FTB 07-31-2021: Widespread Heavy Rainfall, Flash Flooding and Riverine Flooding Expected Today

Issue Date: Saturday, July 31st, 2021
Issue Time: 10:30AM MDT

— A HIGH flood threat has been posted for parts of the Front Range, Urban Corridor, Palmer Ridge, Southeast Mountains, Raton Ridge and Southeast Plains
— The HIGH threat will persist through at least the early overnight hours
— A MODERATE flood threat has been posted for parts of the Northeast Plains, Northern Mountains, Central Mountains and San Juan Mountains
— A LOW flood threat has been posted for parts of the San Luis Valley, Northwest Slope, Grand Valley and Southwest Slope
— An afternoon update is possible today

On the 45th anniversary of the devastating Big Thompson floods that changed the way flood detection was done in Colorado, we are expecting a very active day of heavy rainfall and flooding across major parts of the state. In fact, all 14 of our forecast zones are under some kind of flood threat today.

Let’s start first with the most vulnerable areas, which will be the I-25 corridor south of Denver through the New Mexico border as well as southeast Colorado. A cold front raced through the state overnight, bringing down high temperatures from the recent heat wave. In fact, this will mean instability will not be all that impressive today anywhere in the state, with perhaps up to 1,500 J/kg at most expected in the Southeast Plains. However, the lack of instability will be easily overcome by very high moisture content through the entire atmosphere. Denver’s PW came in at 1.20 inches today, significantly above normal for the date. Up to 1.5 inches of PW is currently estimated in eastern Colorado and this plume of moisture will move southward today after the cold front passage. A decaying mesoscale storm complex, with a distinct anti-cyclonic circulation aloft, was noted along the northeast CO border. The main impact of this will be to limit instability over the I-76 corridor, likely suppressing heavy rainfall activity. Further south and west, enough breaks in the clouds today will support moist adiabat type soundings with extremely efficient rainfall generation expected. With the sheer amount of moisture available, we expect multiple rounds of heavy rainfall along the entire high terrain east of the Continental Divide. Storms will then spill over into the foothills and Southeast Plains by later in the afternoon and persist well into the overnight hours towards the OK/KS border. In particular, the threat today is not so much from short-term 1-hour rainfall but more so the 3-6 hour timeframe where multiple heavy rainfall cores could support up to 4-7 inches of rainfall over parts of the Palmer Ridge, Southeast Mountains, Raton Ridge and Southeast Plains. With the spatial extent of heavy rainfall, in addition to the normal threat of flash flooding, riverine flooding of creeks and streams will be likely today. Base flows on the larger rivers are not all the high, mitigating any large-scale riverine flooding. However, there is a chance of a flood wave, albeit brief, along the Arkansas River this evening and overnight.

Moving west of the Continental Divide, moisture has come down somewhat over the past days, especially over the far western areas. Furthermore, instability will be lower today. Nonetheless, with plenty of moisture and strong heating expected, storms capable of heavy rainfall are expected today mainly over the higher terrain. The primary threats will be isolated flash flooding, debris slides and mud flows, just like with past days.

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

Widespread showers and thunderstorms expected beginning this afternoon and lasting into the overnight hours over the Southeast Plains. Max 1-hour rainfall up to 4.0 inches over the Southeast Plains and up to 3.0 inches elsewhere. Max 6-hour rainfall up to 7.0 inches over the Southeast Plains and 5.0 inches elsewhere. A HIGH flood threat has been posted for large parts of the area for flash flooding, debris slides and mud flows over the steeper terrain as well as creek and stream flooding especially later into the evening as runoff begins to channel into the flood plains.

Primetime: 3PM through 5AM (Southeast Plains)
Primetime: 12PM through midnight (outside of Southeast Plains)

Urban Corridor, Northeast Plains:

Much cooler with scattered to numerous showers and thunderstorms this afternoon and evening, especially closer to the foothills. Max 1-hour rainfall up to 2.9 inches and max 3-hour rainfall up to 4.0 inches supports a MODERATE/LOW flood threat for isolated flash flooding. Creeks and smaller streams could experience minor flooding in areas with the more persistent heavy rainfall.

Primetime: 1PM through 11PM

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

Scattered with numerous showers and thunderstorms expected this afternoon and evening. Highest coverage will be over the foothills and higher elevations right along the Continental Divide. Max 1-hour rainfall up to 1.6 inches supports a MODERATE/LOW flood threat for isolated flash flooding, debris slides and mud flows.

Primetime: 12PM through 10PM

FTB 07-30-2021: Heavy Downpours Expected Over the Mountains & Northern Urban Corridor

Issue Date: Friday, July 30th, 2021
Issue Time: 10AM MDT

PM Update (3:40PM MDT): There is an increased chance for heavy rainfall producing storms anywhere across the Urban Corridor tonight and into the overnight hours. High dew points, increasing PW and extra convergence paired with slow steering flows will set the stage for the possibility of very high rainfall rates. Storms may be capable of producing road flooding, field ponding, low-lying intersection flooding and local stream/creek/gulch flooding. Therefore, the MODERATE threat has been extended eastward to include all of the Denver Metro area. Localized storm totals up to 2.75 inches possible will still be possible. 

MODERATE flood threat has been issued for the Northern Mountains, Central Mountains, and portions of the Northwest Slope, Grand Valley, Southwest Slope, San Juan Mountains, Front Range, Urban Corridor and Northeast Plains
LOW flood threat has been issued for the eastern Palmer Ridge, Southeast Mountains and San Luis Valley

An active rainfall day is ahead with the well-established monsoon surge (green arrow) continuing to rotate sub-tropical moisture around the High. Dew points are quite impressive across the state this morning at 50F+, which should aid in efficient rainfall rates as storms begin to fire over the mountains by early afternoon. PW at Grand Junction has risen to 1.17 inches, nearing the maximum moving average for this time year. While PW to the east, over Denver, was measured at 0.79 inches, it is expected to increase throughout the day to over 1 inch. Storms today will be mostly driven by the diurnal cycle that kicks in by early afternoon, but some weak mid-level lift from embedded shortwaves may help to generate better coverage of thunderstorms across western Colorado with a couple severe thunderstorms possible once again.

There is a much higher chance today that storms will spill into the adjacent eastern plains this evening (Urban Corridor and Northeast Plains). Part of the reason for this is that a cold front is expected to drop south through eastern Colorado around this time. Extra convergence along the front and post-frontal upslope flow will likely keep storms going over the plains and mountains into the overnight hours. Thus, the MODERATE flood threat has been extended into this region as rainfall rates could be very high with the extra convergence and high moisture content. However, over southern Colorado, storms are expected to dissipate a couple hours after sundown as instability decreases and a drier air mass works its way in from the northeast.

As far as the flood threat today, it is widespread and elevated as the storms that develop this afternoon and evening have the potential to produce a lot of rainfall over numerous locations. This is due to the high moisture, slower steering flows and the potential for back-building/training storms. It is also likely that in this moisture-rich environment, outflow boundaries from stronger storms will help to trigger additional storm coverage over the mountains. There is also a threat for riverine flooding today along local creeks and streams, especially around and near recent burn areas. Over steeper terrains, the heavy rainfall has to potential to generate mud flows and debris slides along with road flooding and field ponding possible over the adjacent plains. A large MODERATE threat has been issued for these reasons. Please stay tuned into your local NWS office this afternoon for the latest on real-time warnings that are 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:

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

Over the mountains, max 1-hour rain rates up to 1.75 inches (north) and 1.50 inches (south) will be possible along with a couple severe thunderstorms. A large swath of 1-inch accumulations are likely by morning over the steeper terrains, and isolated totals just over 2 inches are possible further north. Storms will likely increase in coverage throughout the evening and may last into the overnight hours. The flood threat should end just before 10PM (south) & midnight (north) with some light rainfall lingering into tomorrow morning. A MODERATE/LOW flood threat have been issued.

Primetime: 1PM to 2AM

Front Range, Northeast Plains and Urban Corridor:

Max 1-hour rain rates up to 1.75 inches will be possible over the Front Range this afternoon and evening. Widespread areas over the mountains could receive 1-inch of rainfall, which increases the threat for riverine flooding – especially near recent burn areas. As the front drops south this evening, a couple stronger storms could develop along the area of enhance convergence. Max 1-hour rain rates up to 2.50 inches with totals over 3 inches may be possible over the adjacent plains. Storms should linger into the overnight hours with the flood threat ending just around 1AM. For these reasons, a MODERATE flood threat has been issued

Primetime: 1PM to 2AM

Southeast Mountains, Raton Ridge, San Luis Valley, Palmer Ridge & Southeast Plains:

Storms should stick close to the higher terrains this afternoon and be more isolated in nature. Isolated totals over a 1 to 2 hour period up to 1 inch will be possible, so a LOW flood threat has been issued. The clockwise rotation of the storms could push some action into the San Luis Valley with totals up to 0.60 inches by morning in the interior valley.  Impending dry air from the northeast will likely end the rainfall chances by early evening.

Primetime: 1:30PM to 8PM

FTB 07-29-2021: Flood Threat Returns with Isolated Severe Storms Possible

Issue Date: Thursday, July 29th, 2021
Issue Time: 10:15AM MDT

LOW flood threat has been issued for the Northwest Slope, Northern Mountains, Front Range, Grand Valley, Central Mountains, Urban Corridor, Southwest Slope and San Juan Mountains

The monsoon moisture surge continues around the High pressure system shown below (green arrow). Within the plume of moisture, PW remains over 1 inch, so it’s a juicy air mass. In this morning’s Grand Junction sounding, PW was measured at 1.08 inches, and it should hover around this above average value throughout the day. Clockwise rotation around the High will pull the plume and mid-level energy noted by the orange “X” below over the northern Front Range by this afternoon. This is expected to return storm chances to the area, and the more westerly steering flows over this region will likely push storms into the Urban Corridor this evening. Storm chances are lower over the adjacent plains due to a cap in place, but a few storms could break the cap along the I-25 corridor. Drier air, marked by the yellow/orange shades below, should help to limit the rainfall potential over the eastern/central mountains south of the Palmer Ridge and over the adjacent ridges and plains. Outside of some weaker and more isolated storms for the Southeast/Central Mountains, southern and eastern Colorado should be dry and hot once again.

Steering flow speed is expected to be comparable to yesterday and training storms are forecast once again, which should let storms today produce higher-end rainfall accumulations that may lead to local flooding issues. Additionally, a couple low-end severe thunderstorms are possible, and these will be most likely over the northern portion of the state where afternoon instability looks to reach 1,500 to 2,500 J/kg. With all the ingredients for heavy rainfall coming together once again, another Low flood threat has been issued over a broad area. While chances for rainfall are lower over the adjacent, eastern plains with the aforementioned cap in place, should a storm or two break the cap, heavy rainfall will be possible. Thus, the Low threat has been extended eastward into the area that this would be most likely to occur.

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:

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

High moisture continues to stream into the area with the clockwise rotation around the High. While heavy rainfall chances aren’t quite as widespread as yesterday, isolated storms over these regions will be capable of producing rainfall rates up to 1.25 inches. Where training storms develop, 24-hour totals up to 2 inches will be possible. Flooding threats this afternoon and evening include mud flows/debris slides, road flooding and local stream flooding, so another LOW flood threat has been issued. A couple severe storms are also possible over the southern San Juan Mountains and further north over the Northwest Slope/Grand Valley with the threats being 1 inch hail and 60 mph gusts. Guidance indicates some stratiform rainfall over the mountains through the overnight hours, although the flood threat drops off a couple hours after sundown.

Primetime: 1:30PM to 2AM

Front Range, Northern Mountains, Palmer Ridge and Urban Corridor:

With the arrival of the plume and some mid-level energy, the flood threat increases across the Front Range and Northern Mountains today. A couple severe storms will also be possible over these forecast regions with decent CAPE and minimal shear intact. Max 1-hour rain rates up to 1.75 inches will be possible with isolated storm totals up to 2.50 inches. Westerly steering flow will push the storms into the adjacent plains this evening and a storm or two may break through the cap. If this occurs, rainfall rates up to 1.25 inches will be possible, which could create road flooding, field ponding and local stream flooding. A LOW flood threat has been issued.

Primetime: 1:30PM to 10:30PM 

Southeast Mountains, Raton Ridge, San Luis Valley, Northeast Plains & Southeast Plains:

Isolated, high-based storms will be possible over the Southeast Mountains this afternoon and expect many of the storms to only produce virga. Max 1-hour rain rates up to 0.20 inches are forecast, and the main threat from storms that develop will be brief, gusty outflow winds. With more easterly steering flow around the High, storms may spill into the San Luis Valley. Isolated totals up to 0.15 inches will be possible. The plains should stay dry, and all regions should expect another hot day ahead.

Primetime: 2PM to 8:30PM

FTB 07-28-2021: Rainfall Coverage To Increase Over Western Slope

Issue Date: Wednesday, July 28th, 2021
Issue Time: 10:10AM MDT

— A LOW flood threat has been posted for parts of the Central Mountains, Grand Valley, San Juan Mountains and Southwest Slope

A battle of monsoonal versus dry air continues across Colorado, with the Continental Divide roughly acting as the barrier, as seen in the water vapor image below. To the east, Denver’s morning PW of 0.66 inches is up significantly from yesterday’s 0.47 inches. However, subsidence noted by a distinct warm layer around 500 mb and very dry air aloft will continue to place a lid on storms activity in most areas east of the Continental Divide (with the exception being the Southeast Mountains). To the west is where things get interesting. This morning’s PW at Grand Junction came in at an impressive 1.30 inches, which is close the daily record and also within about 10% of the all-time record value in the 1.45 inch range. PW should stay steady or perhaps slightly decrease as some intrusion of dry air is likely around the very high terrain. However, PW well above 1 inch should persist towards the UT border. Surface moisture was equally impressive with dewpoint temperatures in the low 60s over the lower elevations of the Grand Valley and Southwest Slope. Instability up to 1,500 J/kg is expected for this afternoon, which is on the higher range of typical monsoon activity. Generally southeasterly steering flow of ~15mph should keep storms moving, with enough shear present to support a more organized storm complex or two, just like on Tuesday. A once impressive, but now decaying storm complex was noted in southeast UT this morning, and its circulation could prevent storms due to cloud cover along the UT border itself. However, to the east, this decaying circulation could continue to enhance storm activity over the San Juans, Elk Mountains, Grand Mesa and the Flat Tops. With the aforementioned factors and yesterday’s storm totals on the high side of guidance (see State Precipitation Map discussion), a LOW flood threat continues to be warranted over large parts of the western slope for this afternoon through late evening. There is some inclination to go with a higher threat today, but unfortunately the main hesitation is a lack of confidence for any given location.

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:

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

Numerous showers and thunderstorms today, especially over higher elevations above 8,000 feet. Max 30-minute rainfall up to 0.8 inches, with max 1-hour rainfall up to 1.3 inches. Storm total precipitation up to 2.0 inches possible by tomorrow morning. A LOW flood threat has been posted for parts of the region for isolated flash flooding, debris slides and mud flows. Additionally, some area rivers are beginning to see higher flows from the persistent rainfall. Thus, small creek and tributary flooding is also possible for areas with already saturated soils.

Primetime: 12PM through midnight

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

Very hot and mostly dry today with an isolated shower or storm possible especially for the higher elevations and foothills. Max 1-hour rainfall up to 0.5 inches. Flooding is NOT expected today. High temperatures are expected to exceed 100F for the lower elevations of the South Platte and Arkansas river valleys.

Primetime: 1PM through 8PM