Issue Date: Saturday, August 20th, 2016
Issue Time: 9:10AM MDT

— Flooding is NOT expected today.

A strong disturbance and associated cool front responsible for yesterday’s active weather has largely cleared the state. Precipitable water values have fallen substantially from the 0.9 – 1.1 inch range yesterday afternoon into the 0.55 – 0.7 inch range by this morning. A 5,000 foot layer of very stable fall-like air has settled in for most areas east of the Divide (see morning sounding from Denver below), which will act to lower or even eliminate rain chances today. The one exception will be in the far southern parts of the state where the depth of the cool, stable air is very shallow. This will allow for solar heating to create enough instability for scattered thunderstorms this afternoon and evening. However, with only marginal instability and relatively fast storm motions today, heavy rainfall will be too isolated and brief to cause a flood threat. Thus, 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.

Issue Date: Friday, August 19th, 2016
Issue Time: 10:25AM MDT

— MODERATE flood threat for Palmer Ridge and Southeast Plains
— LOW flood threat for Northeast Plains, Urban Corridor, Front Range, Southeast Mountains, Raton Ridge and San Juans

An active weather map is seen across the western US this morning, as shown in the water vapor image below. There are two disturbances located north/west of Colorado today. The main feature will be the trough, and associated fall-like cool front (frost advisories have been posted for higher elevations of south-central WY), currently positioned over Montana that is racing south-southeastward and will enter Colorado during or slightly after peak heating. Morning soundings look tropical-like east of the Divide with precipitable water (PW) values hovering in the 0.8 – 0.9 inch range. A feed of moist air will be continuous over eastern Colorado today ushering in low-level dew points up to 60F, which will raise PWs into the 0.9 – 1.1 inch range. One to two rounds of storms will be likely east of the Divide. Maximum rainfall rates will be dictated by the amount of heating that can prep the atmosphere before rainfall starts. Currently the cloud deck is most expansive over far northern/eastern parts of the state. Thus, storms will likely build off the higher terrain first and then race southeastward, with a potential to spread eastward towards the Kansas border as the AM cloud deck dissipates. Despite the fast storm motion, strong convergence implies that short-term rainfall rates in the 5-30 minute range will be capable of producing a flash flood threat. A given location could see multiple rounds of storms, which will also increase the overall flood threat. A Low flood threat has been posted for parts of eastern Colorado with a Moderate threat for areas where instability is expected to be highest. Severe weather (hail up to 1.25 inches and winds up to 70mph) will be possible with the strongest storms.

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.

 

Issue Date: 8/18/2016
Issue Time: 10:12 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE NORTHERN MOUNTAINS, FRONT RANGE, CENTRAL MOUNTAINS, SOUTHWEST SLOPE, SAN JUAN MOUNTAINS, SOUTHEAST MOUNTAINS, SOUTHEAST PLAINS, URBAN CORRIDOR, PALMER RIDGE, AND NORTHEAST PLAINS.

The weak ridge that has been overtop the state will be pushed aside today as the upper-trough (yellow line) continues to deepen and begins its sweep to the east. Embedded within the mid-/upper-level flow is a weak wave (purple line) that will rotate across Colorado today, combining with daytime heating and orographic effects to kick off another period of unsettled weather. The High Country and Western Slope will be favored for the greatest coverage of showers/storms, while a few more are expected across the lower elevations of eastern Colorado as compared to yesterday. IPW values will tick upward again this afternoon, providing sufficient moisture for periods of moderate rainfall. Slow-moving storms are the main concern for any flooding issues, especially in the higher terrain and across burn scars.

A couple strong-to-severe storms will be possible across the northern fringes of the state, mainly the far northern extents of the Northeast Plains and Urban Corridor. A cool front will be knocking on Colorado’s door this evening, providing a focus for storm development across Wyoming. A couple of these storms may progress far enough southward to sneak into Colorado, with the main threats being large hail (up to 1.5 inches in diameter) and strong winds (gusts up to 50-60 mph). Otherwise, this cool front will continue to push into the state overnight, increasing surface moisture and upslope flow behind it, and keeping a few showers/weak thunderstorms going into the morning hours. The cool front will set the stage for heavy rain tomorrow.

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

Isolated-to-scattered showers/storms are expected, with the best coverage along/near the mountains, as well as far northern extents of the Northeast Plains and Urban Corridor (near the cool front). Most showers/storms will be garden variety, producing light-to-moderate rainfall, but a stronger storm or two will be capable of producing heavy rain, hail, and strong winds. Urban areas with poor drainage will be the main concern for flooding issues today. Maximum rain rates are as follows:

Urban Corridor and Palmer Ridge: 1.0-1.4 inches/hour
Northeast Plains: 1.5-2.2 inches/hour
Southeast Plains and Raton Ridge: 0.5-0.8 inches/hour

Timing: 1 PM – 11 PM, with a couple showers/weak storms continuing into the morning hours (mainly over the Northeast Plains and Urban Corridor)

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

Scattered showers/thunderstorms are expected, favoring the higher terrain, with more widely scattered coverage over the lower valleys. Slow-moving storms will be the main concern behind the low flood threat today, as well as increased moisture promoting more efficient rainfall. Maximum rain rates are as follows:

Front Range, Northern Mountains, Grand Valley, and Southeast Mountains: 0.9-1.3 inches/hour
Central Mountains, Southwest Slope, and San Juan Mountains: 0.8-1.2 inches/hour
San Luis Valley: 0.4-0.7 inches/hour
Northwest Slope: 0.5-0.8 inches/hour

Timing: 11 AM – 10 PM, with a few isolated showers continuing until around midnight

Issue Date: 8/17/2016
Issue Time: 10:03 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SAN JUAN MOUNTAINS, SOUTHWEST SLOPE, CENTRAL MOUNTAINS, SOUTHEAST MOUNTAINS, SOUTHEAST PLAINS, URBAN CORRIDOR, RATON RIDGE, AND FRONT RANGE.

Mid-level moisture continues its presence across the state today, fueling another period of isolated-to-scattered showers/thunderstorms. More so than yesterday, showers and storms will favor the High Country, with scattered coverage expected over the higher terrain, and isolated-to-widely scattered coverage over the valleys and adjacent lower elevations. A couple isolated storms will also be possible further east this evening, favoring areas near the Palmer Ridge, Cheyenne Ridge, and Raton Ridge.

Even though moisture values are fairly average for this time of year, a slight bump this afternoon will push IPW towards 0.8-0.9 inches at the 3 normal reporting stations (Schriever AFB, Boulder, and Grand Junction). This will provide enough moisture for a few bouts with locally moderate-to-heavy rain, mainly in the areas outlined by the low flood threat. Slow storm motions are the main concern driving this threat, as storm organization will not be great and moisture is so-so. The main threats in the low-end, low flood threat will be street/field ponding/minor flooding, as well as burn scar runoff issues. The Hayden Pass Fire burn is especially vulnerable to heavy rain, so that will need to be monitored closely. 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.

Zone-Specific Forecasts

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

Isolated-to-widely scattered showers/thunderstorms expected, mainly along/near the higher terrain. Most activity will be garden variety, resulting in gusty winds, lightning, and periods of light-to-moderate rainfall. One or two stronger storms are possible, mainly across southern extents of the Urban Corridor and western portions of the Southeast Plains. Minor street/field ponding/flooding will be possible underneath stronger storms, thus the inclusion in the low-end, low flood threat. Maximum rain rates are as follows:

Urban Corridor, Southeast Plains, and Raton Ridge: 0.8-1.2 inches/hour
Palmer Ridge and Northeast Plains: 0.5-0.7 inches/hour

Timing: Noon – 10 PM

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

Scattered showers/storms are expected today, favoring the higher terrain, with more isolated-to-widely scattered coverage across the valleys. The best low-level moisture exists across southern Colorado, which will lead to slightly more efficient rainfall. For this reason, plus the concern for slow storm motions, the issuance of the low-end, low flood threat is warranted. Maximum rain rates are as follows:

Northwest Slope, Northern Mountains, San Luis Valley, and Grand Valley: 0.4-0.7 inches/hour
Southwest Slope and Central Mountains: 0.8-1.2 inches/hour
San Juan Mountains: 1.0-1.4 inches/hour
Front Range and Southeast Mountains: 0.6-1.1 inches/hour

Timing: 11 AM – 11 PM, with a few isolated showers continuing into the early morning hours