FTB 07-28-2017: Another Day, Another Round Of PM Heavy Rainfall

Issue Date: Friday, July 28th, 2017
Issue Time: 10:30AM MDT

MODERATE/LOW flood threat for Palmer Ridge, Urban Corridor, Northeast Plains, Southeast Plains, Southeast Mountains, Raton Ridge, San Juan Mountains and Southwest Slope

This morning’s water vapor image, below shows that ample amounts of low-level water vapor are found statewide. Precipitable water (PW) values range from about 0.7 inches over the North Platte basin to 1.1 inches in the southwest corner of the state to 1.3 inches along our far eastern border. PW is expected to remain steady or increase throughout the day as southwesterly flow aloft continues to usher in moist, monsoonal air. Some stronger jet stream level winds are noted along the CO/UT border and weak upper-level divergence is supporting a mid/high cloud deck seen in the southwest part of the state. This will slightly limit instability over the western slope, though very moist low-level air will partially offset this and another round of scattered to numerous storms are expected in the San Juans and surrounding regions.

East of the Continental Divide, a tricky forecast is in store with our high resolution ensemble guidance suggesting many different possibilities today. A weak and shallow wind shift axis is noted along the I-76 axis (also accompanied by localized areas of very dense fog), possibly from last night’s outflow boundaries. This should act as a demarcation between higher heavy rainfall chances to the east and lower chances to the west. Once again, lower precipitation chances are expected north of the Denver metro due to a relative minimum in moisture content and the strongest downsloping flow. Highest storm chances today will likely be in the southeast quadrant of Colorado once again. We expect two separate areas of action. First, diurnal upslope flow combined with plenty of moisture and adequate instability will trigger slow moving storms along the foothills from Denver south through the New Mexico border. The Palmer Ridge will once again likely serve as a hot spot. Second, another separate cluster of storms is expected to fire along the aforementioned wind shift. These storms will have the potential for very heavy rainfall due to their slow motion and possibility of back-building/training of individual cells. A Moderate threat is warranted for both of these areas with a broad Low flood threat surrounding the Moderate.

Once again, with adequate instability expected to persist well after sunset, so will the chance of heavy rainfall. Activity could persist through about 2AM.

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, scroll below the map.

Flood Threat Legend

Zone-Specific Forecasts:

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

Partly cloudy early then scattered to numerous showers and thunderstorms developing by early afternoon. Highest coverage will be along Palmer Ridge and adjacent areas to the east. Max 1-hr rainfall up to 3.0 inches (east) and 1.9 inches (west) will support a Moderate flood threat with flash flooding and small stream flooding likely. Mud flows and debris slides will be a possibility in higher elevation locations. A Low threat has been posted for broad parts of eastern Colorado. The threat is expected to persist into the early overnight hours once again.

Primetime: 1PM to 2AM

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

Mostly cloudy with scattered showers turning to thunderstorms and increasing in intensity. High rainfall coverage will be in southwest areas, especially the western portion of the San Juans. A broad Low flood threat has been posted for max 1-hr rainfall up to 1.3 inches capable of producing isolated mud flows and debris slides. A Moderate flood threat has been posted for parts of the San Juans due to an expected longer duration of heavy rainfall there.

Primetime: 12:30PM to 10PM

FTO 07-27-2017: Streak Of Heavy Rainfall To Continue, Then Substantial Drying

Issue Date: Thursday, July 27, 2017
Issue Time: 3PM MDT
Valid Dates: 7/28-8/11

If you have ever read either our Flood Threat Bulletin or Flood Threat Outlook discussions, you can probably quickly recall how much emphasis we place in analyzing moisture content when developing the flood threat forecast. The reason for this is that sometimes, especially in the summertime, moisture is as good or better predictor for heavy rainfall that any other guidance, including model Quantitative Precipitation Forecasts! For you technical readers, please refer to this paper in the Monthly Weather Review journal for more information. In short, it is not so much that moisture is a good predictor of where or when thunderstorms will develop, or even how long they last, but it is a good predictor of the upper limit of their potential rain rates.

So how has moisture fared across Colorado recently? With little shock, the table below shows significantly above normal amounts over (at least) the past 7 days, as measured by column integrated Precipitable Water at both the Denver and Grand Junction evening soundings (the asterisk in the measurements at Denver signify that the radiosonde did not sample the entire atmosphere, implying that this reading underestimates the actual PW). This was recently punctuated by values over 70% above normal, interestingly coinciding with one of the wettest 48-hour periods observed thus far this summer.

As this afternoon’s water vapor image shows, below, the same familiar players will once again be in charge of flood potential for the next 5 days or so. The first feature is the upper-level ridge that is currently centered just south of Colorado. As long as the western periphery of this ridge does extend too much west of the Four Corners region, this typically allows for the transport of low and mid-level level monsoon moisture into Colorado. This is expected to continue over the next 5 days, keeping elevated moisture and above normal heavy rainfall chances in the picture.

The second, and often overlooked feature, is the presence of surface high pressure features that act to (i) move southward “cool” fronts, especially across eastern Colorado and (ii) more importantly, provide enhanced pulses of moist upslope flow that can interact favorably with the climatological circulation to accentuate and expand the lifetime and eastward extent of the elevated convection over our higher terrain. As shown below in an interesting analysis of surface pressure anomalies available on the Tropical Tidbits model site, another surface high pressure is expected to move southward out of Canada. By Sunday and Monday, a stronger than normal fetch of moist, upslope easterly flow is expected across southeast Colorado. The result will be a pattern conducive for slow moving storms capable of producing very heavy rain rates, much like on Wednesday.

After the passage of this surface high pressure, guidance has now come into good agreement that the upper-level ridge will shift westward, which will drastically reduce moisture levels across most of Colorado. Heavy rainfall and precipitation chances are expected to lessen to below their climatological average accordingly.

For this edition of the 15-day Flood Threat Outlook, we have identified one prolonged Elevated/High flood threat event that is discussed in more detail below.

Event #1: Friday (7/28) through Tuesday (8/1)

Elevated/High Flood Threat for flash flooding, debris slides and mud flows, especially over higher terrain

Above normal moisture will persist statewide through early next week. Precipitable water is expected to remain in the 0.9 – 1.6 inch range, which will support daily bouts of scattered to numerous shower and thunderstorm activity from Friday through Tuesday. Rain rates up to 1.4 inches per hour (west) and 2.9 inches per hour (far east) will be possible, which will support an elevated flood threat. Highest coverage will be over the higher terrain, especially east of the Continental Divide as well as southeast Colorado. On Sunday/Monday, a cool front is expected to enhance rainfall coverage over southeast Colorado and amplify the intensity further, sustaining longer duration heavy rainfall. Current guidance suggests 3-hour rainfall up to 4 inches will be possible in the foothills of the Sangre de Cristo and Wet Mountains. Max 3-hour rainfall up to 2.4 inches will be possible in the San Juan Mountains.

As with the recent heavy rainfall event on Wednesday, this event will not have significant upper-level support implying that sustained heavy rainfall, beyond 3 hours, will not be likely. Thus, currently the main impacts of this event will be mud flows and debris slides in higher elevation locations, as well as small stream flooding associated with the stronger cells. Major riverine flooding is not expected at this time. Please stay tuned to daily Flood Threat Bulletins for more detailed information, especially if you plan on camping and hiking over the weekend.


STP 07-27-2017: Efficient Thunderstorms Drop Impressive Rainfall

Issue Date: Thursday, July 27, 2017
Issue Time: 10:50AM MDT


Yesterday was one of those July Colorado days where you could just feel the moisture when you walked out the door. Dew points were near 60F by noon across the Urban Corridor and Eastern Plains with the mid-levels saturated as well. The High pressure over Oklahoma allowed moisture to pour into the eastern plains. There was some drying throughout the day over the western portion of the state, which meant more isolated thunderstorms when compared to the previous couple days. Storms begin to fire off the high terrain a little later than normal with the upslope flow. Storm motion was more easterly, so the thunderstorms over the western portion of the state were confined to the mountains. In this area, local storm centers were estimated at 1.5 inches of rain. A CoCoRaHS station happened to be positioned over SW Gunnison County that captured that storm center at 3.11 inches. This gage should be read with a grain of salt as we were not able to confirm this 24-hour amount this morning.

The storms that formed over the higher terrain of the Front Range and Southeast Mountains moved off the mountains in the late afternoon and evening. The rainfall amounts from these widespread storms were nothing less than impressive. With the high moisture, multiple rounds of storms were able to fire. The extensive precipitation gage network throughout the Urban Corridor gave us an idea pretty early in the afternoon about how efficient these storms were going to be. Over Douglas County near Russellville Gulch, 2.68 inches fell in 1-hour with 1.14” falling in 10 minute. Another storm was recorded dropping 2.48 inches in 45 minutes. While these were some of the highest rainfall amounts, other storms had well over 1 inch in 30-min. The list of the most impressive storm reports are below. CDOT reported rain washing mud and rocks onto most roads west of Denver, Golden and Boulder including US 285, CO 74, US 40, US 6,  CO 72 and CO 119. In Cripple Creek, CR 11 at US 67 was flooded. Ponding in yards and fields was also reported over the Franktown area. Radar estimates the 3-6 hour totals were just over 3.5 inches with CoCoRaHS stations recording 3.18 inches. A USGS gage recorded 3.05 inches at Fort Carson and 3.78 inches at Franktown. Fountain Creek was running high last night which can be seen at the gage in Colorado Springs reaching action level last evening.

Over the Eastern Plains, the high moisture was paired with larger instability and weak shear. This allowed storms in the SE corner to persist into the night. The 1-hour max precipitation rates were as high as 2 inches. The 3-6 hour totals were up to 4 inches with 24 hour total just over 4 inches in Otero County. Unfortunately, this area isn’t as well gaged, but a CoCoRaHS station recorded 3.5 inches of rain over Otero County for the 24-hour period.

Impressive reported rain totals:

10-min: 1.14” at Russellville Gulch near Franktown

10-min: 0.67 at UDFCD gage in Littleton

15-min: 0.96” at Cherry Hills ASOS

15-min: 0.79” at the Rosedale gage just west of Evergreen

15-min: 0.59” at 4mi Creek south of Deckers

15-min: 0.5” in Park County

18-min: 1.25” at Berthoud

20-min: 0.75” in Falcon near Colorado Springs

30-min: 1.5” at Marston Lake in Littleton

30-min: 0.98” in For Collins; 3 other gages reported over 0.9” in 30-min

30-min: 0.94” Choke Cherry Reservoir near Genesee

30-min: 0.91” at the Rosedale gage just west of Evergreen

30-min: 0.88” at Cripple Creek in Teller County

30-min: 0.5” at Porphyry gage near Jamestown

1-hr: 2.68” at Russellville Gulch near Franktown

1-hr: 1.13” at Fort Carson

To see how much rain fell over your area the last 24-hours, check out our new Precipitation Map below.

FTB 07-27-2017: Heavy Rainfall Continues Along the Eastern Mountains and Adjacent Plains

Issue Date: Thursday, July 27, 2017
Issue Time: 10:40AM MDT

—LOW/MODERATE flood threat for Front Range, Urban Corridor, Palmer Ridge, Central Mountains, Southeast Mountains, Raton Ridge, Southeast Plains

—LOW flood threat for San Juan Mountains, Northeast Plains

Afternoon Update (5PM): The low threat has been extended over the NE corner of the state along the NE, WY, KS border. Storms have been firing off the Cheyenne ridge this afternoon due to strong upslope flow and are moving S/SE over the state line. These storms will move into an area with PW values of 1.3 inches, high instability and 30-45 knots of shear. All of these ingredients combined are favorable for severe thunderstorm development. Threats include heavy local rain, large hail and small stream and street flooding. 1-hour rain rates will be as high as 1.9 inches and 3 hour totals up to 3.5 inches.

On the water vapor image below, the storms that persisted with the moist easterly flow overnight can be seen over the SE Plains. The High pressure has started to build back to the west and has elongated its center over NM, TX and OK. Today it will continue to build itself west and situate itself over the 4 corners region, which will cut off monsoon moisture into the region. The 500mb ridge is currently dipping into northern CO and has gained some strength since yesterday. This ridge will help produce westerly flow that will pull the drier air seen over Utah into western Colorado. Precipitable Water (PW) over the eastern portion of the state will also decrease today in the upper-levels with a more northwesterly/northerly flow. The gradient will decrease from north to south, but will still remain over 1 inch this afternoon and evening, which should be more than enough moisture for another round of heavy rainfall. Diurnal heating patterns will allow thunderstorms to fire over the higher terrain midday. Over the western slope, the limited moisture and westerly upper level flow should cause thunderstorms to be more isolated in nature and confine them to the higher terrains favoring the San Juan Mountains.

Along the Front Range and Southeast Mountains, storms will begin to organize just after noon. As they move off the mountains in the late afternoon and evening, expect a couple rounds of storms similar to yesterday. The steering winds aloft remain weak and storm motion will be S/SE at 15-20 mph. Cloud cover may limit instability over the far eastern plains, but plenty of instability from daytime heating will occur over the adjacent plains and Palmer Divide. So there is a potential for heavy, local rainfall once again today. Over the mountains, local 3 hour rainfall totals of 1.8 inches are possible. Once storms move off the mountains, stronger thunderstorms will favor the higher terrain of the Palmer Divide and could have 3 hour totals up to 3.5 inches. Outflow boundaries from storms will likely help provide low-level convergence for other storms, and an isolated storm may move into the northern portion of the state off the Cheyenne Ridge. Threats include mud flows, debris slides, gusty winds and urban and small stream flooding. Burn scars over the Southeast Mountains should be monitored closely for flash flooding, debris slides and mud flows. Fountain Creek in Colorado Springs ran into Action level yesterday evening, but should make a full recovery before the next round of storms this afternoon and evening.

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, scroll below the map.

Flood Threat Legend

Zone-Specific Forecasts:

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

 Sunny this morning and drier air being entrained at the upper-levels from Utah. Limited moisture and westerly flow aloft should confine storms to the higher terrains this afternoon. Max 1-hour rain rates up to 1 inch are possible with local 24-hour totals up to 2.1 inches in the San Juan Mountains. Debris slide, mud flows and road flooding are the main threats over these saturated soils. A Low threat has been issued. Storm activity should start to subside after sundown.

Primetime: 12PM to 11PM

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

 Partly cloudy over the eastern plains this morning becoming more sunny later this morning. Front Range, Urban Corridor and Southeast Mountains should heat up rather quickly today building instability for multiple rounds of storms again today. Max 1-hour rain rates up to 1.3 inches over the higher terrain and 2.3 inches over the adjacent Plains. Burn scars will need to be monitored closely for mud flows, debris slides and flash flooding. There is a Moderate/Low flood threat issued for today.

Primetime: 12PM to 12AM