FTB 05-22-2015: Sun To Stage Comeback, But Result Is More Storms

Issue Date: 5/22/2015
Issue Time: 10:50 AM

— LOW flood threat for South Platte River from Greeley through the Nebraska state line
— LOW flood threat for Arkansas River from Pueblo through John Martin Reservoir
— LOW flood threat for Palmer Ridge and parts of Northeast Plains

The “spring of the cut-off lows” continues across the Southwest U.S. The water vapor imagery shows yet another cut-off low currently positioned off the California coast. This low pressure system is being supported by not one, but too different subtropical jets. The maximum wind speeds in both jets, about 35,000 feet off the ground is more than 100mph. For Colorado, this implies active weather not only today, but all the way through Sunday. Hikers and campers should closely monitor weather conditions and National Weather Service forecasts this weekend.


Closer to home, a strong disturbance (black dashed line) is passing overhead this morning, and is currently about two-thirds of the way across the state on its way northeastward. This disturbance is supporting general light rain in the northeast, but is quickly exiting the state. In its wake is a sharp line of clearing, meaning that sunshine will break out for most everyone shortly after noon today. However, with that sunshine will come a quick heating of the ground, leading to instability as the saturated ground loses water vapor to the atmosphere through evaporation. Isolated to scattered thunderstorm activity will break out over mostly the northern half of the state by early afternoon.

While most thunderstorms will be rather modest, producing light to moderate rainfall, the Northeast Plains will play host to stronger storms by early afternoon. Some of these stronger storms will be capable of producing very heavy rainfall as they will be working with a large amount of moisture. Precipitable water values are expected to stay in the 0.7-0.9 inch range, but cloud warm layer depths (the portion of a cloud that is above 32F) will be very deep. The forecasted warm layer depth for 3pm is shown below. This results in very efficient rainfall formation, and will support isolated rain rates up to 1.7 inches per hour. Given the saturated soil conditions, this warrants a Low flood threat for parts of the Northeast Plains. Additionally, some storms will be capable of producing large hail up to 1.75 inches.


Lastly, parts of the Arkansas and South Platte Rivers remain elevated. Low flood threats are maintained, but we expect the Arkansas river to subside as warmer and drier temperature evaporate substantial moisture from the ground this afternoon.

Please see below for area-specific flood threat and forecasts (click on “About the FTB” link above to view Colorado region names).

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, Front Range, Northeast Plains and Palmer Ridge:

Showers quickly ending, followed by brief sunshine, followed by scattered thunderstorms by early afternoon. Most thunderstorm rainfall will be 0.7 inches/hour or less, however, along the Palmer Ridge and into the Northeast Plains, isolates storms may produce rainfall up to 1.7 inches per hour. Several storms may pass over a given location, and 3-hour rainfall up to 2.5 inches is possible. A Low flood threat has been posted.

Southeast Mountains, Raton Ridge and Southeast Plains:

Sunshine and much warmer today. Isolated thunderstorms will be possible in the afternoon. However, rainfall rates will remain below 0.7 inches/hour. A low flood threat remains in place for the Arkansas River, but the river should fall below flood stage by later this evening.

Central Mountains, San Juan Mountains and Northern Mountains:

Partly cloudy with scattered rain and snow showers developing by the early afternoon. Hourly rainfall rates will remain below 0.5 inches. No flooding is expected.

Southwest Slope, Northwest Slope, Grand Valley and San Luis Valley:

Sunny to partly cloudy early, with isolated rain showers and a stray thunderstorm developing by early afternoon. Hourly rainfall rates will remain below 0.6 inches/hour and flooding is not expected.