Flood Potential Outlook
Issued by NWS
Issued by NWS
591 FGAK78 PACR 172319 CCA ESFAK Hydrologic Outlook NWS Alaska Pacific River Forecast Center Anchorage AK 400PM AKDT FRI MAY 17 2024 ...UPDATED BREAKUP OUTLOOK USING AN EXPERIMENTAL GRAPHICAL FORMAT... 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/ESFAK_ACR_20240503.pdf ...Spring Breakup and Flood Potential Outlook for Alaska... Current Conditions * As of May 13, the Kuskokwim River was ice free to 10 miles below the Johnson River. Kuskokwim Riverwatch demobilized that day. Last week, several ice jams on the lower Kuskokwim caused flooding in Bethel around Browns Slough, in Kwethluk, Napakiak and Napaskiak. Ice has since flushed out and flooding has subsided. Shorefast ice remains along the Kuskokwim delta coast. * The Middle Yukon Riverwatch team demobilized May 14. Breakup has progressed to past Pilot Station without notable flooding on the Yukon this year. Cooler temperatures this breakup season have kept snowmelt breakup gradual. Cooler temperatures are expected to continue, with this year leaning to a more thermal breakup on the Yukon River. Forecast Conditions * Yukon - Expect continued decay and likely thermal breakup for the rest of the Lower Yukon over the next week. Shorefast ice remains, which may be a complicating factor for Emmonak, Alakanuk and Nunam Iqua. Snowmelt flooding will begin to be the focus over the next week or two in Fort Yukon, due well above normal SWE in the Porcupine basin and forecasted above normal temperatures. * Buckland - expect the river ice to go out during next few days * Kobuk - expect the river ice to go out during the next few days The 2024 spring breakup is trending more towards 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 headwater 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. ...Updates to the previous Spring Breakup Outlook... Breakup is well underway this week. The Tanana is mostly ice-free, the upper and middle Kuskokwim broke up this week, breakup is imminent at Eagle, and many of the low elevation streams and rivers in Southcentral are mostly open. The Yukon is still frozen above Circle, and it is still winter in the Brooks Range and the Arctic. Forecast breakup dates in the table below have been adjusted per recent model analysis, but are largely near normal. ...Spring Breakup Flood Potential along major rivers in Alaska... Spring breakup village flood potential considers the climate outlook, snowpack, ice thickness and condition, historical likelihood of flooding and flood severity, and community knowledge. Village flood potential is reassessed continually as outlooks change and breakup season progresses. Please see most recent Experimental Product for figure at www.weather.gov/media/aprfc/BreakupProducts/ ...River Ice Observations... Late March through mid-April measurements indicate that ice thicknesses were near normal across the state. ...Snowpack... The modeled snow water equivalent map for April 27th, above, shows that melt is well underway in the central interior, but there is still snow to melt in western Alaska, southcentral, and the northern Yukon Territory. Snowpack depth and extent is used to estimate the snowmelt runoff potential across Alaska. ...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 Yukon-Kuskokwim Delta are below normal with highs in the mid 40s to 50s farther inland, and lows in the mid 30s to low 40s. These are good indicators of trending towards a thermal breakup. The Climate Prediction Center temperature outlook for the end of May indicates an increased chance of cooler than normal temperatures for southwest Alaska and normal to above normal temperatures for the eastern interior and north slope. Cooler temperatures in May have the greatest impact on the Lower Yukon River; the key will be where the ice jam front comes into contact with stronger downstream ice. ...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, breakup is expected to be 0-2 days early in the eastern half of Alaska. Western and Southwest Alaska are forecast to break up 0-2 days later, although some locations may trend towards median. 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 24, 2024. $$ CVB