Issue Date: Wednesday, July 27th, 2016
Issue Time: 10:10AM MDT

— Flooding is NOT expected today

An interesting weather pattern is noted this morning across the Rocky Mountain states, as shown by the water vapor image below. The first feature of interest is the high pressure that has developed over the Great Basin. This will quickly strengthen today, and its clockwise circulation will basically shut off the northward transport of new moisture from the south (including the outflow from Tropical Storm Frank). The result will be much lower rain coverage and intensity west of the Divide. The second important feature is a rather strong disturbance, especially by summer standards, that is currently located over Wyoming and Montana. This will race south-southeastward and clip eastern areas of our state, providing favorable dynamics for enhanced upward motion. Scattered thunderstorms will be likely for far eastern areas. Counteracting this will be the rapid drying as downslope winds usher in much lower dewpoints originating off the Cheyenne ridge and then spreading south and east. The disturbance will also promote fast storm motion, up to 40 mph, which will be a negative factor for heavy rainfall in a given location.

The highest low-level moisture will be locked up mainly along the Kansas border, which is where the highest rainfall rates will occur today. However, we expect rainfall to stay just below flood thresholds. 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: 7/26/2016
Issue Time: 9:42 AM

A MODERATE FLOOD THREAT IS ISSUED FOR PORTIONS OF THE NORTHEAST PLAINS AND SOUTHEAST PLAINS.
A LOW FLOOD THREAT IS ISSUED FOR SURROUNDING AREAS AND PORTIONS OF THE URBAN CORRIDOR, PALMER RIDGE, FRONT RANGE, AND SOUTHEAST MOUNTAINS.
A LOW FLOOD THREAT IS ISSUED FOR PORTIONS OF THE SAN JUAN MOUNTAINS, SOUTHWEST SLOPE, AND SAN LUIS VALLEY.

IPW values at all four of the normal reporting stations (Boulder, Grand Junction, Pueblo, and Schriever AFB) are below 1 inch this morning. This reflects the slight drying that has taken place as the lower-levels have become drier over the past few days. With that said, another swing upward is expected this afternoon and evening, pushing moisture values to levels where heavy rain remains a threat.

Over the mountains, storms will be scattered as the elevated terrain heats up and destabilizes the airmass. Most showers/storms will be garden variety, though, with the main threats being gusty winds and lightning, with brief light-to-moderate rainfall. There are two exceptions to this:

1) Front Range and Southeast Mountains: Low-level moisture is best here due to proximity with the eastern plains. Although westerly winds will likely dry the low-levels somewhat, enough of a concern exists for portions to be included in the low flood threat.
2) Southwest Slope and San Juan Mountains: The deepest moisture remains across southern zones as the drier air works in from the northwest. Access to better low-level moisture advecting in from AZ/NM increases the risk for heavy rain. Thus, the issuance of the low flood threat.

Near the eastern border, scattered strong-to-severe thunderstorms are expected, especially across Northeastern Colorado. The presence of a modest upper-level jet streak will be enough to support thunderstorm development, while strong daytime heating combined with sufficient wind shear will organize updrafts/downdrafts, promoting the development of large hail, strong winds, and heavy rain. Eastern portions of the Palmer Ridge may get in on this action, as well.

Over the Urban Corridor and western portions of the Southeast Plains and Palmer Ridge, the atmosphere will be generally capped off, but a few isolated-to-widely scattered storms will move overhead from the adjacent higher terrain. Enough moisture exists to warrant inclusion in the low flood threat area, but the lack of support aloft means it will be tough for storms to reach those thresholds

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

Northeast Plains, Palmer Ridge, and Southeast Plains:

Scattered thunderstorms, some will be strong/severe, are expected this afternoon through tonight. The main threat area will be across the far Northeast Plains and along/near the eastern CO border. For western portions of the Palmer Ridge and Southeast Plains, the storms will first develop over the higher terrain, and then move overhead. Elsewhere, it will be tough for storms to overcome a cap aloft, and only an isolated thunderstorm or two are expected. Maximum rain rates are as follows:

Northeast Plains: 2.0-3.0 inches/hour
Palmer Ridge: 1.2-1.8 inches/hour
Southeast Plains: 1.4-2.0 inches/hour

Timing: 2 PM – 2 AM

Front Range, Southeast Mountains, Urban Corridor, and Raton Ridge:

Isolated-to-scattered showers and thunderstorms expected, with the best coverage over/near the higher terrain. Enough moisture exists for a heavy rain threat, but the main threat from storms will be strong winds and lightning. Maximum rain rates are as follows:

Front Range and Southeast Mountains: 0.8-1.2 inches/hour
Urban Corridor: 1.0-1.4 inches/hour
Raton Ridge: 0.5-0.8 inches/hour

Timing: Noon – Midnight over the mountains and Raton Ridge, 1 PM – 9 PM for the Urban Corridor

San Juan Mountains, Southwest Slope, and San Luis Valley:

Scattered showers/thunderstorms are expected over the higher terrain, moving generally eastward and over adjacent valleys. IPW values near 1 inch exhibit enough moisture for locally heavy rainfall, but drier low-levels will temper rain rates a bit until moisture can be mixed down, either by momentum or rainfall. Maximum rain rates are as follows:

San Juan Mountains: 1.0-1.8 inches/hour
Southwest Slope: 0.8-1.4 inches/hour
San Luis Valley: 0.6-1.2 inches/hour

Timing: 11 AM – 11 PM

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

Isolated-to-scattered showers/storms expected, but drier air will filter in from the northwest. This drier air will temper rain rates, keeping them below flash flooding thresholds. Activity will be garden variety, with the main threats being gusty outflow winds, lightning, and light-to-moderate rainfall. Maximum rain rates will be 0.3-0.6 inches/hour.

Timing: 11 AM – 9 PM for the Central Mountains, Northern Mountains, and Grand Valley, 1 PM – 7 PM for the Northwest Slope

Issue Date: 7/25/2016
Issue Time: 9:15 AM

A LOW FLOOD THREAT IS ISSUED FOR PORTIONS OF THE NORTHEAST PLAINS, URBAN CORRIDOR, PALMER RIDGE, RATON RIDGE, FRONT RANGE, AND SOUTHEAST PLAINS.

A LOW FLOOD THREAT IS ALSO ISSUED FOR PORTIONS OF THE SAN JUAN MOUNTAINS AND SOUTHWEST SLOPE.

Behind the shortwave aloft that passed across the region yesterday, the upper-level ridge will build northward throughout the day today. The upper-high will become centered over the southern Rockies by this afternoon, with a jet max sitting across northern Colorado. Drier air has been filtering in from the west, and IPW values have decreased from yesterday afternoon’s values. 3 of the 4 normal reporting stations are below 0.8 inches IPW, while Pueblo holds on at just above 1 inch.

Generally speaking, with drier air filtering in and an upper-level ridge building overhead, this period will feature fewer showers and thunderstorms. Fighting against the drying, however, will be southeasterly surface flow across eastern Colorado, which will reinforce moisture this afternoon/evening. This will keep the threat of heavy rain in the forecast, albeit to a slightly lesser extent than yesterday. Additionally, enough moisture will hang on across southwest Colorado to keep the threat of locally heavy rain in the forecast for portions of the Southwest Slope and San Juan Mountains. Thus, the issuance of the low flood threat areas.

With southeasterly surface flow and northwest flow aloft over northeast Colorado, enough wind shear will be available to promote a couple isolated strong-to-severe storms this afternoon/evening. The main threats will be hail (up to 1.5 inches in diameter) and strong winds (gusts up to 65 mph), with periods of heavy rainfall. The most likely locations for this activity will be along and near the Cheyenne Ridge, and adjacent locations to the southeast. For more information on timing and rain rates across the state, 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/storms, with the best coverage occurring adjacent to the higher elevations and along/near the preferred terrain of the Cheyenne Ridge, Palmer Ridge, and Raton Ridge. A couple storms will be severe, mainly over the Northeast Plains, namely Weld, Morgan, Logan, and Washington counties. Strong-to-severe storms will be capable of producing hail, lightning, strong winds, and periods of heavy rainfall. Maximum rain rates are as follows:

Urban Corridor: 0.8-1.2 inches/hour
Palmer Ridge: 1.0-1.5 inches/hour
Northeast Plains and Southeast Plains: 1.5-2.5 inches/hour
Raton Ridge: 0.8-1.2 inches/hour

Timing: Noon – 10 PM, with a few storms continuing until midnight/1 AM.

Front Range, San Luis Valley, and Southeast Mountains:

Isolated showers/thunderstorms expected, with the best coverage across southern portions of the Front Range and across the Southeast Mountains where moisture will be best. The San Luis Valley will be mostly sunny, with the one or two storms moving overhead from the surrounding higher terrain. Maximum rain rates are as follows:

Front Range: 0.8-1.2 inches/hour
Southeast Mountains: 0.6-0.8 inches/hour
San Luis Valley: 0.4-0.8 inches/hour

Timing: 11 AM – 11 PM for the mountains, 1 PM – 9 PM for San Luis Valley

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

Isolated, garden variety showers/storms will interrupt the otherwise hot and mostly sunny day across the area. Storms will favor the higher terrain, and only briefly move over adjacent lower valleys. The main threats from storm activity will be gusty winds and lightning, with light-to-moderate rainfall. Maximum rain rates are as follows:

Northwest Slope: 0.1-0.3 inches/hour
Northern Mountains: 0.2-0.5 inches/hour
Central Mountains and Grand Valley: 0.3-0.6 inches/hour

Timing: Noon – 10 PM

San Juan Mountains and Southwest Slope:

Scattered showers/thunderstorms are expected, with the main threats being gusty winds, lightning, and brief periods of moderate-to-heavy rainfall. The higher terrain will be favored, with storms only briefly drifting over adjacent valleys. Maximum rain rates will be 0.8-1.25 inches/hour.

Timing: 11 AM – 11 PM, with primetime for heavy rainfall threat between Noon and 8 PM.

Issue Date: 7/24/2016
Issue Time: 9:51 AM

A MODERATE FLOOD THREAT IS FORECAST FOR PORTIONS OF THE URBAN CORRIDOR, PALMER RIDGE, SOUTHEAST PLAINS, NORTHEAST PLAINS, AND FRONT RANGE.

A LOW FLOOD THREAT IS FORECAST FOR SURROUNDING AREAS, INCLUDING PORTIONS OF THE NORTHERN MOUNTAINS, CENTRAL MOUNTAINS, SOUTHEAST MOUNTAINS, RATON RIDGE, AND SAN JUAN MOUNTAINS.

The current water vapor analysis (below) shows two main features of note: a shortwave tracking across the northern US, and a strong upper-level high across the southern US. The shortwave trough has flattened the ridge and turned the flow aloft to a more westerly direction, which will continue to shift drier air into Colorado from the west. Ahead of this dry air, however, the plume of subtropical air is still situated across Colorado. The moisture aloft will, in itself, provide fuel for scattered showers and thunderstorms, but it is what is occurring at the surface that will drive the low-to-moderate flood threat.

A surface cool front has moved into Northeast Colorado this morning, and will settle along/near the Palmer Ridge this afternoon. North of the front temperatures will be 5-10 degrees cooler than yesterday, and south of the front temperatures will be comparable to yesterday. More importantly for our product is that behind the cool front, moist upslope flow has developed, deepening the moisture available to showers and thunderstorms. IPW values will climb near/above one inch along the Front Range and eastward across the plains during the afternoon and evening hours. Additionally, the cool front will provide a focus for thunderstorm development, and storms anchoring to, or near, this boundary will present both a severe threat and an enhanced heavy rain threat.

Also, as the winds turn southeast behind the front and across the Palmer Ridge, a Denver Cyclone is expected to develop. This will place Douglas and Elbert Counties under the gun for strong/severe thunderstorms producing heavy rain. These areas have been included in the moderate flood threat. Along and west of the Continental Divide, scattered showers/thunderstorms are expected again today, mainly over the higher terrain. Westerly flow aloft will allow storms to move over lower valleys.

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

Scattered showers and thunderstorms are expected, with a few becoming strong/severe. For reasons outlined in the discussion above, the low-to-moderate flood threat is warranted. Training storms along the cool front/Denver Cyclone are a concern, and those concerns have been taken into consideration by the moderate flood threat area. Maximum rain rates are as follows:

Urban Corridor and Palmer Ridge: 1.5-2.0 inches/hour
Southeast Plains and Southeast Plains: 2.0-3.0 inches/hour
Raton Ridge: 1.0-1.6 inches/hour

Timing: 1 PM – 9 PM for most, with scattered thunderstorms continuing across the Southeast Plains and eastern portions of the Palmer Ridge/Raton Ridge until Midnight.

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

Isolated-to-scattered showers and thunderstorms are expected, mainly over the higher terrain. The best coverage will occur across the San Juan Mountains, Central Mountains, Front Range, and Southeast Mountains, with lesser coverage elsewhere. The Northwest Slope and Grand Valley will see the least amount of activity today as drier air aloft continues to move in. Maximum rain rates are as follows:

Front Range and Southeast Mountains: 0.8-1.4 inches/hour
Northern Mountains, Central Mountains, and San Juan Mountains: 0.6-1.2 inches/hour
Northwest Slope and Grand Valley: 0.2-0.5 inches/hour
San Luis Valley: 0.4-0.8 inches/hour

Timing: 11 AM – 10 PM, with a few lingering showers/storms over southern mountains/San Luis Valley into the early morning hours.