ISSUE DATE: May 23, 2015
ISSUE TIME: 10:40AM

— MODERATE flood threat for South Platte River from Greeley through the Nebraska state line
— MODERATE flood threat for Front Range (likely to continue through tomorrow)
— MODERATE flood threat for Northeast Plains
— LOW flood threat for Northeast Plains and parts of Urban Corridor and Front Range
— LOW flood threat for Arkansas River from Pueblo through La Junta

The cut-off low that was over California yesterday is approaching the Four Corners this morning, as shown by the water vapor image below. Two distinct disturbances are noted to its east: one that barely grazes Colorado and is mainly responsible for clouds over the state, and another that is positioned on the Colorado/New Mexico border. It is the latter that will deliver most of the weather impact to day across our state.

This morning, sunshine is more abundant across the state and temperatures have quickly responded, running several degrees above yesterday’s readings east of the Divide. Dewpoint temperatures are in the 40s to low 50s across most of the state. Meanwhile, precipitable water values are only 0.4 inches at Grand Junction, but close to 0.8 at Pueblo. This indicates that the upper atmosphere is somewhat drier today compared to yesterday especially west of the Divide. However, the strong dynamics of the incoming system will be enough to overcome this.

For today, we expect showers and thunderstorms to quickly dot a large portion of the state. Initially, rainfall across most areas will be light. However, as the afternoon heating progresses and good convergence occurs along the higher terrain, heavier rainfall is expected across the Front Range and perhaps spilling over into the Northern and Central Mountains. Storm motion will be fast, at 30-40 mph, but multiple storms may affect the same area. For this reason, in concert with it being Memorial Day weekend with many hikers and campers around, a Moderate flood threat is in place for the Front Range for afternoon and evening storms. This threat will likely persist through tomorrow as the upper-low moves directly overhead.

Farther east, another round of strong to severe thunderstorms is expected across the Palmer Ridge and Northeast Plains. One hour rainfall up to 1.7 inches and 3 hour rainfall up to 3 inches warrants a Moderate flood threat for parts of the area. Saturated soils are a contributing factor.

We have raised the South Platte River flood threat from Low to Moderate in anticipation of substantial afternoon and evening runoff. The Arkansas River is under a Low flood threat from Pueblo to La Junta.

We urge anyone with outdoor activities to stay current with today’s weather evolution. An excellent source of real-time information is the National Weather Service.

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, Northern Mountains and Central Mountains:

Partly cloudy skies with numerous rounds of shower and thunderstorm activity developing by early afternoon. One hour rainfall up to 1.25 inch and 3-hr rainfall up to 2.5 inches will lead to localized flash flooding, debris slides and mud flows. A Moderate flood threat is in place for the Front Range with a Low flood threat elsewhere

Prime time: From 2pm through the overnight hours

Urban Corridor, Northeast Plains and Palmer Ridge:

Partly cloudy skies with showers and thunderstorms developing by early afternoon. One hour rainfall up to 1.7 inches and 3-hour rainfall up to 3 inches will be possible near the Kansas border with lower amounts elsewhere. Severe weather, including large hail up to 1.5 inches, damaging winds up to 75 mph and a weak tornado are all possible. A Moderate flood threat is in place in the far northeast with a Low flood threat elsewhere. Formation of the Denver cyclone will likely limit rainfall amounts in the immediate Denver metro area.

Prime time: 2pm through 10pm

Southeast Mountains, Raton Ridge and Southeast Plains:

Mostly sunny to partly cloudy with scattered storms developing by early afternoon. One hour rainfall up to 1 inch is possible, but a pocket of dry air aloft and fast storm motion is expected to limit the flood threat.

Prime time: 2pm through 7pm

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

Partly cloudy skies with scattered showers and storms developing. Weak instability will limit one-hour rainfall to 0.6 inches so no flood threat is expected. Small hail, up to 0.5 inches, and gusty winds are possible in the stronger thunderstorms.

Issue Date: 5/22/2015
Issue Time: 10:50 AM

— LOW flood threat for South Platte River from Greeley through the Nebraska state line
— LOW flood threat for Arkansas River from Pueblo through John Martin Reservoir
— LOW flood threat for Palmer Ridge and parts of Northeast Plains

The “spring of the cut-off lows” continues across the Southwest U.S. The water vapor imagery shows yet another cut-off low currently positioned off the California coast. This low pressure system is being supported by not one, but too different subtropical jets. The maximum wind speeds in both jets, about 35,000 feet off the ground is more than 100mph. For Colorado, this implies active weather not only today, but all the way through Sunday. Hikers and campers should closely monitor weather conditions and National Weather Service forecasts this weekend.

Closer to home, a strong disturbance (black dashed line) is passing overhead this morning, and is currently about two-thirds of the way across the state on its way northeastward. This disturbance is supporting general light rain in the northeast, but is quickly exiting the state. In its wake is a sharp line of clearing, meaning that sunshine will break out for most everyone shortly after noon today. However, with that sunshine will come a quick heating of the ground, leading to instability as the saturated ground loses water vapor to the atmosphere through evaporation. Isolated to scattered thunderstorm activity will break out over mostly the northern half of the state by early afternoon.

While most thunderstorms will be rather modest, producing light to moderate rainfall, the Northeast Plains will play host to stronger storms by early afternoon. Some of these stronger storms will be capable of producing very heavy rainfall as they will be working with a large amount of moisture. Precipitable water values are expected to stay in the 0.7-0.9 inch range, but cloud warm layer depths (the portion of a cloud that is above 32F) will be very deep. The forecasted warm layer depth for 3pm is shown below. This results in very efficient rainfall formation, and will support isolated rain rates up to 1.7 inches per hour. Given the saturated soil conditions, this warrants a Low flood threat for parts of the Northeast Plains. Additionally, some storms will be capable of producing large hail up to 1.75 inches.

Lastly, parts of the Arkansas and South Platte Rivers remain elevated. Low flood threats are maintained, but we expect the Arkansas river to subside as warmer and drier temperature evaporate substantial moisture from the ground this afternoon.

Please see below for area-specific flood threat and forecasts (click on “About the FTB” link above to view Colorado region names).

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, Northeast Plains and Palmer Ridge:

Showers quickly ending, followed by brief sunshine, followed by scattered thunderstorms by early afternoon. Most thunderstorm rainfall will be 0.7 inches/hour or less, however, along the Palmer Ridge and into the Northeast Plains, isolates storms may produce rainfall up to 1.7 inches per hour. Several storms may pass over a given location, and 3-hour rainfall up to 2.5 inches is possible. A Low flood threat has been posted.

Southeast Mountains, Raton Ridge and Southeast Plains:

Sunshine and much warmer today. Isolated thunderstorms will be possible in the afternoon. However, rainfall rates will remain below 0.7 inches/hour. A low flood threat remains in place for the Arkansas River, but the river should fall below flood stage by later this evening.

Central Mountains, San Juan Mountains and Northern Mountains:

Partly cloudy with scattered rain and snow showers developing by the early afternoon. Hourly rainfall rates will remain below 0.5 inches. No flooding is expected.

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

Sunny to partly cloudy early, with isolated rain showers and a stray thunderstorm developing by early afternoon. Hourly rainfall rates will remain below 0.6 inches/hour and flooding is not expected.

Issue Date: 5/21/2015
Issue Time: 10:45 AM

A LOW FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE PALMER RIDGE, URBAN CORRIDOR, SOUTHEAST PLAINS, AND SOUTHEAST MOUNTAINS. A LOW FLOOD THREAT CONTINUES FOR PORTIONS OF THE CACHE LA POUDRE/SOUTH PLATTE RIVER, AND PORTIONS OF THE ARKANSAS RIVER.

Today will see an uptick in the coverage and relative intensity of showers and thunderstorms as an upper-level low over California ejects a disturbance across Colorado. Accompanying this disturbance will be an increase in both broad-scale ascent and mid-level moisture, providing the ingredients for the increase in showers/thunderstorms.

Widespread heavy rain is not expected as instability and deep moisture will still be relatively limited, especially over areas east of the mountains. The IPW graph below shows the moisture picture well; all four of the normal reporting stations are below 0.65 inches, and for a typical heavy rain day those values need to be closer to, or above, an inch. Most rain rates today will be in the 0.3-0.6 inches/hour range, with the best chance at heavy rain occurring across the low flood threat areas. Max rain rates in the low flood threat area will be between 0.7-1.2 inches/hour; combine this with the moist antecedent conditions and there is enough concern to warrant a low flood threat. Additionally, snow levels will be between 9,000 and 10,000 feet today, so higher mountain passes and mountain peaks in the Central Mountains and San Juan Mountains regions will pick up an additional 4-8 inches of snow today/tonight.

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 (south of Castle Rock), Palmer Ridge, Raton Ridge, Southeast Plains, and Northeast Plains:

For these regions, the counties with the relative greatest risk for moderate-to-heavy rain include El Paso, northwest Pueblo, southern Fremont, Custer, and northwest Huerfano Counties. Max rain rates across these locations (below 9,500 feet) will be in the 0.7-1.2 inches/range, and combined with the moist antecedent conditions, a low flood threat is warranted.

Elsewhere across the regions, any isolated-to-scattered showers/thunderstorms will be relatively weak, producing rain rates in the 0.25-0.5 inches/hour range at best.

Timing: 2 PM – Midnight, with a few light showers continuing into the early morning. A few thunderstorms are expected near the CO/KS border after midnight.

Urban Corridor (along and north of Castle Rock) and Northeast Plains:

A few isolated, showers/weak thunderstorms are expected this afternoon, continuing through the nighttime hours. Near the CO/KS border, a few thunderstorms are expected after midnight tonight as the energy associated with the disturbance crosses the area. Rain rates will typically be in the 0.25-0.5 inches/hour range.

Timing: 3 PM – Midnight, a few showers lingering into the early morning, and a few thunderstorms after midnight near the CO/KS border.

Front Range, Northern Mountains and Northwest Slope:

Isolated-to-scattered showers and weak thunderstorms expected today, diminishing after sunset. Rain rates will maximized at 0.25-0.45 inches/hour, and light snow will fall above 10,000 feet. The far southern extent of the Front Range, i.e., far southeastern Teller County is included in the low flood threat for maximum rain rates in the 0.6-1.0 inches/hour range.

Timing: 11 AM – Midnight, with a shower or two continuing over the highest peaks until 3 AM.

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

Scattered showers and thunderstorms are expected, developing over the higher peaks, and drifting slowly to the northeast with time. Snow levels will be between 9,000 and 10,000 feet, with areas above that gathering 4-8 inches of snow accumulations today and tonight. Below snow level, rain rates are expected to be 0.25-0.5 inch/hour range, with a few stronger thunderstorms over the San Juan Mountains/Southwest Slope pushing 0.5-0.7 inches/hour.

Timing: 11 AM – Midnight, with activity diminishing quickly due to subsidence behind the passing disturbance.

Issue Date: 5/20/2015
Issue Time: 10:01 AM

A LOW FLOOD THREAT HAS BEEN ISSUED FOR THE CACHE LA POUDRE/SOUTH PLATTE RIVER FROM GREELEY TO JULESBURG, AND THE ARKANSAS RIVER FROM AVONDALE TO LA JUNTA.

Colorado will experience a downtick in precipitation today, thanks to declining moisture, an unseasonably cool air mass, and a lack of upper-level forcing. The precipitable water (IPW) graph below shows just how much the moisture has declined since Monday afternoon/Tuesday morning. With all of the normal 4 reporting stations reporting IPW values below 0.5 inches, the chance of heavy rain with any isolated shower/thunderstorm is basically zero.

The cool air mass will go a long ways in keeping the plains mostly dry today, especially as one gets further away from the mountains and Palmer Ridge. Additionally, upslope flow will keep clouds socked in across the Urban Corridor, keeping temperatures from recovering to more seasonable temperatures. Expect highs across the Urban Corridor in the upper 40s/low 50s; downright chilly for this time of year.

The best chances for isolated-to-scattered shower and weak thunderstorm activity will be across the higher terrain. Weak steering winds mean that any activity will struggle to move over the valleys, so today’s action will be confined to mainly the higher peaks. A few isolated showers will work over the Urban Corridor and Palmer Ridge this afternoon, but don’t expect much to come from them as rain rates will be less than 0.25 inches/hour.

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, Raton Ridge, Southeast Plains, and Northeast Plains:

With a cool, stable air mass in place, today will be much quieter across these regions. Most locations will stay underneath mostly cloudy skies, but a few peaks of sun will allow the Southeast Plains and Raton Ridge to warm up into the upper 50s/low 60s. Otherwise, everyone else will be in the upper 40s/low 50s.

A few isolated showers/weak thunderstorms cannot be ruled out over the Urban Corridor, Palmer Ridge, or Raton Ridge this afternoon and evening, but they will not amount to much. Rain rates will be less than 0.25 inches/hour.

Timing: 2 PM – 9 PM

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

The higher elevations will hold the best chance at isolated-to-widely scattered showers and thunderstorms this afternoon and evening. Weak steering winds will keep most, if not all, activity confined to the higher terrain, so chances of precipitation in valleys are very low. Rain rates will mainly be in the 0.1-0.3 inches/hour range, with the maximum rates from the strongest thunderstorms in the 0.25-0.45 inches/hour range.

Timing: 11 AM – 8 PM, with a few isolated showers lingering until 10 PM.