Flood Potential Outlook
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FGAK78 PACR 222026
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HYDROLOGIC OUTLOOK
NWS ALASKA PACIFIC RIVER FORECAST CENTER
ANCHORAGE AK
1 PM AKDT WED APR 22 2020

..SPRING BREAKUP OUTLOOK FOR ALASKA...

Breakup Update: The Forty Mile river upstream of the Taylor Highway
has broken up as well as the Kuskokwim River near Nikolai. The Tozitna
River has broken up with an ice jam forming and releasing. The Aniak
River is reported to have broken up dynamicaly and is mostly open at
the confluence with the Kuskokwim. Ice is beginning to lift on the
Kuskokwim, Yukon and Koyukuk Rivers.

The 2020 Alaska Spring Breakup flood potential is forecast to be
generally above average south of the Brooks Range and average flood
potential for the North Slope. The Flood Potential forecast is based on
observed snowpack, ice thickness reports, and long-range temperature
forecasts and focuses on the Kuskokwim, Tanana , Yukon, Koyukuk,
Copper, and North Slope rivers. Recent observed and forecast
temperatures suggest a generally earlier than normal, more dynamic
type breakup for locations statewide.

Timing of breakup statewide is expected to be several days earlier
than normal. Breakup along the Yukon River upstream of Fairbanks
is expected to be 1 to 3 days earlier than median dates; generally 3 days
earlier at locations downstream from Fairbanks to Anvik; and 3 to 5 days
earlier at locations downstream from Anvik to the mouth. Breakup along the
Kuskokwim River at Nikolia downstream to Tuluksak are expected to be 1
to 2 days earlier than median dates and 2 to 5 days earlier downstream
from Tuluksak to the mouth.

Temperatures - Statewide, temperatures over the past few weeks generally
have been normal to below normal. However, temperatures over the next
several weeks are expected to be normal in areas north of the Brooks
Range and above normal for the remaining parts of the state. Statewide,
temperatures for April and May are expected to be above normal.

Ice - The April 1st ice thickness data indicate that ice thickness is near
normal across the state. A few measurements across interior Alaska
ranged from 32 inches at Galena, which is 82% of average; 33 inches
at Nenana, which is 80% of average; 48 inches at Eagle, which is
117% of average.

Snow - April 1st snowpack by the Natural Resources Conservation Service
(NRCS) indicates greater than normal snowpack across interior Alaska
ranging from 117% in the central Yukon Basin to 197% of average in
the Kuskokwim Basin. The Yukon government is also reporting greater
than normal snowpack across the Upper Yukon River Basin in Canada.
The snowpack is near normal along the North Slope and below normal along
the Gulf Coast and throughout Southeast Alaska.

Climate Outlook:

April and May weather is the most important factor determining the severity
of river ice breakup. Dynamic breakups have a high potential for ice jam
flooding and typically require cooler than average temperatures during March
and into the first few weeks of April followed by an abrupt transition to
warmer than normal temperatures in late April to early May. Thermal breakups
have a low potential for ice jam flooding as river ice generally rots in place.

The Climate Outlook for Spring 2020 suggests a more dynamic type breakup this
year. Observed temperatures in March thru the first week of April were generally
below normal to normal. The second and third weeks of April were generally
warmer than normal.  Temperature forecasts for the last week in April
indicate near normal temperatures statewide with equal chances for above and
below normal for early may. The mid-March 3-month Climate outlook
indicates increased chances of above average temperatures throughout 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:

The two generalized types of river ice breakup are dynamic (mechanical)
and thermal. A dynamic breakup moves from the headwaters of a river
downstream in a somewhat linear fashion. Ice jam flooding occurs more often
during a dynamic type breakup than a thermal type. A thermal type breakup
results in the river ice rotting in place primarily due to gradual to rapid
warm ups and little snowmelt runoff. Thermal type breakups usually result
in fewer ice jams and less chance of flooding.

Statewide, breakup this year is expected to lean generally to the dynamic
type. Temperatures statewide over the past few weeks generally have been
normal to below normal. However, temperatures over the next several weeks
are expected to be normal in areas north of the Brooks Range with above
normal temperatures for the remaining parts of the state. Temperatures
statewide for April and May are expected to be above normal. South of
the Brooks Range, the expected more dynamic type breakup, above normal
temperatures, and above average snowpack has increased the potential for
flooding this year to above average, while flooding potential north of the
Brooks Range is expected to be average.

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 2019 HISTORICAL RECORD AND ADJUSTED
                  TO REFLECT CURRENT CONDITIONS.

*  MEDIAN BREAKUP DATES ARE FOR THE PERIOD 1980 THROUGH 2019 AND ARE
      CALCULATED FOR LOCATIONS WITH AT LEAST 5 YEARS OF DATA.
** ACTUAL BREAKUP DATE FOR THIS YEAR
---------------------------------------------------------------------

                      SNOWMELT    FLOOD     MEDIAN  NO. OF  FORECAST
RIVER - REACH          RUNOFF    POTENTIAL  BREAKUP  YEARS   BREAKUP
                       VOLUME                DATE*    USED    DATE
-------------         ---------- ---------  -------  ------  --------
SOUTHEAST PANHANDLE     ABOVE

KENAI RIVER            AVERAGE    LOW       OPEN TO SKILAK LAKE

ANCHOR RIVER            BELOW     LOW       MOSTLY OPEN

MATANUSKA RIVER       ABOVE AVERAGE  LOW

SUSITNA RIVER         ABOVE AVERAGE
  GOLD CREEK                      MOD
  SUNSHINE                      LOW-MOD     05/02     31  04/26-05/02

YENTNA RIVER          ABOVE AVERAGE
  LAKE CREEK                    LOW-MOD     04/30     29  04/27-05/03

SKWENTNA RIVER        ABOVE AVERAGE
  SKWENTNA                      LOW-MOD     04/29     25  04/23-04/29

COPPER RIVER BASIN     AVERAGE
  GAKONA RVR @ HWY                LOW       04/29     32  04/23-04/29
  GULKANA RVR @ HWY               LOW       04/29     30  04/23-04/29

CHENA RIVER           ABOVE AVERAGE
  CHENA LAKES PROJECT             MOD
  FAIRBANKS                       LOW       04/25     27    OPEN

TANANA RIVER          ABOVE AVERAGE
  CHISANA @ NORTHWAY             LOW-MOD    04/26     27  04/17-04/23
  SALCHA                         LOW-MOD
  FAIRBANKS                      LOW-MOD    04/28     19  04/26-04/02
  NENANA                         LOW-MOD    04/29     40  04/24-04/30
  MANLEY                         LOW-MOD    05/02     29  04/26-05/02

KUSKOKWIM RIVER       ABOVE AVERAGE
  NIKOLAI                        LOW-MOD    04/22     34   **04/21
  MCGRATH                          MOD      05/04     40  04/30-05/06
  STONY RIVER                    LOW-MOD    05/01     32  04/27-05/03
  SLEETMUTE                        MOD      05/01     31  04/27-05/03
  RED DEVIL                        MOD      05/03     34  04/28-05/04
  CROOKED CREEK                    MOD      05/03     34  04/29-05/05
  ANIAK                            MOD      05/04     37  04/29-05/05
  KALSKAG                        LOW-MOD    05/04     31  04/30-05/06
  TULUKSAK                       LOW-MOD    05/06     28  05/01-05/07
  AKIAK                          LOW-MOD    05/08     34  05/01-05/07
  KWETHLUK                         MOD      05/03      8  04/30-05/06
  BETHEL                         LOW-MOD    05/09     40  05/01-05/07
  NAPAKIAK                       LOW-MOD    05/10     25  05/03-05/09

YUKON RIVER (UPPER)   ABOVE AVERAGE
  DAWSON, YT                     LOW-MOD    05/03     40  04/29-05/05
  EAGLE                            MOD      05/03     40  04/28-05/04
  CIRCLE                         MOD-HIGH   05/07     36  05/02-05/08
  FORT YUKON                       MOD      05/10     36  05/04-05/10
  BEAVER                         LOW-MOD    05/09     24  05/04-05/10
  STEVENS VILLAGE                LOW-MOD    05/10     23  05/05-05/11
  RAMPART                        LOW-MOD    05/10     25  05/04-05/10

YUKON RIVER (MIDDLE)   ABOVE AVERAGE
  TANANA                         LOW-MOD    05/07     35  05/01-05/07
  RUBY                           LOW-MOD    05/08     34  05/02-05/08
  GALENA                           MOD      05/10     39  05/04-05/10
  KOYUKUK                          MOD      05/08     14  05/04-05/10
  NULATO                           MOD      05/10     23  05/04-05/10
  KALTAG                         LOW-MOD    05/11     34  05/05-05/11
  ANVIK                          LOW-MOD    05/13     32  05/07-05/13

YUKON RIVER (LOWER)    ABOVE AVERAGE
  HOLY CROSS                     LOW-MOD    05/14     33  05/06-05/12
  RUSSIAN MISSION                LOW-MOD    05/14     34  05/06-05/12
  MARSHALL                         MOD      05/14     28  05/06-05/12
  PILOT STATION                  LOW-MOD    05/13     23  05/07-05/13
  MOUNTAIN VILLAGE               LOW-MOD    05/15     33  05/08-05/14
  ALAKANUK/EMMONAK                 MOD      05/20     32  05/12-05-18

KOYUKUK RIVER          ABOVE AVERAGE
  BETTLES                        LOW-MOD    05/09     38  05/01-05/07
  ALLAKAKET                      LOW-MOD    05/09     33  05/03-05/09
  HUGHES                           MOD      05/10     33  05/03-05/09

SEWARD PENINSULA       ABOVE AVERAGE
  BUCKLAND                         MOD      05/15     30  05/09-05/15

KOBUK RIVER            ABOVE AVERAGE
  KOBUK                            MOD      05/13     36  05/07-05/13
  SHUNGNAK                       LOW-MOD    05/16     29  05/10-05/16
  AMBLER                         LOW-MOD    05/16     35  05/10-05/16

NOATAK RIVER           ABOVE AVERAGE
  NOATAK                         LOW-MOD    05/13     23  05/17-05/19

BROOKS RANGE - NORTH     AVERAGE
  COLVILLE @ UMIAT               LOW-MOD    05/24     19  05/16-05/22
  COLVILLE @ COLVILLE            LOW-MOD    06/02     22  06/26-06/01

SAGAVANIRKTOK RIVER      AVERAGE
  DALTON HWY                     LOW-MOD


For more detail and to see a Flood Potential Map, refer to our Website
at https://www.weather.gov/aprfc/FLOODPOTENTIAL

The next Spring Breakup Outlook is scheduled at 2PM AKDT Friday April 24th, 2020

$$
bcj


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