Issue Date: 5/07/2015
Issue Time: 10:59 AM

A MODERATE FLOOD THREAT IS FORECAST FOR PORTIONS OF THE URBAN CORRIDOR, FRONT RANGE, PALMER RIDGE, AND NORTHEAST PLAINS. A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE NORTHWEST SLOPE, NORTHERN MOUNTAINS, CENTRAL MOUNTAINS, FRONT RANGE, PALMER RIDGE AND SOUTHEAST PLAINS.

The Flood Threat Bulletin is starting to sound a lot like a broken record over the past few days, and the same tune is playing again today. Precipitable water values are in 85th percentile for this date (per this morning’s Denver and Grand Junction soundings) and the IPW graph below shows just how steady the moisture has been.

With that being said, moisture west of the Divide will be less than the previous few days, leading to less potential for heavy rain and shorter duration storms. A few showers are ongoing at this time over the higher terrain thanks to a weak shortwave pushing across the region. As this shortwave moves into eastern Colorado this afternoon, drier/subsident air will begin working across SW Colorado. This means that most showers and thunderstorms this afternoon will be across the higher terrain of the Northern Mountains, Northwest Slope, and Central Mountains.

East of the Divide will be a different story entirely, as moisture will be reinforced this afternoon as weak, low-level easterly flow develops behind a surface front forecast to sit near the CO/NM. This, combined with the approaching shortwave from the southwest, will create an environment capable of producing strong storms with heavy rain. Couple this with the last few days of rain, and we have been given enough reason to issue our first moderate flood threat of the season for portions of the Front Range, Urban Corridor, Palmer Ridge, and Northeast Plains.

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 and Front Range:

Scattered showers and thunderstorms will begin to develop between 11 AM and Noon; a few will become strong-to-severe over the Urban Corridor shortly thereafter. The threat of strong storms will remain until 8-9 PM, with general showers and thunderstorms then continuing into tomorrow morning.

Maximum rain rates will be in the 2-2.5 inches per hour range, and this makes flash flooding a distinct threat underneath the strongest storms, especially considering the saturated grounds from the last few days of rain.

Palmer Ridge, Northeast Plains, and Southeast Plains:

Scattered thunderstorms will begin to develop/move overhead in the 11 AM – Noon timeframe, with storms becoming quickly strong-to-severe, across the Palmer Ridge and Southeast Plains first, then over the Northeast Plains. Heavy rain at 2-3 inches/hour, wind gusts up to 60 mph, and hail greater than 1 inch in diameter will be threats under any of the stronger thunderstorms. A tornado or two will be possible, as well, especially early this afternoon when low-level moisture is maximized. Storm motions over the Plains will limit some of the potential for flash flooding, but street and field ponding already exists in some locations due to the last few days of rain.

Southeast Mountains and Raton Ridge:

Isolated-to-scattered showers and thunderstorms will develop between 11 AM and Noon, with the bulk of the activity ending by 6 PM. A few showers/thunderstorms will continue into the night over northern portions of the region. Maximum rain rates will be in the 0.4-0.8 inch/hour range.

Northern Mountains, Central Mountains, and Northwest Slope:

A few showers and thunderstorms are ongoing across the regions, and will increase in coverage with time into this afternoon. Expect most activity to end just after sunset, with a few weak showers over the higher terrain continuing into tomorrow morning. Rain rates will be in the 0.75-1.2 inches/hour range, so a low flash flood threat is in place for portions of these regions.

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

A few showers will continue throughout the day, though most activity will be shut off due to subsidence/dry air behind todays shortwave. Any showers will likely stay anchored to the higher terrain, with rain rates less than 0.4 inches/hour.

Issue Date: May 6th, 2015
Issue Time: 10:30 AM MDT

— Low flood threat for Northwest Slope, parts of Northern Mountains and parts of Grand Valley
— Low flood threat for Palmer Divide, Urban Corridor and parts of Northeast Plains
— Low flood threat for South Platte River from Greeley through Fort Morgan

This morning’s weather analysis shows quite an active pattern across western United States. A polar jet is identified over the Pacific Northwest while a subtropical jet continues to pulse and propagate slowly eastward near the United States/Mexico border. As shown in the water vapor image below, the disturbance causing yesterday’s rainfall was just leaving northeast Colorado but radar returns still show light to moderate rain across the Northeast Plains with lighter rainfall in the Southeast Plains and also over the Grand Valley. A weak surface low-pressure system was noted across eastern Colorado and some clearing was seen along the New Mexico border. Precipitable water values continue to run abnormally high for early May with this morning’s values coming in at 0.85 in Pueblo and Grand Junction and 0.7 in Boulder.

There are both similarities and differences between today’s weather setup compared to yesterday’s. The similarity is the upper-level pattern will be generally conducive to upward motion and thus rainfall. On the other hand, the organized disturbance causing yesterday’s rainfall is moving out of the state, leaving less organized support for widespread rain today. Another important difference is that we expect more sunshine today especially the farther south one goes, which will allow for warmer temperatures and significantly more instability. Collectively, our high-resolution precipitation probability product below shows a good estimate of which locations will likely be impacted by rainfall today.

In terms of specifics, expect light to moderate rainfall to continue across the north with breaks in the clouds across the south. Showers and then thunderstorms will increase in coverage with the afternoon heating and affect the entire I-70 corridor east and west of the Divide. Due to yesterday’s rainfall, a low flood threat is once again in place in the Northwest part of the state for debris slides and mud flows. Meanwhile, instability, high moisture content and slow storm motion warrant a low flood threat for parts of northeast Colorado as thunderstorms redevelop in the afternoon. A few storms may become strong with small hail possible. Most rainfall activity will quickly subside shortly after sunset. The South Platte River has become elevated from the past few days of moderate rainfall. A low flood threat has been posted from minor riverside flooding from the Greeley to Fort Morgan stretch of the river.

 

Zone-Specific Forecasts

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

Overcast early with showers slowly shrinking in coverage by noon. Breaks in the sun will boost temperatures and allow for a development of thunderstorm activity by mid-afternoon along the foothills of the entire Urban Corridor. Storms will be capable of producing rainfall up to 1.4 inches per hour. This may result in isolated flash flooding due to saturated soils. A low flood threat has been posted for parts of the area as well as the South Platte River from Greeley to Fort Morgan. Activity will subside shortly after sunset.

Southeast Plains, Raton Ridge and Southeast Mountains:

Early morning showers will dissipate leading to clearing skies. Temperatures will rise and allow for isolated to widely scattered thunderstorm activity mainly in the northern parts of the area. Thunderstorms may produce rain rates up to 1 inch per hour. No flood threat is expected.

Central Mountains, San Juan Mountains, San Luis Valley and Southwest Slope:

Morning cloud cover partially break up leading to some sunshine. Showers and isolated weak thunderstorms may redevelop in the afternoon but rain rates are expected to stay at 0.4 inches per hour or below. No flood threat is expected.

Northwest Slope, Grand Valley, and Northern Mountains:

Morning showers and isolated thunderstorms will continue and likely redevelop in coverage by early afternoon. Rain rates will generally be less than 0.5 inches per hour, however later in the afternoon enough instability may allow for stronger thunderstorms with rain rates up to 0.7 inches per hour. Due to yesterday’s widespread moderate rainfall, a low flood threat has been posted for isolated flash flooding, debris slides and mud flows.

Issue Date: May 5th, 2015
Issue Time: 10:55 AM MDT

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

An impressive subtropical jet stream over the eastern Tropical Pacific Ocean and western Mexico continues to send upper-level disturbances north and eastward across Colorado. As the water vapor image shows below, another feature is just entering the state as of this morning. North of this feature, marked by the dashed black line, a plume of moisture, cold upper-level temperatures and dynamical wind forcing will continue to support showers and thunderstorms across our state.

One difference from yesterday’s weather setup is that cloud cover will be more widespread and many areas may not see any sunshine at all. This is in contrast with yesterday, where areas along and south of the Palmer Ridge broke out of clouds, allowing temperatures to quickly jump into the mid-60s. The implication of today’s reduced sunshine will be less instability. Thus, even though precipitable water values remain quite high for this time of year, ranging from 0.7 in Boulder and Grand Junction to 0.8 in Pueblo, the lack of instability will be the main factor that will keep rain rates down. The one exception to this will be in the far eastern portion of the Southeast Plains, where sunshine may bolster temperatures late in the afternoon allowing for rainfall rates to approach 2.0 inches per hour.

However, the combination of yesterday’s widespread moderate to heavy rain amounts, elevated stream levels east of the Continental Divide and today’s steady rain rates of 0.4 to 0.7 inches will require a Low flood threat. Additionally, part of the Northern Mountains and Northwest Slope have been added to a Low flood threat because these areas are likely to experience the most sunshine today, raising temperatures and instability.

Please see below for area-specific forecasts (click on “About the FTB” link above to view Colorado area-specific regions).

 

Zone-Specific Forecasts

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

Overcast with showers developing by early afternoon. Rain rates of 0.4 to 0.7 inches per hour will be possible across this entire area. In the far southeast portion of the Southeast Plains, more sunshine will cause strong to isolated severe thunderstorms capable of producing rain rates up to 2 inches per hour. A low flood threat has been issued for parts of this area.

Central Mountains, San Juan Mountains, San Luis Valley and Southwest Slope:

Morning cloud cover partially break up leading to some sunshine. Rain rates up to 0.5 inches per hour will be possible later in the afternoon as some instability develops. No flood threat is expected.

Northwest Slope, Grand Valley, and Northern Mountains:

Morning showers and isolated thunderstorms will expand in coverage and continue throughout the day. Rain rates will generally be less than 0.5 inches per hour, however later in the afternoon enough instability may allow for stronger thunderstorms with rain rates up to 0.9 inches per hour. A low flood threat has been posted for isolated flash flooding, debris slides and mud flows.

Issue Date: 5/04/2015
Issue Time: 11:08 AM

LOW FLOOD THREAT IS FORECAST FOR TODAY FOR PORTIONS OF THE URBAN CORRIDOR, FRONT RANGE, PALMER RIDGE, NORTHEAST PLAINS, RATON RIDGE, SOUTHEAST MOUNTAINS, SAN JUAN MUONTAINS, CENTRAL MOUNTAINS AND NORTHERN MOUNTAINS. BE SURE TO CHECK THE MAP BELOW FOR SPECIFIC LOCATIONS.

Moisture has been on the increase since yesterday, with IPW values (shown below) climbing to between 0.6-0.85 inches at the normal reporting stations (Boulder – DSRC, Grand Junction – MC01, Pueblo – PUB5, and Shriever AFB – AMC2). This is due to the moisture plume being pulled across the Desert SW and into Colorado by an upper-level low currently centered over southern CA. This places Colorado under a moist, southwesterly flow, which will contribute to the increased coverage of showers/thunderstorms today and tonight. Precipitable Water values, as measured by atmospheric soundings from Grand Junction and Denver at 6 AM, are in the 90th percentile for this date, showing just how “juicy” the atmosphere is today. With that said, though, we still haven’t reached the 1 inch IPW mark this season, which is the benchmark for a typical, summertime heavy rain/flood potential day.

Limiting today’s potential for heavy rain to cause flash flooding is relatively insufficient instability for the most flood prone areas along the Front Range and Urban Corridor. Cloud cover from this morning’s showers is still stuffed into much of northeast Colorado, though it is starting to thin out across the Palmer Ridge and southern Front Range/Urban Corridor regions. This cloud cover will minimize the work that the sun can do to warm up the surface, lessening the amount of instability that it can create in those areas. On the flip side of that same coin, the upper-level low will provide vertical motion to compensate for that lack of instability, and so moderate-to-heavy rain cannot be ruled out.

West of the Divide, much of the cloud cover from last night has thinned, leaving mostly sunny skies to heat the surface and increase instability. Additionally, support from the upper-level low will aid in thunderstorm development. Widespread heavy rain is not likely, but a few isolated thunderstorms producing rain in the 0.8-1.4 inches/hour range is not out of the question.

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 and Northeast Plains:

Scattered showers and thunderstorms capable of producing periods of moderate-to-heavy rain are likely. Rain rates will generally be in the 0.5-0.8 inches/hour range, but the strongest storms could produce rain in the 1.0-1.6 inches/hour range. Limiting the flood potential is the cloud cover still in place over much of the region, though it is thinning out over southern reaches of the area.

Primetime: Noon – Midnight

Palmer Ridge:

Cloud cover is thinning quickly, which will allow the sun to warm the surface. This will create an environment capable of producing moderate-to-heavy rain in the 1.0-1.6 inches/hour range.

Primetime: Noon – 11 PM.

Southeast Plains and Raton Ridge:

Sufficient moisture and instability this afternoon will combine to produce scattered strong thunderstorms, with maximum rain rates in the 1.5-2.25 inches/hour range. The strongest storms will produce small hail (1.0 inch) and gusty winds (45-60 mph).

Primetime: 1 PM – 6 AM for the Southeast Plains, Noon – Midnight for the Raton Ridge

Northern Mountains, Central Mountains, Southeast Mountains, and San Juan Mountains:

Scattered showers and thunderstorms are expected this afternoon and evening, before diminishing after sunset. A few will continue through the overnight hours as the upper-level low continues to track eastward towards the state. Rain rates this afternoon and evening will generally be in the 0.4-0.7 inches/hour range, but the strongest storms could produce rain at 0.7-1.25 inches/hour.

Primetime: 11 AM – 8 PM

Northwest Slope, Southwest Slope, Grand Valley, and San Luis Valley:

Scattered showers and thunderstorms are expected to develop over the adjacent higher terrain, and a few will move overhead throughout today and tonight. Rain rates will generally be in the 0.4-0.8 inches/hour range, but the strongest storms will be capable of 0.7-1.25 inches/hour.

Primetime: 11 AM – 4 AM