Flood Potential Outlook
Issued by NWS Cheyenne, WY

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Probabilistic Hydrologic Outlook
National Weather Service Cheyenne WY
915 PM MST Wed Feb 24 2021

...Spring Flood and Water Resources Outlook Number 2...

...Near Average Potential for Ice Jam Flooding...
...Below Average Potential for Spring Snowmelt Runoff Flooding...

This Spring Flood and Water Resources Outlook is the second of three
issuances for the Cheyenne Hydrologic Service Area (HSA) which
encompasses seven counties in Southeast Wyoming and seven counties
in the Nebraska Panhandle. This outlook includes the North Platte
River mainstem and its tributaries (the Laramie and Medicine Bow
Rivers), the Niobrara and White Rivers, and the Little Snake River.

.Flood Outlook Summary...
Due to the recent Arctic cold outbreak, there is an near average
probability of flooding early this spring due to the potential of
river ice breakup and ice jam flooding along portions of the North
Platte River in the Nebraska Panhandle.

Southeast Wyoming generally does not experience mountain snowmelt
runoff until May and June. A rapid warming of temperatures and rainfall
before the ground thaws may result in faster snowmelt, greater runoff,
and minor flooding. Once the ground has thawed and the ice has melted,
the flood potential for the rest of the spring will be below average.

The potential for flooding is dependent on temperatures, additional
precipitation, and timing of peak river and streamflows.

The spring mountain snowmelt forecast issued February 12 by the
Bureau of Reclamation predicts below average runoff, with the North
Platte River volume projected at 60 percent of the 30-year average.

.Temperatures and Precipitation...
February temperatures across Southeast Wyoming averaged three to as
much as 14 degrees below normal, largely due to the recent Arctic
cold outbreak. Precipitation averaged near to below normal, with
snowfall favoring the higher mountains.

.Snow Cover and Liquid Water Content...
Snow water equivalents ranged from 75 to 95 percent of normal across
Southeast Wyoming. With 25 percent of the snow season remaining,
river basins could receive up to 50 percent of their precipitation
from late season snowstorms during March, April, and May. Across the
plains, snowpack has averaged below normal.

.Soil Conditions and Frost Depths...
Soil moisture remained well below average with moderate to extreme
drought conditions. Most frost depths were one to two feet.

.Lake and River Conditions...
Many of the lakes across Southeast Wyoming and the Nebraska Panhandle
remain ice covered, with increasing thickness due to the very cold
temperatures the past few weeks. Ice recently formed on most rivers
and tributaries. There is concern for river ice breakup and ice jam
flooding as temperatures warm early this spring. Most area streamflows
and volumes were near normal.

.Climate Outlooks...
The latest outlooks issued by the Climate Prediction Center predict
near to slightly above average temperatures and below average
precipitation for the next two weeks. Longer range outlooks through
early spring indicate above average temperatures and below average
precipitation.

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:  02/27/2021  - 05/28/2021

                                       :    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
--------           -----  -----  ----- : ---  ---  ---  ---  ---  ---
:Encampment River
Encampment           6.5    7.5    8.5 :  <5   <5   <5   <5   <5   <5
:North Platte River
Saratoga             8.5    9.5   10.5 :  <5   17   <5   <5   <5   <5
Sinclair             9.0   10.0   11.0 :  <5   12   <5    5   <5   <5
:Medicine Bow River
Hanna                6.0    7.0    8.0 :  <5   <5   <5   <5   <5   <5
:North Platte River
Glenrock             7.0    8.0    9.0 :  <5   <5   <5   <5   <5   <5
Orin                 8.0    9.0   10.0 :  <5   <5   <5   <5   <5   <5
:Laramie River
Woods Landing        6.0    6.5    7.0 :  <5   <5   <5   <5   <5   <5
Laramie              9.0   10.0   11.0 :  <5    7   <5   <5   <5   <5
Bosler               7.5    8.5    9.5 :  <5   <5   <5   <5   <5   <5
Fort Laramie         7.0    8.0    9.0 :  <5   <5   <5   <5   <5   <5
:North Platte River
Henry                5.5    6.5    7.5 :  <5   <5   <5   <5   <5   <5
Mitchell             7.5    8.5    9.5 :  <5   <5   <5   <5   <5   <5
Minatare             7.5    8.5    9.5 :  <5   <5   <5   <5   <5   <5
Bridgeport           9.5   10.0   10.5 :  <5   <5   <5   <5   <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: 02/27/2021  - 05/28/2021
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Encampment River
Encampment            2.7    2.8    3.1    3.3    3.5    4.2    4.6
:North Platte River
Saratoga              4.8    4.9    5.3    5.6    6.0    6.5    8.0
Sinclair              4.0    4.1    4.5    4.7    5.2    5.8    7.8
:Medicine Bow River
Hanna                 2.3    2.4    2.7    3.2    3.8    4.0    4.5
:North Platte River
Glenrock              1.9    1.9    1.9    2.0    2.1    2.6    3.2
Orin                  4.2    4.2    4.2    4.3    4.3    4.9    5.3
:Laramie River
Woods Landing         1.8    1.9    1.9    2.0    2.5    2.8    2.9
Laramie               4.5    4.6    4.8    5.1    6.1    6.9    7.3
Bosler                1.1    1.1    1.1    1.4    1.9    3.0    4.6
Fort Laramie          2.6    2.8    2.9    3.3    3.5    3.5    3.8
:North Platte River
Henry                 2.8    2.8    2.8    2.9    3.0    3.0    3.5
Mitchell              2.5    2.6    2.7    2.8    2.9    3.5    4.4
Minatare              3.1    3.1    3.2    3.3    3.5    4.2    4.8
Bridgeport            4.5    4.5    4.6    4.7    4.8    5.3    5.7

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: 02/27/2021  - 05/28/2021
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Encampment River
Encampment            1.8    1.8    1.8    1.8    1.7    1.6    1.6
:North Platte River
Saratoga              3.0    3.0    3.0    3.0    3.0    3.0    3.0
Sinclair              3.0    3.0    3.0    3.0    3.0    3.0    2.9
:Medicine Bow River
Hanna                 2.1    2.1    1.9    1.8    1.7    1.7    1.7
:North Platte River
Glenrock              0.9    0.8    0.8    0.7    0.7    0.7    0.7
Orin                  3.6    3.5    3.4    3.4    3.4    3.4    3.4
:Laramie River
Woods Landing         1.3    1.3    1.2    1.2    1.0    0.3    0.3
Laramie               3.6    3.6    3.6    3.6    3.6    3.6    3.6
Bosler                0.8    0.8    0.8    0.7    0.6    0.6    0.6
Fort Laramie          2.5    2.5    2.5    2.5    2.5    2.5    2.5
:North Platte River
Henry                 2.1    2.1    2.1    2.1    2.1    2.1    2.1
Mitchell              1.1    0.5    0.3    0.3    0.3    0.3    0.3
Minatare              2.2    2.0    1.8    1.5    1.4    1.2    1.2
Bridgeport            4.0    4.0    3.9    3.8    3.7    3.7    3.7

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 weather.gov/cys for more weather and water information.

The last Spring Flood and Water Resources Outlook will be issued
March 11.

$$



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