FTB 09-27-2021: Isolated Heavy Rainfall Threat Returns To Southwest Colorado

Issue Date: Monday, September 27th, 2021
Issue Time: 10:18AM MDT

— A LOW flood threat has been posted for parts of the San Juan Mountains

Although cut-off lows usually require meteorologists to carry extra ibuprofen, we still secretly like them. This morning’s visible satellite image, below, shows a distinct cut-off circulation over western New Mexico responsible for ongoing showers and weak thunderstorms over a large swath of the Four Corners region. The low has moved slowly eastward since yesterday morning, has deepened a bit and will continue to do so as it begins to feel the synoptic scale lift from a strong incoming trough further to the northwest. Notably colder temperatures aloft have now moved into New Mexico and will enter southwest Colorado today. Yesterday’s QPE up to 0.50-0.75 inches over the San Juan Mountains carried max hourly rain rates of up to a rather impressive 0.5 inches with very limited instability and moisture. In addition, isolated heavy rainfall, near 1.5 inch/hour, was observed over central AZ higher terrain yesterday. Thus, we expect that today’s rainfall intensity will very likely be higher over southwest Colorado. This morning’s Precipitable Water (PW) was 0.61 inches at Grand Junction and 0.48 at Denver. A PW of 0.78 inches was measured at both Albuquerque and Flagstaff, and boundary layer moisture advection will be from the southwest today. Thus, we expect PW up to 0.7-0.85 in the far southwestern portion of the state. With the aforementioned cooling aloft, instability up to 1,000 J/kg will be possible this afternoon especially over areas that see some breaks in cloud cover as is expected. Steering winds will be modest, in the 20-25mph range and will be pushing storms towards the west.

Translating everything into sensible weather, with the strengthening cut-off low, we expect several rounds of showers and thunderstorms over southwest Colorado today. During the afternoon and early evening, there will be a chance for heavy rainfall over the San Juan Mountains roughly south of a South Fork-Silverton line. A LOW flood threat has been posted for this region.

Elsewhere, expect one more day of significantly above normal temperatures before cooling overtakes the state starting tomorrow and especially Wednesday.

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:

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

Mostly cloudy with numerous showers and thunderstorms developing later this morning and persisting into the late evening hours. Max 1-hour rainfall up to 1.1 inches, with max 3-hour rainfall up to 1.5 inches, warranting a LOW flood threat for parts of the region. Isolated flash flooding, mud flows and debris slides will be possible especially over steeper terrain. Gusty winds and hail up to 1 inch could also accompany the very strongest storms.

Primetime: 2PM through 11PM

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

A few clouds this afternoon, along with continued very warm temperatures up to 12F above normal. An isolated shower cannot be ruled out over southern areas later this evening and into the overnight hours. However, flooding is NOT expected today.

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

Partly cloudy with temperature slightly above seasonal normal. An isolated shower or weak storm cannot be ruled out this afternoon and evening over far southern areas, but flooding is NOT expected today.