Issue Date: 5/03/2015
Issue Time: 10:43 AM

THERE IS A LOW FLOOD THREAT FOR PORTIONS OF THE FRONT RANGE AND URBAN CORRIDOR, INCLUDING THE WALDO CANYON, HAYMAN-TELLER, FOURMILE CANYON, AND HIGH PARK BURN SCARS.

Moisture continues to be stuck underneath the flattened, upper-level ridge that has been over the region for a few days now. Precipitable water values are just above average for this date, and isolated-to-scattered showers and thunderstorms are expected again this afternoon/evening across the state, with a few lingering into the overnight hours. Just like yesterday, though, most of the moisture this afternoon and early evening will be confined to the mid-levels, meaning that most shower and thunderstorm activity will produce rain on the light-to-moderate end of the scale. The earliest round of thunderstorms that develop will remain high-based, providing more gusty winds and lightning than rain. West of the Divide, this is how most of the day will play out, as thunderstorms will follow the typical diurnal cycle, developing around noon and ending just after sunset; a few showers will linger overnight, as the mid-levels remain moist.

The bulk of this forecast will sound the same as the previous two days, except for a very important distinction – a weak cool front (shown in the image above) will drop south across eastern Colorado today. This cool front will provide a focal point for convection this afternoon, as well as provide upslope flow to bring a slight uptick in moisture later this afternoon and through the overnight. This will bring a greater risk for heavier rain to develop mainly along and east of I-25, in the 1.0-1.5 inch/hour range. There is potential for storm development over the Front Range and Urban Corridor after cool frontal passage as moisture increases into the mountains, leading to the low flood threat over portions of the Front Range and Urban Corridor, including the Waldo Canyon, Hayman-Teller, Fourmile Canyon, and High Park burn scars.

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.

Zone-Specific Forecasts

Urban Corridor, Front Range, and Palmer Ridge:

The bulk of thunderstorm activity will occur this afternoon ahead of, and in concert with, cool frontal passage. Rain rates will generally be in the 0.25-0.65 inches/hour range. After cool frontal passage, however, moisture will increase into the mountains as easterly, upslope flow develops. This is the reason for the low flood threat from this afternoon and into the overnight hours. Rain rates will be maximized in the 1-1.5 inches/hour range, which can cause flooding issues on burn scars and in urban areas.

Timing: Noon – 6 AM

Northeast Plains, and Southeast Plains:

Isolated-to-scattered showers and thunderstorms will develop/enter the region around 2 PM, and diminish nearly entirely by midnight. Main threats will be gusty winds and lightning, but moderate-to-heavy rain under the strongest storms will be maximized in the 0.8-1.3 inches/hour range. Small hail will be attendant to the strongest few storms.

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

Isolated-to-scattered showers and thunderstorms will follow the typical diurnal cycle today, beginning just before noon, and diminishing after sunset. Most storms will remain over the higher terrain, but a few will drift over adjacent valleys. Maximum rain rates will be in the 0.5-0.7 inches/hour range.

A few showers and thunderstorms will continue overnight over the higher terrain, as the next round of mid-level disturbances rounds into Colorado from the southwest.

Timing: 11 AM – 6 AM.

Issue Date: 5/02/2015
Issue Time: 10:15 AM

NO FLOOD THREAT IS FORECAST.

Moisture is about 60% of what it was 24 hours ago (see IPW image below), and this will lead to a day filled with more sunshine and less coverage of showers and thunderstorms. Three things will provide the “ignition” for isolated showers and thunderstorms this afternoon/evening; daytime heat, a weak mid-level disturbance, and a surface trough over the Northeast and Southeast Plains.

The daytime heat and weak mid-level disturbance will be present as trigger mechanisms statewide, while the surface trough across the Northeast and Southeast Plains will provide additional support for a few stronger thunderstorms to develop. In general, today’s showers and thunderstorms will be of the weaker variety, producing mainly gusty winds and light rain, as most of the moisture lives above 600 mb (3 km above ground level). Where slightly more moisture can hang on, near the CO/KS border, small hail and brief moderate-to-heavy rain will be possible.

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.

Zone-Specific Forecasts

Urban Corridor, Front Range, and Palmer Ridge:

Today will feature more sun than yesterday, with storms being more isolated than yesterday. Expect much more gusty winds than rain from any storm, with drier air in place in the low-levels. Maximum rain rates will be in the 0.25-0.6 inch/hour range. Brief, small hail will be possible over the Urban Corridor and Palmer Ridge, as instability increases to respectable levels through the afternoon due to daytime heating.

Timing: Noon – 8 PM

Northeast Plains and Southeast Plains:

The same trend will follow these regions as the regions noted above, with the noted exception that a surface trough will provide an additional focus for isolated-to-scattered thunderstorms to develop. A bit more moisture will be available to storms in these regions, so rain rates will be maximized around 1-1.5 inches/hour. Small hail will be present with the stronger storms, perhaps reaching severe criteria at 1 inch in diameter. Gusty winds (40-55 mph) and lightning will be the main threats with any strong thunderstorms.

Timing: 3 PM – 10 PM

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

Mostly sunny, with isolated showers and thunderstorms. Activity will mainly be confined to the higher terrain, as weak steering winds won’t be able to “push” storms over the valleys. Rain rates will be low, in the 0.2-0.6 inch/hour range, due to relatively dry air in the lower levels. Gusty winds and lightning will be the main threats with any storm activity.

Timing: Noon-10 PM, with a few weak showers lingering until midnight.

Issue Date: 5/1/2015
Issue Time: 10:20 AM

NO FLOOD THREAT IS FORECAST.

Welcome back to the 2015 edition of the Colorado Flood Threat Bulletin. Mother Nature decided not to allow us a few dry days to get back into the swing of things, and instead has provided Colorado with a bit of moisture to provide some early-season action. Precipitable water (IPW), shown in the image below, tells a story of increasing moisture during, and after, the cool frontal passage yesterday evening. This left-behind moisture will provide the fuel for today’s showers and thunderstorms, as sunshine and a weak mid-level disturbance provide an ignition mechanism.

A closer look at other observations reveals that most of the moisture west of the Continental Divide is confined to the mid-levels, which means that most showers and thunderstorms on that side of the state will struggle to produce moderate rainfall, resulting in mainly gusty winds and light rain at the surface. East of the Continental Divide, however, moisture is spread between the low- and mid-levels of the atmosphere, and will allow for a better chance at stronger storms and moderate-to-heavy rainfall.

The forcing mechanism for today’s showers and thunderstorms will be a combination of daytime heating and a 700-mb shortwave. These will combine to kick off isolated showers and thunderstorms over the higher terrain between 10 am and Noon. As the shortwave progresses east, and daytime heating take place, coverage and intensity will increase as storms develop over the Front Range between Noon and 2 PM. Storm motions today will take storms in a southeasterly direction at about 15-20 mph, across the plains through the afternoon and evening hours. The strongest storms will impact the Front Range, Urban Corridor, Palmer Ridge, and Northeast/Southeast Plains region (forecast zones). Stronger storms will hold the risk for 1-inch diameter hail.

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.

Zone-Specific Forecasts

Urban Corridor and Front Range:

Morning clouds will continue to thin out through this morning, resulting in mostly sunny skies by lunchtime. This will allow sunshine to heat the surface, resulting in ample instability to produce scattered thunderstorms; a few will become marginally severe with the threat of 1-inch hail and 50-55 mph wind gusts. Storm motions and insufficient moisture will preclude the issuance of any flash flood threat, but street ponding will occur underneath the stronger thunderstorms. Maximum rain rates will be in the 1.0-1.75 inch/hour range.

Timing: Scattered showers and thunderstorms will develop over the Front Range between Noon and 2 PM, moving east-southeastward with time, with storms beginning over the Urban Corridor between 2 PM and 3 PM. Activity will come to an end between 8 PM and 9 PM as the sun sets.

Palmer Ridge, Northeast Plains, and Southeast Plains:

Mostly sunny skies early, with scattered showers and thunderstorms moving across the regions in a southeasterly direction beginning around 3 PM. Storms will end, either by diminishing or exiting the state, by midnight. Gusty winds and small hail will be threats with the stronger storms, with moderate-to-heavy rainfall attending, as well. Maximum rain rates will be in the 1-2”/hour range. Some street and field ponding will occur under the strongest thunderstorms. Highest likelihood of storms will be across the Palmer Ridge and Northeast Plains, with probabilities diminishing from north to south across the Southeast Plains.

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

Mostly sunny early, becoming partly sunny this afternoon and evening as isolated, brief, showers and weak thunderstorms develop mainly over the higher terrain (11 AM-9 PM). With most of the moisture combined to the mid-levels, the result will be mainly localized gusty winds (25-35 mph) and light rain. Maximum rain rates will be around 0.25 inch/hour.

Pardon our dust while we make some final changes to the new Flood Threat Bulletin website! If you have any questions, please contact Stu Geiger at sgeiger at dewberry dot com.