Flood Potential Outlook
Issued by NWS Rapid City, SD

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FGUS73 KUNR 151515
ESFUNR

Probabilistic Hydrologic Outlook
National Weather Service Rapid City SD
815 AM MST Thu Feb 15 2024

...Spring Flood and Water Resource Outlook...

...Below-Average Flood Potential This Spring...

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 Keya Paha.

.Flood Outlook Summary...
The flood potential this spring is below average across all of
northeastern Wyoming and western South Dakota.

Ice cover that is on the streams, rivers, and lakes will continue to
melt over the next few weeks. River-ice breakup and the potential
for ice-jam flooding usually occurs in late February and March.
Flooding from snowmelt on the plains typically occurs between March
and May, while snowmelt flooding is later in the Black Hills. 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
this spring. The amount of additional snow and rain, as well as the
timing of peak river and stream flows, will have a significant
effect on the severity of flooding.

.Temperatures and Precipitation...
Temperatures so far this winter have been very warm and above
average. To date, Rapid City and Gillette are the 9th warmest with
an average temperature of 31.2 degrees and 29.3, respectively. The
average temperature in Winner has been 33.8 degrees, making it the
second warmest winter.

Precipitation since December has been slightly below average.
Snowfall this winter season ranks the 10th lowest for Rapid City and
8th lowest for Gillette.

.Snow Cover and Liquid Water Content...
Snow cover across the area is mostly from what we have received this
week (February 13-15). Amounts are mainly a few inches, but with up
to a foot or so in the Black Hills. Snow water equivalent is more
than half of average for this time of year.

The halfway point of the snow season was February 1, but March and
April are typically snowy months, which is when a third of the
annual snowfall occurs. The median peak snow water equivalent in the
Black Hills usually occurs around April 1.

.Soil Conditions and Frost Depths...
Recent rain along with thawed ground has helped to replenish
moisture in the top layers of the ground. Soil moisture is slightly
above average across south-central South Dakota, with below-average
soil moisture in moderate drought areas. The only frost remaining in
the ground is over northwestern South Dakota and higher elevations
of the Black Hills. Frost depths in these areas are mostly less than
a foot. In a typical year, the frost usually comes out of the ground
by the end of March.

.Lake and River Conditions...
Ice thickness on rivers, streams and lakes ranges from open water to
around 1.5 feet of ice at Deerfield. The cool down the next few days
will help keep the ice in place a little longer, but warmer
temperatures next week will slow or cease the ice growth.

.Weather Outlooks...
Water temperatures in the tropical eastern Pacific Ocean are above
average at the surface; however, subsurface water temperatures have
trended to near average. This means El Niño Southern Oscillation
(ENSO) neutral conditions are expected to develop by late spring.
Historically, there is a tendency for La Niña to follow strong El
Niño events. If this does occur, it will likely develop in the June-
July time frame. This weather pattern typically favors drier
conditions across northeastern Wyoming and western South Dakota
through the summer. Temperatures are often a little cooler than
average in the spring, but warmer than average in the summer.

.Numerical River Outlooks...
In Table 1 below, the current (CS) and historical (HS) or normal
probabilities of exceeding minor...moderate...and major flood stage
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 greater 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: 2/17/2024 - 9/30/2024

                                       :    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   44   <5   20   <5    6
:Moreau River
Faith               16.0   18.0   21.0 :  <5   21   <5   12   <5    6
:Belle Fourche River
WY-SD State Line    15.0   16.0   18.0 :  <5   16   <5   <5   <5   <5
:South Belle Fourche River
Sturgis             15.0   17.0   19.0 :  21   26   17   18    8    9
:Belle Fourche River
Elm Springs         19.0   22.0   24.0 :  12   16   <5   <5   <5   <5
:Cheyenne River
Wasta               13.0   15.0   16.0 :  <5    7   <5   <5   <5   <5
Plainview           17.0   19.0   20.0 :  29   48   23   27   14   17
:Bad River
Midland             21.0   24.0   25.0 :  29   39   <5    5   <5   <5
:White River
Kadoka              15.0   16.0   19.0 :  46   53   28   45    6   19
White River         14.0   15.0   17.0 :  35   50   24   39   18   25
Oacoma              15.0   20.0   25.0 :  59   66   15   26   <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: 2/17/2024 - 9/30/2024

Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Little Missouri River
Camp Crook            2.0    2.0    2.1    4.7    6.9   13.9   14.9
:Moreau River
Faith                 2.0    3.3    5.3    7.8   10.7   12.8   13.3
:Belle Fourche River
WY-SD State Line      4.4    4.4    4.4    5.0    7.2   12.4   13.7
:South Belle Fourche River
Sturgis               4.1    4.9    5.7    8.4   13.5   17.6   19.8
:Belle Fourche River
Elm Springs           5.3    6.2    7.2   10.0   15.6   19.3   21.3
:Cheyenne River
Wasta                 0.8    1.7    3.2    4.8    6.6    8.2   10.2
Plainview             9.8   10.1   11.9   14.5   18.5   20.9   21.8
:Bad River
Midland               4.5    4.9    7.0   15.8   21.2   22.6   23.0
:White River
Kadoka                8.1    8.3   11.0   14.3   16.1   18.3   24.8
White River           8.5    9.4   11.4   13.0   14.7   18.7   24.3
Oacoma               11.3   12.5   13.8   15.4   19.0   21.9   24.4

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: 2/17/2024 - 9/30/2024

LOCATION              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Little Missouri River
Camp Crook            1.9    1.8    1.7    1.6    1.5    1.5    1.4
:Moreau River
Faith                 0.8    0.7    0.7    0.7    0.7    0.7    0.7
:Belle Fourche River
WY-SD State Line      3.3    3.2    3.2    3.2    3.1    3.1    3.1
:South Belle Fourche River
Sturgis               2.2    2.2    2.2    2.2    2.2    2.2    2.2
:Belle Fourche River
Elm Springs           3.2    3.2    3.2    3.2    3.2    3.2    3.2
:Cheyenne River
Wasta                 0.6    0.6    0.6    0.6    0.5    0.5    0.5
Plainview             8.4    8.4    8.4    8.3    8.3    8.3    8.3
:Bad River
Midland               2.7    2.7    2.7    2.7    2.7    2.7    2.7
:White River
Kadoka                4.9    4.3    4.1    3.9    3.8    3.8    3.8
White River           4.3    4.1    4.0    3.9    3.9    3.9    3.9
Oacoma                8.6    8.0    7.7    7.5    7.4    7.3    7.3

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.

.Outlook Schedule...

The next outlook will be issued February 29, 2024.

$$

Smith


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