Prognostic Discussion for Monthly Outlook NWS Climate Prediction Center College Park MD 300 PM EDT Mon May 31 2021 30-DAY OUTLOOK DISCUSSION FOR JUNE 2021 The update to the June 2021 temperature and precipitation outlooks are driven primarily by the most recent short-, medium- and extended-range model guidance as well as the most recently available subseasonal and monthly integrated forecast products. The MJO has become less coherent in recent weeks and does not play as much of a role in the updated outlook, while soil moisture anomalies continued to inform the June temperature outlook. Changes to the June temperature outlook are generally small overall. The outlook continues to favor above-normal temperatures in an inverted horseshoe pattern encompassing areas from the Southwest CONUS to the Pacific Northwest across the northern Plains and Great Lakes to the Northeast and mid-Atlantic. Strong ridging anticipated during much of the first half of June increases odds for above-normal temperatures for the Northeast, Great Lakes and mid-Atlantic compared to the mid-month outlook. Also, highest probabilities for above-normal temperatures in the western CONUS have been shifted to the northwest and are now focused in the central Great Basin and western Rockies where ridging and warmer-than-normal temperatures (some potentially extreme) are likely during the first week of June. The most substantial change in the updated June outlook, is the inclusion of favored below-normal temperatures for the southern Plains, lower Mississippi Valley and western Gulf coast. The mid-month outlook did note the uncertainty in this region and potential cooler temperatures with a forecast area of Equal-Chances (EC) due to negative 500-hPa heights indicative of weakened sub-tropical ridging and enhanced cloudiness and precipitation. Model guidance through the end of May 2021 across time scales has become more clear and increased the likelihood of the conditions described above. Substantial further support for this addition is the recent heavy rainfall, especially for parts of east Texas and Louisiana, which has maintained or increased anomalously wet surface conditions across this region. Some minor changes are made in Alaska in the update, but forecast confidence is low and only very slight tilts from climatological odds are depicted for the state. For precipitation, the updated June 2021 outlook illustrates some minor adjustments in general from the mid-month outlook. Favored below-normal precipitation remains for the northern Rockies and Pacific Northwest, but has been removed from parts of the eastern Southwest and southern High Plains. Model guidance favors some rainfall early in the month in this region where normal precipitation totals are relatively low. In addition to the potential MJO influence noted in the mid-month discussion below, model guidance across time scales has increased confidence for above-normal precipitation for a region from Texas to the lower Ohio Valley, mid-South, Gulf coast, Southeast and lower mid-Atlantic. Greatest odds are for parts of eastern Texas and Louisiana. Other minor changes include the addition of a small region of favored above-normal precipitation for parts of the northern Plains and upper Mississippi Valley, but confidence is low. High uncertainty due to varying signals in the available forecast tools across time scales in Alaska has led to a forecast of EC for the state of Alaska in the updated June outlook. ******************************************************************************* ****** Previous mid-month discussion below ****** ******************************************************************************* The June 2021 monthly outlook was prepared based on current surface conditions such as soil moisture and coastal sea surface temperatures (SSTs), potential impacts from the currently active MJO, dynamical model and statistical forecast guidance and newly derived long term trends from the 1991-2020 base period. La Nina conditions present during the winter and spring months has ended as both SSTs in the equatorial Pacific Ocean have warmed and coupling to the atmosphere has weakened. The MJO, however, has been active since mid-March and has been strong and is starting its third "cycle" around the global Tropics. The enhanced phase of the MJO is currently centered in the Indian Ocean and is forecast to continue to propagate eastward to the western Pacific by the beginning of June. The teleconnection to the mid-latitudes from MJO forcing during late spring and summer, even during a moderate to strong event, is typically considerably weaker, has a lower likelihood to occur and impacts highly variable. For this event, the forecast phase of the MJO at the start of June would favor above-normal temperatures for the western CONUS based on composites and statistical forecast tools as well as, but to a considerably lesser degree below-normal temperatures in the vicinity of the Great Lakes, the Ohio Valley and parts of the mid-South. These impacts, however, would be mainly during the first 10 days of the month June and impacts based on MJO composites and other tools are not statistically significant in subsequent MJO phases thereafter. Favored above-normal temperatures for the western CONUS from the MJO perspective is consistent and further supported by overwhelming dynamical model forecast guidance, statistical tools, low soil moisture considerations and positive long term temperature trends. Conflicting and/or weak signals related to the MJO, dynamical model and statistical guidance, areas of very wet surface conditions in addition to favored above-normal precipitation for the month of June as a whole results in low confidence for the eventual category of June monthly mean temperatures for the southern Plains, lower Mississippi Valley and parts of the Southeast. Equal-chances (EC) for either of the three categories is forecast in these areas. Above-normal SSTs in the lower Bering Sea favor above-normal temperatures for southwest Alaska which is somewhat supported by model forecast guidance. Climate signals were weak for most of the remainder of the state. The June 2021 precipitation outlook favors below-normal precipitation for a region from the Pacific Northwest and northern Rockies to the central Rockies southward to include parts of the state of New Mexico and Texas primarily based on consistent dynamical model forecast guidance. Elevated probabilities for above-normal precipitation is forecast for the western portion of Alaska supported by model guidance as well as potentially greater moisture availability for precipitation due to more open and warmer waters in the Bering Sea, especially the first half of the month. The likely MJO phase over the course of the month of June would tend to favor at various times enhanced odds of above-normal precipitation for the Gulf Coast, Southeast and Ohio Valley. This along with support from dynamical model guidance and long term positive precipitation trends favor above-normal monthly total precipitation amounts for these areas. 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 Jul ... will be issued on Thu Jun 17 2021 These outlooks are based on departures from the 1981-2010 base period. $$