Flood Potential Outlook
Issued by NWS Cheyenne, WY

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7

000
FGUS75 KCYS 162046
ESFCYS
WYC001-007-009-015-021-027-031-NEC007-013-033-045-105-123-157-165-
020000-

Probabilistic Hydrologic Outlook
National Weather Service Cheyenne WY
145 PM MST THU FEB 16 2017

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

The 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 far 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...

Mountain snowpack across southern Wyoming was 115 to 125 percent of normal
by early February. Snow water equivalents (SWEs) for the Lower North Platte,
Laramie, Upper North Platte, South Platte and Little Snake basins averaged
between 110 and 150 percent of normal from January through mid-February.
The higher SWEs encompassed the Little Snake Basin, in closer proximity
to the Snowy and Sierra Madre mountains. Snowpack across the plains east
of the Front Range was below average.

Soil moisture values across southeastern Wyoming and western Nebraska
averaged less than 200 mm since January 1. Soil moisture values were
below average throughout portions of the Cheyenne Basin in eastern Wyoming
and Niobrara and White basins in northwestern Nebraska, as well as the
Laramie and South Platte basins in southeastern Wyoming. These areas
coincided with abnormally dry to moderate drought conditions.

Streamflows in the Upper North Platte Basin averaged above the 90th
percentile. Elsewhere, streamflows averaged between the 25th and
75th percentiles.

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

The 90-day outlook for March through May 2017 issued February 16 by the
Climate Prediction Center predicts above normal temperatures, with equal
chances for above, normal and below normal precipitation for the Cheyenne
HSA. The Wyoming Bureau of Reclamation`s first snowmelt forecast issued
February 1 predicts above normal snowmelt. Total April through July runoff
in the North Platte Basin is expected to be 127% of the 30-year average.
The total storage for the North Platte reservoir system on January 31 was
139% 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.

...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:  02/18/2017  - 05/19/2017

                                       :    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 :   8    9    5    5   <5   <5
Sinclair             9.0   10.0   11.0 :   7    8    5    6   <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    6   <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: 02/18/2017  - 05/19/2017
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Encampment River
Encampment            3.0    3.1    3.4    3.7    4.0    4.6    6.5
:North Platte River
Saratoga              6.1    6.3    6.5    6.8    7.0    7.7    9.7
Sinclair              5.5    5.6    5.8    6.2    6.5    8.3   10.1
:Medicine Bow River
Hanna                 2.8    3.0    3.2    3.6    4.0    4.5    5.0
:North Platte River
Glenrock              2.6    2.6    2.7    2.9    3.2    4.2    5.2
Orin                  4.7    4.7    4.8    4.9    5.2    6.0    6.8
:Laramie River
Woods Landing         1.6    1.7    1.8    2.0    2.2    2.7    3.0
Laramie               4.5    4.5    4.8    5.0    5.7    6.7    7.0
Bosler                1.1    1.1    1.2    1.3    1.6    2.1    3.1
Fort Laramie          1.6    1.6    1.6    3.2    3.6    4.4    4.9
:North Platte River
Henry                 3.7    3.8    4.0    4.0    4.1    4.3    5.6
Mitchell              4.4    4.7    5.0    5.2    5.3    5.9    8.2
Minatare              4.9    5.0    5.2    5.3    5.4    5.7    7.2
Bridgeport            5.6    5.7    5.8    5.9    6.0    6.2    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: 02/18/2017  - 05/19/2017
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              4.2    4.2    4.2    4.2    4.2    4.2    4.2
Sinclair              3.3    3.3    3.3    3.3    3.3    3.2    3.2
:Medicine Bow River
Hanna                 2.3    2.3    2.3    2.3    2.2    2.0    1.9
:North Platte River
Glenrock              1.0    0.9    0.9    0.9    0.9    0.9    0.9
Orin                  3.7    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    0.7    0.3
Laramie               4.0    4.0    4.0    3.9    3.8    3.7    3.7
Bosler                0.9    0.8    0.7    0.7    0.6    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    2.2    2.2    2.2    2.2    2.2    2.2
Mitchell              1.1    1.0    0.6    0.5    0.4    0.4    0.4
Minatare              2.4    2.4    2.4    2.3    2.3    2.3    2.3
Bridgeport            3.8    3.8    3.8    3.7    3.7    3.7    3.7

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

Visit our web site at www.weather.gov/cheyenne for more weather
and river information including graphs of probabilistic river
outlooks.

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.

An update to the Spring 2017 Flood and Water Resources Outlook will
be issued March 2.

$$



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.