FTB 05-29-2015: Upper-Level Low + Cool Front Bringing Low Flood Threat

Issue Date: 5/29/2015
Issue Time: 10:32 AM

A LOW FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, SOUTHEAST PLAINS, SOUTHEAST MOUNTAINS, AND RATON RIDGE. A LOW FLOOD THREAT CONTINUES FOR A PORTION OF THE ARKANSAS RIVER.

As is the case on most days, the water vapor imagery paints a good picture for today’s forecast. It doesn’t tell the whole story, but it is a good place to start. A relatively weak upper-level trough will move to the east-southeast across the state today, providing support for another day of isolated-to-scattered showers and thunderstorms. West of the trough axis (black line), the large-scale environment will generally be unsupportive of thunderstorm development; however, a bit of moisture embedded in the northwesterly flow (red arrow) will combine with orographic support for isolated-to-scattered showers and thunderstorms west of the Continental Divide. The number of storms will be noticeably less than yesterday west of the Divide, especially for southern areas. The most activity will occur across the Northern and Central Mountains.

05292015_WV

East of the Continental Divide, another factor will be in play – a passing cool front. This cool front will do a few things:

1.  Provide upslope flow after passage, reinforcing moisture against the mountains, albeit a small net increase over yesterday
2.  Provide a forcing mechanism for thunderstorms to develop along the front
3.  The wind shear due to easterly flow behind the front with northwesterly flow aloft will support a few strong-to-severe thunderstorms, mainly across the Palmer Ridge, southern reaches of the Urban Corridor, Southeast Plains, Raton Ridge, and Southeast Mountains. The probability of occurrence will increase the further southeast you go.

Due to these factors at play, scattered showers and thunderstorms are expected east of the Continental Divide, with heavy rain, hail, and gusty winds being the main threats. For more details on timing and maximum rain rates, be sure to jump down to the Zone-Specific forecasts.

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.
FTB_snapshot_20150530

Zone-Specific Forecasts

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

Scattered showers and thunderstorms are expected today, and a few will be strong-to-severe. Rain rates will generally be in the 0.5-0.8 inches/hour range, but the strongest storms will produce rainfall at rates of 1.2-2.0 inches/hour. The likeliest location for heavy rainfall at that rate will be across the Southeast Plains and Raton Ridge, but strong storms across the Urban Corridor and Southeast Mountains will push rain rates near 1.0-1.5 inches/hour. Burn scars like Waldo Canyon and Black Forest will need to be watched, as well as areas that are still saturated.

Timing: 11 AM – 9 PM, a few thunderstorms continuing over the Southeast Plains until midnight. Storms will develop from north-to-south with the passage of the cool front, and will diminish from north-to-south this evening.

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

Considerably fewer showers and thunderstorms are expected today, with the most activity occurring across the Northern and Central Mountains. Instability will be fairly low, so any showers/thunderstorms will rely on orographic influences to develop/maintain themselves. This means that the higher terrain will see the majority of all precipitation, with lower valleys scraping by on the remainders as the storms move off of the mountains. Above 10,000 feet, it is likely that snow will fall, producing light accumulations mainly across the Northern and Central Mountains. Don’t expect heavy rain to cause any problems, with storms only being able to muster maximum rain rates of 0.2-0.4 inches/hour.

Timing: 11 Am – 8 PM, diminishing quickly thereafter.