PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD 300 PM EDT THU AUG 31 2017 30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2017 THE BACKGROUND CLIMATE DRIVERS REMAIN UNCHANGED FROM THE 0.5-MONTH LEAD FORECAST. THE MJO HAS RECENTLY EMERGED OVER THE INDIAN OCEAN, BUT ITS FUTURE EVOLUTION IS UNCERTAIN. THE DEVELOPMENT OF HURRICANE IRMA IN THE ATLANTIC BASIN IS CONSISTENT WITH THE CURRENT MJO EVOLUTION, BUT BEYOND THAT THERE IS LITTLE TROPICAL VARIABILITY TO HARVEST FOR THE MONTHLY FORECAST AT THIS TIME. THE UPDATED MONTHLY OUTLOOK MAINLY TAKES INTO ACCOUNT THE LATEST FORECASTS FOR THE NEXT ONE TO TWO WEEKS, WITH SOME MINOR INFLUENCE FROM THE LATEST WEEKS 3-4 GUIDANCE. THE FORECAST RIDGE/TROUGH PATTERN THAT WAS DISCUSSED FOR THE 0.5-MONTH LEAD FORECAST IS FORECAST TO BE ESPECIALLY AMPLIFIED DURING THE FIRST 5-10 DAYS OF THE MONTH, THOUGH SLIGHTLY FARTHER EAST THAN WAS FORECAST A COUPLE OF WEEKS AGO. THIS AMPLIFIED PATTERN DOMINATES THE UPDATED MONTHLY FORECAST, WITH SOME TENDENCY TOWARD THE LONG-TERM TREND MORE LIKELY LATE IN THE MONTH. THE IMPACT OF THIS ON THE MONTHLY FORECAST IS TO SHIFT AREAS OF PROBABILITIES EAST, WITH BELOW-NORMAL TEMPERATURES FAVORED OVER MUCH OF THE EAST-CENTRAL CONUS. PROBABILITIES FAVORING ABOVE-NORMAL TEMPERATURES OVER THE EAST COAST HAVE RETREATED FOR ALL BUT FAR EASTERN NEW ENGLAND AND THE FLORIDA PENINSULA. PROBABILITIES FAVORING ABOVE-NORMAL TEMPERATURES OVER THE WESTERN CONUS HAVE BEEN ELEVATED, WITH VERY WARM TEMPERATURES NOW ANTICIPATED EARLY IN THE MONTH AND NO DISCERNIBLE SIGN OF A NOTABLE PATTERN CHANGE COMING LATER IN THE MONTH FROM ANY FORECAST TOOL. PARTS OF THE CENTRAL CONUS RETAIN A GOOD DEAL OF UNCERTAINTY, WITH A BACK-AND-FORTH PATTERN DURING THE FIRST TWO WEEKS OF THE MONTH EXPECTED TO LEAVE PARTS OF THAT REGION SITTING NEAR NORMAL. THE WEEK 3-4 FORECAST GUIDANCE DOES NOT INDICATE ANY STRONG SIGNALS FOR LATE MONTH OVER THE MIDWEST AND MOST OF THE PLAINS, FOR EXAMPLE, AND TRENDS FAVORING ABOVE-NORMAL TEMPERATURES ARE QUITE STRONG. THE PRECIPITATION OUTLOOK IS BROADLY SIMILAR TO THE MID-MONTH OUTLOOK, BUT PROBABILITIES ARE ENHANCED OVER REGIONS EXPECTING HEAVY RAINFALL OVER THE NEXT SEVERAL DAYS. THIS INCLUDES AREAS AFFECTED BY THE REMNANTS OF HURRICANE HARVEY AT THE BEGINNING OF THE MONTH. IN ADDITION, ODDS FAVORING BELOW-NORMAL PRECIPITATION ARE ADDED OVER PARTS OF THE NORTHWEST AND PLAINS. THIS WAS DEBATED FOR THE MID-MONTH OUTLOOK, AND THE VARIOUS FORECAST TOOLS HAVE CONVERGED ON A FAIRLY CONFIDENT BELOW-NORMAL RAINFALL FORECAST CENTERED OVER THE NORTH-CENTRAL PLAINS. KEY AREAS OF UNCERTAINTY INCLUDE THE GULF COAST AND FLORIDA, AS WELL AS PARTS OF THE WEST. THERE IS LIKELY TO BE SOME TROPICAL MOISTURE DIRECTED ALONG AN APPROACHING COLD FRONT EARLY IN THE MONTH OVER THE GULF COAST, AND THE LATEST GUIDANCE FROM THE ECMWF SUGGESTS THAT HURRICANE IRMA COULD IMPACT FLORIDA OR PARTS OF THE GULF COAST. OVER THE NORTHWEST, CLIMATOLOGICAL PRECIPITATION AMOUNTS BEGIN TO INCREASE DURING THE MONTH, WITH ONLY FAIRLY WEAK FORECAST SIGNALS PRESENT FOR THE SECOND HALF OF THE MONTH. PARTS OF THE GREAT BASIN AND CENTRAL ROCKIES ARE FORECAST TO SEE LOCALLY HEAVY RAINFALL EARLY IN THE MONTH, AND THE FORMER COULD BE IMPACTED BY A MOISTURE SURGE ASSOCIATED WITH TROPICAL STORM LIDIA IN THE EAST PACIFIC. THE PREVIOUS DISCUSSION, ISSUED AUGUST 17, FOLLOWS BELOW: ---------------------------------------------------------- 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 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, CONSISTED 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. THE NEXT MONTHLY OUTLOOK...FOR OCT ... WILL BE ISSUED ON THU SEP 21 2017 THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD. $$