FTB 06-03-2020: Drying Trend Continues with High-Based, Afternoon Thunderstorms Forecast

Issue Date: Wednesday, June 3rd, 2020
Issue Time: 9:45AM MDT

— Flooding is NOT expected today

Below is the low-level water vapor imagery, and it’s noteworthy that not much moisture looks be trapped under the ridge over the state. This is indicated by more yellow than blue/white. Westerly flow aloft continues over the majority of the state today as the ridge builds northward, which will help further mix out low-level moisture and help set up a dryline (area of convergence) over the far eastern plains. There is also some mid-level energy noted in the image below (orange “X”). One is to our north, and the other further south. The one to our north may help spark some additional thunderstorms over the Cheyenne Ridge that will likely spill into the Northeast Plains with the clockwise steering winds around the High, and it will help set up the dryline over the Northeast Plains. The other shortwave further south may help spark better thunderstorm coverage over the eastern San Juan Mountains and Southeast Mountains/Raton Ridge than there would otherwise be with the diurnal flow pattern. Storms that form over the mountains and Palmer/Raton ridges will move westward with the steering flow.

Coverage of storms will decrease this afternoon due to the increased dryness, and storms that do form are expected to be high-based with the lower dew points. The large difference in temperature and dew points (dew point depression) means gusty outflow winds will be the main threat from storms. Slightly faster steering flows will also limit the amount of rainfall over an area. Higher accumulations are expected further west along the dryline that sets up, but only moderate rainfall is anticipated. Therefore, flooding is not expected today.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

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

Over the mountains, storms will favor areas south and east of the Continental Divide. Additionally, storms are anticipated to form over the southern Front Range Mountains, Palmer Ridge and Raton Ridge. Activity will likely spill into the adjacent plains with the westward motion. Max 1-hour rain rates up to 0.25 inches will be possible under the stronger storms with gusty outflow winds being the main threat from storms this afternoon and evening.

There will likely be a couple areas of rainfall over the Northeast Plains as storms this afternoon initially form along a moisture boundary. Max 1-hour rain rates up to 0.75 inches will be possible over Phillips, Yuma, and Kit Carson County along with severe hail. Then, additional storms may be possible in the region as storms that form north rotate south around the high. Max 1-hour rain rates for those storms will drop to 0.50 inches. Mid-level energy may bring some lighter rainfall to the Southeast Plains this evening, and totals should remain under 0.25 inches.

Primetime: 1PM to 11PM

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

A bit too dry for widespread coverage of afternoon storms, but the passing shortwave to the north may provide a little extra moisture to the northern border. Inverted-V sounds indicated strong outflow winds will be possible with storms that do fire over the Northern Mountains, northern Central Mountains, and Northwest Slope this afternoon. Elsewhere it’s going remain dry and hot with high temperatures similar to yesterday.

Primetime: 1PM to 9PM

FTB 06-02-2020: Drying for Many, But Heavy Rainfall Still Possible for Some

Issue Date: Tuesday, June 2, 2020
Issue Time: 10AM MDT

— A LOW flood threat has been issued for eastern parts of the Northeast Plains
— A LOW flood threat has been issued for the Spring Creek burn scar in Huerfano and Castillo counties

There are two factors at play that will guide heavy rainfall chances across Colorado over the next 24-hours, as can be seen in the visible satellite imagery below.

First, a rather unusual feature: a band of ongoing shower and weak embedded thunderstorms extending from the Northeast Plains southwestward into the Southeast Mountains and even spilling over into the San Luis Valley. The unusual part being the fact that this is occurring during the morning, implying some active atmospheric dynamics overhead. Loops of overnight satellite images suggests this is a mid-level disturbance that has been cut-off from the main flow, and will be a key feature in organizing this afternoon’s diurnally-enhanced shower and storm activity in southeast Colorado. This morning’s Grand Junction and Denver radiosonde’s showed a Precipitable Water (PW) of 0.48 and 0.65 inches, respectively. This is similar to yesterday. However, some subtle drying is expected across western and central Colorado today, as a small-scale ridge rebuilds over the Four Corners. Thus, expect less storm activity across these areas. East of the Continental Divide, an area of higher moisture was noted on the SPC Mesoanalysis with a bulge of PW exceeding 0.80 inches over the Southeast Plains. Overnight and morning high-resolution model guidance is having a difficult time capturing the small-scale moisture maximum and circulation with ongoing precipitation. Thus, precipitation probabilities and rain rates look too low over the Southeast Mountains and Raton Ridge this afternoon. However, the only area of concern looks to be the Spring Creek burn area, which has been highlighted with a Low flood threat today.

The second feature of note is a strong cool front located over the US Northern Plains, associated with an area of low pressure moving eastward today. Note the choice of the word “cool”, as for our state, this front is more noteworthy for its moisture boundary than its temperature gradient. The moisture boundary is expected to race into Colorado late this afternoon, setting the stage for some severe weather across the far eastern parts of the state. Heavy rainfall will be likely, but the only areas of concern look to be near the Kansas border. Guidance suggests PW will increase up to 1.2 inches, with CAPE up to 2,000 J/kg. Upper-level storm motion will be weak but strong N/NE low-level flow will be capable of producing enough shear for storm cell training. A 3-4 hour window of possibly very heavy rainfall warrants a Low flood threat for this small area.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Southeast Plains, Palmer Ridge, Raton Ridge, Southeast Mountains, Southeast Plains:

Scattered showers and weak storms increasing in coverage and intensity this afternoon and evening, especially across southern areas. Max 1-hr rainfall up to 0.7 inches above 6,000 feet and 1.0 inch below 6,000 feet is possible this afternoon and evening. A Low flood threat has been issued for the Spring Creek fire burn for this afternoon and evening. The strongest storms will also be capable of gusty winds. Primetime is noon through 10PM.

Northeast Plains, Urban Corridor, Front Range:

Variable cloudiness early then isolated to scattered showers possible this afternoon mainly over the higher terrain. Max 1-hr rainfall up to 0.5 inches. Another wave of storm activity is likely with the arrival of a moisture surge after about 5PM. Max 1-hr rainfall up to 2.1 inches (far eastern areas) and 0.7 inches (western areas) are possible. The strongest storms will be capable of producing gusty winds and hail up to 1.5 inches. A Low flood threat has been posted for far eastern areas for flash flooding and street flooding. Primetime is 3PM through 11PM.

San Juan Mountains, Central Mountains, San Luis Valley:

Increasing clouds with isolated to scattered showers developing early this afternoon and into the evening. Max 1-hr rainfall up to 0.6 inches, mainly across eastern areas. Flooding is not expected today.

Northern Mountains, Northwest Slope, Grand Valley:

Mostly sunny and hot today with an isolated thunderstorm possible across southern and eastern areas. Max 1-hr rainfall up to 0.25 inches. Flooding is not expected today.

FTB 06-01-2020: Rainfall Chances Increase Over the Southern Mountains to Start Meteorological Summer

Issue Date: Monday, June 1st, 2020
Issue Time: 9AM MDT

— A LOW flood threat has been issued for the Decker and Spring Creek burn areas

There’s a bit of cloud cover over the state to start the morning, but visible satellite is already showing those clouds breaking up over the eastern plains. By looking at the water vapor imagery below, it looks like moisture is a little weaker in this area as compared to the northwest corner of the state where moisture and lift is a bit stronger. This means it will likely take a bit longer for the sun to shine through over this region. The best moisture remains over areas with the green “X”, but PW remains high at Grand Junction and Denver around 0.70 inches. That means there should be plenty of moisture for another round of diurnally driven storms this afternoon with residual moisture under the ridging pattern to start meteorological summer.

Today’s pattern generally looks the same as Sunday. Westerly flow aloft will continue, but it is expected to weaken, which means there will be little to no steering flow. This will cause storms to be nearly stationary and pulse in place with little upper air dynamics to make them severe. Additionally, this afternoon looks to have a little more widespread rainfall activity that yesterday with storms favoring the areas along and near the Continental Divide in the southern Front Range, Central Mountains, Southeast Mountains and San Juan Mountains. Storms are also likely to form over the western Palmer and Raton Ridge, but activity will likely decrease when compared to yesterday. Accumulations should also be on the upswing with the steering flow so weak. With the Southeast Mountains expecting to have more rainfall activity this afternoon, the Low flood threat has been re-issued for the Decker and Spring Creek burn areas. Elsewhere, rain rates are forecast to be low enough that flooding issues should be avoided.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Front Range, Southeast Mountains, San Juan Mountains, & Central Mountains:

Over western Colorado, storms look to favor the eastern Central Mountains and San Juan Mountains. It looks to be most active further south, which includes increased activity over the Southeast Mountains. Max 1-hour rain rates up to 0.65 inches will be possible over the eastern San Juan Mountains with 1-hour rain rates just over 0.50 inches possible over the Southeast Mountains. This is shaping up to be a nice wetting rain for that drought area in the San Juans. Further north into central Colorado, max 1-hour rain rates up to 0.50 inches will be possible along and near the Divide. A couple stronger storms may produce some brief gusty winds and small hail. Storms should start to end a couple hours after sundown when instability decreases.

These rain rates will be strong enough to cause flash flooding issues over recent burn areas in the Southeast Mountains if storms form directly overhead. Thus, a Low flood threat has been issued for the Decker and Spring Creek burn areas. Threats include local stream flooding, mud flows and debris slides. Note that we have yet to see what flash flooding looks like near and around the Decker burn area, and what rain rates will cause issues for the area. Please exercise extreme caution if you’re participating in recreation near the burn scar.

Primetime: 1PM to 11PM

Raton Ridge, San Luis Valley, Palmer Ridge, Southeast Plains, Urban Corridor, Palmer Ridge, & Northeast Plains:

Enhanced convergence along the western Palmer and Raton Ridge may produce some afternoon storms. Max 1-hour rain rates up to 0.50 inches will be possible with most storms producing virga and total rainfall under 0.25 inches. The edges of the San Luis Valley may also get a little rainfall with totals up to 0.20 inches possible by morning. There is the possibility of some weak rainfall over the Northeast Plains tonight, but totals should remain under 0.25 inches.

Primetime: 1PM to 12AM

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

Looks to remain hot and dry for these regions, especially north. High temperatures over the lower elevations looks to reach the upper 80Fs and 90Fs, so the increase in afternoon cloud cover will be welcomed. There is a chance for some storms to move in from the southeast (around the high) into the southwest corner of the state, but storms should dissipate quickly as they move off the high terrains and into drier air.

FTB 05-31-2020: Diurnal Thunderstorms Return to the Mountains

Issue Date: Sunday, May 31st, 2020
Issue Time: 9:20AM MDT

— A LOW flood threat has been issued for the Deckers and Spring Creek burn areas

Slight pattern shift from the previous few days. The upper low that was off the coast of California is now an open, upper wave over the Pacific Northwest (dashed purple line). There is some lift still associated with the trough (upper jet) as seen by the cloud cover to our northwest. Several small mid-level disturbances are expected to continue to move through the flow around the ridge. In fact, these are what is helping produce the cloud cover currently seen over western Colorado and the mountains. Meanwhile, the axis of the ridge is building to our east with an elongated 500mb high over northern New Mexico and Texas. What does all of this mean for rainfall chances today?

As seen by the cloud cover below, there is still quite a bit of moisture trapped under the ridge. This will help produce another round of diurnally driven thunderstorms. Storms will again favor the mountains for coverage, although with only weak mid-level disturbances moving through the flow, storms are not expected to be as widespread today. Additionally, southwest flow aloft has picked up over the state, which will turn more westerly throughout the day. This will decrease the rain rate efficiency as surface moisture begins to mix out. Therefore, expecting a downtick in dew points, and the main threat from storms today to be wind. With storm motion from east to west this afternoon, storms will likely spill into the adjacent plains again. They have the best chance of survival over the ridges (Cheyenne, Palmer, Raton), and are not expected to survive too far into the eastern plains.

A Low flood threat has been issued for the Deckers and Spring Creek burn areas due to trailing storms increasing totals and semi-saturated soils from rainfall the last few days. Elsewhere, flooding is not expected.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Front Range, Southeast Mountains, San Juan Mountains, Northern Mountains, & Central Mountains:

The Northern Mountains look to remain mostly dry this afternoon, but a few dry thunderstorms may be possible over the northern border. Over the Central and San Juan Mountains, storms will favor the areas near and along the Continental Divide. Max 1-hour rain rates up to 0.50 inches will be possible along with strong downdraft winds. Over the Front Range, rain rates up to 0.75 inches will be possible under the strongest cores. The Southeast Mountains will likely see a drop in rain rates with max 1-hour rain rates just over 0.50 inches possible. Should a storm or multiple storms track directly over burn areas, flash flooding, debris slides and mud flows may be possible. A Low flood threat has been issued for the Deckers and Spring Creek burn areas due to 1-3 hour totals likely exceeding 0.50 inches in this scenario and rain helping to saturate flood prone soils over the last few days. Weak rainfall may persist overnight over the southern mountains, but overnight flooding is not forecast.

Primetime: 1PM to 12AM

Raton Ridge, Southeast Plains, Urban Corridor, Palmer Ridge, & Northeast Plains:

Storms will push off into the plains earlier this afternoon with steering winds turning more westerly. Best chance for rainfall will be over the Raton and Palmer Ridge where isolated max 1-hour rain around 0.75 inches will be possible. Storms are not expected to survive long if they make it over the Northeast and Southeast Plains (lower elevations), but a quick 0.25-0.50 inches still seems feasible. Strong outflow winds and small hail will be the main threats from the storms today.

Primetime: 1:30PM to 12AM

Northwest Slope, Grand Valley, Southwest Slope, & San Luis Valley:

Less chance for rainfall at the lower elevations this afternoon, but chances of storms producing strong wind gusts will go up (50 mph+). Max 1-hour rain rates will be around and under 0.15 inches for storms that do move off the mountains in the lower terrains. The best chance for accumulations will be over the Northwest Slope. As expected, flooding is not forecast.

Primetime: 1PM to 9PM