000 FGAK78 PACR 032303 CCA ESFAK Hydrologic Outlook NWS Alaska Pacific River Forecast Center Anchorage AK 400PM AKDT FRI MAY 3 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 * Upper Yukon River- The main breakup front is upstream of Eagle, between Dawson and the Fortymile River. The ice continues to slowly degrade between the Fortymile and Eagle, and the strongest ice is at Eagle Bluffs, downstream of the Seventy Mile River. Of note, the ice appears to gain strength between Eagle and Circle, * Middle and Lower Yukon - No changes yet, anticipate timing to be close to normal. * Tanana - Breakup front is downstream of Manley Hot Springs. The Tanana at Nenana broke up on 4/27, 3 days ahead of median. The Chena is mostly open. Post breakup, water levels are increasing due to snowmelt along the Tanana. * Kuskokwim - Breakup front is at Kalskag, where minor flooding is occurring on 5/3/2024. Ice is largely in-place downstream of Kalskag and ice free upstream. * Southcentral - Breakup is progressing, with many of the lower elevation streams ice free. Snowmelt flooding is occurring in Glennallen. Forecast Conditions * Yukon - Eagle will be breaking up soon. Because the ice gains strength between Eagle and Circle, hydrologists will be watching the ice front carefully for flood potential in Circle. Satellite imagery of the Porcupine indicates that it is still white ice and has not yet begun melting. Water levels in Galena are coming up slowly, but no signs of breakup yet. * Tanana - hydrologists will be watching the pulse of snowmelt for the coming days. Temperatures in Fairbanks are expected to stay above freezing, even at night. * Kuskokwim - Cooler temperatures will modulate the speed of breakup, however the ice seems generally weak so a complete stall is unlikely. * Southcentral - Snowmelt flooding is expected to continue in Glennallen as snowpack is still present. The 2024 spring breakup is trending more towards a thermal* breakup across the Eastern Interior due to warmed late April temperatures, recent above average temperatures and strong solar input, which has helped 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. The one exception to the thermal trend is the community of Circle, which still appears to have strong ice in place. Across the western part of the state, including the middle/lower Yukon River, breakup is still trending towards a dynamic* breakup with a robust snowpack and intact river ice persisting early May. The Kuskokwim appears to be experiencing a largely thermal breakup. Despite cooler April temperatures, the ice on the upper river was weak, with breakup between Stony River and Kalskag occurring between May 1st and 3rd. *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 usually. Thermal breakups commonly result in fewer and less severe ice jams. ...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 interior are close to climate normals, with highs in the 50s and lows in the 30s. In Bethel, highs will be in the 40s, and lows in the 30s. The relatively normal temperatures and slow warm up are good indicators of trending towards a thermal breakup. The Climate Prediction Center temperature outlook for mid May indicates an increased chance of cooler than normal temperatures for southwest Alaska and normal temperatures for the eastern interior and north slope. Cooler temperatures in mid 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 10, 2024. $$ CVB
