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000
FGAK78 PACR 200039 CCA
ESFAK

Hydrologic Outlook
NWS Alaska Pacific River Forecast Center Anchorage AK
440PM AKDT FRI APR 19 2024


...UPDATED BREAKUP OUTLOOK ISSUED 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 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_20240419.pdf



...Spring Breakup and Flood Potential Outlook for Alaska...

Across Alaska, the 2024 spring breakup is shaping up to be dynamic*.
Temperatures are forecasted to remain below normal through the end
of April, particularly for the western half of the state, and a
robust snowpack persists across northeast and western Alaska.
However, while we anticipate a dynamic breakup in some areas this
year, the conditions aren`t as favorable as they were at the
beginning of the 2023 historic breakup season, which was marked by
numerous significant ice jams and snowmelt floods across the
Interior. If air temperatures remain cold through the remainder of
April and into early May, the chances increase for a dynamic breakup
and localized flooding.

*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...

Ice thickness across Alaska remains near normal through April. April
1 statewide snow summary showed well-above normal snowpack for the
North Slope, Porcupine, Copper, Lower Yukon, and Kuskokwim river
basins. Recent aerial snowpack measurements show very high snow
amounts in the Nushagak Hills near Dillingham and the Holitna River
near Sleetmute basins, as well as in the lower Kuskokwim River below
Aniak. Normal to slightly below normal snowpack was observed in the
Upper Yukon and Tanana river valleys. Climate outlooks favor
elevated chances of above normal temperatures in central and eastern
Alaska and equal chances of above, below, and/or normal temperatures
in western Alaska through the first half of May.


...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...

April 1st river ice observations are available for a limited number
of sites in Alaska. Late March through mid- April measurements
indicate that ice thicknesses are near normal across the state.
Observations across interior Alaska range from 81%-111% of normal.
However, dense jumble ice has been observed on the middle Yukon
River between Rampart and Tanana. Recent UAF Fresh Eyes On Ice team
(FEOI) reconnaissance confirmed that ice this year (2024) along the
middle Yukon and Tanana River was on average thicker than last year
(2023). Yukon River ice thickness at Eagle appears to be thicker
than the past two winters (2022-2023), with river observers noting
significant jumbled ice in the vicinity. Observers on the Kuskokwim
River reported normal to slightly below normal ice thicknesses
between Aniak and Bethel, with recent reports indicating that river
ice has begun to deteriorate, and/or soften in some locations. Ice
thicknesses along the Kuskokwim are, on average, 20% less than this
time last year. No freeze-up jams or mid-winter breakups were
reported in the Yukon or Kuskokwim river basins this winter.


...Freezing Degree Days...

Cumulative freezing degree days (FDD), which can serve as a proxy
for river ice thickness, are near normal across most of Alaska.
Colder conditions were observed across coastal sites along the Gulf
of Alaska (Homer to Sitka), where FDD was reported to be 110% to
200% of normal. Near normal FDD conditions have been observed across
Southcentral and Copper River Valley. The West Coast, Interior, and
North Slope observed near to below normal FDD, ranging from 83% to
101% of normal.


...Snowpack...

April 1 Natural Resources Conservation Service (NRCS) snowpack
analysis indicates an above average snowpack for the majority of the
state.

Although snowpack observations are limited, North Slope winter
precipitation is well-above average indicating likely above average
snowpack north of the Brooks Range. In fact, year-to-date
precipitation at Utqiaġvik Airport and Prudhoe Bay are the highest
on record. The record at Utqiaġvik goes back eighty-nine years.

In the northeastern region of Alaska, the Porcupine basin has a
measured snowpack well-above average, with Old Crow village in the
northern Yukon Territory recording its highest snowpack levels in
over four decades of monitoring. However, as one moves towards the
southern half of the Yukon Territory, snowpack conditions vary, with
areas from Whitehorse to Mayo experiencing average to below-average
levels. Meanwhile, the Upper Yukon basin spanning from Dawson to
Fort Yukon, as well as the Fortymile basin, exhibit snowpack levels
well-above average, measuring around 130% of the 1991-2020 normal.

Further down the Yukon, snowpack remains near average until reaching
the lower Yukon, where the snowpack is ~120-135% of normal. The
Tanana and Upper Kuskokwim basins snowpack ranges from average to
slightly below average. However, in the middle to lower Kuskokwim
snowpack levels are significantly higher, reaching approximately 150-
170% of the April 1 average (1991-2020), as indicated by ERA5 model
analysis. This assessment is corroborated by anecdotal reports and
in-situ observations, notably in Bethel, which has a measured record
high April 1 snow depth based on 100 years of record.

In Southcentral Alaska, the snowpack in the Copper basin stands at
around 140% of the April 1 average. Monitoring sites throughout the
basin are consistently reporting higher-than-average snowpack.
Notably, four sites within the basin rank within the top three of
historical records for snowpack levels as of April 1. However, it`s
important to note that despite this above-average snowpack, the
basin`s snowpack remains lower than the levels observed in 2022
(180%) and 2023 (160%), both of which resulted in flooding in
Glennallen.

Across the rest of Southcentral Alaska, the Susitna basin is
reporting normal snowpack, while the Cook Inlet watershed and Kenai
Peninsula have an above-average snowpack.

Snowpack depth and extent is used to estimate the snowmelt runoff
potential across Alaska.

In summary, the state`s snowpack is generally above average, with
anomalies increasing from average in the east to above average in
the west. A well-above average snowpack is noted in the Porcupine,
Yukon, and Lower Kuskokwim, as well as the Copper basins. Although
above average this year, snowpack across most of the interior is
notably less than the previous two years.

The next NRCS Statewide Snowpack Summary is expected after the first
week of May.


...Climate Outlook...

The most important factor determining the severity of ice breakup
remains the weather during April and May. Dynamic breakups, with a
high potential for ice jam flooding, typically require cooler than
normal temperatures in early April followed by an abrupt transition
to warm, summer-like temperatures in late April to early May.

The amplified pattern across mainland Alaska is expected to quickly
transition from enhanced ridging to moderate troughing over the
eastern Bering Sea and Arctic Ocean through the end of April. Under
this pattern, the Climate Prediction Center’s climate outlook for
the 8-14 day range favors better chances for below normal
temperatures for the western half of the state. Meanwhile over
eastern Alaska, persistent ridging will yield higher chances for
above normal temperatures, primarily along the Yukon border.

The week 3-4 timeframe continues to feature better than average
chances for above normal temperatures for much of eastern Alaska.
For interior and western Alaska, equal chances of above, below,
and/or normal temperatures are predicted. It should be noted that
there is considerable uncertainty during this time period leading to
low confidence in any solution.

Looking ahead at the longer, 3-month outlook, which includes May,
June, and July, there`s an elevated probability (40-60% chance) of
above-normal temperatures across the eastern two-thirds of Alaska.


...Spring Breakup Timing...

Timing of river breakup takes into account 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 later in the eastern half of Alaska, although some
locations may break up early. Western and Southwest Alaska are
forecast to break up 1-4 days later. The North Slope is expected to
break up closer to its median date.


...Flood Potential...

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.

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.

Median breakup dates are for the period 1980 through 2023 and are
calculated for locations with at least 5 years of data.

Forecast breakup timing is expressed as a range based on snowmelt
runoff volume and flood potential. Locations where breakup has
already occurred are identified with two asterisks preceding a
single date; for example Kuskokwim River at Nikolai breakup occurred
on April 16, 2024 (**4/16).


Please refer to the Flood Potential Map at the APRFC website for
timing and flood potential details at:
https://www.weather.gov/aprfc/floodpotential

For additional information please visit:
https://www.weather.gov/aprfc/breakupProducts

The next Spring Breakup Outlook will be published April 26, 2024.


$$
JSO/MSO