Issue Date: 8/13/2015
Issue Time: 9:21 AM

A LOW FLOOD THREAT IS ISSUED FOR PORTIONS OF THE SOUTHEAST PLAINS, PALMER RIDGE, AND URBAN CORRIDOR.

Persistent moisture is still hanging out under the upper-level ridge aloft, leaving precipitable water values above average for this date. As you can see in the IPW trace below, Shriever AFB and Boulder have fallen to near, or just below, 1 inch IPW, while Grand Junction and Pueblo remain above 1.15 inches (as of 8:30 AM). This depth of moisture still suggests that heavy rain is possible with thunderstorms, but the environment overall is fairly unsupportive to that occurring. The exception to this will be over Eastern Colorado, but we will get to that point in just a moment.

Over the higher terrain, isolated-to-widely scattered showers/thunderstorms are expected to increase through the afternoon and evening hours due to daytime heating/orographic effects making use of the residual moisture. The Northern Mountains, Front Range, Central Mountains, and San Juan Mountains will be the most active higher terrain regions, relatively speaking. Brief moderate-to-heavy rainfall will be possible, but probabilities of occurrence are low enough that no flood threat is warranted.

For the lower elevation east of the mountains, a weak shortwave trough will round the upper-level ridge and bring a bit of upper-level support to the environment. This will allow for isolated-to-scattered thunderstorms to develop this afternoon and evening, accompanied with a threat of brief moderate-to-heavy rainfall. A couple will become strong/marginally severe over the Northeast Plains, Southeast Plains, and Palmer Ridge regions where good instability will be in place. Strong winds, large hail, and lightning will be the main threats. For a look at maximum rain rates and timing of storms, jump down to the zone-specific discussions below.

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, jump below the map.

Zone-Specific Forecasts

Urban Corridor, Northeast Plains, Southeast Plains, Palmer Ridge, and Raton Ridge:

Isolated-to-scattered thunderstorms are expected this afternoon and evening. A couple will become strong/marginally severe over the Northeast Plains, Southeast Plains, and Palmer Ridge regions. Maximum rain rates will vary by region:

Urban Corridor: 0.6-1.2 inches/hour
Palmer Ridge, Raton Ridge, and Northeast Plains: 0.8-1.4 inches/hour
Southeast Plains: 1.2-2.0 inches/hour

Timing: 1 PM – 9 PM will be prime time, with a few lingering until midnight.

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

Isolated-to-scattered showers and thunderstorms expected today/this evening, before diminishing after sunset with the loss of daytime heating. The main result will be light rain, gusty winds, and lightning, but brief bouts with moderate-to-heavy rainfall cannot be ruled out. With a lack of general support from the environment, storms will not be long-lived, so flooding rainfall, even if heavy for brief periods of time, is not expected.

Rain rates will generally be 0.25-0.5 inches/hour, but maximum rain rates will push 0.6-1.0 inches/hour.

Timing: 11 AM – 9 PM

Issue Date: 8/12/2015
Issue Time: 10:35AM

— LOW flood threat for the Palmer Divide, Front Range, Northwest Slope and parts of Central Mountains, Northern Mountains

This morning, Colorado is positioned directly under the axis of a stout upper-level ridge, as seen in the water vapor image below. The return flow of very moist monsoonal moisture has been temporarily directed to the west of our state, into Arizona, Utah and Idaho. Typically, this overall setup would imply that we would be lucky to see a drop of rain. However, with the extremely high amount of residual moisture in place, we expect isolated to scattered thunderstorm activity today. Although we do expect a noticeable downtick in coverage, compared to the last few days. Overnight, mid-level temperatures warmed by 3-5 degrees Celsius across the state. Thus, stability this afternoon is expected to be much weaker than yesterday, especially away from higher terrain. However, precipitable water values in the 1.0 – 1.4 inch range (about as high as they get for CO) implies that any storm that does manage to form will be capable of short-term torrential rainfall.

For today, we expect mostly clear skies to quickly give way to scattered thunderstorms over the higher terrain of the Central Mountains, Northern Mountains, as well as the Northwest Slope where return will allow for a continual feed of high moisture content. Isolated storms will have the capability to produce flash flooding, hence a Low flood threat for this region. These storms should diminish rapidly by sunset. Elsewhere a Low flood threat is also in place over the Palmer Ridge where activity moving east and south off the higher terrain could tap into higher moisture and produce isolated flash flooding as well. This activity could last into the late evening hours.

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, jump below the map.

Zone Specific Forecasts

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

Sunny early, then partly to mostly cloudy with isolated to scattered thunderstorms developing by early afternoon. Max 1-hr rain rates up to 1.25 inch will be possible, causing isolated flash flooding and street flooding. Small hail, up to 0.5 inches and gusty winds up to 45 mph may accompany the strongest storms. Storm coverage will likely be highest from mid-afternoon through around sunset.

Primetime: 1pm to 8PM

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

Partly cloudy early, then isolated to scattered showers and thunderstorms forming over the highest terrain by early afternoon. Some storms will be able to move off the mountains and into the plains, especially over the Palmer Ridge. There, max 1-hr rain rates up to 1.8 inches could cause isolated flash flooding and street flooding. A Low flood threat is in place. Activity could persist into the late evening hours as storms find their way into higher moisture and some residual instability.

Primetime: 2PM through 11PM

Southwest Slope, San Juans, San Luis Valley:

Sunny early, then partly cloudy with isolated to widely scattered showers and thunderstorms possible, especially over the higher terrain. Subsidence and unfavorable dynamics will limit 1-hr rain rates to about 0.5 inches. Thus, flooding is not anticipated. Small hail, up to pea-size and gusty winds up to 45 mph could accompany the strongest storm. Activity will subside by sunset.

Issue Date: 8/11/2015
Issue Time: 10:05 AM

A HIGH IMPACT FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE URBAN CORRIDOR AND FRONT RANGE.

A MODERATE FLOOD THREAT IS ISSUED FOR PORTIONS OF THE URBAN CORRIDOR, PALMER RIDGE, NORTHEAST PLAINS, SOUTHEAST PLAINS, RATON RIDGE, SAN JUAN MOUNTAINS, SOUTHWEST SLOPE, AND GRAND VALLEY.

A LOW FLOOD THREAT IS ISSUED FOR PORTIONS OF THE NORTHEAST PLAINS, SOUTHEAST PLAINS, PALMER RIDGE, RATON RIDGE, URBAN CORRIDOR, FRONT RANGE, SOUTHEAST MOUNTAINS, CENTRAL MOUNTAINS, NORTHERN MOUNTAINS, NORTHWEST SLOPE, SAN JUAN MOUNTAINS, SOUTHWEST SLOPE, AND GRAND VALLEY.

Today will be another active day as the depth of moisture has really become impressive across the state. The increase in moisture is fairly easy to pick out on the IPW graph below. Beginning around 6 PM Sunday, the monsoonal moisture plume began to move back over Colorado, and the increase has continued through this morning with all four normal reporting stations (Boulder, Grand Junction, Pueblo, and Shriever AFB) near, or above, 1.0 inches. The 1.0 inch IPW mark is a rough threshold for heavy rain concerns in Colorado; there will be plenty of moisture available to storms.

Scattered thunderstorms are expected across much of the High Country regions (including their lower elevations), Urban Corridor/Front Range south of I-70, Palmer Ridge, Raton Ridge, and Southeast Plains. More isolated storm coverage is expected over the Northeast Plains, and Urban Corridor/Front Range north of I-70. With the amount of moisture available, most thunderstorms will invoke efficient precipitation processes, thus the large spatial extent of flood threat area today. For more information on maximum rain rates and timing, be sure and check the zone-specific forecast discussions below.

The reasoning behind issuing the first High Impact Flood Threat really hinges on four things: 1) High maximum rain rates of 2-3 inches/hour due to deep moisture, 2) Saturated soils from the previous 36-48 hours of rainfall, 3) Relatively slow storm motion, and 4) Population Density. A couple of water rescues were performed across the area as cars became stranded in high water over roads, debris/fast-flowing water from the Waldo Canyon Burn Scar inundated roads, damaged business in Manitou Springs, and damaged cars in western Colorado Springs. With scattered storms capable of producing equal, or greater, rain rates, and on top of saturated soil, runoff will occur quicker today than yesterday. Be prepared if a strong thunderstorm moves over areas that received heavy rainfall yesterday, and remember: Turn Around, Don’t Drown.

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, jump below the map.

Zone-Specific Forecasts

Urban Corridor/Front Range north of I-70, and Northeast Plains:

Isolated storms are expected, with chances decreasing from west to east. Maximum rain rates will be 1.2-1.8 inches/hour over the Northeast Plains, and 0.9-1.3 inches/hour over the Urban Corridor/Front Range north of I-70.

Timing: Noon/1 PM – Midnight

Urban Corridor/Front Range along and south of I-70, Raton Ridge, Palmer Ridge, and Southeast Plains:

Scattered thunderstorms capable of heavy rainfall are expected today and tonight, with maximum rain rates breaking down like this:

Urban Corridor south of I-70, and Palmer Ridge: 2-3 inches/hour
Front Range south of I-70: 1.2-2.2 inches/hour
Raton Ridge and Southeast Plains: 1.6-2.4 inches/hour

Not every location will receive rainfall, but those that do will likely experience at least brief heavy rainfall. Slow-storm motions are a concern, especially for moderate and high impact flood threat areas.

Timing: Noon – Midnight, with one or two showers/thunderstorms lingering into the early morning hours.

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

Scattered showers and thunderstorms are ongoing, and are expected to continue throughout the day and into the nighttime hours. There will be a few scattered showers/weak thunderstorms that continue into the early morning, especially across southwestern Colorado. Maximum rain rates break down like this:

Northern Mountains: 0.8-1.1 inches/hour
Central Mountains: 0.9-1.2 inches/hour
Southeast Mountains: 1.4-1.8 inches/hour
San Juan Mountains, Grand Valley, and Southwest Slope: 1.2-1.8 inches/hour

Timing: Currently – Midnight, with a few showers/storms lingering into the early morning hours tomorrow

Northwest Slope and San Luis Valley:

Isolated showers and thunderstorms expected, otherwise will be partly sunny. Maximum rain rates will be 0.8-1.2 inches/hour for the strongest storms, mainly over/near the higher terrain. Moisture is less across the Northwest Slope than any other region in Colorado, and thus escapes much of the flood threat.

Timing: 11 AM – 10 PM, with one or two isolated showers continuing into the early morning hours.

Issue Date: 8/10/2015
Issue Time: 10:05 AM

A MODERATE FLOOD THREAT IS ISSUED FOR PORTIONS OF THE URBAN CORRIDOR, FRONT RANGE, PALMER RIDGE, SOUTHEAST PLAINS, SOUTHEAST MOUNTAINS, RATON RIDGE, AND NORTHEAST PLAINS.

A LOW FLOOD THREAT IS ISSUED FOR PORTIONS OF THE NORTHEAST PLAINS, SOUTHEAST PLAINS, PALMER RIDGE, URBAN CORRIDOR, FRONT RANGE, NORTHERN MOUNTAINS, CENTRAL MOUNTAINS, SOUTHEAST MOUNTAINS, RATON RIDGE, SAN JUAN MOUNTAINS, AND SOUTHWEST SLOPE.

Today’s forecast discussion could drag on for hours with the set-up in place, but I will attempt to keep it as clear and concise as possible. The water vapor imagery sets up the background environment quite well: upper-level high over Texas, upper-level low just off the west coast, mid-/upper-level disturbance extending across AZ/NV/CA/OR (black line), and monsoonal moisture streaming into NM/AZ/CO (green arrow).

The depth of the monsoonal moisture will increase, especially across southern and central Colorado, throughout today and tonight as the upper-level high builds westward and pushes the moisture stream directly overhead. In addition to this moisture increase, areas along and east of the Front Range and Southeast Mountains will see surface moisture reinforced as the winds turn southeasterly through the afternoon and evening hours. The depth of moisture will allow scattered thunderstorms to produce locally moderate-to-heavy rainfall.

An additional concern arises this afternoon/evening, as the southeasterly surface flow will likely develop a Denver Convergence Zone. If the convergence zone develops, a couple of well-organized, heavy rainfall producing storms will be possible across portions of the Urban Corridor, Palmer Ridge, and adjacent plains this afternoon/evening. In general, thunderstorms will develop along the Front Range and Southeast Mountains early this afternoon, spreading eastward with time through the evening hours.

Elsewhere over western Colorado: Scattered thunderstorms are expected over the San Juan Mountains and Central Mountains, with more isolated coverage over the higher terrain of the Northern Mountains, San Luis Valley, and Southwest Slope. The Grand Valley and Northwest Slope regions will remain mainly dry. The Central Mountains and San Juan Mountains will be the likeliest regions to see moderate rainfall, while the other regions will mainly experience light rainfall and gusty winds from any thunderstorm activity. Rain rates are discussed below in the zone-specific discussions.

Overnight, a repeat of overnight last night appears possible for southern portions of the Urban Corridor, Palmer Ridge, Southeast Plains, and to some extent the Northeast Plains. Isolated-to-scattered slow-moving thunderstorms will be possible as easterly/southeasterly flow continues into the early morning hours. Locally heavy rainfall will be a threat if storms can develop. Probability of occurrence for this scenario is roughly 40%.

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, jump below the map.

Zone-Specific Forecasts

Urban Corridor, Northeast Plains, Raton Ridge, Palmer Ridge, and Southeast Plains:

Scattered thunderstorms expected through the afternoon and evening hours, with a few isolated thunderstorms lingering into the overnight hours, mainly along and south of I-70. See discussion above for more details. Maximum rain rates will be 1.2-2.0 inches/hour near the mountains, and 2.0-3.0 inches/hour further east.

Timing: 11 AM – 6 AM

Front Range, Northern Mountains, Central Mountains, Southeast Mountains, San Juan Mountains, San Luis Valley, and Southwest Slope:

Isolated-to-scattered thunderstorms expected to develop around 11 AM, and continue through the afternoon and evening hours. Overnight, a few isolated showers/thunderstorms will continue over the higher terrain of the San Juan Mountains, Southwest Slope, and Southeast Mountains. Maximum rain rates are as follows:

Front Range and Southeast Mountains: 1.2-2.0 inches/hour
Northern Mountains, Central Mountains, San Juan Mountains, and Southwest Slope: 0.8-1.2 inches/hour
San Luis Valley: 0.4-0.8 inches/hour

Timing: 11 AM – Midnight

Northwest Slope and Grand Valley:

Mostly sunny early, becoming partly sunny this afternoon and evening. An isolated shower/thunderstorm or two cannot be ruled out over the higher terrain, but the day for most areas will be marked by dry conditions.