Prognostic Discussion for Monthly Outlook NWS Climate Prediction Center College Park MD 300 PM EDT Thu Apr 30 2020 30-DAY OUTLOOK DISCUSSION FOR MAY 2020 The updated monthly temperature and precipitation outlooks for May 2020 are based on the latest dynamical model guidance, WPC temperature and precipitation forecasts during the first week of the month, the CPC 6-10/8-14 day temperature and precipitation outlooks, and climate linkages to current soil moisture. The enhanced phase of the Madden-Julian Oscillation (MJO) propagated east across Africa and the Indian Ocean during mid to late April. Despite the coherent, ongoing MJO and the likelihood of continued eastward propagation through early May, it is unclear how much influence the MJO will have on the mid-latitude circulation pattern. If the MJO continues to shift eastward over the Western Hemisphere, the large-scale environment would become favorable for tropical cyclone (TC) development in the East Pacific later in May and perhaps the Gulf of Mexico heading into June. Please refer to CPC’s Global Tropics Hazards for updates on the MJO along with TC development forecasts. Dynamical model forecasts are in excellent agreement for this time of year and feature a high amplitude ridge (trough) over western (eastern) North America during the first two weeks of May. This highly amplified ridge aloft increases chances of above normal temperatures throughout the West extending to the central and southern Great Plains. The largest probabilities (70 percent or more) of above normal temperatures are forecast across the Southwest due to anomalous heat likely to begin the month and also long-term trends. Rapidly drying topsoil supports those large probabilities extending east to the southern High Plains. An area of increased chances of below normal temperatures was added to the Great Lakes, eastern Corn Belt, and parts of the Northeast due to a persistent and highly amplified upper-level trough early in the month. Week 3-4 tools such as the CFS and ECMWF (initialized on Apr 27) suggest that temperatures across these areas would moderate or even transition to warmer-than-normal during the latter half of May. Equal chances of below, near, or above normal temperatures extend from the northern Great Plains southeast to the southern Appalachians and Carolinas where temperatures are expected to be highly variable during the month. Although the amplified upper-level trough over eastern North America is expected to result in a period of near or below normal temperatures across Florida during early May, above normal temperatures remain the most likely outcome for the entire month, especially the southern half of the Peninsula. The high amplitude ridge/trough pattern over North America, forecast during early May, is a complicating factor in the revised precipitation outlook. This longwave pattern is a relatively dry one for the CONUS, while the outlook (released on April 16) featured a broad area with elevated probabilities of above normal precipitation throughout much of the Great Plains, Mississippi Valley, and Southeast. Since there is likely to be at least a week of mostly dry weather across the northern half of the Great Plains and parts of the Southeast, the coverage of elevated probabilities of above normal precipitation was reduced. The Weeks 3-4 tools (as of Apr 27) along with the CFS monthly continue to support either near or above normal precipitation across the Gulf Coast States. The slightly elevated chances of above normal precipitation, centered across the Ozarks to the middle Mississippi Valley, is associated with predicted rainfall at the beginning of the month. During the second week of May, model guidance continues to trend towards a wetter solution from the upper Rio Grande Valley north into New Mexico as return flow from the western Gulf spreads northwest. Given that these areas are relatively dry this time of year, above normal precipitation is slightly favored for parts of the Big Bend of Texas and New Mexico. A wet start to the month and many of the precipitation tools support increased chances of above normal precipitation across the Northeast. Given that the first half of the month is likely to be quite dry with anomalous northerly flow, below normal precipitation is most likely across the Great Lakes and northwest Corn Belt. To the north of the dry climatological areas of the Southwest, below normal precipitation is favored across the Great Basin due in part to the persistent, upper-level ridging early in the month. Although near to below normal temperatures are forecast to prevail across Alaska at the beginning of May, the high amplitude ridge is likely to result in the development of above normal temperatures by the second week of the month. Once this upper-level ridge establishes itself, it is expected to continue through at least mid-May. This persistent ridge aloft and good agreement among Weeks 3-4 tools support increased chances of above normal temperatures across Alaska. This evolving longwave pattern also elevates probabilities of below normal precipitation across western mainland Alaska. ----------- Previous message (from Apr 16) is below ------------ The May 2020 temperature and precipitation outlooks are based primarily on dynamical model guidance, statistical tools, and current soil moisture conditions. ENSO is not a factor in the monthly outlooks since it is predicted to remain neutral. Since the beginning of April, a large-scale area of enhanced convection propagated rapidly eastward across the global tropics from the Maritime Continent to the Western Hemisphere. As of April 15, the anomalous upper-level divergence and associated enhanced convection are overspreading Africa. Although a coherent Wave-1 pattern of enhanced (suppressed) convection is observed over the Western Hemisphere (Indian Ocean and Maritime Continent), this anomalous convection is likely more related to a convectively coupled Kelvin due to its fast eastward propagation. Based on the latest dynamical model forecasts of the RMM index, Kelvin and equatorial Rossby waves are expected to contribute to anomalous tropical convection heading into May. Therefore, the Madden-Julian Oscillation (MJO) is unlikely to influence the mid-latitude circulation pattern. Weekly sea surface temperature (SST) anomalies, centered on April 8 are running as much as 2.5 degrees C above normal in parts of the Gulf of Mexico. Since SSTs (as of April 13) are near or slightly above 27 degrees C in the southern Gulf of Mexico, the future progression of Kelvin waves will have to be closely monitored for an early season tropical cyclone later in May into the beginning of June. During the final week of April, the GFS, Canadian, and ECMWF ensemble means feature a less amplified longwave pattern compared to early to mid-April. Also, there is large spread among ensemble members which is not surprising due to the lower amplitude. The increased chances of above normal temperatures forecast across the western CONUS, Gulf Coast, and most of the East Coast is based on the North American Multi-Model Ensemble (NMME) and is also consistent with long-term trends during the spring season. The largest probabilities (60 percent of higher) of above normal temperatures are forecast across central and south Florida due to: unanimous support from the dynamical models, long-term trends, and much above normal SST anomalies surrounding the Florida Peninsula. Although the NMME indicates increased chances of above normal temperatures across the Great Lakes, equal chances (EC) of below, near, or above normal temperatures are forecast for this region since the ECMWF model features below normal temperatures during early May. Soil moisture currently ranks above the 90th percentile from the Missouri River Basin south to the Ozarks and Tennessee Valley. This high soil moisture content is likely to have a cooling effect on temperatures for these areas during May. The EC forecast for temperatures across the Great Plains, Mississippi Valley, and Corn Belt is consistent with these ongoing soil moisture conditions along with the calibrated NMME. The dynamical models are in good agreement and feature increased chances of above normal precipitation across the central Rockies, much of the Great Plains, and Southeast. This wet pattern is also supported by the CFS and ECMWF models (initialized on Apr 13) with their depiction of an upper-level trough over the Rockies and high Plains during early May. Despite the good model agreement, the probabilities of above normal precipitation are tempered due to predictability in the monthly precipitation outlook at this time lead. Also, precipitation becomes more convective and less predictable later in the spring. The NMME supports increased chances of below normal precipitation across the coastal Pacific Northwest. Modifications to the precipitation outlook are likely with the updated release on April 30. The dynamical model guidance is in good agreement with increased chances of above normal temperatures across Alaska. In addition, the CFS and ECMWF models feature above normal temperatures during early May. Probabilities of above normal temperatures were reduced across northern mainland Alaska, due to a deep snowpack for this time of year. The only notable signal among the precipitation tools is a slight lean towards below normal precipitation across the Aleutians and Alaska Peninsula during May. FORECASTER: Brad Pugh 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 Jun ... will be issued on Thu May 21 2020 These outlooks are based on departures from the 1981-2010 base period. $$