PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
830 AM EDT THU SEP 20 2018


30-DAY OUTLOOK DISCUSSION FOR OCTOBER 2018

ON MOST OCCASIONS, PREDICTABILITY REACHES A GENERAL MINIMUM OVERALL DURING THE 
MONTH OF OCTOBER FOR THE U.S. FORECAST DOMAIN AS IT IS IN THE MIDDLE OF THE 
SHOULDER SEASON (SEP-OCT-NOV) THAT RESIDES BETWEEN THE TRANSITION FROM THE 
SUMMER MONTHS TO THE COLD SEASON. THE HALF-MONTH LEAD MONTHLY FORECAST SUCH AS 
THE OCTOBER TEMPERATURE AND PRECIPITATION OUTLOOK PRESENTED HERE INCLUDES 
INFORMATION ACROSS MULTIPLE TIME SCALES (THE EXTENDED RANGE, SUBSEASONAL, AND 
THE SEASONAL BACKGROUND STATE). THESE FACTORS INFLUENCE THE EVENTUAL OBSERVED 
MONTHLY MEAN TEMPERATURE AND TOTAL PRECIPITATION AMOUNTS. THE TYPICAL GREATER 
UNCERTAINTY RESULTS IN A RATHER SIZABLE FORECAST AREA DEPICTING EQUAL CHANCES 
(EC) FOR THE INITIAL OCTOBER OUTLOOK.

A REVIEW OF THE MJO CURRENTLY INDICATES THAT IT REMAINS GENERALLY POORLY 
DEFINED AND THE ANOMALOUS TROPICAL CONVECTION IN RECENT WEEKS HAS BEEN 
DOMINATED BY HIGHER FREQUENCY TROPICAL SUBSEASONAL VARIABILITY SUCH AS 
ATMOSPHERIC KELVIN WAVES AND BURSTS IN TROPICAL CYCLONE ACTIVITY. SOME MODELS 
DO INDICATE POTENTIAL IMPROVED MJO ORGANIZATION ACROSS THE WESTERN HEMISPHERE 
AS WE ENTER EARLY OCTOBER, BUT UNTIL THAT PROVES TO BE THE CASE, POTENTIAL MJO 
INFLUENCE WAS NOT LARGELY UTILIZED IN THE CURRENT OUTLOOK. 

THE OCTOBER TEMPERATURE OUTLOOK FAVORS ABOVE NORMAL TEMPERATURES FOR ALL OF 
ALASKA AND MOST OF THE WESTERN CONUS WITH HIGH PROBABILITIES FOR MUCH OF THE 
ALASKA REGION INCLUDING GREATER THAN 80% PROBABILITY FOR PORTIONS OF NORTHWEST 
MAINLAND ALASKA. THE TRENDS ASSOCIATED WITH LATE ARRIVAL OF SEA ICE, MUCH ABOVE 
NORMAL SEA SURFACE TEMPERATURES (SSTS) ALONG MUCH OF THE COASTAL AREAS OF THE 
STATE, AND STRONG RIDGING AND SOUTHERLY FLOW ANTICIPATED EARLY IN THE MONTH ARE 
THE BASIS FOR THIS FORECAST. FOR THE WESTERN CONUS, NEARLY UNANIMOUS DYNAMICAL 
MODEL FORECAST GUIDANCE, LARGE AREAS OF BELOW NORMAL SOIL MOISTURE CONDITIONS 
AND STRONG LONG TERM TEMPERATURE TRENDS SUPPORT THE FORECAST IN THIS REGION.

ABOVE NORMAL TEMPERATURES ARE ALSO MOST LIKELY FOR AREAS OF THE SOUTHEAST, 
TENNESSEE VALLEY, AND MID-ATLANTIC AS SUBTROPICAL RIDGING, ABOVE NORMAL 500-HPA 
HEIGHT DEPARTURES, AND SURFACE MEAN SOUTHERLY FLOW ARE STILL ANTICIPATED TO 
PLAY A ROLE DURING THE MONTH. THE PROBABILITIES ARE MORE MODEST, HOWEVER, AS 
DYNAMICAL MODEL FORECAST GUIDANCE IS LESS DEFINITIVE IN THIS REGION AND SOME 
AREAS HAVE SUBSTANTIAL POSITIVE SOIL MOISTURE DEPARTURES AND PERHAPS REMAINING 
STANDING WATER FROM AN OBSERVED VERY WET SUMMER AND, MOST RECENTLY, IMPACTS 
FROM HURRICANE FLORENCE.

THE MOST CHALLENGING ASPECT OF THE TEMPERATURE OUTLOOK RESIDES ACROSS THE 
NORTHERN AND CENTRAL PLAINS, GREAT LAKES AND OHIO VALLEY REGIONS. INDICATIONS 
IN LATE SEPTEMBER ARE THAT A ROBUST RIDGE WILL DEVELOP ACROSS ALASKA WHICH MAY 
FAVOR DOWNSTREAM TROUGHING ON AVERAGE DURING EARLY PORTIONS OF OCTOBER. THE 
LATEST WEEK 3-4 FORECAST GUIDANCE FROM THE CFS AND ECMWF FAVOR THIS TO PERSIST 
SIGNIFICANTLY INTO OCTOBER WITH ELEVATED ODDS FOR BELOW NORMAL TEMPERATURES. IN 
ADDITION TO THE GENERAL UNCERTAINTY FOR THE FINAL TWO WEEKS IN OCTOBER, THIS IS 
ALSO AT ODDS WITH NMME DYNAMICAL MODEL GUIDANCE AND LONG TERM TRENDS SO EC IS 
CURRENTLY FORECAST FOR MUCH OF THIS AREA, HOWEVER, A REGION IS HIGHLIGHTED 
FAVORING BELOW NORMAL TEMPERATURES IN THE CENTRAL PLAINS AT THIS TIME. 

AREAS SHOWN IN WHITE LABELED EC DEPICT A FORECAST WHERE THE PROBABILITIES FOR 
EITHER OF THREE CATEGORIES, BELOW-, NEAR- OR ABOVE-NORMAL MONTHLY MEAN 
TEMPERATURE ARE NOT SIGNIFICANTLY DIFFERENT FROM CLIMATOLOGICAL PROBABILITIES.

THE OCTOBER PRECIPITATION OUTLOOK WAS PARTICULARLY DIFFICULT THIS MONTH AS 
SIGNALS IN FORECAST GUIDANCE PRODUCTS ARE WEAK AND OFTEN CONFLICTING IN NATURE. 
THE OUTLOOK DEPICTS AN ENHANCED LIKELIHOOD FOR ABOVE NORMAL PRECIPITATION FOR 
MOST OF WESTERN AND CENTRAL ALASKA. MORE OPEN WATERS AND ABOVE NORMAL SSTS AS 
WELL AS GENERALLY CONSISTENT DYNAMICAL MODEL GUIDANCE ARE THE RATIONALE FOR THE 
OUTLOOK IN THIS REGION. POTENTIAL TROUGHING AS NOTED ABOVE ACROSS THE INTERIOR 
OF NORTH AMERICA WITH SOME EXTENSION INTO THE CONUS FAVORS ABOVE NORMAL 
PRECIPITATION FOR SIGNIFICANT PORTIONS OF THE SOUTHERN TIER OF THE CONUS AND 
THE EASTERN SEABOARD.  THIS IS CONSISTENT TO FIRST ORDER WITH THE LATEST WEEK 
3-4 DYNAMICAL MODEL GUIDANCE FROM THE ECMWF AND CFS. LONG TERM POSITIVE 
PRECIPITATION TRENDS ALSO CONTRIBUTE TO THE OUTLOOK ALONG PARTS OF THE EASTERN 
SEABOARD. ANTICIPATED RIDGING ON AVERAGE DURING THE FIRST HALF OF THE MONTH 
FAVORS BELOW NORMAL PRECIPITATION IN AREAS OF THE PACIFIC NORTHWEST DOING THE 
MONTH OF OCTOBER. 

AREAS SHOWN IN WHITE LABELED EC DEPICT A FORECAST WHERE THE PROBABILITIES FOR 
EITHER OF THREE CATEGORIES, BELOW-, NEAR- OR ABOVE-NORMAL MONTHLY TOTAL 
PRECIPITATION ARE NOT SIGNIFICANTLY DIFFERENT FROM CLIMATOLOGICAL PROBABILITIES.

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 SUN SEPTEMBER 30 2018

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