PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK 
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD 
300 PM EDT SAT SEP 30 2017 
 
30-DAY OUTLOOK DISCUSSION FOR OCTOBER 2017 
 
THE UPDATE TO THE OCTOBER TEMPERATURE AND PRECIPITATION OUTLOOK REQUIRED 
SUBSTANTIAL CHANGES FROM THE PRELIMINARY OUTLOOK. THIS IS NOT SURPRISING GIVEN 
THAT THIS TIME OF YEAR TENDS TO OBSERVE SOME OF THE HIGHEST VARIABILITY, LARGE 
UNCERTAINTY AND SO OFTEN THE LOWEST PREDICTABILITY DURING ANY TIME OF THE YEAR. 
IT APPEARS NOW THAT FOR MUCH OF THE FIRST HALF OF THE MONTH THE UPPER LEVEL 
PATTERN FAVORED IS FOR A PERIOD OF ROBUST TROUGHING ACROSS THE WEST AND LATER 
WEST-CENTRAL CONUS AND RIDGING AND LARGE POSITIVE HEIGHT DEPARTURES FOR MUCH OF 
THE CENTRAL AND EASTERN CONUS. LATER DURING THE SECOND HALF OF THE MONTH, THERE 
ARE SOME SIGNS THAT THIS PATTERN MAY RETROGRADE SO THAT TROUGHING FOCUSES 
CLOSER TO THE WEST COAST AND MORE RIDGING MAY RETURN TO THE CENTRAL PORTION OF 
THE CONUS. THE MONTHLY UPDATE IS BASED PRIMARILY ON BIAS-CORRECTED AND 
CALIBRATED DYNAMICAL MODEL GUIDANCE SPANNING MULTIPLE TIME SCALES AND TO A 
LESSER EXTENT CONSIDERATIONS FROM LONG TERM TRENDS. 
 
BASED ON THE FAVORED PATTERN NOTED ABOVE DURING THE FIRST HALF OF THE MONTH, 
THE TEMPERATURE OUTLOOK HAD LARGE CHANGES AND CURRENTLY DEPICTS ELEVATED ODDS 
FOR BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE NORTHERN AND CENTRAL ROCKIES 
WITH ABOVE NORMAL TEMPERATURES NOW FAVORED FOR THE SOUTHEAST QUARTER OF THE 
CONUS. ABOVE NORMAL TEMPERATURES CONTINUE TO BE MOST LIKELY FOR THE NORTHERN 
TIER OF THE CENTRAL AND EASTERN CONUS, BUT WITH CONSIDERABLY HIGHER 
PROBABILITIES ESPECIALLY IN THE NORTHEAST. THE TEMPERATURE OUTLOOK FOR ALASKA 
IS UNCHANGED. 
 
CONSIDERABLE ADJUSTMENTS ARE REQUIRED FOR THE OCTOBER PRECIPITATION OUTLOOK AS 
WELL AS FAVORED BELOW NORMAL PRECIPITATION ONCE INDICATED IN THE INTERIOR OF 
THE CONUS HAS BEEN SHIFTED SOUTHEASTWARD WITH ABOVE NORMAL PRECIPITATION NOW 
MOST LIKELY ACROSS AREAS OF THE NORTH CENTRAL CONUS - A RESULT OF FAVORED 
UPSTREAM TROUGHING DURING MUCH OF THE FIRST PORTION OF THE MONTH. THE OUTLOOK 
FOR ABOVE NORMAL PRECIPITATION IN PROXIMITY TO FLORIDA IS MAINLY UNCHANGED FROM 
THE PRELIMINARY OUTLOOK (ONLY A SLIGHT INCREASE IN ODDS FOR FLORIDA). DUE TO 
THE SHIFT IN THE NOW MORE LIKELY PATTERN FROM WHAT WAS ANTICIPATED EARLIER IN 
SEPTEMBER, THE HIGHLIGHTED AREAS OF ABOVE-NORMAL AND BELOW NORMAL PRECIPITATION 
FOR THE PACIFIC NORTHWEST / ALASKA PANHANDLE AND SOUTH COAST OF ALASKA 
RESPECTIVELY ARE NO LONGER INDICATED IN THE UPDATED OUTLOOK. ABOVE NORMAL 
PRECIPITATION IS FAVORED FOR SOUTHWEST ALASKA DUE TO ANTICIPATED TROUGHING 
UPSTREAM OF THIS REGION DURING THE FIRST PART OF OCTOBER. 
 
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PREVIOUS DISCUSSION FROM EARLIER IN THE MONTH IS BELOW 
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THE OCTOBER 2017 TEMPERATURE AND PRECIPITATION OUTLOOKS ARE BASED ON A NUMBER 
OF FACTORS THAT INCLUDE DYNAMICAL MODEL GUIDANCE SPANNING A FEW DIFFERENT 
FORECAST TARGET PERIODS, THE INFLUENCE OF LONG TERM TRENDS, STATISTICAL TOOLS, 
AND TO VARYING DEGREES AND IN SOME AREAS SEA SURFACE TEMPERATURE (SSTS) AND 
SOIL MOISTURE CONDITIONS. ALTHOUGH ODDS FOR LA NINA HAVE INCREASED RECENTLY, 
ANY IMPACT AT THIS STAGE OF ITS POTENTIAL DEVELOPMENT, THE CURRENT TIME OF THE 
YEAR, AND THE MONTHLY OUTLOOK TARGET LENGTH CONSIDERABLY MARGINALIZES ANY 
INFLUENCE ON THE OUTLOOK AT THIS TIME. ALTHOUGH SUBSEASONAL TROPICAL 
VARIABILITY (ATMOSPHERIC KELVIN WAVE ACTIVITY) HAS BEEN EVIDENT DURING 
SEPTEMBER, THE EMERGENCE OF A MORE ROBUST, GLOBAL PROPAGATING MJO SIGNAL HAS 
BEEN DIFFICULT TO ORGANIZE AND FORECASTS OF ITS EVOLUTION IN THE COMING WEEKS 
DO NOT INDICATE THAT THIS WILL CHANGE. 
 
DYNAMICAL MODEL GUIDANCE FOR MUCH OF OCTOBER TENDS TO INDICATE, ON AVERAGE, AN 
ANOMALOUS TROUGH - RIDGE - TROUGH PATTERN EXTENDING ACROSS THE NORTH PACIFIC 
AND CONUS. THERE IS SOME DIFFERENCE IN THE LOCATIONS OF THE SOME OF THE 
FEATURES AMONG THE GUIDANCE, BUT TO FIRST ORDER THIS APPEARS TO BE PREFERRED 
PREDICTED PATTERN. ABOVE NORMAL TEMPERATURES ARE MOST LIKELY FOR THE WESTERN 
CONUS, CONSISTENT WITH ANOMALOUS RIDGING, AND THIS HIGHLIGHTED AREA EXTENDS 
EASTWARD ACROSS THE NORTHERN TIER OF THE CONUS TO THE NORTHEAST. LONG TERM 
POSITIVE TEMPERATURE TRENDS ARE STRONG ACROSS MUCH OF THE CONUS THIS TIME OF 
THE YEAR AND PLAYED A ROLE IN THE OUTLOOK IN ALL OF THESE AREAS. TEMPERATURE 
GUIDANCE WAS CONSISTENT FOR AN ENHANCED LIKELIHOOD FOR ABOVE NORMAL 
TEMPERATURES IN THESE AREAS FROM THE WEEK 3-4 PERIOD AND MONTHLY PERIOD AS A 
WHOLE AS DEPICTED FROM THE CFS, NMME AND IMME SUITE OF MODELS. 
 
STRONG POSITIVE TEMPERATURE TRENDS FAVOR HIGH PROBABILITIES FOR ABOVE NORMAL 
TEMPERATURES FOR THE NORTH SLOPE OF ALASKA DUE TO NEGATIVE TRENDS IN SEA ICE 
COVERAGE AND SLOWER RETURN OF SEA ICE COMPARED TO THE LONGER PERIOD 
CLIMATOLOGY. PROSPECTS FOR ANOMALOUS TROUGHING AND NEGATIVE 500-HPA HEIGHT 
ANOMALIES, WET SOIL MOISTURE CONDITIONS AND CONSISTENT SIGNALS IN CALIBRATED 
DYNAMICAL MODEL TEMPERATURE GUIDANCE FAVOR A SMALL REGION OF BELOW NORMAL 
TEMPERATURES IN THE TENNESSEE VALLEY. THERE IS MARGINAL SUPPORT FOR A SLIGHT 
TILT IN THE ODDS OF ABOVE NORMAL TEMPERATURES FOR THE SOUTHERN HALF OF THE 
FLORIDA PENINSULA. 
 
THE PRECIPITATION OUTLOOK IS PRIMARILY DERIVED FROM THE FAVORED ANTICIPATED 
CIRCULATION PATTERN DESCRIBED EARLIER AND CALIBRATED DYNAMICAL MODEL GUIDANCE 
FROM THE WEEK 3-4 PERIOD, AS WELL AS MONTHLY PREDICTIONS FROM THE CFS, NMME AND 
IMME MODEL SUITE. ABOVE NORMAL PRECIPITATION IS FAVORED FOR PARTS OF THE 
PACIFIC NORTHWEST, ALASKA AND EXTREME SOUTHEAST CONUS, PRIMARILY FLORIDA. 
FAVORED UPPER LEVEL CONVERGENCE ACROSS AREAS IN THE INTERIOR OF THE CONUS TO 
THE GULF COAST FAVOR BELOW NORMAL PRECIPITATION IN THIS REGION WHILE MARGINAL 
SUPPORT EXISTS FROM CALIBRATED DYNAMICAL MODEL GUIDANCE FOR BELOW NORMAL 
PRECIPITATION FOR PARTS OF THE SOUTH COAST OF ALASKA. 
 
 
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. 
 
 
 
THE NEXT MONTHLY OUTLOOK...FOR NOV ... WILL BE ISSUED ON THU OCT 19 2017 
 
 
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD. 
 
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