Hydrologic Outlook
Issued by NWS Denver/Boulder, CO
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000 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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