638  
FXUS07 KWBC 191254  
PMD30D  
PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
830 AM EDT THU APR 19 2018  
 
30-DAY OUTLOOK DISCUSSION FOR MAY 2018  
 
THE MAY 2018 IS ISSUED AGAINST THE BACKDROP OF A TRANSITIONING ENSO STATE (TO  
ENSO-NEUTRAL) AND FOLLOWING A PROLONGED STRETCH (INCLUDING FORECASTS THAT  
EXTEND THROUGH THE END OF APRIL) OF BELOW-NORMAL TEMPERATURES ACROSS MANY PARTS  
OF THE CONUS. AS WITH ANY CLIMATE FORECAST, WE MUST BE COGNIZANT OF LONG-TERM  
TRENDS; BOTH A 15-YEAR OCN AND A LEAST-SQUARES LINEAR FIT REVEAL STRONG WARMING  
TRENDS ACROSS THE SOUTHERN AND EASTERN CONUS DURING LATE SPRING AND EARLY  
SUMMER. SLIGHTLY NEGATIVE TEMPERATURE TRENDS ARE OBSERVED FOR A SMALL PORTION  
OF THE NORTHERN HIGH PLAINS AND NORTHERN ROCKIES. PRECIPITATION TRENDS ARE NOT  
NEGLIGIBLE THIS TIME OF YEAR EITHER; AN INCREASING (DECREASING) TREND IN LATE  
SPRING AND EARLY SUMMER PRECIPITATION HAS BEEN OBSERVED ACROSS MUCH OF THE  
NORTHERN AND EASTERN CONUS (SOUTHWESTERN CONUS). THESE LONG-TERM TRENDS FORM  
THE STARTING POINT FOR THE MAY 2018 OUTLOOK, AT WHICH POINT WE CONSIDER  
SUBSEASONAL TO SEASONAL CLIMATE VARIABILITY, INCLUDING ENSO, SOIL MOISTURE, AND  
VARIOUS STATE-OF-THE-ART DYNAMICAL MODEL FORECASTS.  
 
THE TEMPERATURE OUTLOOK IS CONSTRUCTED WITH THE AFOREMENTIONED TRENDS IN MIND,  
BUT TWEAKED COLDER ACROSS PARTS OF THE EAST-CENTRAL CONUS CENTERED ON THE  
MISSISSIPPI RIVER. THIS IS DUE TO VERY HIGH SOIL MOISTURE CONTENT AND THE  
LATEST MODEL GUIDANCE FROM THE NMME AND ECMWF. OVER THE MID-ATLANTIC AND  
NORTHEAST THE OBJECTIVE GUIDANCE, TAKEN IN ITS ENTIRETY, STILL EXHIBITS A  
SLIGHT TILT TOWARD ABOVE-NORMAL TEMPERATURES DESPITE DYNAMICAL MODEL WEEK 3-4  
GUIDANCE, FOR EXAMPLE, THAT TILTS COLD THROUGH THE FIRST HALF OF MAY.  
PROBABILITIES FAVORING ABOVE-NORMAL TEMPERATURES ARE HIGHEST OVER THE  
SOUTHWEST, WHERE LONG-TERM TRENDS, NEGATIVE SOIL MOISTURE ANOMALIES, LAGGED  
ENSO REGRESSIONS, AND DYNAMICAL MODEL GUIDANCE ARE ALL IN AGREEMENT. OVER THE  
NORTHERN HIGH PLAINS, A SIMILAR ALIGNMENT OF GUIDANCE POINTS MORE TOWARD  
BELOW-NORMAL TEMPERATURES. THERE IS BROAD CONSENSUS FAVORING ABOVE-NORMAL  
TEMPERATURES FOR ALASKA, WITH THE HIGHEST PROBABILITIES NEAR THE NORTHERN AND  
WESTERN COASTS, WHERE ICE BREAKUP IS LIKELY TO OCCUR EARLIER THAN NORMAL.  
 
THE PRECIPITATION OUTLOOK IS CONSTRUCTED IN SIMILAR FASHION TO THE TEMPERATURE  
OUTLOOK, STARTING WITH LONG-TERM TRENDS AND ADJUSTING FOR SUBSEASONAL TO  
SEASONAL VARIABILITY MANIFEST IN EITHER STATISTICAL OR DYNAMICAL GUIDANCE.  
LAGGED ENSO REGRESSIONS AND THE PREPONDERANCE OF DYNAMICAL GUIDANCE  
CONSTRUCTIVELY INTERFERE WITH LONG-TERM TRENDS OVER MUCH OF THE WEST, SO  
BELOW-NORMAL PRECIPITATION IS MODESTLY FAVORED FOR PARTS OF THE WEST EXTENDING  
FROM THE NORTHWEST COAST SOUTHEASTWARD TO PARTS OF THE SOUTHERN HIGH PLAINS.  
ABOVE-NORMAL PRECIPITATION IS FAVORED SLIGHTLY OVER THE NORTHERN HIGH PLAINS  
WHERE LONG-TERM TRENDS ARE FAIRLY STRONG, AND CALIBRATED GUIDANCE FROM THE NMME  
HAS A COHERENT WET SIGNAL. OVER THE EASTERN CONUS THERE ARE SOME CONFLICTING  
SIGNALS, BUT THE MAJORITY OF FORECAST GUIDANCE, INCLUDING THE NMME, LONG-TERM  
TRENDS, AND THE ECWMF, TILTS SLIGHTLY TOWARD ABOVE-NORMAL PRECIPITATION. THE  
VARIOUS TOOLS ARE IN QUITE GOOD AGREEMENT OVER ALASKA, BROADLY FAVORING  
ABOVE-NORMAL PRECIPITATION, WITH THE BEST ODDS IN SOUTHWESTERN ALASKA.  
 
FORECASTER: STEPHEN BAXTER  
 
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 MON APRIL 30 2018  
 
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD.  

 
 
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