292  
FXUS02 KWBC 240505  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1204 AM EST SAT FEB 24 2018  
 
VALID 12Z TUE FEB 27 2018 - 12Z SAT MAR 03 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 ACROSS NORTH AMERICA WILL AT LEAST PARTIALLY  
REORGANIZE DURING THE MEDIUM RANGE. A BUILDING UPPER RIDGE ACROSS  
THE HIGH LATITUDES OF THE NORTH ATLANTIC WILL PROMOTE DEVELOPMENT  
OF A REX BLOCK ALONG APPROXIMATELY 30 DEG W BY DAY 3 (TUE).  
THROUGH THE MEDIUM RANGE, THE POSITIVE HEIGHT ANOMALY CENTER WILL  
GRADUALLY SHIFT WESTWARD, REACHING THE LABRADOR SEA BY NEXT  
FRI-SAT, WITH THE AXIS OF THE REX BLOCK DIPOLE SHIFTING WEST AS  
WELL. MEANWHILE, POSITIVE HEIGHT ANOMALIES WILL ALSO BUILD NORTH  
FROM THE PACIFIC ACROSS ALASKA AND INTO THE ARCTIC OCEAN. THE  
IMPLICATIONS OF THIS HEMISPHERIC PARTIAL REALIGNMENT 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,  
ALTHOUGH LIKELY BEYOND THE END OF THIS FORECAST PERIOD. OUT WEST,  
THE IMPLICATIONS SEEM CLEARER, WITH STRENGTHENING RIDGING ACROSS  
THE NORTH PAC/ALASKA SUPPORTING PERSISTENT TROUGHING FROM THE WEST  
COAST TO THE NORTHERN ROCKIES. ADDITIONALLY, RISING HEIGHT APPEAR  
LIKELY ACROSS THE SOUTH CENTRAL U.S., AS THE SUBTROPICAL RIDGE  
SHIFTS WEST ACROSS THE GULF OF MEXICO.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES
 
 
MODELS/ENSEMBLE ARE IN DECENT AGREEMENT IN SHOWING AN UPPER LOW  
BRIEFLY CUTTING OFF ACROSS THE SOUTHWESTERN U.S. TUE/TUE NIGHT,  
BEFORE OPENING INTO A WAVE AND QUICKLY MOVING EAST ACROSS THE  
CENTRAL/EASTERN STATES WED-THU. SOLUTIONS CONTINUE TO DIVERGE AS  
TO HOW QUICKLY THE LOW BEGINS TO OPEN AND MOVE EAST. THE ECMWF HAS  
SHOWN THE POOREST RUN-TO-RUN CONTINUITY, AND AFTER SPEEDING THE  
SYSTEM UP WITH THE 00Z FRI RUN, THE 12Z RUN SLOWED THE SYSTEM DOWN  
AGAIN RATHER SIGNIFICANTLY. THE GFS HAS BEEN CONSISTENTLY ON THE  
FASTER SIDE OF THE SPREAD WITH HOW QUICKLY THIS OCCURS, WHILE THE  
CMC HAS REMAINED ON THE SLOW SIDE OF THE SPREAD. OF THE MODEL  
GUIDANCE AVAILABLE FOR THIS FORECAST, THE 12Z UKMET SEEMED THE  
MOST MIDDLE GROUND SOLUTION FOR FOR THE TIMING OF THIS SYSTEM AS  
IT EXITS THE SOUTHERN ROCKIES AND CROSSES THE CENTRAL U.S.  
WED-THU. AS THE UPPER SHORTWAVE REACHES THE OH VALLEY ON THU,  
MODELS GENERALLY AGREE THAT SOME DEGREE OF PHASING OCCURS WITH  
ADDITIONAL NORTHERN STREAM ENERGY, RESULTING IN A RAPID  
AMPLIFICATION AND DEVELOPMENT OF AN UPPER LOW, ALONG WITH PERHAPS  
A RELATIVELY DEEP SURFACE CYCLONE ACROSS THE MIDWEST/ GREAT LAKES.  
THE ECMWF SOLUTION HAS VARIED FROM RUN-TO-RUN, BUT SEEMS TO BE  
TRENDING TOWARD A DEEPER SURFACE LOW ACROSS THE MIDWEST ON THU,  
MORE LIKE THE GFS/UKMET. THE FORECAST WAS INITIALLY BASED HEAVILY  
ON A MULTI-MODEL DETERMINISTIC BLEND DURING DAYS 3-5, INCLUDING  
THE 12Z ECMWF/UKMET AND 18Z GFS. GIVEN THE ABOVE CONSIDERATIONS,  
THE UKMET WAS WEIGHTED MORE HEAVILY THAN THE ECMWF OR GFS THROUGH  
MUCH OF THIS TIME PERIOD.  
 
FROM DAY 6 ONWARD, SPREAD BEGINS TO INCREASE DRAMATICALLY. THIS IS  
LIKELY DUE, AT LEAST IN PART, TO THE HIGHER LATITUDE FLOW PATTERN  
REORGANIZATION DISCUSSED ABOVE. WHILE MODELS/ENSEMBLES ARE IN  
GENERAL AGREEMENT ON THE DESCRIBED PATTERN EVOLUTION, ANY LARGE  
SCALE DIFFERENCES AT THAT TIME SCALE CAN RESULT IN QUITE  
SIGNIFICANT SMALLER SCALE DIFFERENCES. MODELS SUGGEST THAT ANOTHER  
ROUND OF SURFACE CYCLOGENESIS IS POSSIBLE ALONG/OFF THE EAST COAST  
ON FRI, BUT SOLUTIONS REMAIN WIDELY VARYING WITH RESPECT TO THE  
SPECIFICS OF ANY POTENTIAL LOW PRESSURE SYSTEM. FARTHER WEST,  
CONFIDENCE IS A BIT HIGHER THAT ADDITIONAL UPPER-LEVEL ENERGY WILL  
RAPIDLY AMPLIFY AS IT TRAVERSES THE EASTERN PERIPHERY OF THE  
UPPER RIDGE, ALONG THE WEST COAST FROM THU NIGHT ONWARD. SOME  
SOLUTIONS SHOW HEIGHTS QUICKLY FALLING ACROSS THE NORTHERN  
INTERMOUNTAIN WEST/GREAT BASIN BY FRI NIGHT/SAT, WHILE OTHERS KEEP  
A MORE CONSOLIDATED UPPER LOW JUST OFF THE WEST COAST, WITH LESS  
MOVEMENT INLAND. 500 HPA ENSEMBLE SPAGHETTI PLOTS SHOW A VERY  
LARGE AMOUNT OF SPREAD ALONG THE WEST COAST BY DAY 7, SO PREFERRED  
TO LEAN HEAVILY ON ENSEMBLE MEANS VERY LATE IN THE PERIOD. THUS,  
THE WPC FORECAST SHOWED AN INCREASE IN ENSEMBLE MEAN (12Z  
ECENS/NAEFS) WEIGHTING ON DAY 6, WITH DETERMINISTIC SOLUTIONS  
ELIMINATED ENTIRELY BY DAY 7.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
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.  
 
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. HEAVY RAIN WILL ALSO BE POSSIBLE ACROSS  
PORTIONS OF THE MID-ATLANTIC OR NORTHEAST THU-FRI, WITH SNOW  
POSSIBLE FOR NEW ENGLAND. CONFIDENCE IS LOW AS TO EXACTLY WHERE  
THE HEAVIEST PRECIP WOULD FALL, HOWEVER, WHICH WILL BE DEPENDENT  
ON THE SPECIFICS OF ANY SURFACE CYCLOGENESIS THAT OCCURS AS THE  
SYSTEM CROSSES THOSE REGIONS. 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. COOLER  
TEMPERATURES WILL PREVAIL ACROSS MUCH OF THE CENTRAL/EASTERN U.S.  
BY LATE NEXT WEEK IN THE WAKE OF A SURFACE COLD FRONT.  
 
RYAN  
 

 
 
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