FTO 08-31-2017: Summer To Leave With A Few Storms And Lots Of Heat; Fall Cool Front To Bring Relief

Issue Date: Thursday, August 31, 2017
Issue Time: 12:20PM MDT
Valid Dates: 9/1-9/15

The afternoon’s water vapor imagery, below, shows a disturbance has finally penetrated the previously impressive upper-level ridge. This disturbance will cross Colorado on Friday, bringing widespread shower and storm activity statewide. This is identified as Event #1 in today’s 15-day Flood Outlook. However, the ridge will not cede its ground that easily, and will return with a vengeance over the weekend. High temperatures up to 15F above normal are likely statewide with a few lower elevation locations likely to top 100F. For reference, here are the latest dates that long-term Colorado weather stations have recorded a 95F reading:

  • Alamosa – July 5 (only happened once),
  • Colorado Springs – September 17,
  • Denver – September 19,
  • Pueblo – September 25 (though 94F has been observed as late as mid-October), and
  • Grand Junction – September 18.

The upper-level ridge re-establishment will be temporary as a strong fall cold front will race southward out of Canada starting Monday. The frontal passage, identified as Event #2, will cause in increase in shower and storm coverage. However, as shown in the forecasted Precipitable Water plumes, below, we do not foresee a heavy rainfall threat due to limited moisture and quick storm motion. Thereafter, a slow decrease in available moisture will limit heavy rainfall chances, as well as precipitation in general.

Below we describe each of the two identified precipitation events in more detail.

Event #1: Friday (9/1)

Low-end Elevated Flood Threat as shortwave moves across Colorado

A shortwave will continue moving across Colorado on Friday, temporarily increasing rainfall coverage and intensity. Afternoon and evening showers and thunderstorms are expected. Highest coverage will be in the foothills east of the Continental Divide, though some activity will make it farther east especially in the Southeast Plains. With the upper-level support, scattered thunderstorms are also expected across the eastern plains. A severe threat, mainly for large hail, will accompany the isolated heavy rainfall. Max 1-hour rainfall up to 1.1 inch (west) and 1.5 inch (east) can be expected. These rates are right at flood threat intensity and a low-end flood threat is expected mainly for isolated flash flooding. The most vulnerable areas are likely to be burn scars in the Southeast Mountains (Junkins, Beulah Hill, Hayden Pass).

Legend

Event #2: Monday (9/4) through Wednesday (9/6)

No Apparent Flood Threat as front passage likely, but return moisture limited

Guidance is in excellent agreement today regarding the passage of a strong fall-like cool front starting on Monday, 9/4. An increase in rainfall coverage is expected mainly east of the Continental Divide. However, moisture will be limited, and showers and the main precipitation type. Isolated thunderstorms remain a possibility and it is not impossible that a low-end flood threat will develop as the amount of moisture becomes more certain. The best chances of precipitation exceeding 0.5 inches will be in south-central Colorado.

Legend

SPM 08-31-2017: Scattered Storms Mainly Across High Country And Nearby Foothills

Issue Date: Thursday, August 31, 2017
Issue Time: 9:50AM MDT

Summary:

Afternoon and evening shower and storm activity was once again mainly limited to the High Country and nearby foothills where instability was greatest. With marginal moisture available for thunderstorms, rain rates were again limited, though a few storms produced short-term heavy rainfall. CoCoRaHS observers in Larimer, Fremont and Arapahoe counties noted just over 0.75 inches of rainfall, occurring in about an hour or less. Radar supplemented estimates suggest up to 1.5 inches over Morgan County, and up to 1 inch in Custer County. A cluster of storms moved across the Hayden Pass burn scar in the late afternoon, prompting a Flash Flood Warning. Western and eastern portions of the burn scar were estimated to have received 0.25 to 0.5 inches of rain. Fortunately, flooding was not reported.

A local storm report of gusty winds was received in Gunnison County, likely associated with a thunderstorm’s rain cooled downdraft.

The increased cloud cover caused a slightly downtick in afternoon high temperatures, though readings up to 10F above average were still observed west of the Continental Divide.

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

Click Here For Map Overview

The map below shows radar-estimated, rainfall gage-adjusted Quantitative Precipitation Estimates (QPE) across Colorado. The map is updated daily during the operational season (May 1 – Sep 30) by 11AM. The following six layers are currently available: 24-hour, 48-hour and 72-hour total precipitation, as well as maximum 1-hour, 2-hour and 6-hour precipitation over the past 24 hour period (to estimate where flash flooding may have occurred). The accumulation ending time is 6AM of the date shown in the bottom right corner. Also shown optionally are fire burn areas since 2012. The home button in the top left corner resets the map to the original zoom.

Note: We have identified a possible underestimation in QPE over the southwest part of the state. We are working to on this issue, and will provide an update as soon as possible.

FTB 08-31-2017: Disturbance To Trigger Widespread Showers/Storms, But Intensity Limited

Issue Date: Thursday, August 31, 2017
Issue Time: 9:35AM MDT

MODERATE flood threat for Junkins and Beulah Hill burn scars
LOW flood threat for Hayden Pass burn scar

Changes are on the way this morning for Colorado’s weather. As shown in the water vapor image, below, a strong shortwave disturbance has made it to the Great Basin as of this morning. The dynamics with this disturbance are forcing upward motion to its east – seen by the wide swath of mainly mid-level clouds across UT, WY and northern CO. Precipitable water values have increased slightly to 0.82 inches at Grand Junction (mostly mid-level moisture with surface dew point of only 36 F) and 0.65 inches at Denver. As the disturbance continues to move eastward, it will generate widespread afternoon and evening showers and thunderstorms. The highest coverage will be over the High Country where instability will be uncapped. Farther east of the Plains, limited instability will only allow for isolated to widely scattered showers and perhaps a weak thunderstorm.

Rainfall rates this afternoon are expected to stay below flood threat levels, with the only exception being across the burn scars of the Sangre de Cristo and Wet mountains. These fresh scars only require 0.5 inches per hour of rainfall to cause mud flows and flash flooding. This appears probable today, especially over the Wet Mountains. Thus, a Moderate flood threat has been posted for the Junkins and Beulah Hill scars, while a Low threat has been posted for the Hayden Pass scar.

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.

Flood Threat Legend

Zone-Specific Forecasts:

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

Partly cloudy this morning, then increasing cloudiness with numerous showers and thunderstorms possible. Highest coverage will be over the higher terrain east of the Continental Divide as well as the Cheyenne, Palmer and Raton ridges. Farther east, coverage will be isolated to scattered. Max 1-hour rain rates up to 0.9 inch closer to the foothills and 1.3 inches across the eastern plains. Max 24-hour rainfall up to 1.3 inches possible across the High Country. Gusty winds could accompany the strongest storms. A Moderate flood threat has been issued for the Junkins and Beulah Hill scars, while a Low threat has been issued for the Hayden Pass scar for mud flows and flash flooding.

Primetime: 1PM to 9PM

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

Partly to mostly cloudy with numerous showers and thunderstorms likely this afternoon. Highest coverage will be over the foothills and higher terrain. Max 1-hour rainfall up to 0.75 inches is possible, with max 24-hour rainfall up to 1.2 inch especially over higher elevation locations. Gusty winds will accompany the strongest cells. Flooding is not expected today.

Primetime: 12PM to 9PM

SPM 08-30-2017: Isolated Storms With Short-term Heavy Rainfall; Western Heat Continues

Issue Date: Wednesday, August 30, 2017
Issue Time: 11AM MDT

Summary:

With just enough moisture remaining in the boundary layer, combined with plenty of morning sunshine to heat things up, isolated to widely scattered thunderstorms roamed Colorado’s landscape once again on Tuesday. With mid-level subsidence as a backdrop, storms had a hard time growing in size and lasting much longer than 1 hour. However, lucky locations in the High Country and nearby foothills received a quick 0.25 – 0.5 inches in less than 1 hour. The highest storm coverage was likely over the Palmer Ridge (specifically El Paso County), Weld County as well as the San Juans. The highest CoCoRaHS observation was 0.41 inches in Montezuma County. However, radar-based estimates suggest up to 1 inch may have fallen in remote parts of Weld County. A brief, precautionary Areal Flood Advisory was issued for the Hayden Pass burn scar, however, flooding was not reported as of this morning. The eastern parts of the scar likely received up to 0.25 inches of rain, which could be enough to cause runoff over the sensitive ground depending on how fast the precipitation fell.

Across the Western Slope, heat continued in full force. Grand Junction tied a record high temperature for the date with a reading of 98F as boundary layer moisture mixed out and allowed the sun to efficiently heat the surface. Relative humidity values briefly dropped below 15% across the High Country and lower elevations west of the Continental Divide.

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

Click Here For Map Overview

The map below shows radar-estimated, rainfall gage-adjusted Quantitative Precipitation Estimates (QPE) across Colorado. The map is updated daily during the operational season (May 1 – Sep 30) by 11AM. The following six layers are currently available: 24-hour, 48-hour and 72-hour total precipitation, as well as maximum 1-hour, 2-hour and 6-hour precipitation over the past 24 hour period (to estimate where flash flooding may have occurred). The accumulation ending time is 6AM of the date shown in the bottom right corner. Also shown optionally are fire burn areas since 2012. The home button in the top left corner resets the map to the original zoom.

Note: We have identified a possible underestimation in QPE over the southwest part of the state. We are working to on this issue, and will provide an update as soon as possible.