SPM 08-26-2017: A Few Thunderstorms Rumbled

Issue Date: Saturday, August 26, 2017
Issue Time: 10:55 AM MDT

Summary:

The hot August sun battled the upper-level ridge to produce isolated showers and garden-variety thunderstorms yesterday. Most locations that received rainfall were treated to light accumulations of less than 0.1 inches. However, a convergence zone set up just to the east/southeast of the South Platte River in eastern Colorado, forcing a couple of strong thunderstorms to produce brief periods of moderate-to-heavy rainfall and pockets of hail. According to NWS local storm reports, 0.75 inch diameter hail was reported 9 miles ESE of Holyoke (Phillips County) and 1.0 inch diameter hail was reported 6 miles SE of Holyoke (Phillips County).

Flash flooding was not reported on Friday. For a look at precipitation estimates in your area, please see our State Precipitation Map below.

FTB 08-26-2017: Isolated Mountain & Far Eastern Plains Storms

Issue Date: 8/26/2017
Issue Time: 9:00 AM

NO FLOOD THREAT IS FORECAST.

The big weather story across the country is Harvey, currently a Category 1 hurricane that is expected to meander slowly across Texas. Here at home, however, a mainly dry day is on tap, with only a few isolated storms over the mountains and far eastern plains. In the water vapor imagery below, I have denoted the presence of the mid/upper high pressure centered over the Great Basin. This high pressure ridge will strengthen and expand over the region today, lowering our precipitation chances and keeping our temperatures near or just above average for this time of year. Enough moisture will be present over the high country for orographic effects to produce isolated showers/weak thunderstorms over the mountains during the afternoon and evening hours, with the preferred regions being the San Juan Mountains, Southwest Slope, Front Range, and Southeast Mountains.

Also denoted on the water vapor image is the presence of a mid-level disturbance, which will rotate around the periphery of the high pressure ridge, scraping by the far eastern plains during the afternoon and evening hours. Most of the thunderstorm activity associated with this disturbance will remain in Nebraska and Kansas, but a couple isolated thunderstorms cannot be ruled out for the Northeast and Southeast Plains, mainly east of a line from Sterling to Las Animas. For more details on rain rates and timing, please see the zone-specific forecast discussions below.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below (hover over threat areas for more details). For Zone-Specific forecasts, jump below the map.

Flood Threat Legend

Zone-Specific Forecasts

Urban Corridor, Palmer Ridge, Raton Ridge, Northeast Plains, and Southeast Plains:

Mainly dry and mostly sunny conditions will prevail for the majority of the area throughout the day today, with a slight chance for a mountain shower drifting overhead locations adjacent to the mountains and the risk for isolated strong-to-severe thunderstorms over the far eastern plains. The main threat from those thunderstorms will be strong winds and hail, with brief periods of moderate-to-heavy rainfall. Rain rates are as follows:

Urban Corridor, Palmer Ridge, and Raton Ridge: 0.05-0.15 inches/hour
Northeast Plains and Southeast Plains: 0.6-1.2 inches/hour

Timing: 1 PM – 9 PM

Northern Mountains, Northwest Slope, Front Range, Southeast Mountains, Central Mountains, San Luis Valley, San Juan Mountains, Grand Valley, and Southwest Slope:

Mainly dry and mostly sunny conditions will be the name of the game for most areas today. Increasing clouds near/over the higher terrain are expected this afternoon/evening as orographic effects work on residual moisture to develop isolated mountain showers/weak thunderstorms. Preferred regions for this activity will be the San Juan Mountains, Southwest Slope, Front Range, and Southeast Mountains. Rain rates will remain well below flash flood thresholds with maximum rates of 0.15-0.35 inches/hour.

Timing: 11 AM – 9 PM

SPM 08-25-2017: Isolated Heavy Rainfall Across Eastern Plains

Issue Date: Friday, August 25, 2017
Issue Time: 11AM MDT

Summary:

With elevated moisture remaining across eastern Colorado on Thursday, scattered showers and thunderstorms were once again seen mainly east of the Continental Divide. Closer to the mountains, storms produced light to moderate rainfall, up to 0.91 inches in Gilpin County and 0.84 inches in Fremont County. Farther east, a few cells were a bit more impressive. In Washington County, a late afternoon slow-moving storm produced over 1.5 inches per hour (from radar estimates) and prompted an Areal Flood Advisory. In addition, hail up to 1 inch was reported. Another smaller storm produced over 1 inch of rainfall in an hour or less in Lincoln County, also prompting an Areal Flood Advisory.

West of the Continental Divide, conditions were generally quieter though light to moderate rainfall, in the form of showers and weak storms, was reported in the climatologically favored southern parts of the San Juan mountains and surrounding valleys. Amount of 0.25 to 0.5 inch range were common in La Plata and Montezuma counties, with slightly higher amounts up to 0.75 inches in the higher elevations.

Flooding was not reported on Thursday. For rainfall estimates in your area, check out our State Precipitation Map below.

FTB 08-25-2017: A Few Isolated Showers And Weak Storms Amidst General Drying

Issue Date: Friday, August 25, 2017
Issue Time: 10:40AM MDT

— Flooding is NOT expected today

The water vapor image, below, shows a weak ridge is currently positioned near the Four Corners. This ridge is expected to modestly strengthen today and support large-scale subsidence to its east. Morning Precipitable Water (PW) values at the Denver and Grand Junction radiosondes measured 0.73 and 0.61 inches, respectively. With the exception of far eastern parts of the state, PW is expected to drop into the 0.5 – 0.65 inch range by this afternoon.

With clear skies this morning and a still strong late-August sun, expect adequate instability for isolated to widely scattered showers and thunderstorms mainly east of the Continental Divide. A weak surface low pressure was noted in southeast Wyoming this morning and with the diurnal lee side trough development, we expect a weak convergence zone to setup just south of the South Platte River. This will be the focal point for the highest rainfall chances (in addition to favored higher elevation locations east of the Continental Divide). However, as already noted, coverage should be lower than Thursday. Short 15-30 minute bursts of heavy rainfall will be possible with the strongest storms especially towards the KS border. However, flooding is not expected today.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below (hover over threat areas for more details). For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

Front Range, Urban Corridor, Palmer Ridge, Northeast Plains, Southeast Mountains, Raton Ridge, Southeast Plains:

Sunny this morning, then partly cloudy with isolated to widely scattered showers and weak thunderstorms possible. Highest coverage will be over the higher terrain as well as parts of the Northeast Plains. Max 1-hour rain rates up to 1.1 inches (east) and 0.5 inches (west). Flooding is not expected today.

Primetime: 12PM to 9PM

Northwest Slope, Grand Valley, Southwest Slope, Northern Mountains, Central Mountains, San Juan Mountains, San Luis Valley:

Sunny early with isolated showers and a weak thunderstorm possible this afternoon. Highest coverage will be over the San Juans. Max 1-hour rainfall up to 0.5 inches is possible. Flooding is not expected today.

Primetime: 12PM to 8PM