Flood Potential Outlook
Issued by NWS North Platte, NE

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Probabilistic Hydrologic Outlook
National Weather Service North Platte, NE
433 PM CST Thu Feb 15 2018

...Spring Flood and Water Resource Outlook...

...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 from March 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 February 15, three to as much as eight inches of snowpack
remained across portions of north central Nebraska, and across
southwest into central Nebraska. Portions of the southeast panhandle
into the central Sandhills had either no snow cover or up to two
inches. Average temperatures are forecast to fall to below average
next week. Therefore, some exiting snow cover is likely to 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 inter
months, with inflows coming from melted 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 mid-February, the snowpack in the North Platte and South
Platte River Basins in Colorado and Wyoming were well below average,
and much lower 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 the central Sandhills is above average with
near average soil moisture elsewhere. Soil temperature sensors
indicate most areas have 4 inch soil temperatures in the upper 20s
to lower 30s. Therefore, the frost depth is minimal and runoff will
be able to be easily absorbed into the ground.

.River and Lake Ice Conditions...
Most of the ice on the larger lakes remains, with only a small
amount of open water reported. Most of the ice on rivers and streams
also remains, with a substantial amount of shore ice remaining.
River flows are mostly in the normal range.

.Seasonal Precipitation...
Precipitation so far this water year, since October 1, 2017, has
been from two to five inches, with higher amounts near 7 inches in
parts of central Nebraska. This is about one to two inches, to as
much as 3 inches above normal for this time of year.

.Weather Outlooks...
Water temperatures in the tropical Pacific Ocean are slightly below
average, indicating La Nina conditions. The outlook for the spring
and summer is for the waters to slightly warm to near average, which
would allow La Nina conditions to trend toward El Nino Southern
Oscillation (ENSO) neutral conditions. ENSO neutral conditions
generally bring a warmer and wetter pattern to the southern half of
the United States, with Nebraska being in the transition zone
between the air masses and the polar and subtropical jet streams.

According to the Climate Prediction Center, the latest 8 to 14 day
outlook for February 23 through March 1st calls for near normal
temperatures and near normal precipitation.

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

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

.Numerical Weather Outlooks...
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
--------           -----  -----  ----- : ---  ---  ---  ---  ---  ---
:North Platte River
Lisco                4.0    5.0    6.0 :   6   13   <5   10   <5    7
Lewellen             7.5    8.5    9.5 :  53   33    5   12   <5    7
North Platte         6.0    6.5    7.0 :  34   27   12   21    6   11
:South Platte River
Roscoe               9.0   11.0   13.0 :   9   10    6    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 :  <5   <5   <5   <5   <5   <5
:Platte River
Brady North Chann    7.5    9.0   11.0 :  15   19   10   10   <5    6
:Frenchman Creek
Palisade             7.0    8.0    9.0 :   8   <5    5   <5   <5   <5
:Stinking Water Creek
Palisade 2NW        10.0   11.0   12.0 :  <5   <5   <5   <5   <5   <5
: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: 02/17/2018  - 05/18/2018
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:North Platte River
Lisco                 2.0    2.0    2.0    2.7    3.0    3.2    4.3
Lewellen              6.7    6.7    6.7    7.5    7.7    7.9    8.5
North Platte          4.5    4.5    4.5    5.4    6.1    6.6    7.2
:South Platte River
Roscoe                4.5    4.5    4.6    5.2    6.4    8.3   11.2
North Platte          7.9    7.9    7.9    8.1    9.0   11.0   13.0
:Elkhorn River
Ewing 1N              3.1    3.6    4.2    4.8    5.7    6.7    7.7
:Platte River
Brady North Chann     2.5    2.5    3.1    4.6    6.0    9.0   10.1
:Frenchman Creek
Palisade              2.8    2.8    2.9    3.3    4.6    6.7    8.0
:Stinking Water Creek
Palisade 2NW          3.0    3.1    3.2    3.6    4.6    6.9    8.5
:Niobrara River
Sparks                2.9    3.0    3.0    3.1    3.2    3.4    3.4

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%
--------            ------ ------ ------ ------ ------ ------ ------
:North Platte River
Lisco                 1.3    1.2    1.2    1.2    1.2    1.1    1.1
Lewellen              5.8    5.8    5.7    5.7    5.7    5.6    5.6
North Platte          4.5    4.5    4.3    4.1    4.0    3.9    3.8
:South Platte River
Roscoe                3.7    3.4    3.0    2.9    2.9    2.9    2.9
North Platte          7.1    7.0    7.0    6.9    6.9    6.9    6.9
:Elkhorn River
Ewing 1N              2.8    2.8    2.8    2.7    2.6    2.5    2.5
:Platte River
Brady North Chann     2.4    2.4    2.4    2.4    2.4    2.4    2.4
:Frenchman Creek
Palisade              2.7    2.7    2.7    2.7    2.7    2.7    2.7
:Stinking Water Creek
Palisade 2NW          2.9    2.9    2.8    2.5    2.4    2.3    2.2
:Niobrara River
Sparks                2.8    2.8    2.8    2.7    2.6    2.6    2.6

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.

This is the first spring flood outlook and water resource outlook
for 2018. Long-range probabilistic outlooks are issued near the
middle of the month throughout the year. The next scheduled outlook
will be issued on March 1, 2018.

$$


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