FTB 07-31-2016: Low Flood Threat Returns to Western Colorado

Issue Date: 7/31/2016
Issue Time: 9:15 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE GRAND VALLEY, SOUTHWEST SLOPE, SAN JUAN MOUNTAINS, CENTRAL MOUNTAINS, NORTHWEST SLOPE, NORTHERN MOUNTAINS, NORTHEAST PLAINS, PALMER RIDGE, AND SOUTHEAST PLAINS.

Deeper moisture has overspread much of western Colorado, as evidenced by the sharp rise in Grand Junction IPW (see chart below, green line). Values above 1 inch, especially across the Western Slope, signify a developing flood threat, and such is the case today. East of the mountains, moisture is less, mainly due to a drier low-level profile. This morning’s (6 AM MDT) sounding from Denver shows that much of the moisture resides between 3.5 km and 5.5 km aloft, which will temper rain rates along the Front Range/Southeast Mountains/adjacent lower elevations a bit. Storm coverage will be scattered-to-widespread across the mountains, with areas along/near the mountains seeing scattered showers/storms.

IPW_20160731

Further east, low-level moisture is better across the plains. Precipitable water values above 1 inch will be widespread across the eastern third of Colorado, with the far eastern plains pushing 1.25-1.4 inches. Thunderstorm coverage will be fairly isolated across the eastern third of Colorado due to weak capping aloft and generally meager instability, so no flood threat attends much of the area.

The exception is the far Northeast Plains, and perhaps far enough south to graze the Palmer Ridge and Southeast Plains, where a weak disturbance is forecast to move overhead this afternoon and evening, providing the best opportunity for strong storms across eastern Colorado. The moisture described above will fuel fairly efficient rainfall, and slow-moving storms are a concern as the steering flow will only be about 10-15 mph. This scenario is the driving factor behind the low flood threat for these areas. For more information regarding 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.
20160731_LFT

Flood Threat Legend

Zone-Specific Forecasts

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

Scattered showers/thunderstorms are expected, producing garden variety results. Brief periods of moderate rainfall, gusty winds, and lightning are the main threats. Slow-moving storms may result in street ponding in areas with poor drainage, but flooding that would warrant the issuance of a flood threat is not expected. Maximum rain rates will be 0.4-0.6 inches/hour.

Timing: 11 AM – 1 AM for the mountains, 1 PM – 9 PM for the Raton Ridge, and Noon – 10 PM for the Urban Corridor

Northeast Plains, Palmer Ridge, and Southeast Plains:

Isolated-to-widely scattered storms are expected, with the best coverage occurring across the Northeast Plains. A disturbance aloft will help kick off stronger thunderstorms, producing bouts with heavy rain, strong winds, and hail. Slow-moving storms are the main concern behind the low flood threat. Maximum rain rates are 1.4-1.8 inches/hour.

Timing: 1 PM to 1 AM for the Northeast Plains, 3 PM – 10 PM for the Palmer Ridge, and 1 PM – Midnight for the Southeast Plains

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

Scattered showers/thunderstorms are expected throughout today and into tonight as deeper moisture fuels activity. Locally heavy rainfall is a concern, especially for steeper terrain areas that received rainfall yesterday and/or this morning. Rain rates will not be particularly impressive due to generally weak instability, but slow-moving storms are a concern. Maximum rain rates will be 0.8-1.2 inches/hour.

Timing: 11 AM – 10 PM, with a few showers/storms lingering over the higher terrain in southern regions into the early morning hours

STP 07-31-2016: Deeper Moisture Nudged in from the West

Issue Date: Sunday, July 31st, 2016
Issue Time: 9:00 AM MDT

Summary:

As the high pressure aloft shifted to the east, deeper moisture was allowed to enter the state from the west. The increase in moisture fueled an increase in thunderstorms across the High Country and Western Slope, especially over the higher terrain across southwestern Colorado. Heavy rain and 0.50 inch diameter hail was reported 5 miles E of Norwood (San Miguel County). Additionally, 0.50 inch diameter hail was reported 3 miles WNW of Breen (La Plata County), where moderate rain also lasted long enough to just wet the ground. Scattered showers and weak thunderstorms continued overnight across portions of the Grand Valley, Southwest Slope, and San Juan Mountains. A few are still ongoing for those regions, and an additional wave has brought showers to the Northwest Slope this morning.

A few isolated-to-scattered thunderstorms rumbled over eastern Colorado, but much less in numbers and intensity than previous days. For these regions, the most active were the Raton Ridge and Southeast Plains, where a couple of stronger thunderstorms tapped into deeper moisture to produce hail and heavy rain. 2.40 inches of rain and 0.88 inch diameter hail was reported 10 miles SSW of Kim (Las Animas County).

No flash flooding was reported. For a closer look at rainfall totals in your area, please see the STP map below for a look at 24-hour precipitation totals.

20160731_STPImage
Storm Total Precip Legend

FTB 07-30-2016: High Pressure Shifting East

Issue Date: 7/30/2016
Issue Time: 9:20 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE NORTHEAST PLAINS, PALMER RIDGE, AND SOUTHEAST PLAINS.

Current water vapor analysis (below) shows the upper-level high centered just off the southwest coast of California, an upper-level trough (red line) digging along the west coast of Canada, and the mean flow pattern of the jet stream (black line and arrows). The upper-high will shift to the east today as the upper trough digs, nudging better moisture into western Colorado. This will lead to an uptick in thunderstorms over the High Country and Western Slope, especially across the higher terrain of the Southwest Slope, San Juan Mountains, and Southeast Mountains. Further to the north and east, the atmosphere will be a bit drier, and only isolated thunderstorms are expected.

WV_20160730

With the jet stream positioned well to the north and east of Colorado, small disturbances in the flow will not have the same magnitude of effect on eastern Colorado storms. Instead, this period’s thunderstorms will mostly originate over the mountains and move generally eastward over the lower elevations. The exception to this will be over the far Northeast Plains, Palmer Ridge, and Southeast Plains, where there will be just enough forcing to kick off isolated-to-scattered strong/severe thunderstorms. Decent low-level moisture will provide the opportunity for bouts with heavy rain, but downslope winds from the west will work to dry the lower-levels. This drying will enhance thunderstorm wind gust potential, so be on the lookout for that, as well as large hail (up to 1.5 inches in diameter). For details regarding 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.
20160730_LFT

Flood Threat Legend

Zone-Specific Forecasts

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

Isolated-to-scattered showers/thunderstorms expected, with the best coverage near the mountains and across the far Northeast Plains. Most storms will be garden variety, producing moderate rainfall, gusty winds, and lightning. A few strong/severe storms over the plains will result in heavy rain, strong winds, and large hail. The areas outlined by the low flood threat are where heavy rain concerns are greatest, relatively speaking. Maximum rain rates are as follows:

Urban Corridor and Raton Ridge: 0.4-0.8 inches/hour
Palmer Ridge: 1.0-1.5 inches/hour
Northeast Plains and Southeast Plains: 1.5-2.0 inches/hour

Timing: 1 PM – 11 PM for the Urban Corridor and Raton Ridge, 2 PM – Midnight for the Palmer Ridge and Southeast Plains, and 3 PM – 2 AM for the Northeast Plains

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

Isolated-to-widely scattered thunderstorms are expected over the higher terrain today, with the best coverage expected over the Southwest Slope, San Juan Mountains, and Southeast Mountains. Moisture is mainly confined between 4 km and 6 km aloft, so not much rain is expected from any activity.

Generally speaking, the main impacts will be gusty winds and lightning. Brief bouts with moderate rainfall cannot be ruled out near the CO/NM border, or across the Front Range/Southeast Mountains, where moisture is best. Maximum rain rates will be 0.4-0.8 inches/hour across the Front Range, Southeast Mountains, San Juan Mountains, and Southwest Slope regions, and 0.2-0.4 inches/hour elsewhere.

Timing: 1 PM – 9 PM for most areas, with a few showers/storms lingering across the higher terrain of the San Juan Mountains, Southwest Slope, Grand Valley, and Central Mountains until the early morning.

STP 07-30-2016: Northwest Flow Aloft Promotes another Large Hail Day

Issue Date: Saturday, July 30th, 2016
Issue Time: 9:00 AM MDT

Summary:

Dry air out west kept areas west of the Front Range and Southeast Mountains mainly dry and hot, with only a couple isolated, high-based showers/thunderstorms over the higher terrain. These storms produced very little rain, if any, and mainly resulted in gusty outflow winds. To the east, low-level moisture was better, and isolated-to-widely scattered thunderstorms rumbled. Bouts with heavy rain, large hail, and strong winds accompanied the strongest storms. It was Weld County’s turn to take the brunt of the hail, as the severe storm reports below show:

2.5 inch hail: 2 miles ESE of Milton Reservoir (Weld)
2.00 inch hail: Natural Fort Rest Area (Weld), 4 miles E of Lucerne (Weld), 1 mile NE of Fort Lupton (Weld)
1.75 inch hail: 1 mile NW of Ault (Weld), 2 miles NE of Milton Reservoir (Weld), 3 miles N of Greeley (Weld), Ault (Weld), 2 miles NW of Nunn (Weld), 4 miles E of Pierce (Weld), 7 miles SE of Carr (Weld)
1.50 inch hail: S Carr (Weld), Eaton (Weld), NW Pierce (Weld), 1 mile SSW of Fort Lupton (Weld)
1.25 inch hail: Roggen (Weld), ENE Eaton (Weld)
1.0 inch hail: Cripple Creek (Teller), 4 miles W of Carr (Larimer), 1 mile E of Greeley (Weld)

No flash flooding was reported. For a closer look at rainfall totals in your area, please see the STP map below for a look at 24-hour precipitation totals.

20160730_STPImage
Storm Total Precip Legend