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Southwestern Storm to Bring Rain and Thunderstorm Potential, and Snow in the Mountains

A storm in the Southwest will produce rain, locally heavy with the potential for flash flooding this weekend. Mountain snow will spread across the Southwest, Great Basin, and into the South/Central Rockies. The whole storm shifts into the Plains later in the weekend while a new system brings rain and mountain snow from the Northwest to the Northern Rockies. Read More >

PROBABILISTIC HYDROLOGIC OUTLOOK
NATIONAL WEATHER SERVICE BISMARCK ND
1029 AM CST THU FEB 13 2020


...SPRING FLOOD AND WATER RESOURCES OUTLOOK...

This Spring Flood and Water Resources Outlook is for the Missouri
and James River basins of North Dakota and covers the period from
mid-February through the middle of May. This outlook is the first in
the Spring Flood and Water Resources Outlook series and will be
updated on 27 February and 13 March. After those updates, the NWS
will revert back to the regular monthly issuance on the fourth
Thursday of each month.

The following message has four sections. The first provides some
discussion on the highlights of this outlook and conditions
affecting the local hydrology. The second section gives the current
and normal/historical risks of flooding as determined by the
Ensemble Streamflow Forecast model of the National Weather Service.
The third section gives the current probabilities for reaching flood
stage at the listed forecast locations. And finally, the fourth
section covers the risk of the river sites falling below the listed
stages.

...Flood Outlook Highlights...
The likelihood of widespread spring flooding across western and
central North Dakota has generally fallen over the past few weeks.
In short, all but the James River basin is at, or below, normal
risk levels. Water equivalent received since the January outlook
has been unremarkable. In fact, received moisture is probably less
than or equal to the sum of observed melt, evaporation, and
sublimation, especially along and west of the Highway 83 corridor.

In the northwest corner of North Dakota, streams like the Little
Muddy River have a somewhat below normal, but certainly not zero,
risk of significant flooding. Other small streams in the area, such
as the White Earth River, Little Knife River, and Deep Creek in
McLean county also have at least some risk of flooding this spring.
The biggest driver behind the existing risk is the high soil
moisture, and not the existing snowpack. Wet and frozen soils in
this area are nearly impermeable and will enhance runoff from any
early spring rains until such time as they are thawed to the
point they again allow infiltration.

Farther to the south and west of the Missouri River, forecast points
on the Knife River and Spring Creek have near normal risks of
reaching at least Minor flood stage. This would suggest that
tributaries to the Knife River are also all at some elevated risk
of high water. The lowering of risk along the Knife is largely due
to an early February melt that removed a significant fraction of the
snow. Soil conditions are still conducive to producing strong runoff
from early spring rains and that risk will remain until the soils
thaw.

Near and south of I-94, the Missouri River tributaries forecast
points and small streams like the Heart and Cannonball rivers along
with Painted Woods, Hay, Burnt, Apple, Cedar, and Beaver creeks are
now considered at near normal risk for minor flooding. Other
streams in the southwest, such as the Green River, Big Muddy, and
Sweet Briar Creek should also be thought of as near normal risk of
minor flooding due to wet and frozen soils.

Farther to the east and in the James River Basin, flood risk in the
very upper part of the basin near Harvey is probably the lowest and
near normal. However, the risk of at least Minor flooding increases
quickly as one gets closer to Pingree and Grace City. Jamestown and
Pipestem dams continue to have well above normal discharge and that
is expected to continue until the commencement of the spring runoff.
Downstream of Jamestown, wet, frozen soils and an already above
normal Snow Water Equivalent (SWE) are enough to present widespread
flood concerns. Bear and Bonehill creek along with the Maple River
and essentially all other small streams have enough SWE to present
problems this spring. Perhaps the one risk factor that is not
working against the lower James River basin is that the early and
heavy snowpack has kept frost depths to a minimum. Soils underneath
the heaviest snowcover have been well insulated this winter and
have the least frost depth, with some areas having unfrozen ground.
A gentle spring will greatly reduce runoff in areas with the
shallowest frost depths.

The Prairie Pothole region, including at least parts of Sheridan,
Wells, Kidder, Stutsman, Logan, McIntosh and Dickey counties have
enough SWE to suggest overland flooding will be a concern going
into spring. Most small wetlands and ponds were filled by the wet
fall and are going into spring with water levels that are near
their normal summer time high water mark. Low lying roads next
to some of these wetlands are at risk of extended closures this
spring and summer.

Lastly, the Missouri River itself. New this spring is the
inclusion of the Missouri River near Williston in the extended
streamflow predictive modeling. Modeling results suggest a near
normal risk of Minor flooding along the Missouri River west of
Williston. This risk is consistent with a now slightly above
normal snowpack in the Yellowstone basin and a near normal snowpack
in the Missouri river basin. One of the items of interest not
included in the tables below is the overall runoff for the Missouri
and Yellowstone Rivers. The combination of plains snowpack and that
in the upper basin would suggest a relatively normal runoff event
even without much spring rainfall. A normal snowmelt season would
see little in the way of problems along the Missouri and Yellowstone
rivers west of Williston. Ice jam related flooding has historically
been a considerable risk in western North Dakota and is often due to
a rapid warming trend during the initial spring melt season.

Importantly, the probabilities shown in Tables 1 and 2 do not
include the risks associated with ice jam related flooding. Ice jams
defy mathematical predictions, but are known to increase during
early and rapid spring runoffs where the ice becomes mobilized, only
to get stuck at some downstream constriction such as bridges and
bends in the river.

...Current Conditions...
The general consensus is that western and central North Dakota`s
snowpack sits atop a crust of fairly wet and frozen soil. This
creates a nearly impermeable surface and will enhance runoff from
a given snowpack. Rivers and streams are likely to hold their full
ice through February, but there is a chance of seeing an early
spring melt. This tends to increase the risk of ice jam related
flooding IF a sudden change in temperature brings on a rapid melt
in early March.

A person`s visual meausuring of the snow across the state may give
a somewhat misleading estimate for how much SWE is on the ground.
A considerable amount of compaction and consolidation has already
taken place from the western James River basin all the way out to
the Badlands. Indeed, the Badlands and Missouri River tributaries
west of the Missouri River have already experienced melting
conditions and some runoff.

...Weather Outlook...
The near term 6-10 and 8-14 day outlooks suggest a continuation of
the very benign weather pattern the area has experienced over the
past few weeks. This below normal precipitation pattern and near
normal to above normal temperatures will help lower flood risks
even more as we go into March. Looking farther into the future, the
3-month outlooks suggest a slightly cooler and wetter than normal
pattern for the entire February, March, and April period.


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.


                 Chance of Exceeding Stages at Specific Locations
                     Valid Period:  02/15/2020  - 05/15/2020

                                       :    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
--------           -----  -----  ----- : ---  ---  ---  ---  ---  ---
:Pipestem Creek
Pingree             11.0   13.0   15.0 :  51   17   10    7   <5   <5
:James River
Grace City          12.0   14.0   15.0 :  47   16   17    9   15    7
Lamoure             14.0   16.0   18.0 : >98   10   77    6   32   <5
:Missouri River
Williston           22.0   24.0   26.0 :   6    7   <5   <5   <5   <5
:Cannonball River
Regent              22.0   24.0   26.0 :  <5   <5   <5   <5   <5   <5
:Cedar Creek
Raleigh             12.0   14.0   16.0 :   7   <5   <5   <5   <5   <5
:Cannonball River
Breien              10.0   20.0   23.0 :  43   39   <5   <5   <5   <5
:Beaver Creek
Linton              12.0   14.0   16.0 :  23   29    5   14   <5    8
:Little Muddy River
Williston           10.0   12.0   14.0 :  41   55   19   27   <5   12
:Little Missouri
Marmarth            18.0   23.0   30.0 :  <5   <5   <5   <5   <5   <5
:Little Missouri River
Medora              15.0   18.0   20.0 :   8   13   <5    7   <5   <5
Watford City        20.0   24.0   30.0 :  <5   <5   <5   <5   <5   <5
:Knife River
Manning             15.0   17.0   20.0 :  16    9   <5   <5   <5   <5
:Spring Creek
Zap                 14.0   18.0   20.0 :   9   10   <5   <5   <5   <5
:Knife River
Hazen               21.0   24.0   25.0 :  14   22    7   10    5    6
:Heart River
Mandan              17.0   23.0   28.0 :   8    9   <5   <5   <5   <5
:Apple Creek
Menoken             15.0   16.0   17.0 :  34   39   29   34   <5   16

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: 02/15/2020  - 05/15/2020
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Pipestem Creek
Pingree               9.3    9.9   10.5   11.1   11.8   13.0   13.4
:James River
Grace City            9.1   10.1   10.5   11.7   13.4   16.4   18.2
Lamoure              15.5   15.7   16.1   16.9   18.1   18.6   19.7
:Missouri River
Williston            17.0   17.1   17.6   18.8   20.9   21.5   22.9
:Cannonball River
Regent                5.2    5.7    7.2    8.5   12.0   15.7   19.7
:Cedar Creek
Raleigh               3.2    3.5    4.6    5.8    7.5   10.1   13.6
:Cannonball River
Breien                5.6    6.4    7.9    9.3   11.6   16.1   18.8
:Beaver Creek
Linton                6.5    7.3    8.8   10.1   11.8   12.9   13.9
:Little Muddy River
Williston             4.6    5.3    7.6    9.4   11.5   12.6   13.3
:Little Missouri
Marmarth              1.9    2.7    3.9    6.0    9.1   13.1   14.8
:Little Missouri River
Medora                2.6    4.1    5.5    8.4   11.3   14.2   17.3
Watford City         -0.5    1.4    2.1    5.3    8.8   10.5   12.7
:Knife River
Manning               6.0    6.4    7.6    9.0   13.8   15.7   16.1
:Spring Creek
Zap                   4.8    5.4    6.4    8.8   11.6   13.6   16.9
:Knife River
Hazen                 1.9    4.2    8.3   13.9   18.4   23.2   25.0
:Heart River
Mandan                1.8    2.8    5.6    8.1   12.3   15.6   19.2
:Apple Creek
Menoken               6.3    6.6    8.8   13.7   16.1   16.4   16.6

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: 02/15/2020  - 05/15/2020
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:Pipestem Creek
Pingree               5.3    5.3    5.2    5.2    5.2    5.2    5.2
:James River
Grace City            5.0    5.0    5.0    5.0    5.0    4.9    4.9
Lamoure               8.7    8.7    8.7    8.3    7.9    7.8    7.6
:Cannonball River
Regent                4.8    4.8    4.8    4.8    4.8    4.8    4.8
:Cedar Creek
Raleigh               1.2    1.1    1.0    1.0    0.8    0.7    0.6
:Cannonball River
Breien                2.9    2.8    2.5    2.4    2.3    2.2    2.2
:Beaver Creek
Linton                4.2    4.2    4.2    4.2    4.2    4.2    4.1
:Little Muddy River
Williston             4.4    4.4    4.4    4.4    4.4    4.4    4.4
:Little Missouri
Marmarth              1.3    1.3    1.3    1.3    1.2    1.2    1.2
:Little Missouri River
Medora                1.7    1.7    1.7    1.7    1.6    1.5    1.3
Watford City         -1.1   -1.1   -1.1   -1.1   -1.2   -1.2   -1.3
:Knife River
Manning               6.0    6.0    6.0    6.0    6.0    5.9    5.8
:Spring Creek
Zap                   4.8    4.8    4.8    4.8    4.8    4.8    4.8
:Knife River
Hazen                 0.4    0.4    0.3    0.3    0.3    0.3    0.3
:Heart River
Mandan               -1.6   -1.6   -1.6   -1.6   -1.6   -1.6   -1.6
:Apple Creek
Menoken               5.1    5.1    5.1    5.1    5.0    5.0    5.0

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 weather.gov/bis for more weather and water
information.

The next outlook will be issued February 27.