Prognostic Meteorological Discussion
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000 FXUS07 KWBC 301934 PMD30D PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD 300 PM EDT SAT APR 30 2016 30-DAY OUTLOOK DISCUSSION FOR MAY 2016 THE UPDATED MONTHLY OUTLOOK FOR MAY IS QUITE SIMILAR TO THAT ISSUED ON APRIL 21. THE FORECAST IS MODIFIED BY TAKING ADVANTAGE OF SHORT-TERM WEATHER AND CLIMATE FORECASTS ISSUED WITHIN THE PAST ONE TO TWO DAYS. SINCE THE IMPLEMENTATION OF THE EXPERIMENTAL WEEKS 3+4 FORECAST, THE ZERO-LEAD MONTHLY FORECASTS LARGELY SERVE THE ROLE OF AGGREGATING FORECASTS FOR WEEK-1 ISSUED BY WPC AND THE WEEKS 2-4 FORECASTS ISSUED BY CPC. AT THIS TIME, SUCH AN EFFORT IS LARGELY SUBJECTIVE, BUT BENEFITS FROM THE CFS FORECASTS, FOR WHICH WE HAVE AN EXPLICIT MAY FORECAST BASED ON THE MOST RECENT MODEL RUNS. EARLY IN THE MONTH, ANOMALOUS TROUGHING IS FORECAST FOR MUCH OF THE CONUS FROM THE SOUTHWEST TO THE EASTERN U.S. THIS IS EXPECTED TO CONTRIBUTE TO BELOW-NORMAL TEMPERATURES AND ABOVE-MEDIAN PRECIPITATION FOR PARTS OF THE GREAT BASIN, ROCKIES, SOUTHWEST, AND SOUTHERN PLAINS. UNSETTLED WEATHER AND COOLER-THAN-NORMAL TEMPERATURES ARE FORECAST TO EXTEND TOWARD THE EAST COAST AND GREAT LAKES EARLY IN THE MONTH. HOWEVER, A WARMING TREND IS EXPECTED DURING WEEK-2 FOR MUCH OF THE EASTERN U.S., AND DYNAMICAL AND STATISTICAL GUIDANCE FOR WEEKS 3+4 FAVORS A WARMER-THAN-NORMAL PATTERN CENTERED ON THE EAST-CENTRAL CONUS. COMBINING THESE EFFECTS FAVORS A MODEST TILT TOWARD ABOVE-NORMAL TEMPERATURES FOR THE EASTERN CONUS FOR THE MONTH. FARTHER WEST, PROBABILITIES OF BELOW-NORMAL TEMPERATURES ARE ENHANCED RELATIVE TO THE MID-MONTH OUTLOOK, WHERE MODEL GUIDANCE STRONGLY FAVORS BELOW-NORMAL 500-HPA HEIGHTS FOR THE MONTH. THE PRECIPITATION OUTLOOK TAKES ADVANTAGE OF HEAVY PRECIPITATION FORECAST OVER THE NEXT FEW DAYS, WHILE ALSO ATTEMPTING TO UTILIZE LOW-FREQUENCY CLIMATE VARIABILITY. THEREFORE, PROBABILITIES ARE HIGHEST WHERE THERE IS A CONFLUENCE OF SHORT-TERM PRECIPITATION ANOMALIES AND LOWER FREQUENCY CLIMATE SIGNALS. AS WITH TEMPERATURE, THE SPATIAL DISTRIBUTION OF PROBABILITIES IS QUITE SIMILAR TO THE 0.5-MONTH LEAD OUTLOOK. THE PREVIOUS DISCUSSION ISSUED WITH THE 0.5-MONTH LEAD FORECAST FOLLOWS. ------------------------------------------------------------------------- THE MAY 2016 OUTLOOK IS CONSTRUCTED BASED ON THE LOW-FREQUENCY CLIMATE SIGNALS MANIFEST IN THE DYNAMICAL AND STATISTICAL GUIDANCE. MOST GUIDANCE IS IN GOOD AGREEMENT ON THE GENERAL STRUCTURE OF FORECAST PROBABILITIES, AND SHORTER TERM OUTLOOKS ARE CONSULTED TO REFINE THE FORECAST. THERE IS LITTLE IN THE WAY OF COHERENT MJO ACTIVITY TO INFLUENCE THE OUTLOOK; THE MJO`S INFLUENCE IS DIMINISHED IN THE WARM SEASON, AND PLAYS NO ROLE HERE IN INFORMING THE MONTHLY FORECAST. SOIL MOISTURE, HOWEVER, IS TAKEN INTO ACCOUNT, AS INCREASING INCIDENT SOLAR RADIATION ALLOWS IT TO EXERT MORE INFLUENCE ON LOCAL TEMPERATURES. THE CALIBRATED NMME TEMPERATURE PROBABILITIES INDICATE A FAIRLY WEAK TILT TOWARD WARMER-THAN-NORMAL TEMPERATURES ACROSS THE NORTHERN TIER OF THE CONUS, ALASKA, AND PARTS OF THE EASTERN CONUS. A NOTABLE WEAKNESS IS FORECAST OVER THE SOUTH-CENTRAL AND SOUTHWESTERN U.S. THIS IS GENERALLY CONSISTENT WITH LAGGED ENSO REGRESSIONS (USING LATE WINTER NINO 3.4 VALUES), OBJECTIVE COMPOSITE ANALOGS BASED ON THE RECENTLY OBSERVED SSTS IN THE TROPICAL AND NORTHERN PACIFIC AND ATLANTIC BASINS, AS WELL AS STATISTICALLY DERIVED LOCAL SOIL MOISTURE IMPACTS. THE SECULAR TREND EXPLAINS LESS VARIANCE ON MONTHLY TIMESCALES THAN ON SEASONAL TIMESCALES, BUT GENERALLY SUPPORTS THE SPATIAL STRUCTURE OF FORECAST TEMPERATURE PROBABILITIES. ENHANCED PROBABILITIES OF ABOVE-NORMAL TEMPERATURES ARE INDICATED FOR MUCH OF THE WESTERN, NORTHERN, AND EASTERN CONUS, BASED ON THE ABOVE FACTORS. THE LOWEST PROBABILITIES ARE FORECAST OVER NORTHERN NEW ENGLAND AND THE NORTHERN PLAINS. THE FORMER IS DUE TO HIGHER-FREQUENCY PATTERNS THAT FAVOR ANOMALOUS NORTHERLY FLOW INTO EARLY MAY FOR THAT REGION, WHILE THE LATTER IS DUE TO TRENDS AND SOIL MOISTURE TOOLS. THE LATEST CFS FORECASTS, WEEK 3/4 GUIDANCE FROM BOTH THE CFS AND ECMWF, SOIL MOISTURE CONSIDERATIONS, AND LAGGED ENSO REGRESSIONS FAVOR ENHANCED PROBABILITIES OF BELOW-NORMAL TEMPERATURES ACROSS A PORTION OF THE SOUTHWESTERN AND SOUTH-CENTRAL CONUS. ALL OF THE AFOREMENTIONED OBJECTIVE GUIDANCE IS IN FAIRLY GOOD AGREEMENT ON THE FORECAST PRECIPITATION OUTLOOK AS WELL. MODEST PROBABILITIES FAVORING ABOVE-MEDIAN PRECIPITATION ARE DEPICTED FOR MUCH OF THE CENTRAL AND SOUTHERN CONUS, EXTENDING WESTWARD TO PARTS OF CALIFORNIA AND THE GREAT BASIN. FORECASTER: STEPHEN BAXTER THE CLIMATIC NORMALS ARE BASED ON CONDITIONS BETWEEN 1981 AND 2010, FOLLOWING THE WORLD METEOROLOGICAL ORGANIZATION CONVENTION OF USING THE MOST RECENT 3 COMPLETE DECADES AS THE CLIMATE REFERENCE PERIOD. THE PROBABILITY ANOMALIES FOR TEMPERATURE AND PRECIPITATION BASED ON THESE NEW NORMALS BETTER REPRESENT SHORTER TERM CLIMATIC ANOMALIES THAN THE FORECASTS BASED ON OLDER NORMALS. THE NEXT MONTHLY OUTLOOK...FOR JUN ... WILL BE ISSUED ON THU MAY 19 2016 THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD. $$

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