FTB 08-21-2017: Rainfall Coverage Continues to Increase

Issue Date: Monday, August 21, 2017
Issue Time: 10:45AM MDT

LOW flood threat for portions of the Front Range, Urban Corridor, Palmer Ridge, Southeast Mountains, Southeast Plains, Raton Ridge, Central Mountains, San Juan Mountains, Southwest Slope

Visible satellite shows mostly clear skies across the state except over the southern border and SE Plains. Mid and high-level cloudiness is strongest over the SE corner of the state and is associated with the overnight, northward movement of the disturbance in NM. There seems to be a break in the cloud over northern NM now. The hope is that these clear skies will move into the heaviest clouded region this morning and that some of the clouds will be able to partially break up for the eclipse. However, ongoing convection over NM may cause high clouds to move into the state and partially or fully obstruct of the eclipse.

The monsoon plume will continue to raise Precipitable Water (PW) values in Colorado. A mid-level Low pressure center is currently located over southern California while a weak surface High will set up over Texas today. A strong southerly component of the flow from the trough/ridge gradient will continue to advect a plume of elevated moisture over the state. PW at Denver and Grand Junction this morning are 0.67 and 0.72 inches, respectfully. PW is expected to increase to 1.2 inches over the eastern plains this afternoon. Mid-level drying will be strongest over the NW corner of the state. Paired with lower PW values, thunderstorms west of the Continental Divide should be confined to south Central Mountains, San Juan Mountains and southern Southwest Slope.

Storms will begin to fire over the mountains today just after noon. East of the Continental Divide, storms should be more numerous over the higher terrain and the Palmer Ridge when compared to yesterday. With the upper-level ridge over the Pacific Northwest, there will be weak mid-level steering winds. This will cause storms that form to slowly drift west over the SW corner of the state and drift E/SE over the eastern mountains and immediate adjacent plains. The slow moving storms and increased moisture will cause local, heavy rainfall. Burn scar areas will be most at risk today for mud flows, debris slides and small stream flooding. Road flooding and field ponding is also possible over the adjacent plains. There is a low flood threat for today.

This afternoon, a convergence boundary will also help storms form over the eastern plains. With higher instability, stronger storms may produce hail up to 0.75 inches and strong winds. Although the high moisture environment will help produce heavy precipitation, should not produce 1-hour rain fall rates greater than 1.5 inches. Thus, there is no flood threat for the eastern plains. Activity is expected to end around midnight, but some lingering showers over the southern mountains is possible overnight.

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, scroll below the map.

Flood Threat Legend

Zone-Specific Forecasts:

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

Most sunny this morning become partly cloudy this afternoon. An increase in thunderstorm activity is expected over the SE Mountains and Front Range. Threats include brief, heavy precipitation, gusty winds and small hail. Storms that form near burn scars need to be monitored closely. 1-hour rain rates will be up to 1.5 inches (east) and 1.2 inches (west). A Low flood threat has been issued.

Primetime: 12PM to 12AM

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

Mostly sunny with showers over the Central Mountains and SW corner of the state. Stronger storms may produce 1-hour rain rates up to 1.1 inches with storm totals up to 1.5 inches. Threats include mud flows, debris slides and road and small stream flooding. A low flood threat has been issued.

Primetime: 12PM to 11PM