142 FXUS07 KWBC 301901 PMD30D Prognostic Discussion for Monthly Outlook NWS Climate Prediction Center College Park MD 300 PM EDT Wed Apr 30 2025 30-DAY OUTLOOK DISCUSSION FOR MAY 2025 The updated outlook for May represents a challenging forecast with much lower than normal confidence. One of the main culprits for this uncertainty is the prospect of a cut-off low across the West during the first week of the month. This forecast cut-off low is likely to lead to a cooler and more unsettled period across much of the West relative to guidance available during the previous mid-month outlook. Additionally, this cut-off low combined with a stalled frontal boundary supports a much more active period across the Southern Plains during the first half of May. Moreover, the Madden Julien Oscillation (MJO) remains weak and is forecast to remain so into much of May, reducing the utility of the MJO as a reliable forecast tool for this upcoming month. Finally, as we progress further into Spring, forecast signals from the El Nio Southern Oscillation (ENSO) become less reliable in many areas and ENSO-neutral conditions are forecast to prevail during the month. Due to these combination of factors, the main drivers for the updated May outlooks were WPCs short range outlooks, CPCs 6-10 day, 8-14 day, and weeks 3-4 outlooks and dynamical model guidance from the CFSv2, GEFS, and ECMWF. Antecedent conditions such as soil moisture anomalies, extratropical sea surface temperatures in nearby waters, and sea ice extent were also utilized in the outlook where appropriate. These combinations of tools signal a significant change from the mid-month May outlook in many areas. Similar to the half month outlook for May, above normal temperatures remain favored across the majority of the Contiguous United States (CONUS). However, probabilities of above normal temperatures are significantly reduced across much of the West due to the predicted cut-off low early in the month. Conversely, increased confidence in above normal temperatures is indicated for much of the north-central and northeastern CONUS as anomalous ridging is favored across these areas for much of the first half of the month and above normal 500-hPa heights continue to be favored into the weeks 3-4 period. A reduction in above normal temperature probabilities is noted for the Florida Peninsula in response to a cool signal represented in CPCs 6-10 day and 8-14 day outlooks. Moreover, probabilities of above normal temperatures were reduced for much of the Southern Plains as a wet pattern is likely to take hold for much of the first half of the month. The forecast for enhanced above normal temperature probabilities for southwestern Alaska in the half month lead remains on track in the updated May outlook. However, equal chances now replaces the tilt toward above normal temperatures across the North Slope of Alaska as both the 6-10 day and 8-14 day CPC outlooks tilt cold across this region. The cut-off low expected over the West early in May significantly alters the precipitation outlook across many areas. Precipitation amounts from this system are expected to approach or even exceed typical monthly totals across parts of the interior West during the first week of May alone. The same holds true for much of the Southern Plains as a combination of this cut-off low and a stalled front are expected to result in an active pattern during the first half of May. As such, probabilities of above normal precipitation were increased significantly relative to the half month lead for both the interior West and the Southern Plains. An active pattern is expected to be especially prolonged for the Southern Plains leading to probabilities of above normal precipitation exceeding 60 percent in some areas. Conversely, the area favored to be drier than normal is expanded east to include the western Great Lakes as ridging is expected to dominate much of the northern tier of the CONUS for much of the month. Uncertainty is high along the Eastern Seaboard as a wet period associated with a frontal boundary is expected early in May followed by a dry period. This changeable pattern leads to an Equal Chances (EC) forecast for the East Coast. Farther to the north and west, an active period is likely for much of Alaska as a persistent low pressure system is expected to linger in the Gulf of Alaska for much of Early May. This should favor above normal precipitation for much of southeastern Alaska southward to parts of the Pacific Northwest. Probabilities of above normal precipitation exceed 50 percent for Southeast Alaska as increased wetness is favored to persist deeper into May in CPCs weeks 3-4 outlooks. ******************************************************************************* ****** Previous mid-month discussion below ****** ******************************************************************************* The May 2025 outlooks are issued against the backdrop of transition from the cold phase to the neutral phase of the El Nio Southern Oscillation (ENSO). The latest Nio 3.4 weekly sea surface temperature (SST) departures stood at -0.1 degrees Celsius which puts us firmly in ENSO-Neutral territory. ENSO-neutral conditions are expected to continue during the month of May. Meanwhile, the magnitude of the Real-time Multivariate (RMM) based Madden Julian Oscillation (MJO) index is weak. However, there is some evidence of a re-emergence of the MJO across the Western Pacific by late April. Uncertainty in this signal is high due to potential Kelvin Wave interference with this potential MJO re-emergence. Due to the current weakness and uncertainty in its evolution, the MJO did not play a major role in the May 2025 outlook. Farther to the north, across the extratropics of the Northern Hemisphere, the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), and the Pacific North America (PNA) index have all been positive for much of the past 10 days. However, all have trended less positive during the past few days with the AO and NAO both negative as of the most recent observations. Combined natural analog composites derived from the recent evolution of tropical and extratropical indices depict a 500-hPa flow pattern dominated by above normal heights across the Aleutians and southwestern Mainland Alaska as well as over the southwestern Contiguous United States (CONUS) for the month of May. Anomalous troughing is generally favored across the Great Lakes and Northeast as well as parts of Southeast Alaska. Meanwhile, trends during the last 15 years favor above normal heights across most of the eastern CONUS, eastern Alaska, and the Aleutians with trends toward below normal heights across much of the southwestern CONUS and parts of southwestern Alaska. As the sun angle increases during the month of May, antecedent soil moisture conditions become more of a factor in the outlook. Currently, soil moisture deficits are observed across much of the southwestern and southeastern CONUS, much of the Plains, and much of the Mid-Atlantic and Northeast. Soil moisture surpluses are currently observed for much of the Ohio, Tennessee, and Lower Mississippi Valleys, parts of the Upper Great Lakes, and parts of northern California and Oregon. Sea surface temperatures (SSTs) are currently above normal adjacent to the South Coast of Alaska, much of the West and Gulf Coasts of the CONUS, and the southern half of the Eastern Seaboard. Below normal SSTs are observed near the Northeast/northern Mid Atlantic coasts and near extreme southern California. Natural analog composites, trends, and dynamical guidance from the Copernicus Climate Change Service (C3S), North American Multi-Model Ensemble (NMME), the Couple Forecast System Version 2 (CFSv2), and the Forecast Consolidation Tool (CON) (which includes both dynamical model input and statistical guidance) form the basis of the May outlooks. The latest CPC Weeks 3-4 Outlooks and associated ensemble guidance from the European Centre for Medium Range Weather Forecasts (ECMWF), Global Ensemble Forecast System (GEFS), and the CFS also contributed. Antecedent conditions such as extratropical SSTs, sea ice extent, and soil moisture conditions were also considered where appropriate. Above normal temperatures are favored for most of the western CONUS, consistent with anomalous ridging. The strongest probabilities (greater than 50 percent) of above normal temperatures are indicated from western portions of the Rio Grande Valley northward to the Four Corners region, where dynamical and statistical guidance show the strongest agreement. A tilt toward above normal temperatures also extends northeastward to parts of the Northern and Central Plains consistent with observed below normal soil moisture. Probabilities of warmer than normal conditions are tempered across parts of the northwestern CONUS, due to cold signals depicted by ENSO-based natural analogs. Farther to the south and east, enhanced above normal temperature probabilities are indicated across the Southern Plains, Lower Mississippi Valley, and the Southeast, due to generally good agreement among dynamical and statistical model guidance. Probabilities of above normal temperatures exceed 40 percent for much of the Gulf Coast region and Southeast and exceed 50 percent over southern Florida, where SSTs in adjacent waters are warmer than normal. Equal chances (EC) of above, near, and below normal temperatures are indicated for the Northeast, Great Lakes, northern Mid Atlantic, as well as the Upper and Middle Mississippi Valley, the Ohio Valley, and the Tennessee Valley. Dynamical models generally depict warm or weak signals in these areas while statistical guidance generally favors cooler outcomes. Above normal soil moisture across the Ohio, Tennessee, and parts of the Middle and Lower Mississippi valleys reduce chances of above normal temperatures across these areas. Similarly, below normal SSTs adjacent to the northern Mid-Atlantic and Northeast increase chances of back-door cold fronts common for this time of year, resulting in a reduction of above normal temperature probabilities relative to what is depicted by dynamical model guidance such as the NMME and C3S and recent trends. Above normal temperatures are favored for southwestern Alaska and the North Slope, consistent with the consensus of dynamical and statistical model guidance. Above normal SSTs and below normal Arctic sea ice also contributed to increased above normal temperature probabilities for these areas. However, EC is indicated for much of the remainder of Alaska due to conflicting signals among dynamical and statistical model guidance and as well as the presence of lingering sea ice adjacent to parts of western mainland Alaska. Above normal precipitation is favored across the northern Mid-Atlantic region and southern portions of the Northeast, extending southwestward across the Ohio Valley and parts of the Lower and Middle Mississippi Valleys. The wetness across parts of the Mid-Atlantic and Northeast aligns with CPCs Weeks 3-4 outlooks, the final consolidation (for the Mid-Atlantic), and the potential for anomalous troughing depicted by the natural analogs. The southwestern extension of the area of enhanced above normal precipitation probabilities across much of the Ohio and Middle and Lower Mississippi Valleys is driven largely by abnormally high soil moisture content. Conversely, below normal precipitation is likely across much of the West due to good agreement among dynamical model guidance, natural analogs, and CPCs weeks 3-4 guidance. An tilt toward below normal precipitation extends to much of the Northern Plains consistent with currently observed below normal soil moisture. Similarly, a slight tilt toward below normal precipitation is indicated for much of Florida and adjacent areas of the Southeast consistent with below normal soil moisture and CPCs weeks 3-4 outlooks. Farther to the north, below normal precipitation is favored for southwestern Alaska consistent with CON guidance and increased ridging suggested by the natural analogs. A weak tilt toward above normal precipitation is indicated for eastern and northern Alaska due to monthly guidance from the CON, C3S, and recent trends. FORECASTER: Scott Handel 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 Jun ... will be issued on Thu May 15 2025 These outlooks are based on departures from the 1991-2020 base period. $$