FGUS75 KRIW 141712 ESFRIW WYC003-013-017-019-023-025-029-035-037-039-043-010000- Probabilistic Hydrologic Outlook National Weather Service Riverton WY 1110 AM MST Thu Mar 14 2024 ...Spring Flood and Water Resource Outlook... ...Below Average Flood Potential This Spring For Western And Central Wyoming But Confidence Remains Low... This spring flood and water resource outlook is for the Riverton Hydrologic Service Area (HSA), which covers western and central Wyoming. .Flood Outlook Summary... Confidence remains low in the flood potential outlook for this spring. There are still one to two months of snow accumulation remaining, and additional snow and rain will greatly influence river flows. The speed of the spring warm-up also greatly influences how much water makes it into rivers, with a slower and more modest warm- up allowing more water to be absorbed by the ground after frost melts. In short, many important variables are still unknown in mid March. All that being said, high elevation snowpack varies across our HSA east of the Continental Divide. Mountains that feed the Shoshone and Upper Bighorn Basins contain above normal SWE, while the Wind River Range remains near normal. The Bighorn Range has below to well below normal snowpack. Low elevation snowpack is below average for most basins. Statistical analysis and probabilistic forecasts in the tables below indicate near normal to below normal chances for flooding. This lines up well with local experience given current below normal snowpack conditions. .Temperatures and Precipitation... Temperatures over the past three months have been above to much above normal for most of central and western Wyoming. A lack of deep lower-elevation snowpack and absence of cold air advection bringing in Arctic air masses has allowed temperatures to remain mild. The Wind River Basin is the one exception, where temperatures have been near normal over the past three months. Precipitation over the past three months has varied across the region. The Bighorn Basin and Wind River Basin have seen near to slightly above normal precipitation. Most mountain ranges, the Upper Yellowstone Basin, and the Powder River Basin have had below to much below normal precipitation. .Snow Cover and Liquid Water Content... High-elevation SWE ranges from 55 to 170 percent of normal. The lowest values are in the Upper Yellowstone Basin, North Platte Basin, and eastern Powder River Basin, with values between 55 to 80 percent of normal. The western Bighorn Basin sits between 120 to 170 percent of normal and the Wind River Basin sits right around normal. Low-elevation snowpack is below normal for most basins. The snowiest basin is the Wind River Basin, where far southwest portions of the basin have snow depth in the 4 to 8 inch range, with a trace to up to 2 inches of SWE. Most of the Wind River Basin, and other central Wyoming basins have less than 1 inch of SWE and a trace to a few inches of snow depth. Its worth noting that for most locations across central Wyoming, there are around one to two more months of snow accumulation, with peak mountain snow depths not occurring until mid-April to mid-May. Late winter into spring is normally one of the wetter times of the year, so the snow cover and SWE values listed above could quickly become non-representative. .Soil Conditions and Frost Depths... Soil moisture for most of central Wyoming is in the 80 to 95 percent range. Exceptions to this are in the Upper Yellowstone Basin and around the North Platte River near Casper, where soil moisture is lower, and ranges from 40 to 60 percent. In the last month, frost depths have been about 1 to 2 feet for central Wyoming. .Lake and River Conditions... A majority of the higher elevation lakes and streams across the region are ice covered while many lower elevation rivers have some open water areas with some being completely ice free. A majority of river gauges are ice affected, but those that are operating indicate normal to slightly above normal flows for this time of year. Its worth noting that the spring melt has not begun yet, so most of the observed river flows are not representative of spring conditions at this time. .Weather Outlooks... The Climate Prediction Center's monthly outlook for March indicates a slightly higher than average chance of above normal precipitation, along with a slightly higher than average chance of above normal temperatures. The seasonal precipitation outlook covering meteorological spring trends toward equal chances of above or below normal precipitation, which is another way of saying that climatological normals are the best forecast. The seasonal temperature outlook covering meteorological spring trends toward a slightly higher than average chance of above normal temperatures. .Numerical River Outlooks... 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. ...Table 1--Probabilities for minor, moderate, and major flooding... Valid Period: 03/16/2024 - 09/30/2024 : 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 -------- ----- ----- ----- : --- --- --- --- --- --- :Wind River Dubois 5.0 5.5 6.0 : 21 22 9 12 6 6 Red Creek 9.5 10.0 10.5 : <5 <5 <5 <5 <5 <5 Crowheart 10.0 10.5 11.0 : 5 17 <5 7 <5 <5 Kinnear 9.0 10.0 11.0 : <5 9 <5 <5 <5 <5 Riverton 9.0 11.0 12.0 : 20 25 <5 8 <5 <5 :Little Wind River Riverton 8.0 10.0 11.0 : 21 16 5 <5 <5 <5 :Bighorn River Basin 10.5 11.0 13.0 : <5 9 <5 8 <5 <5 Greybull 92.0 93.5 95.0 : <5 10 <5 7 <5 <5 :North Fork Shoshone River Wapiti 8.0 9.0 10.0 : 8 16 5 10 <5 <5 :South Fork Shoshone River Buffalo Bill Rese 9.5 10.5 12.0 : <5 6 <5 <5 <5 <5 :Shoshone River Lovell 11.0 11.5 12.0 : <5 20 <5 13 <5 12 :Yellowstone River Fishing Bridge 9.0 10.0 11.0 : <5 <5 <5 <5 <5 <5 :Lamar River Tower Junction 15.0 16.0 17.0 : <5 <5 <5 <5 <5 <5 :Powder River Sussex 11.0 12.0 13.0 : 5 13 <5 11 <5 10 :North Platte River Casper 8.0 9.0 10.0 : <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: 03/16/2024 - 09/30/2024 Location 95% 90% 75% 50% 25% 10% 5% -------- ------ ------ ------ ------ ------ ------ ------ :Wind River Dubois 3.9 4.0 4.0 4.3 4.9 5.4 6.1 Red Creek 5.7 6.1 6.3 7.0 7.4 8.0 8.1 Crowheart 8.4 8.5 8.7 9.2 9.5 9.9 10.0 Kinnear 6.3 6.4 6.7 7.3 7.8 8.3 8.5 Riverton 5.7 6.2 6.7 7.8 8.7 9.6 10.0 :Little Wind River Riverton 4.4 5.0 5.7 6.5 7.7 8.9 10.1 :Bighorn River Basin 5.2 5.2 5.5 7.4 8.7 9.4 9.6 Greybull 84.5 84.8 85.5 88.3 89.7 91.0 91.4 :North Fork Shoshone River Wapiti 5.7 5.9 6.1 6.3 6.7 7.5 9.1 :South Fork Shoshone River Buffalo Bill Rese 6.7 6.8 7.1 7.7 8.4 8.6 9.0 :Shoshone River Lovell 8.3 8.3 8.3 9.2 9.6 9.9 10.0 :Yellowstone River Fishing Bridge 6.1 6.2 6.4 6.5 7.1 7.4 7.7 :Lamar River Tower Junction 5.5 5.6 6.0 6.6 7.3 8.1 9.0 :Powder River Sussex 3.1 3.2 3.5 4.5 6.6 8.2 11.0 :North Platte River Casper 3.4 3.4 3.4 3.4 3.5 3.9 4.8 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: 03/16/2024 - 09/30/2024 Location 95% 90% 75% 50% 25% 10% 5% -------- ------ ------ ------ ------ ------ ------ ------ :Wind River Dubois 2.3 2.3 2.3 2.3 2.3 2.3 2.3 Red Creek 2.1 2.1 2.1 2.1 2.1 2.1 2.1 Crowheart 5.0 5.0 5.0 5.0 5.0 5.0 4.9 Kinnear 3.2 3.2 3.1 3.1 3.1 3.1 3.1 Riverton 1.9 1.9 1.8 1.4 1.1 1.1 1.1 :Little Wind River Riverton 2.4 2.4 2.4 2.2 2.1 2.1 2.1 :Bighorn River Basin 4.0 3.9 3.8 3.6 3.1 3.0 3.0 Greybull 82.4 82.4 82.4 82.1 81.4 81.1 80.9 :North Fork Shoshone River Wapiti 2.8 2.8 2.8 2.8 2.8 2.8 2.8 :South Fork Shoshone River Buffalo Bill Rese 4.1 4.1 4.0 4.0 3.8 3.3 3.0 :Shoshone River Lovell 4.1 3.7 3.4 3.3 3.3 3.3 3.3 :Yellowstone River Fishing Bridge 2.7 2.7 2.7 2.7 2.7 2.7 2.7 :Lamar River Tower Junction 0.8 0.8 0.8 0.8 0.8 0.8 0.8 :Powder River Sussex 2.7 2.7 2.7 2.6 2.6 2.5 2.5 :North Platte River Casper 0.6 0.6 0.6 0.6 0.6 0.5 0.5 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 then National Weather Service's Advanced Hydrologic Prediction Service. Visit our web site weather.gov/riw for more weather and water information. This is the third scheduled spring flood and water resources outlook for 2024. $$ Hensley