FTB 09-14-2017: Upper-Level Dynamics will Produce Widespread Rainfall for Western Colorado

Issue Date: Wednesday, September 14, 2017
Issue Time: 10:30 AM MDT

LOW flood threat for Grand Valley, Central Mountains, Southwest Slope, San Juan Mountains

Finally some changes to the weather pattern as what’s left of Irma moves off to the NE. This will allow the persistent ridging pattern to be pushed eastward as the low pressure system off the coast of California ejects across the plains. This morning, we start with some higher clouds over the mountains and northwest corner as seen in the visible satellite image below. The approaching weather system can be seen over southwest Utah, which is expected to track NE throughout the day. Currently, PW water values at Grand Junction and Denver are at 0.72 inches. Low-level moisture is expected to increase over the western portion of the state as the disturbance moves into the area this afternoon. A slight increase in PW over the eastern portion of the state is expected later this afternoon and evening. With the arrival of the mid-level energy over western Colorado around the same time as the storms initiate over the mountains due to the daily heating pattern, shower and thunderstrom coverage is forecasted to be very widespread.

Shear profiles this afternoon are also impressive over the SW corner of the state. With moderate instability building over this same region, some isolated severe thunderstorms are possible. Threats include gusty outflow winds, brief, heavy rainfall and hail up to 1 inch in diameter. The approaching upper-level dynamics will support multiple rounds of showers and thunderstorms this afternoon, so a Low flood threat has been issued. Isolated 24-hour rainfall amounts up to 2 inches are possible. Threats include mud flows, debris slides, small stream and road flooding.

Storms are also expected to be more numerous over the Front Range and Palmer Ridge today. With stronger SW steering winds aloft and increasing moisture, storms that form over the higher terrains are expected to move into the Northeast Plains later this afternoon and evening. Low-level moisture will still be lacking, so expect strong outflow winds with these storms. Faster steering winds will also limit flooding potential, but max 1-hr rain rates up to 0.8 inches are possible further east. Tonight, rain over the western portion of the state is expected to end around midnight. A couple hours later, showers over the Northeast Plains will conclude. Some light showers over the mountains are possible through 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, scroll below the map.

Flood Threat Legend

Zone-Specific Forecasts:

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

Sunny this morning becoming partly cloudy this afternoon. Shower and thunderstorm initiation over the mountains will be greatest over the Front Range and Palmer Ridge. These storms are expected to move over the Northeast Plains with SW steering winds aloft. Storms will likely produce gusty winds and max 1-hr rain rates up to 0.8 inches are possible further east. Flooding is not expected.

Primetime: 2PM to 2AM

Northwest Slope, Grand Valley, Southwest Slope, Northern Mountains, Central Mountains, San Luis Valley, San Juan Mountains:

Partly cloudy over the northwest corner this morning with increasing clouds expected throughout the afternoon. Multiple rounds of widespread showers are likely this afternoon with the approaching weather system, especially further south. Some of these storms over the southwest corner may become severe. Threats include gusty winds, brief, heavy rainfall and hail up to 1 inch in diameter. A Low flood threat has been issued for the 24-hour rainfall amounts. Over the San Juan/Central Mountains and Southwest Slope, isolated 24-hour amounts up to 2 inches are possible. Showers and thunderstorms should come to an end around midnight, but a few light showers may linger over the southern high terrains through the night.

Primetime: 12PM to 12AM