PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK 
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD 
300PM EDT TUESDAY APR 30 2013 
 
30-DAY OUTLOOK DISCUSSION FOR MAY 2013 
 
THE UPDATED MAY OUTLOOK UTILIZED SHORT TERM AND EXTENDED RANGE MODEL GUIDANCE 
TO ADJUST FORECAST AREAS DEPICTED AT HALF MONTH LEAD. CHANGES WERE NECESSARY 
FOR BOTH TEMPERATURE AND PRECIPITATION OUTLOOKS. 
 
SHORT TERM WEATHER FORECASTS SUGGEST BELOW-NORMAL TEMPERATURES DURING AT LEAST 
THE FIRST TEN DAYS OF MAY ACROSS MUCH OF THE PLAINS EXTENDING TO THE GULF COAST 
AND SOUTHEAST. IN ADDITION, HIGH FORECAST PROBABILITIES FOR BELOW-NORMAL 
TEMPERATURES FROM THE CFS 30 DAY FORECAST FOR MUCH OF THE SAME AREA RESULTED IN 
A CHANGE FROM ELEVATED CHANCES OF ABOVE-NORMAL TEMPERATURES FOR PARTS OF THE 
CENTRAL AND SOUTHERN PLAINS TO AN ENHANCED LIKELIHOOD OF BELOW NORMAL 
TEMPERATURES FOR SOME OF THIS AREA. ALTHOUGH THE ABOVE-AVERAGE SNOWPACK IN THE 
NORTHERN PLAINS HAS LARGELY MELTED, THERE REMAINS SOME POTENTIAL FOR 
ABOVE-AVERAGE SOIL MOISTURE CONDITIONS AND SO SOME FEEDBACK TO AIR TEMPERATURE. 
THIS IN COMBINATION WITH THE PROSPECTS OF NEAR-TO-BELOW AVERAGE TEMPERATURES 
DURING THE FIRST 10 DAYS OF MAY MAKE RETAINING AN AREA OF ELEVATED CHANCES OF 
BELOW NORMAL TEMPERATURES ACROSS THIS AREA REASONABLE. 
 
THERE REMAINS STRONG SUPPORT FROM SHORT-TERM AND EXTENDED RANGE FORECAST 
GUIDANCE ALONG WITH THE CFS FOR ENHANCED ODDS OF ABOVE-NORMAL TEMPERATURES FOR 
MOST OF THE WESTERN THIRD OF THE CONUS, AS WELL AS THE ADDITION OF AN AREA FOR 
NEW ENGLAND. 
 
LATE SEASON LOW ELEVATION SNOW AND THE CFS 30 DAY TEMPERATURE FORECAST SUPPORT 
THE ADDITION OF AN AREA OF FAVORED BELOW-NORMAL TEMPERATURES FOR THE SOUTHEAST 
CORNER OF MAINLAND ALASKA. 
 
THE UPDATED MAY PRECIPITATION OUTLOOK EXPANDS THE REGION OF FAVORED 
BELOW-MEDIAN PRECIPITATION FOR THE NORTHWEST QUARTER OF THE CONUS AND MAINTAINS 
AN AREA OF ELEVATED CHANCES FOR ABOVE-MEDIAN PRECIPITATION FOR THE ALASKA 
PANHANDLE. THERE IS NO LONGER SUPPORT FOR AN AREA OF BELOW-MEDIAN PRECIPITATION 
FOR THE LOWER MIDWEST AS HIGHLIGHTED AT THE HALF-MONTH LEAD, SO THIS AREA WAS 
REMOVED. SHORT TERM MODEL GUIDANCE INDICATES THE POTENTIAL FOR HEAVY 
PRECIPITATION DURING THE FIRST WEEK OF MAY FOR AN AREA CENTERED NEAR IOWA. THIS 
REGION MAY EXPERIENCE PRECIPITATION GREATER THAN 50% OF ITS TYPICAL MAY TOTAL 
PRECIPITATION IN THE FIRST WEEK. THERE IS ALSO GOOD AGREEMENT BETWEEN SHORT 
TERM AND EXTENDED RANGE MODEL GUIDANCE, AS WELL AS FROM THE CFS FOR ELEVATED 
ODDS OF ABOVE-MEDIAN PRECIPITATION FOR MUCH OF THE SOUTHEAST U.S. INCLUDING 
FLORIDA. THE HIGHEST PROBABILITIES ARE WHERE ALL THREE TOOLS OVERLAP. 
 
PREVIOUS DISCUSSION 
 
THE OCEAN AND ATMOSPHERIC STATES ACROSS THE EQUATORIAL PACIFIC OCEAN CONTINUE 
TO REFLECT ENSO NEUTRAL CONDITIONS. SEA SURFACE TEMPERATURE ANOMALIES REMAIN 
SMALL (LESS THAN 0.5 DEGREES C ABSOLUTE MAGNITUDE) THROUGHOUT MUCH OF THE BASIN 
AND IN FACT THE LATEST WEEKLY NINO3.4 SST ANOMALY IS NEAR ZERO, INDICATING NO 
DEPARTURE FROM ITS CLIMATOLOGICAL VALUE. THE MADDEN-JULIAN OSCILLATION HAS BEEN 
ACTIVE OVER THE LAST FEW MONTHS, AT VARYING STRENGTHS, BUT RECENT OBSERVATIONS 
INDICATE THAT IT IS NOW LESS COHERENT AND MOST FORECASTS OF THE MJO DO NOT 
FAVOR A ROBUST SIGNAL DURING THE REMAINDER OF APRIL INTO THE MONTH OF MAY. 
MOREOVER, TELECONNECTION ASSOCIATED WITH THE MJO INTO THE MID-LATITUDES WEAKENS 
CONSIDERABLY MOVING TOWARDS THE MONTH OF MAY. ENSO AND MJO DID NOT PLAY A ROLE 
IN THE MAY OUTLOOK. THE MAY OUTLOOK WAS BASED ON A COMBINATION OF STATISTICAL 
FORECAST TOOLS, DYNAMICAL MODEL FORECASTS AS WELL AS SNOW COVER AND SOIL 
MOISTURE CONSIDERATIONS. 
 
THE MAY TEMPERATURE OUTLOOK FAVORS ABOVE NORMAL TEMPERATURES FOR MUCH OF THE 
WESTERN CONUS AND PARTS OF THE CENTRAL AND SOUTHERN PLAINS. THERE IS GOOD 
AGREEMENT FROM THE FORECAST TOOLS IN THIS AREA WHERE STATISTICAL TOOLS (CCA, 
SMLR AND OCN) INDICATE ENHANCED ODDS FOR ABOVE NORMAL TEMPERATURES PRIMARILY 
FOR THE SOUTHERN PORTIONS OF THIS AREA. DYNAMICAL MODEL FORECASTS FAVORED ABOVE 
NORMAL TEMPERATURES PRIMARILY ACROSS CENTRAL AND NORTHERN SECTIONS OF THE 
HIGHLIGHTED REGION. DRY SOIL MOISTURE CONDITIONS ARE ALSO EVIDENT ACROSS MUCH 
OF THIS AREA AND FURTHER SUPPORT THE ENHANCED LIKELIHOOD OF ABOVE-NORMAL 
TEMPERATURES. THE GREATEST PROBABILITIES ARE FORECAST FOR PARTS OF THE CENTRAL 
ROCKIES WHERE THE MAJORITY OF TOOLS FAVOR ABOVE-NORMAL TEMPERATURES. 
 
THERE IS AN INCREASED LIKELIHOOD FOR BELOW NORMAL TEMPERATURES FOR PARTS OF THE 
NORTHERN PLAINS WHERE UNSEASONABLY LATE SNOWFALL AND/OR MELTING OF THE SNOWPACK 
IN MANY AREAS LIKELY WILL KEEP TEMPERATURES COOLER THAN NORMAL EITHER BY 
REMAINING SNOWPACK IN EARLY MAY AND/OR RESIDUAL ELEVATED SOIL MOISTURE 
CONDITIONS THROUGHOUT MUCH OF THE MONTH, ALTHOUGH THERE IS UNCERTAINTY FOR HOW 
MUCH SNOWMELT WILL BE IN THE FORM OF RUNOFF VERSUS INFILTRATION. 
 
LARGE DIFFERENCES BETWEEN STATISTICAL TOOLS AND DYNAMICAL MODEL FORECASTS LEAD 
TO AN EQUAL CHANCES (EC) FORECAST FOR ALASKA. AREAS WHERE THE LIKELIHOODS OF 
MONTHLY MEAN TEMPERATURE ARE SIMILAR TO CLIMATOLOGICAL PROBABILITIES OF BELOW-, 
NEAR- AND ABOVE-AVERAGE TEMPERATURE, EQUAL CHANCES (EC) IS INDICATED. 
 
THE MAY PRECIPITATION OUTLOOK HAS VERY LOW COVERAGE GIVEN OFTEN CONFLICTING AND 
WEAK CLIMATE SIGNALS FROM THE FORECAST TOOLS AVAILABLE. THREE AREAS ARE 
HIGHLIGHTED AND THESE ARE PRIMARILY BASED ON A CONSENSUS OF DYNAMICAL MODEL 
FORECASTS. ENHANCED ODDS OF ABOVE-MEDIAN PRECIPITATION ARE INDICATED FOR THE 
ALASKA PANHANDLE WHILE BELOW-MEDIAN PRECIPITATION IS FAVORED FOR PARTS OF THE 
PACIFIC NORTHWEST AND LOWER MIDWEST. 
 
AREAS WHERE THE LIKELIHOODS OF MONTHLY ACCUMULATED PRECIPITATION AMOUNTS ARE 
SIMILAR TO CLIMATOLOGICAL PROBABILITIES OF BELOW-, NEAR- AND ABOVE-AVERAGE 
PRECIPITATION, EQUAL CHANCES (EC) IS INDICATED. 
 
THE MAIN FACTORS WHICH 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) PERSISTANTLY 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 (CFS) 
AND FOR ZERO-LEAD UPDATE FORECASTS THE NCEP GLOBAL ENSEMBLE FORECAST SYSTEM 
(GEFS). THE EARLIER VERSION OF THE CFS MODEL WAS DISCONTINUED AS OF OCTOBER, 
2012. AN EXPERIMENTAL FORECAST SYSTEM, THE NATIONAL MULTI-MODEL ENSEMBLE 
(NMME), COMPRISED OF SEVERAL DYNAMICAL MODELS IS ALSO USED. 
 
 
FORECASTER: JON GOTTSCHALCK 
 
 
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 16 2013 
 
 
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD. 
 
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