Flood Potential Outlook
Issued by NWS Denver/Boulder, CO

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FGUS75 KBOU 071905 CCA
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093-095-115-117-121-123-212300-

Probabilistic Hydrologic Outlook
National Weather Service Denver/Boulder CO
1155 AM MST Thu Mar 7 2019

...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...
* The spring flood potential due to snowmelt is near to slightly
  above average. Areas prone to minor flooding in the past may
  experience issues again this year.
* Significant flooding due to mountain snowmelt alone is not
  expected.
* Minor ice jam flooding is possible early this spring primarily on
  the North Platte River in Jackson County. Break-up ice jam
  flooding in the North Platte basin is primarily dependent on
  temperatures, the rate of snowmelt, and river levels.

Significant mountain snowmelt runoff typically starts in late April
to early May. Mountain snowmelt runoff generally peaks in mid-May to
mid-June. Therefore, it is still early to make long range
predictions on the magnitude of flooding due to mountain snowmelt
runoff.

The mountains are about three-quarters of the way through a typical
snow accumulation season so conditions may change before the runoff
begins. 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...
The snowpack increased significantly the past two weeks, and is now
above average across Colorado. The Colorado River basin snowpack
above Kremmling was around 125 percent of normal on March 7th. The
mountain snowpack was 124 and 118 percent of normal, respectively,
in the South Platte and North Platte River basins east of the
Continental Divide. The snow water equivalent in the mountain
snowpack ranged from 7 to 45 inches. At lower elevations, across the
Front Range urban corridor and northeast plains, patchy snow or snow
up to 4 inch depths remained.

Basin reservoir storage continued to hold fairly steady at the end
of February. The upper Colorado River basin storage was 90 percent
of average or 59 percent of capacity. Combined reservoir storage in
the South Platte basin was 102 percent of average or 75 percent of
capacity.

Over half of the U.S. Geological Survey stream gages continued to
have normal to above normal flows on March 7th. Frost depths were
generally less than one foot on the eastern plains.

Due to the wetter conditions, Severe Drought /D2/ was removed
recently from Clear Creek, Elbert, Grand, Lincoln, and Summit
Counties, and all but the southern tip of Park County on the U.S.
Drought Monitor. Moderate Drought /D1/ was removed from Jackson
County in the North Platte River basin. Moderate /D1/ continued from
Grand/Gilpin Counties southward and also on the east central plains.
Abnormally Dry conditions /D0/ endured elsewhere in north central
and northeast Colorado.

The U.S. Seasonal Drought Outlooks indicate drought removal is
likely in the north central mountains and east central plains this
spring.

.Temperatures and Precipitation...
Precipitation the past two weeks has generally been above to much
above normal. Temperatures varied from 2 to 5 degrees below normal
west of the Continental Divide to 15 to 25 degrees colder than
normal on the far eastern plains.

The outlooks for the month of March have a tilt toward below normal
temperatures and above normal precipitation. There is a 55 percent
chance that weak El Nino conditions will continue through 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: 3/09/19 - 6/07/19

                                       :    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    8.5    9.5 :  <5   <5   <5   <5   <5   <5
Denver              11.0   13.0   15.0 :  <5   <5   <5   <5   <5   <5
Henderson           11.0   13.0   15.0 :  <5   <5   <5   <5   <5   <5
Fort Lupton         11.0   13.0   15.0 :  <5   <5   <5   <5   <5   <5
Kersey              10.0   11.5   13.5 :   6    6    5    5   <5   <5
Weldona             10.0   11.0   12.0 :   5   <5   <5   <5   <5   <5
Fort Morgan         17.5   19.5   21.5 :   6    5   <5   <5   <5   <5
Balzac              10.0   11.0   12.5 :   5    5    5   <5   <5   <5
Atwood              11.5   13.0   14.5 :   5   <5   <5   <5   <5   <5
Julesburg           10.0   11.0   12.5 :  <5   <5   <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   10.5 :  <5   <5   <5   <5   <5   <5
Sheridan             8.0   10.0   10.5 :  <5   <5   <5   <5   <5   <5
:Clear Creek
Golden               8.5    9.5   10.5 :  <5   <5   <5   <5   <5   <5
Derby               10.5   11.5   12.5 :  <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    6   <5   <5   <5   <5
:North Platte River
Northgate            8.0    9.5   11.0 :   5    8   <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: 3/09/19 - 6/07/19
Location              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:South Platte River
South Platte          3.9    4.0    4.1    4.3    4.7    5.7    6.0
Denver                4.2    4.4    4.8    5.1    6.0    6.4    6.8
Henderson             5.7    5.9    6.5    6.8    8.2    8.7    9.4
Fort Lupton           4.4    4.8    5.4    5.9    7.8    8.5    9.6
Kersey                4.1    4.4    4.8    5.9    7.5    8.5   11.3
Weldona               3.1    3.6    4.1    5.1    6.9    7.5    9.5
Fort Morgan          10.1   10.3   10.8   11.9   13.9   15.3   18.4
Balzac                3.2    3.5    4.1    5.6    7.0    8.0   10.4
Atwood                4.2    4.2    4.9    6.4    7.7    8.8   11.1
Julesburg             5.4    5.4    5.6    6.2    7.2    7.8    8.9
:Plum Creek
Sedalia               3.8    3.9    4.1    4.2    4.5    5.0    6.3
:Bear Creek
Morrison              6.4    6.5    6.7    6.8    7.1    7.3    7.3
Sheridan              2.8    3.0    3.3    3.7    4.2    4.5    4.7
:Clear Creek
Golden                4.9    5.1    5.4    5.5    5.8    6.3    6.5
Derby                 3.3    3.4    3.4    3.9    4.5    4.8    5.0
:N Fk Big Thompson River
Drake                 2.6    2.6    2.8    2.9    3.2    3.5    3.6
:Cache la Poudre River
Fort Collins 9NW      4.1    4.1    4.3    4.7    5.1    5.9    6.5
Fort Collins          2.2    2.3    2.8    3.4    4.4    6.2    7.2
Greeley               1.9    2.0    2.6    3.9    5.8    7.6    8.2
:North Platte River
Northgate             3.8    4.1    4.5    5.1    5.9    7.0    7.9

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: 3/09/19 - 6/07/19
LOCATION              95%    90%    75%    50%    25%    10%     5%
--------            ------ ------ ------ ------ ------ ------ ------
:South Platte River
South Platte          2.9    2.9    2.9    2.9    2.9    2.9    2.9
Denver                2.8    2.8    2.8    2.8    2.8    2.8    2.8
Henderson             3.4    3.3    3.3    2.4    2.4    2.4    2.4
Fort Lupton           3.4    3.4    3.2    3.2    3.1    3.1    3.1
Kersey                3.6    3.4    3.1    2.6    2.3    2.2    2.1
Weldona               2.4    2.3    2.2    2.1    2.1    2.0    2.0
Fort Morgan           9.2    9.0    8.8    8.4    8.4    8.4    8.4
Balzac                2.2    1.6    1.1    0.6    0.6    0.6    0.6
Atwood                3.3    2.5    2.0    2.0    2.0    2.0    1.9
Julesburg             5.1    1.0    1.0    1.0    1.0    1.0    1.0
:Plum Creek
Sedalia               3.5    3.5    3.4    3.4    2.7    0.5    0.5
:Bear Creek
Morrison              5.7    5.7    5.7    5.6    5.6    5.5    5.4
Sheridan              2.2    2.2    2.2    2.1    2.1    2.1    2.1
:Clear Creek
Golden                3.4    3.4    3.4    3.3    3.3    3.3    3.3
Derby                 2.9    0.9    0.9    0.9    0.9    0.9    0.9
:N Fk Big Thompson River
Drake                 2.0    2.0    2.0    2.0    2.0    2.0    2.0
:Cache la Poudre River
Fort Collins 9NW      2.8    2.8    2.8    2.8    2.8    2.8    2.8
Fort Collins          0.8    0.7    0.7    0.2    0.2    0.2    0.2
Greeley               1.6    1.5    1.1    1.1    1.1    1.1    1.1
:North Platte River
Northgate             2.4    2.4    2.4    2.4    2.4    2.4    2.4

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      15           92
 Spinney Mtn Res Inflow    Apr-Sep      53          103
 11-Mile Canyon Res Inflow Apr-Sep      59          104
 Cheesman Lake Inflow      Apr-Sep      96           91
 South Platte              Apr-Sep     181           96

Bear Creek
 Morrison                  Apr-Sep      14           78

Clear Creek
 Golden                    Apr-Sep      92           75

St Vrain Creek
 Lyons                     Apr-Sep      81           89

Boulder Creek
 Orodell                   Apr-Sep      51          103

South Boulder Creek
 Eldorado Springs          Apr-Sep      26           70

Cache la Poudre
Canyon Mouth               Apr-Sep     186           93

North Platte River
 Northgate                 Apr-Sep     313          102

Colorado River
 Lake Granby               Apr-Jul     225          102

Willow Creek
 Willow Creek Res          Apr-Jul      52          111

Fraser River
 Winter Park               Apr-Jul      19.5        101

Williams Fork
 Williams Fork Res         Apr-Jul      94           98

Blue River
 Dillon Res                Apr-Jul     180          110
 Green Mtn Res             Apr-Jul     290          105

Muddy Creek
 Wolford Mtn Res Blw       Apr-Jul      62          115

Colorado River
 Kremmling                 Apr-Jul     900          105

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 second scheduled spring flood and water resources
outlook for 2019. Long-range probabilistic outlooks are issued
toward the end of each month. Water supply outlooks are issued the
second week of each month through June. Additional outlooks will be
issued as we get closer to the flood season.

$$
tlh


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