Prognostic Meteorological Discussion
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000 FXUS02 KWBC 011559 PMDEPD EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1158 AM EDT WED APR 01 2015 VALID 12Z SAT APR 04 2015 - 12Z WED APR 08 2015 ...OVERVIEW... WILL BE IN A TRANSITION PERIOD THIS MEDIUM RANGE PERIOD. COLD TROUGH OVER MARITIME CANADA AND NEW ENGLAND EXITS---AND A PACIFIC TROUGH ENTERS THE LOWER 48...AND SPREADS AN ACTIVE WEATHER PATTERN EASTWARD INTO THE CENTRAL STATES AROUND DAY 6. AHEAD OF THIS PACIFIC TROUGH AND STRENGTHENING BUT COMPLEX SURFACE CYCLONE IN THE CENTRAL PLAINS---SHORTWAVE RIDGING AND MILDER AIR BRIEFLY RETURNS TO THE EASTERN THIRD OF THE NATION. ...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT... OVERVIEW COMPLETELY OVER-SIMPLIFIES THE TRANSITION PROCESS FROM THE WEST COAST TO CONTINENTAL DIVIDE---AND THE `SCOURING OUT` OF COLD AIR OVER NEW ENGLAND AND THE EASTERN LAKES-UPPER OHIO VALLEY---BEHIND THE DAY 3 FRONTAL PASSAGE. HERE...PRECIPITATION PATTERN FOR DAY 4 (SUNDAY) STILL ANTICIPATED ACROSS APPALACHIA---WITH THE SURFACE FRONT IN THE WESTERN ATLANTIC BY LATE DAY 3 (SATURDAY NIGHT) AND A WEST-TO-NORTHWEST FLOW ALOFT ALONG AND SOUTH OF 40N LATITUDE. 1/00Z DETERMINISTIC GUIDANCE HAS BACKED OFF ON THE QPF TOTALS ACROSS THE SOUTHEAST US AND THAT SEEMED REASONABLE. THEN...IT`S THE PACIFIC SYSTEM---HAVING MAINTAIN DECENT CONSISTENCY THROUGH DAY 4---WITH MOST OF THE DETERMINISTIC GUIDANCE SWINGING SOME MID-LEVEL ENERGY IN COUNTER-CLOCKWISE FASHION ACROSS NORTHERN CALIFORNIA AND INTO SOUTH CENTRAL IDAHO. BY DAY 5...DETERMINISTIC GUIDANCE WAS WIDELY VARIED---AND PARTICULARLY OUT OVER THE PACIFIC NORTHWEST AND NORTHERN HIGH PLAINS. THE 1/00Z GFS DID LOOK TO BE "ON ITS OWN" IN EASTERN MONTANA ON DAY 5---AND ACROSS THE NORTHERN PLAINS ON DAY 6. THE 1/00Z UKMET WAS DEEP...FAST AND INTENSE ALOFT OVER CALIFORNIA DAY 5-6...BUT NOT OVER MONTANA. AND THE ECMWF INCONSISTENT ALTOGETHER---WITH ITS ENERGY ALONG THE WEST COAST OVER ITS PAST THREE RUNS. THIS POINTS TO EXTREME RUN-TO-RUN VOLATILITY OVER THE PLAINS AND GREAT LAKES DAYS 5-7. TO WHAT EXTENT A RETURN GULF MOISTURE STREAM CAN TRANSPORT NORTHWARD---IS UNCLEAR AND PERHAPS THE GREATEST CHALLENGE WITH THIS TRANSITION PERIOD FORECAST. FOR THE MOST PART...THE CHALLENGE WILL BE WITH POCKETS OF DEEP CONVECTION...AND INCONSISTENT SOUTHWARD MIGRATION OF COOLER CANADIAN AIR FROM THE NORTHERN HIGH PLAINS AND UPPER MISSOURI VALLEY. DETERMINISTIC GUIDANCE SUGGESTS A SLOW-EVOLVING AND COMPLEX SERIES OF SURFACE WAVES---SOME CONVECTIVELY-INDUCED FEATURES AND SOME LEE-SIDE IN NATURE. BOTH WILL ATTEMPT TO POOL SOME COOLER AIR---AT TIMES---OVER THE LOWER HALF OF THE MISSOURI VALLEY. AT THE SAME TIME---THE CONVECTION SHOULD GENERATE MOISTURE AND DEW POINT BOUNDARIES---FURTHER MUDDLING THE SURFACE PATTERN DOWNSTREAM FROM THE OZARKS TO THE DELMARVA. THE MODELS HAVE STRUGGLES WITH HOW TO HANDLE THE MID-LEVEL ENERGY MIGRATING DOWNSTREAM OF THE DIVIDE---WHERE THE PRESENCE OF THE UPPER-LEVEL TROUGH OVER EASTERN CANADA MAINTAINS INFLUENTIAL BETWEEN DAYS 4-6---IE A NORTHERN STREAM GRADUALLY BECOMING EMBEDDED WITH PACIFIC FLOW. WAS ABLE TO MAINTAIN WPC CONTINUITY HEADING INTO DAY 5---USING THE 1/00Z ECENS-NAEFS AND EITHER OF THE 1/00Z DETERMINISTIC GFS/ECMWF RUNS. GIVEN THE DIFFERENCES IN THE NORTHERN ROCKIES...NORTHERN PLAINS...NEW ENGLAND AND CALIFORNIA (ALOFT AND SURFACE) BETWEEN THE GFS/ECMWF/CANADIAN AND UKMET---TAPERED AWAY FROM THE DETERMINISTIC SOLUTIONS BY 7/00Z---TO A 40/60 ECENS-NAEFS BLEND. THIS WILL ALLOW FOR SOME FUTURE ADJUSTMENT TO THE LEAD SURFACE WAVE AND ITS STRENGTH/TRACK ACROSS THE MID-MISSOURI VALLEY AND UPPER MIDWEST (DAY 6)---WITHOUT TOTALLY MINIMIZING ITS SENSIBLE WEATHER EFFECTS. IN THE WEST...THE MEANS ACTUALLY CLUSTER---AS THE DEEPER SOLUTION OFF THE OREGON COAST VERSUS SOME OF THE DETERMINISTIC GUIDANCE (THE 1/00Z GFS/ECMWF) LATE IN THE PERIOD. THIS WOULD SUGGEST A CUTOFF-LIKE FEATURE MEANDERING ALONG 130W THAT WOULD LIKELY EJECT ADDITIONAL WEAK MID-LEVEL WAVE ENERGY INLAND BEYOND DAY 4. ...SENSIBLE WEATHER HIGHLIGHTS... TEMPERATURE EXTREMES MAY BE LIMITED DURING THE PERIOD... WITH MAINE BEING HIGHLIGHTED AS A POTENTIAL COLD SPOT ON MONDAY. RECORD LOW MINS AND MAXES MAY BE CHALLENGED AND HAVE BEEN CONSISTENTLY SHOWING UP IN THE FORECAST GUIDANCE. A RETURN SOUTHERLY LOW-LEVEL FLOW---FROM THE NORTHWEST GULF OF MEXICO---WILL BE ON THE INCREASE LATE IN THE FORECAST PERIOD. THIS LOW-LEVEL MOISTURE APPEARS TO BECOME ENTRAINED INTO A DEEPENING SURFACE CYCLONE AND MIGRATORY PACIFIC TROUGH ON DAYS 6-7. LOOKS LIKE SOME EVIDENCE OF A `SPRING` DRY-LINE PATTERN IS BEGINNING TO SET UP ACROSS THE SOUTHERN HIGH PLAINS AFTER DAY 5. VOJTESAK $$

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