FTB 05-22-2018: Widespread Thunderstorm and Shower Activity for Western Colorado and Mountainous Terrain

Issue Date: Tuesday, May 22, 2018
Issue Time: 9:10AM MDT

— Flooding is NOT expected today

Looking at the visible imagery below, we can see a lot of low clouds and fog over the majority of Colorado with much clearer skies over the Grand Valley and Northwest Slope. These clear skies will help build instability over this area for thunderstorm development this afternoon. The current fog is expected to burn off this morning as daytime heating occurs after sunrise. Today, the closed low over Nevada begins to move to the north/northeast and become an open wave. The release of shortwaves and upper support will aid in triggering widespread thunderstorms west of the Continental Divide this afternoon into this evening. Southwest flow, marked below, will pull in drier air over southwest Colorado, so the highest rainfall totals (west) will be limited to the northwest corner. Storm motion will be north/northwest at 20 knots, so this is another reason heavy rainfall over one area is not expected. Accompanying the stronger thunderstorms will be gusty winds, small hail and lightning.

East of the Continental Divide, thunderstorm development will start over the Southeast Mountains and spread north into the Front Range by the early afternoon. A few storms will roll off the Southeast Mountains to give the Southeast Plains some rainfall as well. However, the best moisture is confined to the CO/KS border, so don’t expect any flooding this afternoon but rather gusty winds and small hail. Over the northern Front Range, upslope flow will help pull in some decent moisture from the east. Storm motion will be more northerly, so storms will be confined to the west today and only affect the Urban Corridor. Highest moisture (dew points in the upper 40s) will occur over the northern Front Range/Urban Corridor, so expecting the highest totals for the day in this region. After sundown, thunderstorm activity will begin to die down with all activity ending around 10PM. Flooding is not expected today.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

Raton Ridge, Southeast Mountains, Southeast Plains, San Luis Valley, San Juan Mountains, Southwest Slope:

Storms will begin to fire over the higher terrains by noon (west) and 1PM (east). Best coverage will be over the eastern high terrains due to entrainment of dry air to the west. Max 1-hour rain rates over the higher terrains will be 0.15 inches/hour. Over the Raton Ridge and Southeast Plains, 1-hour max rain rates up to 0.75 inches are possible. Rain rates aren’t expected to meet flood criteria over the burn scars on the Southeast Mountains (<0.5 inches/hour). Thus, flooding is not expected today.

Primetime: 11AM to 9PM

Northwest Slope, Central Mountains, Grand Valley, Northern Mountains:

Widespread thunderstorms and showers expected this afternoon. Max 1-hour rain rates between 0.4 and 0.5 inches/hour are possible under the stronger storms over the Northwest Slope. Small hail and gusty winds (up to 50mph) are likely with all thunderstorms due to limited moisture. Flooding is not expected today.

Primetime: 1PM to 10PM

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

Isolated showers and thunderstorms are expected over the higher terrains this afternoon and may spill into the Urban Corridor. Heavier rainfall will be confined further north where 1-hour rain rates could be up to 0.7 inches/hour under the stronger storm cores. There is no flood threat today.

Primetime: 2PM to 10 PM