FTB 09-27-2019: The Next Trough Begins to Dig South and Returns Rainfall Chances to Northern Colorado

Issue Date: Friday, September 27th, 2019
Issue Time: 9:25AM MDT

— Flooding is NOT expected today

Taking a look at the water vapor imagery below there are a couple of things happening today. First the cut off low, which is now an open wave, finally starts to move eastward and will be located over southern New Mexico by tonight. It’s a little bit too far south to push much mid-level energy or extra PW into the state, but it should help enhance storm coverage over the eastern San Juan Mountains this afternoon by providing a slight increase in moisture. Westerly flow aloft will scour out most of the moisture over the San Luis Valley, Southeast Mountains and adjacent eastern plains, but a weak shower or two be possible over the mountains with the main threat being wind due to the large spread in temperature and dew point.

To our north, the next trough starts to dig south over the west coast, which will push a shortwave into the state, which is currently over the Utah/Idaho border (orange “X” with arrow). As the next trough has been digging south, there has been a slight increase in moisture (that will continue) over the northern portion of the state. Denver measured 0.64 inches in this morning’s sounding, which is up 0.10 inches from this time yesterday. The shortwave is expected to help spark more widespread coverage of showers and weak thunderstorms over the Front Range and Northern Mountains this afternoon. The jet also began to move into northern Colorado this morning, so this will similarly help fuel storms this afternoon and will likely support weak showers over the high terrains near the Divide tonight. With fast westerly steering flows aloft, expecting the storms to push into the adjacent plains by late this afternoon. With low dew points (upper 30°Fs to 40°Fs), despite a cold front that pushed through overnight, gusty outflow winds will be the main threat from storms today with only moderate to light rainfall forecast. Flooding is not expected from storms today due to the fast storm motion and lack of low level moisture.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

Front Range, Urban Corridor, Palmer Ridge, Northern Mountains, Northeast Plains:

Fast storm motion and lack of low level moisture will keep storms at moderate to light rainfall accumulations with scattered coverage. Storms will fire a little later this afternoon, so they are expected to spread into the adjacent plains around 4PM. Max 1-hour rain rates up to 0.4 inches are possible with 24-hour totals up to 0.5 inches possible along the northern Continental Divide and northern Weld County. Most storms will produce rain rates at or below 0.25 inches favoring the Palmer Ridge and southern Cheyenne Ridge over the adjacent plains. A few lingering showers will be possible over the high terrains this evening, but flooding is not forecast. The main threats from today’s storms will be gusty outflow winds with the jet overhead. This is especially true over the Northern Mountains and Front Range where gusts could reach 40-45 mph.

Primetime: 3PM to 4AM

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

Storms today will be confined to the high terrains, although the increased cloud cover at the lower elevations may produce some light sprinkles for western Colorado. Slight increase in moisture over the eastern San Juan Mountains and north Central Mountains from the open wave and approaching trough. Max 1-hour rain rates up to 0.5 inches (south) and 0.3 inches (north) will be possible favoring the western facing slopes. Westerly wind will scour out most of the moisture east of the Divide, so only a few showers will be possible over the Southeast Mountains, again favoring the west facing slopes. Flooding is not forecast for storms today, and all activity should end a few hours after sundown.

Primetime: 2:30PM to 11PM