Issue Date: 7/8/2015
Issue Time: 9:45AM

— Moderate flood threat for Palmer Ridge and Southeast Plains
— Low flood threat for Grand Valley, Northwest Slope, Northern Mountains, Front Range, Urban Corridor and parts of Northeast Plains

A disturbance will trek across Colorado today, sparking more thunderstorm activity. This disturbance is a “child” of the parent low-pressure system that continues to rotate off the California coast, as shown by the water vapor imagery below. Another disturbance is noted on the UT/NV border that may influence northwestern Colorado later this afternoon. With the favorable dynamics described above, we now look at today’s moisture content. As in past days, it is quite high with dewpoint temperatures in the 40s and 50s across the state. Precipitable water values remain at or above an inch almost statewide, further supporting the heavy rain threat.

As the disturbance currently over the state treks slowly eastward, heating of the day will cause plenty of instability to fuel thunderstorm activity. The highest threat today will be over the Palmer Ridge and extending towards the Kansas border. A Moderate flood threat has been posted for the area. Low cloud activity and a stable atmosphere will continue to limit rainfall chances in the Northeast Plains for the second day in a row. In northwest Colorado, a Low flood threat is in place for short-term heavy downpours that may cause brief flash flooding, mud flows and debris slides. West of the Divide, activity will subside around sunset. East of the Divide, storms may persist through midnight.

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

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

Thunderstorms will develop by early afternoon over the higher terrain and spread east and south through the day. Over the higher terrain, 1-hr rain rates up to 1.5 inches are possible, increasing to as high as 2.4 inches over the Palmer Divide. Over the Palmer Divide and Southeast Plains, 3-hr rain rates up to 3.5 inches will be possible. A Moderate flood threat is in place for part of the region, with a Low flood threat elsewhere except for the far northeast corner of the state. Activity will subside by early evening closest to the mountains, but may persist through midnight towards the Kansas border.

Primetime: 12pm to midnight

Northwest Slope, Grand Valley, Northern Mountains:

Ongoing thunderstorms growing in coverage and intensifying by early afternoon. One hour rain rates up to 1.2 inches will be possible. A Low flood threat is in effect for isolated flash flooding, mud flows and debris slides. Soils remain rather wet and several mud slides have been reports across the Grand Valley during the past few days. Activity will subside by sunset.

Primetime: 11am to 8pm

Raton Ridge, Southeast Mountains, San Luis Valley, Central Mountains, Southwest Slope, San Juans:

Partly cloudy with isolated to scattered thunderstorms developing by early afternoon. One hour rain rates up to 0.8 inches will be possible, but flooding is not expected. Activity will subside shortly after sunset.

Issue Date: 7/7/2015
Issue Time: 10:45AM

— Moderate flood threat for Northwest Slope, Grand Valley, Southwest Slope, San Juan Mountains, Central Mountains and Northern Mountains
— Low flood threat for Front Range, San Luis Valley, Southeast Mountains and Raton Ridge

The incredibly moist spring has transitioned into an equally moist summer across our state with many Colorado residents waking up to dewpoints exceeding 50F on a near-daily basis. Today is no exception. In addition to the plentiful surface moisture, the atmosphere is moist throughout its entire column (roughly up to 40,000 feet). The best measure of this is precipitable water (PW), which was 1.0-1.2 inches across all four sites that we regularly monitor: Pueblo, Colorado Springs, Boulder and Grand Junction. We expect PWs to stay in the 1.0-1.4 inch range throughout the course of the day. In short, these values are about as high as we see here in Colorado, so it is clear that a heavy rainfall threat exists today.

Coupled with the high moisture content today, atmospheric dynamics are also conducive for heavy rainfall. The water vapor image below shows that a deep upper-level low is positioned west of California. However, its influence extends into Colorado with several shortwaves noted across western CO and over UT. As these features propagate eastward, they will spark numerous rounds of shower and thunderstorm activity from late this morning into the late evening hours. Most of the higher terrain will once again be in a Moderate flood threat today. In the lower elevations east of the Divide, atmospheric instability is limited so no flood threat is anticipated today.

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

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

Mostly cloudy this morning with ongoing thunderstorm activity intensifying and growing in coverage. Maximum 1-hour rain rates will be 1.25 inches. Multiple storms crossing one area will result in maximum 6-hour rain rates up to 2 inches. Heavy rainfall will bring a threat of flash flooding, debris slides and mud flows. A Moderate flood threat is in place over most of the region with a Low threat elsewhere. Hail up to 0.75 inches may accompany the strongest storms. Please stay tuned to National Weather Service warnings for specific local information.

Primetime: 11am to 11pm

Front Range, Palmer Ridge, Southeast Plains, Raton Ridge, Southeast Mountains:

Overcast this morning with some sunshine by early afternoon, leading to thunderstorms. Isolated to scattered activity will be likely across the higher terrain. One hour rain rates up to 2 inches will be possible, resulting in isolated flash flooding, debris slides and mud flows. Street flooding will be possible as well.

Primetime: 2pm to 10pm

Urban Corridor, Northeast Plains:

Overcast and cool conditions with some breaks in the cloud cover possible by late afternoon. An isolated thunderstorm cannot be ruled out along the I-25 corridor in the late afternoon. The strongest storm will be capable of rainfall up to 1 inch per hour. No flooding is expected due to isolated storm coverage and limited runoff.

Primetime: 2pm to 9pm

Issue Date: 7/6/2015
Issue Time: 10:00 AM

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

The once strong upper-level ridge has been flattened out by the upper-level trough that continues shifting eastward across the US/Canada border. Attendant of this upper-level trough was surface cool front that continues to move south across Colorado, with upslope flow developing behind it. Surface dewpoints continue to run high statewide; readings are in the upper 50s and low 60s across lower elevations of eastern Colorado, mid-to-upper 50s across the valleys of western Colorado, and in the 40s across the High Country. Coupled with this surface moisture is plenty of deep, subtropical moisture, which has pushed precipitable water values above 1 inch at each of the normal 4 reporting stations (Boulder, Grand Junction, Pueblo, and Shriever AFB).

The moist, southwesterly flow will continue through this afternoon, as the upper-level low off the coast of California continues to push onshore. Embedded in the general flow is a couple of weak disturbances that will slide across the state this afternoon and overnight, providing a bit of focus for thunderstorm activity. However, orographics and daytime heating will arguably be the main focus for thunderstorm development this afternoon and evening.

The best coverage of showers and thunderstorms during this forecast period will be across the Central Mountains, Front Range, Southeast Mountains, Urban Corridor, Raton Ridge, and Southeast Plains. These zones will be the focus for heavy rain concerns today, as well. Isolated showers and thunderstorms are expected across all other zones, including the Northeastern Plains, Palmer Ridge, San Luis Valley, Northern Mountains, Northwest Slope, San Juan Mountains, Grand Valley, and Southwest Slope. For more details, check out the Zone-Specific discussions below the threat map.

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, Palmer Ridge, and Northeast Plains:

Isolated-to-scattered showers and thunderstorms expected this afternoon and into the late evening hours, with a few lingering showers overnight. Due to upslope flow behind the surface cool front, the best relative chances for heavy rain will be along and west of I-25 and into the foothills. Maximum rain rates will be 0.8-1.4 inches/hour.

Timing: 11 AM – 10 PM will be primetime, but expect a few showers and a weak thunderstorm or two to continue into the early morning hours tomorrow.

Front Range, Central Mountains, Southeast Mountains, Southeast Plains and Raton Ridge:

Scattered showers and thunderstorms are likely throughout the day and into the overnight hours, with a few showers continuing into the early morning hours tomorrow. With the significant amount of moisture in place across the area, heavy rain is likely, and flash flooding/mudslides are a concern, especially for areas that have experienced rainfall the past few days. Fairly quick storm motions will help mitigate the flood risk, but a moderate flood threat is warranted. Maximum rain rates will 1.5-2.5 inches/hour over the Raton Ridge and Southeast Plains, and 1.0-1.6 inches/hour over the Front Range, Central Mountains, and Southeast Mountains.

Timing: 11 AM – Midnight, with a few showers and thunderstorms continuing into the early morning hours.

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

Isolated-to-scattered showers and thunderstorms expected, mainly over the higher terrain and adjacent valleys. Precipitable water values have backed off a bit, but will still be sufficiently high (0.75-1.0 inch) to produce efficient rainfall. Storm motions will help mitigate flood risk, but areas that have received rainfall over the past few days, and streams that are flowing near bank-full, will bear monitoring for issues with additional rainfall. Rain rates will typically be 0.8-1.25 inches/hour under thunderstorms, with one or two stronger thunderstorms holding the potential to produce rain 1.0-1.6 inches/hour.

Timing: 11 AM – Midnight, with a few showers continuing over the higher terrain into the morning hours.

Issue Date: 7/5/2015
Issue Time: 10:41 AM

A MODERATE FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE NORTHWEST SLOPE, NORTHERN MOUNTAINS, FRONT RANGE, CENTRAL MOUNTAINS, GRAND VALLEY, SOUTHWEST SLOPE, AND SAN JUAN MOUNTAINS. A LOW FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS SURROUNDING THE HIGH COUNTRY MODERATE FLOOD THREATS, AS WELL AS PORTIONS OF THE URBAN CORRIDOR, PALMER RIDGE, NORTHEAST PLAINS, AND SOUTHEAST PLAINS.

This morning’s water vapor imagery is very useful in diagnosing the forecast for this period, so let’s break it down…

1. The upper-level ridge (black dashed line) continues to flatten out and slide east in advance of a shortwave (circled in orange) moving across Utah and into western Colorado at this time.
2. Ahead of this shortwave trough, subtropical moisture is streaming into the state. Precipitable water values have been pushed above 1 inch along the CO/UT border, and nearing 0.8-1.0 inches elsewhere. In fact, a new daily 12Z (6 AM) record has been set for Grand Junction, where 1.29 inches of precipitable water was observed. This is a deep, subtropical moisture plume. Monsoon season has arrived.
3. A second wave, currently dropping out of British Columbia (circled in blue), will bring additional, large-scale forcing to Colorado this afternoon and evening. This will contribute to widespread showers and thunderstorms across northern portions of the High Country, namely the Northwest Slope, Northern Mountains, and Front Range, as well as scattered coverage across the Urban Corridor and Northeast Plains.
4. Drier air will work in slowly from the west during evening hours (also circled in orange, associated with the backside of the first wave), bringing showers to an end from SW to NE across western Colorado. A few showers will persist over the higher terrain of the Northwest Slope and Northern Mountains into the early morning hours tomorrow, as those areas remain under weak forcing from the second wave.

Not noticeable on the water vapor imagery, but associated with the upper-level trough working across the northern United States, a cool front will work southward across Colorado, providing shallow, upslope flow to Eastern Colorado. This will continue the threat of isolated showers and thunderstorms into the overnight hours.

For more details on today’s forecast, please see the Zone-Specific Forecast discussions below the threat map.

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, Palmer Ridge, and Northeast Plains:

Scattered showers and thunderstorms expected this afternoon and evening, and stronger storms will produce periods of moderate-to-heavy rainfall. Maximum rain rates will be 0.8-1.2 inches/hour over the Urban Corridor, 1.0-1.4 inches/hour over the Palmer Ridge, and 1.0-1.6 inches/hour over the Northeast Plains. There is potential for one or two severe storms to develop over the far Northeast Plains, namely Sedgwick, Logan, Morgan, Washington, and Yuma counties; strong winds and large hail will be the main threats. Storm motions will help limit flooding concerns, but urban and low-lying areas will need to be monitored for localized flooding.

Timing: Noon – 9 PM, with a few showers and weak thunderstorms persisting into the early morning hours following cool frontal passage.

Southeast Plains and Raton Ridge:

Isolated-to-scattered showers and thunderstorms are expected this afternoon and evening as deep moisture continues to overspread the state. A thunderstorm or two will continue into the overnight hours across the Southeast Plains, mainly east of a North-South line following the Bent/Otero county boundary. Maximum rain rates will be 0.4-0.8 inches/hour from general showers/thunderstorms, with a stronger thunderstorm pushing 1.2-2.0 inches/hour.

Timing: 3 PM – 10 PM

Front Range, Northern Mountains, Northwest Slope, and Central Mountains:

Widespread showers and thunderstorms expected, with locally heavy rain and localized flooding a concern. With precipitable water values as high as they are, rain rates will generally be in the 0.6-1.0 inches/hour range, with maximum rates as high as 1.0-1.5 inches/hour. Storm motions will help limit flooding potential, but due to coverage and terrain, a moderate flood threat is warranted. The most activity and main threat period will occur during the afternoon and evening hours, but scattered showers will continue overnight.

Timing: 11 AM – 10 PM, but showers capable of moderate rainfall will continue overnight and into tomorrow morning.

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

Widespread showers and thunderstorms expected over the San Juan Mountains, Southwest Slope, and Grand Valley, with more isolated-to-scattered coverage over the San Luis Valley and Southeast Mountains. Burn scars will warrant the most concern for flooding/debris slide/mudslide issues, but mudslides/localized flooding will be a concern for places that have received rainfall over the last few days. Maximum rain rates will be 0.4-0.8 inches/hour over the Southeast Mountains and San Luis Valley, with rates pushing 0.8-1.2 inches/hour over the San Juan Mountains, Southwest Slope, and Grand Valley. Showers and thunderstorms will come to an end from southwest to northeast later this evening as drier air works in from the west, but a few will persist over the higher terrain until midnight.

Timing: 11 AM – 10 PM