Prognostic Meteorological Discussion
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000 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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