STP 07-26-2016: Fewer Storms Overall, One Thunderstorm Complex Provided the Main Show

Issue Date: Tuesday, July 26th, 2016
Issue Time: 9:00 AM MDT

Summary:

As the title of this post suggests, yesterday saw a downtick in the number of thunderstorms as compared to the previous day. Isolated-to-scattered storms were still found over the High Country and Western Slope, while eastern Colorado focused on one main storm complex. This thunderstorm complex began first over Douglas and Elbert counties, then moving in a general south-southeastward direction across El Paso, Pueblo, Lincoln, and Crowley counties. It then lost its steam, becoming more of a general rainfall event as it moved slowly to the east-southeast, dissipating over Otero, Bent and Kiowa counties. Storm reports from this complex are as follows:

Thunderstorm wind gust: 63 mph (4 miles NE of Blende, Pueblo County), 60 mph (3 miles NE of Castle Rock, Douglas County), 55 mph (4 miles S of Cheraw, Otero County)
0.88 inch hail: 2 miles SSE of Fountain (El Paso County)
0.75 inch hail: 2 miles NW of Crowley (Crowley County)
Heavy Rain: 2.0 inches (5 miles WSW of Blende, Pueblo County), 0.87 inches/35 minutes (2 miles SSE of Fountain, El Paso County), 0.83 inches (1 mile SE of Pueblo West, Pueblo County)

No flash flooding was reported, but street and field ponding/flooding likely occurred underneath the stronger thunderstorms. For a complete look at 24-hour precipitation totals, please take a look at the Storm Total Precipitation map below.

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Storm Total Precip Legend

FTO 07-25-2016: Elevated Flood Threat in Short-term, Extended Wet Period Awaiting?

Issue Date: 7/25/2016
Issue Time: 1:32 PM

Threat_Timeline_20160725

For Tuesday (Event #1), the upper-level ridge over the Desert Southwest will continue to rebound and build across the region. However, residual/monsoonal moisture (orange arrow in WV image below), and weak west/northwest flow aloft (while waiting on the ridge to rebound) will keep showers and thunderstorms in the forecast. Activity will be isolated over the mountains and widely scattered over the plains, similar to today’s FTB forecast. There will be sufficient instability and wind shear to produce a few isolated strong-to-severe thunderstorms, especially over the Northeast Plains. These storms will continue to have the threat of heavy rain, thus the “elevated” flood threat tag. Stay tuned to tomorrow’s FTB for more specific information.

Event #2, marked in the water vapor image below, is a shortwave disturbance that will push across the Northern Rockies on Thursday/Friday. At the surface, a cool front will push southward across eastern Colorado. This will allow for low-level moisture to move back into the plains, and perhaps back into the Front Range foothills. At this time, it looks like instability and wind shear will be sufficient for a couple strong/severe storms over the plains. With deepening low-level moisture behind the front with upslope flow, the threat for heavy rain will exist, thus the “elevated” flood threat designation. After Event #2, there will be a short break before the beginning of Event #3. During this break, typical summertime weather is expected as residual moisture and sunshine/orographic influences will promote isolated thunderstorms over the mountains and adjacent valleys/plains.

WV_20160725

Event #3 was discussed during the previous two FTO’s as a possibility, and it is beginning to come into focus. The upper-level low (and multiple shortwave troughs associated with it) will move into the Pacific Northwest by Sunday, July 31. The flow aloft will turn southwest across Colorado, bringing the next surge of monsoonal moisture. At the same time, the upper-level low will eject disturbances across the region, although their exact timing and placement are still uncertain. Also uncertain, but equally important, will be the placement of the high pressure center over the central US. Too far north and west, and the best moisture will miss Colorado and a dry period will ensue. If it slides far enough to the south and east, then deep moisture will overspread Colorado, and it will be an extended wet period with multiple days of flood threats. This will continue to be monitored, so please stay tuned for more information in Thursday’s FTO.

Event #1: Tuesday (07-26-2016)

Elevated Flood Threat as Ridge Rebounds

Residual/monsoonal moisture and weak west/northwest flow aloft will keep isolated-to-widely scattered showers and thunderstorms in the forecast. There will be enough instability and wind shear to organize a couple isolated strong-to-severe storms across the Northeast Plains. The threat of heavy rain is still present, and antecedent conditions are a concern in some areas. Stay tuned to tomorrow’s FTB for updated information.

Legend

Event #2: Thursday (07-28-2016) and Friday (07-29-2016)

Elevated Flood Threat as a Shortwave and Cool Front Join Forces

A shortwave disturbance will push across the Northern Rockies on Thursday/Friday. At the surface, a cool front will push southward across eastern Colorado, allowing low-level moisture to return to the plains, and perhaps back into the Front Range foothills. Instability and wind shear will likely be sufficient for a couple strong/severe storms over the plains. Local details will be of utmost importance in the location of the flood threat, so stay tuned to the FTB for each day.

Legend

FTB 07-25-2016: Fewer Storms, Heavy Rain Still a Threat

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.

IPW_20160725

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.
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Flood Threat Legend

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.

STP 07-25-2016: Strong-to-Severe Thunderstorms Rumbled Across Eastern Colorado

Issue Date: Monday, July 25th, 2016
Issue Time: 9:00 AM MDT

Summary:

The combination of a cool front, disturbance aloft, and sufficient instability/wind shear produced an afternoon and evening filled with scattered thunderstorms; a few were severe. The storm of the day, as far as heavy rain/flooding is concerned, occurred over Morgan County. Significant street flooding was reported within Fort Morgan, as well as flooded homes. 15 miles to the north of Fort Morgan, 2.75 inch hail was reported around 5:15 PM MDT. Other severe storm reports from around the state include:

Thunderstorm wind gust: 74 mph (Denver International Airport, Denver County), 63 mph (4 miles S of Cheraw, Otero County), 59 mph (Sterling, Logan County), 58 mph (4 miles SSW of Keenesburg, Weld County), 58 mph (8 miles SSW of Grover, Weld County), 57 mph (4 miles SW of Watkins, Arapahoe County)
Non-thunderstorm wind gust: 60 mph (Centennial, Arapahoe County)
Thunderstorm wind damage: Greenhouse was flattened (Byers, Arapahoe County), Large tree limbs broken (Hudson, Weld County)
1.75 inch hail: 7 miles SW of Grover (Weld County)
1.50 inch hail: 9 miles W of New Raymer (Weld County)
1.0 inch hail: 5 miles S of Wiggins (Morgan County)

Heavy rain, 1.20 inches in 1 hour, occurred 3 miles S of Vallecito (La Plata County), but no flash flooding issues were reported. For a complete look at 24-hour precipitation totals, please take a look at the Storm Total Precipitation map below.

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Storm Total Precip Legend