175 FXUS07 KWBC 301901 PMD30D Prognostic Discussion for Monthly Outlook NWS Climate Prediction Center College Park MD 300 PM EDT Tue Apr 30 2024 30-DAY OUTLOOK DISCUSSION FOR MAY 2024 The May 2024 Temperature and Precipitation Outlooks are updated considering current Weather Prediction Center (WPC) Outlooks for week 1, Climate Prediction Center (CPC) Extended Range and Subseasonal Outlooks, dynamical model guidance for week 2 and weeks 3-4, soil moisture, background climate states, and recent Climate Forecast System version 2 (CFSv2) monthly forecasts of temperature and precipitation. Currently, equatorial sea surface temperatures (SSTs) are above average across the central and east-central Pacific Ocean and El Nio conditions are observed. However, tropical Pacific atmospheric anomalies are weakening and a transition from El Nio to ENSO-neutral is likely by April-June 2024, with the odds tilted toward La Nia developing by June-August 2024. Despite the weakening of El Nio some lingering impacts are expected into May. The Madden Julian Oscillation (MJO) weakened during mid to late April, but dynamical model forecasts favor a re-emerging MJO across the eastern Indian Ocean, with propagation across the Maritime Continent and Western Pacific forecast during early May. Some weak impacts due to MJO are possible in early May, though teleconnections tend to be weak in spring. CPC 6-10 Day Outlooks (early May) feature a 500 hPa trough over the Bering sea and Alaska and down the west coast, while ridging is forecast over the central and eastern Contiguous United States (CONUS). This pattern is persistent but less amplified in the CPC 8-14 Day Outlook period (through mid-May). By weeks 3-4, CFSv2, GEFSv12, and ECMWF 500 hPa predictions generally favor ridging over the western CONUS and troughing over the east, with differences in placement of ridge or trough axes and strength of the height pattern. This indicates a fairly transient pattern expected throughout the month of May, with troughing moving eastward and becoming less certain in terms of placement and amplitude by the end of the month. The updated Monthly Temperature Outlook remains similar to the previously released Outlook over the eastern CONUS where above normal temperatures are favored, though there are some changes to the spatial extent. Ridging over the eastern CONUS through mid-May supports the tilt toward above normal temperatures, and good model agreement exists over parts of the Southwest and Texas as well as the Great Lakes and parts of New England, thus probabilities remain similar to the previously released Outlook. Though short term forecasts through mid-May are overall warm over the eastern CONUS, model runs for week 3-4 begin to introduce troughing and cooler temperatures by the end of the month stretching from the Central Plains to the Mid-Atlantic. However, the below normal temperature probabilities are weak due to uncertainty in the forecasted mid-level height pattern. Given the warm beginning and middle of the month, but weaker and uncertain below normal probabilities by the end of May, 33 to 40% probabilities are maintained for much of the region. The largest change from mid-month is to the northwestern edge of the above normal probabilities. Shorter term forecasts favor below normal temperatures from the west coast overspreading through the Central and Northern Plains from early to mid-May beneath favored troughing that is forecasted to shift eastward with time. Given the forecasted cooler start to the month over the west that could stretch into the central and north central CONUS by mid-month, the western edge of above normal temperature probabilities is shifted eastward compared to the initial release. The forecasted cooler start and transition to above normal temperatures by the end of the month supports removal of the above normal probabilities over the Northwest in favor of Equal Chances of Above, Near, and Below Normal Temperatures (EC). Near normal is maintained over southern California and parts of the the Southwest where earlier North American Multi-Model Ensemble (NMME) and Copernicus Model Suite (C3S) forecasts were in good agreement, but is shifted toward the coast where we might see mixed temperatures throughout May due to the potential for marine layer intrusions. Over Alaska, below normal temperatures are indicated over the southwest where EC was previously indicated due to persistent below normal sea surface temperature (SST) anomalies and better model agreement in updated runs. Several changes are made in the updated May 2024 Precipitation Outlook. Given the transient pattern forecast throughout May, the Precipitation Outlook is relatively uncertain, and areas of EC are indicated where signals are weak and inconsistent in forecasts throughout May. EC is now favored in place of the weak tilt toward below normal precipitation previously indicated over northern Washington, Idaho, and Montana due to the potential for above normal precipitation in the beginning of May, and weak and inconsistent signals throughout the remainder of the month despite the expected below normal El Nio influence. A weak tilt toward below normal precipitation is still forecast over the Southwest where early to mid-May forecasts tilt toward below normal along with CFSv2 monthly forecasts, but probabilities are damped considering Weeks 3-4 Outlooks that indicate EC or above normal precipitation over parts of the region. Above normal precipitation is indicated from the Central and Southern Plains eastward to the Mid-Atlantic coast, but tilts toward EC along coastal areas of Gulf States where forecasts vary throughout the month of May, and models show weak or inconsistent signals. Below normal precipitation probabilities are favored over Peninsular Florida where below normal forecasts are more consistent in models, and where the CPC Week-2 Hazards Outlook indicates Rapid Onset Drought Risk. A weak tilt toward above normal precipitation is depicted over eastern North Dakota, Minnesota, and the western Great Lakes due to forecasted early to mid-May above normal precipitation and consistent signals in most recent CFSv2 runs. Precipitation probabilities over Alaska remain similar to the mid-month Outlook. ****************************************************************************** ***** Previous mid-month discussion below ***** ****************************************************************************** El Nio conditions are currently observed, and equatorial sea surface temperatures (SSTs) are above average across the much of the central and eastern equatorial Pacific Ocean, reaching 0.9 degrees Celsius above normal in the Nio3.4 region, 0.6 degrees Celsius above normal in the Nio3 region, but have decreased to -0.2 degrees Celsius below normal in the Nio1+2 region. Above average SSTs have weakened across most of the equatorial Pacific Ocean in the last 4 weeks, and tropical Pacific atmospheric anomalies are weakening as well. Given the weakening signals in the equatorial Pacific, a transition from El Nio to El Nio Southern Oscillation (ENSO)-neutral conditions is likely by April-May-June 2024. The Madden Julian Oscillation (MJO) has continued to weaken recently and though there has been eastward propagation in the past week, the amplitude has dropped. Dynamical model forecasts of the Realtime Multivariate MJO (RMM) index through close to the end of April are weak and incoherent due to an emerging low-frequency feature over the western Indian Ocean which appears to be interfering with the MJO. While we may see some lingering impacts to temperature and precipitation due to El Nio, the influence is expected to weaken throughout the next few months. Moreover, the weak amplitude of the MJO does not support much of a teleconnection response over the Contiguous United States (CONUS). As such, the May 2024 Temperature and Precipitation Outlooks are mainly supported by the lingering influence of El Nio, local SST anomalies, influence of soil moisture anomalies, dynamical model guidance from the North American Multi-Model Ensemble (NMME), Copernicus model suite (C3S), and the Climate Forecast System version 2 (CFSv2), as well as statistical models that include the influence of trend and ENSO. The Week 3-4 Outlook for the early part of May was also considered, as was the expected evolution from the Week 2 forecast period. The May 2024 Temperature Outlook favors above normal temperatures over the Northwest, much of the eastern half of the CONUS, and southeastern Alaska. A relatively small region of near normal temperatures is indicated over parts of California, Arizona, Nevada, and Utah. The highest probabilities of above normal temperatures, 50 to 60 percent, are located over Washington, the Great Lakes, northern New England, southeastern New Mexico and parts of southwestern Texas, the southern tip of Florida, and southeastern Alaska. Probabilities are enhanced over these regions given good consistency among tools on above normal probabilities. Decadal trends also support above normal probabilities over the northwest and northern New England. Anomalously warm Great Lakes temperatures provide additional support for above normal probabilities over the region. Lower than normal soil moisture along with forecasted below normal rainfall in dynamical models, and strong probabilities for above normal temperatures in recent CFSv2 forecasts of May temperatures adds to confidence over the Southwest. Signals were more mixed over western Alaska given cooler SST anomalies and sea ice, but NMME and C3S favor a transition to warmer temperatures over the southeast part of the state. Near normal and equal chances of above, near and below normal temperatures (EC) are indicated over southern California and the Four Corners where there are observed coastal below normal to neutral SSTs, near normal temperature forecasts from NMME and C3S, and a below normal CFSv2 temperature forecast. While tools tilt toward above normal temperatures over the Gulf States (particularly coastal regions), partially supported by warmer Gulf of Mexico SSTs, high amounts of recent rainfall over eastern Texas and Louisiana have resulted in high soil moisture, and as such probabilities are still above normal but damped. EC is favored where models were inconsistent and/or where there was a lack of support from statistical tools. There is more uncertainty in the May 2024 Precipitation Outlook than the Temperature Outlook, as evidenced by lower overall probabilities and larger areas of EC. Models favor above normal precipitation over parts of the central and southern CONUS, including parts of the Mid-Atlantic, Middle Mississippi and Ohio Valley regions. Some lingering influence from El Nio further supports the tilt toward above normal precipitation over the Southeast, as well as enhanced soil moisture. Though El Nio can lead to below normal precipitation over the Great Lakes, Middle Mississippi Valley, and Ohio Valley, this influence is overshadowed by above normal precipitation trend. EC is indicated over the Great Lakes where models had weak or inconsistent signals and trend and El Nio influence are opposite. However, over the Mississippi and Ohio Valley regions C3S and NMME favored above normal rainfall and thus a weak tilt toward above normal is indicated. Dynamical models and dry soil moisture led to the favored probability of below normal precipitation over parts of the southwest, which is also indicated in the Seasonal Outlook for May-July 2024. However, this is at odds with the recent Weeks 3-4 Precipitation Outlook for early May that tilted weakly toward above median precipitation. We expect below normal precipitation over the region by mid- to late- May. Tools were again mixed over the northwest, but influences from El Nio and trend tilt the odds toward below normal. Finally, over Alaska, above normal precipitation is favored over the southwest where there has been recent above normal precipitation, with a weak tilt toward below normal precipitation over southeastern Alaska due to expected influence from El Nio. FORECASTER: Johnna Infanti 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 16 2024 These outlooks are based on departures from the 1991-2020 base period. $$