Flood Potential Outlook
Issued by NWS

Home |  Current Version |  Previous Version |  Text Only |  Print | Product List |  Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13
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