Issue Date: Sunday, June 20th, 2021
Issue Time: 9:10AM MDT

— Flooding is NOT expected today, but moderate rainfall is expected overnight mainly over northeast Colorado

The summer solstice will occur at 9:31PM MDT tonight, and will ironically be accompanied by a very strong cool front moving in from the north. In fact, an initial cool front, seen primarily by a wind shift, has already moved into northeast Colorado this morning. This will knock down afternoon high temperatures by 5F or so across the Northeast Plains. However, the stronger cool front, accompanied by a moisture surge and notable synoptic scale forcing in the middle and upper atmosphere, will approach the WY/CO border right around sunset. Until then, we expect mostly sunny to partly cloudy conditions across the state. The highest chances of rain during the afternoon and early evening will be over the Raton Ridge, Southeast Mountains and San Juan Mountains, though only widely scattered coverage of weaker thunderstorms is expected.

As the strong cold front approaches this evening, instability will be generally lacking but moisture will be plentiful. This morning’s PW at Denver was only 0.48 inches, but is expected to spike to near 1.0 inch overnight with the frontal passage. With limited instability, we expect more of a widespread, moderate-intensity type rainfall over a 3-6 hour duration for most locations of northeast Colorado. Thus, flooding is NOT expected.

West of the Continental Divide, it will continue to be windy and very dry today, maintaining an elevated wildfire threat through this evening. Stay tuned to local National Weather Service office Red Flag Warnings for more information. Unfortunately, the cold front will not make much progress west of the Continental Divide.

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:

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

Becoming partly cloudy and slightly cooler today with isolated afternoon showers and weak storms possible over the higher terrain. A stronger cool front moves in around sunset, generating widespread shower and thunderstorm activity, with higher rainfall intensity for eastern areas. Max 1-hour rainfall up to 1.1 inches (east), and 0.7 inches (west) with max 3-hour rainfall up to 2.2 inches possible for far eastern areas. However, flooding is NOT expected today.

Primetime: 7PM through 6AM

Raton Ridge, Southeast Mountains, Southeast Plains, San Luis Valley and San Juan Mountains:

Partly cloudy and continued hot with isolated to widely scattered showers and thunderstorms this afternoon and evening. Max 1-hour rainfall up to 0.8 inches (below 5,500 feet) and 0.5 inches (above 5,500 feet). Flooding is NOT expected today.

Primetime: 12PM through 8PM

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

Mostly sunny and continued hot, with windy conditions developing this afternoon. An isolated shower or weak storm is possible for the highest elevations, but max 1-hour rainfall limited to 0.4 inches. Flooding is NOT expected today. A Red Flag Warning has been issued for parts of the region, so stay tuned to local National Weather Service offices for updates.

Primetime: 12PM through 6PM

Issue Date: Saturday, June 19th, 2021
Issue Time: 9:45AM MDT

— A LOW flood threat has been posted for parts of the Northeast Plains, Southeast Plains and Palmer Ridge

As shown in the visible satellite image, below, a much weaker version of the long-lasting upper-level ridge continues to be centered over the Four Corners. The big change over the past 24-hours across Colorado has been an increase in moisture statewide. Dewpoint temperatures have risen 4-6F both east and west of the Continental Divide. PW has increased from 0.59 to 0.66 inches in Denver, and has held steady around 0.78 inches at Grand Junction (which is above normal for this time of year). However, as has been the case for the better part of a week, this moisture continues to be very shallow in the boundary layer, and concentrated mainly in the mid and upper levels. Nonetheless, the increase in moisture will support more widespread shower and storm activity this afternoon and evening. In addition to moisture, there is a pronounced shortwave disturbance entering northwest Colorado. This is already causing showers and a few weak thunderstorms in a moisture-starved environment. As this feature moves east-southeast and encounters more moisture and instability, expect higher intensity and more widespread rainfall. There are a few factors mitigating the heavy rainfall threat today. First, cloud cover over northern Colorado will likely limit instability to localized pockets, which will limit thunderstorm core size and thus make storms more prone to dry air entrainment. This will likely affect most of the Front Range and Urban Corridor throughout the day. Additionally, steering winds will increase markedly this afternoon over northern Colorado, and will help limit rainfall at any given location.

In terms of sensible conditions, we expect a quick increase in shower and storm activity, first over the higher terrain, then spreading eastward into the plains during early afternoon. Multiple rounds of rainfall are likely as the June sunshine will quickly recover any lost instability after a storm passes through. Two areas have been placed under a Low flood threat. First, over the Palmer Ridge, steering flow will be weaker and instability is expected to be maximized with limited cloud cover. With dewpoint temperatures exceeding 50F east of Colorado Springs, storms will be capable of heavy rainfall and isolated flash flooding at the 1-hour duration. Second, along the KS border of the Northeast Plains, we expect moderately strong moisture advection from the southeast this afternoon and evening to provide for a 3-6 hour period of heavy rainfall, warranting a Low flood threat for isolated flash flooding. Large hail, damaging wind and an isolated tornado will also be possible here.

Finally, west of the Continental Divide, although rain chances will increase mainly over the higher terrain, the result will be bittersweet as dry lightning will become a major concern this afternoon due to limited boundary layer moisture. A Red Flag Warning has been posted for most of the region through early evening.

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:

Northeast Plains, Southeast Plains and Palmer Ridge:

Partly to mostly cloudy with widespread showers and thunderstorms this afternoon and into the early overnight hours, especially over northern areas. Max 1-hour rainfall up to 2.1 inches (north) and 1.3 inches (south) warrants a Low flood threat for isolated flash flooding and higher elevation debris slides and mud flows. Max 3-hour rainfall up to 3.2 inches will be possible along the KS border. Large hail, damaging winds and an isolated tornado will also be possible, mainly over the Northeast Plains.

Primetime: 12PM through 2AM

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

Mostly cloudy with scattered to numerous showers and thunderstorms this afternoon and evening. Max 1-hour rainfall up to 1.0 inch could cause nuisance street flooding in urban areas, but that aside, flooding is NOT expected. Large hail and damaging winds could accompany the strongest storms.

Primetime: 12PM through 9PM

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

Variable cloudiness with scattered to numerous higher elevation showers and weak thunderstorms this afternoon and early evening. Max 1-hour rainfall up to 0.6 inches. Dry lightning is a major concern this afternoon, and a Red Flag Warning has been issued mainly for lower elevations. Flooding is NOT expected today.

Primetime: 12PM through 8PM

Issue Date: Friday, June 18th, 2021
Issue Time: 10:25AM MDT

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

Slightly cooler temperatures are forecast today over eastern Colorado as a cold front continues to push through the state this morning (blue dashed line below). The passing Low that helped flatten the ridge to our north and the upper High itself has returned moisture to the state (blue/white shades below). Deeper surface moisture is expected behind the front, and PW has already begun to rise at Denver (0.59 inches). Back to the west, the moisture is located more in the mid and upper levels of the atmosphere. Grand Junction’s sounding measured PW at 0.77 inches, which is expected to help produce moderate rainfall accumulations over the Central and San Juan Mountains this afternoon with training storms.

Over eastern Colorado, afternoon post-frontal upslope flow with the moisture return is expected to produce scattered to numerous storms over the mountains. As storms move off the mountains to the east with the westerly flow aloft, they will move into higher moisture and instability. The stronger storms that develop will also be capable of producing heavy rainfall, which could produce some localized flash flooding issues. Best chance for these storms will be over the elevated ridges. The front looks to stall somewhere over the Southeast Plains late this afternoon. Additional storms may develop along it later tonight, and the extra convergence may help to produce training storms capable of heavy rainfall. Thus, a 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 Mountains, Front Range, Urban Corridor, Palmer Ridge & Raton Ridge:

Numerous storms are forecast develop over the mountains this afternoon with the post-frontal upslope flow. Max 1-hour rain rates up to 0.25 inches will be possible over the southern Front Range and Southeast Mountains. Westerly steering flow will push the storms east where they will encounter some higher moisture. As this occurs, stronger storms should begin to develop that will be capable of producing heavy rainfall, hail and strong outflow winds. Best chance for the more severe storms will be over the elevated ridges where max 1-hour rain rates may reach 1.25 inches. A Low flood threat has been issued for the adjacent plains where road flooding and ponding in low-lying areas may occur.

Primetime: 3PM to 9PM

Northeast Plains & Southeast Plains:

Deeper moisture return is expected over these areas as well as an increase in convergence along the front (south). Max 1-hour rain rates with training storms (south) may reach up to 2.25 inches. Further north, max 1-hour rain rates up to 1.75 inches will be possible. A Low flood threat has been issued for possible flash flooding of roads, fields and local streams. The severe storms that develop will also likely produce hail and strong outflow winds. Storms may linger into early tomorrow morning.

Primetime: 8PM to 3AM

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

Increased mid and upper level moisture, paired with the diurnal flow and mid-level energy, will help to return scattered afternoon storms to the Central/San Juan Mountains as well as the elevated plateaus of the Northwest/Southwest Slopes. While some evaporation will occur, moderate accumulations will still be possible with training storms. Max 1-hour rain rates up to 0.20 inches (west), 0.50 inches (Central Mountains) and 0.75 inches (San Juan Mountains) will be possible. Strong outflow winds and lightning will be the main threats. Flooding is NOT forecast.

Primetime: 2PM to 9PM

 

Issue Date: Thursday, June 17th, 2021
Issue Time: 9:20AM MDT

— Flooding is NOT expected today

Today will be the last day of the recent record-breaking heat wave, as the upper-level ridge will finally begin to weaken this afternoon. Unfortunately, this means another day of record or near-record temperatures for most of the state. As shown in the visible satellite image, below, the ridge is centered over southwest Colorado this morning and has weakened slightly over the past 24 hours. Some monsoonal-type mid and high-level moisture has found its way around the northern periphery of the ridge this morning. Grand Junction’s PW has increased from 0.48 to 0.58 inches over the past 24 hours with most of the increase coming in the mid and upper-levels. Nonetheless, this will support a slight increase in storm coverage west of the Continental Divide, although only light precipitation is expected there today.

East of the Continental Divide, dewpoint temperatures have generally dropped by 2-4F since yesterday morning. Denver’s PW is up slightly from 0.49 to 0.54 inches this morning driven mainly by increases above the boundary layer. Boundary layer moisture continues to be very shallow, and slight drying since yesterday, especially along the KS/NE border, suggests a reduction in storm coverage and peak rainfall intensity today.

Overall, we expect another round of isolated to widely scattered shower and thunderstorm activity mainly limited to the higher terrain as well as the Southeast and Northeast Plains. Weak steering flow and marginal instability implies storms will have a hard time staying intact for more than 30 minutes. Pulses of moderate rainfall are expected today, but there is no flood threat. Isolated large hail will also be possible with the strongest storms in the eastern plains.

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:

Northeast Plains and Southeast Plains:

Continued very hot today with widely scattered showers and thunderstorms this afternoon and evening. Max 1-hour rainfall up to 1.4 inches, locally. Large hail could accompany the strongest cells.

Primetime: 1PM through 10PM

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

Continued very hot with isolated to widely scattered showers and thunderstorms this afternoon and evening. Max 1-hour rainfall up to 0.5 inches above 6,000 feet and 0.8 inches for lower elevations. Flooding is NOT expected today.

Primetime: 12PM through 9PM

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

Partly cloudy and continued very hot today with isolated to widely scattered showers and weak storms possible, mainly over the San Juan Mountains and Central Mountains. Max 30-minute rainfall up to 0.5 inches. Flooding is NOT expected today.