FTB 08-06-2018: Severe Weather Returns to the Eastern Plains

Issue Date: Monday, August 6th, 2018
Issue Time: 09:15AM MDT

— A MODERATE flood threat has been issued for the Palmer Ridge and portions of the Front Range and Urban Corridor

— A LOW flood threat has been issued for the Front Range, Urban Corridor, Northeast Plains and Southeast Plains

Visible satellite shows some widespread, dense fog over far Northeast Plains to start off Monday morning. Dew points in this area are already in the 60Fs this morning and should remain throughout the day. In fact, dew points today will remain above 50F over the adjacent plains of eastern Colorado. A passing shortwave trough over WY/UT will move through the state this afternoon before the ridge begins to build back over the Pacific Northwest for the rest of the week. This should arrive to the eastern half of Colorado around the same time as peak heating, which will make shower and thunderstorm coverage more widespread along the Front Range and eastern plains.

 

Precipitable Water (PW) values at Grand Junction and Denver are 0.61 inches and 1.01 inches, respectfully. The passing of the cold front overnight returned low-level moisture east of the Continental Divide. Currently, the front is over the Southeast Plains with a surface low near the CO/KS border. This should promote moist, upslope flow this afternoon behind it. This afternoon, high instability and shear will promote some supercells and clusters of strong convection along and just east of I-25. As is common in moist and unstable environments, outflow boundaries from the first storms may help spark more convection. Thunderstorms are also expected form along the Cheyenne Ridge, which if they do, would bring strong storms to the northern counties of Colorado including Weld and Morgan County. With the exception of the Palmer Ridge/Front Range intersect, heavy rainfall should remain east of the higher terrains this afternoon, which is good news for the recent burn scars. Stronger storms will be possible over the Waldo Canyon area, so storms that track into this area will need to be monitored closely.

Threats with the thunderstorms today include heavy rainfall, large hail and strong winds. Storm motion will be to the ESE at 20-25 mph, so widespread flooding is not anticipated. However, multiple storms over one area may be possible, which would increase the 24-hour totals and runoff in the area. Street flooding, field ponding and local stream flooding are all possible. A Moderate flood threat has been issued for the Palmer Ridge, southern Urban Corridor and Southern Front Range. A Low flood threat has been issued for the Front Range, Urban Corridor, Northeast Plains and Southeast Plains.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

Flood Threat Legend

Zone-Specific Forecasts:

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

The heavy fog over the Northeast Plains should decrease by late morning. However, dew points are expected to remain in the 60Fs over the eastern plains with at least 50F dew points over the Urban Corridor. This will set the stage for some heavy rainfall this afternoon; however, storm motion should be fast enough to prevent widespread flooding. Max 1-hr rain rates up to 1.85 inches/hour are possible further east with 1 inch/hour possible over the Front Range. Localized 24-hour totals just over 2 inches are also possible east of I-25, but dry soils should be able to absorb a lot of moisture and also help prevent widespread flooding. Threats with the stronger thunderstorms today include severe hail, strong winds and local, heavy rainfall. Street and small stream flooding may be possible, especially over low-lying roads. A Moderate and Low flood threat has been issued.

Primetime: 2PM – 4AM

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

Skies should be mostly clear with a few scattered clouds over the higher terrains. Dry air and westerly flow should limit rainfall chances to the higher terrains and along the Continental Divide this afternoon. Rainfall totals are expected to be under 0.05 inches with plenty of virga. Temperatures and smoke will also be on the increase, with lower elevations nearing 100F.

Primetime: 2PM – 7PM

FTB 08-05-2018: Unsettled Weather Continues, a Couple Severe Storms Possible

Issue Date: 8/5/2018
Issue Time: 8:05 AM

NO FLOOD THREAT IS FORECAST TODAY.

A group of showers is moving across northern Colorado this morning in response to a weak, passing disturbance. This activity will dissipate/exit the state to the east by lunchtime, just in time for the next round of showers/thunderstorms to begin. This second round will be the main focus for today’s forecast, as it will hold the potential for the strongest storms and heaviest (relatively speaking) rainfall. This second disturbance will kick off storms around lunchtime over the High Country, while upslope flow promotes the additional development of a couple showers/ thunderstorms along/near the Front Range/Urban Corridor between Noon and 2 PM.

As showers/storms move quickly east of the mountains, they will encounter an environment supportive a couple severe thunderstorms, with the main threats being strong winds (up to 65 mph), hail (up to 1.25 inches in diameter), and brief heavy rainfall. Due to the relatively quick storm motions, the threat of flash flooding is not really a concern. Minor areas of street/field ponding can be expected under the strongest storms, but nothing that warrants the issuance of a flood threat area. Showers/storms will end from west to east as the disturbances passes through; information on timing, along with rain rates, can be found in 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, Northeast Plains, Southeast Plains, and Raton Ridge:

The area of showers ongoing over portions of the Urban Corridor, Northeast Plains, and Palmer Ridge will continue to move eastward, dissipating/exiting the state around Noon. Today’s main event will kick off around the same time as the next disturbance enters the area, aided by upslope flow. The first showers/thunderstorms will develop over the Urban Corridor, adjacent to the Front Range, between Noon and 2 PM, with storms moving relatively quickly eastward with time. A few of the storms will reach severe thresholds, capable of producing strong winds (up to 65 mph), hail (up to 1.25 inches in diameter), and brief heavy rainfall. Due to the quick storm motions, flash flooding will not be a threat, but minor instances of street/field ponding can be expected under the stronger storms. Maximum rain rates are as follows:

Urban Corridor and Raton Ridge: 0.25-0.5 inches/hour
Palmer Ridge: 0.6-1.0 inches/hour
Northeast Plains and Southeast Plains: 1.0-1.5 inches/hour

Timing: Urban Corridor: Noon – 9 PM. Palmer Ridge: Noon – 10 PM. Northeast Plains: 1 PM – 10 PM. Southeast Plains: 1 PM – 11 PM. Raton Ridge: 1 PM – 10 PM.

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

The area of showers ongoing across portions of the Northern Mountains, Front Range, and Central Mountains will exit the area to the east by 10 AM, which will turn our attention to the second round of showers/storms expected for this afternoon/evening. The next round will kick off around Noon, with showers/storms initially favoring the higher terrain. Showers/storms will track eastward with time, moving over mountains valleys along the way. Coverage and intensity of storms will peak during the afternoon hours, with storms dissipating around 6 PM as the disturbance moves east of the area. A few lingering showers/weak thunderstorms will continue until just after sunset, mainly east of the Continental Divide, ending by 9 PM-10 PM. Generally speaking, rain rates will be in the 0.05-0.15 inch/hour range, with the strongest storms producing rates up to 0.2-0.4 inches/hour.

Timing: Northwest Slope, Grand Valley, and Southwest Slope: Noon – 7 PM. Northern Mountains, Central Mountains, San Juan Mountains, and San Luis Valley: Noon – 8 PM. Front Range and Southeast Mountains: Noon – 10 PM.

FTB 08-04-2018: High-Based Showers and Thunderstorms Expected

Issue Date: 8/4/2018
Issue Time: 8:13 AM

NO FLOOD THREAT IS FORECAST TODAY.

Upper-level high pressure will build across the region today, leading to a drier and warmer day across Colorado. Meanwhile, a plume of mid-level moisture riding the top of the building ridge will stream across the state, providing enough fuel for isolated-to-scattered, high-based showers and thunderstorms. Showers/thunderstorms will kick off over the High Country around lunchtime, with storms moving quickly to the east, moving over the Urban Corridor and western portions of the plains/Palmer Ridge regions by 1-2 PM. Generally speaking, storm coverage will peak during the late afternoon/early evening hours coinciding with peak daytime heat, then diminishing as the sun sets. A few lingering showers will continue into the overnight/early morning hours over the High Country.

Rain rates will be tempered by drier air below cloud bases, causing storms to result in more gusty winds than rain. The bulk of shower/thunderstorm activity is expected over the higher terrain, where orographic effects will aid daytime heating in production of storms. East of the mountains, activity will be more isolated. For information 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, Northeast Plains, Southeast Plains, and Raton Ridge:

Isolated-to-widely scattered, high-based showers/thunderstorms expected to dot the area today, moving quickly from west to east. Dry air below cloud bases will cut back on precipitation efficiency, resulting in maximum rain rates of 0.2-0.4 inches/hour, with general rain rates less than 0.1 inches/hour. Gusty winds will be the main impact today, with gusts up to 25-45 mph from garden-variety showers/thunderstorms, with stronger thunderstorms producing gusts up to 45-65 mph.

Timing: 1 PM – 9 PM, with a few lingering showers/storms until, or just after, Midnight

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

Scattered high-based showers/thunderstorms are expected today, producing more gusty winds than rain. Wind gusts up to 65 mph can be expected from stronger storms, with wind gusts 20-40 mph from garden-variety activity. Dry air below cloud bases will reduce precipitation efficiency, with general rain rates less than 0.1 inches/hour. Maximum rain rates will be 0.2-0.4 inches/hour. Storms will move quickly from west to east, effectively eliminating the flood threat over even the most sensitive burn scars in coordination with the marginal rain rates.

Timing: 11 AM – 9 PM, with a few lingering showers over the higher terrain overnight and into the early morning hours

FTB 08-03-2018: Monsoon Moisture Returns; Along with Widespread Rainfall

Issue Date: Friday, August 3rd, 2018
Issue Time: 09:00AM MDT

— A MODERATE flood threat has been issued for the following burn scars: 416/Burro, Weston Pass, Spring Creek, Hayden Pass, Junkins

— A LOW flood threat has been issued for portions of each region with the exception of the San Luis Valley

Monsoon moisture is back, which will return the heavy rainfall threat for Friday. The west coast, upper-level trough began its migration eastward today, which shifted the ridge axis with it. This has allowed a plume of subtropical moisture to make its way into the state from west to east. The trough has also released some extra energy, and this vorticity max will help set the stage for more widespread and a bit stronger thunderstorms this afternoon. Currently, the main wave of energy is over western Colorado (marked with orange “X”) and is producing moderate rain and showers over the northwest corner of the state. Without much instability to work with, the lift from this wave is much stronger than the last couple of days where no early morning showers were present. This will reach the eastern portion of the state by early afternoon, just as the upslope flow regime sets up.

Precipitable Water (PW) values at Grand Junction and Denver are 1.16 inches and 0.92 inches, respectfully. This means rainfall intensities should be on the uptick this afternoon and evening. As the vorticity max moves eastward, this will help increase the PW values even more. Storms today over eastern Colorado will be widespread over the mountains and are expected to move into the adjacent plains by early afternoon. Storm coverage over western Colorado, especially the mountains, will increase throughout the morning as well. More than one round of storms are possible as upper-level lift and outflow boundaries will help trigger additional storms in the moist atmosphere. A stronger line of thunderstorms is expected over the far eastern plains, and with higher low-level moisture intensities should also increase. Small hail and strong winds are possible, though limited instability and shear may only produce one or two severe thunderstorms.

With high cloud cover already working its way into eastern Colorado, instability will be limited this afternoon, so not expecting severe thunderstorms over the mountains or immediate adjacent plains. Storm motion should be fairly quick as well, which will limit heavy, local rainfall. Storms may still be able to produce a quick 0.5 inches in 30 minutes though thanks to the abundant low-level moisture. However, without much rain the last couple of days, the soils should be able to absorb a lot of the precipitation and decrease runoff. A Low flood threat has been issued. Threats today include mud flow, debris slides (higher terrains) and street/small stream flash flooding. Burn scars will be especially susceptible today as max 1-hour rain rates will exceed 0.5 inches/hour and multiple storms may track over one area with the more widespread coverage. A Moderate Flood threat has been issued for the following burn scars: 416/Burro, Lake Christine, Hayden Pass, Junkins, Spring Creek and Weston Pass.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

Flood Threat Legend

Zone-Specific Forecasts:

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

Moderate and light rain showers to kick off today over the northwest corner of the state and San Juan Mountains. High cloud cover over this region and showers should keep high temperatures below normal for the beginning of August. Haze is still be reported in the Central Mountains. Showers and weak thunderstorms are expected throughout the day and a second round is expected this evening. Max 1-hour rain rates up to 0.75 inches/hour are possible with 24-hour totals up to 1 inch over the mountains. A Low flood threat has been issued with a Moderate flood threat for the Lake Christine and 416/Burro burn scars.

Primetime: 11AM – 12AM

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

High clouds to start the day should keep high temperatures relatively cool to the west. More instability will be able to gather over the eastern plains, so 1 or two storms may become severe this evening and produce local, heavy rainfall. Max 1-hour rain rates up to 0.85 inches/hour (west) and 1.3 inches/hour (east) are possible. 24-hour totals just over 1 inch are possible in isolated areas of the mountains. Localized 3-hour totals up to 1.75 inches are possible over the far eastern counties (Yuma, Kit Carson and Cheyenne). A Low flood threat has been issued with a Moderate flood threat for recent burn scars.

Primetime: 11AM – 2AM