FTB 09-27-2017: Widespread Showers for Colorado as Upper Level Low Increases Moisture and Lift

Issue Date: Wednesday, September 27, 2017
Issue Time: 10:45 AM MDT

LOW flood threat for the Hayden Pass and Junkins burn scars

As expected, the upper level Low over AZ has drawn in a lot of low level moisture over Colorado as seen in the water vapor (WV) imagery below. The increase in low level moisture allowed showers and snow (at the higher elevations) to persist overnight and into the morning along the Front Range and Southeast Mountains. There is also an overall increase in cloud cover across the eastern portion of the state and the mountains, which kept low temperatures overnight warmer than normal. Precipitable Water (PW) this morning at Denver was measured at 0.73 inches, and in Grand Junction it was 0.4 inches. With southwest surface winds becoming more southerly throughout the day, moisture will continue to increase with the highest moisture expected over the southern portion of the state.

The upper level low is expected to slowly track north today and will be positioned over Utah by tonight. As the low tracks north, increased shortwave activity and upper air support from the jet stream will bring multiple rounds of showers and snow. Snow should be confined to elevations above 11,000 feet today, but as temperatures decrease tonight, the snow line is expected to drop to 10,000 feet. The initial wave of showers occurring now will be confined to the higher terrains, but by this afternoon, showers will increase in intensity and spread into the adjacent plains and valleys. Thunderstorm activity is expected to be marginal and more isolated due to cloud cover limiting instability.

The highest accumulations for the next 24-hour period are expected to be over the Southeast/San Juan Mountains and southern Central/Front Range Mountains. Over the highest peaks, greater than 6 inches of snow is possible, and a Winter Weather Advisory has been issued for elevations about 11,000 feet. With limited instability, max 1-hour rain rates are expected to be under 1 inch today. However, 3-hour rain rates up to 0.9 inches are possible. There is enough confidence that multiple areas over the Southeast Mountains will have 24-hour totals exceeding 2 inches, so a Low Flood Threat has been issued for the Hayden Pass and Junkins burn scars. Threats include road and small stream flooding, mud flows and debris slides. Shower activity is expected to decrease after midnight over the San Juan Mountains, but rain will continue into the morning over the other southern mountains.

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:

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

Cloudy this morning with showers over the high terrains. More widespread showers expected this afternoon. Showers are also expected to increase in intensity throughout the day. Max 1-hr rainfall rates up to 0.6 inches and max 3-hour rainfall rates up to 0.9 inches possible. 24-hour amounts up to 2.75 inches are possible over the Southeast and San Juan Mountains. Elevations greater than 11,000 feet will likely see all snow all day with the snow line dropping to 10,000 feet overnight. Snow totals greater than 6 inches are likely at the higher elevations. A Low flood threat has been issued for the Hayden Pass and Junkins burn scars for sufficient confidence 24-hour totals will exceed 2 inches over multiple areas. Threats include mud flows, debris slides and small stream and road flooding.

Primetime: 11AM to 7AM

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

Cloudy this morning over the high terrains and east of the Continental Divide. Currently, showers are confined to the higher terrains, but this afternoon are expected to increase in intensity and spread into the adjacent plains and valleys. Over the higher terrains, max 3-hour rain rates up to 0.8 inches with 24-hour totals up to 1.75 inches (south) and up to 1 inch (north) are possible. 24-hour amounts up to 0.8 inches are possible over the adjacent plains and valleys further south.

Primetime: 12PM to 12AM