506 FXUS07 KWBC 311900 PMD30D Prognostic Discussion for Monthly Outlook NWS Climate Prediction Center College Park MD 300 PM EDT Sat Aug 31 2024 30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2024 The updated September 2024 monthly temperature and precipitation outlooks are based on the latest dynamical model guidance, including the CFSv2 and GEFSv12 forecasts for the month of September, as well as the Weather Prediction Center (WPC) temperature and precipitation forecasts for the first week of the month, the CPC 6-10 day and 8-14 day temperature and precipitation outlooks, and the latest CPC Week 3-4 Outlook (valid September 14-27). Although the El Nio Southern Oscillation (ENSO) remains in an ENSO-neutral state, a La Nia is predicted to likely develop over the next few months. However, atmosphere and ocean conditions remain neutral at present, and impacts of a developing La Nia were only considered in the update to the September outlook through the influence of the tropics on dynamical and statistical model forecasts. The Madden-Julian Oscillation (MJO) emerged over the Western Hemisphere in early August and propagated into the Indian Ocean. The MJO lost amplitude recently due to interference with other tropical variability, but is forecast by dynamical models to strengthen and propagate eastward across the Maritime Continent over the next few weeks. If the MJO regains amplitude and propagates into the Western Pacific, this would support less favorable conditions for tropical cyclone development over the Eastern Pacific and tropical Atlantic. However, conditions are expected to remain favorable for tropical cyclone development in the next few weeks due to other climate factors. The MJO was only considered in the monthly outlook to the extent that impacts may be associated with variability in the dynamical model forecasts for the month of September. Dynamical models predict a large-scale wave pattern extending across the North Pacific into North America in the first week of September, with a mid-level ridge to the west of Alaska, a trough over the North Pacific, a ridge over western North America and the contiguous U.S. (CONUS), and a trough over eastern North America. While this pattern evolves and breaks down in model forecasts through the first week of the forecast period, this pattern is reestablished by the start of the 6-10 day period, with a weaker trough over the eastern CONUS. By the third and fourth week of September, dynamical model forecasts from the ECMWF and GEFS predict primarily positive 500-hPa height anomalies across most of the CONUS, with a ridge centered over the north-central CONUS and a trough extending from southwestern Alaska along the Pacific coast of North America to near or over the Pacific Northwest. The updated September temperature Monthly Outlook favors below normal temperatures for the west coast of Mainland Alaska and parts of the eastern Aleutians, due to persistent negative sea surface temperature (SST) anomalies in the region. The updated Monthly Outlook favors above normal temperatures across eastern Mainland Alaska consistent with the latest dynamical model guidance and Week 2 and Week 3-4 Outlooks. Above normal temperatures are likely for much of the western half of the CONUS, consistent with the latest 6-10 Day, Week 2, and Week 3-4 Outlooks. Equal chances (EC) of below, near and above normal temperatures are indicated for the Pacific coastal regions of the CONUS ahead of a predicted trough, similar to the Week 3-4 Outlook. A large area of EC is now indicated over much of the eastern CONUS, where the presence of a trough for much of the first half of September leads to forecasts of below normal temperatures and the Week 3-4 Outlook favors above-normal temperatures. Below normal temperatures are slightly favored for parts of the Mid-Atlantic region for the month of September, where cooler temperatures early in the month are likely to outweigh warmer temperatures later in the month in the September average. Above normal temperatures continue to be likely for the Gulf Coast, where sea surface temperatures are much above long-term averages. Above-normal temperatures continue to be likely for parts of the Northeast for the month of September, where potentially warmer temperatures later in the month outweigh cooler temperatures early in the month in model forecasts from the CFSv2 and GEFS for the entire month. The updated September precipitation Monthly Outlook favors above normal precipitation for Southeast Alaska, ahead of a predicted trough and consistent with the Week 3-4 Outlook. Below normal precipitation continues to be slightly favored from parts of the Southwest northward across the Central Rockies into Montana, consistent with the latest dynamical model guidance. Below normal precipitation is likely from the northern Great Plains across the Great Lakes region, consistent with CFSv2 precipitation forecasts for September and the Week 2 and Week 3-4 Outlooks. Above normal precipitation is likely for parts of southeastern New Mexico, western Texas, and north-central Texas where WPC precipitation forecasts predict significant precipitation amounts in the first week of the month. Above normal precipitation is favored across the southeastern CONUS consistent with dynamical model guidance from the CFSv2 and the Week 3-4 Outlook. ******* Previous discussion, released on August 15, is below ******* The September 2024 monthly temperature and precipitation outlooks are made under neutral El Nio Southern Oscillation (ENSO) climate conditions. The most recent weekly Nio 3.4 sea surface temperature (SST) anomaly is at 0.0 degrees Celsius. SST anomalies for the Nio 3.4 region of the east-central equatorial Pacific Ocean have remained steady over the last two-to-three months. In recent weeks, lower level wind anomalies at 850 hPa were easterly over the east-central and eastern equatorial Pacific Ocean, while upper level wind anomalies at 200 hPa were easterly over the east-central equatorial Pacific Ocean and westerly over the eastern equatorial Pacific Ocean. Outgoing longwave radiation (OLR) anomalies are weak across the equatorial Pacific Ocean, indicating near-average convection. These ocean and atmosphere conditions indicate neutral ENSO conditions have persisted. A potential transition to La Nia conditions is forecast and may be a driver of temperature and precipitation patterns over North America in the near future and in the September temperature and precipitation outlooks. On subseasonal timescales, the Madden Julian Oscillation (MJO) is active in phase 1 near the threshold of phase 2 with enhanced convection over Africa. Dynamical model forecasts indicate that the MJO will propagate eastward into the Indian Ocean and Maritime Continent in the next couple weeks. Lagged composites indicate that this active MJO would increase temperatures over the western United States in early September, with potentially cooler temperatures over parts of the eastern contiguous U.S. (CONUS). Impacts of the MJO were generally considered in the September monthly outlook through dynamical model forecasts for the week 3-4 period that overlap with the first half of September. In addition to the teleconnections over North America, more favorable conditions for tropical cyclone activity over the Atlantic at the start of September are predicted. The September temperature and precipitation outlooks were based primarily on dynamical model forecasts and a combined consolidation of the statistical and dynamical model forecasts. Dynamical model forecasts for the month of September are from the North America Multimodel Ensemble (NMME). In addition, the full consolidation includes a consolidation of the statistical models: the Canonical Correlation Analysis (CCA), the Constructed Analog (CA), and an ENSO OCN tool, that combines the impact of ENSO, based on the CPC SST consolidation predicted median Nio 3.4 SST anomaly, with the Optimum Climate Normal (OCN) representing decadal trends. Daily initialized forecasts from the NCEP CFSv2 dynamical model and the most recent ECMWF and GEFS dynamical model forecasts for the week 3-4 period that overlaps the beginning of the month of September were also considered. Recent boundary conditions, including coastal SSTs, and soil moisture anomalies, were additional factors considered. The September temperature outlook favors below-normal temperatures along the west coast of Mainland Alaska, consistent with dynamical model guidance from the NMME including the CFSv2, plus ECMWF and GEFS forecasts for the week 3-4 period ending September 10th, as well as well below average SSTs along the coast. Above-normal temperatures are likely along the coast of the North Slope, consistent with well below average sea ice extent and above average SSTs. Above-normal temperatures are likely across most of the CONUS, supported primarily by the consolidation of statistical and dynamical forecast tools. The probability of above-normal temperatures exceeds 60 percent for a large area of the West from the Four Corners region to the eastern Great Basin and central Rockies, where dynamical models, statistical models, and decadal trends all indicate a strong temperature signal. The probability of above-normal temperatures exceeds 60 percent for parts of northern New England, where adjacent SSTs are above average, ECMWF week 3-4 forecasts favor above-normal, and decadal temperature trends are strong. Above average adjacent SSTs lead to a 60 percent probability of above-normal temperatures for the Florida Peninsula. Equal chances (EC) of below-, near- and above-normal temperatures are indicated along the central and southern California coast, where temperatures are moderated by near-to-below average SSTs. EC is also indicated for parts of the Pacific Northwest along the northern tier into the Northern Plains and Upper Mississippi Valley, where the PAC-calibrated NMME temperature forecast shows moderation of probabilities due to lower skill and/or weaker signals, while the full consolidation forecast predicts near-normal is favored in some of the same areas. The September precipitation outlook slightly favors above-normal precipitation for parts of western Mainland Alaska, supported primarily by dynamical model forecasts from the NMME. Above-normal precipitation is favored for a small coastal area of the Pacific Northwest, consistent with the CFSv2 forecast and possible early impacts of La Nia. Below-normal precipitation is favored over a large area of the western CONUS from eastern areas of the Desert Southwest northward to the High Plains of Montana and eastward into much of the Great Plains and the Central Mississippi Valley, consistent with negative precipitation anomaly forecasts from the NMME multi-model ensemble mean and impacts from potential La Nia conditions. Above-normal precipitation is favored for the entirety of the Gulf Coast and northward along the Atlantic Coast to Maine, consistent with the peak month of a predicted active Atlantic tropical cyclone season in addition to favorable conditions for Atlantic tropical activity at the start of September related to the predicted propagation of the MJO. The probability of above-normal precipitation exceeds 50 percent for parts of the Florida Peninsula, supported by NMME forecasts for September. FORECASTER: Dan Collins 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. The next monthly outlook...for Oct ... will be issued on Thu Sep 19 2024 These outlooks are based on departures from the 1991-2020 base period. $$