Issue Date: 7/4/2015
Issue Time: 9:46 AM
A MODERATE FLOOD THREAT IS IN PLACE FOR PORTIONS OF THE SAN JUAN MOUNTAINS. A LOW FLOOD THREAT IS IN PLACE FOR PORTIONS OF THE SOUTHWEST SLOPE, SAN JUAN MOUNTAINS, CENTRAL MOUNTAINS, NORTHWEST SLOPE, GRAND VALLEY, SOUTHEAST MOUNTAINS, AND RATON RIDGE.
Changes are afoot today as the upper-level ridge shifts to the east, which will result in two main consequences: 1) Temperatures at the surface and aloft will rise a few degrees over yesterday, especially east of the Divide, and 2) West-northwest flow aloft will persist through the afternoon across western Colorado, but moist, southerly flow will return overnight and into tomorrow morning. So, what does this mean for our flood threat forecast?
First, the rising temperatures at the surface and aloft across Eastern Colorado will tend to suppress thunderstorms, resulting in more isolated coverage. However, there is still sufficient moisture remaining under the ridge, so strong surface heating will still result in thunderstorms developing first over the mountains and foothills (between 11 AM and Noon), moving east over the plains during the afternoon and evening. Cloud bases will tend to be higher today than previous days, so the main threats east of the mountains will be lightning, strong outflow winds, and brief moderate rainfall. The exception to this, because of course there is an exception, will be across the far Northeast and Southeast Plains, where better low-level moisture and instability will reside east of a surface trough.
Second, west-northwest flow aloft will keep precipitable water values over western Colorado well above-normal, as there is still plenty of moisture available upstream. This will result in another afternoon and evening of scattered thunderstorms, mainly over the higher terrain and adjacent lower valleys. Lightning, strong outflow winds, small hail, and brief heavy rain will all be threats with thunderstorm activity this afternoon and evening. The best relative chance for heavy rain will be over southern regions, i.e., Southwest Slope and San Juan Mountains. As we head into the overnight hours and the upper-level ridge continues to slide east, moist, southwesterly flow will return to western Colorado, pushing precipitable water values at, or above, 1 inch for tomorrow. Far western Colorado, near the CO/UT border and 4-corners region, will hold the flood threat into tomorrow morning.
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.
Zone-Specific Forecasts
Front Range, Urban Corridor, Palmer Ridge, Northeast Plains, Southeast Plains, and Raton Ridge:
Isolated-to-widely scattered thunderstorms will be possible this afternoon and evening. Otherwise, mostly sunny skies and hot temperatures will be the main story today. Far eastern portions of the Northeast Plains and Southeast Plains will hold the opportunity for severe thunderstorms this afternoon and evening, as the best moisture, instability, and shear will reside along the developing surface trough. For other areas, storms will be higher based than the past few days, resulting in the main threats being lightning, gusty outflow winds, and brief moderate rainfall. Maximum rain rates will vary, and are as follows:
Northeast Plains, Southeast Plains, and Palmer Ridge: 1.5-2.5 inches/hour.
Front Range and Urban Corridor: 0.6-1.2 inches/hour
Raton Ridge: 0.8-1.2 inches/hour
Central Mountains, San Luis Valley, Southeast Mountains, Northern Mountains, and Northwest Slope:
Scattered thunderstorms expected for the Central Mountains, San Luis Valley, and Southeast Mountains, mainly over the higher terrain and adjacent valleys this afternoon and evening. There will be more coverage over the Northern Mountains and Northwest Slope than previous days, but will still be in the isolated category. During the day, lightning, small hail, gusty outflow winds, and brief heavy rain will be the main threats. Overnight, the concern will shift to more widespread coverage over the Northwest Slope, with thunderstorms producing efficient rainfall. Max rain rates are as follows:
Central Mountains, Southeast Mountains, Northern Mountains, and San Luis Valley: 0.5-1.0 inches/hour
Northwest Slope: 0.25-0.5 inches/hour during the day, and then 0.6-1.0 inches/hour overnight.
San Juan Mountains, Southwest Slope, and Grand Valley:
Scattered thunderstorms expected through this evening, mainly across the higher elevations and adjacent valleys. As the upper-level ridge shifts eastward, moisture values will climb to near record levels tomorrow morning. Even though daytime heating will not be able to help thunderstorms overnight and during the early morning hours, the support will be provided by a passing shortwave embedded in the southwest flow. More widespread coverage is expected, with thunderstorms producing efficient rainfall. Maximum rain rates will be in the 1.0-1.5 inches/hour during the afternoon and evening, shifting to 0.7-1.1 inches/hour overnight.
