Issue Date: Saturday, September 24, 2017
Issue Time: 10:20 AM MDT

— LOW flood threat for the Southeast Plains, Raton Ridge, Northeast Plains

— Timing of flood threat is later than normal, and persists into the overnight hours

Heavy and widespread rainfall yesterday across the state. Much cooler temperatures in the wake of the cold front with 40s over the lower elevations and below freezing temperatures in the mountains. Heavy cloud cover over the eastern portion of the state with some convective showers still occurring over the Southeast Plains. A lot of fog was reported due to the increase in low level moisture and cooler temperatures. Currently, the western portion of the state is placed in the dry slot, so only a few clouds over the Northern Mountains. The upper trough will continue to move slowly eastward today and tonight. Colorado will continue to be under southwest flow aloft, which will draw in some drier air over western Colorado and inhibit rainfall. East of the Continental Divide, higher moisture will continue to reside, especially over the southeast corner of the state. Dew points over the plains are expected to reach 60F, so more heavy rain can be expected today and tonight.

Weaker showers are expected today over the higher terrains with snow at the higher elevations (9,500 feet). Upslope flow will continue behind the cold front with multiple upper level shortwaves enhancing lift. Expected scattered showers today and tonight. The highest accumulations are expected be in the Northern Mountains and northern Front Range. Over the southeast corner of the state, models hint at instability this afternoon with some decent shear. This could trigger a round of more convective storms capable of strong winds and small hail. Lift is expected to continue overnight, so expecting another round of showers in the moisture rich environment. High moisture paired with instability will aid in another 24-hour period of heavy rainfall accumulations. On top of accumulations yesterday, and increased runoff due to saturated soils, a Low flood threat has been issued. Threats include small stream and arroyo flooding, road flooding and field ponding.

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.

Zone-Specific Forecasts:

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

Cloudy this morning with increasing showers and thunderstorms over the northern, high terrains this afternoon. Some showers will move off the higher terrains into the Urban Corridor. Max 1-hr rain rates up to 0.5 inches possible. More convective rain is expected this afternoon over the Southeast Plains. Max 1-hour rain rates up to 1.5 inches are possible with isolated 24-hour rain totals up to 2.5 inches. Due to increased runoff from already saturated soils and high accumulations from yesterday, a Low flood threat has been issued. Threats include small stream and arroyo flooding, road flooding and field ponding. This is another overnight threat.

Primetime: 11AM to 7AM

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

Partly cloudy over the northern high country with sunshine increasing as drier air works its way in from the southwest. Some light precipitation and cloud cover is likely over the Northwest Slope as increased moisture rotates around the upper level Low. Max 1-hour rain rates up to 0.3 inches are possible over the northwest corner of the state.

Primetime: 11AM to 8PM

Issue Date: Saturday, September 23, 2017
Issue Time: 10:55 AM MDT

— MODERATE flood threat for the Southeast Plains, Northeast Plains and Raton Ridge

— LOW flood threat for portions of the Front Range, Urban Corridor, Southeast Mountains, Palmer Ridge, Raton Ridge, Southeast Plains, Northeast Plains

— Timing of flood threat is later than normal, and persists into the overnight hours

The visible satellite imagery below shows quite a bit of rainfall over the state as the trough moves in from the west. Beginning early this morning, the low pressure system over northwest Kansas began to pull in very moist air from Texas and Oklahoma into the eastern portion of the state. The dew points this morning over the Southeast Plains have already made it into the 60s, which is incredibly high for this time of year. Precipitable Water (PW) this morning at Denver was measured at 0.71 inches. Over Grand Junction, PW was 0.64 inches. Dodge City PW is probably a little high, but a better estimate for the SE Plains right now, which is at 1.26 inches. PW greater than 1 inch is marked in the map below. Aloft, expecting south and southwest winds, which will continue to pull moisture north and give storms a N/NE storm motion.

The upper low pressure system will slowly continue to slowly propagate eastwards with the majority of Colorado being under positive vorticity advection throughout the forecast period. Paired with high moisture, this lift will translate into the widespread showers over the state. Currently over the western portion of the state, the jet streak is creating a strong line of convection from north to south. Thunderstorms and widespread showers are likely through midnight until this feature begins to move to the northwest and western Colorado is under the influence of the dry slot.

As the upper low tracks to the northeast, showers and thunderstorms over the mountains are expected to increase. With colder temperatures behind the cold front, rain should transition into snow later this evening for elevations around 9,000 feet. With colder temperatures above 10,000 feet already, precipitation this afternoon will likely be all snow. That could mean a couple of inches by morning the higher elevations in the Northern and Central Mountains. East of the Continental Divide, the higher rainfall amounts will be over the southern portion of the high terrain due to the increase of moisture in this area. Southerly flow is expected to enhance precipitation along the Palmer Ridge/Teller County high elevation intersect. Over the Southeast Mountains, there is enough confidence that rain rates will reach 0.5 inches an hour near the Hayden Pass and Junkins burn scar, so storms that form in this area need to be monitored closely. A Low Flood threat has been issued for the higher terrains of the Palmer Ridge, Raton Ridge and Southeast Mountains. Threats include mud flows, debris slides and small stream and road flooding.

Over the eastern plains, enhanced thunderstorm and shower activity is likely this afternoon as the upper level energy, high low level moisture and lift along the slow moving cold front combine. With PW values over 1 inch, heavy rainfall is expected over the Southeast Plain beginning this afternoon. Rain is expected to continue over the far eastern plains into early morning, which warrants a Moderate Flood threat. Threats include stream and road flooding as well as field ponding.

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.

Zone-Specific Forecasts:

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

Cloudy this morning and increasing showers and thunderstorms throughout the day. Max 1-hr rain rates up to 0.7 inches possible (west) and 1.5 (east). Rain over the mountains will begin decreasing around midnight. Continuous rain is expected over the eastern plains through tomorrow making this an overnight threat. 24-hour totals up to 3.5 inches are possible over the far Southeast Plains. There is enough confidence 1-hour rain rates for storms near the Junkins and Hayden Pass burn scars will be over 0.5 inches. A Low Flood Threat has been issued. Threats include mud flows and debris slides (west), road and stream flooding (west/east) and field ponding (east). A Low/Moderate flood threat has been issued.

Primetime: 12PM to 7AM

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

Cloudy and rainy this morning with showers continuing to increase throughout the day. By late morning, more showers will form over the higher terrains. Southern high terrains could see 24-hour amounts up to 1.8 inches. Max 1-hour rain rates up to 0.5 inches are possible (south) and 0.3 inches (north).

Primetime: 11AM to 12AM

Issue Date: Friday, September 22, 2017
Issue Time: 8:45AM MDT

— LOW flood threat for Hayden Pass burn scar
— LOW flood threat for Southeast Plains and Raton Ridge (along with chance of severe weather)
— Timing of flood threat is later than normal, and persists into the overnight hours

It has been the center of attention for several days now, and is finally here. As shown in the water vapor image, below, a strong upper-level trough has begun to enter Colorado from the west. It is being supported by a large-scale jet stream with several embedded jet streaks exceeding 120 mph. Its sensible impact is already being felt in the form of mid-level clouds and scattered showers and even thunderstorms west of Continental Divide. One key variable is lacking right now, however. Precipitable water (PW) is only 0.5 inches at Grand Junction and less than that at Denver. However, much moister air resides just across the border into OK and KS with PW exceeding 1 inch there. At the surface, a cool front has entered northeast Colorado and is positioned right along the South Platte River valley as of 8AM MDT. With plenty of upper air support, expect this low pressure to intensify this afternoon, drawing in some of this moister air into its warm sector over the Southeast Plains.

In all, we expect isolated to scattered showers and weak thunderstorms to increase in coverage and intensity. While coverage will be almost the entire state, the only areas with potential for heavy rainfall will be in the Southeast Plains. Expect a round of thunderstorm associated with peak heating this afternoon, followed by another round of showers and thunderstorms this evening and into the overnight hours as numerous upper-level impulse combined with a strong moisture advection. There are two areas of concern today. First, in the far southeast part of the state, thunderstorms will be capable of producing heavy rainfall and isolated flash flooding. Severe weather will also be possible. Second, precipitation coverage is expected to increase during the overnight hours across central Colorado. Adequate moisture warrants a Low flood threat for the Hayden Pass fire scar for both short-term (< 1 hour) and medium term (3 hour) rainfall rates.

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.

Zone-Specific Forecasts:

Southeast Mountains, Raton Ridge, Southeast Plains:

Partly cloudy early, then increasing clouds with scattered showers and thunderstorms developing this afternoon and continuing into the evening/overnight hours. Max 1-hr rainfall up to 1.6 inches (far southeast) and 1.2 inches (Raton Ridge) with up to 0.7 inches elsewhere. A Low flood threat has been posted. In addition, severe weather will be possible with hail up to 2.25 inches during the afternoon storms and up to 1.5 inches with the overnight activity. Gusty winds up to 75 mph are also possible. A separate Low flood threat has been posted for the Hayden Pass fire scar where max 1-hour rainfall up to 0.5 inches and max 3-hour rainfall up to 0.8 inches is possible, enough to trigger mud flows and debris slides over the sensitive soil.

Primetime: 3PM to 6AM

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

Partly cloudy early, then increasing clouds with scattered showers and weak thunderstorms developing this afternoon and continuing into the evening/overnight hours. Max 1-hour rainfall up to 0.7 inches. Hail up to 0.75 inches and gusty winds up to 60mph could accompany the strongest cells on the Palmer Ridge and Northeast Plains. Snow showers will be possible at the highest elevations. Flooding is not expected today.

Primetime: 3PM to 6AM

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

Mostly cloudy with scattered showers and weak thunderstorms increasing in coverage and continuing into the late evening and overnight hours. Max 1-hour rainfall up to 0.5 inches with max 24-hour totals up to 1.6 inches in favored southwest facing slopes. Snow will be possible at the highest elevations. Flooding is not expected today.

Primetime: 11AM to 11AM

Issue Date: Thursday, September 21, 2017
Issue Time: 10:25 AM MDT

— Flooding is NOT expected today

The strong trough to our west continues to push south and begins to move a little east today. The elongated and positively tilted trough is forecasted to become an upper level, closed Low pressure system by this afternoon. Maximum wind speeds in the jet streak are forecasted to be 120+ knots. Aloft this means more southwesterly flow over the state. As seen in the water vapor imagery below, this flow will continue to advect drier air into Colorado and promote downsloping winds, which will further dry out the atmosphere as it is adiabatically warmed.

With a tight surface gradient and upper level winds mixing down to the surface, winds today are also expected to increase. The winds are expected to increase the most over the higher terrains and southern/eastern plains. A Red Flag Warning has been issued for the eastern plains as well as the southwest corner of Colorado due to low relative humidity and wind gusts this afternoon reaching 20-30 mph. Over the mountains, the fast moving upper level winds will mix down once again, so gusts in the 60-70 mph over the high elevation passes are likely. High winds over the mountains may continue through the night.

With the extremely dry atmosphere, zero to trace amounts of precipitation are expected over the state today. There is some increase in precipitable water late tonight over the southwest corner of the state. This is the same time shortwave activity increases as the trough moves east. The enhanced lift is expected to initiate some late night/morning showers over the San Juan Mountains. Flooding is not expected today or overnight.

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.

Zone-Specific Forecasts:

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

Sunny again today with winds increasing this afternoon over the eastern and southern plains. Once again, high-mountain passes are expected to have gusts around 60 mph as the fast moving, upper level winds mix down. The dry atmosphere will limit coverage in cloud cover and no rain is expected today. A Red Flag Warning has been issued over the plains until this evening due to critical fire weather. Flooding is not expected today.

Primetime: 5PM to 10PM

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

Sunny this morning with winds increasing this afternoon. Higher elevation passes could see wind gusts in the 60-70 mph range. A Red Flag warning has been issued for the southwest corner of the state through this evening. Overnight, precipitable water will increase over the southwest corner of the state at the same time shortwave activity increases from the trough. A few late night/early morning showers are likely over the San Juan Mountains. Storm totals up to 0.5 inches could occur by late morning. Flooding is not expected today or tonight.

Primetime: 12AM to 11AM