FTB 06-29-2015: Another Day of Isolated-to-Scattered Showers and Thunderstorms

Issue Date: 6/29/2015
Issue Time: 10:22 AM


The forecast has been fairly consistent over the last few days as the upper-level ridge continues to be anchored across the Western US, centered over Utah. This is has left Colorado under the influence of northerly flow aloft, creating mean storm motions from north to south, and leaving the bulk of shower/thunderstorm activity over the High Country. Moisture has not only remained, but maintained, near-average to above-average, and today is no different; precipitable water is above average for this date, running between 0.75 and 1.0 inches at the normal four reporting stations shown in the graph below (Boulder – blue, Grand Junction – green, Pueblo – pink, and Shriever AFB – red).


Scattered showers and thunderstorms are expected across the High Country, following the diurnal pattern of beginning around lunchtime, maximizing coverage during the late afternoon and early evening, before diminishing just after sunset as daytime heating ends and radiational cooling begins. Just as yesterday, a few isolated thunderstorms are expected over the adjacent lower elevations of the Urban Corridor, Palmer Ridge, Northeast Plains, and Southeast Plains, with the relative best chances occurring near the preferred terrain of the Front Range, Palmer Ridge, and Southeast Mountains. Adding an additional complexity to today’s forecast is a weak surface boundary that will scrape across the far Northeast and Southeast Plains, providing support for isolated afternoon/evening thunderstorms. Primary concerns with most thunderstorm activity will be gusty outflow winds, lightning, small hail, and brief moderate-to-heavy rain. For more specifics regarding rain rates, check out the zone-specific forecasts below.

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

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

Mostly sunny and hot will be the main story today, as high temperatures remain in the mid-80s to low-90s across the regions. Isolated thunderstorms are expected this afternoon and evening, thanks to the conditions outlined above. The main threats will be lightning, strong outflow winds, hail (up to 1.5 inches in diameter), and brief moderate-to-heavy rain. Rain rates will typically be 0.4-0.8 inches/hour, but a couple strong/marginally severe storms could produce rain up to 0.9-1.3 inches/hour.

Timing: Noon – 10 PM

Front Range and Northern Mountains:

Isolated showers and thunderstorms expected, with the main threats being gusty outflow winds, small hail, and lightning. The Front Range south of I-70 will hold the best chance for bouts with brief heavy rain, with rain rates in the 0.5-0.8 inches/hour range. With most of the moisture confined to the mid-levels, and relatively shallow low-level moisture, dry air beneath the cloud base will eat away at precipitation before reaching the surface. However, an outflow boundary from another storm interacting with a thunderstorm over the southern Front Range could push maximum rain rates to 0.8-1.2 inches/hour. This is a relatively low probability, but cannot be ruled out, thus the inclusion in the outlined Low Flood Threat area.

Timing: Noon – 8 PM

Northwest Slope and Grand Valley:

Mostly sunny and hot will be the main story of the day, with an isolated thunderstorm or two possible over the higher terrain near the Central Mountain and Southwest Slope regions. The main threats from any isolated thunderstorms will be lightning and gusty outflow winds, as rain rates will stay below 0.25 inches/hour.

Timing: Noon – 6 PM

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

Scattered showers and thunderstorms expected, mainly over the higher terrain. With mean storm motions from north to south, activity will move over lower elevations and valleys with time. Rain rates will typically be in the 0.3-0.6 inches/hour range, but brief heavy rain over the San Juan Mountains (0.9-1.3 inches/hour) and Central Mountains (0.8-1.2 inches/hour) is possible. The main threats from thunderstorms will be gusty outflow winds, lightning, and small hail. There have been numerous reported injuries in the mountains from lightning over the last few weeks, so please be aware of changing weather conditions and seek shelter, if necessary.

Timing: 11 AM – 9 PM