FTB 07-03-2015: DRIER AIR PUSHING IN FROM THE NORTH, BUT HEAVY RAIN STILL POSSIBLE TODAY

Issue Date: 7/3/2015
Issue Time: 9:36 AM

LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, CENTRAL MOUNTAINS, SAN JUAN MOUNTAINS, SOUTHEAST MOUNTAINS, PALMER RIDGE, RATON RIDGE AND SOUTHEAST PLAINS.

With an introduction that seems like a broken record, the upper-level ridge over the western US remains entrenched, however subtle changes are underway. The most notabe change is the elongation of the high pressure circulation (shown with the blue arrows) as a result of the flattening of the ridge. This is allowing for drier air to slowly work in from the north (circled in orange), which will push the best moisture southward. In short, this means the heaviest rainfall will occur with storms south of I-70 today. However, with sufficient instability in place this afternoon as daytime heating warms the surface, isolated strong thunderstorms cannot be ruled out north of I-70 along the Front Range foothills and Urban Corridor.

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The main threat from heavy rain will exist across southern portions of the Front Range and Urban Corridor, as well as the San Juan Mountains, Southeast Mountains, Raton Ridge, and Southeast Plains. The best moisture will reside across these locations, with precipitable water values hanging near, or just above, 1 inch this afternoon and evening. Overnight, there will be another weak, mid-level disturbance rotate across eastern Colorado, providing widely scattered showers and thunderstorms into the early morning hours.

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.

Flood Threat Legend

Zone-Specific Forecasts

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

Scattered showers and thunderstorms expected, with high temperatures a few degrees warmer than yesterday. A few strong-to-severe storms will be possible, as instability and wind shear remain favorable for a few supercells capable of large hail (1+ inch) and strong winds. Brief heavy rainfall will also be a threat with the stronger storms, but storm motions should be quick enough to mitigate potential flooding impacts. With that said, the maximum rain rates are as follows:

Northeast Plains: 0.6-1.2 inches/hour
Front Range and Urban Corridor, north of I-70: 0.8-1.2 inches/hour
Front Range and Urban Corridor, along and south of I-70: 1.0-2.0 inches/hour
Palmer Ridge, Southeast Plains, and Raton Ridge: 1.5-2.5 inches/hour

Timing: 11 AM – 10 PM, with a few lingering overnight with the passage of a weak, mid-level disturbance

Northern Mountains, Northwest Slope, and Grand Valley:

Isolated thunderstorms, mainly over the higher terrain. A few will move over adjacent valleys thanks to southerly storm motions. Maximum rain rates will be 0.4-0.6 inches/hour, with the main threats from thunderstorms being lightning and strong outflow winds.

Timing: 11 AM – 7 PM

Central Mountains, San Juan Mountains, San Luis Valley, Southeast Mountains, Southwest Slope:

Scattered showers and thunderstorms expected across these regions, mainly over the higher terrain. Adjacent low elevations/valleys will see a few thunderstorms move overhead due to southerly storm motions. Moisture will remain deepest over these southern regions, so brief heavy rainfall will be a threat, as well as hail (0.5-1.0 inch), lightning, and gusty winds. Storm motions will help to mitigate the flooding threat. Maximum rain rates are as follows:

Central Mountains: 0.6-1.2 inches/hour
Southeast Mountains: 1.5-2.5 inches/hour
San Juan Mountains: 1.2-1.8 inches/hour
San Luis Valley: 0.6-1.0 inches/hour
Southwest Slope: 0.4-0.8 inches/hour

Timing: 11 AM – 10 PM, with a few lingering into the early morning hours over the San Juan Mountains and Southwest Slope.