Issue Date: Sunday, May 23rd, 2021
Issue Time: 9:30AM MDT

— A LOW flood threat has been issued for parts of the Northeast Plains and Southeast Plains

The strong upper-level trough, a staple of our weather since Thursday, has nearly stalled out over the Great Basin as shown in the visible satellite image, below. Colorado continues to be under its influence with deep southerly flow over the state, along with numerous disturbances embedded within that flow. Ongoing elevated convection is moving rapidly northward in eastern, with more redevelopment later this morning. However, there is a major difference in today’s atmosphere compared to Saturday. Very dry air has finally overtaken the western portion of the state. For example, this morning’s Grand Junction sounding had a PW of only 0.16 inches. Although this air will continue to progress eastward through the day, it will have resistance. Denver’s morning PW was 0.80 inches, which is much above average for late May. PW around 1 inch was noted along the Kansas border, some of which will make it back into northeast Colorado courtesy of southeastern boundary layer flow. The dry air will eventually win out, but not before another active afternoon mainly in the Northeast Plains and the northern part of the Southeast Plains.

Just like the past few days, the main concern regarding heavy rainfall is not necessarily the shorter duration 1-hour amounts, but the 2-hour and 3-hour durations given that storm motion will continue to favor training. On Saturday evening, this kind of scenario yielded more than 3 inches of rain in Las Animas county over a 2-3 hour period. For today, the area of concern is in the northeast quadrant of Colorado, along the NE and KS border, where up to 2.9 inches of rainfall over a 2-3 hour period will again be possible. Additionally, severe weather, notably large hail and damaging winds, will likely accompany the earlier storms, similar to Saturday but more isolated in nature.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Northeast Plains, Southeast Plains

Scattered to widespread showers and thunderstorms continuing into the late evening hours. Max 1-hour rain up to 1.6 inches, with max 3-hr rain up to 2.9 inches. Isolated flash flooding, street and field flooding will be possible. Large hail and damaging winds are also possible with the earlier round of thunderstorms. A Low flood threat has been issued for parts of the region.

Primetime: 12PM through 11PM

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

Isolated to scattered showers and thunderstorms mainly through the late afternoon, then mostly sunny and much drier. Max 1-hour rainfall up to 0.7 inches. Flooding is NOT expected today.

Primetime: 12PM through 6PM

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

Warmer and drier with gusty winds in the afternoon. Flooding is NOT expected today.

Issue Date: Saturday, May 22nd, 2021
Issue Time: 9:30AM MDT

— A LOW flood threat has been issued for parts of the Northeast Plains, Palmer Ridge and Southeast Plains

Another day of active weather is anticipated for Colorado, especially in eastern areas. As shown in the water vapor image, below, a large trough of low pressure is located over the Great Basin, leading to strong southerly flow across Colorado. Numerous disturbances within this flow will help trigger scattered to widespread showers and thunderstorms east of the Continental Divide. PW has generally increased in eastern areas, with values above 1 inch now noted along the eastern Colorado border. The Denver morning sounding showed a PW of 0.50 inch, right in line for this time of year, also indicating a very large gradient across the eastern part of the state. This, along with southerly steering flow will anchor today’s storms across a rather narrow corridor in eastern Colorado. Also of note was a strong cool front that pushed into CO from WY overnight, leading to a very stable airmass roughly north of I-76. Although some of this airmass will be pushed back northward, heavy rainfall chances will be capped north of this boundary.

We expect multiple rounds of showers and thunderstorms today, mainly east of the Continental Divide. With dewpoint temperatures as high as the low 60s F southeast of the cool front, there will be the potential for heavy rainfall. A Low flood threat has been issued, primarily for rainfall in the 1-hr to 3-hr duration. Isolated flash flooding, street and field flooding will be possible. In addition, severe weather in the form of large hail, damaging straight line winds and isolated tornadoes will also be possible especially with the earlier round of thunderstorms.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

Northeast Plains, Palmer Ridge, Southeast Plains

Scattered to widespread showers and thunderstorms beginning this afternoon, and continuing into the early overnight hours. Max 1-hour rain up to 1.7 inches, with max 3-hr rain up to 3.0 inches. Isolated flash flooding, street and field flooding will be possible. Large hail, damaging winds and isolated tornado is also possible with the earlier round of thunderstorms. A Low flood threat has been issued for parts of the region.

Primetime: 1PM through 3AM

Front Range, Urban Corridor, Southeast Mountains, Raton Ridge:

Isolated to scattered showers and thunderstorms this afternoon and early evening. Max 1-hour rainfall up to 0.8 inches. Large hail and damaging winds will be possible with the strongest storms. Flooding is NOT expected today, although street ponding will be possible in urban areas.

Primetime: 12PM through 7PM

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

Warmer and drier with isolated to widely scattered showers and perhaps a thunderstorm especially in the San Juan Mountains and Central Mountains. Max 1-hour rainfall up to 0.3 inches. Flooding is NOT expected today.

Issue Date: Friday, May 21st, 2021
Issue Time: 9:45AM MDT

— Flooding is NOT expected today

The upper Low (shown below) will continue to dig in-place today just to our west. A jet on the east side of the Low has is producing southeasterly and southerly winds, which has pulled a nice plume of moisture into western Colorado and eastern Utah. Not surprisingly, PW at Grand Junction has rebounded quite a bit to 0.63 inches with this band of moisture, which is around the 90^th percentile for this time of year. The combination of the two, along with some weak mid-level energy, is helping to produce widespread showers and snow (~9-10K feet) across the western portion of the state this morning. Expecting this precipitation to pick up intensity (north) and coverage throughout the day as mid-level energy continues to move through the flow into the moisture-rich environment. However, a fairly dry surface layer, mild lapse rates, drier spring soils and a mixed precipitation at the highest elevations will limit the heavy rainfall and runoff threat. This should be a nice soaking through this evening with the highest totals by morning expected over the south and southwest facing slopes.

Over the eastern portion of the state, another dryline is forecast set up over the plains near Highway 59 with lee cyclone development. This means storms that develop on the west side of this line (eastern mountain and ridges) are expected to produce lower rainfall totals with plenty of virga/wind. Mid-level lift moving through the flow should help pop storms along the dryline this afternoon in an environment ripe for severe thunderstorms. The main threat from storms will be large hail, strong outflow winds and perhaps a tornado before a line of convection develops. Although there will be plenty of moisture in the area for high rain rates in the storm cores, fast storm motion with the jet overhead, should limit the flood threat. Thus, flooding is NOT forecast this Friday.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

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

Expect storms to become more widespread across these regions by early afternoon as instability and increased lift develop over the area. South, ongoing rainfall and cloud cover will limit the amount of instability that can build, so only mild rainfall rates are forecast. Over the higher elevations, a snow/rain mix should also help limit the flood threat. Isolated precipitation totals up to 2.50 inches will be possible by morning with max 1-hour rain rates up to 0.40 inches.

Further north, some embedded convection is likely this afternoon with heating and mid-level energy moving through the flow. Max 1-hour rain rates up to 0.40 inches will be possible with snow for the highest elevations. Isolated storm totals up to 1 inch will be possible over the western Central Mountains/Grand Valley higher terrains by morning. Flooding is NOT forecast.

Primetime: Ongoing to Midnight  

Palmer Ridge, Southeast Plains & Northeast Plains:

Severe weather returns to the plains with brisk southerly winds forecast throughout the day with the tightening pressure gradient. Expect storms to develop along a dryline this afternoon and form a line of convection by this evening. Although local heavy rainfall is possible with the storm cores, storm motion should be quick enough to limit the flood threat. Max 1-hour rain rates up to 1.50 inches (north) and 1.25 inches (south) will be possible. Large hail, strong outflow winds and a couple tornadoes are also possible with the severe storms that develop. Flooding is NOT forecast.

Primetime: 4PM to Midnight

Raton Ridge, Front Range, San Luis Valley, Southeast Mountains, & Urban Corridor:

Another warm day is ahead with breezy conditions. Isolated storms may develop over the Front Range and northern Southeast Mountains favoring areas along the Continental Divide. With surface moisture decreasing and strong upper-level steering flow, storms that are able to pop will have limited rainfall potential. Isolated totals up to 0.20 inches (north) will be possible with lower totals further south. As storms move off the high terrains, high cloud bases mean the main threat will be wind with the virga producing storms. Flooding is NOT forecast.

Primetime: 12:30PM to 8PM

 

Issue Date: Thursday, May 20th, 2021
Issue Time: 8:50AM MDT

— Flooding is NOT expected today

The drying trend continues across the state, which can be seen by the clear skies this morning. Outside of some fair weather cumulus over the mountains and spotty fog over the adjacent plains, it’s a warm and sunny start to the day. Today, CO will become sandwiched between a departing system (orange “X”) and incoming system from the west. Weak westerly flow will begin to turn more southwesterly throughout today and diffluence flow aloft will develop. This flow is forecast to send a weak shortwave across the state, which will mix with some residual moisture over the northeast corner of the state and eastern plains (associated with the departing disturbance). Scattered storms look to favor the Front Range and Palmer/Raton Ridges for develop by early afternoon. With low level moisture decreasing at the surface over the adjacent plains, storms are forecast be high-based and produce some brief outflow winds with only light rainfall. As storms move into the eastern plains, a couple low-end severe storms are possible along a dryline that sets up. The main threats from the severe storms would be hail, wind and brief heavy rainfall. Flooding is NOT forecast today.

Back over western Colorado, large dew point depressions mean the main threat from the isolated mountain storms today will be strong winds with plenty of virga. However, early tomorrow morning, the jet starts to poke into the southwest corner of the state from the approaching system, and this will draw some moisture northward from the Pacific/Gulf of California. These two features are forecast to produce some early morning showers over the San Juan and Central Mountains, which are expected to continue and expand in coverage tomorrow. Finally, some measurable rainfall in the forecast.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. If there is a threat, hover over the threat areas for more details, and click on burn areas to learn more about them. For Zone-Specific forecasts, scroll below the threat map.

Zone-Specific Forecasts:

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

Warm day ahead with highs in the 80Fs across the lower elevations and mid-60Fs in the mountain valleys. Storms will favor development over the Front Range and adjacent ridges this afternoon where there is some residual moisture to work with. High-based showers mean only light rainfall is forecast over the mountains and adjacent plains with 1-hour rain rates up to 0.25 inches possible. As the storms move into the adjacent plains along and near the dryline, slightly heavier rainfall is forecast – if they can break the cap. Max 1-hour rain rates up to 1 inch (north) and 0.75 inches (south) will be possible. Flooding is NOT forecast.

Primetime: 1PM to 8PM

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

Warm day ahead with plenty of sunshine and afternoon cloud cover over the mountains. Highs over the lower elevations will reach from the mid 70Fs to mid 80Fs with 70Fs for the higher elevation valleys. Outside of some sprinkles, rainfall is not forecast this afternoon. Additionally, it’s going to get windy with a tightening surface gradient. Red Flag Warnings and Wind Advisories have been issued. Showers are forecast begin late tonight/early tomorrow morning over the San Juan and Central Mountains. Rain rates will generally be between 0.05 and 0.10 inches per hour.

Primetime: 2AM to ongoing