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FGAK78 PACR 122353 CCA
ESFAK

Hydrologic Outlook
NWS Alaska Pacific River Forecast Center Anchorage AK
4PM AKDT FRI APR 12 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_20240412.pdf



...SPRING BREAKUP AND FLOOD POTENTIAL OUTLOOK FOR ALASKA...

Across the state, this year`s breakup is shaping up to be dynamic,
with temperatures forecasted to remain below normal through the end
of April, particularly for the western half of the state, with a
robust snowpack present across the northeast and western parts of
the state.  However, while some areas anticipate a dynamic breakup,
conditions aren`t as favorable as they were during last year`s
historic breakup season, marked by numerous significant ice jams and
snowmelt floods across the interior.  If temperatures remain cold in
the last half of April and into early May, it will increase chances
for a dynamic breakup and flooding.

*The two generalized types of river ice breakup are dynamic
(mechanical) and thermal. A dynamic breakup moves from headwaters
downstream in a somewhat linear fashion. Ice jam flooding occurs
more often during a dynamic breakup. A thermal breakup occurs when
the ice rots in place usually caused by a gradual warm up resulting
in very few and only minor ice jams.


...Updates to the previous Spring Breakup Outlook...

April 1 ice thickness data across the state continue to show near
normal values. April 1 snowpack measurements continue to show well
above normal snowpack for the North Slope, Porcupine River Basin,
Copper River Basin, and the Lower Yukon and Kuskokwim River Basins.
Recent aerial snowpack measurements show very high snow amounts in
the Nushagak Hills and Holitna River Basin, south of the middle
Kuskokwim River.  Normal to slightly below normal snowpack is
observed in the Upper Yukon and Tanana River Valleys.  Climate
outlooks are indicating temperatures to remain colder than normal
across most of the state through the end of April, especially across
the western half of the state.  If temperatures remain cold in the
last half of April and into early May, it will increase chances for
a dynamic breakup and flooding.


...River Ice Observations...

April 1st river ice observations are available for a limited number
of observing 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).
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 across the Yukon and 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 normal FDD, ranging from 89% to 100% of
normal.


...Snowpack...

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

Along the North Slope, though snowpack observations are limited,
winter time 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 part of the state, the Porcupine Basin reports a
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 normal.

Further down the Yukon Basin, snowpack levels remain near average
until reaching the lower Yukon, where the snowpack is ~120-135% of
normal. The Tanana River Basin and Upper Kuskokwim River Basin
report snowpack levels ranging from average to slightly below
average. However, in the middle to lower Kuskokwim River Valley,
snowpack levels are significantly higher, reaching approximately 150-
170% of the April 1 average, as indicated by ERA5 analysis. This
assessment is corroborated by anecdotal reports and in-situ
observations, notably in Bethel, which has recorded a record high
snow depth for April, marking the highest measurement for the month
in 100 years of records.

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 and Kenai Peninsula
are experiencing above-average snowpack.

In summary, the state`s snowpack is generally above average, with
anomalies increasing from average in the east to above average in
the west. Well above average snowpack is noted in the Porcupine
Basin, Lower Kuskokwim and Yukon Basins, as well as the Copper
Basin. 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.

A brief warming trend is expected by the middle of next week for
most of the mainland with daytime temperatures expected to warm into
the 50s in the valleys across the eastern interior and the Canadian
Yukon. However, NOAA’s Climate Prediction Center (CPC) April 12th
outlooks favor colder than normal temperatures for the northwestern
third of the state with normal temps expected for the rest of
mainland through the third week in April. By the 4th week in April,
the CPC is favoring a strong chance of below average temperatures
across nearly the entire mainland, especially over the western half
of the state. By the beginning of May, the temperature outlook
becomes less uncertain with the CPC favoring a slight chance of
below normal temperatures across the southwest portion of the state
with equal chances of above and below normal temperatures across the
rest of the mainland.

Looking ahead into May and June, there`s an elevated  probability
(40-60% chance) of above-normal temperatures across all of Alaska.


...Timing of River Ice Breakup...

Timing of breakup is expected to be 1-2 days late for the eastern
half of the state and 2-5 days late for the western half of the
state. The North Slope is expected to breakup near the median date.

For more detail and to see the Flood Potential Map refer to the
APRFC website at: https://www.weather.gov/aprfc/floodpotential

The next Spring Breakup Outlook will be published April 14, 2024.


$$
KVP