898
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