PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD 300PM EDT THURSDAY APR 30 2015 30-DAY OUTLOOK DISCUSSION FOR MAY 2015 THE REVISED MONTHLY OUTLOOK FOR MAY 2015 CONTAINS SIGNIFICANT DIFFERENCES OVER THE NORTHEAST QUADRANT OF THE CONTIGUOUS 48 STATES. THE DIFFERENCES ARE SUPPORTED BY MOST DYNAMICAL MODELS AND SHORT-TERM OUTLOOKS. ADDITIONALLY, THE CORRELATIONS WITH THE PACIFIC DECADAL OSCILLATION AND ENSO HAVE WEAK SIGNALS OVER THE NORTHEAST. PROBABILITIES ARE STILL MODEST, DUE TO UNCERTAINTY BECAUSE OF WEAK CORRELATIONS BETWEEN TEMPERATURES OVER THE NORTHEAST AND CURRENTLY OBSERVED ATMOSPHERIC AND OCEANIC PATTERNS. THROUGH THE FIRST TWO WEEKS OF MAY, MOST DYNAMICAL MODELS PREDICT ABOVE NORMAL HEIGHTS OVER THE NORTHERN TIER OF THE CONUS AND BELOW NORMAL HEIGHTS OVER THE SOUTHERN TIER. THAT PATTERN IS GENERALLY CONSISTENT WITH THE PATTERN INDICATED IN CORRELATIONS OF MID-LEVEL HEIGHTS AND THE NINO3.4 INDEX. THE UPPER-LEVEL PATTERN IN THE MODELS SUPPORTS ABOVE NORMAL TEMPERATURES FOR THE WESTERN CONUS AND NORTHEAST QUADRANT, WITH BELOW NORMAL TEMPERATURES OVER THE SOUTHERN GREAT PLAINS AND GULF COAST. CORRELATIONS OF AIR TEMPERATURE WITH THE NINO3.4 INDEX SUPPORT ABOVE NORMAL TEMPERATURES OVER THE PACIFIC NORTHWEST AND BELOW NORMAL TEMPERATURES OVER THE SOUTHWEST AND SOUTHERN GREAT PLAINS. CORRELATIONS OF AIR TEMPERATURE WITH THE PDO SHOW LITTLE SIGNAL OVER THE CONUS. BALANCING THE CORRELATIONS OF AIR TEMPERATURE WITH ATMOSPHERIC AND OCEANIC PATTERNS, AGAINST THE DYNAMICAL MODEL OUTPUT, RESULTED IN THE EXPANSION OF THE AREA WHERE BELOW NORMAL TEMPERATURES ARE FAVORED TO COVER THE GULF COAST AND FLORIDA FROM THE SOUTHERN GREAT PLAINS. THE INTRODUCTION OF THE AREA WHERE ABOVE NORMAL TEMPERATURES ARE FAVORED IS FROM SHORT-TERM AND MONTHLY DYNAMICAL MODEL GUIDANCE, WHERE LITTLE TO NO SIGNAL IS PRESENT IN THE CORRELATION MAPS OF EITHER PDO OR NINO3.4 VERSUS SURFACE AIR TEMPERATURE. A SLIGHT REDUCTION IN PROBABILITIES FOR ABOVE NORMAL TEMPERATURES IS MADE TO THE CALIFORNIA COAST, WHERE THE CORRELATIONS WITH ENSO BEGIN TO WEAKEN. AN ANTICIPATED WET START TO THE MONTH ACROSS THE PLAINS SUPPORTED EXPANDING THE AREA WHERE ABOVE MEDIAN PRECIPITATION IS FAVORED. BEYOND THE EARLY PORTIONS OF THE MONTH, FORECASTS INDICATE AN ACTIVE SOUTHERN STREAM AND A GENERALLY WET PATTERN FOR MUCH OF THE CONUS AND SOUTHERN ALASKA. THE AREA WHERE ABOVE MEDIAN PRECIPITATION IS FAVORED WAS EXPANDED ACROSS THE GREAT PLAINS AND SOUTHEAST. DYNAMICAL MODEL GUIDANCE AND CORRELATIONS WITH NINO3.4 AND PDO INDEX VALUES FAVOR ABOVE MEDIAN RAINS ACROSS MUCH OF THE ROCKIES, SO THAT FORECAST IS RETAINED FROM THE 0.5 MONTH LEAD OUTLOOK. A DRY START TO MAY, WHEN AVERAGE MONTHLY PRECIPITATION AMOUNTS ACROSS THE PACIFIC NORTHWEST ARE STILL GREATER THAN 5 INCHES, RESULTS IN SLIGHTLY ENHANCED ODDS FOR BELOW MEDIAN PRECIPITATION THERE, WHICH IS A CHANGE FROM THE 0.5 MONTH LEAD OUTLOOK. CORRELATIONS OF PRECIPITATION RATE AND THE NINO3.4 INDEX ARE WEAKLY NEGATIVE OVER THAT AREA, ALTHOUGH MOST OF THE COUNTRY SHOWS A POSITIVE CORRELATION WITH NINO3.4 INDEX VALUES, WHICH WOULD BE A WET SIGNAL GIVEN THE WARM SST VALUES. THE PREVIOUS DISCUSSION, ISSUED APRIL 16, FOLLOWS: EQUATORIAL SEA-SURFACE TEMPERATURE (SST) ANOMALIES REMAINED ABOVE NORMAL FOR MUCH OF THE EQUATORIAL PACIFIC, AND ALONG THE WEST COAST OF MUCH OF NORTH AMERICA. COUPLING BETWEEN THE ATMOSPHERE AND THE OCEAN OVER THE EQUATORIAL PACIFIC HAS INCREASED, WITH NEGATIVE OLR ANOMALIES NEAR THE DATE LINE. THE LATEST NINO3.4 INDEX VALUE IS +0.7 C, WHILE NINO4 IS +1.2 C. THE PACIFIC DECADAL OSCILLATION (PDO) INDEX REMAINED POSITIVE AT 2.0. THE ATLANTIC MULTIDECADAL OSCILLATION (AMO) INDEX CALCULATED AT NOAA/ESRL/PSD IS -0.109, WHICH IS THE LOWEST SINCE APRIL 2009. THE MJO WAS VERY STRONG DURING THE MIDDLE OF MARCH, BUT HAS WEAKENED CONSIDERABLY DURING THE PAST WEEK. THE MJO IS FORECAST TO REMAIN WEAK DURING THE 2 WEEKS, AND BY SOME MODELS, REMAIN WEAK FOR THE NEXT 30 DAYS, AND THEREFORE NOT LIKELY TO PLAY A SIGNIFICANT ROLE IN THE MAY 2015 TEMPERATURE AND PRECIPITATION PATTERNS OVER THE U.S. DYNAMICAL MODEL GUIDANCE GENERALLY REFLECTED ONGOING EL NINO CONDITIONS. THE THOUGHT AMONG THE FORECAST GROUP IS THAT THE NMME MEAN AND PROBABILITIES WERE GENERALLY TOO WARM THIS MONTH. THE PRECIPITATION OUTLOOKS FROM THE MODELS WERE GENERALLY WETTER THAN IN PREVIOUS MONTHS, BUT DID NOT LOOK UNREASONABLE. THE MAY TEMPERATURE OUTLOOK SHOWS AN ENHANCED LIKELIHOOD OF ABOVE NORMAL TEMPERATURES FOR ALASKA AND FROM THE NORTHERN ROCKIES, ACROSS THE GREAT BASIN, TO SOUTHERN CALIFORNIA AND WESTERN ARIZONA. THE STATE OF PDO AND ENSO, AND THE DYNAMICAL MODEL GUIDANCE, SUPPORT THE OUTLOOK OVER THOSE REGIONS, WHILE ADDITIONAL SUPPORT COMES FROM THE ONGOING DROUGHT CONDITIONS ACROSS THE WESTERN CONUS. BELOW AVERAGE TEMPERATURES ARE FAVORED OVER THE SOUTH-CENTRAL GREAT PLAINS AND WESTERN PORTIONS OF THE SOUTHEAST. REGRESSIONS OF TEMPERATURE ON PDO, NINO3.4, AND AMO, AS WELL AS DYNAMICAL MODEL OUTPUT, SUPPORT BELOW NORMAL TEMPERATURES ACROSS THIS REGION. BELOW NORMAL TEMPERATURES ARE FAVORED FROM THE GREAT LAKES TO INTERIOR NEW ENGLAND, A SIGNAL INDICATED BY REGRESSIONS OF SURFACE AIR TEMPERATURES ON NINO3.4 AND PDO INDICES. ABOVE MEDIAN PRECIPITATION IS FAVORED OVER WESTERN ALASKA, CONSISTENT WITH DYNAMICAL MODEL OUTPUT AND REGRESSIONS OF PRECIPITATION ON THE PDO INDEX. FROM THE CENTRAL ROCKIES TO THE SOUTHERN GREAT PLAINS AND ALONG THE GULF COAST, ABOVE MEDIAN PRECIPITATION IS FAVORED. THAT SIGNAL IS EVIDENT IN MANY DYNAMICAL MODEL SOLUTIONS, AND IS CONSISTENT WITH NINO3.4 AND AMO BASED CORRELATIONS. BELOW MEDIAN PRECIPITATION IS FAVORED OVER A SMALL PORTION OF THE GREAT LAKES REGION. THAT SIGNAL IS EVIDENT IN CORRELATIONS WITH NINO3.4, PDO, AMO, AND APPEARED IN ABOUT HALF OF THE NMME MODEL GUIDANCE. IN AREAS WHERE CLIMATE SIGNALS ARE WEAK, SUCH THAT THE LIKELIHOODS OF MONTHLY MEAN TEMPERATURES OR MONTHLY ACCUMULATED PRECIPITATION AMOUNTS ARE SIMILAR TO CLIMATOLOGICAL PROBABILITIES, EQUAL CHANCES (EC) IS INDICATED. THE MAIN FACTORS THAT USUALLY INFLUENCE THE MONTHLY CLIMATE OUTLOOK INCLUDE: 1) EL NINO AND LA NINA - WHICH COMPRISE ENSO. IMPACTS OF THESE EVENTS ARE SUMMARIZED BY SEPARATING 3-MONTH OBSERVATIONS FROM THREE OR MORE DECADES INTO EL NINO, NEUTRAL AND LA NINA SETS, AVERAGING EACH SEPARATELY, AND THEN COMPUTING ANOMALIES. THESE ARE CALLED "COMPOSITES" AND USED AT TIMES TO SUBJECTIVELY MODIFY THE FORECAST. 2) TRENDS - APPROXIMATED BY THE OCN TOOL AS THE DIFFERENCE BETWEEN THE MOST RECENT 10-YEAR MEAN OF TEMPERATURE OR 15-YEAR MEAN OF PRECIPITATION FOR A GIVEN LOCATION AND TIME OF YEAR AND THE 30-YEAR CLIMATOLOGY PERIOD (CURRENTLY 1981-2010). 3) THE TROPICAL 30-60 DAY OSCILLATION - CALLED THE MADDEN JULIAN OSCILLATION (MJO) - AFFECTS CLIMATE VARIABILITY WITHIN SEASONS. 4) THE ARCTIC OSCILLATION (AO), THE SIMILAR NORTH ATLANTIC OSCILLATION (NAO), AND THE PACIFIC NORTH AMERICAN (PNA) PATTERNS - WHICH AFFECT THE TEMPERATURE ANOMALY PATTERN ESPECIALLY DURING BOREAL WINTER AND ARE GENERALLY CONSIDERED TO HAVE LOW PREDICTABILITY. 5) THE PACIFIC DECADAL OSCILLATION (PDO) - AN ENSO-LIKE PATTERN OF CLIMATE VARIABILITY AFFECTING THE TROPICS AND THE NORTH PACIFIC AND NORTH AMERICAN REGIONS, BUT WHICH VARIES ON A MUCH LONGER TIME-SCALE THAN ENSO. 6) PERSISTENTLY DRY OR WET SOILS IN THE SPRING AND SUMMER AND SNOW AND ICE COVER ANOMALIES IN THE WINTER. THESE FACTORS TEND TO PERSIST FOR LONG PERIODS AND ACT AS A KIND OF MEMORY IN THE CLIMATE SYSTEM. 7) STATISTICAL FORECAST MODELS - CANONICAL CORRELATION ANALYSIS (CCA), SCREENING MULTIPLE LINEAR REGRESSION (SMLR), CONSTRUCTED ANALOG (CA), AND ENSEMBLE CCA (ECCA). 8) DYNAMICAL FORECAST MODELS - INCLUDING THE NCEP CLIMATE FORECAST SYSTEM VERSION 2 (CFS) AND FOR THE ZERO-LEAD UPDATE FORECASTS THE NCEP GLOBAL ENSEMBLE FORECAST SYSTEM (GEFS). AN EXPERIMENTAL FORECAST SYSTEM, THE NATIONAL MULTI-MODEL ENSEMBLE (NMME), COMPRISED OF SEVERAL DYNAMICAL MODELS IS ALSO USED. FORECASTER: MATTHEW ROSENCRANS 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 21 2015 THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD. $$