SRAK48 PACR 261732 CCB RVAAK Alaska Spring Breakup Summary NWS Alaska Pacific River Forecast Center Anchorage 1000 AM AKDT SUN MAY 26 2024 The Breakup Summary is issued seasonally after breakup begins. The Alaska-Pacific River Forecast Center is updating the format of its breakup outlook and summary products. The new experimental format proposes to move to a more graphical presentation, away from the historically text-based product below.  The likelihood of flooding from snowmelt and/or ice jams is initially calculated based on the flood frequency for the current 2000 to 2021 historical record and adjusted to reflect current conditions. The experimental graphical format Alaska Spring Breakup Outlook products are posted to the Alaska-Pacific River Forecast Center website at: www.weather.gov/aprfc/breakupProducts A direct link to the latest graphical product is: www.weather.gov/media/aprfc/BreakupProducts/RVAAK_ACR_20240525.pdf ..Spring Breakup and Flood Potential Summary... Current Conditions as of May 25: * The Yukon River breakup front had pushed through Emmonak and Alakanuk in the south channel, and past Kotlik to the north. * Last week, an ice jam upstream of Hill Island caused minor flooding in Pilot Station; this jam released on May 20. * Another ice jam near Emmonak caused minor flooding for Emmonak and Alakanuk from May 23-25; this jam released on May 25. * On May 24, Fort Yukon water levels began to rise due to snowmelt runoff from the Porcupine River. Minor flooding continues and will persist for the next several days. * North Slope rivers have started flowing water. * Kobuk and Noatak rivers have broken up at all communities but stranded ice remains in the lower reaches. * Buckland - local breakup occurred last week. Intact ice remains in place upstream. No ice jam flooding is expected. The 2024 spring breakup has behaved generally more as a thermal* breakup across much of Alaska. In the Eastern Interior late April temperatures were warm, helping to deplete low elevation snowpack and degrade river ice across the middle and upper Tanana River as well as portions of the Upper Yukon River. Across the western part of the state, including the middle/lower Yukon River, breakup has been slow because temperatures are still gradually easing out of winter. *The two generalized types of river ice breakup are dynamic (or mechanical) and thermal. A dynamic breakup is characterized by cold early spring air temperatures followed by rapid warming, and can be compounded by above average headwaters snowpack and river ice thicknesses, and generally moves the breakup ice front downstream in a somewhat linear fashion. Ice jam flooding occurs more often during a dynamic breakup. A thermal breakup occurs from gradually warming air temperatures, where the ice simply rots in place. Thermal breakups does not mean no flooding, ice jams do occur but they are commonly less severe. Forecast Conditions * Yukon - Some shorefast ice persists near the mouth, but any flood threat for Kotlik and Nunam Iqua remains low. Expect likely thermal breakup for the very final stretch of the Lower Yukon in the next few days. Snowmelt flooding will remain the focus over the next week or two in Fort Yukon, due well above normal SWE in the Porcupine/Coleen basin and forecasted above normal temperatures. ...Climate Outlook... The most important factor determining the severity of ice jam breakup remains weather immediately prior to and during breakup. Dynamic breakups, with the high potential for ice jam flooding typically require an abrupt warm up in temperature. In the near term, temperatures in the lower Yukon continue below normal with highs in the low 40Fs to 50Fs farther inland, and lows near 30F. For the Arctic, conditions continue to be below normal with highs in the 30Fs and lows in the mid 20Fs. These are good indicators of continuing a thermal breakup. The Climate Prediction Center temperature outlook for the beginning of June indicates an increased chance of cooler than normal temperatures for southwest Alaska and normal temperatures for the northwest and north slope. Cooler temperatures in late May and early June could lead to further delaying breakup north of the Brooks Range. ...Spring Breakup Timing... Timing of river breakup considers all previously described variables and relates it to the historical median breakup date for individual locations. Based on this evaluation, the North Slope is expected to break up closer to its median date. ...Flood Potential... Please refer to the Flood Potential Map at the APRFC website for timing and flood potential details at: https://www.weather.gov/aprfc/floodpotential Tables in the Experimental Product give an estimation of snowmelt runoff volume, flood potential, and forecast breakup date range for various locations across the state. To view the tables and for additional information please visit: https://www.weather.gov/aprfc/breakupProducts The next Spring Breakup Outlook will be published May 31, 2024. $$ JO/MO