####018012334#### FGUS75 KCYS 142220 ESFCYS NEC007-013-033-045-105-123-157-165-WYC001-007-009-015-021-027-031- 141900- Probabilistic Hydrologic Outlook National Weather Service Cheyenne WY 420PM MST Thu Mar 14 2024 ...Spring Flood and Water Resources Outlook Number 3... ...Below-Average Potential for Ice Jam Flooding This Spring... ...Below-Average Potential for Spring Snow melt Runoff Flooding... This Spring Flood and Water Resources Outlook is the third of three routine issuances for the Cheyenne Hydrologic Service Area which comprises seven counties in southeast Wyoming and eight counties in the Nebraska panhandle. This outlook includes the North Platte River main stem and its tributaries (the Laramie, Encampment, and Medicine Bow Rivers), the Niobrara, White and Little Snake Rivers. .Flood Outlook Summary... There is a below-average potential for flooding early this spring due to low-elevation snow melt, ice break up, and ice jams. These normally occur between February and April. The winter has brought extended periods of above-normal temperatures along with limited low-elevation snow pack. This limits the chances for flooding due to low-elevation snow melt or ice jams during periods of ice break-up on area lakes and rivers. The potential for flooding due to snow melt runoff in southeast Wyoming and the Nebraska Panhandle this spring is near-normal. At this time, the snow pack in the southern Laramie Range is only 52 to 62 percent of the long-term median. This will limit the potential for snow melt flooding in the South Platte river basin. In the Laramie and North Platte River basins the snow packs are at 101 and 109 percent of the median. In the Little Snake basin the snow pack is at 115 percent of the median. These numbers have improved over the last four weeks. Near-normal snow packs in southeastern Wyoming generally translate into a low probability of flooding. However, a low probability of flooding is relatively normal for this region. There remains considerable opportunity for further accumulation of snow pack before the expected high-elevation snow melt in May and June. A normal or even low probability of flooding does not mean there is no chance of flooding. Even a lighter snow pack can generate flooding if there is a period of extremely warm temperatures and/or significant rainfall before the ground thaws. Once the ground has thawed, the flood potential for the rest of the spring and early summer will remain below normal. While the snow pack is currently near normal. The potential for flooding, early or late in the season, will depend on future snowfall, temperature patterns, and spring rainfall. All of these factors affect the total amount of water stored in the snow pack and the speed with which it is released to the rivers. .Observed and Forecast Temperatures and Precipitation... Weather patterns since November have been extremely warm. In the last 60 days they have also been extremely wet. This sets off a predominantly dry period earlier this winter. .Snow Cover and Liquid Water Content... Mountain snow pack across southeast Wyoming is below normal to normal across most of southeast Wyoming. In southeastern Wyoming Basin snow water equivalents, measured at the Natural Resource Conservation Service (NRCS) SNOTEL stations, ranged from 52 percent of median in the South Platte River basin in southeast Wyoming to 115 percent of median in the Little Snake River basin. There remains almost a month until the median date of peak snow pack in most basins across the area. March and April can have a disproportionate affect on the final snow pack accumulation for the season so water supply forecasts can still change. Warm, windy temperatures or late winter and early spring storms can cause dramatic, late-season, changes in total snow pack. Snow cover across the Nebraska Panhandle counties is extremely light and inconsistent. Snow pack there contains relatively small amounts of water and should not be a major factor in Spring flooding. .Soil Conditions and Drought... Soil moisture in the area, as portrayed by the CPC, varies from near median to above median with a small exception of below median soil moisture in south-central Wyoming. Normal or above-normal conditions are prevalent across southeastern WY and the Nebraska Panhandle. Moderate severe drought conditions continue to linger in Carbon and Albany counties but have improved considerably in the last month. Dry conditions have expanded southward into Converse and Niobrara Counties but remain only abnormally dry and have not yet developed into drought. No areas in Nebraska are shown to be affected by abnormally dry or drought conditions. .Lake and River Conditions... It has been an exceptionally warm winter with only short periods of extremely cold weather. Despite this, most of the higher elevations lakes are iced over, although the ice thickness might be less than in most years. Lower elevations rivers and lakes have less ice than normal with large areas of open water. .Climate Outlooks... Outlooks by the Climate Prediction Center (CPC) for the period from 6 to 14 days indicate conditions that favor below-normal temperatures and above-normal precipitation. The latest one and 3-month outlooks for March through May indicate a moderate to strong wet signal early in the period and then a return to near-equal chances of above, below, or near normal precipitation. There is only a weak signal for above-normal temperatures during the 3-month period. 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: 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 -------- ----- ----- ----- : --- --- --- --- --- --- :Encampment River Encampment 6.5 7.5 8.5 : <5 <5 <5 <5 <5 <5 :North Platte River Saratoga 8.5 9.5 10.5 : 33 40 11 17 <5 <5 Sinclair 9.0 10.0 11.0 : 27 31 11 17 5 6 :Medicine Box River Hanna 6.0 7.0 8.0 : <5 <5 <5 <5 <5 <5 :North Platte River Glenrock 7.0 8.0 9.0 : <5 <5 <5 <5 <5 <5 Orin 8.0 9.0 10.0 : <5 <5 <5 <5 <5 <5 :Laramie River Woods Landing 6.0 6.5 7.0 : <5 5 <5 <5 <5 <5 Laramie 9.0 10.0 11.0 : 13 21 <5 6 <5 <5 Bosler 7.5 8.5 9.5 : <5 8 <5 <5 <5 <5 Fort Laramie 7.0 8.0 9.0 : <5 7 <5 <5 <5 <5 :North Platte River Henry 5.5 6.5 7.5 : <5 <5 <5 <5 <5 <5 Mitchell 7.5 8.5 9.5 : 6 14 <5 9 <5 5 Minatare 7.5 8.5 9.5 : <5 6 <5 <5 <5 <5 Bridgeport 9.5 10.0 10.5 : <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% -------- ------ ------ ------ ------ ------ ------ ------ :Encampment River Encampment 4.2 4.6 4.8 4.9 5.3 5.5 6.1 :North Platte River Saratoga 6.3 6.7 7.5 8.2 8.9 9.6 10.2 Sinclair 5.6 6.0 7.0 8.0 9.1 10.2 11.2 :Medicine Box River Hanna 3.2 3.2 3.7 4.2 4.4 5.0 5.2 :North Platte River Glenrock 2.9 2.9 3.0 3.1 3.3 3.5 4.5 Orin 4.9 4.9 5.0 5.1 5.2 5.6 5.9 :Laramie River Woods Landing 2.0 2.2 2.6 3.2 3.5 4.2 4.8 Laramie 4.7 5.1 6.8 7.9 8.3 9.2 9.5 Bosler 1.3 1.6 2.3 4.4 5.7 6.5 6.9 Fort Laramie 3.0 3.2 3.4 3.5 4.3 5.7 6.4 :North Platte River Henry 3.6 3.6 3.7 3.8 4.0 4.1 4.5 Mitchell 2.8 2.9 3.0 3.5 4.3 5.9 7.7 Minatare 3.5 3.6 3.9 4.4 4.8 5.6 6.8 Bridgeport 4.9 5.0 5.2 5.5 5.9 6.4 7.2 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% -------- ------ ------ ------ ------ ------ ------ ------ :Encampment River Encampment 1.7 1.7 1.7 1.7 1.6 1.5 1.4 :North Platte River Saratoga 3.6 3.5 3.5 3.5 3.5 3.5 3.4 Sinclair 3.1 3.1 3.1 3.1 3.1 3.1 2.9 :Medicine Box River Hanna 2.1 2.0 2.0 1.9 1.9 1.8 1.7 :North Platte River Glenrock 0.9 0.9 0.9 0.9 0.9 0.9 0.8 Orin 3.5 3.5 3.5 3.5 3.5 3.5 3.5 :Laramie River Woods Landing 1.2 1.2 1.2 1.1 0.3 0.3 0.3 Laramie 3.5 3.5 3.5 3.4 3.4 3.3 3.3 Bosler 1.0 0.9 0.8 0.7 0.7 0.7 0.7 Fort Laramie 2.3 2.3 2.3 2.3 2.3 2.3 2.3 :North Platte River Henry 2.1 2.1 2.0 2.0 2.0 2.0 2.0 Mitchell 0.9 0.6 0.5 0.4 0.3 0.3 0.3 Minatare 1.8 1.7 1.6 1.5 1.4 1.4 1.4 Bridgeport 3.8 3.8 3.7 3.7 3.6 3.6 3.6 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/cys for more weather and water information. This is the final winter Spring Flood Outlook of the season. Other outlooks may be issued this spring as needed. $$ AJA