Prognostic Discussion for Monthly Outlook NWS Climate Prediction Center College Park MD 300 PM EDT Mon Sep 30 2019 30-DAY OUTLOOK DISCUSSION FOR OCTOBER 2019 The updated October 2019 temperature and precipitation outlook builds upon some of the factors outlined in the mid-September discussion below and is modified by the latest suite of dynamical model guidance from the short-range, medium-range, extended-range and complete monthly forecast integrations. The start of October is anticipated to see an amplified pattern across the CONUS with a strong trough-ridge pattern oriented across the CONUS. Over the first 7 days or so, this pattern is favored to shift eastward, become less amplified and then make a transition to a more zonally oriented flow over the CONUS. During the second week of October, a warmer pattern is favored to develop for the eastern two-thirds and Southwest CONUS as a result of this more zonal oriented flow and increasing coverage of positive 500-hPa height departures along the southern tier and eastern CONUS. Residual troughing is favored to remain across the Pacific Northwest. A number of factors point to elevated odds for above-normal temperatures for most of the CONUS for the second half of October. First, long term trends are positive in nearly all areas of the forecast domain, ongoing MJO activity favors above-normal temperatures for most of the central and eastern CONUS and Southeast Canada and Week 3-4 dynamical model guidance from the CFS, ECMWF and JMA favor above-normal temperatures for much of the second half of October. Also, short-term or regime based bias corrections (i.e., based on past 45 days) show that some model forecasts for below-normal temperatures across the CONUS have been overestimated to some degree. Forecast of favored below-normal temperatures for October are focused over some areas of the Pacific Northwest to the northern High Plains. These depictions only show modest increases in odds as compared to climatology and are related to early in the month cold air intrusion during the first week of October that may potentially keep mean October temperature anomalies below the lower one-third of the historical range. High odds for above-normal temperatures are depicted for the Southeast, lower Appalachians and lower mid-Atlantic - a result of nearly all information primarily pointing to above-normal temperatures and also where flash drought conditions are ongoing and favored to continue to deteriorate during the first 10 days of October. The updated October precipitation outlook is able to increase forecast coverage from very low areal numbers released in the mid-September initial outlook. The general themes, to first order, are maintained with the forecast coverage for above-normal precipitation depicted in the initial outlook along the northern tier of the CONUS increased with modest odds depicted for the Pacific Northwest and Northeast and greater probabilities depicted over the northern Plains, upper Mississippi Valley and western Great Lakes. This area of favored above-normal precipitation is extended southward in the updated outlook as a plume of moisture from the sub Tropics is likely to spread northward and eastward around anticipated ridging located over the Southeast and then interact with troughing and the general westerlies along the Canadian border. Above-normal precipitation remains most likely for areas of the Southwest associated with potential influence from eastern Pacific tropical disturbances or cyclones that may increase moisture at times during the month. These odds are on modestly increased from climatology. Forecast guidance for the first week in October indicates very little precipitation for much of the Southeast from Texas to the lower Atlantic seaboard. Week 3-4 forecast tools, long term trends and MJO impacts do not indicate with any consistency or reliability, robust signatures for above-normal precipitation during the second half of October. Based on this combined information below-normal precipitation is depicted for this region. Elevated odds for above-normal precipitation remain forecast for parts of the Florida Peninsula, however. There is high uncertainty for Alaska so the initial outlook was modified to now favor enhanced odds for above-normal precipitation for the west coast of Alaska. ******************************************************************************* ** Previous discussion from mid-month outlook made in mid-September is below ** ******************************************************************************* The initial half month lead October monthly outlook always represents a considerable challenge in short term climate prediction. A two week lead monthly outlook during a transition season period, such as October, integrated over only a monthly period, historically is one in which predictability is low. Reviewing the latest climate conditions and status of major climate modes, there are a few items to note, however. During the summer, we entered ENSO-neutral conditions across the Pacific Basin so that the ENSO phase did not play any role in the October outlook. However, there is coherent subseasonal tropical variability (MJO / atmospheric Kelvin wave activity) ongoing and is considered in preparing the outlook. Although impacts from this subseasonal tropical variability at higher latitudes this time of the year are tenuous and unreliable, this forcing can play a role in October conditions for portions of the southern CONUS - mainly related to potential enhancement of tropical cyclone activity in the East Pacific and Atlantic basins. It is also noted that soil moisture anomalies are substantial in some areas. This includes large soil moisture surpluses in the northern Plains and upper Midwest as above-normal rainfall continued through the summer months. Also, the CONUS devoid of much drought for most of the year, has shown an increase in drought conditions for parts of the Southeast, Texas and Southwest. The soil moisture anomalies, although noteworthy, played a generally minor role when preparing the October outlook. The October temperature outlook favors above-normal monthly mean temperatures for the entire forecast domain. The largest probabilities are depicted for the western and northern areas of Alaska due to continued well above-normal ocean surface temperatures, feedback due to earlier loss and lower coverage of nearby sea ice extent, as well as dynamical model guidance from a number of sources. Odds for above-normal temperatures are modestly elevated for the southwest corner of the CONUS due to strong positive temperature trends during this time of year, consistent model guidance and to a lesser extent generally drier than normal conditions in some areas. The increase in probabilities for above-normal temperatures from climatological values across the central portion of the country are small. In these areas, there was considerable uncertainty as available forecast tools and dynamical model guidance were in conflict and in locations of historically low skill so that forecast confidence is low. A potential cooler start to October in the Pacific Northwest and northern Rockies along with surpluses in soil moisture in the northern Plains and Upper Midwest tempered above-normal probabilities in these areas as well. There is high uncertainty, low forecast confidence and so small forecast coverage in the October precipitation outlook. Monthly total precipitation amounts are forecast to be above-normal for small regions of the forecast domain. This includes the Alaska Panhandle and areas of the Pacific Northwest, Southwest, northern Plains and central and southern eastern seaboard. Model guidance indications of northward shifted westerlies support forecasts of above-normal precipitation for the highlighted areas along the northern tier of the CONUS. Long term trends and the potential for an active period of tropical cyclone activity in the Atlantic, consistent with MJO/subseasonal variability noted earlier, tilts the odds for above-normal precipitation for the eastern seaboard from the mid-Atlantic southward. Similar prospects for continued enhanced tropical cyclone activity in the eastern Pacific basin increases the potential for Gulf surges and tropical moisture to enter the far Southwest. Some consistency in dynamical model guidance was the primary basis for the favored below-normal rainfall highlighted area in the southern Plains. 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 17 2019 These outlooks are based on departures from the 1981-2010 base period. $$