Flood Potential Outlook
Issued by NWS Rapid City, SD

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6
000
FGUS73 KUNR 161719
ESFUNR

Probabilistic Hydrologic Outlook
National Weather Service Rapid City SD
1019 AM MST Thu Feb 16 2017

..Spring Flood and Water Resource Outlook...

This spring flood and water resource outlook is for the Rapid City
Hydrologic Service Area (HSA) which covers northeastern Wyoming and
western South Dakota. The main river basins include the Little
Missouri...eastern Powder...Belle Fourche...Grand...Moreau...
Cheyenne...Bad...White and Keyapaha River Basins.

.Flood Outlook Summary...
At this time, the probability of spring flooding from March through
June is:
* Below average in the Little Missouri, Belle Fourche, and upper
  Cheyenne River basins due to dry conditions last fall.
* Near average in the Grand, Moreau, Bad, and lower Cheyenne River
  basins due to ample snowpack over the winter months.
* Above average in the White River basin due to saturated soil
  conditions and above average precipitation over the winter.

For the Black Hills and higher elevations...flooding from snowmelt
typically begins in late April and May. Flooding on the plains due
to snowmelt and ice jams typically occurs between February and May.

The potential for rainfall-induced flash flooding is not quantifiable
because this type of flooding is usually caused by localized
thunderstorms during the spring and summer.

The flood potential is still very dependent on weather conditions
from now until the spring melt. The amount of additional snow and
rain and the timing of peak flows will have a significant effect on
the flooding.

.Precipitation...
Since October 1 2016 precipitation has varied from below average
across most of northeastern Wyoming and portions of far western
South Dakota to above average across central South Dakota.

.Snow Cover and Liquid Water Content...
Snowpack remains across portions of northwestern South Dakota and in
the higher elevations of the Black Hills and Bear Lodge Mountains. A
few scattered drifts also remain across the western South Dakota
plains and in the draws and creek bottoms.

.Soil Conditions and Frost Depths...
Soil moisture ranges from dry across northeastern Wyoming to above
average in south central South Dakota. Frost depths are minimal
where the ground is clear to 18 inches where some of the snowpack
remains.

.Lake and River Conditions...
Ice thickness on lakes varies, but is around average for this time
of the year. Recent reports indicate ice thickness is 16 to 20
inches in the bays on the larger lakes and on streams in
southwestern South Dakota. Ice thickness over 3 feet is common on
lakes and rivers in northwestern South Dakota.

.Weather Outlooks...
Water temperatures in the tropical Pacific Ocean are around average,
indicating neutral conditions. The outlook for the spring and summer
is for neutral conditions to continue. Neutral conditions generally
bring a warmer and wetter pattern to the southern half of the United
States with a cold polar jet stream diving across Canada into the
northeastern United States. The northern plains will be in the
transition zone between the two air masses that would create varying
conditions for the northern plains.

Above average temperatures and mainly dry conditions will continue
through the middle of next week. There is a chance for showers
Sunday Night and Monday and then another chance for rain and snow
later next week.

The outlook for March indicates odds tilted toward below average
temperatures with odds favoring above average precipitation.

The three month outlook for March, April, and May calls for a
somewhat increased chance for normal to slightly above normal
temperatures with odds favoring above average precipitation.

.Numerical River Outlooks...
In Table 1 below...the current (CS) and historical (HS) or normal
probabilities of exceeding minor...moderate...and major flood stages
are listed for the valid time period.

CS values indicate the probability of reaching a flood category
based on current conditions.

HS values indicate the probability of reaching a flood category
based on historical or normal conditions.

When the value of CS is more than HS...the probability of exceeding
that level is higher than normal. When the value of CS is less than
HS...the probability of exceeding that level is lower than normal.

Table 1--Probabilities for minor...moderate and major flooding
               Valid Period:  02/18/2017  - 05/19/2017

                                       :    Current and Historical
                                       :     Chances of Exceeding
                                       :       Flood Categories
                                       :      as a Percentage (%)
                      Categorical      :
                   Flood Stages (FT)   :   Minor    Moderate   Major
Location           Minor   Mod   Major :  CS   HS   CS   HS   CS   HS
--------           -----  -----  ----- : ---  ---  ---  ---  ---  ---
:Little Missouri River
Camp Crook          12.0   17.0   19.0 :  15   34    8    6   <5   <5
:Moreau River
Faith               16.0   18.0   21.0 :  <5    6   <5   <5   <5   <5
:Belle Fourche River
WY-SD State Line    15.0   16.0   18.0 :  <5   <5   <5   <5   <5   <5
Sturgis             15.0   17.0   19.0 :   5    6   <5   <5   <5   <5
Elm Springs         19.0   22.0   24.0 :  <5   <5   <5   <5   <5   <5
:Cheyenne River
Wasta               14.0   16.0   17.0 :  <5   <5   <5   <5   <5   <5
Plainview           16.0   17.0   19.0 :  19   22   17   14    8    5
:Bad River
Midland             21.0   24.0   25.0 :  15   17   <5   <5   <5   <5
:White River
Kadoka              13.0   15.0   19.0 :  31   17   10    9   <5   <5
White River         14.0   15.0   17.0 :  39   20   14   15   <5   <5
Oacoma              15.0   20.0   25.0 : >98   40    6    8   <5   <5

Legend
CS = Conditional Simulation (Current Outlook)
HS = Historical Simulation
FT = Feet

In Table 2 below...the 95 through 5 percent columns indicate the
probability of exceeding the listed stage levels (FT) for the valid
time period.

Table 2--Exceedance Probabilities

                               Chance of Exceeding Stages
                                  at Specific Locations
                          Valid Period: 02/18/2017  - 05/19/2017
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Little Missouri River
Camp Crook            5.2    5.2    5.8    7.6    8.9   15.1   17.7
:Moreau River
Faith                10.3   10.3   10.3   10.4   14.2   15.5   15.9
:Belle Fourche River
WY-SD State Line      4.4    5.0    5.5    6.8    8.1    9.6   12.5
Sturgis               3.9    4.0    5.0    6.3    8.4   13.7   15.0
Elm Springs           5.2    5.3    6.2    8.7    9.5   15.5   17.3
:Cheyenne River
Wasta                 2.0    2.4    3.4    4.5    6.4    8.9   11.1
Plainview            10.3   10.6   11.3   13.6   14.4   18.2   19.5
:Bad River
Midland               6.6    6.6    7.2   13.4   19.7   22.5   22.5
:White River
Kadoka               11.2   11.2   11.2   11.3   13.5   15.1   16.3
White River          12.6   12.6   12.6   13.1   14.6   15.4   15.9
Oacoma               15.6   15.6   15.6   15.8   17.8   19.3   20.2

In Table 3 below...the 95 through 5 percent columns indicate the
probability of falling below the listed stage levels (FT) for the
valid time period.

Table 3--Nonexceedance Probabilities...

                            Chance of Falling Below Stages
                                 at Specific Locations
                          Valid Period: 02/18/2017  - 05/19/2017
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Little Missouri River
Camp Crook            2.5    2.4    2.3    2.1    2.1    2.0    1.9
:Moreau River
Faith                 1.0    1.0    0.8    0.6    0.5    0.5    0.5
:Belle Fourche River
WY-SD State Line      3.9    3.9    3.8    3.8    3.8    3.7    3.7
Sturgis               2.9    2.9    2.9    2.9    2.9    2.9    2.8
Elm Springs           4.1    4.1    4.1    4.1    4.1    4.1    4.1
:Cheyenne River
Wasta                 0.9    0.9    0.9    0.8    0.8    0.7    0.6
Plainview             9.2    9.2    9.2    9.1    9.1    9.0    9.0
:Bad River
Midland               3.6    3.4    3.3    3.2    3.2    3.2    3.2
:White River
Kadoka                4.1    4.0    3.8    3.5    3.3    3.1    3.0
White River           5.4    5.2    4.9    4.6    4.4    4.1    4.1
Oacoma                9.1    9.0    8.6    8.2    7.9    7.7    7.6

These long-range probabilistic outlooks contain forecast values that
are calculated using multiple season scenarios from 30 or more years
of climatological data...including current conditions of the
river...soil moisture...snow cover...and 30 to 90 day long-range
outlooks of temperature and precipitation. By providing a range of
probabilities...the level of risk associated with long-range planning
decisions can be determined. These probabilistic forecasts are part
of the National Weather Service`s Advanced Hydrologic Prediction
Service.

Visit our web site weather.gov/unr for more weather and water
information.

This is the first spring flood and water resource outlook for 2017.
Long-range probabilistic outlooks are issued near the middle of the
month throughout the year. The next scheduled outlook will be issued
on March 2 2017.

$$


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.