICE FOR LOCAL STATEMENTS (INCLUDING WATCHES AND WARNINGS). ROSA  
IS EXPECTED TO WEAKEN TO A TROPICAL STORM AS IT LIFTS TOWARD BAJA  
CALIFORNIA AND SOUTHERN ARIZONA EARLY NEXT WEEK BUT WILL STILL  
BRING SIGNIFICANT MOISTURE TO THE SOUTHWEST. COMBINED WITH A  
PRE-ROSA SURGE THROUGH THE GULF OF CALIFORNIA AND AN INCOMING  
PACIFIC FRONT INTO CALIFORNIA, THIS MAY SET THE STAGE FOR A  
WIDESPREAD MODEST AND LOCALLY HEAVY MULTI-DAY RAINFALL EVENT FOR  
PORTIONS OF SOUTHERN CALIFORNIA EASTWARD AND NORTHEASTWARD TO THE  
FOUR CORNERS REGION. WHILE THE LOCATION OF THE HEAVIEST RAINFALL  
AXIS (OR AXES) WILL LIKELY CHANGE OVER THE NEXT FEW DAYS, IT  
REMAINS PRUDENT TO STAY ABREAST OF THE LATEST FORECAST INFORMATION  
AS THIS SYSTEM HAS THE POTENTIAL FOR SIGNIFICANT HYDROLOGICAL  
IMPACTS INCLUDING HEAVY RAINFALL AND LOCAL RUNOFF ISSUES AS  
ENHANCED BY TERRAIN.  
 
   
..PATTERN OVERVIEW AND WEATHER HIGHLIGHTS
 
 
A HIGHLY AMPLIFIED FLOW PATTERN IS EXPECTED TO PERSIST FROM THE  
NORTH PACIFIC ACROSS NORTH AMERICA WITH A BLOCKING RIDGE CENTERED  
OVER ALASKA DRIFTING NORTHWARD INTO THE ARCTIC, MEAN TROUGHING  
OFF/ALONG THE U.S. WEST COAST AS WELL AS HUDSON BAY, AND  
PERSISTENT SUBTROPICAL RIDGING ANCHORED OVER THE SOUTHEAST.  
ADDITIONALLY, THE EXTRATROPICAL TRANSITION OF TYPHOON TRAMI NEAR  
EASTERN ASIA THIS WEEKEND SHOULD SUPPORT CONTINUED UPPER FLOW  
AMPLIFICATION DOWNSTREAM OVER THE NORTH PACIFIC AND ALASKA,  
SUGGESTING A CONTINUATION OF THE HIGHLY AMPLIFIED/BLOCKED NORTH  
AMERICAN FLOW REGIME INTO THE END OF NEXT WEEK.  
 
IN THIS PATTERN, AMPLE TROUGHING NEAR THE WEST COAST SHOULD ACT TO  
STEER HURRICANE ROSA NORTHEASTWARD TOWARD BAJA CALIFORNIA EARLY  
NEXT WEEK AS A TROPICAL STORM PER THE LATEST NATIONAL HURRICANE  
CENTER TRACK, WITH A WEAKENING ROSA THEN LIFTING INTO THE  
SOUTHWEST.  
THE OVERALL FLOW PATTERN ALSO FAVORS SOUTHWARD TRANSPORT OF POLAR  
AIR FROM THE NORTHERN ROCKIES EAST TO THE GREAT LAKES THEN  
NORTHEAST AND MORE SUMMER-LIKE CONDITIONS FROM THE GULF COAST TO  
THE SOUTHEAST. EXPECT LINGERING RAINS FOR THE GULF COAST AND  
SOUTHEAST INTO MIDWEEK WITHIN A MOISTENED EASTERLY LOW LEVEL  
FETCH. A PERSISTENT STRONG FRONTAL ZONE IN BETWEEN THESE AIR  
MASSES WILL FOCUS PERIODS OF ENHANCED PRECIPITATION PER DEEPENED  
MOISTURE. COLDER THAN AVERAGE TEMPERATURES WILL PREVAIL FROM THE  
NORTHERN ROCKIES TO THE UPPER MIDWEST NEXT WEEK WITH MAXIMUM  
TEMPERATURES AS COLD AS 10-25 DEG F BELOW AVERAGE ARE POSSIBLE  
(40S AND 50S). WAVES ALONG A STRONG AND REINFORCING NORTH-CENTRAL  
U.S. FRONTAL BOUNDARY MAY PRODUCE HEAVY RAINFALL FOR THE UPPER  
MIDWEST/GREAT LAKES INTO EARLY-MID NEXT WEEK. EJECTING  
MOISTURE/ENERGY FROM THE REMNANTS OF ROSA MAY ACT TO ENHANCE  
RAINFALL RATHER EFFICIENCY ALONG THE FRONT. SUFFICIENTLY COLD AIR  
IN PLACE NORTH OF THE FRONTAL BOUNDARY MAY ALSO SUPPORT SOME  
NORTHERN ROCKIES TO THE NORTHERN HIGH PLAINS SNOWS (I.E., MONTANA)  
AS REINFORCING TROUGHING DIVES SOUTHWARD THROUGH WESTERN CANADA TO  
THE EAST OF THE ALASKAN UPPER RIDGE.  
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
THE WPC MEDIUM RANGE PRODUCT SUITE WAS MAINLY DERIVED FROM A BLEND  
OF THE DETERMINISTIC MODELS TO START WITH A PREFERENCE TOWARD THE  
06Z GFS AND 00Z PARALLEL GFS WITH RESPECT TO ROSA. THE 00Z  
ECMWF/ECENS MEAN AND 06Z GFS/GEFS MEAN THEN OFFERED A REASONABLE  
BLENDED SOLUTION FOR THE WED-FRI PERIOD AS THE TROUGHING IN THE  
WEST LIFTS EAST-NORTHEASTWARD. THE STRENGTH OF THE SOUTHEASTERN  
RIDGING WILL PLAY A PART IN HOW FAR EAST THE TROUGHING MAY  
PROGRESS AND HAS BEEN A POINT OF CONTENTION WITHIN THE ENSEMBLES  
FOR SEVERAL DAYS. THE TREND SEEMS TO BE TO HOLD ONTO THE RIDGING  
LONGER THAN PREVIOUS FORECASTS WHICH WOULD KEEP THE FRONTAL  
BOUNDARY CLOSER TO THE U.S./CANADIAN BORDER EAST OF THE  
MISSISSIPPI. HOWEVER, THE UNCERTAIN UPSTREAM FLOW OVER THE  
NORTHEASTERN PACIFIC WILL ALSO MODULATE THE FLOW ALONG 50N  
INCLUDING UPPER RIDGE BUILDING BETWEEN HAWAII AND CALIFORNIA. FOR  
NOW, OPTED TO SPLIT THE DIFFERENCE BETWEEN THE QUICKER ECMWF/GFS  
AND SLOWER ENSEMBLE MEANS GIVEN THE UNCERTAINTY.  
 
FRACASSO/SCHICHTEL  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page