FTB 09-22-2017: Strong Disturbance Enters Colorado

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.

Flood Threat Legend

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

FTB 09-21-2017: Increasing Winds as the Trough Strengthens

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

FTB 09-20-2017: Mostly Sunny with Continuing Downslope Winds

Issue Date: Wednesday, September 20, 2017
Issue Time: 10:35 AM MDT

— Flooding is NOT expected today

Currently, cooler temperatures after the passage of the cold front yesterday evening and a few clouds over the northwest corner of the state. The cold front dramatically cooled temperatures 15-20F yesterday and produced a lot of blowing dust during the evening. It also helped moisten the lower levels of the atmosphere a bit. This moistening helped produce light showers overnight in the northern high terrains and is responsible for the continued cloud cover this morning. Despite temperatures in the 40s and low 50s this morning, high temperatures will return to average this afternoon.

The trough to our northwest will begin to dig south today, and the jet stream will lift to our north. This should decrease wind speeds throughout the day, especially when compared to yesterday. Higher dew points and cooler temperatures will also give us a little relief from fire danger. However, windy conditions will still exist today, so use extra caution with open flames. Aloft, winds will shift from west to southwesterly as the day continues. Not much in the way of mid-level energy this afternoon, but models hint at a weak shortwave over the southwest corner of the state, which may help initiate some showers. Low dew points and mid-level drying will limit how much rainfall reaches the surface, but isolated totals up to 0.25 inches are possible. Elsewhere across the state, downsloping winds will keep skies clear and continue to promote atmospheric drying. Surface wind speeds will be highest today over the NW corner, mountain peaks and the Palmer Ridge. Isolated gusts are expected to be 35 mph over the lower elevations and up to 50 mph over the higher terrains. Flooding is not expected today.

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 throughout the day with increasing winds this afternoon, especially over the higher peaks and the Palmer Ridge. Downsloping winds will continue to promote atmospheric drying, so no rainfall is expected. No Red Flag Warning today, but gusty conditions could still easily spread fires and extra caution should be used with any open flame. 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:

Clouds over the northern mountains expected to clear later this morning. Gusty winds over the northwest corner of the state will decrease throughout the day as the jet lifts north. This afternoon some showers could develop over the San Juan Mountains with the arrival of a weak shortwave. However, mid-level drying and low surface dew points will limit how much rain reaches the surface. Isolated amounts up to 0.25 inches are possible. Flooding is not expected today.

Primetime: 4PM to 11PM

FTB 09-19-2017: Gusty Winds and High Fire Danger

Issue Date: Tuesday, September 19, 2017
Issue Time: 10:20 AM MDT

— Flooding is NOT expected today

High winds expected today as the upper level trough brushes Colorado’s northern border. The approaching jet stream has streaks up to 140 knots with 110-120 knot values over the northwest corner of the state. Higher elevations in the mountains and along the Palmer Divide could see gusts in the 40-50 mph range. The 700mb direction and speed today is one associated with stronger winds along the Front Range as well. Thus, downslope westerly/southwest flow is expected across the state today, which should keep things quite dry. The exception is north over the CO/WY border where a shortwave moves through early this evening, which may help initiate some isolated storms. With low dew point values and high-cloud bases, most precipitation will evaporate before reaching the ground. However, some measurable precipitation could fall along and near the Gore Range.

The increased downslope flow will create very low relative humidity this afternoon and increase temperatures a few degrees just east of the mountains. Low humidity combined with fast wind speeds will generate extreme fire danger across the state today. Burning should be avoided and extra caution should be used with any open flame. A wind advisory is in effect until 7PM tonight for the northwest corner of the state as the jet stream moves in and mixes down through the afternoon. Sustained southwest winds of 20 to 35 mph and gusts up to 50 mph are expected. Winds should begin to die down late tonight with the decoupling of the upper and lower atmospheres. Flooding is not expected today.

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 throughout the day with strong SW winds. The downsloping winds will cause temperatures closer to the mountains to increase a few degrees. A Red Flag Warning has been issued through this evening, so please avoid burning and use extreme caution with any open flame. An isolated storm along the northern Front Range with trace amounts of precipitation may be possible this evening. 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; especially over the northwest corner of the state. Winds are expected to be 20-35 mph with winds gusting as high as 50 mph. Some clouds and minimal rain are possible over the Northern Mountains (Gore Range) this evening. Precipitation amounts are expected to be under 0.25 inches. Flooding is not expected today.

Primetime: 5PM to 10PM