Issue Date: 7/5/2018
Issue Time: 8:15 AM

A HIGH FLOOD THREAT IS FORECAST TODAY FOR PORTIONS OF THE FRONT RANGE AND SOUTHEAST MOUNTAINS, NAMELY FOR THE SPRING FIRE, CHATEAU FIRE, AND ADOBE FIRE BURN AREAS.
A MODERATE FLOOD THREAT IS FORECAST TODAY FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, SOUTHEAST MOUNTAINS, RATON RIDGE, AND SOUTHEAST PLAINS.
A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SOUTHEAST PLAINS, RATON RIDGE, SOUTHEAST MOUNTAINS, URBAN CORRIDOR, PALMER RIDGE, NORTHEAST PLAINS, FRONT RANGE, CENTRAL MOUNTAINS, AND NORTHERN MOUNTAINS.
A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SOUTHWEST SLOPE AND SAN JUAN MOUNTAINS, NAMELY THE 416 FIRE AND BURRO FIRE BURN AREAS, AS WELL AS THE WEST FORK COMPLEX BURN AREAS.

The broad upper-level ridge over the central United States will continue to retrograde west through the period, becoming centered over Colorado during the afternoon/evening. This will place Colorado under weak flow aloft, which will set the stage for slow-moving showers/thunderstorms. At the same time, deeper moisture is setting up along/east of the Continental Divide due to the combination of mid-level moisture from the east/southeast and easterly flow from the Great Plains. In combination, these two factors will allow for showers/thunderstorms with efficient rainfall production to have increased residence time over a location, enhancing the flooding threat.

Overall, the main factor behind this period’s high/moderate flood threat is the number of recent burn scars potentially being impacted by slow-moving showers and thunderstorms today/tonight. For new burn scars, it will likely take no more than 0.25-0.4 inches/hour rain rates to cause flash flooding, debris slides, and mud flows, and for older burn scars such as the Hayden Pass and Junkins, it will only take slightly higher rain rates. In addition to the area burn scars and steep terrain, the Colorado Springs area has been included in the Moderate Flood Threat due to the heavy rain they experienced last night. Any excessive rainfall today will have enhanced runoff due to the saturated soils in the area.

The reason the 416 Fire and Burro Fire are NOT included in the moderate flood threat, but rather are under a low flood threat, is due to a relative lack of good moisture in the low-levels over the area. Evaporation as precipitation falls will play a role in reducing rain rates over the area, but it only takes one slow-moving storm, or a second storm over an area that became saturated due to another storm’s outflow to mitigate this effect. Thus, they deserve a watchful eye today.

For more information on timing and rain rates, 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

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

Scattered-to-widespread showers/thunderstorms expected today/tonight, with the bulk of the activity occurring over the Front Range, Urban Corridor, Palmer Ridge, Southeast Mountains, Raton Ridge, and western portions of the Northeast Plains and Southeast Plains. As the evening turns to nighttime, the main area of precipitation will shift eastward towards the CO/KS/NE border. Maximum rain rates are as follows:

Front Range: 0.8-1.2 inches/hour
Urban Corridor and Palmer Ridge: 1.2-1.8 inches/hour
Northeast Plains and Southeast Plains: 1.5-2.0 inches/hour
Raton Ridge: 1.2-1.8 inches/hour
Southeast Mountains: 0.9-1.3 inches/hour

Timing: 11 AM – 11 PM over the Front Range and Southeast Mountains, 2 PM – Midnight for the Urban Corridor, and 2 PM – 1 AM for the Southeast Plains, Northeast Plains, and Raton Ridge (with a couple lingering showers/thunderstorms into the early morning hours)

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

Scattered showers/thunderstorms are on tap today/tonight across the area, with the bulk of the activity favoring the higher terrain of the Southwest Slope, San Juan Mountains, Central Mountains, and Northern Mountains. More isolated activity is expected over the higher terrain elsewhere.

Storms will drift over adjacent lower elevations with time. Most showers/thunderstorms will be of the garden-variety, producing light rain and gusty winds, but a stronger storm or two will be capable of producing brief moderate rainfall, especially over the Southwest Slope, San Juan Mountains, Northern Mountains, and Central Mountains, where moisture and support will be best. Maximum rain rates are as follows:

Northern Mountains and Central Mountains: 0.7-1.1 inches/hour
Northwest Slope and Grand Valley: 0.2-0.4 inches/hour
Southwest Slope, San Juan Mountains, and San Luis Valley: 0.5-0.8 inches/hour

Timing: 11 AM – 9 PM, with isolated lingering showers/thunderstorms into the early morning hours south of I-70.

Issue Date: 7/4/2018
Issue Time: 7:55 AM

NO FLOOD THREAT IS FORECAST TODAY.

Colorado finds itself on the western periphery of an upper-level ridge, with weak, relatively dry, southwesterly flow overlaying the state. At the surface, much of the state remains dry, especially west of the Continental Divide, but a cool front working into eastern Colorado from the north will reinforce moisture in that part of the state. It is those areas along/east of the Front Range and north of Highway 50 that will experience the bulk of thunderstorm activity today/tonight, as scattered thunderstorms will work off the moisture, daytime heating, upslope flow, and surface convergence provided by the cool front and any subsequent outflow boundary collisions.

Over the High Country, cumulus clouds will develop during the afternoon/evening hours over the higher elevations thanks to daytime heating and orographic lift. A couple isolated high-based showers/weak thunderstorms will dot the sky, with the main result being plenty of virga and sporadic gusty winds, with the best chance at wetting rainfall over eastern portions of the Central Mountains.

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

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

For areas south of Highway 50, including the Raton Ridge, an isolated thunderstorm or two will be possible east of the dryline, which includes the counties of Bent, Prowers, Baca, and eastern Las Animas. Periods of brief moderate-to-heavy rain, small hail, and gusty winds will be the main threats.

Areas to the north of Highway 50 will be the locations that experience the bulk of thunderstorm activity in the state today, as the cool front reinforces some low-level moisture and provides surface convergence and a bit of weak upslope flow. Generally speaking, the main threat from thunderstorms today will be hail and strong winds. A relative lack of good wind shear and deep moisture will prevent storms from producing the heavy rainfall necessary for a flood threat, but one or two storms will be capable of producing brief periods of moderate-to-heavy rainfall. A bit of street/field ponding in poorly drained areas, or those areas where hail accumulation clogs drainage, is possible, but that will be about it.

Maximum rain rates are as follows:

Front Range: 0.3-0.6 inches/hour
Urban Corridor: 0.4-0.8 inches/hour
Palmer Ridge: 0.5-1.0 inches/hour
Northeast Plains and Southeast Plains: 0.8-1.25 inches/hour
Raton Ridge: 0.3-0.6 inches/hour

Timing: 2 PM – 10 PM, with a few storms lingering until midnight (or just after) across the eastern plains

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

Mostly sunny skies and warm temperatures will mark Independence Day across the area, with a few isolated high-based showers/weak thunderstorms dotting the sky during the afternoon/evening hours. Any storm activity will remain confined to the higher elevations, mainly over the Southeast Slope, San Juan Mountains, Central Mountains, Southeast Mountains, and Northern Mountains. Streaks of virga and gusty winds will be the main result, with very little rainfall reaching the surface. The best chance at wetting rainfall will be over eastern portions of the Central Mountains, where rain rates will be 0.05-0.15 inches/hour. Rain rates will remain below 0.05 inches/hour elsewhere.

Timing: Noon – 9 PM

Issue Date: Tuesday, July 3, 2018
Issue Time: 09:10AM MDT

— Flooding is NOT expected today

Visible satellite imagery below shows a couple upper atmospheric disturbances marked in orange. The first one over Utah is associated with the trough positioned over the Pacific Northwest. This local area of lift should stay north and west of the state due to the 500mb ridge building west. The second disturbance over southwest Colorado will help bring increased cloud cover and maybe some light rain to the San Juan Mountains early this afternoon. However, soundings in the area shows inverted-V profiles, so the more likely scenario is breezy conditions and only trace amounts of rainfall. This small increase in moisture will keep the Red Flag Warning further north where the tightening gradient of the Pacific Northwest trough will mix down some windy conditions at the surface at the lower elevations.

The diurnal flow pattern will initiate thunderstorms over the higher terrains and Palmer Ridge by this afternoon. Low-level atmospheric moisture is still limited, so not expecting much rain with these storms. There is decent mid-level moisture in Denver’s sounding this morning, so forecasting some cumulus that should provide some decent shade to the heat this afternoon. As the showers move to the northeast/east, they will encounter some better moisture (for more rainfall) over the northeast corner of the state where 50F dew points are able to hold on. Dew points will be much lower over the Southeast Plains as the low-level moisture gets mixed out to the east, so not expecting as much rainfall here. Light showers and weak thunderstorms may initiate after sundown as moisture returns westward to the eastern plains and multiple outflow boundaries provide lift. Flooding is not forecast today.

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.

Zone-Specific Forecasts:

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

High temperatures should be close to yesterday with more breezy conditions over the Southeast Mountains and adjacent plains. Without too much low-level moisture to the west, expecting more virga and cloud cover rather than rainfall. Max 1-hour rain rates should be under 0.1 inches. Over the Northeast Plains, higher dew points will help produce rain rates up to 0.9 inches/hour, while the Southeast Plains should only see rain rates up to 0.4 inches/hour. The HRRR is showing lots of smoke over these regions, so hazy conditions will continue with all the ongoing fires in the state. Flooding is not expected today.

Primetime: 1PM to 1AM

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

Increased cloud cover for the southwest corner of the state thanks to a disturbance traveling north from NM/AZ. This increase in moisture is enough to lift the Red Flag Warning over the higher terrains today west of the Divide. Not expecting too much rainfall, but with this disturbance, localized totals up to 0.15 inches over the San Juan Mountains are possible. However, inverted-V atmospheric profiles will more so produce gusty winds with these storms than measurable rainfall. Over the northwest corner of the state at the lower elevations, increased surface winds and dry, warm air will continue to keep this area under a Red Flag Warning. Flooding is not expected today, but smoky conditions will continue over southwest Colorado.

Primetime: 11:30AM to 9PM

Issue Date: Monday, July 2, 2018
Issue Time: 09:50AM MDT

— Flooding is NOT expected today

The ridge begins to build to the east of Colorado today, which will give westerly flow aloft this morning more of a southwesterly component this afternoon. This will pull in dry, warm air from the desert southwest, which will help high temperatures to climb a few more degrees today. This will also help to dry out the atmosphere and limit rainfall totals over the elevated terrain this afternoon with storms that develop under the diurnal upslope flow process. Without any upper level support, expecting only isolated showers over the Southwest, San Juan and Southern Front Range Mountains with plenty of virga and some gusty winds.

Later this afternoon, a weak shower or two are forecast move off the mountains into the Urban Corridor/Northeast Plains. A lee trough over the eastern plains is expected to pull in moisture-rich air with southerly flow on its north and east sides. Thus, as storms move northeast/east into the Northeast Plains, the higher low-level moisture (marked with the green below) will increase rainfall totals. Another line of convergence (dry line) will promote thunderstorm growth over the Southeast Plains. The highest surface dew points are expected to be east of Colorado, so there should be plenty of strong wind gusts associated with these thunderstorms. As all the thunderstorms move towards the eastern border today, their chance for becoming severe increases. The main threats will be severe hail and wind gusts greater than 60 mph. Rainfall rates are expected to stay under flood threat criteria, so there is no flood threat today.

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.

Zone-Specific Forecasts:

Front Range, Urban Corridor, Palmer Ridge, San Luis Valley, Southeast Mountains, Raton Ridge, San Juan Mountains:

Temperatures should be on the increase a few ticks today with southwesterly flow aloft. Expect isolated, diurnal storms to fire over the higher terrains this afternoon under upslope flow. Without too much low-level moisture, expecting more virga and gusty winds than measureable rainfall. A few of these storms will move into the Urban Corridor and Northeast Plains this afternoon. Max 1-hour rain rates in the adjacent plains could reach 0.5 inches over the eastern Urban Corridor. Flooding is not expected today.

Primetime: 1PM to 9 PM

Southeast Plains, Northeast Plains:

Southerly flow will allow low-level moisture to be a bit higher over the eastern plains this afternoon. Storms that are able to make it off the mountains into the Northeast Plains could have max 1-hour rain rates up to 1.25 inches. Over the Southeast Plains, larger spread between the dew point and temperature will promote gusty winds. Localized, max 1-hour rain rates up to 1.4 inches/hour are possible. Flooding is not expected today.

Primetime: 3PM to Midnight

Northern Mountains, Southwest Slope, Grand Valley, Central Mountains, Northwest Slope:

The arid southwest flow will continue to dry out the atmosphere today, so rain is not forecast. Expecting some increasing cloud cover over the higher terrains this afternoon as well as over the Northwest Slope. Plentiful sunshine should allow high temperatures to be a couple degrees warmer when compared to yesterday.