FTB 08-26-2015: Monsoon Disturbance On the Way

Issue Date: 8/26/2015
Issue Time: 9:10 AM



An increase in shower/thunderstorm coverage today is expected over the mountains as a monsoon disturbance rotates across Colorado from the west/southwest. Ahead of this disturbance, moisture has increased fairly significantly over western Colorado, with IPW readings greater than 1 inch at Grand Junction. Widespread showers and thunderstorms are expected over the High Country and adjacent low valleys, with areas east of the mountains looking to remain mainly dry and hot today.

With the increase in moisture and support from the monsoonal disturbance, showers and thunderstorms will produce rainfall fairly efficiently today, resulting in locally moderate-to-heavy rainfall. Instability will not be great due to extensive cloud cover and storm motions will be decent; both of which will help to limit the flood threat somewhat.


Along the Urban Corridor, as well as western portions of the Northeast Plains, Southeast Plains, Raton Ridge, and Palmer Ridge, gusty outflow winds from mountain convection will be the main impact today, but a very low chance of isolated thunderstorms will exist during the afternoon and evening hours. Further east, mostly sunny skies and hot temperatures will be the name of the game.

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

Front Range, and Southeast Mountains:

Isolated-to-scattered showers and thunderstorms expected, though moisture will be slightly less than the deep moisture found over points to the west. Brief, localized, moderate rainfall is possible, with rain rates breaking down like this:

Front Range: 0.5-1.0 inches/hour
Southeast Mountains: 0.3-0.6 inches/hour

Timing: Noon – 10 PM, with a few showers/weak thunderstorms lingering into the early morning hours tomorrow

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

Aside from low chances of an isolated storm or two across the Urban Corridor, and western portions of the other regions, these zones will stay mostly sunny and hot today. The mixing out of moisture will keep rain rates low, with maximum rates of 0.25-0.6 inches/hour from any thunderstorms.

Timing: 2 PM – 8 PM, with a shower/weak thunderstorm or two during the early morning hours as the disturbance rotates across eastern Colorado.

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

Widespread showers and thunderstorms expected today and tonight, with locally heavy rainfall possible. Widespread cloud cover will keep temperatures much cooler than yesterday. Maximum rain rates will be 1.0-1.5 inches/hour, 1.8-2.2 inches/3 hours. The best chances of realizing these rates will be over the San Juan Mountains, Southwest Slope, Grand Valley, Northwest Slope, and Northern Mountains regions.

Timing: 11 AM – 11 PM will be the prime time, but scattered showers/thunderstorms will continue into tomorrow morning. For that reason, the flash flood threat areas will be valid through 11 AM tomorrow.

STP 08-26-2015: Uptick in Thunderstorm Coverage and Rainfall

Issue Date: Wednesday, August 26th, 2015
Issue Time: 9:00 AM MDT


The upper-ridge shifted ever so slightly to the east yesterday, allowing more moisture to return to Colorado. The increase in moisture helped to fuel an uptick in thunderstorm coverage, as well as more efficient rainfall production to said thunderstorms. Brief, locally heavy rainfall was observed under the strongest storms, but no flash flooding issues were reported. From CoCoRaHS observers, the following counties were the big winners with rainfall reports:

El Paso County: 0.47 inches
La Plata County: 0.32 inches
Gunnison County: 0.13 inches
Douglas County: 0.12 inches

Be sure and check the STP map below for a statewide overview on rainfall totals.

Storm Total Precip Legend

STP 08-25-2015: High Pressure Ridge Remained In Charge, Very Little Rainfall

Issue Date: Tuesday, August 25th, 2015
Issue Time: 9:00 AM MDT


Upper-level ridging remained over Colorado yesterday, leading to a day full of mostly sunny skies and warmth. Hazy conditions continued thanks to smoke from western US wildfires, delaying flights at Denver International Airport. On the precipitation side of the coin, the best moisture stayed trapped to the south and west of the state, unable to make it into Colorado with the strengthening high pressure. Mid-level warm temperatures also fought to keep the environment capped, and overall, yesterday did not produce much precipitation.

No flash flooding was reported yesterday.

Storm Total Precip Legend

FTB 08-25-2015: Shifting of the Ridge Brings Changes to the Forecast

Issue Date: 8/25/2015
Issue Time: 8:35 AM


Subtle changes to the environment compared to yesterday will bring slight changes to the forecast, but no flood threat will unfold. The mid-/upper-level ridge axis, depicted on the water vapor imagery below, will shift slightly eastward, allowing a bit more moisture to make its way into Colorado this afternoon and evening. The moisture will bring a slight uptick in thunderstorm activity over/near the mountains, including the Urban Corridor and Palmer Ridge regions. The Northeast and Southeast Plains will hold a low possibility for isolated thunderstorms, with chances diminishing from west to east.


Overall, most showers and thunderstorms will be of the garden variety. Scattered coverage is expected across the Central Mountains, Southwest Slope, San Juan Mountains, Grand Valley, and Southeast Mountains regions, with more isolated coverage over the Northern Mountains, Northwest Slope, and Front Range. The San Luis Valley should stay dry, with the surrounding mountains experiencing scattered showers/thunderstorms.

Further east, the Urban Corridor, western portions of the Southeast Plains, and Palmer Ridge should see a couple isolated thunderstorms move overhead, with the best relative chances over the Palmer Ridge. Outflow boundaries from mountain convection will try and force development of an isolated thunderstorm or two further east over the Plains regions, but will struggle to do so with mid-level temperatures being difficult to overcome.

Activity will tend to diminish after sunset, especially for low elevations, but a few mountain showers/thunderstorms will remain through the night as moisture continues to be transported into Colorado from the west/southwest.

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

Isolated thunderstorms expected this afternoon and evening, with the best chances near the mountains and over preferred terrain of the Palmer Ridge. Gusty outflow winds and lightning will be the main threats, but localized, brief, moderate-to-heavy rainfall will be possible. Otherwise, mostly sunny skies and warm temperatures will be experienced.

Timing: 2 PM – 11 PM

Front Range, Northern Mountains, and Northwest Slope:

Isolated showers/thunderstorms expected, with the best relative chances across the Front Range. Gusty outflow winds, light rain, and lightning will be the main threats, but localized, brief moderate rainfall will be possible across southern portions of the Front Range. Rain rates will general be 0.15-0.35 inches/hour, but a strong storm over the southern Front Range could drop 0.5-0.8 inches/hour.

Timing: Noon – 8 PM

Central Mountains, San Juan Mountains, Southwest Slope, Southeast Mountains, San Luis Valley, and Grand Valley:

Scattered showers and thunderstorms expected over/near the mountains. Lower valleys will stay mainly dry, with only a couple isolated storms drifting off the mountains and overhead. Activity will begin to increase shortly after 11 AM, and continue into the overnight hours across the mountains where moisture continues to increase from the west-southwest. Localized brief moderate-to-heavy rainfall will be possible with stronger thunderstorms across the Southwest Slope and San Juan Mountains, but storm motions will be quick/erratic enough to mitigate any potential flood threat. Near bank-full conditions may be found in smaller streams in those regions, but flash flooding is not expected. The flash flood threat will increase tomorrow as moisture continues to uptick and the additional support of a shortwave disturbance is expected.

Timing: 11 AM – 11 PM, with a few showers and thunderstorms continuing overnight and into tomorrow morning across the southwestern quadrant of Colorado.