Flood Threat Bulletin

FTB 08-24-2016: Clouds and Drier Air To Limit Heavy Rainfall For Most, But Not All

August 24, 2016 by Dima Smirnov ·

Issue Date: Wednesday, August 24th, 2016
Issue Time: 10:45AM MDT

— HIGH flood threat for Hayden Pass fire burn area
— LOW flood threat for San Juans, Southeast Mountains and parts of Southeast Plains

A rather complicated weather map is seen this morning, see the marked-up water vapor image below. The trough, and attendant surface low pressure, that has been at the center of attention the past few days has broadened. Just like spokes on a bicycle wheel, there are now at least three distinct disturbances that are part of the bigger trough. The far southeast one is the main focus for Colorado’s weather today. As of this morning, it was roughly oriented along the Continental Divide with upward motion to its east and subsidence to its west. Accordingly, morning visible satellite imagery showed this contrast with clear skies across the western slope but thick clouds and light to moderate shower activity to the east. Cloud cover and drier air entraining from the northeast will be a strong limiting factor on rainfall rates. However, with more sunshine a few southern areas will be the exception to this and that’s where a Low flood threat has been issued this morning. The Southeast Plains will see a threat of severe weather in addition to heavy rainfall. Additionally, the Hayden Pass burn scar has been a trouble spot for the past few days. It will continue to be an issue today with moderate rainfall rates exceeding 0.5 inches per hour likely. A High flood threat is in effect for the scar area.

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, Southeast Mountains, San Luis Valley, Hayden Pass fire scar:

Partly cloudy this morning then scattered to widespread showers and thunderstorms developing by early afternoon. Max 1-hr rain rates up to 1 inch possible for the San Juans with up to 1.2 inches possible for the Southeast Mountains. A Low flood threat has been issued for the region due to antecedent 48-hour rainfall that will lead to quicker runoff. A High flood threat is in effect for the Hayden Pass fire scar because rainfall rates exceeding 0.5 inches per hour are likely today leading to more runoff and potential mud flows.

Primetime: 12PM to 8PM, except through midnight for the Hayden Pass burn scar

Southeast Plains:

Partly cloudy then scattered thunderstorm and shower activity developing by mid afternoon. Maximum 1-hour rain rates up to 2.0 inches will be possible, along with hail up to 1.5 inches and wind gusts up to 75 mph. A Low flood threat has been issued for isolated flash flooding and field/roadway flooding.

Primetime: 3PM to 11PM

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

Sunny early then isolated to scattered showers and weak thunderstorms developing by early afternoon. Maximum 1-hour rain rates up to 0.4 inches are possible, with maximum 24-hour rainfall up to 0.7 inches. Flooding is not expected today.

Primetime: 12PM to 7:30PM

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

Overcast with scattered to widespread moderate shower activity on and off throughout the day, diminishing after sunset. Maximum 1-hr rain rates up to 0.5 inches possible. An isolated strong storm is possible for southern areas especially near the foothills where up to 0.9 inches per hour of rainfall is possible. However, flooding is not expected today.

Primetime: 2PM to 7:30PM for southern areas with higher storm chances

Filed Under: Flood Threat Bulletin ·

FTB 08-23-2016: More Heavy Rainfall On The Way For Western Slope, But Instability Is a Limiting Factor

August 23, 2016 by Dima Smirnov ·

Issue Date: Tuesday, August 23rd, 2016
Issue Time: 10:00AM MDT

— LOW flood threat for San Juans, Grand Valley, Central Mountains, Southwest Slope and Southeast Mountains

Today’s weather map, shown overlaid on the water vapor image below, has not changed much since yesterday. The broad trough is now centered over southern Alberta and has only slowly progressed eastward. However, a more noticeable southern end circulation is seen today over northern Arizona. This will be the most notable feature for Colorado’s weather today. A rather broad cloud deck is noted this morning over the southwest part of the state, stretching back into UT and AZ. This cloud deck is an indicator of the synoptic scale upward motion being caused by the disturbance. As it slowly treks northeast into northern NM and CO, solar heating will cause widespread showers and thunderstorms across a similar region as yesterday. However, despite fairly high moisture content with Grand Junction’s morning precipitable water reading around 0.9 inches, the broad thick cloud deck will likely limit instability more so than yesterday. Although isolated heavy rainfall is expected, today’s flood threat is kept at a Low status due to the expected short duration of such rainfall. Instead, light to moderate rainfall is more likely for most of the afternoon and evening hours for most of the western slope and into the northern parts of the Southeast Mountains. East of the Continental Divide, dry air from Wyoming will once again limit rainfall chances, although a shower or weak storm will likely prevail closer to the foothills and 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, Grand Valley, Central Mountains, Southeast Mountains, San Luis Valley:

Partly cloudy early with widespread showers and thunderstorms quickly developing by early afternoon. Max 30-min rain rates up to 0.6 inches, 1-hour rain rates up to 1.0 inch and 6-hour rates up to 2.2 inches are possible. A Low flood threat has been posted for parts of the region. Showers and a weak storm will continue into the overnight hours. Hail up to 0.5 inches may accompany the stronger afternoon and evening cells.

Primetime: 12PM to 10PM, but rainfall continuing overnight

Northwest Slope, Northern Mountains:

Partly cloudy early then scattered showers and weak thunderstorms developing by early afternoon. Maximum 1-hour rain rates up to 0.5 inches are possible, with maximum 24-hour rainfall up to 0.8 inches. Flooding is not expected today. Scattered showers may continue into the overnight hours.

Primetime: 1PM to 7:30PM

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

Mostly sunny early and continued warm. Isolated to scattered showers and thunderstorms are possible across higher terrain and foothills. Max 1-hour rain rates up to 0.5 inches are possible, except up to 0.8 inches in the Southeast Plains. Flooding is not expected. Scattered showers and a weak storm will likely continue into the overnight hours closer to the mountains and foothills as upslope flow develops from the incoming disturbance.

Primetime: 1PM to 9PM

Filed Under: Flood Threat Bulletin ·

FTB 08-22-2016: Scattered Heavy Rainfall Likely Across Southwest

August 22, 2016 by Dima Smirnov ·

Issue Date: Monday, August 22nd, 2016
Issue Time: 10:24AM MDT

— MODERATE flood threat for San Juans, Grand Valley and Central Mountains
— LOW flood threat for Southwest Slope

Changes are on seen on this morning’s weather map, overlaid on the water vapor image below. A large-scale disturbance, with its center located over British Columbia, is carrying a long trough that extends southward through the Mexico border. Of importance for Colorado is the southern fringe of this activity, which is being maintained by a strong subtropical jet stream. As the disturbance moves east today, it will promote widespread lift and thus, shower and thunderstorm activity west of the Divide. Some of this can already been seen on morning radar and visible satellite loops across UT and western CO. Precipitable water values have jumped from below 0.4 inches to over 0.8 inches at Grand Junction and Durango. North of the San Juans, most of this moisture is found above the boundary layer so it will take several hours before moisture is transported to the surface through rainfall and moisture transport from the south. However, dew point temperatures at or over 50F are found just south of the NM/CO border so there is a nearby source of higher moisture. Despite marginal instability today, convective temperatures will be reached with help from dynamical forcing. Numerical weather prediction guidance suggests moderate to high probabilities of exceeding 1 inch of rainfall across the southwest, and there is no reason to doubt this given the overall pattern. In all, a Moderate flood threat is in place for parts of the San Juans and Central Mountains, with a Low flood threat surrounding those areas. The main impact will be from heavy rainfall at durations of 30-minutes to 3-hours.

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:

San Juan Mountains, Southwest Slope, Grand Valley, Central Mountains:

Mostly cloudy early with scattered showers giving way to widespread showers thunderstorms developing by early afternoon. Max 30-min rain rates up to 0.7 inches,1-hour rain rates up to 1.1 inch and 3-hour rates up to 2.4 inches are possible. A Moderate flood threat has been posted for parts of the regions with a Low flood threat surrounding that region. Hail up to 0.5 inches may accompany the stronger cells.

Primetime: 1PM to 9PM

Northwest Slope, Northern Mountains, San Luis Valley:

Partly cloudy early then scattered showers and weak thunderstorms developing by early afternoon. Maximum 1-hour rain rates up to 0.6 inches are possible, with maximum 24-hour rainfall up to 1 inch. Flooding is not expected today.

Primetime: 1PM to 7:30PM

Southeast Plains:

Sunny early, then partly cloudy with isolated to scattered thunderstorms possible. Max 1-hour rainfall up to 1.4 inches is possible. However, flooding is not expected today. Hail up to 1.25 inch may accompany the strongest storms.

Primetime: 3PM to 9PM

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

Filed Under: Flood Threat Bulletin ·

FTB 08-21-2016: Low Flood Threat Returns For San Juans and Southeast Mountains

August 21, 2016 by Dima Smirnov ·

Issue Date: Sunday, August 21st, 2016
Issue Time: 10:00AM MDT

— LOW flood threat for San Juan Mountains, Southeast Mountains, Raton Ridge and San Luis Valley

Most of Colorado will turn much warmer, but stay dry today. Just like yesterday, the one exception to this will be far southern areas of the state. The water vapor image, below, shows that a strong subtropical jet is seen this morning stretching across the southern parts of CA, AZ and into central New Mexico. This jet is very high in the atmosphere, above 30,000 feet. As we move through the day, the jet will retain strength and slowly expand northward to impact the southern tier of our state. The main impact from the jet will be to allow for an efficient “venting” of thunderstorm activity. Adequate residual moisture in place for southern areas. Surface dew point temperatures in the upper-40s to mid-50s F and precipitable water will hold in the 0.7 – 0.9 range. Thus, with instability not an issue today, we expect scattered thunderstorm activity with heavy rainfall potential to develop across favored parts of the Sangre de Cristos, Wet Mountains and San Juans during the afternoon and early evening hours. A Low flood threat is in place for these areas.

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, San Juan Mountains, San Luis Valley:

Sunny early then becoming partly cloudy with scattered thunderstorms developing shortly after noon. Most action today will be limited to the higher terrain and foothills. Max 30-min rain rates up to 0.7 inches and 1-hour rain rates up to 1.1 inch are possible. A Low flood threat has been posted for parts of the area, including the southern part of the San Luis Valley. Hail up to 0.75 inches and gusty winds up to 60 mph may accompany the stronger cells.

Primetime: 12PM to 8:30M

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

Partly to mostly sunny and much warmer today. Expect fair weather cumulus clouds developing by late morning. Isolated showers and weak thunderstorms are possible closer to the higher terrain during the afternoon hours. Max 1-hour rain rates up to 0.4 inches are possible. Flooding is not expected today.

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

Mostly sunny early then partly cloudy and much. An isolated shower or storm cannot be ruled out for far southern areas. Max 1-hour rain rates up to 0.3 inches. Flooding is not expected today.

Filed Under: Flood Threat Bulletin ·