FTO 06-30-2016: Elevated Flood Threat Continues, 3 Events Total

Issue Date: 6/30/2016
Issue Time: 1:10 PM

Threat_Timeline_20160630

The upper-level low (denoted by a red “L” in the water vapor image) will shift slowly east through this coming weekend, ejecting weak disturbances into the mid-/upper-level flow. These disturbances will slide across the central/northern Rockies, working with the monsoonal moisture stuck underneath the high pressure ridge (blue “H”). Friday and Saturday (July 1/July 2) will be the wettest days of the first event, with moisture subsiding/shifting east slowly through the weekend.

By Monday (July 4), the moisture will subside and dry westerly flow will be overhead as high pressure builds across the region. This warming and drying period will continue through Thursday (July 7), only being broken up by a few afternoon high country showers/thunderstorms as sunshine and orographic effects work on residual moisture. This dry period will be hot, especially over the lower elevations of eastern Colorado. The Arkansas River Valley could see multiple afternoons with high temperatures over 100 F.

WV_20160630

After the warming/drying period, another moisture surge awaits (Event #2) for the end of next week. This surge will be of the Gulf moisture type, transported into the state by south/southeasterly flow across the US plains. Moisture will be confined essentially along/east of the Continental Divide, with the strongest storms over the eastern plains. A cool front will escort this moisture eastward again Sunday/Monday (July 10/July 11), bringing another dry/warming period to Colorado. By the middle-to-end of that week (July 12-15), there are hints of monsoonal moisture returning to the state as the eastern Pacific becomes active with tropical activity. This would mainly impact areas along and west of the Continental Divide. This surge will be discussed as Event #3 below.

Event #1: Friday (07-01-2016) through Sunday (07-03-2016)

Elevated Flood Threat Continues

Event #1 is a continuation of Event #1 as discussed in Monday’s FTO. Multiple disturbances will eject from the upper-level low, currently over the Gulf of Alaska (but shifting eastward), providing upper-level support for scattered-to-widespread showers and thunderstorms through Saturday. Moisture will begin to subside and slide eastward on Sunday as the upper-level ridge builds across the region and dry, westerly flow encroaches on Colorado. Due to these factors, Sunday will see a downtick in thunderstorm intensity/coverage as compared to Friday and Saturday.

Legend

Event #2: Friday (07-08-2016) through Sunday (07-10-2016)

No Apparent Flood Threat as Best Moisture Remains Over the Plains

A Gulf moisture surge is forecast to move into eastern Colorado late next week, continuing through the weekend, as south/southwesterly low-level flow asserts itself across the central US. The moisture will be essentially confined along/east of the Continental Divide. The strongest storms will likely remain east of I-25 and over the plains, especially the Northeast Plains. Due to the higher threshold for flash flooding over the plains, no flood threat is apparent. If sufficient moisture can move back into the Front Range/Urban Corridor, a flood threat could emerge. Stay tuned.

Legend

Event #3: Tuesday (07-12-2016) through Friday (07-12-2016)

No Apparent Flood Threat due to Uncertainty Surrounding Timing of Monsoon Surge

The strength of the summer ridge, as well as the timing of the tropical activity in the eastern Pacific will play a big role in the eventual emergence of Event #3. At this current time, it looks more probable to occur late in the week, with the best moisture/support for storms over the High Country. Due to the uncertainties surrounding this event, no precipitation map will be issued. Stay tuned for an update on this time period in Monday’s FTO.

FTB 06-30-2016: Widespread Showers/Thunderstorms Statewide

Issue Date: 6/30/2016
Issue Time: 10:15 AM

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

A LOW FLOOD THREAT IS ISSUED FOR SURROUNDING AREAS, INCLUDING PORTIONS OF THE NORTHERN MOUNTAINS, NORTHWEST SLOPE, CENTRAL MOUNTAINS, GRAND VALLEY, SOUTHWEST SLOPE, SAN JUAN MOUNTAINS, AND SOUTHEAST MOUNTAINS.

Instead of discussing the water vapor imagery, today is good opportunity to show one image from our model guidance that gets to the nitty-gritty of this forecast. The image shows the chance of precipitation across Colorado, and it’s fairly easy to see just how widespread the activity will be today. Monsoonal moisture, plus the presence of a cold frontal boundary/reinforcing outflow boundaries, will combine to leave a moist air mass in place across the state. Precipitable water (PWAT) values will be at (or above) 1 inch nearly everywhere along/east of the Front Range and Southeast Mountains, as well as far western Colorado. This signals just how moist the air mass is, and heightens the concern for flash flooding across the state.

POP_20160630

Scattered showers are ongoing over the high country and over the eastern plains. Activity will increase around 11 AM, first over the mountains, and then spreading eastward over the Urban Corridor and then across the plains. A couple storms across the plains will be strong/severe, with strong winds, hail (up to 1.5 inches), and heavy rain being the main threats. Scattered showers/thunderstorms will continue overnight and into tomorrow morning. For more details, including timing and maximum rain rates, 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

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

Widespread showers/thunderstorms are expected today and tonight, with isolated-to-scattered showers/thunderstorms continuing into the morning hours tomorrow. With precipitable water values at, or above, 1 inch, heavy rain is a heightened concern. Storm motions will help mitigate the flood threat somewhat, but that “help” is neutralized a bit with the past few days of rain saturating soils. Maximum rain rates are as follows:

Front Range: 1.0-1.5 inches/30-45 minutes
Urban Corridor: 1.2-2.0 inches/30-45 minutes
Palmer Ridge, Northeast Plains, and Southeast Plains: 1.8-2.5 inches/30-45 minutes

Timing: 11 AM – 4 AM

Southeast Mountains and Raton Ridge:

Scattered showers/thunderstorms expected to begin around lunchtime and continue through the evening, with a few isolated-to-scattered showers/storms lasting into the early morning hours. Although surface moisture will be a bit lower here, and it will be a bit removed from the best mid-level forcing, the environment will still support periods of locally heavy rainfall. Maximum rain rates will be 0.8-1.4 inches/hour, but most rain will fall at a rate of 0.4-0.8 inches/hour.

Timing: 11 AM – 3 AM

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

Widespread showers/thunderstorms are expected today and tonight, with scattered showers/thunderstorms continuing into tomorrow morning. Gusty winds, lightning, and locally heavy rainfall are the main threats. Maximum rain rates are as follows:

Northwest Slope and Grand Valley: 0.8-1.4 inches/hour
Northern Mountains and Central Mountains: 0.6-1.2 inches/hour
San Luis Valley: 0.6-1.4 inches/hour
Southwest Slope and San Juan Mountains: 1.2-1.6 inches/hour

Timing: 11 AM – 11 AM (tomorrow)

Note: Notice that some portions of central Colorado are not included in the low flood threat. This is due to the best instability/moisture existing east and west, so heavy rainfall causing flash flooding is not enough of a concern to warrant inclusion in the low flood threat.

STP 06-30-2016: More Widespread Activity, a Couple Were Strong/Severe

Issue Date: Thursday, June 30th, 2016
Issue Time: 9:00 AM MDT

Summary:

The mid-/upper-level ridge over the ridge weakened slightly, allowing the moisture underneath the ridge to fuel more widespread thunderstorms as compared to previous days. This increase in coverage was noticed most over the mountains as monsoonal moisture crept in from the southwest. Generally speaking most storms over the higher terrain were garden variety, resulting in gusty winds, lightning, and brief light-to-moderate rainfall. A few stronger storms produced brief moderate-to-heavy rainfall, but not enough to result in any flash flooding issues. East of the mountains, and mainly east of I-25, scattered strong/severe storms rumbled, producing hail, strong winds, lightning, and bouts with heavy rain. No flash flooding was reported with these storms either, but they likely resulted in some street/field ponding under the heaviest rain.

Storm reports are as follows:

Funnel Cloud: 3 miles SSW of Montrose (Montrose County)
1.0 inch hail: 5 miles W of Hoyt (Weld County), 1 mile NE of Sugar City (Crowley County)
0.75 inch hail: 2 miles S of Durango (La Plata County)
Thunderstorm Wind Damage: 4 miles NW of Grover (Weld County)
60 mph Wind Gust: 4 miles S of Burlington (Kit Carson County)

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

FTB 06-29-2016: Another Period of Showers/Thunderstorms On Tap

Issue Date: 6/29/2016
Issue Time: 10:12 AM

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

Current water vapor analysis shows the (slightly weakening) mid-/upper-level ridge still in place across the region today. Underneath the ridge, sufficient mid-level moisture remains across the state, while monsoonal moisture continues to seep into Colorado from the west/southwest. Embedded within the flow is a weak disturbance that will pivot out of Utah this morning and into Colorado this afternoon/evening. The disturbance will provide the support for more widespread showers/thunderstorms today across the High Country, as well as another day of scattered thunderstorms across eastern Colorado.

Across eastern Colorado, the concern for strong/severe storms exists again today, mainly across the Northeast Plains, Palmer Ridge, and northern/eastern extents of the Southeast Plains. The Urban Corridor has a lesser chance of getting in on the strong/severe storms, but the potential extends to those regions, as well. Generally speaking, the best coverage of showers/storms will be along/near the higher terrain, as well as over the preferred terrain of the Palmer Ridge, Cheyenne Ridge, and Raton Ridge, while the strongest storms will be further east where instability/moisture/wind shear will be best.

WV_20160629

From the Front Range/Southeast Mountains and westward, scattered-to-widespread showers and thunderstorms are expected this afternoon and evening, with a few showers lingering into the overnight hours. The main threats from these storms will be gusty winds and lightning, but locally heavy rainfall is a concern over southwestern Colorado where moisture will be best, thus the issuance of the low flood threat.

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:

Scattered showers and thunderstorms are expected to begin this afternoon, continuing into the evening and overnight hours. Ingredients are favorable for the production of isolated strong/severe thunderstorms during the afternoon hours, with the main threats being hail and strong winds, as well as locally heavy rainfall.

The main event looks to come after 8 PM tonight in the form of a thunderstorm complex rolling in from WY/NE and into northeast Colorado. Much like yesterday’s strong storms, this complex will follow much of the same path, producing large hail, strong/damaging winds and heavy rain.

Timing: 1 PM – 1 AM, with lingering showers/thunderstorms along/south of the Palmer Ridge into the early morning hours

Front Range and Southeast Mountains:

Scattered showers/thunderstorms are expected, beginning around lunchtime and continuing into the evening and nighttime hours. Isolated showers/weak thunderstorms will continue into the early morning hours. The Front Range will have the potential to see one or two strong/severe storms this afternoon/evening, producing strong winds, small hail, and dangerous lightning. Locally heavy rainfall is also a threat, thus the inclusion of the Front Range in the low flood threat. Maximum rain rates will be 1.0-1.6 inches/hour for the Front Range, and 0.6-0.8 inches/hour for the Southeast Mountains.

Timing: 11 AM – 10 PM, with a few lingering into the early morning

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

Scattered-to-widespread showers/thunderstorms are expected, beginning around 11 AM and continuing into the evening hours. Overnight, showers/weak thunderstorms will become more isolated, but a few will linger into tomorrow morning. Generally speaking, the main threats from thunderstorms will be gusty winds, dangerous lightning, and small hail, but locally heavy rainfall is possible across southwestern Colorado. Maximum rain rates are as follows:

Northwest Slope, Northern Mountains, and Central Mountains: 0.4-0.8 inches/hour
San Luis Valley: 0.6-1.0 inches/hour
Grand Valley: 0.8-1.2 inches/hour
Southwest Slope and San Juan Mountains: 1.0-1.6 inches/hour

Timing: 11 AM – 10 PM, with isolated showers/weak thunderstorms continuing into tomorrow morning