326  
FXUS02 KWBC 241600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST SAT FEB 24 2018  
 
VALID 12Z TUE FEB 27 2018 - 12Z SAT MAR 03 2018  
 
...ANOTHER HEAVY RAIN EVENT FOR PARTS OF THE MID-MISSISSIPPI  
VALLEY/MID-SOUTH AND ONGOING FLOOD CONCERNS FROM SATURATED  
SOILS/SWELLING RIVERS...  
 
   
..PATTERN OVERVIEW
 
 
EXPECT SOME ADJUSTMENT IN THE NORTH AMERICAN FLOW PATTERN DURING  
THE MEDIUM RANGE PERIOD. A BUILDING UPPER RIDGE ACROSS THE HIGH  
LATITUDES OF THE NORTH ATLANTIC WILL PROMOTE DEVELOPMENT OF A REX  
BLOCK ALONG APPROXIMATELY 30 DEG W LONGITUDE AT THE START OF THE  
FORECAST DAY 3 TUE. THE POSITIVE HEIGHT ANOMALY CENTER WILL  
GRADUALLY SHIFT WESTWARD AND LIKELY REACH THE LABRADOR SEA BY NEXT  
FRI-SAT, WITH THE AXIS OF THE REX BLOCK DIPOLE SHIFTING WEST AS  
WELL. AT THE SAME TIME POSITIVE HEIGHT ANOMALIES WILL ALSO BUILD  
NORTH FROM THE EAST-CENTRAL PACIFIC ACROSS ALASKA AND INTO THE  
ARCTIC OCEAN. THE IMPLICATIONS OF THIS EVOLUTION FOR WEATHER  
ACROSS THE CONUS ARE SOMEWHAT UNCLEAR. TELECONNECTIONS WITH  
RESPECT TO TEMP/PRECIP ANOMALIES ACROSS THE CONUS ARE HIGHLY  
SENSITIVE TO THE EXACT POSITION OF THE NORTH ATLANTIC RIDGE. IF  
THIS NEGATIVE NAO/AO PATTERN CAN PERSIST FOR LONG ENOUGH, IT WOULD  
FOSTER A COLDER AND DRIER PERIOD FOR THE EASTERN SEABOARD THOUGH  
LIKELY BEYOND THE END OF THIS FORECAST PERIOD. THERE IS AT LEAST  
LESS SENSITIVITY TOWARD THE IDEA OF THE CORE OF NEGATIVE HEIGHT  
ANOMALIES REMAINING WELL OFF THE EAST COAST TOWARD 60W LONGITUDE.  
OUT WEST, THE IMPLICATIONS SEEM CLEARER FOR A TIME WITH  
STRENGTHENING RIDGING ACROSS THE NORTH PACIFIC/ALASKA SUPPORTING  
PERSISTENT TROUGHING FROM THE WEST COAST TO THE NORTHERN ROCKIES.  
HOWEVER A GRADUAL WESTWARD SHIFT IN EMPHASIS OF STRONGEST POSITIVE  
HEIGHT ANOMALIES OVER THE NORTH PACIFIC COULD LEAD TO SOME  
FLATTENING OF EASTERN PACIFIC/WESTERN U.S. FLOW (IN MULTI-DAY MEAN  
TERMS) AROUND OR AFTER THE END OF THE PERIOD. MEANWHILE RISING  
HEIGHTS APPEAR LIKELY ACROSS THE SOUTH CENTRAL U.S. AS THE  
SUBTROPICAL RIDGE SHIFTS WEST ACROSS THE GULF OF MEXICO.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES
 
 
CONFIDENCE HAS ACTUALLY DECREASED A BIT FOR SPECIFICS OF  
CENTRAL-EASTERN U.S. EVOLUTION DAY 4 WED ONWARD. THE 00Z ECMWF  
HAS FURTHERED ITS TREND INTRODUCED IN ITS 12Z/23 RUN, FEATURING  
NORTHERN TIER U.S. FLOW THAT IS SUFFICIENTLY PROGRESSIVE TO RESULT  
IN SLOWER TIMING OF THE UPPER LOW ENERGY EJECTING FROM THE EXTREME  
SOUTHWEST TUE-WED. THE ECMWF SCENARIO ALOFT LEADS TO  
SLOWER/SOUTHWARD LOW PRESSURE FROM THE PLAINS EASTWARD AND THEN  
INTERACTION WITH NORTHERN STREAM ENERGY PRODUCES A VERY  
DEEP/WESTWARD SYSTEM JUST OFF THE EAST COAST BY EARLY DAY 7 SAT.  
THE 00Z UKMET ADJUSTED TO THE SLOWER TIMING THROUGH THE DAY THU.  
THE ENSEMBLE MEANS, AND THE ECMWF MEAN IN PARTICULAR, HAVE  
PROVIDED THE MOST STABLE FORECAST THUS FAR IN MAINTAINING A PHASED  
UPPER TROUGH WHICH SUPPORTS A SURFACE LOW TRACK FROM THE  
SOUTH-CENTRAL PLAINS THROUGH MID-MS VALLEY AND LOWER GREAT LAKES  
FOLLOWED BY SECONDARY DEVELOPMENT OFF THE EAST COAST. THE GFS/CMC  
HAVE LOOSELY FOLLOWED THIS IDEA IN PRINCIPLE ALBEIT WITH SOME  
SIGNIFICANT DIFFERENCES IN TIMING/STRENGTH. SOME GFS RUNS MAY BE  
A LITTLE FAR NORTHWARD WITH THE LOW PRESSURE TRACK OVER THE  
NORTHEAST QUADRANT OF THE U.S. IN LIGHT OF THE RIDGE RETROGRADING  
ACROSS SOUTHERN GREENLAND. THE 00Z GFS COMPARES BETTER TO  
CONSENSUS IN THIS REGARD THAN THE 06Z RUN. OVER THE PAST FEW DAYS  
ECMWF RUNS HAVE BEEN VERY INCONSISTENT WITH EJECTION OF THE  
SOUTHWEST U.S. UPPER LOW/TROUGH AND THE MEAN PATTERN APPEARS TO  
FAVOR REASONABLE PROGRESSION. THUS FOR THE PURPOSES OF A SINGLE  
DETERMINISTIC FORECAST, PREFER TO MAINTAIN A SOLUTION CLOSE TO OR  
SLIGHTLY DEEPER THAN CONTINUITY BY WAY OF THE 06Z GEFS MEAN (MUCH  
BETTER DEFINED THAN THE 00Z RUN) AND 00Z ECMWF MEAN ALONG WITH  
SOME 00Z GFS/CMC INPUT. SOLUTIONS ARE CLOSE ENOUGH TO ALLOW FOR  
SOME ECMWF INCLUSION TUE INTO WED.  
 
ALONG THE WEST COAST THE 06Z GFS COMPARES LESS FAVORABLY TO MOST  
GUIDANCE WITH A FARTHER SOUTH/SOUTHEAST TRACK OF UPPER  
ENERGY/SURFACE LOW PRESSURE NEARING THE PACIFIC NORTHWEST BY LATE  
DAY 4 WED. SO AS WITH THE FORECAST BLEND FARTHER EAST, PREFERENCE  
GOES TO THE 00Z RUN FOR THE GFS COMPONENT. THEN THERE IS  
REASONABLE AGREEMENT THAT AMPLIFYING FLOW ALOFT WILL SUPPORT A  
FAIRLY DEEP UPPER LOW SETTLING JUST OFF THE PACIFIC NORTHWEST  
COAST BY DAY 6 FRI. TOWARD THE LATTER HALF OF THE PERIOD  
MODEL/ENSEMBLE SOLUTIONS BEGIN TO DIVERGE WITH RESPECT TO HOW MUCH  
UPSTREAM ENERGY CUTS THROUGH THE EAST-CENTRAL PACIFIC RIDGE AND  
THE EFFECT OF THIS ENERGY ON WEST COAST TROUGH PROGRESSION BY  
EARLY NEXT WEEKEND. GFS RUNS ARE STILL A BIT ON THE  
STRONG/PROGRESSIVE SIDE WITH THE UPSTREAM ENERGY-- EVENTUALLY  
LEADING TO A FASTER WESTERN TROUGH ALOFT-- BUT THEY ARE MUCH  
IMPROVED OVER THEIR RUNS FROM 24 HOURS AGO WHICH HAD NEARLY FLAT  
FLOW OVER THE EAST PACIFIC/WESTERN U.S. BY FRI. EVEN IF THE GFS  
IS SOMEWHAT EXTREME, THERE ARE GROWING SIGNALS FROM MODEL/ENSEMBLE  
GUIDANCE AS A WHOLE THAT ENOUGH ENERGY MAY COME THROUGH THE RIDGE  
TO RESULT IN MORE WEST COAST TROUGH PROGRESSION BY NEXT SAT THAN  
ADVERTISED BY THE 00Z CMC MEAN. OVERALL PREFERRED A 00Z MODEL  
CONSENSUS TRANSITIONING TOWARD THE 06Z GEFS/00Z ECMWF MEANS. LATE  
IN THE PERIOD MINORITY INPUT FROM THE 00Z GFS/CMC USED FOR THE  
EASTERN SYSTEM OFFSET FAST/SLOW TRAITS RESPECTIVELY FOR THE WEST  
COAST TROUGH TO KEEP THE FORECAST ANCHORED NEAR THE MEANS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
MID-LATE WEEK UPPER TROUGH SETTLING ALONG THE WEST COAST AND  
POSSIBLE EMBEDDED CLOSED LOW NEARING THE PACIFIC NORTHWEST WILL  
BRING INCREASING POTENTIAL FOR AREAS OF HEAVY RAIN/HIGHER  
ELEVATION SNOW OVER PORTIONS OF THE WEST. EXPECT HIGHEST 5-DAY  
TOTALS TO EXTEND FROM THE COASTAL/CASCADE AREAS OF THE PACIFIC  
NORTHWEST DOWN INTO NORTHERN CALIFORNIA AND THE SIERRA NEVADA  
RANGE. SOME ENHANCED PRECIP IS ALSO POSSIBLE ALONG THE REST OF  
THE CA COAST AND FARTHER INLAND INTO THE ROCKIES. ARRIVAL OF  
UPPER TROUGHING WILL LEAD TO A DECREASE IN SNOW LEVELS. AHEAD OF  
THIS LARGE SCALE TROUGH, A SHORTWAVE/EMBEDDED CLOSED LOW TRACKING  
THROUGH THE EXTREME SOUTHWEST EARLY IN THE PERIOD WILL BE  
ACCOMPANIED BY SOME RAIN AND HIGHER ELEVATION SNOW. EXPECT BELOW  
NORMAL TEMPS TO PERSIST OVER MUCH OF THE WEST INTO THE NORTHERN  
HIGH PLAINS THROUGH THE PERIOD, WITH SOME LOCATIONS SEEING ONE OR  
MORE DAYS OF MINUS 10-20F ANOMALIES.  
 
WIDESPREAD CONVECTION AND POTENTIALLY HEAVY RAINS WILL ONCE AGAIN  
POSE A THREAT ACROSS PORTIONS OF THE LOWER MS VALLEY INTO THE TN  
VALLEY, AND PERHAPS SOMEWHAT NORTH OR SOUTH OF THIS REGION, 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 LOW PRESSURE  
TRACKING ACROSS THE PLAINS/MIDWEST/GREAT LAKES WED-THU WOULD BRING  
CHANCES OF SNOWFALL TO LOCATIONS NORTH OF THE LOW TRACK.  
CONFIDENCE IS LOW IN THE SPECIFICS AT THIS POINT GIVEN SIGNIFICANT  
DIFFERENCES IN SOME GUIDANCE, BUT POTENTIAL EXISTS FOR HEAVY RAIN  
TO EXTEND INTO PORTIONS OF THE MID-ATLANTIC OR NORTHEAST THU-FRI  
WITH SNOW POSSIBLE FOR NEW ENGLAND. BROAD POSITIVE HEIGHT  
ANOMALIES/UPSTREAM SURFACE LOW PRESSURE WILL KEEP TEMPERATURES  
ABOVE AVERAGE FOR MUCH OF THE CENTRAL/EASTERN U.S. TUE-WED WITH  
MIN TEMPS IN PARTICULAR LIKELY TO BE MUCH ABOVE NORMAL (POSSIBLY  
EXCEEDING RECORD WARM VALUES AT SOME LOCATIONS). DEPARTURE OF LOW  
PRESSURE INTO THE WESTERN ATLANTIC SHOULD BRING TEMPS DOWN CLOSER  
TO NORMAL BY FRI-SAT.  
 
RAUSCH/RYAN  
 

 
 
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