Flood Potential Outlook
Issued by NWS North Platte, NE

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Probabilistic Hydrologic Outlook
National Weather Service North Platte, NE
755 AM CST Thu Mar 7 2019

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

...Average Flood Potential this Spring...

This Spring Flood and Water Resource Outlook is for the North Platte
Hydrologic Service Area, which covers western and north central
Nebraska. The river basins include: the North Platte and South
Platte Rivers and the Platte River in western Nebraska, Frenchman
Creek and Stinking Water Creek in southwest Nebraska, the Loup and
Dismal Rivers in the Sandhills of Nebraska, and portions of the
Elkhorn and Niobrara Rivers in north central Nebraska.

.Flood Outlook Summary...
At this time, the probability of spring flooding through May is
generally around average for most of central and western Nebraska.
The potential for flooding in southwestern Nebraska, including the
Frenchman and Stinking Water Creeks, is below average.

The potential for ice jam related flooding will remain high until
the ice is off the rivers, streams and lakes. Recent cold weather
has kept lakes and rivers mostly ice covered, especially along
portions of the North and South Platte Rivers.

The potential for rainfall induced flash flooding is not
quantifiable. This type of flooding is most likely to occur during
the late spring and summer months.

.Snow Cover and Liquid Water Content...
As of March 7, snow depth ranged from 3 to 7 inches across southwest
Nebraska to 6 to 15 inches across portions of north central
Nebraska. Most locations were averaging about 6 inches of snow. Snow
was equivalent across the area was between a half inch to an inch.
With cold conditions expected the next two weeks, snow cover will
persist.

.Reservoir and Mountain Snowpack Conditions...
Normal operations are ongoing at reservoirs along the North Platte
River for this time of year. Releases from these dams have been
limited through the winter months, with inflows generally coming
from melting snow. Because of these operations, reservoir levels
have risen though the winter months. The current reservoir storage
across Wyoming, as well as Lake McConaughy, are slightly above
average for this time of year.

As of March 7, the snowpack in the North Platte and South
Platte River Basins in Colorado and Wyoming were above average,
ranging near 110 percent of average. These values are higher than
last year. Flooding from snowmelt runoff is highly dependent on how
quickly it comes out of the low to mid elevations of the mountains,
typically 8500 feet and lower.

.Soil Conditions and Frost Depths...
Soil moisture across western and north central Nebraska is above
average, with the driest conditions across the panhandle. Soil
temperature sensors indicate most areas have 4 inch soil
temperatures in the upper 20s to around 30 with a frost depth around
15 inches.

.River and Lake Ice Conditions...
Most of the ice on the larger lakes, rivers and streams remains in
place. A substantial amount of shore ice with only a minimal amount
of open water has been reported so far.

.Seasonal Precipitation...
Precipitation so far this water year, since October 1, 2018, has
ranged from near average in the eastern panhandle to 1 to 4 inches
above average across southwest and north central Nebraska. Amounts
ranged from around 2 inches in the eastern panhandle to 3 to 6
inches southwest into north central, with higher amounts near 7
inches parts of central Nebraska.

.Weather Outlooks...
Water temperatures in the tropical Pacific Ocean continue to be
slightly above average, indicating weak El Nino conditions. The
outlook indicates weak El Nino conditions continuing this spring,
before diminishing this summer. This weather pattern will typically
bring warmer temperatures to the western half of the United States,
with wetter conditions over the central and southern plains. For the
Summer, predictability is low for any type of overall weather
pattern.

According to the Climate Prediction Center, the latest 8 to 14 day
outlook for March 14th through March 20th calls for below average
temperatures and below average precipitation.

The latest 30 day outlook for March indicates there is increased
chances for below average temperatures and above average
precipitation.

The latest 90 day outlook for March, April, and May indicates there
are equal chances for above, below, and average temperatures and
precipitation conditions this spring.

.Numerical Weather Outlooks...

For the North Platte, South Platte, Platte, Elkhorn, and Niobrara
Rivers...long range probabilistic outlooks are issued for the water
year from December through May. All other months and locations are
90 day probabilistic outlooks.

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:  03/09/2019  - 06/07/2019

                                       :    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
--------           -----  -----  ----- : ---  ---  ---  ---  ---  ---
:North Platte River
Lisco                4.0    5.0    6.0 :  <5   <5   <5   <5   <5   <5
Lewellen             7.5    8.5    9.5 :   8   20   <5   <5   <5   <5
North Platte         6.0    6.5    7.0 :   5   14   <5   <5   <5   <5
:South Platte River
Roscoe               9.0   11.0   13.0 :  24   30    5    5   <5   <5
North Platte        13.0   14.0   15.0 :   5   <5   <5   <5   <5   <5
:Elkhorn River
Ewing 1N             9.0   10.0   12.0 :  16   <5    9   <5    6   <5
:Platte River
Brady North Chann    7.5    9.0   11.0 :  37   33   14   15    8    6
:Niobrara River
Sparks               6.0   10.0   12.0 :   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: 03/09/2019  - 06/07/2019
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:North Platte River
Lisco                 1.0    1.0    1.3    1.6    2.1    2.4    3.5
Lewellen              5.6    5.6    5.9    6.2    6.8    7.4    7.8
North Platte          4.6    4.6    4.7    4.9    5.3    5.7    6.0
:South Platte River
Roscoe                5.2    5.4    6.6    8.0    8.8   10.4   11.4
North Platte          5.8    5.9    7.5    9.1   10.4   11.4   12.6
:Elkhorn River
Ewing 1N              5.2    5.3    6.0    6.8    8.0   10.0   12.3
:Platte River
Brady North Chann     4.8    4.9    5.2    7.0    8.3   10.3   11.7
:Niobrara River
Sparks                2.9    2.9    3.0    3.1    3.2    4.0    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: 03/09/2019  - 06/07/2019
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:North Platte River
Lisco                 1.0    1.0    1.0    0.8    0.7    0.6    0.6
Lewellen              5.5    5.5    5.5    5.4    5.3    5.2    5.2
North Platte          4.4    4.3    4.2    4.2    4.1    4.1    4.0
:South Platte River
Roscoe                3.2    3.1    3.0    2.9    2.9    2.9    2.9
North Platte          5.5    5.5    5.5    5.5    5.4    5.4    5.4
:Elkhorn River
Ewing 1N              2.3    2.3    2.3    2.2    2.0    1.7    1.6
:Platte River
Brady North Chann     2.5    2.5    2.5    2.4    2.4    2.4    2.4
:Niobrara River
Sparks                2.5    2.5    2.4    2.4    2.3    2.3    2.2

Table 1--Probabilities for minor, moderate and major flooding
               Valid Period:  03/09/2019  - 06/07/2019

                                       :    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
--------           -----  -----  ----- : ---  ---  ---  ---  ---  ---
:Frenchman Creek
Palisade             7.0    8.0    9.0 :  12   12    5    6   <5   <5
:Stinking Water Creek
Palisade 2NW        10.0   11.0   12.0 :  <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: 03/09/2019  - 06/07/2019
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Frenchman Creek
Palisade              2.7    2.7    2.8    3.7    5.6    7.2    7.9
:Stinking Water Creek
Palisade 2NW          2.7    2.8    3.0    3.5    4.6    5.4    6.1

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: 03/09/2019  - 06/07/2019
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Frenchman Creek
Palisade              2.7    2.7    2.7    2.6    2.5    2.5    2.5
:Stinking Water Creek
Palisade 2NW          2.7    2.7    2.6    2.3    1.7    1.7    1.7

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 moistue, 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.

This is the second and final spring flood outlook and water resource
outlook for 2019. Long-range probabilistic outlooks are issued near
the middle of the month throughout the year.

$$


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