Autumn outlook 

Posted: 9:40 am Friday, August 17th, 2018

By Kirk Mellish

No signs of anything too extreme this Fall one way or the other. That’s nice, because I think autumn is often one of our most lovely seasons in these parts.

First some modeling data.

The CFSv2 most recent run did an about face from a warm Fall to a cool one:

cfs_anom3_t2m_noram_201809_1

The ECMWF Ensemble next 6 weeks:

eps_t2m_1104h_conus_185

CANADIAN MODEL MONTHLIES:

cansips_anom_t2m_noram_2cansips_anom_t2m_noram_3cansips_anom_t2m_noram_4

The JAMSTECH MODEL FROM JMA FOR SEPT-NOVEMBER:

Screen Shot 2018-08-17 at 9.03.47 AMScreen Shot 2018-08-17 at 9.05.18 AM

The SEPTEMBER-NOVEMBER CONSENSUS OF AMERICAN/CANADIAN MODELS:

Screen Shot 2018-08-17 at 9.08.35 AM

Screen Shot 2018-08-17 at 9.09.06 AM

The NMME shows October temperatures about average then the following two months a tad above-normal. As for rainfall the NMME shows above-normal rain for September then near-normal for the following two months.

THE CONSENSUS OF INTERNATIONAL MODELS FOR SEPTEMBER-DECEMBER:

Screen Shot 2018-08-17 at 9.17.48 AM

International models consensus monthly breakdown:

Screen Shot 2018-08-17 at 9.20.53 AM

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THE NWS/NOAA/CPC FALL OUTLOOK SEPTEMBER-NOVEMBER:

off01_tempoff01_prcp

So in short it looks like the summer pattern will continue into the Fall for Georgia, but obviously average high and low temperatures are now declining. It looks like the 3-month period will average out near-normal to slightly warmer than normal with rainfall near-average to a little above-average in the mean. Not every day or every week but taken as a whole over the autumn season.

Here is the discussion from the Climate Prediction Center concerning their outlook:

THE SEPTEMBER-OCTOBER-NOVEMBER (SON) 2018 TEMPERATURE OUTLOOK INDICATES LIKELY 
ABOVE NORMAL TEMPERATURES ACROSS A MAJORITY OF THE FORECAST DOMAIN, ALTHOUGH 
EQUAL CHANCES OF BELOW, NEAR, OR ABOVE NORMAL TEMPERATURES ARE FORECAST FOR 
PARTS OF THE SOUTHEAST REGION. THE HIGHEST PROBABILITIES FOR ABOVE NORMAL 
TEMPERATURES OVER THE CONUS EXCEED 50 PERCENT FOR MUCH OF THE SOUTHWEST AND THE 
NORTHEAST. PROBABILITIES GREATER THAN 70 PERCENT FOR ABOVE NORMAL TEMPERATURES 
ARE INDICATED FOR NORTHWESTERN ALASKA. THE SON 2018 PRECIPITATION OUTLOOK 
INDICATES ENHANCED PROBABILITIES FOR ABOVE NORMAL PRECIPITATION FOR COASTAL 
STATES OF THE SOUTHEAST, AS WELL AS OVER THE AREA EXTENDING FROM SOUTHERN 
CALIFORNIA AND THE DESERT SOUTHWEST INTO THE GREAT BASIN AND CENTRAL ROCKIES. 
INCREASED CHANCES FOR ABOVE NORMAL PRECIPITATION ARE ALSO FORECAST ACROSS 
MAINLAND ALASKA. BELOW NORMAL PRECIPITATION IS MORE LIKELY FOR MUCH OF THE 
PACIFIC NORTHWEST. 
 
DURING THE AUTUMN AND WINTER 2018-19, THE TEMPERATURE AND PRECIPITATION 
OUTLOOKS ARE CONSISTENT WITH THE ELEVATED PROBABILITY OF EL NINO DEVELOPMENT 
AND ITS IMPACTS, WITH ADJUSTMENTS RELATED TO MODEL FORECASTS AND DECADAL 
TIMESCALE CLIMATE TRENDS. TEMPERATURE OUTLOOKS FOR WINTER 2018-2019 WERE 
MODIFIED OVER PARTS OF THE CENTRAL PLAINS AND SOUTHWEST REGIONS TO REPRESENT 
THE MODERATION OF PROBABILITIES FOR ABOVE NORMAL TEMPERATURES BY A POTENTIAL 
SHIFT OF THE JET STREAM AND STORM TRACKS SOUTHWARD, DUE TO THE IMPACTS OF A 
POTENTIAL EL NINO. PRECIPITATION OUTLOOKS FOR WINTER 2018-2019 WERE MODIFIED TO 
REMOVE ABOVE NORMAL PRECIPITATION AREAS FROM THE CENTRAL PLAINS, WHERE THE 
SIGNAL DUE TO EL NINO IS WEAK. AREAS OF PROBABLE ABOVE NORMAL PRECIPITATION IN 
LATE WINTER 2018-2019 AND EARLY SPRING 2019 OUTLOOKS WERE EXPANDED WESTWARD 
INTO SOUTHERN CALIFORNIA, REPRESENTING IMPACTS OF THE POTENTIAL SHIFT IN THE 
STORM TRACK DUE TO EL NINO. 
 
EQUAL CHANCES (EC) ARE FORECAST IN AREAS WHERE THE LIKELIHOOD OF SEASONAL MEAN 
TEMPERATURES OR SEASONAL ACCUMULATED PRECIPITATION AMOUNTS ARE EXPECTED TO BE 
SIMILAR TO CLIMATOLOGICAL PROBABILITIES. 
CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS 
 
OCEANIC AND ATMOSPHERIC OBSERVATIONS ACROSS THE EQUATORIAL PACIFIC INDICATE 
ENSO-NEUTRAL CONDITIONS HAVE PERSISTED INTO EARLY AUGUST. OBSERVED WEEKLY SSTS 
INDICATE POSITIVE ANOMALIES OVER ALMOST THE ENTIRE EQUATORIAL PACIFIC OCEAN, 
WITH ANOMALIES EXCEEDING 0.5 C GENERALLY WEST OF THE INTERNATIONAL DATE LINE. 
POSITIVE SUBSURFACE TEMPERATURE ANOMALIES HAVE PERSISTED INTO EARLY AUGUST, AT 
DEPTHS OF 50 TO 300 METERS FROM 130 E TO 130 W LONGITUDE AND NEAR THE SURFACE 
IN THE EASTERN PACIFIC. PERSISTENT SUBSURFACE POSITIVE HEAT ANOMALIES 
CONTRIBUTE TO THE CONTINUED LIKELIHOOD OF THE DEVELOPMENT OF EL NINO CONDITIONS 
BY AUTUMN 2018. 
 
OUTGOING LONGWAVE RADIATION (OLR) ANOMALIES INDICATED SUPPRESSED CONVECTION 
OVER PARTS OF THE WESTERN MARITIME CONTINENT AND NEAR THE INTERNATIONAL DATE 
LINE. LOW-LEVEL WINDS WERE NEAR AVERAGE OVER MOST OF THE EQUATORIAL PACIFIC, 
WHILE UPPER-LEVEL WESTERLY WIND ANOMALIES WERE OBSERVED OVER THE EASTERN 
TROPICAL PACIFIC OCEAN. OVERALL, ATMOSPHERIC CONDITIONS INDICATE THE 
CONTINUATION OF ENSO-NEUTRAL CONDITIONS.
THE CPC SST CONSOLIDATION (CON) FOR THE NINO 3.4 REGION PREDICTS THE LIKELIHOOD 
OF TEMPERATURE ANOMALIES GREATER THAN POSITIVE 0.5 DEGREES C TO BE GREATER THAN 
50 PERCENT BY ASO WITH POSITIVE ANOMALIES PEAKING AT ABOUT POSITIVE 1.0 DEGREES 
C DURING NDJ 2018-19. THE SST CONSTRUCTED ANALOG, MARKOV MODEL, AND CFS 
INDICATE PEAK ANOMALY VALUES GREATER THAN POSITIVE 1.0 DEGREES C THIS UPCOMING 
WINTER. THE CPC/IRI CONSENSUS FORECAST FAVORS THE ONSET OF EL NINO DURING THE 
NORTHERN HEMISPHERE AUTUMN WITH PROBABILITIES OF EL NINO AROUND 70 PERCENT 
DURING NDJ AND DJF 2018-19. 
 
PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS 
 
THE CONTINUED LIKELIHOOD OF EL NINO DEVELOPMENT IS A LARGE FACTOR IN THE AUTUMN 
2018 AND WINTER 2018-19 OUTLOOKS. TOOLS USED FOR THE SEASONAL OUTLOOKS INCLUDED 
DYNAMICAL MODEL FORECAST GUIDANCE SUCH AS THE NORTH AMERICAN MULTI-MODEL 
ENSEMBLE (NMME), AS WELL AS STATISTICAL GUIDANCE SUCH AS THE ENSO-OCN 
REGRESSION FORECAST, THE SST-BASED CONSTRUCTED ANALOG, AND CANONICAL 
CORRELATION ANALYSIS (CCA). A RELATIVELY NEW TOOL IN THE CPC SUITE IS THE CBAM, 
OR CALIBRATION, BRIDGING, AND MERGING FORECAST TOOL. THIS USES A BAYESIAN 
APPROACH TO THE NMME MODEL CALIBRATION AND CONSOLIDATION, AS WELL AS A HYBRID 
STATISTICAL-DYNAMICAL FORECAST, IN WHICH NMME CONSTITUENT-MODEL NINO 3.4 
FORECASTS ARE STATISTICALLY BRIDGED TO PREDICTED TEMPERATURE AND PRECIPITATION 
USING A BAYESIAN MODEL. THE OUTLOOKS DURING THE 2019 WARM SEASON WERE BASED 
PRIMARILY ON THE ENSO-OCN TOOL, WHICH INCORPORATES DECADAL CLIMATE TRENDS AND 
ANY RESIDUAL IMPACTS OF ENSO VARIABILITY.
TEMPERATURE 
 
ABOVE NORMAL TEMPERATURES ARE LIKELY FOR MUCH OF THE CONUS AND ALASKA DURING 
SON 2019 WITH THE HIGHEST PROBABILITIES OVER THE CONUS FORECAST FOR THE FOUR 
CORNERS AND NORTHEAST REGIONS, BASED ON CALIBRATED NMME FORECASTS, USING 
PROBABILITY ANOMALY CORRELATIONS, AND DECADAL CLIMATE WARMING. THE CBAM 
FORECASTS INDICATE GREATER UNCERTAINTY AND WEAKER PROBABILITIES FOR AREAS OF 
THE NORTHWEST CONUS THAN THE PREVIOUS OUTLOOK. PROBABILITIES OF ABOVE NORMAL 
TEMPERATURES WERE ALSO DECREASED SLIGHTLY FOR PARTS OF THE SOUTHWEST IN THE DJF 
AND JFM 2019 OUTLOOKS, BASED ON THE LATEST CALIBRATED NMME MODEL GUIDANCE. 
EQUAL CHANCES FOR BELOW, NEAR, OR ABOVE NORMAL TEMPERATURES ARE FORECAST FOR 
PARTS OF THE SOUTHEAST REGION FROM AUTUMN INTO WINTER OUTLOOKS, WHERE 
TEMPERATURES ARE MODERATED BY THE POTENTIAL IMPACTS OF A DEVELOPING EL NINO, 
AND UNCERTAINTY IS GREATEST. 
 
THE ELEVATED CHANCES FOR EL NINO DEVELOPMENT WERE A FACTOR IN THE TEMPERATURE 
OUTLOOKS, BEGINNING IN THE AUTUMN AND BECOMING MORE PROMINENT DURING THE WINTER 
2018-19. BASED ON REGRESSIONS FROM THE CPC NINO 3.4 SST CONSOLIDATION 
FORECASTS, PROBABILITIES FOR ABOVE NORMAL TEMPERATURES ARE GREATEST, EXCEEDING 
50 PERCENT, FOR PARTS OF THE PACIFIC NORTHWEST AND OVER PARTS OF THE NORTHERN 
PLAINS AND GREAT LAKES REGION, DURING WINTER SEASONS. AN EXPECTED LACK OF SEA 
ICE ELEVATES CHANCES FOR ABOVE NORMAL TEMPERATURES FOR WESTERN AND NORTHERN 
COASTAL AREAS OF ALASKA DURING THE AUTUMN SEASONAL OUTLOOKS. TEMPERATURE 
OUTLOOKS AT LONGER LEAD TIMES, FOLLOWING THE FMA 2019 SEASON, INDICATE ELEVATED 
PROBABILITIES OF ABOVE NORMAL TEMPERATURES FOR MOST OF THE FORECAST DOMAIN, 
PRIMARILY BASED ON DECADAL CLIMATE TRENDS. CLIMATE TREND SIGNALS ARE WEAKEST 
FOR PARTS OF THE NORTHERN CENTRAL CONUS FROM MJJ THROUGH ASO 2018, WHERE EQUAL 
CHANCES OF BELOW, NEAR AND ABOVE NORMAL TEMPERATURES ARE INDICATED. 
 
PRECIPITATION 
 
THE PRECIPITATION OUTLOOKS ARE BASED PRIMARILY ON THE LATEST DYNAMICAL MODEL 
GUIDANCE FROM THE NMME AND IMPACTS OF A POTENTIAL EL NINO, FOR SON 2018 THROUGH 
MAM 2019. ABOVE NORMAL PRECIPITATION IS LIKELY FOR A LARGE AREA OF THE 
SOUTHWEST CONUS IN THE SON 2018 OUTLOOK, EXTENDING NORTHWARD AS FAR AS SOUTHERN 
IDAHO AND WYOMING, WITH THE PREDICTION OF AN ENHANCED MONSOON, AS WELL AS THE 
POTENTIAL FOR MOISTURE SURGES FROM THE EASTERN TROPICAL PACIFIC, WHICH EXPANDS 
THE AREA OF PROBABLE ABOVE NORMAL PRECIPITATION INTO SOUTHERN CALIFORNIA. 
OUTLOOKS FOR OND 2018 THROUGH MAM 2019 RESEMBLE THE CANONICAL PATTERN OF EL 
NINO IMPACTS, WITH ENHANCED CHANCES OF ABOVE NORMAL PRECIPITATION FOR PARTS OF 
THE SOUTHERN CONUS, AND BELOW NORMAL PRECIPITATION LIKELY IN THE PACIFIC 
NORTHWEST THROUGH DJF 2018-2019 AND FOR THE MIDWEST REGION LATER IN THE 
UPCOMING WINTER. THE LATEST PRECIPITATION FORECASTS FROM THE NMME AND CBAM 
INCREASE PROBABILITIES OF ABOVE NORMAL PRECIPITATION FOR PARTS OF THE SOUTHWEST 
IN EARLY WINTER 2018-2019. PROBABILITIES OF ABOVE NORMAL PRECIPITATION ARE 
ENHANCED FOR SOUTHERN ALASKA FROM SON 2018 THROUGH FMA 2019, AS INDICATED BY 
TOOLS BASED ON THE IMPACTS OF EL NINO, SUCH AS THE ENSO-OCN AND BRIDGED 
NMME-BASED CBAM FORECASTS. PROBABILITIES OF ABOVE NORMAL PRECIPITATION ARE 
ENHANCED FOR OTHER AREAS OF ALASKA, INCLUDING WESTERN AND NORTHERN COASTAL 
REGIONS, DUE TO POSITIVE DECADAL TRENDS IN PRECIPITATION RELATED TO POSITIVE 
ATMOSPHERIC TEMPERATURE TRENDS AND REDUCED SEA ICE COVER. 
 
THE PRECIPITATION OUTLOOKS FOR AMJ 2019 THROUGH SON 2019 ARE PRIMARILY BASED ON 
DECADAL CLIMATE TRENDS. PRECIPITATION TRENDS ENHANCE THE CHANCES OF ABOVE 
NORMAL PRECIPITATION FOR PARTS OF THE NORTHEAST CONUS FROM AMJ THROUGH SON 
2019, AND ENHANCE THE CHANCES OF BELOW NORMAL PRECIPITATION FOR PARTS OF THE 
NORTHWEST CONUS IN MJJ THROUGH ASO 2019. 

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