Flood Potential Outlook
Issued by NWS Rapid City, SD

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FGUS73 KUNR 071752
ESFUNR

Probabilistic Hydrologic Outlook
National Weather Service Rapid City SD
1052 AM MST Thu Mar 7 2019

..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
above average, and there will also be a high potential for ice jam
related flooding. Cold temperatures over the past couple of months
have created thick ice on rivers and streams. When warmer conditions
return and the ice begins to break up, these large chunks of ice may
cause restrictions in the channel, causing water to back up and
overflow the river banks. Also, significant snow cover over most of
the area will add to the flooding. 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 flows, will
have a significant effect on the amount of 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.

.Temperatures and Precipitation...
Since October 1 2018, precipitation has been around average and
temperatures have been below average. In February, temperatures were
been well below average, with above average precipitation and
snowfall. In fact, it was one of the coldest Februarys on record for
many locations. For most stations the only colder year was 1936. For
February, average high temperatures typically range from the lower
30s over northwestern South Dakota to the lower 40s across
southwestern South Dakota. Average lows generally range from the
lower teens across northwestern South Dakota to near 20 across
southwestern 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.

.Snow Cover and Liquid Water Content...
Almost all areas are snow-covered, with snow depths ranging from a
couple of inches across the plains of northeastern Wyoming to over
two feet across portions of northwestern South Dakota and the Black
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...
The soil moisture profile indicates the top layer of the ground is
wet, due to the rain and melting snow that occurred in December
before the ground froze. The lower sub-surface layers are drier, due
to the lack of moisture last fall. Frost depth generally ranges from
2 to 5 feet across the area.

.Lake and River Conditions...
Almost all lakes, rivers, and streams remain ice-covered, with the
exception of fast-moving creeks in the Black Hills. Thick ice
remains in place due to the cold temperatures this past month.

.Weather Outlooks...
Water temperatures in the tropical Pacific Ocean continue to be
slightly above average, indicating weak El Nino conditions. The
outlook indicates weak El Nino conditions will continue this spring,
before diminishing this summer. This weather pattern will typically
bring warmer conditions to the western half of the United States,
with wetter conditions over the central and southern plains during
the spring. For the summer, the predictability is low for any type
of overall weather pattern.

The outlook for the next two weeks indicates cold temperatures will
continue. Precipitation will be above average for the next week,
before drying out after the middle of the month.

The three month outlook for March, April, and May calls for
equal chances for above, below, and near average temperatures
and 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:  03/09/2019  - 09/30/2019

                                       :    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 :  83   34   48   12   18    6
:Moreau River
Faith               16.0   18.0   21.0 :  60   12   36   <5   20   <5
:Belle Fourche River
WY-SD State Line    15.0   16.0   18.0 :  21   13   14   <5   <5   <5
Sturgis             15.0   17.0   19.0 :  20   18   13   12   10    5
Elm Springs         19.0   22.0   24.0 :  11    8    9   <5   <5   <5
:Cheyenne River
Wasta               13.0   15.0   16.0 :   6   <5   <5   <5   <5   <5
Plainview           17.0   19.0   20.0 :  47   24   32   12   16    5
:Bad River
Midland             21.0   24.0   25.0 :  56   27   <5   <5   <5   <5
:White River
Kadoka              13.0   15.0   19.0 :  74   32   47   18   13   <5
White River         14.0   15.0   17.0 :  89   39   72   30   22    6
Oacoma              15.0   20.0   25.0 :  91   54   64   17   21   <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: 03/09/2019  - 09/30/2019
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Little Missouri River
Camp Crook            6.6    7.8   13.1   16.9   18.6   19.5   20.2
:Moreau River
Faith                 9.6   10.1   12.4   17.1   20.3   23.4   25.1
:Belle Fourche River
WY-SD State Line      5.3    5.7    6.8   10.6   14.4   16.6   16.7
Sturgis               6.3    7.7    8.7   10.6   13.8   19.3   20.9
Elm Springs           8.9    9.0   10.1   11.9   15.5   20.7   23.1
:Cheyenne River
Wasta                 5.1    5.4    6.5    7.9    9.9   11.5   13.4
Plainview            14.2   14.5   15.7   16.9   19.3   21.5   21.8
:Bad River
Midland              10.8   12.7   17.2   21.7   22.6   23.2   23.3
:White River
Kadoka                9.5   11.4   12.9   14.9   17.8   20.0   25.3
White River          12.5   13.7   14.8   15.6   16.9   18.4   19.2
Oacoma               13.6   15.4   17.9   21.0   23.8   26.9   28.4

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: 03/09/2019  - 09/30/2019
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Little Missouri River
Camp Crook            2.4    2.4    2.4    2.3    2.2    1.4    1.4
:Moreau River
Faith                 1.4    1.4    1.4    1.4    0.7    0.5    0.5
:Belle Fourche River
WY-SD State Line      3.4    3.4    3.4    3.4    3.4    3.3    3.3
Sturgis               2.1    2.1    2.1    2.1    2.1    2.1    2.1
Elm Springs           4.3    4.3    4.3    4.3    4.3    4.3    4.3
:Cheyenne River
Wasta                 0.6    0.6    0.6    0.5    0.5    0.4    0.4
Plainview             9.1    9.1    9.1    9.1    9.0    9.0    9.0
:Bad River
Midland               3.2    3.2    3.2    3.2    3.1    3.1    3.1
:White River
Kadoka                3.0    3.0    2.9    2.4    2.3    2.3    2.3
White River           4.2    4.2    4.1    3.7    3.7    3.6    3.6
Oacoma                7.5    7.5    7.5    7.2    7.1    7.1    7.1

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 second scheduled  spring flood and water resource
outlook for 2019. Long-range probabilistic outlooks are issued near
the middle of the month throughout the year. Additional outlooks
will be issued as we get closer to the flood season.

$$


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