PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK 
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD 
830 AM EDT THU SEP 21 2017 
 
30-DAY OUTLOOK DISCUSSION FOR OCTOBER 2017 
 
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 AND SURFACE HIGH PRESSURE 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. 
 
 
 
AN UPDATED MONTHLY OUTLOOK... FOR OCT WILL BE ISSUED ON SAT SEPTEMBER 30 2017 
 
 
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD. 
 
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