Issue Date: Monday, August 7, 2017
Issue Time: 10:55AM MDT

— HIGH riverine flood threat Fountain Creek and Arkansas River to Lamar

— LOW/MODERATE flood threat Northwest Slope, Northern Mountains, Grand Valley, Urban Corridor, Central Mountains, Front Range, Palmer Ridge, Southeast Plains, Southeast Mountains

Afternoon update likely

A moist low-level atmosphere and cooler temperatures are present throughout the state this morning. Cloud cover or fog are present over nearly the entire state with the exception of the SW corner where drier air can be seen on the water vapor imagery below. The overnight mesoscale convective system (MCS) is exiting the eastern plains over the KS/OK border this morning. Moisture rich air is beginning to regenerate showers over the Northeast Plains this morning with convective temperatures around 65F today. With moisture intact throughout the day and multiple shortwaves moving through the state, expect cloud cover and rain to continue throughout the morning and afternoon over the NE portion of the state. Once again, the 500mb ridge will continue to provide NW/W flow over the state and multiple mid-level waves for lift to generate storms throughout the day. We will continue to monitor the strength and efficiency of the waves throughout the day and likely provide a PM update.

Further south, east of the Continental Divide, storms will begin to fire over the higher terrain later this morning from a shortwave within the NW/W flow. A second wave of storms will initiate over the higher terrain this afternoon, with a possible third wave this evening. As the storms move off the mountains, they will initially favor the higher terrain of the Palmer Ridge. Slow easterly/southeasterly flow with the multiple rounds of storms will lead to heavy rainfall over the mountains and immediate adjacent plains. Flood threats include small stream and road flooding, mud flows and debris slides especially over burn scars. Over the more urban areas, road flooding and field ponding are likely. The multiple rounds of storms in the high-moisture environment warrant a Moderate flood threat today. Eventually the storms will form a MCS, similar to yesterday, as they move over the far eastern plains later this evening. The strength of the MCS is forecasted to be relatively weak with lack of instability, but rainfall totals up to 4 inches could still be possible. Once again the flood threat for the eastern plains continues overnight.

West of the Continental Divide, the embedded shortwaves in the westerly flow will also help trigger widespread storms in the moisture rich atmosphere. Westerly steering flows should confine the heavier rain producing storms to the higher terrains. Along the southern border, the drier air seen the water vapor imagery should limit rainfall totals. A shower or two in the Grand Valley cannot be ruled out as convection from the west moves into the state. The Flat Tops and Roan Plateau are particularly susceptible today to mud flows, debris slides and road flooding with storm totals as high as 2 inches.

As mentioned in earlier posts, the Arkansas River is running extremely high from the July rainfall. Current flows from the headwaters downstream to the Kansas border are in the 75^th to 90+ percentiles. Riverine flooding is likely with the saturated soils and multiple rounds of rainfall today. A High threat has been issued from Fountain Creek (vulnerable to flooding) south to Pueblo and along the Arkansas to Lamar for rapidly rising waters and flooding starting midday.

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.

Zone-Specific Forecasts:

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

Low-level clouds and rain this morning with multiple rounds of precipitation expected. Storms will become more severe where sun is able to break through and create instability. Max 3-hour rainfall up to 2.2 inches (east), 1.5 inches (west). Localized max 24-hour rainfall over 4 inches possible (east) with 24-hour amounts up to 3 in (west). Riverine flooding along Fountain Creek and the Arkansas River to Lamar is likely with saturated soils and widespread rainfall. A Moderate and High flood threat has been issued today.

Primetime: 10:30AM to 3AM

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

High moisture and embedded shortwaves will create multiple rounds of rainfall today. 3-6 hour totals up to 2 inches over the Flat Tops and Roan Plateau put this area at particular risk for mud flows, debris slides and road flooding. 3-hr rain totals up to 2 inches possible (north) and 1.2 inches (south). There is a Moderate/Low flood threat for today.

Primetime: 11AM to 10PM

Issue Date: Sunday, August 6, 2017
Issue Time: 10:15AM MDT

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

— LOW flood threat for Central Mountains, Raton Ridge

Fog and low-level clouds this morning for the northeast portion of the state are the result of moist and cooler air over this region. The cooler air and increased cloud cover will help limit high temperatures over this region today, but instability should still be able to build later this morning and afternoon for widespread PM showers. Precipitable Water (PW) is currently over 1 inch over the eastern plains and is forecasted to remain intact throughout the day with the cool, moisture rich air mass moving south. West of the Continental Divide, the best moisture will be over southwest region. However, westerly flow at the mid-levels should draw in some drier air from southern Utah that will limit heavy rainfall. The large-scale trough will provide west/northwest flow over Colorado and help usher in shortwaves that will enhance convection initiated by upslope flow and outflow boundaries. There is an upper-level wind maximum that moves south over our state this afternoon and tonight, that will also help provide lift for showers over the eastern plains into the early morning hours.

Moisture rich upslope flow is expected to initiate thunderstorms over the higher terrain early this afternoon. Storm coverage will widespread east of the Continental Divide. As storms move off the mountains, they will initially favor the high terrain of the Palmer and Raton Ridge and have storm motion to the east/southeast. The storms are expected to quickly form into widespread, slow-moving convection over the eastern plains and be amplified by a mid-level distrubance. Heavier rainfall is likely near the highest instability, which will be located where daytime heating is the greatest throughout the day. Small hail and gusty winds may be present with the more severe storms. Over the mountains, rainfall rates greater than 1 inch are likely over the Southeast Mountains. Threats include mud flows, debris slides and flash flooding especially over burn scars. Over the far eastern plains, localized 24-hour rainfall amounts over 4.5 inches may be possible. Threats include small stream and arroyo flooding, field ponding and road flooding. The threat for the eastern plains last into the overnight hours, and the cluster of storms is expected to exit the state around 3AM. Lingering showers and cloud cover over the plains and southeastern mountains are likely Monday morning.

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.

 

Zone-Specific Forecasts:

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

Low-level clouds this morning becoming partly sunny to fully sunny this afternoon. Expecting scattered showers and thunderstorms to develop over the mountains around 1-2PM and heavier convection over the plains lasting overnight. Max 1-hour rainfall up to 2.3 inches (east), 1.2 inches (west). Localized max 24-hour rainfall over 4.5 inches on the eastern plains cannot be ruled out. A Moderate flood threat has been issued today, and it is an overnight threat for the eastern plains.

Primetime: 1:30PM to 3AM

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

Sunny and near normal temperatures over the NW corner of the state. Scattered thunderstorms expected throughout the afternoon over the San Juan Mountains and Central Mountains with 24-hour totals up to 1.5 inches. Max 1-hour rainfall up to 0.6 inches (north) and 0.8 inches (south). There is no flood threat for today.

Primetime: 1PM to 10PM

Issue Date: Saturday, August 5, 2017
Issue Time: 10:45AM MDT

— LOW flood threat for Northeast Plains, Urban Corridor, San Juans, Southwest Slope, Grand Valley, Northwest Slope

As mentioned in previous Flood Threat Bulletins and Outlooks, an active weather pattern has setup across Colorado. The central feature supporting this is the large-scale trough located across the US Great Plains. As shown in this morning’s water vapor image below, a weak jet stream lies just north of Colorado. Within it and in close proximity are numerous ripples, or shortwaves (or even mini-shortwaves) that are supporting mid to high level clouds this morning. In particular, the feature in southwest Colorado is likely a remnant of yesterday’s convection and will be a factor in supporting heavy rainfall as instability develops this afternoon over the western slope. Moisture is somewhat lower today across central Colorado with Precipitable Water (PW) as low as 0.6 inches. However, PW greater than 1 inch is found in far eastern Colorado as well as the western parts of the state. This morning’s Grand Junction sounding showed a PW of 1.03, which is above normal for this time of year. Steady to slightly increasing PW is expected in western Colorado due to moisture advection from the west.

At the surface, a high pressure cell was located over southwest Colorado, which will promote some enhanced low-level moisture advection into western facing foothills of the San Juans, Elk Mts. And Grand Mesa. Much like yesterday, we expect scattered to numerous showers and thunderstorms this afternoon. A Low flood threat has been posted, mainly for mud flows and debris slides. Farther east, downsloping flow will once again prevail closer to the mountains. However, a weak boundary is expected to develop along the South Platte River this afternoon. As storms form of the Cheyenne Ridge, they will move southeast and encounter higher instability and higher moisture. The strongest cells will have the potential to produce heavy short-term rainfall, and a Low flood threat has been posted for isolated flash flooding. Large hail will also be possible with these cells.

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.

Zone-Specific Forecasts:

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

Scattered showers and thunderstorms developing by early afternoon. Highest coverage will be over the Northeast Plains as well as in the Southeast Mountains/Raton Ridge. Max 1-hr rain rates up to 1.8 inches over northeast areas and up to 0.9 inches in southern areas. A Low flood threat has been posted for isolated flash flooding in the Northeast Plains and surrounding areas. Large hail up to 1.5 inches and gusty winds up to 65 mph could accompany the strongest cells.

Primetime: 1PM to 10PM

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

Partly cloudy early with scattered to numerous showers and thunderstorms developing by afternoon. Highest coverage will be over western foothills where moisture is higher, specifically the San Juans, Elk Mountains, Grand Mesa and Roan Plateau. Max 1-hour rainfall up to 1.1 inches (max 30-min up to 0.7) will support isolated mud flows and debris slides. Gusty winds up to 50 mph could accompany the strongest cells. Activity will quickly subside after sunset.

Primetime: 12PM to 8PM

Issue Date: Friday, August 4, 2017
Issue Time: 10:00AM MDT

— Flooding is NOT expected today

Northwest and westerly flow will remain over the state again today with the upper-level ridge centered north and just west of Colorado. This will usher in drier air at the mid-levels that will help mix out some of the moisture at the lower levels over the NW portion of the state. Over the eastern portion of the state, the early morning fog and low-level clouds should quickly wear off after some morning heating. Low-level southerly flow will pull some moisture into the eastern plains today. Precipitable Water at both Denver and Grand Junction this morning are about 0.75 inches, with the eastern plains increasing some this afternoon. Over the northeastern portion of the state, shear will be greatest with a bit of upper-level upper air support. If storms forming off the Cheyenne Ridge and northern Front Range are able to move into this area, they may become severe with large hail and strong winds as the main threats. Rain rates should stay below flood threat criteria.

Storms will begin to fire off the southern higher terrain midday with the diurnal heating pattern. The synoptic scale downsloping flow today will limit the coverage of the storms over the eastern mountains. Where upslope flow will still prevail is over the southern Front Range, Raton Ridge and San Juan Mountains. Over the San Juan Mountains, 3-hour totals up to 1.2 inches are possible as higher moisture air is pulled in from southern Utah and northern Arizona. As storms move S/SE off the mountains into the eastern plains, they will likely dissipate except for a few isolated showers that may survive along the Palmer and Raton Ridge. Rainfall rates, again, will be below flood threat criteria. Storms over the eastern plains will exit the state around 11PM, while the main storms over the mountains will end by 9:30 PM.

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.

 

Zone-Specific Forecasts:

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

Low-level clouds early this morning but will become sunny through the day. Expecting isolated showers and thunderstorms to develop over the mountains around midday and last into the evening hours over the plains. Highest coverage will be along the southern Front Range, Palmer Ridge and Raton Ridge. If storms forming off the Cheyenne Ridge and Northern Front Range are able to make it to the Northeast Plains, they may become severe with the more favorable thunderstorm environment. Max 1-hour rainfall up to 1.5 inches (east) and 1 inch (west). No flood threat has been issued for today.

Primetime: 12PM to 11PM

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

Mostly sunny and warmer today over the NW corner of the state. Scattered thunderstorms expected throughout the afternoon over the San Juan Mountains with 3-hour totals up to 1.2 inches. Max 1-hour rainfall up to 0.7 inches (north) and 0.9 inches (south). There is no flood threat for today.

Primetime: 12PM to 10PM