PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
830 AM EDT THU AUG 17 2017

30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2017

THE SEPTEMBER 2017 TEMPERATURE AND PRECIPITATION OUTLOOKS ARE BASED ON 
DYNAMICAL MODEL GUIDANCE, STATISTICAL TOOLS, AND TO A LESSER EXTENT, SOIL 
MOISTURE ANOMALIES. ENSO IS NOT A FACTOR IN THESE OUTLOOKS SINCE IT IS IN A 
NEUTRAL STATE AND IS PREDICTED TO REMAIN NEUTRAL. PLEASE SEE THE ENSO 
DIAGNOSTIC DISCUSSION AT: 
HTTP://WWW.CPC.NCEP.NOAA.GOV/PRODUCTS/ANALYSIS_MONITORING/ENSO_ADVISORY/ENSODISC
.SHTML FOR A MORE DETAILED ANALYSIS OF SEA SURFACE TEMPERATURES ACROSS THE 
EQUATORIAL PACIFIC. THE MADDEN-JULIAN OSCILLATION (MJO) WAS ACTIVE DURING THE 
SECOND HALF OF JULY AND INTO VERY EARLY AUGUST, BUT HAS WEAKENED OVER THE PAST 
COUPLE OF WEEKS. THE LATEST DYNAMICAL MODEL GUIDANCE SUGGESTS THAT A MORE 
COHERENT SIGNAL MIGHT EMERGE OVER THE INDIAN OCEAN OVER THE NEXT WEEK OR TWO. 
WHILE THIS MAY PLAY A ROLE IN MODULATING TROPICAL CYCLONE ACTIVITY DURING OVER 
THE COURSE OF THE COMING WEEKS, THERE IS LITTLE TO BE HARVESTED IN TERMS OF 
TEMPERATURE AND PRECIPITATION PREDICTION FOR SEPTEMBER 2017.

THE LATEST RUNS OF THE CFS, AS WELL AS WEEK 3-4 GUIDANCE FROM THE ECMWF, DEPICT 
A REASONABLY HIGH AMPLITUDE TROUGH-RIDGE-TROUGH PATTERN EXTENDING FROM THE 
NORTHEASTERN PACIFIC THROUGH EAST-CENTRAL NORTH AMERICA AT LEAST THROUGH 
MID-SEPTEMBER. THIS PATTERN IS LARGELY CONSISTENT WITH THE LOW-FREQUENCY 
PATTERN OBSERVED OVER NORTH AMERICA OVER THE PAST 45-90 DAYS. SUCH A PATTERN 
FAVORS ABOVE-NORMAL TEMPERATURES OVER ALASKA AND MUCH OF THE WESTERN CONUS, AS 
WELL AS OVER PARTS OF THE NORTHEAST AND THE IMMEDIATE EASTERN SEABOARD AND GULF 
COAST REGION. THOSE LATEST DYNAMICAL TOOLS FAVOR BELOW-NORMAL TEMPERATURES OVER 
MUCH OF THE SOUTH-CENTRAL AND INTERIOR SOUTHEASTERN CONUS. THE DYNAMICAL TOOLS 
THAT WERE INITIALIZED EARLY IN THE MONTH FAVOR THE SAME GENERAL PATTERN, BUT 
WITH REDUCED PROBABILITIES FOR BELOW-NORMAL TEMPERATURES OVER PARTS OF THE 
SOUTHEASTERN AND SOUTH-CENTRAL U.S. SOIL MOISTURE DECREASES IN IMPORTANCE AS A 
LAGGED PREDICTOR OVER THE SOUTH-CENTRAL CONUS FROM AUGUST THROUGH OCTOBER. SOME 
MODEST COOLING EFFECT IS POSSIBLE WHERE POSITIVE SOIL MOISTURE ANOMALIES ARE 
THE GREATEST FOR PARTS OF THE CENTRAL AND SOUTHERN U.S., ESPECIALLY THROUGH THE 
FIRST HALF OF SEPTEMBER.

THE TEMPERATURE FORECAST IS BASED LARGELY ON THE CALIBRATED NMME GUIDANCE, BUT 
TWEAKED TOWARD THE LATEST DYNAMICAL GUIDANCE THAT FAVORS A CONTINUATION OF THE 
WARM SEASON LOW-FREQUENCY PATTERN INTO AT LEAST THE FIRST HALF OF SEPTEMBER. 
THIS LEADS TO A LOW AMPLITUDE TWO-CATEGORY CHANGE OVER THE CENTRAL CONUS, WHERE 
BELOW-NORMAL TEMPERATURES ARE SLIGHTLY FAVORED DURING SEPTEMBER AND 
ABOVE-NORMAL TEMPERATURES ARE SLIGHTLY FAVORED FOR THE SEP-OCT-NOV SEASON. THE 
LOW-FREQUENCY TREND SIGNAL IS MORE LIKELY TO EMERGE IN OCTOBER AND NOVEMBER AS 
WE UNDERGO SEASONAL TRANSITION. OVER ALASKA, THE GREATEST PROBABILITIES FOR 
ABOVE-NORMAL TEMPERATURES ARE ALONG COASTAL REGIONS, WHERE WARMER-THAN-NORMAL 
SSTS ARE LIKELY TO HAVE SOME INFLUENCE ALONG WITH ANOMALOUSLY LOW SEA ICE 
EXTENT.

THE PRECIPITATION OUTLOOK FOR SEPTEMBER HAS LIMITED COVERAGE GIVEN THE 
GENERALLY POOR SKILL AT THESE LEAD TIMES THIS TIME OF YEAR. THE CALIBRATED NMME 
OUTLOOK IS USED AS A STARTING POINT, WITH THE COVERAGE OF PROBABILITIES 
FAVORING ABOVE-NORMAL RAINFALL ADJUSTED TO ALIGN WITH THE REGION FORECAST TO BE 
JUST EAST OF THE FORECAST ANOMALOUS TROUGH AXIS. THERE IS SOME REASON TO FAVOR 
BELOW-NORMAL PRECIPITATION OVER PARTS OF THE NORTH-CENTRAL AND NORTHWESTERN 
CONUS GIVEN THE LOW-FREQUENCY PATTERN AND THE FORECAST CIRCULATION PATTERN 
THROUGH MID-SEPTEMBER (DISCUSSED ABOVE). HOWEVER, FORECAST TOOLS THAT WERE 
INITIALIZED EARLIER, FOR WHICH INITIAL ATMOSPHERIC CONDITIONS ARE LESS 
IMPORTANT, SHOW LARGE UNCERTAINTY OVER THOSE REGIONS. GIVEN THE SIGNIFICANCE OF 
THE ONGOING DROUGHT OVER PARTS OF THE NORTH-CENTRAL U.S., THIS AREA WILL BE 
MONITORED CLOSELY FOR THE UPDATED OUTLOOK ISSUED ON AUGUST 31. OVER ALASKA, THE 
TOOLS WERE IN REASONABLY GOOD AGREEMENT FAVORING ABOVE-NORMAL PRECIPITATION 
ALONG THE SOUTHERN COAST, WHICH IS ALSO CONSISTENT WITH THE FORECAST 
CIRCULATION PATTERN. THE CALIBRATED NMME FAVORS ABOVE-NORMAL PRECIPITATION FOR 
THE NORTH SLOPE, CONSISTENT WITH FORECAST ABOVE-NORMAL TEMPERATURES AND 
INCREASED MOISTURE AVAILABILITY.


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 SEP WILL BE ISSUED ON THU AUGUST 31 2017


THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD.

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