PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK
NWS CLIMATE PREDICTION CENTER CAMP SPRINGS MD
830 AM EDT THU APR 15 2010

30-DAY OUTLOOK DISCUSSION FOR MAY 2010

EL NINO CONDITIONS CONTINUE ACROSS THE EQUATORIAL PACIFIC OCEAN WITH SEA 
SURFACE TEMPERATURE (SST) DEPARTURES OF +1.0 DEGREE C OR GREATER ACROSS MUCH OF 
THE CENTRAL PACIFIC. OCEAN SUB-SURFACE TEMPERATURE ANOMALIES ALSO REMAIN 
POSITIVE FROM THE SURFACE TO A DEPTH OF 75 TO 100 METERS FROM JUST WEST OF THE 
DATE LINE ALONG THE EQUATOR ACROSS THE PACIFIC TO NEAR THE SOUTH AMERICAN 
COAST. THEREFORE EL NINO OCEANIC CONDITIONS ARE EXPECTED TO CONTINUE AT LEAST 
INTO MAY 2010.

CURRENT ATMOSPHERIC CONDITIONS CONSISTENT WITH EL NINO IN THE TROPICS HAVE 
BECOME LESS PRONOUNCED. ENHANCED TROPICAL CONVECTION ACROSS THE PACIFIC OCEAN 
NEAR THE DATE LINE IS EVIDENT, THOUGH EARLIER IN THE MONTH, ANOMALOUS LOW-LEVEL 
(850-HPA) WESTERLY WIND ANOMALIES HAVE RECENTLY SWITCHED SIGN OVER PORTIONS OF 
THE WEST-CENTRAL PACIFIC DURING THE PAST 30 DAYS. BASED ON THE PREVIOUSLY 
MENTIONED OCEANIC AND ATMOSPHERIC CONDITIONS, EL NINO REMAINS A FACTOR, ALBEIT 
A WEAKENING ONE, IN THE MAY 2010 MONTHLY OUTLOOK FOR THE U.S.

AS WAS INDICATED IN LAST MONTH'S DISCUSSION, WE NEED TO EMPHASIZE THAT ENSO 
IMPACTS DEPEND ON SEASON. WE MAY SEE EL NINO TYPE FORCING IN THE TROPICS AND AN 
ATMOSPHERIC RESPONSE IN THE MIDDLE LATITUDES FOR MAY, BUT LIKELY LESS SO THAN 
IN PREVIOUS MONTHS. AS THIS RESPONSE CONTINUES TO WEAKEN, WE EXPECT SOIL 
MOISTURE ANOMALIES TO PLAY A MORE DOMINANT ROLE LATER IN THE SPRING.

BOTH STATISTICAL AND DYNAMICAL FORECAST TOOLS WERE CONSIDERED FOR THE MAY 2010 
OUTLOOK, ALTHOUGH THE STATISTICAL TOOLS MAY HAVE LITTLE SIGNAL FOR A PERIOD AS 
SHORT AS A MONTH. ENSO COMPOSITES FOR AMJ 2010 (CENTERED ON THE TARGET MONTH OF 
MAY) WERE ALSO CONSULTED, BUT THEIR APPLICATION IN MAY IS THOUGHT TO BE FAIRLY 
MINIMAL.

THE OFFICIAL CPC 30-DAY TEMPERATURE OUTLOOK CALLS FOR ENHANCED CHANCES OF 
ABOVE-NORMAL MEAN TEMPERATURES FOR THE NORTHWEST, MUCH OF THE DESERT SOUTHWEST 
AND GREAT LAKES REGION, AND ALMOST ALL OF ALASKA. THE OUTLOOK ALSO CALLS FOR 
ENHANCED CHANCES OF BELOW-NORMAL MEAN TEMPERATURES ACROSS THE SOUTHERN GREAT 
PLAINS, AND THE GULF AND SOUTHERN ATLANTIC COASTS, INCLUDING ALL OF FLORIDA. 
ELSEWHERE, WHICH INCLUDES A SIZABLE PORTION OF THE CONUS, EQUAL CHANCES (EC) OF 
BELOW-, NEAR-, AND ABOVE-NORMAL MEAN TEMPERATURES ARE ANTICIPATED. 

GREATER THAN CLIMATOLOGICAL ODDS FOR ABOVE-NORMAL TEMPERATURES EXPECTED OVER 
THE PACIFIC NORTHWEST AND NORTHERN ROCKIES ARE BASED ON NEARLY ALL OF THE 
AVAILABLE TOOLS, INCLUDING THE CAS AND ENSO COMPOSITES. FOR THE SOUTHWEST, 
THERE IS A STRONG CONTRAST BETWEEN THE CFS, AND THE SMLR, CCA, AND ESPECIALLY 
OCN TOOLS. THE CFS SUPPORTS BELOW-NORMAL TEMPERATURES ACROSS THIS AREA, WHILE 
THE STATISTICAL TOOLS PREFER ABOVE-AVERAGE TEMPERATURES. GIVEN SUCH A STRONG 
TREND FOR ANOMALOUS WARMTH ACROSS THE SOUTHWEST DURING THE PAST 10 YEARS, IT IS 
THOUGHT THAT THE CFS COLD SIGNAL IS SIGNIFICANTLY LESS LIKELY TO VERIFY. THE 
ENHANCED CHANCES OF BELOW-NORMAL MEAN TEMPERATURES EXPECTED OVER THE SOUTHERN 
GREAT PLAINS ARE LARGELY A FUNCTION OF POSITIVE SOIL MOISTURE ANOMALIES AND 
ASSOCIATED RE-CIRCULATION OF REGIONAL MOISTURE. THE FAVORED BELOW-NORMAL MEAN 
TEMPERATURES INDICATED ACROSS THE GULF AND SOUTHERN ATLANTIC COASTAL AREAS, 
INCLUDING ALL OF FLORIDA, IS ATTRIBUTED TO UNSEASONABLY COLD WATER TEMPERATURES 
IN THE GULF OF MEXICO. IN ALASKA, GREATER THAN CLIMATOLOGICAL ODDS FOR 
WARMER-THAN-NORMAL MEAN TEMPERATURES ARE FORECAST OVER MOST OF THE STATE, 
ESPECIALLY SOUTH-CENTRAL ALASKA. THIS IS PRIMARILY BASED ON THE SMLR, CCA, AND 
OCN TOOLS, AND THE WARM SIGNAL FOR SOUTH-CENTRAL ALASKA ALSO PASSES THROUGH THE 
CFS SKILL MASK. 

THE PRECIPITATION OUTLOOK FOR MAY 2010 IS BASED ON VERY WEAK SIGNALS. SLIGHTLY 
ELEVATED CHANCES OF ABOVE-MEDIAN PRECIPITATION ARE FORECAST OVER UTAH, 
COLORADO, AND PORTIONS OF ADJACENT STATES, BASED LARGELY ON CFS INDICATIONS. 
THE FAVORED BELOW-MEDIAN PRECIPITATION AREA PREDICTED ACROSS THE GREAT LAKES 
COMES PARTLY FROM THE CFS AND CAS TOOLS, AND PARTLY FROM THE PERSISTENT AND 
SEVERE LONG-TERM DROUGHT IN PARTS OF THAT REGION. THE END OF THE MONTH UPDATE 
WILL HAVE THE ADVANTAGE OF BEING ABLE TO UTILIZE DYNAMICAL NWP MODEL GUIDANCE 
OUT THROUGH MID-MAY, ALLOWING FURTHER REASSESSMENT OF THE LIKELIHOOD OF DROUGHT 
REMAINING IN THIS AREA. AN INTERESTING SITUATION ARISES IN THE SOUTHEASTERN 
U.S. WHERE ENSO COMPOSITES INDICATE ELEVATED CHANCES FOR WETTER-THAN-MEDIAN 
CONDITIONS, ESPECIALLY FROM SOUTH CAROLINA AND GEORGIA SOUTHWARD THROUGH 
FLORIDA. HOWEVER, THE CFS AND OCN HAVE MIXED, OR AT BEST WEAK, INDICATIONS FOR 
BELOW-MEDIAN RAINFALL THERE, AND THE CAS TOOL SUGGESTS BELOW-MEDIAN RAINFALL 
THERE AS WELL. GIVEN THE RAPID DRYING OUT ACROSS THE SOUTH IN RECENT WEEKS, AND 
THE WEAKENING AND SOUTHWARD SUPPRESSED SUBTROPICAL JET STREAM AFFECTING MOSTLY 
SOUTHERN FLORIDA, IT IS THOUGHT THAT THERE IS NOT ENOUGH JUSTIFICATION TO 
WARRANT AN AREA OF ANOMALOUS WETNESS ACROSS THE SOUTHEAST. FINALLY, IN 
SOUTH-CENTRAL ALASKA, THERE ARE WEAK INDICATIONS FROM THE CFS FOR BELOW-MEDIAN 
PRECIPITATION FOR MAY. ELSEWHERE, WHICH INCLUDES THE REMAINING, LARGE PORTIONS 
OF THE CONUS AND ALASKA, EQUAL CHANCES (EC) OF BELOW-, NEAR-, AND ABOVE-MEDIAN 
PRECIPITATION ARE ANTICIPATED.

FORECASTER: ANTHONY ARTUSA

NOTES: AREAS OF UNCERTAINTY THAT FORMERLY WERE LABELED CL (CLIMATOLOGICAL 
PROBABILITIES) ARE NOW LABELED AS EC (EQUAL CHANCES).

AN UPDATED MONTHLY OUTLOOK... FOR MAY WILL BE ISSUED ON FRI APRIL 30 2010

THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1971-2000 BASE PERIOD.

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