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Prognostic Discussion for Monthly Outlook NWS Climate Prediction Center College Park MD 830 AM EDT Thu Sep 21 2023 30-DAY OUTLOOK DISCUSSION FOR OCTOBER 2023 El Nino conditions remain strongly in place across the equatorial Pacific Ocean both in the ocean as well as the atmosphere and so is considered in the preparation of the October outlook. Potential impacts, however, are less likely and generally weaker than that sometimes observed during winter and early spring. Recent MJO activity has not been strong in recent weeks as compared to earlier in 2023. Some model forecasts of the RMM index and other MJO forecast tools indicate potential strengthening in the western Pacific (WPAC), but there is considerable uncertainty overall in this outcome especially given ongoing robust El Nino conditions. Eventual evolution of the MJO amplitude and phase notwithstanding, conditions in the WPAC remain quite conducive for tropical cyclone development over the next few weeks in the WPAC as forecast in the latest CPC Global Tropics Hazards Outlook (GTH). Since late September and October is near the peak for extratropical circulation influencing WPAC re-curving typhoons, predictability and overall forecast confidence is generally at a minimum for October within the seasonal cycle. The October outlook will be updated on the last day of September utilizing the latest information including that related to the above uncertainties. The October 2023 temperature outlook favors above-normal monthly mean temperatures for the majority of the forecast domain with the only exceptions being Southeast Alaska and the Alaska Panhandle as well as a region from the central Rockies to the central Plains where Equal-Chances (EC) of either of the three categories are equally likely. The greatest probabilities for above-normal temperatures are forecast for areas in northern and western Alaska and for the Great Lakes, Northeast, mid-Atlantic and parts of the Southeast and Gulf coast. For Alaska, reduced sea ice coverage and above-normal sea surface temperatures (SSTs) in surrounding waters elevate odds for warmer than normal conditions as well as El Nino considerations and dynamical model guidance from the NMME and C3S contributing ensemble systems. Chances for above-normal temperatures decrease moving south and east across Alaska. Through a considerable period of the first half of the month of October, model forecasts from extended-range and subseasonal systems (ECMWF, GEFSv12, JMA, CFS and others) indicate the likelihood of substantial positive 500-hPa height anomalies associated with both ridging as well as a general northward shift in the mean westerlies over central North America including the northern CONUS. As a result, above-normal temperatures are most likely for the northern tier of the U.S., the eastern CONUS and southern Plains. Considerably above-normal SSTs in the Gulf of Mexico and along the Atlantic seaboard also support favored above-normal temperatures in these areas. Short-term climate prediction models as part of the NMME and C3S ensemble systems indicate this forecast outcome is maintained in the monthly mean temperature forecast. Elevated probabilities for above-normal temperatures for much of the western CONUS are low due to uncertainty in model guidance and other forecast tools at the start of October. Conflicting forecast tools and El Nino considerations (i.e., potential below-normal temperatures) led to a forecast of EC for a region in the interior CONUS. For precipitation, elevated odds of above-normal October monthly total precipitation amounts are forecast from the Southwest eastward to include parts of the central Rockies, central and southern Plains, Ohio Valley and mid-Atlantic. The forecast for the center of this region is mainly supported by the likelihood of more unsettled conditions underneath forecast ridging to the north as noted above and this is consistent with much of the extended-range and short-term climate prediction model guidance. Although only slightly elevated, above-normal precipitation is forecast for some areas in the Southwest due to potential tropical influences of overall enhanced moisture, tropical cyclone impacts and El Nino considerations. Potential unsettled conditions underneath ridging, partial model guidance support and positive precipitation long term trends slightly favor above-normal precipitation for the Ohio Valley and mid-Atlantic. Above-normal precipitation is also favored for western and northern Alaska supported by more open and warmer waters both over a larger area in the northeast Pacific as well as more nearby waters of the Bering Sea, Chukchi Sea and Beaufort Sea. Northward shifted westerlies and ridging as indicated by extended-range and subseasonal model guidance in early October and short-term climate prediction models (NMME and C3S) elevate odds for below-normal precipitation for parts of the Pacific Northwest and an area from the northern Great lakes to New England. El Nino considerations (i.e., potential below-normal precipitation) also supports the forecast for the area highlighted in the Pacific Northwest. FORECASTER: Jon Gottschalck The climatic normals are based on conditions between 1991 and 2020, 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 2023 These outlooks are based on departures from the 1991-2020 base period. $$
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