FTB 09-11-2021: Scattered Storms Return to the Forecast, Minimal Rainfall Expected

Issue Date: Saturday, September 11th, 2021
Issue Time: 9:15AM MDT

— Flooding is NOT expected today

Isolated to scattered storms return to the forecast today as subtropical moisture and mid-level energy move across the state. The ridge, that has been bringing us dry and hot weather the last few days, will start to break down over the next 24-hours from incoming shortwaves associated with a trough to our north. Even with the breakdown of the ridge beginning, another day of hot temperatures are forecast statewide. Currently, south/southwest flow aloft around the High has brought some subtropical moisture northward into the state (blue in the water vapor image below). Looking at this mornings’ soundings, PW has risen to 0.68 inches and 0.53 inches in Grand Junction and Denver, respectively. This should increase a little more throughout the day (west), and it is expected to combine with mid-level energy and the diurnal flow to produce scattered storms mostly along and north of I-70.

Despite the return of storms and rainfall, there will be two limiting factors to the flood threat today. The first is a very dry boundary layer, which will cause storms to produce more wind than rainfall. The second factor will be very fast steering flows (20-30 mph), which should keep storms from accumulating meaningful rainfall over any given area. Therefore, flooding is NOT expected. Dry thunderstorms are also a concern this afternoon and evening, which will increase in fire danger with lightning and gusty outflow winds forecast. It has gotten dry over the last few weeks, especially with the very hot temperatures we saw this last week. A Red Flag Warning has been issued for the Northwest Slope and Northern Mountains from noon to 9PM today.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Grand Valley, Northwest Slope, Northern Mountains, Front Range, Central Mountains, Northeast Plains, Urban Corridor & Palmer Ridge:

Best chance for storms today will be along and north of I-70. Some isolated, high-based showers may be possible just south of this area along and near the Continential Divide and Palmer Ridge. For the most part, storms will be quick moving and produce little rainfall/plenty of outflow winds as they travel to the northwest. Training storms over the Northwest Slope and Northern Mountains will help produce slightly higher accumulations, and isolated storm totals up to 0.40 inches will be possible by morning. Elsewhere, max 1-hour rain rates should remain around or below 0.25 inches for a brief period. A Red Flag Warning has been issued for the Northwest Slope and the Northern Mountains where dry thunderstorms are expected to produce lightning and strong outflow winds from noon to 9PM. Some linger lighter showers may be possible over the northwest corner of the state tonight. Flooding is NOT expected today.
Primetime: 2PM to Midnight

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

A couple high based storms may develop over the Raton Ridge area this afternoon, but more cloud cover than rainfall is expected. Elsewhere, dry conditions are forecast with another round of hot temperatures expected with the center of the High near the area. Expect daily record highs to be broken once again.