Issue Date: 5/30/2018
Issue Time: 9:03 AM

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

A shortwave disturbance embedded in southwesterly flow aloft will invade Colorado today, triggering scattered showers and thunderstorms across the state. The disturbance shows up distinctly in the water vapor image below, and is accompanied by plenty of mid-level moisture (blue/white colors) to support the activity. For eastern Colorado, the low-level moisture will be sufficient to produce periods of moderate-to-heavy rainfall. Combined with a concern for small hail clogging drainage, and the potential for multiple thunderstorms impacting the same location, a low flood threat is warranted. A few thunderstorms will be on the strong-to-severe side, with the potential to produce large hail (up to 1.5” in diameter), strong winds, and an isolated tornado.

West of the Continental Divide, low-level moisture will be a limiting factor on rainfall, keeping rain rates below flash flood thresholds. The main impacts from storms will be gusty winds, light rainfall, and lightning, with plenty of virga to be noted across the sky. For more information on rain rates and timing, please see the zone-specific forecast 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

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

Scattered showers and thunderstorms are expected this afternoon and evening, with a few isolated storms continuing until around midnight. Most will be garden-variety, producing periods of light-to-moderate rainfall, gusty winds, and lightning. The atmosphere will support a couple strong-to-severe storms today east of the Mountains, with the potential to produce periods of heavy rain, large hail (up to 1.5 inches in diameter), strong winds, and an isolated tornado. Maximum rain rates are as follows:

Front Range and Southeast Mountains: 0.5-1.0 inches/hour
Northeast Plains, Urban Corridor, Palmer Ridge, and Raton Ridge: 1.0-1.2 inches/hour
Southeast Plains: 1.3-1.6 inches/hour

Timing: 1 PM – 10 PM, with a couple isolated storms rumbling until Midnight-1 AM

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

Scattered showers/thunderstorms are expected this afternoon and evening, with the best coverage over the higher terrain of west-to-east oriented ranges. “Inverted-V” soundings characterized the environment today (low-level moisture is lacking), so the main impacts from storms today will be light rain, gusty winds, and lightning. Generally speaking, rainfall rates will be in the 0.1-0.2 inches/hour range, with maximum rates around 0.2-0.3 inches/hour.

Timing: 11 AM – 8 PM for the Southwest Slope and San Juan Mountains, Noon – 10 PM for the other regions

Issue Date: Tuesday, May 29, 2018
Issue Time: 10:00AM MDT

— Flooding is NOT expected today

The upper wave marked in the water vapor imagery below will slowly track to the northeast throughout the day, which will put us under northwestly flow for most of the day. This should allow enough low-level moisture to hang on for another round of garden variety showers and thunderstorms over northern Colorado and the eastern plains. High temperatures will be down a bit as well over northeastern Colorado. Tonight and tomorrow the flow will gain more of a westerly component as the open wave moves out of Wyoming. This will begin drying out the atmosphere for the rest of the week and limit Colorado’s chances for afternoon rain.

Precipitable Water (PW) this morning at Denver was measured at 0.56 inches, which is quite a bit lower than yesterday. This means the drying process has begun and thunderstorms today will produce lighter rainfall rates and stronger winds. PW at Grand Junction is around the same with fairly decent moist mid-level moisture. Despite the decrease in moisture, dew points should be high enough to support scattered thunderstorms and showers this afternoon and evening over the northern high terrains and Northwest Slope.

The diurnal heating pattern and upper low should help trigger thunderstorms over the mountains just after noon. Coverage in the mountains will be greatest over the Northern Front Range and Northern Mountains, but more isolated storms will also fire further south. Small hail may occur, but limited instability should keep this around pea size. Over the eastern plains and Palmer Ridge, a dry line is expected to spark thunderstorm development later this afternoon. Numerous outflow boundaries from yesterday could also spin up a few thunderstorms. Higher dew points will increase rainfall amounts, but storm motion will be about 15-20 knots. This should limit heavy rainfall over any area. Thus, there is no flooding expected today. Without any upper support, storms will quickly fizzle out after sundown as instability rapidly decreases.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

Palmer Ridge, Front Range, Urban Corridor, Northern Mountains, Central Mountains, Northeast Plains, Southeast Plains, Northwest Slope:

More scattered thunderstorms are expected over the northern high terrains and Northwest Slope this afternoon. More isolated thunderstorms and showers for the Central Mountains and southern Front Range. Max 1-hour rain rates up to 0.3 inches (north) and 0.15 inches (south) are possible.  Out over the eastern plains, more moisture could cause max 1-hour rain rates up to 1.1 inches. Decent storm motion and lack of high moisture should limit the flooding potential. Threats for stronger storms include hail up to 1.25 inches and wind gusts of 50 mph. Flooding is not expected today.

Primetime: 1PM – 8PM

San Luis Valley, San Juan Mountains, Southwest Slope, Southeast Mountains, Raton Ridge, Grand Valley:

More isolated showers and thunderstorms are expected over southern and western Colorado. Rain rates over the Southeast/San Juan Mountains and Raton Ridge should be under 0.1 inches/hour. Expect more afternoon clouds over the higher terrains than rainfall. Should be sunny, beautiful day elsewhere. There is no flood threat today.

Primetime: 2PM – 8PM

Issue Date: Monday, May 28, 2018
Issue Time: 10:40AM MDT

— MODERATE flood threat for portions of the Front Range, Urban Corridor, Northeast Plains and Southeast Plains

— LOW flood threat for the Front Range, Palmer Ridge, Northeast Plains, Urban Corridor, Southeast Plains, Northern Mountains and Northwest Slope

Another round of severe weather and heavy rainfall is expected this afternoon, evening and overnight for the northern portion of the state. Below is the water vapor imagery with several key features for today’s forecast marked. First is the upper low over NV/Utah. Today this will release upper air support in the form of shortwaves and jet streak for more widespread showers and thunderstorm coverage. It will also help maintain showers and thunderstorms overnight throughout the Northern Mountains and northeast corner of the state. Also marked is a surface low, which will help pull in moisture into the state on both to the north and east sides. The moisture axis is drawn in green with higher moisture residing to the east of the line. Dew points will be in the 50s (west) and 60s (east), which will support heavy rainfall. Precipitable Water was measured at 0.75 inches at Denver this morning, and this is expected to increase throughout the day with low-level, moist easterly flow.

Further south, over the southern Southeast Plains, the eastward progression of the dry line throughout the day will limit the heavy rainfall east of Colorado. Low relative humidity and increased winds to the west of the dry line will cause critical fire conditions. A Red Flag Warning is in place through this evening for the San Luis Valley, Southeast Mountains and Raton Ridge. This evening, a MCS sets up over the northeastern plains, with heavy rainfall falling extending south into Cheyenne and Kiowa counties.

This afternoon, upslope flow and instability will help initiate thunderstorms over the Northern Mountains, Front Range and Palmer Ridge. East of the Continental Divide, these storms will begin to move to the northeast into the Urban Corridor and Northeast Plains later in the afternoon. As they move east, they will encounter better instability and shear, which will increase the chances of severe thunderstorms. Threats include large hail (up to 2 inches), strong winds (60-65 mph), a couple tornadoes and heavy rainfall. A Moderate flood threat has been posted for portions of the Urban Corridor due to the potential for street and small stream flooding this afternoon. Max 1-hour rain rates up to 1.2 inches per hour are possible with these thunderstorms.

This evening and overnight another MCS is expected to set up over the Northeast Plains where heavy rainfall occurred yesterday. With already saturated soils, flooding of fields, roads and small streams is likely, so a Moderate flood threat has been issued. 24-hour storm totals could be up to 3.5 inches in this area.

To the west of the Continental Divide, moisture is also forecasted to increase this afternoon with the greatest increases over the Northwest Slope and Northern Mountains. Lifting from the upper wave is expected to initiate more widespread shower and thunderstorm coverage today when compared to yesterday. Convection may persist overnight in the Northern Mountains, but the flood threat will end after sundown. For this afternoon, a Low flood threat has been issued. Threats include road and small stream flooding with the potential for mud flows and debris slides over the steeper terrains.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

 

Zone-Specific Forecasts:

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

Instability and upper dynamics will support severe thunderstorms and heavy rainfall this afternoon into the night. 1-hour rain rates up to 1.2 inches are possible over the Urban Corridor, so a Moderate flood threat has been issued. Further east, 1-hour rain rates up to 2.2 inches are possible due to high low-level moisture today. Isolated 24 hour totals over the far eastern plains will exceed 3.5 inches due to thunderstorms continuing through the night. Due to heavy rainfall last night, a Moderate flood threat has been issued. Threats include street, field and small stream flooding. East of the Front Range, severe weather is expected this afternoon with large hail (up to 2 inches), strong winds (60-65 mph) and, possibly, a couple of tornadoes.

Primetime: 1PM – 4AM

Southeast Plains, Southeast Mountains, Raton Ridge:

A few thunderstorms may form over the Southeast Mountains this afternoon. Moisture this far south will be restricted, so 1-hour rain rates will be limited to 0.25 inches/hour. Low relative humidity and increasing winds will cause critical fire danger to the west. Please use caution with any sort of spark through this evening. A Red Flag Warning is in place. Overnight rainfall could extend south into Kiowa County where 24-hour totals up to 2.0 inches are possible.

Primetime: 2PM – 8PM

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

Increases in moisture and upper level support will help trigger numerous thunderstorms over the Northwest Slope and Central Mountains this afternoon. Max 1-hour rain rates up to 1 inch are possible, so a Low flood threat has been issued. Threats include small stream and street flooding with the possibility of mud flows and debris slides over the steeper terrain of the Northern Mountains. There may be a few linger showers overnight, but the flood threat should end around sundown.

Primetime: 1PM – 8PM

Issue Date: 5/27/2018
Issue Time: 8:35 AM

A LOW FLOOD THREAT IS FORECAST TODAY FOR PORTIONS OF THE NORTHEAST PLAINS, SOUTHEAST PLAINS, URBAN CORRIDOR, FRONT RANGE, AND PALMER RIDGE.

Today’s forecast across Colorado will be defined by those with access to moisture and those without access to moisture. In the surface plot shown below, a thick green line has been drawn to denote the location of the 45°F dewpoint contour. To the right of the line, ample moisture is already in place to fuel a fairly active strong-to-severe thunderstorm day. Strong instability and “good enough” shear will coincide with this low-level moisture, which will promote quickly strengthening thunderstorms capable of producing large hail (up to 1.5” diameter), strong winds (up to 70-80 mph), an isolated tornado or two, and periods of heavy rainfall. Precipitable water values will push to near, or exceed, 1 inch across the far eastern plains by this evening, so that makes our ears perk up with regards to flash flooding concerns. Storm motions should keep storms moving quickly enough to avoid flash flooding, but the possibility of training thunderstorms is enough to warrant a low flood threat. Portions of the Front Range, Urban Corridor, and Palmer Ridge are also included in the low flood threat due to easterly flow likely pushing the 45°F dewpoint line back into the area during the afternoon/evening hours, providing enough moisture for a quick 0.5”/30 min rainfall – this is enough to cause street flooding in urban areas.

To the left of the green line (not including the Urban Corridor/Front Range for reasons described above), mainly dry conditions will win out. The exception to this rule will be across the higher terrain of the Northwest Slope and Northern Mountains where enough moisture will sneak into the state from the north to work with orographic support and result in isolated showers/weak thunderstorms.

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, Southeast Plains, Palmer Ridge, and Raton Ridge:

Strong Isolated-to-scattered thunderstorms will rumble this afternoon/evening, with a few reaching severe thresholds. The main threats will be large hail (up to 1.5” diameter), strong winds (up to 70-80 mph), and periods of heavy rainfall, with the potential for an isolated tornado or two. The Raton Ridge will remain mostly dry, but an isolated thunderstorm or two will result in gusty winds, dangerous lightning, and a brief period of light rainfall.

Training storms over the Northeast Plains and quick bursts of heavy rain over urban areas and steep(er) terrain of the Front Range are the driving forces behind today’s low flood threat. Maximum rain rates are as follows:

Front Range, Urban Corridor, and Palmer Ridge: 0.4-0.6 inches/30 minutes
Northeast Plains and Southeast Plains: 1.5-2.0 inches/hour
Raton Ridge: 0.15-0.3 inches/hour

Timing: Noon – 10 PM, with an isolated storm or two rumbling until midnight

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

Dry and warm, though temperatures will be a few degrees cooler than yesterday. The exception to the dry rule will be over the higher terrain of the Northwest Slope and Northern Mountains where enough low-level moisture will be in place to fuel orographically-forced showers/weak thunderstorms. Rain rates will be low, with maximum rates of 0.15-0.25 inches/hour

Timing: Noon – Midnight