National Weather Service United States Department of Commerce

127
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