Flood Potential Outlook
Issued by NWS Rapid City, SD

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FGUS73 KUNR 151701
ESFUNR

Probabilistic Hydrologic Outlook
National Weather Service Rapid City SD
1000 AM MST Thu Feb 15 2018

..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 through June is
below average. However, the flood potential is still very dependent
on weather conditions this spring. The amount of additional snow and
rain and the timing of peak flows will have a significant effect on
the flooding.

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.

.Precipitation...
Since October 1 2017, precipitation has varied from near average
across northeastern Wyoming to below average over much of the
western and south central South Dakota plains. Across the Black
Hills, the northern half of the Black Hills has seen below average
precipitation, while the southern half of the Black Hills has
received above average precipitation. Temperatures have been near
average across the area since October 1.

So far in February, temperatures have been well below average and
precipitation has been above average. Average high temperatures for
February range from the lower 30s in northwestern South Dakota to
the lower 40s across southwestern South Dakota. Average lows range
from the lower teens across northwest South Dakota to near 20 across
southwest South Dakota. Average precipitation typically ranges from
a quarter to a half inch on the plains to an inch and a half over
the northern Black Hills.

.Snow Cover and Liquid Water Content...
Snow cover across the plains ranges from little to no snowpack
across southern portions of northeast Wyoming and parts of southwest
and south central South Dakota to six to twelve inches across far
northeast Wyoming and northwest South Dakota. Snow water equivalent
is one inch to around two inches across far northeast Wyoming and
northwest South Dakota. Across the Black Hills and Bear Lodge
Mountains, snowpack and snow water equivalent are near average for
this time of year. March and April are typically the two snowiest
months, when a third of the seasonal snowfall usually happens.

.Soil Conditions and Frost Depths...
Soil moisture is below average due to the ongoing drought
conditions. Frost depth generally ranges from 20 inches to 40 inches
across the area.

.Lake and River Conditions...
Most lakes, rivers, and streams remain ice covered, with the
exception of fast moving creeks in the Black Hills. Ice thickness
ranges from 10 to 20 inches on the lakes. According to the Bureau of
Reclamation, reservoir levels are all above average, except for
Shadehill Reservoir, which is below average. Most river basins in
the area are reporting below normal streamflows.

.Weather Outlooks...
Sea surface temperatures in the tropical Pacific Ocean are below
average, indicating La Nina conditions. The outlook for the spring
and summer is for La Nina conditions to trend toward El Nino
Southern Oscillation (ENSO) neutral conditions. For the Northern
Plains, this pattern typically favors near average temperatures and
precipitation through the spring.

The outlook for the rest of February shows below average
temperatures and above average precipitation. The three month
outlook for March, April, and May calls for near to below average
temperatures and near 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:  02/17/2018  - 05/18/2018

                                       :    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 :  13   34    7    6   <5    5
:Moreau River
Faith               16.0   18.0   21.0 :  <5    7   <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 :  13   22   10   13   <5    5
:Bad River
Midland             21.0   24.0   25.0 :  14   17   <5   <5   <5   <5
:White River
Kadoka              13.0   15.0   19.0 :  10   17   <5    9   <5   <5
White River         14.0   15.0   17.0 :  14   20   <5   15   <5   <5
Oacoma              15.0   20.0   25.0 :  33   39   <5    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: 02/17/2018  - 05/18/2018
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Little Missouri River
Camp Crook            4.3    5.3    6.7    7.8    9.4   15.0   18.0
:Moreau River
Faith                 3.2    4.7    6.3    8.8   12.3   13.9   14.1
:Belle Fourche River
WY-SD State Line      3.6    4.1    5.0    6.2    7.5    9.4   12.0
Sturgis               2.9    3.3    4.7    6.1    7.8   13.0   14.4
Elm Springs           4.3    4.8    6.0    8.3    9.1   14.9   16.6
:Cheyenne River
Wasta                 0.7    0.9    1.6    3.0    4.9    6.4    7.5
Plainview             9.3    9.5   10.8   12.7   13.5   17.1   18.5
:Bad River
Midland               3.5    4.3    6.3   10.0   17.3   22.2   22.5
:White River
Kadoka                3.2    3.6    5.5    8.9   11.4   13.0   13.5
White River           4.5    5.0    6.6   10.4   12.8   14.4   14.9
Oacoma                8.9    9.1    9.9   13.0   15.2   16.8   17.5

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: 02/17/2018  - 05/18/2018
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Little Missouri River
Camp Crook            2.5    2.5    2.5    2.5    2.5    2.5    2.4
:Moreau River
Faith                 1.0    1.0    1.0    1.0    1.0    0.6    0.5
:Belle Fourche River
WY-SD State Line      3.5    3.5    3.5    3.5    3.5    3.4    3.4
Sturgis               2.7    2.7    2.7    2.7    2.7    2.7    2.7
Elm Springs           4.2    4.2    4.2    4.2    4.2    4.2    4.2
:Cheyenne River
Wasta                 0.6    0.6    0.6    0.5    0.5    0.4    0.3
Plainview             9.1    9.1    9.1    9.1    9.1    9.0    9.0
:Bad River
Midland               2.8    2.6    2.6    2.6    2.6    2.6    2.6
:White River
Kadoka                2.9    2.7    2.5    2.1    2.0    2.0    2.0
White River           4.2    4.2    4.0    3.8    3.5    3.5    3.5
Oacoma                7.5    7.4    7.3    7.1    7.0    7.0    7.0

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 2018.
Long-range probabilistic outlooks are issued near the middle of the
month throughout the year. The next scheduled outlook will be on
March 1 2018.

$$



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