PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK 
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD 
830 AM EDT THU AUG 16 2018 
 
 
30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2018 
 
AS WE ENTER THE AUTUMN MONTHS, FORECAST PREDICTABILITY OFTEN DECREASES 
ESPECIALLY FOR THE MONTHS OF SEPTEMBER AND OCTOBER. THESE ISSUES ARE FURTHER 
ENHANCED WHEN CONSIDERING A HALF-MONTH LEAD MONTHLY FORECAST SUCH AS THE 
SEPTEMBER TEMPERATURE AND PRECIPITATION OUTLOOK PRESENTED HERE AS INFORMATION 
ACROSS TIME SCALES (THE EXTENDED RANGE, SUBSEASONAL, AND SEASONAL PERIODS) CAN 
INFLUENCE THE EVENTUAL OBSERVED FINAL MONTHLY TEMPERATURE AND PRECIPITATION 
PATTERNS. THESE CONSIDERATIONS RESULT IN RATHER SIZABLE AREAS DEPICTING EQUAL 
CHANCES (EC) FOR THE THREE FORECAST CATEGORIES. 
 
THE SEPTEMBER OUTLOOK DEPICTS ELEVATED ODDS OF ABOVE-NORMAL MONTHLY MEAN 
TEMPERATURES FOR MOST OF ALASKA BASED ON SIGNIFICANTLY ABOVE-AVERAGE SSTS OFF 
THE WEST COAST OF THE STATE (LOCATION OF THE GREATEST PROBABILITIES) AND THE 
MAJORITY OF DYNAMICAL MODEL GUIDANCE. FAVORED ABOVE-NORMAL TEMPERATURES ARE 
ALSO NOTED FOR THE FAR WESTERN CONUS AS SUBSEASONAL AND SEPTEMBER MONTHLY 
INTEGRATED FORECAST GUIDANCE ARE CONSISTENT IN DEPICTING WARMER THAN NORMAL 
CONDITIONS IN THESE AREAS. 
 
THE FORECAST FOR THE SOUTHWEST CONUS INDICATES ONLY SLIGHTLY INCREASED 
PROBABILITIES FROM CLIMATOLOGY FOR ABOVE NORMAL TEMPERATURES DUE TO CONFLICTING 
INFORMATION. STRONG POSITIVE LONG TERM TRENDS ARE OFFSET BY FORECAST GUIDANCE 
FAVORING ABOVE NORMAL RAINFALL AND BELOW NORMAL TEMPERATURES IN SOME AREAS. 
 
MOVING EASTWARD TO THE INTERIOR CONUS, EC IS FORECAST FOR THIS REGION DUE TO 
COMPETING FACTORS. THESE INCLUDE EARLY IN THE MONTH SUBSEASONAL FORECAST 
GUIDANCE FAVORING BELOW NORMAL TEMPERATURES, HIGH FORECAST UNCERTAINTY AND 
LIKELY LARGE VARIABILITY DURING THE SECOND HALF OF THE MONTH, AREAS OF DRY SOIL 
MOISTURE CONDITIONS AND LOW-FREQUENCY IMPACTS SOME OF WHICH WOULD TEND TO 
SUPPORT WARMER THAN NORMAL CONDITIONS IN SOME AREAS. STRONG POSITIVE LONG TERM 
TRENDS AND SOME FORECAST GUIDANCE INFORMATION PRIMARILY FAVOR ELEVATED ODDS OF 
ABOVE NORMAL TEMPERATURES ALONG THE GULF COAST AND IN THE NORTHEAST CONUS. 
 
THE SEPTEMBER PRECIPITATION OUTLOOK FAVORS ABOVE NORMAL TOTAL MONTHLY 
PRECIPITATION AMOUNTS FOR WESTERN ALASKA ASSOCIATED WITH POTENTIAL ELEVATED 
MOISTURE DUE TO LARGE POSITIVE SST DEPARTURES FROM CLIMATOLOGY IN COASTAL AREAS 
AS WELL AS SIGNATURES FROM MODEL FORECAST GUIDANCE. 
 
THE SOUTHWEST MONSOON IS ANTICIPATED TO REMAIN NEAR TO ABOVE AVERAGE DURING ITS 
FINAL MONTH AS ABOVE NORMAL SSTS OFF THE WESTERN COAST OF NORTH AMERICA ARE 
LIKELY TO MAINTAIN, ON AVERAGE, AN ACTIVE EAST PACIFIC OCEAN ITCZ AND TROPICAL 
CYCLONE SEASON RESULTING IN POTENTIAL MOISTURE SURGES INTO THE SOUTHWEST 
PORTION OF THE CONUS. 
 
STRONG SUBTROPICAL ATLANTIC RIDGING AND ASSOCIATED MEAN FLOW TO THE SOUTHEAST 
AND MID-ATLANTIC COAST FAVORS ENHANCED ODDS OF ABOVE NORMAL PRECIPITATION IN 
THESE AREAS. FORECAST GUIDANCE FAVORS BELOW NORMAL PRECIPITATION IN THE PACIFIC 
NORTHWEST WHILE CONDITIONS ACROSS THE CARIBBEAN AND ATLANTIC MAIN DEVELOPMENT 
REGION (MDR) LOWER THE ODDS FOR MOISTURE TRANSPORT TO THE GULF COAST AND LONG 
TRACK TROPICAL SYSTEMS TO ENTER AND PERSIST IN THE GULF OF MEXICO. THESE 
FACTORS FAVOR BELOW NORMAL MONTHLY PRECIPITATION AMOUNTS FOR THE TEXAS AND 
LOUISIANA COAST. 
 
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. 
 
AN UPDATED MONTHLY OUTLOOK... FOR SEP WILL BE ISSUED ON FRI AUGUST 31 2018 
 
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD. 
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