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. ------------------------------------------------------------------------------- PREVIOUS DISCUSSION FROM EARLIER IN THE MONTH IS BELOW ------------------------------------------------------------------------------- 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. $$