Flood Potential Outlook
Issued by NWS Denver/Boulder, CO

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000
FGUS75 KBOU 012135
ESFBOU
COC001-005-013-014-019-031-035-039-047-049-057-059-069-073-075-087-
093-095-115-117-121-123-101200-

Probabilistic Hydrologic Outlook
National Weather Service Denver/Boulder CO
234 PM MST Thu Mar 1 2018

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

This outlook is for north central and northeast Colorado including
the Colorado...North Platte and South Platte Rivers.

...Flood Outlook Summary...

At this time, the probability of snowmelt flooding through June is
below average. However...the flood potential is dependent on the
amount of snow and rain this spring and the timing of the peak
flows. Conditions could change before the runoff begins. It is
important to keep in mind that an extended period of much above
normal temperatures or heavy rainfall during the melt period can
cause or exacerbate flooding problems in any year.

The potential for flash flooding is not quantifiable since this type
of flooding is usually caused by localized thunderstorms producing
heavy rain during the spring and summer.

...Current Hydrologic Conditions...

On March 1st liquid water in the high mountain snowpack ranged from
3 to 30 inches. The north central Colorado mountain snowpack ranged
from 83 percent of normal west of the Continental Divide to 89
percent of normal east of the divide. Future snow would need to be
123, 135 and 153 percent of normal respectively, this spring for the
South Platte, North Platte and upper Colorado River basins to reach
their normal peak snowpack. Most of the NRCS SNOTEL gages used for
determining basin snowpack are above 9000 feet. It has been drier in
the mountains and foothills below 8500 feet; with the snowpack
farther below normal. A couple of winter storms dropped snowfall of
2 inches or more over the Front Range Urban Corridor and eastern
plains during February. By March there was patchy to little snow
remaining at lower elevations.

Nearly 70 percent of the U.S. Geological Survey stream gages had
normal to above normal flows on March 1st.

...Climate Summary...

Precipitation since mid-February has generally been within a half an
inch of normal, except highest elevations east of the divide had a 1
to 2 inch deficit. Temperatures the past two weeks have been up to
10 degrees below normal. There have been no changes recently in
north central or northeast Colorado on the U.S. Drought Monitor.
Areas of Abnormally Dry Conditions /D1/ and Moderate Drought /D2/
continue. Areas farthest north in Colorado continued to be wetter
and colder than locations south.

...Weather and Drought Outlooks...

March precipitation is predicted to be below normal with
temperatures near to above normal. La Nina conditions are present.
There is a 55 percent chance of a transition from La Nina to ENSO-
neutral conditions this spring.

...Numerical River Outlooks...

In Table 1 below, the current (CS) and historical (HS) or normal
probabilities of exceeding minor...moderate...and major flood stage
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 greater 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: 3/03/2018 - 6/01/2018

                                       :    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
--------           -----  -----  ----- : ---  ---  ---  ---  ---  ---
:South Platte River
South Platte         7.0    9.0   11.0 :  <5   <5   <5   <5   <5   <5
Denver              11.0   12.5   14.0 :  <5   <5   <5   <5   <5   <5
Henderson           10.0   11.0   15.0 :  <5   <5   <5   <5   <5   <5
Kersey              10.0   11.5   13.0 :   8    9   <5    6   <5   <5
Weldona             10.0   11.0   12.0 :   7    8   <5    6   <5   <5
Balzac              10.0   11.0   12.5 :   7    8   <5    5   <5   <5
Julesburg           10.0   11.0   12.0 :   7    7   <5   <5   <5   <5
:Plum Creek
Sedalia              8.0   10.0   12.0 :  <5   <5   <5   <5   <5   <5
:Bear Creek
Morrison             9.0   10.0   11.0 :  <5   <5   <5   <5   <5   <5
Sheridan             8.0   10.0   11.0 :  <5   <5   <5   <5   <5   <5
:Clear Creek
Golden              10.0   13.0   14.0 :  <5   <5   <5   <5   <5   <5
Derby                9.0   10.5   11.5 :  <5   <5   <5   <5   <5   <5
:N Fk Big Thompson River
Drake                5.5    6.0    7.0 :  <5   <5   <5   <5   <5   <5
:Cache la Poudre River
Fort Collins 9NW     7.5    9.0   10.5 :  <5   <5   <5   <5   <5   <5
Fort Collins        10.5   11.5   13.0 :  <5   <5   <5   <5   <5   <5
Greeley              8.0    9.5   10.5 :  <5   <5   <5   <5   <5   <5
:North Platte River
Northgate            8.0    9.5   11.0 :  <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: 3/03/2018 - 6/01/2018
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:South Platte River
South Platte          3.8    3.8    3.9    4.0    4.3    4.7    5.3
Denver                4.0    4.1    4.4    5.1    5.5    6.4    7.1
Henderson             4.9    5.0    5.4    6.2    7.2    8.3    9.4
Kersey                4.0    4.4    5.0    6.4    8.0    9.0   11.1
Weldona               3.0    3.2    3.6    4.8    6.7    7.8   10.6
Balzac                2.6    2.9    3.2    4.8    6.7    8.1   10.7
Julesburg             5.6    5.6    6.1    7.2    8.0    8.9   10.5
:Plum Creek
Sedalia               4.1    4.2    4.5    4.7    5.0    5.5    7.3
:Bear Creek
Morrison              6.1    6.3    6.5    6.6    6.9    7.4    7.8
Sheridan              2.7    2.8    3.1    3.2    3.7    4.6    4.9
:Clear Creek
Golden                4.5    4.6    5.0    5.2    5.4    5.6    6.1
Derby                 3.3    3.4    3.5    3.7    4.1    4.5    5.2
:N Fk Big Thompson River
Drake                 4.0    4.1    4.2    4.3    4.4    4.6    4.7
:Cache la Poudre River
Fort Collins 9NW      3.9    4.0    4.1    4.3    4.7    5.2    5.7
Fort Collins          1.8    1.8    1.9    2.2    3.0    4.2    5.7
Greeley               1.8    1.9    2.0    2.8    3.8    6.0    7.4
:North Platte River
Northgate             3.9    4.2    4.6    5.2    6.3    7.2    7.8

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: 3/03/2018 - 6/01/2018
LOCATION              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:South Platte River
South Platte          2.7    2.7    2.7    2.7    2.7    2.7    2.7
Denver                2.9    2.8    2.8    2.7    2.7    2.7    2.7
Henderson             3.7    3.7    3.6    3.6    3.6    3.6    3.6
Kersey                3.2    3.0    2.7    2.6    2.5    2.5    2.4
Weldona               2.2    2.1    1.9    1.7    1.5    1.4    1.4
Balzac                1.9    1.8    1.8    1.8    1.8    1.8    1.8
Julesburg             2.5    2.5    2.5    2.5    2.5    2.5    2.5
:Plum Creek
Sedalia               3.7    3.6    3.6    3.6    3.5    3.5    3.5
:Bear Creek
Morrison              5.6    5.6    5.6    5.6    5.5    5.5    5.5
Sheridan              2.3    2.2    2.1    2.1    1.3    1.3    1.3
:Clear Creek
Golden                3.4    3.4    3.4    3.4    3.4    3.4    3.4
Derby                 0.9    0.9    0.9    0.9    0.9    0.9    0.9
:N Fk Big Thompson River
Drake                 3.7    3.7    3.7    3.7    3.7    3.7    3.7
:Cache la Poudre River
Fort Collins 9NW      2.7    2.7    2.7    2.7    2.7    2.7    2.7
Fort Collins          0.6    0.6    0.4    0.1    0.1    0.1    0.1
Greeley               1.2    1.1    1.1    1.1    1.1    1.1    1.1
:North Platte River
Northgate             2.6    2.6    2.6    2.6    2.6    2.5    2.5

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/bou for more weather and water
information.

This will be the last Spring Flood Outlook issued for 2018. Long-
range probabilistic outlooks are issued toward the end of each month.

$$
tlh



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