FTB 06-16-2018: Rare High Impact Flood Threat Issued – Read Below

Issue Date: 6/16/2018
Issue Time: 9:10 AM

8:15 PM UPDATE:

The event has not evolved exactly as expected, as cloud cover limited instability during the afternoon and early evening hours. This limited instability, combined with a more aggressive dry slot in the mid-level moisture has kept precipitation over the eastern Southwest Slope and San Juan Mountains to a minimum, and kept most of the precipitation to the west and north.

With that said, the High Impact Flood Threat remains in effect for the 416 Fire and the High Flood Threat remains in effect for the Burro Fire. Moisture is expected to rebound during the overnight hours, coupled with broad, mid-level support. This will set up an axis of showers and one or two thunderstorms, oriented southwest-to-northeast across western Colorado. This axis of showers/thunderstorms will be the one to watch. If it sets up to the west of the High Impact/High Flood threat areas, a bullet will have been dodged. If this sets up over the 416 Fire and Burro Fire, it will likely produce enough rain for all of the threats discussed earlier (flash flooding, mud flows, and debris slides). The areas are not out of the woods yet, so to speak. The prime time for this threat is Midnight – 6 AM, but the flood threat will remain in effect until 11 AM.

A HIGH IMPACT FLOOD THREAT IS FORECAST TODAY FOR PORTIONS OF THE SOUTHWEST SLOPE AND SAN JUAN MOUNTAINS – NAMELY FOR THE 416 FIRE AND LOCATIONS IMMEDIATELY DOWNSTREAM.

A HIGH FLOOD THREAT ENCOMPASSES THE BURRO FIRE AND LOCATIONS IMMEDIATELY DOWNSTREAM.

A LOW FLOOD THREAT SURROUNDS THESE AREAS AND INCLUDES PORTIONS OF THE NORTHERN MOUNTAINS, NORTHWEST SLOPE, GRAND VALLEY, CENTRAL MOUNTAINS, SOUTHWEST SLOPE, AND SAN JUAN MOUNTAINS.

Note: If necessary, a PM Update will be issued if the forecast deviates significantly.

Today begins the start of a wetter and cooler period for Colorado, and on most counts that is a good thing. 67% of the state, according to the United States Drought Monitor, is experiencing drought conditions. Rain is good. However, because of the dry conditions, two wildfires in particular have raged – the Burro Fire and the 416 Fire – and these burn scars (and points immediately downstream) are under the gun for flash flooding, mud flows, and debris slides through this FTB period.

A few major features, highlighted in the water vapor image below, are coming together to result in today’s flood threat. The remnants of Hurricane Bud are being pulled northward into Colorado by the upper-level low over the Pacific Northwest and a secondary, upper-level low circulation over the Californian Baja. These upper-level lows will provide ample, broad scale support for showers and thunderstorms across the state today/tonight, and the depth of the moisture being transported into the state will lead to efficient rainfall production. Pockets of precipitable water values over an inch have already begun invading southwest Colorado this morning, and those values will continue to spread northward throughout the day.

Additionally, for eastern Colorado, low-level moisture will be pulled in from the Great Plains. This will mainly impact the Northeast Plains and Urban Corridor, where a couple strong-to-severe storms will also be possible.

Burro Fire and 416 Fire burn areas:

Both fire burn areas, and locations immediately downstream, are under the High/High Impact Flood Threat today. It will only take rain rates of 0.25-0.5 inches/hour to produce flash flooding, mud flows, and debris slides, and those rates occur often when precipitable water values exceed 1 inch. Residents near these burn scars, especially those along the Highway 550 corridor including Hermosa need to be prepared. Durango, out of an abundance of caution since this is the first rainfall event over these burn scars, needs to be prepared, as well.

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, jump below the map.

Flood Threat Legend

Zone-Specific Forecasts

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

Scattered showers and thunderstorms are expected today/tonight. Most will be garden-variety, producing gusty winds and light-to-moderate rainfall. A few stronger thunderstorms will rumble, mainly north of I-70, producing small hail, strong winds, and brief periods of heavy rainfall. Most activity will come to an end by midnight, but a few lingering showers/weak thunderstorms will continue into the overnight/early morning hours. Relatively quick storm motions preclude the issuance of a flood threat. Maximum rain rates are as follows:

Front Range, Urban Corridor, and Palmer Ridge: 0.4-0.8 inches/hour
Northeast Plains: 1.0-1.5 inches/hour
Southeast Mountains: 0.25-0.5 inches/hour
Raton Ridge and Southeast Plains: 0.5-0.8 inches/hour

Timing: Noon – 11 PM, with a few lingering showers/weak thunderstorms into the early morning hours

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

Scattered-to-widespread showers and thunderstorms are expected. The Burro Fire and 416 Fire burn areas are under the gun today, due to their sensitivity to rainfall and the likely multiple rounds of showers/storms that will impact the area throughout this FTB period. 0.25-0.5 inches/hour rain rates are all it will take, and those rates will likely be exceeded by thunderstorm activity in the area. Flash flooding, mud flows, and debris slides are all threats in those areas.

Elsewhere across the High Country, other areas with steep terrain and burn scars will need to be monitored. The low flood threat is a result of this event being more of a 24-hour event rather than a 1-hour rainfall event. Widespread areas of 0.25-0.75 inches of rain are likely, with localized areas of 1-2 inches of rainfall possible.

Maximum rain rates are as follows:

Southwest Slope, San Juan Mountains, Central Mountains, Grand Valley, and Northern Mountains: 0.5-1.0 inches/hour
Northwest Slope: 0.4-0.7 inches/hour
San Luis Valley: 0.2-0.5 inches/hour

Timing: 11 AM – 11 AM

FTB 06-15-2018: Dry Thunderstorms to the West and Moderate Rainfall for the Northeast Plains

Issue Date: Friday, June 15, 2018
Issue Time: 09:30AM MDT

— Flooding is NOT expected today

Everything is coming together for the cool and wet weekend. In the water vapor imagery below you can see the trough that will start to drop into Nevada/Northern California later today. You can also see the subtropical moisture from Bud starting to creep north over the southwest. But for today, that low level moisture won’t quite make it into western Colorado for the next round of afternoon thunderstorms and showers.

Soundings at both Grand Junction and Denver show quite a bit spread between the temperature and dew point at the surface, which should lead to another day of gusty winds and limited rainfall from the thunderstorms this afternoon. A Red Flag warning is in place over Western Colorado through this evening due to dry thunderstorms (strong outflows and lightning) creating critical fire conditions. Rainfall coverage and totals over the southwest corner of the state are expected to increase overnight as remnants of Bud moisten the lower levels. Please check back to the FTB tomorrow for more specific details as a Flash Flood Watch and High Flood Threat Outlook have been issued for the San Juan Mountains and 416/Burro burn scars on Saturday.

Over the Northeast Plains, behind the cold front, dew points are increasing into the mid 50Fs. A bit of this moisture will mix out this afternoon, but there is still enough in place for moderate rainfall this afternoon. The front is expected to stall out over the Palmer Ridge later this morning, so these higher dew points won’t quite reach the Southeast Plains. This will make showers and thunderstorms more isolated over the Southeast Mountains. Behind the front, upslope flow will increase as will thunderstorm coverage over the Front Range. More storms are forecast to form over the Palmer and Raton Ridges later this afternoon. Storm motion will be to the northeast at 15-20 knots, so flooding is not a concern. Threats today include gusty winds, dry thunderstorms and small hail. Storm activity over the eastern plains should end by 11PM. There is no flood threat today.

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:

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

Upslope flow will return to the region favoring the Front Range, which should initiate thunderstorms over the higher terrains by this afternoon. Max 1-hour rain rates up to 0.75 inches/hour are possible near the CO/WY/NE border. However, the higher totals are expected to stay north of Colorado. Further south over the Palmer Ridge max 1-hour rain rates up to 0.6 inches/hour are likely. Due to limited low level moisture over the Southeast Plains, the storms that fire along the Raton Ridge will only have 1-hour rain rates up to 0.3 inches/hour. Storms will move east of Colorado by 11PM.

Primetime: 2PM – 11PM

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

Dry thunderstorms this afternoon will lead to critical fire conditions as they will produce lightning and gusty outflow winds (45-60 mph range). Thus, a Red Flag Warning is in effect until 9PM tonight. Max 1-hour rain rates this afternoon will be in the 0.05-0.15 inches/hour range with the highest rain rates over the southern/central mountains. After 10PM, the low levels will begin moisten as the subtropical moisture reaches the southwest corner of the state. This will increase rainfall coverage and intensity overnight in western Colorado with the most widespread activity over the San Juan Mountains. Totals over the San Juans by tomorrow morning will be in the 0.25-0.4 inch range favoring the south facing slopes. Flooding is not expected overnight, but please tune back into the FTB tomorrow morning as a Flash Flood Watch has been issued for Saturday over the San Juan Mountains and 416/Burro burn scars.

Primetime:  12:30PM – 11AM

FTB 06-14-2018: More Wind than Rain Expected from Thunderstorms

Issue Date: 6/14/2018
Issue Time: 7:40 AM

NO FLOOD THREAT IS FORECAST TODAY.

Mid-level moisture is streaming into the state from the southwest as a result of high pressure aloft over New Mexico and an upper-level trough digging into the Pacific Northwest. This stream of moisture is sufficient enough to produce plenty of mid-level clouds this morning, and will support scattered thunderstorms across the state this afternoon and evening. Unfortunately for those looking for wetting rain, you will not find much of it today. The low-levels will remain too dry, especially over the High Country, where dry thunderstorms will produce plenty of virga, gusty winds, and cloud-to-ground lightning which could spark new fires.

East of the mountains, a little bit better low-level moisture is present, but not nearly enough to result in heavy rainfall. Strong, gusty winds and lightning will be the main impacts from thunderstorms today, with only brief periods of light-to-moderate rainfall as storms race off to the east-northeast. A couple of grass fires were started in Weld County last night, burning over 8,000 acres before being contained. Grass fires sparked by cloud-to-ground lightning will be a concern today, as well.

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, jump below the map.

Flood Threat Legend

Zone-Specific Forecasts

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

Scattered high-based showers and thunderstorms are expected, producing more wind and lightning than rain. Plenty of virga will paint the sky, with brief periods of light-to-moderate rainfall underneath thunderstorms. Otherwise, the main story will be that high temperatures are a few degrees warmer than yesterday, with highs in the mid-to-upper 90s across the plains, and even a few 100+ readings in the Arkansas River Valley. Maximum rain rates from thunderstorms are as follows:

Front Range and Southeast Mountains: 0.05-0.15 inches/hour
Urban Corridor, Palmer Ridge, and Raton Ridge: 0.25-0.5 inches/hour
Northeast Plains and Southeast Plains: 0.4-0.8 inches/hour

Timing: Noon – 11 PM, with a few storms rumbling over the Eastern Plains until 1 AM

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

Isolated-to-scattered showers/thunderstorms are expected today, producing very little (if any) rainfall. Most of the activity will remain south of I-70, but a few isolated storms over the Northern Mountains and Northwest Slope cannot be ruled out. Gusty winds and cloud-to-ground lightning will be the main concerns, with Red Flag Warnings in place across much of the area. Rain rates will remain below 0.15 inches/hour.

Timing: 11 AM – 11 PM

FTB 06-13-2018: Battle Between Dry Air and Moist Air

Issue Date: 6/13/2018
Issue Time: 8:12 AM

A LOW FLOOD THREAT IS FORECAST TODAY FOR PORTIONS OF THE NORTHEAST PLAINS, URBAN CORRIDOR, PALMER RIDGE, AND SOUTHEAST PLAINS.

Colorado is ground-zero for a battle between dry air and moist air, both in the mid-levels and at the surface. Shown in the water vapor imagery below is the mid-level battle, where moist air from the south is attempting to nudge northward into western Colorado. This moist air will make a valiant effort, pushing north towards I-70 over the High Country and Western Slope. This moisture will be sufficient enough to produce isolated, high-based showers/thunderstorms over southern and central portions of the High Country, producing very little rain (if any), with plenty of lightning and virga painting the sky.

The battle in the low-levels will take place east of the mountains, where east-southeasterly flow has pushed moisture rich low-level air into eastern Colorado. Dry air from the west will try to scour this moisture, with a dry line setting up along, or just east of, the foothills. The moisture, combined with daytime heating, will be enough to produce isolated-to-scattered thunderstorms, with the best coverage and intensity occurring east of I-25. A couple of the storms will become severe, with the potential to produce hail up to 1.5-2 inches in diameter and strong winds greater than 55 mph.

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, jump below the map.

Flood Threat Legend

Zone-Specific Forecasts

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

Isolated-to-scattered thunderstorms will rumble today/tonight. Without a “trigger” in the mid-/upper-levels, it will be up to low-level convergence and daytime heating to produce adequate forcing for thunderstorm development. With south-southeasterly winds at the surface, the Denver cyclone is expected to aid thunderstorm development, likely kicking off a couple thunderstorms in the vicinity of the Denver metro area. Most storms will struggle to produce heavy rain thanks to dry air in the mid-levels zapping some of the potential, but hail clogging drainage and brief periods of heavy rainfall will likely produce a couple instances of street/field ponding in poorly drained areas. This is the culprit behind the low flood threat. Maximum rain rates are as follows:

Front Range: 0.05-0.25 inches/hour
Urban Corridor, Palmer Ridge, and Raton Ridge: 0.5-1.0 inches/hour
Northeast Plains and Southeast Plains: 1.0-1.5 inches/hour

Timing: 2 PM – 11 PM, with a couple lingering thunderstorms over the Northeast and Southeast Plains until 2 AM

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

Dry and hot will be the main weather story today, with only a few isolated, high-based thunderstorms rumbling during the afternoon and evening hours south of I-70 where mid-level moisture from the south can have an impact. Dry air in the low-levels will keep rain rates low, so no flash flooding is expected. Maximum rain rates will be 0.05-0.10 inches/hour. North of I-70, expect dry conditions and mostly sunny skies to rule the day.

Timing: 1 PM – Midnight