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Official 90-day Outlooks are issued once each month near mid-month at 8:30am Eastern Time. Please consult the schedule of 30 & 90-day outlooks for exact release dates.

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More Outlooks
    0.5mn MJJ 2015
    1.5mn JJA 2015
    2.5mn JAS 2015
    3.5mn ASO 2015
    4.5mn SON 2015
    5.5mn OND 2015
    6.5mn NDJ 2015
    7.5mn DJF 2015
    8.5mn JFM 2016
    9.5mn FMA 2016
   10.5mn MAM 2016
   11.5mn AMJ 2016
   12.5mn MJJ 2016
    0.5mn May 2015


Tools Used (see Discussion for explanation)
   CCA
   OCN
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   SMT
   POE
 
HOME> Outlook Maps>Seasonal Forecast Discussion
 
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.

$$

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