FTB 05-30-2020: Heavy Rainfall Forecast for the Mountains & Urban Corridor

Issue Date: Saturday, May 30th, 2020
Issue Time: 10AM MDT

— A MODERATE flood threat has been issued for the Front Range, Urban Corridor, Palmer Ridge, Northeast Plains and 416 burn area

— A LOW flood threat has been issued for the Northern Mountains, Central Mountains, San Juan Mountains, Southeast Mountains, portions of the Northwest Slope. This includes the Spring Creek, Deckers, and Lake Christine burn areas.

Same general pattern as yesterday with a few minor differences, which will be pointed out in the water vapor imagery below. There looks to still be plenty of moisture trapped underneath the ridging pattern, which is defined by the yellow line. PW in the soundings this morning was measured at 0.80 inches and 0.75 inches at Grand Junction and Denver, respectively, which means there is actually a slight uptick since yesterday. In the image, you can also see a surface low over the eastern plains as indicated by the blue arrows (surface flow). This will help reinforce and slightly increase surface moisture on its eastern and northern side (northeast Colorado). Dew points are forecast in the 50Fs, which is pretty juicy for Colorado.

Today, the 500mb low off the coast of California will start to move northward up the coast. While this feature is well to our west, there will likely be some enhanced lift over the state out in front of it, which will help storms that fire over the mountains this afternoon be more widespread than yesterday. Additionally, as the low starts to move towards the Pacific Northwest, the axis of the ridge will start to shift eastward. This will place the state under southwest flow loft today. This should allow storms that form (from the southwest to northeast over the mountains) to push into the adjacent plains this afternoon favoring the Palmer Ridge and Urban Corridor for accumulations. With storm motion slower than yesterday, paired with increasing moisture and trailing/pulsing storms, the flood threat increases.

A Low flood threat for the majority of the mountain areas, which includes the aforementioned burn areas above. A Moderate flood threat has been issued for the Front Range, Urban Corridor and Palmer Ridge as rain rates will increase in the moisture rich environment along a tongue of convergence and instability. Isolated 24-hour totals over the San Juan Mountains will likely reach 1-inch with the multiple rounds of rainfall through tomorrow morning, so for caution, a Moderate flood threat has been issued for the 416 burn area. Threats over all burn areas include flash flooding, mud flows and debris slides.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Front Range, Urban Corridor, Palmer Ridge, & Northeast Plains:

Nearly stationary storms this afternoon will increase the chances for flooding over the Front Range. Other threats include severe hail and strong outflow gusts. Max 1-hour rain rates up to 1 inch will be possible with isolated storm totals up just over 1.50 inches by morning. This could cause local flooding issues such as mud flows, debris slides, field ponding and local stream flash flooding.

As storms move off the mountains, moisture increases, but the hail threat likely decreases somewhat. However, small hail will still be possible. With nearly stationary storms, very heavy, local rainfall is forecast. Guidance is indicating that the greatest threat for flooding will be from 3PM to 8PM. Max 1-hour rain rates up to 1.50 inches will be possible under the storm cores with isolated storm totals up to 1.90 inches possible. This would cause street flooding, field ponding and local stream/gulch flash flooding. Storms are expected to start dissipating around midnight.

Primetime: 12PM to 2AM

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

Although there was an inverted-V sounding west of the Divide (indicating strong winds with storms again), the lower atmosphere and multiple rounds of rainfall will increase totals for these regions as the boundary layer moistens. Isolated max 1-hour rain rates up to 0.60 inches will be possible with 24-hour totals up to 1-inch over the San Juan Mountains along with gusty outflow winds. Thus, a Low flood threat has been issued for the Deckers, Spring Creek, and Lake Christine burn areas. A Moderate Threat has been issued for the 416 burn area.

With 1-2 hour rain rates around 0.75 inches, a Low flood threat has been issued for other mountainous regions. Lower elevations over western Colorado and the San Luis Valley will likely get in on the rain action as well with instability rotating around the ridge. Totals between 0.15 and 0.25 inches are the most likely with strong outflow winds (40-50 mph).

Primetime: 12PM to 6AM

Raton Ridge & Southeast Plains:

Due to storm motions, not thinking there will be much rainfall threat over these areas today with the eastern Raton Ridge receiving the majority of the rainfall. Max 1-hour rain rates up to 0.50 inches will be possible. A couple overnight storms may be possible over the Southeast Plains, but lower moisture will reduce spatial coverage and totals. Isolated totals just under 0.50 inches by morning seem reasonable.

Primetime: 12:30PM to 3AM