PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
830AM EDT THURSDAY APR 16 2015

30-DAY OUTLOOK DISCUSSION FOR MAY 2015

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.



AN UPDATED MONTHLY OUTLOOK... FOR MAY WILL BE ISSUED ON THU APRIL 30 2015


THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD.

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