Prognostic Discussion for Monthly Outlook NWS Climate Prediction Center College Park MD 300 PM EDT Sun Mar 31 2024 30-DAY OUTLOOK DISCUSSION FOR APRIL 2024 The update to the 0.5-month lead April 2024 temperature and precipitation outlooks are based primarily on official precipitation outlooks for Week-1 (from WPC) and for Week-2 and Week- 3/4 (from CPC), the latest dynamical model output that extends two weeks into the future, current boundary conditions (snow cover/depth and soil moisture), lagged temperature composites from the Madden-Julian Oscillation (MJO), and historical temperature and precipitation trends. Lagged El Nino impacts from the rapidly weakening El Nino are expected to extend well into April. The updated temperature outlook for April favors above-normal monthly mean temperatures from the Pacific Northwest and Northern California generally eastward across the Northern Rockies, Northern Plains, central and eastern portions of the Central and Southern Plains, the Mississippi-, Tennessee-, and Ohio Valleys, Great Lakes region, Central and Northern Appalachians, Northeast, and most of the Mid-Atlantic region. Warmer-than-normal temperatures are also favored over most of Alaska, excluding southwest portions of the Mainland, the Alaska Peninsula, and the Aleutians, where Equal Chances (EC) of below-, near-, and above-normal monthly mean temperatures are favored. This widespread area of favored above-normal temperatures represents a subjective compromise between the various constituent WPC and CPC temperature outlooks, which together comprise the April monthly mean, and on dynamical model guidance from the GEFS and ECMWF ensemble means which extend through the first half of April. The modest tilt in the odds towards above-normal temperatures near the Northeast and Upper Mid-Atlantic coast is due to anticipated tempering effects of soil moisture, where current ranked percentiles exceed the 95th historical percentile. Large negative anomalies of snow cover and depth across the interior Northeast, Great Lakes region, and Upper Midwest contribute towards heightened probabilities for above-normal temperatures in those areas. The large region of EC indicated across the Southwest is due to an expected relatively cool first half of April, followed by EC for the second half. A mid-level ridge is forecast to build across this region later in the month, which works in tandem with positive temperature trends. When the month as a whole is considered, EC is thought to be the most likely scenario for the Southwest. The updated precipitation outlook for April favors above-normal precipitation amounts from the southwestern Contiguous U.S (CONUS) across Wyoming, eastern portions of the Central and Southern Plains, and continuing eastward to the Atlantic Coast from southern Maine in the north to southern Florida in the south. The exceptions include southern Virginia, the Carolinas, and northeast Georgia where EC is favored. Maximum probabilities for above-normal precipitation (greater than 50%) extend from south-central Illinois eastward across Indiana, Ohio, and western Pennsylvania. This is primarily based on WPC’s Week-1 precipitation forecast, with 2-4 inches (locally heavier) of rain expected within the first few days of April, related to a quasi-stationary front accompanied by several waves of surface low pressure. During the second week of the month, above-normal precipitation is expected to shift to the middle portion of the CONUS (Central and Southern Plains, and Middle to Lower Mississippi Valley) with an eastward extension across much of the Southeast. This is consistent with CPC’s Week-2 precipitation outlook, and on total precipitation amounts for that 7-day period from the GEFS and ECMWF ensembles. For southern Texas, the precipitation amounts received during the first two weeks is expected to register as above-normal, despite an anticipated drier finish to April. It is unclear as to whether or not these rainfall amounts will be enough to justify some improvement in the regional drought conditions there. Across the Southwest CONUS, there are increased chances for above-normal precipitation based on WPC’s Week-1 precipitation forecast, and CPC’s Week-2 and Week-3,4 precipitation outlooks. Wetter-than-normal conditions are typically observed across the Southwest during the final stages of a strong El Nino, as atmospheric impacts tend to lag a bit even as the positive sea-surface temperature anomalies weaken rapidly over the equatorial Pacific. Below-normal precipitation amounts are favored over much of the northern and central Pacific Coast states, and over Northern Maine. This is generally based on WPC and CPC precipitation guidance throughout the month of April. The small area of favored relative dryness depicted over Northern Maine is fairly uncertain, being not that far displaced from the heavy precipitation axis forecast to the south during Week-1 as noted above. Wetter-than-normal conditions are favored over southern Mainland Alaska and the Alaska Peninsula, based on a general consensus of model guidance, and on a significant mid-level trough expected to approach the state early in the period. Elsewhere in Alaska and the Lower 48 states, EC is favored. ---- Previous message for 0.5-month lead April 2024 Outlook is shown below ----- The April 2024 monthly temperature and precipitation outlooks are based on various climate factors (such as ENSO and to some degree, the Madden-Julian Oscillation (MJO)), boundary conditions (snow depth and soil moisture), dynamical and statistical model guidance, and historical temperature and precipitation trends. Sea-surface temperature (SST) anomalies across the central and eastern equatorial Pacific have significantly decreased during the past few weeks, and a relatively small extent of anomalously cool water has appeared over the eastern Pacific related to the upwelling phase of an oceanic Kelvin wave. Cooler-than-average subsurface temperatures have developed over much of the eastern Pacific. By mid-March, enhanced trade winds have returned to a large portion of the equatorial Pacific. All these factors are consistent with a rapidly weakening El Nino, which has been in place for just over one year. El Nino is forecast to transition to ENSO-neutral during the April-May-June (AMJ) season. However, El Nino impacts are expected to linger well into the month of April across the United States, and these expected impacts were considered in the construction of the April outlook. Another climate factor to consider is the currently robust MJO. The enhanced convective phase of the MJO is located over the Western Pacific (Phase 7 in RMM space), and the suppressed phase of the MJO is located over the Indian Ocean (Phases 2 and 3 in RMM space). Model forecasts of the MJO index propagate the MJO signal into the Western Hemisphere over the next two weeks. Within the first week of April, lagged MJO composites for Phase 7 favor anomalously cool temperatures over at least the central third of the contiguous U.S. (CONUS). In addition to the climate factors noted above, there is an extensive area of anomalously low snow depth from Montana eastward across the Northern Great Plains, Great Lakes region, and Northeast. From eastern Montana across the Northern Plains snow depth departures are generally 8 inches or less, though in the vicinity of Great Falls, Montana, snow depth departures range from 2-3 feet (locally greater) below normal. From the Upper Mississippi Valley eastward across the Great Lakes and New England snow depth ranges anywhere from 8-30 inches below normal. Unusually low snow depth means less water will be available for staggered release into soils during the beginning weeks of the growing season. Much of this region is also experiencing relatively low soil moisture values. The exception is the Northeast, where recent precipitation has resulted in ranked soil moisture percentiles in excess of the 95th percentile. For the April temperature outlook, there are elevated chances for above-normal temperatures from most of the West Coast states eastward and northeastward into the Northern Plains, continuing eastward and now southeastward across the Upper and Middle Mississippi Valley, Great Lakes region, Ohio Valley and neighboring portions of Tennessee, most of the Appalachians, and the Atlantic coastal plain from Maine to northern North Carolina. The favored above-normal temperatures are generally supported by the Final Consolidation tool (Final-CON) which skill-weights a consolidation of dynamical models (NMME-CON) and a consolidation of statistical models (Stat-CON). These temperatures are also generally supported by the Constructed Analog on Soil Moisture Tool (CA-SMT), and to a lesser degree the International Multi-Model Ensemble (IMME, also referred to as C3S or Copernicus), CFS, calibrated NMME (PAC), and GFDL SPEAR models. Most of the favored above-normal temperatures were derived from dynamical model guidance rather than statistical model guidance. Expected lagged El Nino impacts also played a role in this forecast, as well as above-normal SSTs along the West Coast. The boundary conditions noted earlier played a significant role in the temperature forecast from Montana eastward across the Great Lakes and Northeast. The probabilities favoring above-normal temperatures across New England and eastern New York state were tempered by anomalously high soil moisture values. The 15-year Optimal Climate Normals (OCN) temperature tool, a proxy to the last 15 years of historical temperature trends, favored relative warmth across portions of the Far West, Four Corners states, southern Texas, and the Mid-Atlantic and Northeast coastal plain. There are increased chances for above-normal temperatures over the southern Rockies and southern High Plains, associated with anomalously dry soils, the three CON tools (noted above), and a high sun-angle. Most model guidance supported above-normal temperature chances for most of Alaska, with the exception of the Aleutians where Equal Chances (EC) of below, near, and above-normal monthly mean temperatures are favored. The April precipitation outlook is based on the same climate factors, boundary conditions, and model guidance as the April temperature outlook. Wetter-than-normal conditions are favored from approximately the southeastern quadrant of the CONUS northwestward into the north-central High Plains. This is generally consistent with the Final and NMME CONs, uncalibrated NMME, IMME, historical precipitation trends, and lagged El Nino impacts. Well below-normal snow depth, anomalously low soil moisture, and expected lagged El Nino impacts favor below-normal precipitation for the Upper Great Lakes region. Anomalously low soil moisture, high sun-angle, and guidance from the Final and Stat CONs favor below-normal precipitation across southwestern Colorado, New Mexico, and most of West Texas. Below -normal precipitation is also favored for parts of the Northwest, based on model support and lagged El Nino impacts. In Alaska, there were significant differences in the precipitation forecasts, though the IMME, GFDL SPEAR, and Canadian CanCM4i models supported an elevated chance of above-normal precipitation across the southern part of the state, including the Alaska Peninsula. FORECASTER: Anthony Artusa 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 May ... will be issued on Thu Apr 18 2024 These outlooks are based on departures from the 1991-2020 base period. $$