Flood Potential Outlook
Issued by NWS Cheyenne, WY

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FGUS75 KCYS 052248
ESFCYS
WYC001-007-009-015-021-027-031-NEC007-013-033-045-105-123-157-165-062248-

Probabilistic Hydrologic Outlook
National Weather Service Cheyenne Wy
348 PM MST THU MAR 5 2015

 ...Spring 2015 Flood and Water Resources Outlook Update #2...

This outlook includes the North Platte River and its tributaries
(Laramie and Medicine Bow Rivers) in southeastern Wyoming and the
western Nebraska Panhandle, the Niobrara and White Rivers of extreme
northwestern Nebraska, and the Little Snake River.

Southeast Wyoming and western Nebraska generally do not experience any
significant mountain snowmelt runoff until early to middle May, with
mountain snowmelt runoff peaking in early June. At this time, it is
early to make any long range prediction on the magnitude of flooding due
to mountain snowmelt runoff.

...Below normal snowmelt streamflow volumes are expected across major
basins in Wyoming and western Nebraska...

...There is a low risk for significant flooding due to mountain snowmelt
alone...

The risk of flooding for the remainder of winter and into mid-spring
is near to below normal. River levels for the past few weeks have been
running near normal to slightlt below normal. Minor fluctuations were
observed at some of the forecast points on the North Platte River due
to ice.

Light to moderate amounts of winter precipitation have kept shallow
surface soil depths near to slightly wetter than normal. Frost depths
averaged less than a foot.

Mountain snowpack conditions were classified as near normal. Estimates
from SNOTEL sites on Mar 2-3 indicated one to two feet of snow fell over
the Snowy and Sierra Madre ranges. Snow depths across the eastern Wyoming
plains ranged from one to four inches.

The Spring 2015 water supply outlook issued in February for southeast
Wyoming and western Nebraska Panhandle main stem rivers and tributaries
indicates 75 to 90 percent of average flow.

The 90-day outlook issued February 19 for March through May indicated
equal chances of above...below and normal temperatures with above normal
precipitation.

The following three tables provide more detailed information about the
probabilities for flooding this spring and summer.

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:  3/7/2015 - 9/30/2015

                                       :    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   13    5    5   <5   <5
:North Platte River
Saratoga             8.5    9.5   10.5 :  25   20    5    7   <5   <5
Sinclair             9.0   10.0   11.0 :  11   27    5   15   <5    5
:Medicine Bow River
Hanna                6.0    7.0    8.0 :  <5   <5   <5   <5   <5   <5
:North Platte River
Glenrock            10.0   11.0   12.0 :  <5   <5   <5   <5   <5   <5
Orin                 8.0    9.0   10.0 :  <5    6   <5   <5   <5   <5
:Laramie River
Woods Landing        6.0    6.5    7.0 :  <5   <5   <5   <5   <5   <5
Laramie              5.0    6.0    7.0 :  11   39   <5   19   <5   <5
Bosler               7.5    8.5    9.5 :  <5   <5   <5   <5   <5   <5
Fort Laramie         7.0    8.0    9.0 :  <5   16   <5    5   <5   <5
:North Platte River
Henry                5.5    6.5    7.5 :   5   18   <5   13   <5   11
Mitchell             7.5    8.5    9.5 :   8   19    5   17   <5   12
Minatare             7.5    8.5    9.5 :   5   27   <5   17   <5    9
Bridgeport           9.5   10.0   10.5 :  <5    9   <5    6   <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: 3/7/2015 - 9/30/2015
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Encampment River
Encampment            4.1    4.2    4.5    5.0    5.5    5.8    6.8
:North Platte River
Saratoga              6.6    6.8    7.2    7.8    8.5    9.1    9.7
Sinclair              5.7    6.0    6.6    7.3    8.4    9.2   10.5
:Medicine Bow River
Hanna                 3.6    3.9    4.1    4.4    4.8    5.4    5.7
:North Platte River
Glenrock              2.8    2.9    2.9    3.0    3.7    5.0    6.1
Orin                  4.8    4.8    4.9    5.0    5.7    6.3    7.3
:Laramie River
Woods Landing         2.1    2.2    2.6    3.0    3.3    4.1    4.3
Laramie               1.5    1.8    2.5    3.2    3.7    5.2    5.5
Bosler                1.5    1.7    2.8    3.9    5.1    6.1    6.4
Fort Laramie          3.6    3.6    3.8    3.9    4.6    6.0    6.3
:North Platte River
Henry                 4.1    4.2    4.2    4.2    4.3    5.0    5.5
Mitchell              5.3    5.3    5.4    5.5    5.7    6.7    8.5
Minatare              5.1    5.1    5.2    5.3    5.5    6.4    7.4
Bridgeport            5.6    5.6    5.7    5.8    6.0    6.7    7.6


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: 3/7/2015 - 9/30/2015
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Encampment River
Encampment            1.9    1.9    1.8    1.8    1.8    1.8    1.7
:North Platte River
Saratoga              4.7    4.7    4.6    4.6    4.6    4.5    4.5
Sinclair              3.4    3.4    3.4    3.3    3.3    3.2    3.2
:Medicine Bow River
Hanna                 2.2    2.2    2.1    2.0    2.0    2.0    2.0
:North Platte River
Glenrock              1.0    1.0    0.9    0.8    0.8    0.7    0.7
Orin                  3.6    3.6    3.5    3.5    3.4    3.4    3.4
:Laramie River
Woods Landing         1.3    1.2    1.1    0.8    0.3    0.3    0.3
Laramie               0.4    0.4    0.4    0.3    0.3    0.3    0.3
Bosler                1.1    1.1    1.0    0.9    0.8    0.8    0.8
Fort Laramie          3.2    3.2    3.2    3.2    3.2    3.2    3.2
:North Platte River
Henry                 2.1    2.0    2.0    2.0    2.0    2.0    2.0
Mitchell              0.6    0.5    0.2    0.2    0.2    0.2    0.2
Minatare              2.0    1.9    1.6    1.4    1.4    1.3    1.3
Bridgeport            3.6    3.5    3.4    3.3    3.2    3.2    3.2


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 at www.weather.gov/cheyenne for more weather
and river information including graphs of probablilistic river
outlooks. Graphical forecasts are available at:
http://water.weather.gov/ahps2/index.php?wfo=cys

$$


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