STP 06-29-2016: Strong/Severe Thunderstorms Rumbled Across Eastern Colorado

Issue Date: Wednesday, June 29th, 2016
Issue Time: 9:00 AM MDT

Summary:

It was an active day across eastern Colorado as scattered thunderstorms rumbled, generally moving from north/northwest to south/southeast. The most intense storm(s) of the day occurred over the Northeast Plains, producing severe winds and large hail, along with flash flooding (more on this in the final paragraph). Much attention surrounded the severe thunderstorm that moved across the Denver metro area during the afternoon, in which hail up to 1.5 inches in diameter was reported. Strong winds blew down trees, while heavy rain generated underpass and intersection ponding/flooding. Additionally, 4 people had to be rescued from high water along Cherry Creek Trail.

Meanwhile, across western Colorado, monsoonal moisture began to creep into the region, setting off isolated-to-scattered showers/thunderstorms. Most activity was garden variety, with lightning and gusty winds being the most notable effects. Locally moderate rainfall attended the strongest storms, relatively speaking.

Severe storm reports are as follows:

Severe Wind Gusts: 7 miles NNW of Alvin (65 mph, Yuma County), 13 miles S of Eckley (60 mph, Yuma County)
1.75 inch hail: 6 miles E of Kirk (Yuma County)
1.50 inch hail: 2 miles NW of Sheridan (Denver County)
1.0 inch hail: Various locations across Washington, Denver, and Yuma counties

Rainfall winners, according to CoCoRaHS observers (reports as of 7:00 AM MDT):

Jefferson County: 1.89 inches
Denver County: 1.58 inches
El Paso County: 1.03 inches
Yuma County: 0.97 inches
Prowers County: 0.77 inches
Kit Carson County: 0.75 inches
Douglas County: 0.74 inches

Flash flooding was reported 6 miles E of Kirk (Yuma County) as 1.54 inches of rain fell in 45 minutes. For a statewide 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

FTB 06-28-2016: Two Distinct Low Flood Threat Areas

Issue Date: 6/28/2016
Issue Time: 10:21 AM

A LOW FLOOD THREAT IS ISSUED FOR PORTIONS OF NORTHEAST PLAINS, SOUTHWEST SLOPE, AND SAN JUAN MOUNTAINS.

Two distinct areas, for two distinct reasons, will feature low flood threats during this period. The Northeast Plains will experience a period very similar to yesterday’s isolated-to-scattered thunderstorms, which included a few severe storms and bouts with moderate-to-heavy rainfall. The other distinct area will be across southwestern Colorado, namely the Southwest Slope and San Juan Mountains regions. There, better moisture aloft is being transported in from the west/southwest (as shown by the water vapor image/annotations below).

During the afternoon, isolated-to-scattered showers/thunderstorms will be high-based across southwestern Colorado, mainly over the higher terrain. Gusty outflow winds in combination with the high-based thunderstorms producing lightning will heighten fire concerns, mainly over the lower elevations of the Southwest Slope. With that said, as the afternoon turns to evening, moisture will mix down as outflow helps to saturate the lower levels. This increase in the depth of moisture, particularly near the surface, will allow for locally heavy rainfall. Storm motions will help mitigate the flood threat somewhat, but a low flood threat is warranted.

WV_20160628

East of the mountains, today will be very similar to yesterday. The main difference is that moisture will be a bit less in comparison, especially along/near the higher terrain. This, combined with the northwest flow regime, will keep the stronger storms further to the east, mainly over the Northeast Plains. For the other regions (Palmer Divide, Urban Corridor, Southeast Plains, and Raton Ridge), isolated showers/thunderstorms will bring mainly gusty winds and lightning, with brief bouts of moderate rainfall. Relatively speaking, the Southeast Plains and Raton Ridge will see the least amount of activity.

For more details, please see the zone-specific 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, Northeast Plains, Palmer Ridge, Raton Ridge, and Southeast Plains:

Isolated-to-scattered thunderstorms expected. The best coverage will occur over the Northeast Plains, as will the strongest storms. The main threats from storms over the Northeast Plains will be strong winds up to 60 mph, large hail (up to 2.5 inches in diameter), and a brief isolated tornado or two.

The Urban Corridor, Palmer Ridge, Raton Ridge, and Southeast Plains will see more isolated coverage, with more “garden variety” intensity. The main threats from these storms will be gusty winds, lightning, and small hail. Maximum rain rates are as follows:

Northeast Plains: 2.0-2.8 inches/hour
Urban Corridor, Palmer Ridge, and Raton Ridge: 0.4-0.8 inches/hour
Southeast Plains: 1.0-1.5 inches/hour

Timing: 2 PM – Midnight, with a couple isolated storms continuing over the Plains into the early morning hours

Front Range, Northern Mountains, Central Mountains, San Luis Valley, and Southeast Mountains:

Isolated-to-scattered showers/thunderstorms expected. Storms will develop over the higher terrain, briefly drifting over lower valleys. The main impacts from any activity will be gusty winds, lightning, and light rain. Maximum rain rates will be 0.5-0.8 inches/hour.

Timing: 11 AM – 10 PM

Northwest Slope and Grand Valley:

Mostly sunny and hot will be the main weather story today. Over the higher terrain, isolated, high-based, dry thunderstorms will develop this afternoon/evening. This will elevate fire concerns in these regions as gusty outflow winds and very little rain will accompany any cloud-to-ground lightning.

Timing: 2 PM – 10 PM

San Juan Mountains and Southwest Slope:

Isolated-to-scattered thunderstorms are expected, mainly over the higher terrain. During the afternoon, storms will be high-based, producing more wind and lightning than rain. As the afternoon turns to evening, however, moisture will mix down into the low-levels, providing the opportunity for locally heavy rainfall. Thus, the issuance of the low flood threat. Steep terrain, burn scars, and low lying areas will be the main areas of concern for any flood/heavy rain issues. Maximum rain rates will be 1.0-1.5 inches/hour.

Timing: 11 AM – 9 PM

STP 06-28-2016: Isolated-to- Scattered Thunderstorms Rumbled, a Few were Strong/Severe

Issue Date: Tuesday, June 28th, 2016
Issue Time: 9:00 AM MDT

Summary:

Ingredients came together yesterday to produce another round of showers and thunderstorms, mainly east of the Continental Divide. Generally speaking, the most intense storms occurred over the Northeast Plains where the best combination of strong instability, wind shear, and deep moisture overlapped. Severe storm reports are as follows:

2.75 inch hail: 2 miles WSW of Paoli (Phillips County), 3 miles WNW of Fairfield (Sedgwick County)
2.5 inch hail: 5 miles W of Paoli (Phillips County), Paoli (Philips County)
1.0 – 1.75 inch hail: Various locations across Logan, Sedgwick, Morgan, Weld, Phillips, and Prowers counties
Tornado: Paoli (Phillips County), 8 miles SE of Julesburg (Sedgwick County)
55 mph wind gust: Crook (Logan County)

Rainfall winners, according to CoCoRaHS observers (reports as of 7:00 AM MDT):

Sedgwick County: 1.00 inches
Bent County: 0.97 inches
Kit Carson County: 0.78 inches
Prowers County: 0.55 inches
Lincoln County: 0.33 inches
Elbert County: 0.31 inches

No flash flooding was reported yesterday. For a statewide 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 06-27-2016: First Breath of Monsoonal Moisture

Issue Date: 6/27/2016
Issue Time: 12:12 PM

Threat_Timeline_20160627

A persistent upper-level low near the Gulf of Alaska will be the main player during this FTO period, accompanied by what appears to be the first real gasp of monsoonal moisture this season. The persistent low will keep the flow above Colorado west/northwesterly, ejecting disturbances across the region. Meanwhile, from the south/southwest, the North American Monsoon is awakening, beginning to transport moisture into the southwest US. As of this writing, Yuma, AZ, IPW is sitting above 1.4 inches. This moisture will work its way north-northeastward into the Four Corners region over the next few days.

WV_20160627

You’ll notice in the threat timeline at the top of the page, that Event #1 is a gradient of “No Apparent Threat” and “Elevated Threat.” I did this to give a visual note on how the threat will evolve. The first two days (Tuesday/Wednesday, June 28/29), will precede the deepest moisture moving in from the southwest. The end of the week and into the weekend (June 30 – July 3) will see the best combination of moisture/instability/disturbances, thus the transition to the “Elevated Flood Threat.” By Monday (July 4), the moisture will subside and dry westerly flow will be overhead, once again.

After the warming and drying period of Tuesday through Friday (July 5 – July 8), the next moisture surge awaits. Meanwhile, the persistent upper-level low will not be persistent anymore; instead, it will slide eastward along the US/Canada border. Depending on its eventual track will determine how much upper-level support is available for showers/thunderstorms. This will be monitored closely in the coming days. Also, there are hints in the extended range of tropical cyclone activity in the eastern North Pacific, which could have profound impacts in the extended range. As evidenced by the explanation above, there is some uncertainty surrounding the time frame of Event #2, so be sure and check back in with the FTO on Thursday.

Event #1: Tuesday (06-28-2016) through Monday (07-04-2016)

Elevated Flood Threat as First Monsoonal Moisture Surge Arrives

Awaiting the arrival of the main moisture surge, the first two days of this period will see progressively more shower/thunderstorm coverage. Temperature/dewpoint spreads in the low-levels suggest the heavy rain threat will be isolated through Wednesday. By Thursday, and lasting through Sunday, the moisture will be sufficiently deep enough to support scattered heavy rain events across much of the state as disturbances pivot across the region. Moisture will subside by late Sunday/early Monday, and dry westerly flow will be overhead.

Legend

Event #2: Saturday (07-09-2016) through Tuesday (07-12-2016)

No Apparent Flood Threat due to Uncertainty Surrounding Key Features

The upper-level low’s movement, timing of the next moisture surge, and any potential tropical cyclone activity in the eastern North Pacific are key factors in the extended range. Uncertainty exists with all three, and all will be monitored closely. Regardless, this looks to be the next opportunity for a wet period. Keep in mind that the precipitation map below represents the current thinking and will likely need adjustments moving forward. Stay tuned.

Legend