FTB 07-04-2020: Widespread Thunderstorms Return to the State for the 4th of July

Issue Date: Saturday, July 4th, 2020
Issue Time: 9:15AM MDT

— A MODERATE flood threat has been issued for the Spring Creek and Decker burn areas

— A LOW flood threat has been issued for the Front Range, Southeast Mountains, San Juan Mountains, portions of the Central Mountains, Palmer Ridge, Raton Ridge, Southeast Plains, and southern Urban Corridor

The moisture plume has nicely settled over the state, which can be seen by the widespread cloud cover in the satellite image below. This will set the stage for more widespread thunderstorm activity statewide on this 4th of July with action kicking off a little earlier than usual. PW has increased to 0.78 inches over Grand Junction, and although the lower 3km of the atmosphere is still a little dry, the moisture present throughout the upper layers of the atmosphere should allow storms to produce some measurable rainfall over the western mountains this afternoon. High dew points are present east of the green dashed line below, and PW was measured at 0.76 inches in Denver. There will be some mixing out of this low-level moisture over the ridges, mountains, and northern Urban Corridor this afternoon, but easterly and southeasterly surface flow is expected to help keep the higher dew points intact over the eastern plains.

No real distinct shortwaves in the flow, and the earlier onset of storms over the mountains may limit the amount of instability that can build. This should lower the severe thunderstorm threat back west, although hail between 0.75 and 1 inch may still be possible under a storm or two over the southern I-25 corridor. As for the heavy rainfall threat, slow, counterclockwise motion around the High will allow storms to drop moderate to heavy rainfall today. Storms will be more pulse-like in nature over the mountains and immediate adjacent plains this afternoon, but lower rain rates over the mountains should allow more gradual, wetting rainfall. However, since it is the holiday weekend (increased outdoor recreation) and storms will sit for a while, a Low flood threat has been extended back west as storms will still be capable of producing 1-inch rain totals over an hour or two. A Moderate flood threat has been issued for the Spring Creek and Decker burn areas due to higher probability of a storm forming directly over a scar today, which could cause flash flooding of local streams, mud flows and debris slides. Dangerous cloud to ground lightning is also expected from the afternoon storms, so be sure to plan your hikes accordingly.

There will be an increasing chance for storms to become severe thunderstorms as they move into the eastern plains. Higher instability out east, once again, will allow the severe storms to produce large hail (1.5” to 2”) and damaging outflow winds (up to 60 mph) on top of heavy rainfall as dew points will be in the upper 50Fs to 60Fs. A Low flood threat has been issued for the Southeast Plains/Palmer Ridge, and 1-hour rain rates up to 1.75 inches will be possible.

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:

Southeast Mountains, Front Range, San Juan Mountains, & Central Mountains:

Storms are expected to be more widespread and kick off just before noon today. Dangerous cloud to ground lightning is expected, so plan your outdoor recreation carefully. Max 1-2 hour rain rates up to 1 inch will be possible, which could cause local flooding issues – especially if you are camped near water. There is also a chance that a storm will park itself over a burn area, which could cause mud flow and debris slides on top of high water in local streams/rivers. A Low flood threat has been issued through about 9PM with Moderate flood threats issued for the Spring Creek and Decker burn areas.

Primetime: 11:30AM to 9PM

Southeast Plains, Northeast Plains, Palmer Ridge, Raton Ridge, & Urban Corridor:

As storms move off the mountains by early afternoon, they will encounter better instability along and south of the Palmer Ridge as well as over the eastern plains. A few severe storms will be possible over the eastern plains with the main threats being heavy rainfall, large hail and damaging winds. Smaller hail (up to 1 inch) will be possible along the southern half of the I-25 corridor under a couple of the storms. Slow steering winds and high moisture will allow moderate to heavy rainfall to drop over all or portions these regions, so a Low flood threat has been issued for this reason. Flood threats include arroyo flooding, small stream flooding, road flooding, flooding of low-lying areas, and field ponding. Use caution when driving at fast speeds as hydroplaning was reported in Kit Carson yesterday on I-70. Storms should come to an end just after midnight.

Primetime: 1:30PM to 2AM

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

Moisture isn’t quite as high over the Northern Mountains and Northwest Slope, but a few weak storms will still be possible along the elevated regions (mountains, Roan Plateau, etc). Expect more gusty outflow winds (45 mph range) than wetting rainfall. Some very light rainfall will also be possible over the valleys as storm move off the high terrains this afternoon. Again, more wind than wetting rain is expected. There is also the chance for some light rainfall over the southern San Luis Valley due to the proximity of the moisture plume and some weak mid-level energy rotating around the high. Totals are expected to remain under 0.10 inches. Flooding is not forecast.