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FGAK78 PACR 201709 CCA
ESFAK
HYDROLOGIC OUTLOOK NWS ALASKA PACIFIC RIVER FORECAST CENTER
ANCHORAGE AK 1PM ADT FRI APRIL 19 2019
..SPRING BREAKUP OUTLOOK FOR ALASKA...
The Spring Breakup flood potential in Alaska is forecast to be below
average with the exception of North Slope rivers and Northwest rivers
including the Koyukuk River where the flood threat is average. This
forecast is based on observed snowpack, ice thickness reports, and
long-range temperature forecasts. Recent observed and forecast
temperature analysis suggests an earlier than normal breakup for all
locations statewide.
Timing of breakup for Yukon River locations are expected to range
between 5 to 10 days earlier than the average breakup dates. Breakup
for Kuskokwim River locations are expected to range between 1 to 3
weeks earlier than average. With the expected cooler temperatures
over the next several weeks, breakup at many locations could be
several days or more later than the initial spring outlook. New dates
are relfected in the table below.
Temperatures - Temperatures across the state were significantly above
normal during March and the 2 weeks of April which caused significant
melt of low elevation snowpack. In addition, the warmer than normal
temperatures initiated river ice decay much earlier than normal.
However, current and forecast temperatures starting the third week of
April have changed from well above normal through most of the state
to normal or below normal. The largest below normal temperatures are
expected to occur over western and interior parts of the state over
the next five days. Cooler temperatures will slow down the progress
of breakup.
Ice - April ice thickness data are available for a limited number of
observing sites in Alaska. Recent measurements indicate that ice
thickness is below average across the state with locations generally
between 60-90 percent of normal. A few exceptions are at Eagle where
the Yukon River ice thickness was 112% of normal and on the Kuskokwim
River at Bethel where the ice thickness was 19% of normal.
Snow - The April 1 analysis of snowpack by the Natural Resources
Conservation Service (NRCS) has above normal snowpack in the
northwest part of the state including the Koyukuk and Kobuk river
basins as well as on the Northslope. The remainder of the state has a
below normal snowpack with the the Tanana and Upper Yukon basins
about 75% of normal.
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
average temperatures for most of April followed by an abrupt
transition to warm summer-like temperatures in late April to early
May. The most recent 6-10 day temperature forecast and 8-14 day
temperature forecast indicate cooler than normal temperatures across
much of the state which would last through the end of April. The One
month outlook for May then shows warmer than normal temperatures over
the entire state with the greatest chances for above normal
temperature in Southwest Alaska.
Further information can be found at the following web sites:
Weather-
https://www.weather.gov/aprfc/AGAK78PACR
Snow-
https://www.weather.gov/aprfc/nrcs_ak_swe
https://www.weather.gov/aprfc/snow_depth
https://www.wcc.nrcs.usda.gov/snow/
Ice thickness-
https://www.weather.gov/aprfc/icethickness
Climate prediction-
http://www.cpc.ncep.noaa.gov/
River Ice Breakup Flood Threat:
A dynamic breakup moves from the headwaters of a river downstream in
a somewhat linear fashion and often during the process ice jams occur
that can cause flooding. A thermal breakup is where the river ice
rots in place primarily due to a gradual warm up and little snowmelt
runoff. A thermal breakup usually results in fewer ice jams and less
chance of flooding. Due to the warmer than normal March and early
April along with thinner than normal river ice a thermal breakup is
highly likely this year along the Yukon and Kuskokwim Rivers with
less potential than usual for ice jams to form that cause flooding.
It is still possible for flooding ice jams to occur but the
probability is less than normal. This trend diminishes for rivers on
the northslope and northwest Alaska, including the Koyukuk River,
which did not experience significnat ice degradation during this
warmer period. These areas also have a deeper than normal snowpack
resulting in an average threat of flooding in these areas.
Recent Observed River Conditions as of Friday April 18th:
Breakup has occurred on the lower Kuskokwim River from the Mouth of
the River up through Kwethluk with some ice in place at Akiak and
Akiakchak. Ice is mostly in place upstream of Akaik on the Kuskokwim.
Ice is out on most South Central rivers with some ice possibly
remaining on the lower Susitna. The Copper River is generally ice
free with ice remaining in some tributaries. The entire Tanana River
is generally some open to mostly ice free. The Yukon River remains
ice covered with some short open areas reported near tributaries.
THE FOLLOWING TABLE GIVES AN ESTIMATION OF FLOOD POTENTIAL FOR
VARIOUS LOCATIONS AROUND THE STATE AND BASIN RUNOFF VOLUMES.
---------------------------------------------------------------------
SNOWMELT RUNOFF VOLUME...EXPECTED WATER VOLUME FROM SNOWMELT DURING
THE MELT SEASON.
FLOOD POTENTIAL...THE LIKELIHOOD OF FLOODING FROM SNOWMELT AND/OR
ICE JAMS. THIS IS INITIALLY CALCULATED BASED ON
THE FLOOD FREQUENCY FOR THE 2000 TO 2017 HISTORICAL
RECORD AND ADJUSTED TO REFLECT CURRENT CONDITIONS.
* AVERAGE (MEDIAN) BREAKUP DATES ARE FOR THE PERIOD 1980 THROUGH
2018 AND ARE CALCULATED FOR LOCATIONS WITH AT LEAST 5 YEARS OF
DATA.
** ACTUAL BREAKUP DATE FOR THIS YEAR
---------------------------------------------------------------------
SNOWMELT FLOOD AVERAGE NO. OF FORECAST
RIVER - REACH RUNOFF POTENTIAL BREAKUP YEARS BREAKUP
VOLUME DATE* USED DATE
------------- ---------- --------- ------- ------ --------
SOUTHEAST PANHANDLE BELOW
KENAI RIVER BELOW LOW
ANCHOR RIVER BELOW LOW
MATANUSKA RIVER BELOW LOW
SUSITNA RIVER BELOW
GOLD CREEK LOW
SUNSHINE LOW 05/03 30 OPEN
YENTNA RIVER BELOW
LAKE CREEK/FISH CREEK LOW 05/01 27 **04/15
SKWENTNA RIVER BELOW
SKWENTNA LOW 04/30 24 **04/12
CHENA RIVER BELOW
FAIRBANKS LOW 04/26 27 OPEN
TANANA RIVER BELOW
CHISANA @ NORTHWAY LOW 04/26 28 **04/10
SALCHA LOW
FAIRBANKS LOW 04/29 19 **04/16
NENANA LOW 04/30 39 **04/14
MANLEY LOW 05/03 28 04/18-04/24
KUSKOKWIM RIVER BELOW
NIKOLAI LOW 04/23 34 **03/31
MCGRATH LOW 05/05 39 04/26-05/03
STONY RIVER LOW 05/02 31 04/24-05/01
SLEETMUTE LOW 05/02 30 04/26-05/03
RED DEVIL LOW 05/04 33 04/26-05/03
CROOKED CREEK LOW 05/04 33 04/26-05/03
ANIAK LOW 05/06 36 04/25-05/01
KALSKAG LOW 05/06 30 04/24-04/30
TULUKSAK LOW 05/07 27 04/23-04/29
AKIAK LOW 05/09 33 04/23-04/29
KWETHLUK LOW 05/05 7 **04/07
BETHEL LOW 05/10 39 **04/12
NAPAKIAK LOW 05/11 24 **04/14
YUKON RIVER (UPPER) BELOW
DAWSON, YT LOW 05/04 39 04/24-05/01
EAGLE LOW 05/04 38 04/25-05/01
CIRCLE LOW 05/08 35 04/29-05/05
FORT YUKON LOW 05/11 35 04/27-05/03
BEAVER LOW 05/10 23 04/29-05/05
STEVENS VILLAGE LOW 05/11 22 05/02-05/08
RAMPART LOW 05/11 24 04/30-05/06
YUKON RIVER (MIDDLE) BELOW
TANANA LOW 05/08 34 04/26-05/02
RUBY LOW 05/09 33 04/27-05/03
GALENA LOW 05/11 38 04/29-05/05
KOYUKUK LOW 05/10 13 04/28-05/04
NULATO LOW 05/11 23 05/01-05/07
KALTAG LOW 05/12 33 05/01-05/07
ANVIK LOW 05/14 32 04/29-05/05
YUKON RIVER (LOWER) BELOW
HOLY CROSS LOW 05/15 32 04/23-05/04
RUSSIAN MISSION LOW 05/16 33 04/25-05/01
MARSHALL LOW 05/15 27 04/28-05/04
PILOT STATION LOW 05/14 22 05/02-05/08
MOUNTAIN VILLAGE LOW 05/17 32 05/01-05/07
ALAKANUK/EMMONAK LOW 05/21 32 05/03-05/09
KOYUKUK RIVER ABOVE
BETTLES LOW 05/10 37 04/29-05/05
ALLAKAKET LOW 05/10 33 04/28-05/04
HUGHES LOW-MOD 05/11 32 04/28-05/04
SEWARD PENINSULA ABOVE
BUCKLAND LOW-MOD 05/19 29 05/04-05/10
KOBUK RIVER ABOVE
KOBUK LOW-MOD 05/14 35 04/29-04/30
SHUNGNAK LOW 05/17 29 05/03-05/09
AMBLER LOW 05/17 35 05/01-05/07
NOATAK RIVER ABOVE
NOATAK LOW 05/19 23 05/04-05/10
BROOKS RANGE - NORTH ABOVE
COLVILLE @ UMIAT LOW 05/25 18 05/15-05/22
COLVILLE @ COLVILLE LOW 06/04 21 05/26-06/02
SAGAVANIRKTOK RIVER ABOVE
DALTON HWY LOW-MOD
FOR MORE DETAIL AND TO SEE A FLOOD POTENTIAL MAP REFER TO OUR WEB
SITE AT HTTPS://WWW.WEATHER.GOV/APRFC/FLOODPOTENTIAL
THE NEXT SPRING BREAKUP OUTLOOK IS SCHEDULED AT 2 PM ADT Wednesday
April 24th 2019.
$$
bcj
