Issue Date: Wednesday, May 19th, 2021
Issue Time: 9:40AM MDT

— A NWS Flood Warning has been issued for the Arkansas River at Avondale

The upper Low will begin to weaken and move to the northeast throughout the day. That means that flow aloft will turn from northerly to more westerly by this evening and then southwesterly by tomorrow morning, which will start to entrain a drier air mass from west to east (yellow shades). There should be enough residual moisture (blue/gray shades) for scattered storms to develop over the mountains as the upslope flow kicks in this afternoon. Since widespread lift and moisture will be decreasing, storms are expected to be more scattered than widespread in nature with accumulation mostly occurring over the eastern mountains. Slow south and southeast movement of storms along with subsidence over the adjacent plains should anchor the scattered storms to the high terrains. With portions of the Southeast Mountains and Raton Ridge receiving between 3 to 5 inches over the last 3 days (saturated soils), increased runoff is expected with storms that develop over these areas. However, rain rates are forecast to decrease, so flooding issues should be avoided. Additional weak thunderstorms may form over the far eastern board with the main threats being small hail and outflow winds. Flooding is NOT forecast 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:

Raton Ridge, Palmer Ridge, Front Range, San Juan Mountains, Southeast Mountains, Northern Mountains & Central Mountains: 

Scattered storms are possible over the high terrains again this afternoon with the residual moisture hanging around. To the west, accumulation will be favored over the southern/eastern San Juan Mountains where rain rates up to 0.25 inches per hour will be possible. Over the eastern mountains, max 1-hour rain rates up to 0.5 inches per hour will be possible. This could cause some excessive runoff further south and east where rainfall has been falling the last few days. However, flooding is NOT anticipated.

Primetime: 1PM to 10PM

Southeast Plains, Northeast Plains & Urban Corridor:

The Urban Corridor should stay dry this afternoon with storms anchored to the high terrains. A couple scattered thunderstorms may be possible over the central, eastern border counties as the Low lifts to the northeast. The main threats from thunderstorms that develop will be small hail and gusty outflow winds. Rain rates up to 0.75 inches per hour may be possible, so flooding is NOT forecast.

Primetime: Noon to 6PM

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

A beautiful day is ahead with a couple scattered storms possible this evening over the Northwest Slope. Wind from storms that develop will be more of a threat than rainfall, but isolated totals up to 0.20 inches will be possible. The San Luis Valley should also get some more rainfall with 24-hour totals around 0.25 inches possible. Flooding is NOT forecast. 

Primetime: 5:30PM to 10PM

Issue Date: Tuesday, May 18th, 2021
Issue Time: 9:50AM MDT

— A MODERATE flood threat has been issued for parts of the Raton Ridge
— A LOW flood threat has been issued for parts of the Palmer Ridge, Southeast Plains and Southeast Mountains

This morning’s visible imagery, below, shows that the slow-moving, cut-off upper-level low pressure has finally made it into the state. A clear surface low is noted this morning over the CO/NM border. Many metrics related to heavy rainfall potential have decreased markedly since yesterday morning. For example, instability will be limited to the southeast quadrant of the state today. Mid-level lapse rates have fallen substantially, which also places a bit of a constraint on moisture convergence, and thus high-end rainfall rates. Moisture has also generally decreased with Denver’s PW dropping from 0.76 to 0.56 inches. Although Grand Junction’s has increased from 0.40 inches to 0.61, boundary layer moisture is still lacking for heavy rainfall activity west of the Continental Divide. Lastly, cooler temperatures will lower the snow line to below 9,000-10,000 feet, further decreasing the concern of excessive runoff.

However, over the Raton Ridge and surrounding areas, the combination of strong daytime heating (see clearing on visible satellite image), deep, moist, upslope flow and relatively slow steering winds will contribute to a Moderate/Low flood threat. A very small corridor of Moderate flood threat has been issued on the eastern face of the Wet Mountains, where antecedent rainfall above 2 inches will cause very effective runoff of any new rainfall. Isolated flash flooding will be possible today in the threat areas. Additionally, an elevated concern for mud flows and debris slides in steeper terrain will persist through the day and into the evening, as these processes can often show a delayed response to long duration heavy precipitation.

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:

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

Much cooler today with widespread showers and thunderstorms, increasing in coverage and intensity through the afternoon. Max 1-hour rainfall up to 1.2 inches will be possible, with max 3-hour rainfall up to 2.1 inches. The juxtaposition of the new rainfall with yesterday’s high amounts warrants a narrow Moderate flood threat for the eastern face of the Wet Mountains. Elsewhere, a Low flood threat has been issued. A severe storm or two is possible along the KS, OK and NM borders with the primary threat being large hail.

Rainfall intensity will subside this evening, but the threat of mud flows and debris slides will persist into the night.

Primetime: Now through 6AM, with highest rainfall intensity between 12PM and 7PM

Northern Mountains, Front Range, Urban Corridor, Central Mountains and Northeast Plains, San Juan Mountains and San Luis Valley:

Cooler and variable cloudiness with scattered showers and a weak thunderstorm possible in the afternoon and evening. Max 1-hour rainfall up to 0.8 inches eastern areas and 0.5 inches in western areas. However, flooding is NOT expected today.

Grand Valley, Southwest Slope and Northwest Slope:

Partly cloudy and cooler with isolated showers, and perhaps a weak thunderstorm possible this afternoon. Max 1-hour rainfall up to 0.3 inches. Flooding is NOT expected today.

Issue Date: Monday, May 17th, 2021
Issue Time: 10:15AM MDT

PM Update (4PM): Heavy rainfall is occurring over the Southeast Mountains and eastern Raton Ridge with a couple gauges already recording 1 inch. With rainfall forecast to continue for the next 3-5 hours, the Moderate threat has been pulled back to the Southeast Mountains and Raton Ridge. Tonight’s heavy rainfall threat has also been pulled back into the western Southeast Plains (Pueblo, Crowley and Otero Counties). Along the eastern border, Kiowa, Bent and northern Prowers will continue to be under the Moderate flood threat due to the potential for increased runoff from any storms that develop in that area.

— A MODERATE flood threat has been issued for portions of the Palmer Ridge and Southeast Plains
— A LOW flood threat has been issued for the Front Range, Raton Ridge, Southeast Mountains, Palmer Ridge and portions of the Central Mountains and Northeast Plains

It’s a warm and nearly sunny start to the day over western Colorado, and a cool and cloudy start to the day over eastern Colorado. The upper-level Low is currently over the Desert Southwest and will track eastward throughout the day helping to produce counter-clockwise winds. Moisture wrapped around this Low, mid-level lift in front of it and persistent upslope flow will produce widespread storms over the mountains this afternoon and evening. With weak steering flows once again and storms pulsing in intensity (embedded convection), decent accumulations are forecast. A Low flood threat has been issued for the Front Range and Southeast Mountains where 1 to 2 hour totals up to 1.50 inches are possible, which could cause local flooding issues.

The upslope component over eastern Colorado will help reinforce the moisture already in place, and PW this morning at Denver increased to 0.76 inches. This is about as high as it gets for mid-May, so it’s juicy out there. Lift out in front of the Low as it moves eastward will help produce additional storm activity over the adjacent eastern plains and Southeast Plains. Stronger instability will be able to build over the Raton Ridge and far eastern Plains, so a couple severe storms may develop with the main threats being hail and outflow winds. Finally, a local vorticity maximum over the Northeast Plains and plenty of outflow boundaries from the last couple days will help produce a third set of storms over this region. Similar to the last couple of days, heavy rainfall is anticipated with the thunderstorms that develop over the eastern plains. Thus, a Moderate/Low flood threat has been issued.

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, Northeast Plains, Raton Ridge & Palmer Ridge:

Expect slow northeast movement of the storms as the Low traverses east. A couple severe storms are possible this afternoon with the main threats being severe hail and strong outflow winds. Heavy local rainfall is possible late this afternoon, evening and overnight with the high moisture and slow steering flow, and isolated totals up to 3 inches will be possible once again. Some of these regions have received quite a bit of rainfall over the last couple of days. With saturated soils, if rainfall tracks over the same area, field ponding, road flooding and excessive rainfall is possible. A Low/Moderate flood threat has been issued. Expect rainfall to linger into tomorrow morning.

Primetime: Noon to Ongoing

Front Range, Southeast Mountains & Urban Corridor:

A Low flood threat has been issued for these zones due to the consistent upslope flow generating multiple rounds of rainfall. Just enough daytime heating could help produce some embedded convection within the widespread rainfall. One to three hour totals could reach up to 1.50 inches, which could cause some local flooding issues. Expect storms to move into the Urban Corridor by late afternoon. The intensity of these storms is still a bit unknown due to the cloud cover. Similar to yesterday, if the sun can poke through, max 1-hour rain rates up to 1.10 inches per hour will be possible. Thus, the Low flood threat has been issued.

Primetime: 4PM to 11PM

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

Finally, some wetting rainfall for the high terrains this afternoon. Small hail and brief outflow winds will be possible with the stronger thunderstorms that develop. Max 1-hour rain rates up to 0.75 inches per hour (Central Mountains) and 0.50 inches will be possible (Northern/San Juan Mountains). Best chance for accumulations will be over the Central Mountains with more spotty totals around 0.5 inches for the other mountain ranges. Portions of the San Luis Valley could receive up to 0.30 inches. Shower activity should begin to end a couple hours after sundown as instability decreases.

Primetime: 1PM to 10PM

Issue Date: Sunday, May 16th, 2021
Issue Time: 9:45AM MDT

PM update (5:15 PM): Lack of daytime heating due to today’s cloud cover has decreased the flood threat across the Urban Corridor. The heavy rainfall forecast for the eastern plains looks to be on track, although totals will likely be slightly lower than guidance hinted at this morning. Nonetheless, persistent and heavy rainfall is expected to cause flooding issues. 

 
— A MODERATE flood threat has been issued for portions of the Palmer Ridge, Northeast Plains and Southeast Plains
— A LOW flood threat has been issued for the Urban Corridor and portions of the Front Range

Plenty of moisture remains over the eastern portion of the state this morning. Dew points are being measured in the upper 40Fs and 50Fs, and PW was measured at 0.60 inches in Denver and 1.15 inches at North Platte (NE). Visible satellite below indicates fog over the Northeast Plains and cloud cover pushing in from our north (green arrow). As this boundary pushes south (marked by the cloud cover), it looks to stall out and will likely support and anchor heavy rainfall over the eastern plains this afternoon and evening. The environment to the north of this boundary (northeast corner) will likely be capped, which means heavy rainfall chances are limited unless there are some breaks in the cloud cover.

Weak mid-level energy will continue to move through the flow as the cutoff low approaches the four corners region. This will help spark numerous storms over the elevated Ridges and Front Range by mid-afternoon. Slow westerly steering flows are forecast once again, so the last big ingredient needed for heavy rainfall is instability. This should be no problem over the eastern plains where there already is plenty of sunshine. Thus, a Moderate flood threat has been issued for this area with increased lift coming later this evening. However, back along the Urban Corridor and Front Range, there needs to be some breaks in the cloud cover throughout this day for instability to build. If this occurs, thunderstorms that develop will be capable of producing very heavy rainfall over a 1 to 2 hour period. Thus, a Low flood threat has been issued over this area.

Over western Colorado, the approaching trough and slight increase in moisture will help produce weak storms over the high terrains. Best chance for accumulation will be along and near the Continental Divide; however, accumulations will be minimal.

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, Northeast Plains & Palmer Ridge:

A Moderate flood threat has been issued for this region. Slow steering winds, high moisture and increased lift over the area will come together this afternoon and evening for widespread thunderstorm development. Isolated rainfall totals up to 3.75 inches will be possible with more general storm core totals between 2 and 2.5 inches. This could cause road flooding, field ponding and local stream flooding, and additional rainfall is likely over areas that already received a lot of precipitation yesterday. A couple severe storms will be possible with the main threats being severe hail and strong outflow winds, although a weak tornado cannot be ruled out.

Primetime: 5PM to 3AM

Front Range, Southeast Mountains, Raton Ridge & Urban Corridor:

A Low flood threat has been issued for portions of these regions. Rainfall is likely this afternoon over the Front Range with only light showers forecast for the foothills of the Southeast Mountains. Better chance for storms development (south) over the Raton Ridge where max 1-hour rain rates just under 1 inch will be possible. Further north, if instability is able to form with breaks in cloud cover, heavy rainfall will be possible as storms move slowly off the mountains and into the Urban Corridor. In this scenario, storm totals up to 2 inches will be possible, which would cause road, underpass and local stream flooding. Severe threat is lower today, but some small hail may be possible with the stronger storms that develop.

Primetime: 3PM to 10PM

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

There should be an increase in cloud cover this afternoon as the trough approaches. While moisture has increased some, dry lower levels continue. This means the main threat from storms that develop again today will be wind. Accumulations up to 0.10 inches will be possible for areas along and near the Continental Divide. Flooding is NOT forecast.