Flood Potential Outlook
Issued by NWS Cheyenne, WY

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053
FGUS75 KCYS 151753
ESFCYS
Probabilistic Hydrologic Outlook
National Weather Service Cheyenne WY
1053 AM MST Thu Feb 15 2018

 ...Spring 2018 Flood and Water Resources Outlook...

This outlook is for the Cheyenne Hydrologic Service Area which
emcompassses seven counties across south central and southeast Wyoming
and seven counties across the Nebraska Panhandle. The outlook includes
the North Platte River mainstem and its tributaries (the Laramie and
Medicine Bow Rivers), tributaries to the Missouri River (Niobrara and
White Rivers) of far northwest 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.

...Near to slightly below normal snowpack water values this winter...

...Below average soil moisture conditions...

...Low potential for flooding due to snowmelt runoff this spring...

...Below to near normal streamflow volumes this spring...

...Antecedent Conditions...

Mountain snow water equivalents (SWE) across south central and southeast
Wyoming since January 1 averaged 70 to 100 percent of normal. The higher
SWEs encompassed the Laramie and South Platte basins in southeast Wyoming.
Soil moisture averaged less than 200 mm since January 1. The U.S. Drought
Monitor indicated abnormally dry conditions across the Niobrara and White
River basins, with abnormally dry to moderate drought conditions over the
Little Snake River basin in southwest Carbon County Wyoming. The basin
streamflows averaged between the 25th and 75th percentile.

...Spring 2018 Climate/Water Supply Outlooks...

The 90-day outlook for March through May 2018 issued February 15 by the
Climate Prediciton Center predicts above normal temperatures over the
Little Snake, Laramie, Upper North Platte, and South Platte basins, with
equal chances for above, normal and below nornal temperatures for the
Niobrara and White basins. There are equal chances for above, normal and
below normal precipitation this spring. The Seasonal Drought Outlook
issued Febraury 15 indicates a continuation of drought conditions over
the Little Snake basin through May.

The Wyoming Bureau of Reclamation snowmelt runoff forecast issued
February 9 indicates below normal snowment runoff for the North Platte
River basin this spring. The total April to July runoff above Glendo
Dam is expected to be 735,000 acre-ft, which is 81 percent of the
30-year average. As of January 31, storage content in the North Platte
Reservoir system was 135 percent of the 30-year average. Current
releases are 530 cubic feet per second (cfs) from Seminoe Reservoir
through the Miracle Mile, 500 cfs from Gray Reef Reservoir, and 0 cfs
from Guernsey Reservoir.

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:  02/17/2018  - 05/18/2018

                                       :    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
--------           -----  -----  ----- : ---  ---  ---  ---  ---  ---
: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 :   7    9   <5   <5   <5   <5
Sinclair             9.0   10.0   11.0 :  <5    5   <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    8   <5   <5   <5   <5
Orin                 8.0    9.0   10.0 :  <5   10   <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   <5   <5   <5   <5   <5
:North Platte River
Henry                5.5    6.5    7.5 :   5   13   <5   10   <5    7
Mitchell             7.5    8.5    9.5 :   6   14    5   12   <5   10
Minatare             7.5    8.5    9.5 :  <5   11   <5    9   <5    6
Bridgeport           8.0    9.0   10.0 :  <5   10   <5    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: 02/17/2018  - 05/18/2018
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Encampment River
Encampment            2.8    3.0    3.3    3.8    4.1    4.4    6.4
:North Platte River
Saratoga              5.5    5.6    6.0    6.5    6.8    7.3    9.1
Sinclair              4.5    4.8    5.3    5.8    6.2    6.8    8.4
:Medicine Bow River
Hanna                 2.7    2.7    3.1    3.5    3.8    4.1    4.5
:North Platte River
Glenrock              2.1    2.2    2.2    3.1    3.3    3.9    4.9
Orin                  4.5    4.5    4.6    5.2    5.4    5.7    6.8
:Laramie River
Woods Landing         1.8    1.9    2.1    2.5    2.7    3.2    3.7
Laramie               4.6    4.8    5.2    6.0    6.6    7.5    8.0
Bosler                1.3    1.4    1.5    1.8    2.5    4.1    4.6
Fort Laramie          2.6    2.6    2.8    3.0    3.2    4.0    4.8
:North Platte River
Henry                 3.2    3.2    3.2    3.9    4.2    4.4    5.6
Mitchell              3.5    3.5    3.5    5.1    5.8    6.2    8.3
Minatare              4.1    4.1    4.1    5.3    5.7    6.0    7.2
Bridgeport            5.0    5.0    5.0    5.9    6.2    6.4    7.4

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: 02/17/2018  - 05/18/2018
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Encampment River
Encampment            1.8    1.8    1.8    1.8    1.8    1.7    1.7
:North Platte River
Saratoga              4.3    4.2    4.2    4.2    4.2    4.2    4.2
Sinclair              3.4    3.3    3.3    3.3    3.3    3.3    3.2
:Medicine Bow River
Hanna                 2.4    2.4    2.4    2.2    2.0    2.0    2.0
:North Platte River
Glenrock              0.9    0.9    0.9    0.9    0.9    0.9    0.9
Orin                  3.7    3.7    3.7    3.7    3.7    3.7    3.7
:Laramie River
Woods Landing         1.4    1.4    1.4    1.4    1.3    1.3    1.1
Laramie               4.0    4.0    3.9    3.7    3.4    3.3    3.2
Bosler                1.2    1.1    0.9    0.8    0.7    0.7    0.7
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              1.2    1.1    0.8    0.5    0.2    0.2    0.2
Minatare              2.5    2.4    2.4    2.3    2.3    2.2    2.2
Bridgeport            3.8    3.7    3.7    3.7    3.6    3.6    3.6

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 (AHPS).

Visit our web site weather.gov/cys for more weather and water
information.

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

The update to the Spring 2018 Flood and Water Resources Outlook will
be issued March 1.

$$



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