Flood Potential Outlook
Issued by NWS Denver/Boulder, CO

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FGUS75 KBOU 151941
ESFBOU
COC001-005-013-014-019-031-035-039-047-049-057-059-069-073-075-087-
093-095-115-117-121-123-031200-

Probabilistic Hydrologic Outlook
National Weather Service Denver/Boulder CO
1240 PM MST Thu Feb 15 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...
* Near average risk of minor snowmelt flooding in the North and
  South Platte River basins east of the Continental Divide. Areas
  prone to minor flooding in the past may experience issues again
  this year.
* Slightly below average risk of snowmelt flooding in the
  headwaters of the Colorado River west of the divide.
* Significant flooding due to mountain snow runoff alone is not
  expected. However...it is still early in the snow accumulation
  season and conditions could change before the runoff begins.

Above normal temperatures have melted the snowpack in the lower
foothills and diminished frozen ground. Near to below normal
subsurface soil moisture was observed. Mountain snowpack above 9000
feet east of the divide has improved to near normal the past month.
The snowpack below 9000 feet and also west of the divide remains
below normal.

Above normal future snow accumulations combined with heavy rains and
rapid melt will increase the current flood potential while below
normal future precipitation and gradual or intermittent freezing and
thawing will decrease the magnitude of the current assessment. Near
average risk suggests that locations which typically experience
minor spring time flooding may flood again this year and those that
typically don`t will not.

...Current Hydrologic Conditions...
We are around two-thirds of the way through a typical snow
accumulation season. On February 15th the South Platte basin had 95
percent of its normal snowpack and 57 percent of its normal peak
snowpack. The mountain snowpack in the North Platte basin was 90
percent of normal and upper Colorado River headwaters had 84 percent
of their normal snowpack. Liquid water in the high mountain snowpack
ranged from 5 to 25 inches. Future snow would need to be around 108,
124 and 131 percent of normal respectively, through 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 should be
noted that below 9000 feet it has been drier with the snowpack
generally farther behind normal. The normal peak date is in April so
there is still plenty of time for the snowpack to reach its normal
peak. Little to no snow was observed along the Front Range Urban
Corridor and eastern plains due to temperatures reaching the 60s
midweek.

Basin reservoir storage continued to hold steady at the end of
January. The upper Colorado River basin storage was 113 percent of
average or 82 percent of capacity. Combined reservoir storage in the
South Platte basin was 114 percent of average or 74 percent of
capacity. 60 percent of the U.S. Geological Survey stream gages had
normal to above normal flows mid-February.

With a wetter pattern developing the past month, areas in Moderate
Drought /D1/ and Abnormally Dry Conditions /D2/ have been slowly
shrinking across the northern third of Colorado.

...Climate Summary...
Since January 1st the wettest area compared to normal was the
northeast plains with precipitation 120 to 150 percent of normal.
The driest area was along the Palmer Divide and east central plains
where precipitation was between 50 and 90 percent of normal.
Temperatures the past 45 days have been above normal with the
exception of the northeast corner of Colorado.

...Weather and Drought Outlooks...
The 30 day outlook calls for equal chances of near...above or below
normal precipitation and a tilt toward below normal temperatures.
The 90 day outlook has slightly higher probabilities of below normal
precipitation and above normal temperatures. The U.S. Seasonal
Drought Outlook valid February 15th through May 31st indicates
drought conditions persisting along and south of a line from
southeast Jefferson County to Adams County to southwest Washington
County.

...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: 2/17/2018 - 5/18/2018

                                       :    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
--------           -----  -----  ----- : ---  ---  ---  ---  ---  ---
: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    7   <5    5   <5   <5
Julesburg           10.0   11.0   12.0 :   6    6   <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: 2/17/2018 - 5/18/2018
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:South Platte River
South Platte          3.8    3.8    3.8    3.9    4.0    4.6    4.9
Denver                3.9    4.0    4.2    4.7    5.2    6.4    7.1
Henderson             4.7    4.8    5.0    5.9    6.6    8.2    9.4
Kersey                4.1    4.1    4.5    5.8    7.5    8.7   11.2
Weldona               3.0    3.0    3.2    4.0    6.0    7.4   10.6
Balzac                2.8    2.8    3.0    3.9    5.5    7.5   10.4
Julesburg             5.6    5.6    5.8    6.7    7.6    8.6   10.2
:Plum Creek
Sedalia               4.0    4.1    4.3    4.7    4.9    5.5    7.3
:Bear Creek
Morrison              6.1    6.2    6.4    6.6    6.9    7.4    7.8
Sheridan              2.6    2.7    2.8    3.2    3.6    4.6    4.9
:Clear Creek
Golden                4.1    4.2    4.4    4.7    5.0    5.4    5.7
Derby                 3.2    3.3    3.4    3.5    4.0    4.4    5.2
:N Fk Big Thompson River
Drake                 3.9    4.0    4.0    4.1    4.2    4.4    4.6
:Cache la Poudre River
Fort Collins 9NW      3.3    3.4    3.6    3.8    4.0    4.8    5.7
Fort Collins          1.1    1.2    1.3    1.6    2.8    3.9    6.2
Greeley               1.8    1.9    2.0    2.6    3.5    5.1    7.7
:North Platte River
Northgate             3.4    3.5    3.8    4.2    4.5    5.3    6.5

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: 2/17/2018 - 5/18/2018
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                2.8    2.8    2.8    2.8    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    2.6    2.5    2.5
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               3.6    3.6    3.6    3.6    3.5    3.5    3.5
:Bear Creek
Morrison              5.7    5.6    5.6    5.6    5.6    5.5    5.5
Sheridan              2.3    2.3    2.2    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          0.4    0.4    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.

Following are streamflow forecasts for selected locations:
                              Most Probable Forecast
                                      Volume      Percent
Stream and Station         Period     1000 AF      of Avg
__________________         ______     _______     _______
South Platte River
 Antero Reservoir Inflow   Apr-Sep       9          58
 Spinney Mtn Res Inflow    Apr-Sep      29          49
 11-Mile Canyon Res Inflow Apr-Sep      32          49
 Cheesman Lake Inflow      Apr-Sep      64          52
 South Platte              Apr-Sep     121          58

Bear Creek
 Evergreen Abv             Apr-Sep      12          62

Clear Creek
 Golden                    Apr-Sep      90          81

St Vrain Creek
 Lyons                     Apr-Sep      66          69

Boulder Creek
 Orodell                   Apr-Sep      41          76

South Boulder Creek
 Eldorado Springs          Apr-Sep      27          73

Cache la Poudre
 Canyon Mouth              Apr-Sep     173          84

North Platte River
 Northgate                 Apr-Sep     255         112

Colorado River
 Lake Granby               Apr-Jul     180          82

Willow Creek
 Willow Creek Res          Apr-Jul      40          85

Fraser River
 Winter Park               Apr-Jul      17.5        90

Williams Fork River
 Williams Fork Res         Apr-Jul      80          83

Blue River
 Dillon Res                Apr-Jul     140          86
 Green Mtn Res             Apr-Jul     230          84

Muddy Creek
 Wolford Mtn Res Blw       Apr-Jul      33          61

Colorado River
 Kremmling                 Apr-Jul     680          79

The streamflow forecasts above reflect natural flow only. Actual
observed flow will likely be affected by upstream water management.

Visit our web site weather.gov/bou for more weather and water
information.

This is the first spring flood and water resources outlook for 2018.
Long-range probabilistic outlooks are issued toward the end of each
month. The next scheduled spring flood outlook will be issued
Thursday March 1st.

$$
tlh



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