Flood Potential Outlook
Issued by NWS Rapid City, SD

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FGUS73 KUNR 151721
ESFUNR

Probabilistic Hydrologic Outlook
National Weather Service Rapid City SD
1120 AM MDT Thu Mar 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:

* Near average for the White River downstream of Kadoka, SD
* Near average for streams and rivers in and around the Bear Lodge
  Mountains and the Wyoming Black Hills
* Below average across the rest of western South Dakota

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 now 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.

.Temperatures and Precipitation...
During the Fall of 2017, precipitation was below average and
temperatures were above average across northeast Wyoming and western
South Dakota. However, since December 1 most areas have seen above
average precipitation and below average temperatures.

In February, temperatures were well below average and precipitation
was above average. In fact, it was one of the coldest, wettest, and
snowiest Februarys` on record for most locations. For February,
average high temperatures typically range from the lower 30s over
northwest South Dakota to the lower 40s across southwestern South
Dakota. Average lows generally range from the lower teens across
northwest South Dakota to near 20 across southwest South Dakota.
Average precipitation for February typically ranges from a quarter
to a half inch on the plains to an inch and a half over the northern
Black Hills.

So far in March, temperatures have been well below average and
precipitation has been above average. Average high temperatures for
March are in the lower to middle 40s with average lows around 20.
Average precipitation typically ranges from around an inch on the
plains to around two inches in the 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 5 to 12 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 slightly above
average for this time of year.

.Soil Conditions and Frost Depths...
Soil moisture is below average due to the ongoing drought
conditions. Frost depth generally ranges from 2 to 5 feet 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
still ranges from 5 to 15 inches on the lakes.

According to the Bureau of Reclamation, February end of month
reservoir elevations were above average at Angostura, Belle Fourche,
Deerfield, and Keyhole, while Pactola was near average and Shadehill
Reservoir was below average.

According to the U.S. Geological Survey 28-day average streamflow
compared to historical streamflow for the day of the year indicates
normal conditions across much of the area.

.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 below average temperatures and
above average precipitation through the early spring.

The outlook for April shows below average temperatures and above
average precipitation. .

The three month outlook for April, May, and June calls colder and
wetter than average conditions in April trending toward warmer and
drier conditions by June.

.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:  03/03/2018  - 06/01/2018

                                       :    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 :  23   34    8    7   <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 :   8   13    6    7    5   <5
Elm Springs         19.0   22.0   24.0 :   6    6   <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 :  18   26   15   21    8   13
:Bad River
Midland             21.0   24.0   25.0 :  21   18   <5   <5   <5   <5
:White River
Kadoka              13.0   15.0   19.0 :  18   21    7   13   <5   <5
White River         14.0   15.0   17.0 :  20   24   11   18   <5    5
Oacoma              15.0   20.0   25.0 :  37   43    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: 03/03/2018  - 06/01/2018
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Little Missouri River
Camp Crook            7.0    7.0    7.8    9.5   11.4   15.8   18.6
:Moreau River
Faith                 6.7    7.9    8.8   11.1   14.2   14.9   15.2
:Belle Fourche River
WY-SD State Line      4.6    5.0    6.2    7.2    9.3   12.9   14.7
Sturgis               3.9    3.9    5.6    6.3    9.7   14.3   18.5
Elm Springs           5.2    5.4    6.6    8.3   10.3   16.3   19.7
:Cheyenne River
Wasta                 0.7    0.9    2.5    3.1    5.6    8.0    8.6
Plainview             9.9   10.4   11.5   12.8   15.1   18.6   19.7
:Bad River
Midland               5.3    5.5    7.7   12.1   19.8   22.5   22.6
:White River
Kadoka                5.0    5.4    6.1   10.0   12.0   13.9   16.1
White River           5.7    6.8    8.1   12.2   13.8   15.3   16.5
Oacoma                9.3    9.6   11.1   14.5   15.5   18.3   19.9

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: 03/03/2018  - 06/01/2018
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Little Missouri River
Camp Crook            2.5    2.5    2.5    2.5    2.5    2.4    2.4
:Moreau River
Faith                 1.0    1.0    1.0    1.0    0.8    0.6    0.6
:Belle Fourche River
WY-SD State Line      3.5    3.5    3.5    3.5    3.5    3.5    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    0.6    0.5    0.4    0.3
Plainview             9.1    9.1    9.1    9.1    9.1    9.0    9.0
:Bad River
Midland               2.6    2.6    2.6    2.6    2.6    2.6    2.6
:White River
Kadoka                2.9    2.8    2.4    2.1    2.0    2.0    2.0
White River           4.2    4.2    4.0    3.7    3.6    3.5    3.4
Oacoma                7.3    7.3    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 last scheduled spring flood and water resource outlook
for 2018. Long-range probabilistic outlooks are issued near the
middle of the month throughout the year.

$$



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