Flood Potential Outlook
Issued by NWS Cheyenne, WY

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Probabilistic Hydrologic Outlook
National Weather Service Cheyenne WY
1237 PM MST THU MAR 2 2017

 ...Spring 2017 Flood and Water Resources Outlook Update...

This outlook is for the Cheyenne Hydrologic Service Area (HSA) which
comprises seven counties across southeastern Wyoming and seven counties
across western Nebraska. The outlook includes the North Platte River
mainstem and its tributaries (the Laramie and Medicine Bow Rivers)
in southeastern Wyoming and western Nebraska, tributaries to the
Missouri River (Niobrara and White Rivers) of northwestern Nebraska,
and the Little Snake River.

The outlook is a generalized summary of spring flood potential. Above
normal future precipitation combined with heavy rain and rapid snowmelt
will increase the current flood potential, while below normal future
precipitation and gradual freezing and thawing will decrease the flood
potential.

...Antecedent Conditions...

Warmer than normal temperatures during the period February 15-22 depleted
the snow pack/snow water equivalents (SWE) by 10 to 20 percent, but increased
by 5 to 10 percent with last week`s snowstorm. Current SWEs across the HSA
on March 2 ranged from 72 percent of normal for basins in eastern Wyoming and
northwestern Nebraska, to 125 to 130 percent of normal across southeastern
Wyoming.

Soil moisture was below normal throughout portions of the Cheyenne Basin in
eastern Wyoming and Niobrara and White basins in northwestern Nebraska, as
well as the South Laramie and South Platte basins in southeastern Wyoming
and southern Nebraska panhandle. These areas coincided with abnormally dry
to moderate drought conditions. Elsewhere, soil moisture was normal.

Streamflows along the South Platte Basin in the southern Nebraska panhandle
average between the 10th and and 25th percentile. Elsewhere, streamflows
averaged between the 25th and 75th percentile.

...Spring/Summer Climate/Water Supply Outlooks..

The outlook for March issued February 28 by the Climate Prediction Center
(CPC) indicates above normal temperatures and below normal precipitation.
The 90-day outlook for March through May 2017 issued February 16 by the
CPC predicts above normal temperatures, with equal chances for above, normal
and below normal precipitation. The Wyoming Bureau of Reclamation`s snowmelt
forecast issued February 1 predicts above normal snowmelt. Total April through
July runoff in the North Platte Basin is expected to be 127 percent of the
30-year average. The total storage for the North Platte reservoir system on
January 31 was 139 percent of the 30-year average. The Wyoming Water Supply
Outlook issued February 8 calls for above average water volumes for the North
Platte, Snake, and Laramie basins and their tributaries for April through July.

...Wyoming carryover reservoir storages are 115 to 125 percent of average for
early March...

...Above normal snowmelt streamflow volumes are expected across almost all
major basins in Wyoming...

...There is a low to moderate potential for flooding due to snowmelt this
runoff season...

...Widespread significant flooding is not likely due to snowmelt alone...

...Minor flooding is possible along the North Platte Basin in western
Nebraska due to above average snowpack and snowmelt into the headwaters
and reservoirs...

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:  03/04/2017  - 06/02/2017

                                       :    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    6   <5    5   <5   <5
:North Platte River
Saratoga             8.5    9.5   10.5 :  24   24    6    9   <5   <5
Sinclair             9.0   10.0   11.0 :  17   19    7   11    5   <5
:Medicine Bow River
Hanna                6.0    7.0    8.0 :  <5    6   <5   <5   <5   <5
:North Platte River
Glenrock             7.0    8.0    9.0 :  <5   10   <5   <5   <5   <5
Orin                 8.0    9.0   10.0 :   6   11   <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    5   <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    6   <5   <5   <5   <5
:North Platte River
Henry                5.5    6.5    7.5 :   9   15    7   12    6    9
Mitchell             7.5    8.5    9.5 :  11   16    8   14    7   12
Minatare             7.5    8.5    9.5 :   8   14    7   11    6    7
Bridgeport           8.0    9.0   10.0 :   7   11    6    7    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: 03/04/2017  - 06/02/2017
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Encampment River
Encampment            4.0    4.1    4.3    4.5    5.1    5.6    6.3
:North Platte River
Saratoga              6.6    6.9    7.2    7.8    8.4    9.2    9.7
Sinclair              5.9    6.3    6.7    7.4    8.4    9.6   10.8
:Medicine Bow River
Hanna                 3.3    3.5    4.0    4.2    4.7    5.4    5.7
:North Platte River
Glenrock              3.0    3.1    3.2    3.5    4.6    5.2    5.6
Orin                  5.0    5.0    5.1    5.3    6.0    6.6    8.4
:Laramie River
Woods Landing         1.9    2.1    2.2    2.6    3.0    3.3    3.8
Laramie               5.0    5.7    6.0    6.7    7.3    7.8    8.4
Bosler                1.3    1.4    1.8    2.6    3.8    5.1    5.7
Fort Laramie          3.4    3.5    3.8    4.0    4.5    5.1    7.0
:North Platte River
Henry                 3.3    3.4    4.0    4.2    4.5    5.4    9.0
Mitchell              3.6    3.8    4.9    5.5    5.9    7.7   13.9
Minatare              4.3    4.4    5.2    5.7    5.9    7.0   10.5
Bridgeport            4.9    5.0    5.6    5.9    6.2    7.0    9.7

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: 03/04/2017  - 06/02/2017
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Encampment River
Encampment            2.0    2.0    2.0    2.0    1.9    1.9    1.8
:North Platte River
Saratoga              4.3    4.3    4.3    4.3    4.3    4.3    4.2
Sinclair              3.3    3.3    3.3    3.3    3.3    3.3    3.3
:Medicine Bow River
Hanna                 2.5    2.5    2.5    2.5    2.5    2.4    2.3
:North Platte River
Glenrock              0.9    0.9    0.9    0.9    0.9    0.9    0.9
Orin                  3.6    3.6    3.6    3.6    3.6    3.6    3.6
:Laramie River
Woods Landing         1.3    1.3    1.3    1.2    1.1    0.6    0.3
Laramie               4.1    4.0    4.0    4.0    3.9    3.8    3.7
Bosler                1.1    0.9    0.8    0.7    0.7    0.6    0.6
Fort Laramie          1.6    1.6    1.6    1.6    1.6    1.6    1.6
:North Platte River
Henry                 2.3    2.3    2.3    2.3    2.3    2.3    2.3
Mitchell              1.2    1.2    1.2    0.6    0.6    0.4    0.4
Minatare              2.3    2.3    2.3    2.3    2.3    2.3    2.3
Bridgeport            3.6    3.6    3.6    3.6    3.6    3.6    3.6

This long-range probabilistic outlook contains forecast values that
are calculated using multiple season scenarios from 30 or more years
of climatolgical data including current conditions of the river, snow
cover, soil moisture, and the 30 to 90 day long-range outlooks of
temperature and precipitaion. By providing the complete 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
services (AHPS).

Graphical forecasts are available at:
http://water.weather.gov/ahps2/index.php?wfo=cys
Visit weather.gov/cheyenne and select the rivers & lakes tab for
graphical forecasts and additional information.

$$



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