May provided near normal conditions throughout the Great Salt Lake Basin. Stream flows increased to above average for the latter half of the month after a cool start in May. Maximum temperatures were 3-5 degrees F below normal and daily mins were mostly normal. Consequently runoff was highly variable and below average except during a 4 day period from May 18-22, when temperatures reached the low 90's and runoff peaked, before it snowed again. Monthly precipitation for the Great Salt Lake basin was 96% of average with seasonal precipitation at 93% of average. The snowpack is nearly gone except on due north slopes and areas above 9000 feet where pockets of late season snow still exist. SNOTEL stations from these sites report a basin total of 101% of average.

Short term weather guidance from National Weather Service models shows a low pressure system bringing rain and snow too the area starting on June 4, which will again cool temperatures to below normal and slow the runoff before it picks up again. This is a continuation of what we've been experiencing for the last 2 months. Longer range temperature forecast call for above average probabilities of below normal conditions through the 6 to 10 day period. Most streams in the area, with the exception of the Big and Little Cottonwood creeks and the American Fork river have already peaked, reducing the threat of flooding. Of course, a heavy rain even could again raise the threat during the next several weeks. Most streams will now begin a slow decline toward their summer base flows conditions after a brief rise during the next 2 weeks.

Volumetric forecasts for the Great Salt Lake Basin decreased slightly again this month and now range from 64% at Deer Creek Reservoir to 128% percent of average for Chalk Creek at Coalville. The average forecast from 32 forecast points was 92% of the 1971-2000 average.

Conditions across the Bear River Basin were normal for the month of May. Snowpack are nearly gone in the Bear but at a few location still show 107% of average. Snow is currently melting at a rate of 1 to 1.5 inches a day and will be completely gone within a week or two. Monthly precipitation was 107% of average while seasonal numbers rose slightly to 92% of average. Streamflows at Bear Utah-Wyoming State Line was 101% of average last month and only 65% of average for the Logan River Near Logan. Large swings of temperature were responsible for this disparity as the higher elevation regions were colder than average for the majority of the month. Forecasts in the Bear ranged from 47% at Stewart Dam near Montpelier (observed flow forecast) to 96% at the Utah-Wyoming State Line.

The Weber and Ogden River basins received below average precipitation for the month of May. Monthly precipitation was recorded at 76% of average and seasonal precipitation dropped to 94% of average. Snow water equivalent numbers are 110% of average, however 10 of the 16 stations now have no snow and may not really represent what we consider normal conditions. Streamflows were 98% of average for the Weber at Oakley and 93% of average for Pineview Reservoir. Most snow now lies above 8500 feet and is diminishing rapidly. The April through July volume forecasts range from 72% of average at Lost Creek Reservoir near Croydon to 143% of average at Wheeler Creek near Huntsville.

Monthly precipitation over the Six Creeks drainages was below average for May at 85% of average. Seasonal precipitation totals dropped to 98% of average as a result. Indexed snow numbers for Six Creeks remains much above average at 178%, however the areal extent is very small now and confined to the uppermost elevations, skewing the meaning of this standard measure. Steamflow in Big Cottonwood Creek near Salt Lake City was 75% of average and never really reach average flow numbers due to the below average maximum temperatures during the month of May and are likely to rise once temperatures return to normal, however we do not expect high peaks with the current extent of snow cover. April through July volume forecasts range from 78% percent of average at Parley's near Salt Lake City to 108% percent of average for Little Cottonwood Creek near Salt Lake City.

Monthly precipitation for Utah Lake Basin was 99% of average and seasonal precipitation totals 100% of average. Currently the indexed snow for the Utah Lake Basin is 146% of average. This months SWE values are much the same as last month but because of the delayed melt, the daily averages appear to be going up which is not the case. This is not really a representation of building snow but more of how cool our temperatures have been this spring at the upper elevations. Forecasts for Utah Lake ranged from 64% at Deer Creek Reservoir to 100% for the American Fork near the UP Power Plant, American Fork Canyon.

Differences between the full period forecasts and the residual forecasts may not exactly equal the actual observed volumes due to rounding conventions (see Definitions section).

10% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 10% chance of being exceeded. Previously referred to as "Reasonable Maximum Forecast".

50% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 50% chance of being exceeded. Previously referred to as "Most Probable Forecast".

90% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 90% chance of being exceeded. Previously referred to as "Reasonable Minimum Forecast".

Acre-Foot (af): The volume equal to one acre covered one foot deep (43,560 cubic feet). See kaf below.

Average: The arithmetic mean. The sum of the values divided by the number of values. Values from 1971-2000 are used for this publication.

Categories: Much Above Average=Greater than 130%, Above Average=111-130%, Near Average=90-110%, Below Average=70-89%, Much Below Average=Less than 70%.

CBRFC: Colorado Basin River Forecast Center.

Forecast Period: The period from April 1 through July 31, unless otherwise noted.

kaf: Thousand Acre-Feet. See Acre-Foot above.

Inflow: The volume of water that flows into a reservoir or lake.

Median: The middle value of an ordered set of values. Half of the values are higher and half of the values are lower. When the set contains an even number of values the median is the average of the two middle numbers.

NOAA: National Oceanic and Atomospheric Administration.

NWS: National Weather Service.

Water supply forecasts take into consideration present hydrometeorological conditions and use average basin temperatures and precipitation for the forecast period. As the forecast season progresses, a greater portion of the future hydrologic and climatic uncertainty becomes known and monthly forecasts become more accurate. For more information on the tools we use, consult Water Supply Forecasting Tools.

Volume forecasts represent adjusted flows; that is, observed flows with upstream water use taken into account. Adjusted flows will closely approximate natural or unimpaired flows. However, not all upstream diversions or impoundments are measured or quantifiable. For specific adjustments used with each forecast point, consult the Guide to Water Supply Forecasting.

The Water Supply Outlook is issued monthly January through May by the Colorado Basin River Forecast Center. It represents a coordinated effort between the National Weather Service, Natural Resources Conservation Service, Bureau of Reclamation, U.S. Geological Survey and local water district managers.

Note: Data used in this report are provisional and are subject to revision.

For more information, or to be included on the mailing list, please contact:

Colorado Basin River Forecast Center
2242 W North Temple
Salt Lake City, UT 84116
(801) 524-5130
www.cbrfc.noaa.gov