Summary: May 22, 2018
The atmosphere has been in a more active pattern over the last couple weeks for the east side of the Intermountain West, but remaining drier in the Upper and Lower Colorado River Basins. Several thunderstorm complexes blew up over northeast Colorado from Thursday through Saturday, and left a heavy mark with 2.00-4.00" of moisture falling over eastern Weld, Morgan, Logan, and northern Washington Counties. Totals from these thunderstorms helped to uphold strong May moisture numbers for eastern Wyoming and northeast Colorado. The southeast plains of Colorado fared better over the last week than many previous weeks, but moisture totals stayed largely in the 0.50-1.00" range. These kinds of totals in May can keep drought from worsening, but are insufficient for busting an existing drought. The Upper Colorado River Basin was by and large dry, but we are now in the dry season west of the Continental Divide.
The Polar Jet receded to higher latitudes earlier in the spring season than normal this year, leaving warmer than normal air across the Intermountain West. Consequentially, evaporative demand has been high across much of the IMW, and thus areas not receiving moisture are paying a higher price for it. Remotely sensed surface vegetation conditions are much drier than normal across the UCRB for this time of year, and showing primarily pre-to-moderate drought east of the divide.
Snowpack has been melting faster than average with above average temperatures across the Intermountain West through the month of May. The North Platte Basin shows this fast melt signal plainly. While the basin peaked at average snowmelt, it is already down to 50% of average. Southern Basins such as the San Juan, Rio Grande, and Arkansas peaked at much below normal snowpack, and are melting out more quickly than normal. This, combined with a forecast for above average temperature, is about the worst possible scenario going into fire season.
Streamflows have already peaked for the Gunnison River basin and everything south. Flows have remained below to more below normal throughout the peak season for the San Juan, Dolores, and Gunnison Rivers. This is leading to much below normal boosts to reservoirs supplies for the Navajo, McPhee, and Rio Grande Reservoirs.
This warmer weather is expected to persist over the next two weeks. The forecast continues to be wet for northern and central Wyoming with a decent chance at near average or average moisture for eastern Colorado and New Mexico over the next week. The 8-14 day outlook is less certain, but does show a tilt towards greater than average moisture into southeast Colorado. These precipitation forecasts are less certain than the forecast for above average temperatures.
UCRB degradations (Utah): It is recommended that D1 be advanced up to the crest of the Uintah Range in northeast Utah. This recommendation is not strongly based on what the area has experience in May. Rather, a large gradient exists in SNOTEL water year to date precipitation percentiles between stations on the north and south end of the crest (drier to the south). It is also recommended that a one-category degradation be administered directly to the south of that. Once again, based on SNOTEL percentiles, drought depiction is a little behind where it should be for northeast Utah.
UCRB degradations (Colorado): It is recommended that D0 be advanced up to Vail Pass on the east side of Eagle County. There was some transformative moisture in the area in the first week of April, but the area has relapsed into a dry pattern since.
It is recommended that D2 be added to southeast Chaffee County, western Fremont County, and southwest Park County. SNOTEL water year to date precipitation in the area is below the 10th percentile, and long-term SPIs in the Arkansas River valley appear to match.
Eastern CO improvements: It is recommended that all improvements made in draft one be upheld. This includes removal of D0 in eastern Weld, Morgan, and southern Washington Counties.
It is recommended that D1 be downgraded to D0 in northern Douglas County. The area is still abnormally dry on long timescales, but has benefitted from recent storms. Open rangelands look green and lush.
It is recommended that D2 be downgraded to D1 in central Elbert County. This area has received more than 2.00" of precipitation for the month of May to date, and was the site of Colorado’s first tornado of the year just last night.
Eastern CO degradations: It is recommended that D3 be added to western Kiowa County, southern Lincoln County, and Crowley County in eastern Colorado. This area missed out on the thunderstorms over the last seven days, receiving less than a quarter of an inch of new precipitation. Reports from FSA indicate that there is no visible difference in surface conditions from southern Otero County (currently D3) up to southern Lincoln County (currently D2). Rangelands in this area essentially look the same way they did in February, no green up to speak of.
Drought Designation: Now that 6-month SPIs and beyond are showing as dry in southeast Colorado it seems appropriate to label the entire IMW drought as 'SL.'
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