Issue Date: Monday, July 20th, 2020
Issue Time: 9:20AM MDT

— Flooding is NOT expected today

High dew points again to start the morning over eastern Colorado, and much drier conditions back to the west. Denver’s morning sounding showed PW decreasing (0.67 inches) with very shallow surface moisture again, so expecting afternoon storms to be high-based with gusty winds possible over the adjacent eastern plains and mountains today. Northerly surface winds along the northern, adjacent plains will keep the precipitation chances higher south of the Denver Metro area. However, the more northerly steering flow aloft could track a storm across the border that develops along the Cheyenne Ridge. Again, the main threat will be strong outflow winds.

Best surface moisture will be east of the green line below on the eastern and northern side of a developing lee cyclone. So, expect additional, stronger storms over the far eastern plains to develop along a line of convergence associated with the surface low. Lack of shear this afternoon will keep the chances for severe thunderstorms lower than yesterday, but very strong outflow winds are still forecast. While rain rates are expected to intensify in the more moisture rich environment, storm cores should be smaller and rain rates should remain just under flood threat criteria. Therefore, no flood threat has been issued. Overnight, an MCS looks to develop near the NE/CO border, but the heavy rainfall looks to remain just east of the state. At this time, it does not look like this feature will build back to the west. If later runs show an increasing flood threat, an afternoon update will be issued.

Over western Colorado, recycled moisture will produce another round of weak storms in similar fashion to yesterday. However, PW values have dropped off a bit, so storms are forecast to produce less rainfall. Large dew point depressions, as indicated by the Grand Junction sounding this morning, will cause the main threat from storms to be gusty outflow winds.

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:

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

Over the eastern mountains, storms should start to form by early afternoon favoring the southern Front Range, northern Southeast Mountains, and Palmer Ridge for development. Coverage of storms will be less today, and max 1-hour rain rates will drop to 0.25 inches. There’s an isolate chance for a max 1-hour rain rate of 0.50 inches over the adjacent plains along the Urban Corridor. Flooding is not expected.

Additional storms will form over the far southeast plains. Max 1-hour rain rates up to 1.40 inches will be possible, but small storm cores should limit the heavy rainfall threat. Thus, no flood threat has been issued. The main threat from storms will be small hail and very strong outflow winds with the larger dew point depression today (up to 60 mph). An MCS looks to form near the CO/NE border, but heaviest rainfall looks to remain east of the state. If this feature looks like it will build back to the west by this afternoon, an update will be issued.

Primetime: 1PM to 11PM

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

Outside of some scattered convection over the San Juan Mountains and Central Mountains, it should remain dry this afternoon. Storms will favor areas along and near the Continental Divide and be capable of producing some strong outflow winds (40 to 50 mph). Max 1-hour rain rates up to 0.20 inches will be possible with morning totals up to 0.40 inches possible over the southern San Juan Mountains from a couple rounds of rain. Flooding is not forecast.

Primetime: 1PM to 8PM

Issue Date: Sunday, July 19th, 2020
Issue Time: 9:35AM MDT

— A MODERATE flood threat has also been issued for the Northeast Plains
— A LOW flood threat has also been issued for Palmer Ridge, Raton Ridge, Southeast Plains, southern Front Range, southern Urban Corridor and the Spring Creek burn area

It is quite moist over the eastern half of Colorado behind the frontal passage that occurred last night. This can be seen by the stratus deck in the visible satellite imagery below. For the most part, dew point observations are greater than 55F east of the green dashed line (low 50Fs for the elevated ridges), and PW was measured at 0.89 inches in Denver this morning. While some of this moisture will mix out to the east under weak westerly/northwesterly flow aloft, plenty will remain for widespread, post-frontal rain this afternoon. As storms move off the eastern mountains and into the higher moisture, slow steering winds will increase the threat for heavy rainfall. Furthermore, with the cloud deck already starting to break up, higher instability will cause the coverage of thunderstorms to increase as they track east.

A few severe thunderstorms can be expected this afternoon, especially over the Northeast Plains. Therefore, a Moderate flood threat has been issued for maximum instability, moisture, and lift over the region, and a Low flood threat has been issued for the eastern plains and elevated ridges. The Low flood threat was extended back to the west to include the Spring Creek burn area.

Back to the west, moisture will continue to decrease as indicated by the PW value at Grand Junction dropping to 0.73 inches. Cloud cover over the area (seen below) indicates that there is still plenty of residual moisture in the region for another round of afternoon and evening convection over the mountains. However, lighter rain rates are anticipated this afternoon and the main threat from thunderstorms will be strong outflow winds. Storms will again favor the central and southern San Juan Mountains and better coverage over the Central Mountains should be near and along the Continental Divide.

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:

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

Widespread showers and thunderstorms are expected to pop over the eastern mountains by around noon and push in to the Urban Corridors shortly after. Storm coverage will be best south of Boulder County and increase into the Southeast Mountains. There’s high confidence in widespread storms over the Spring Creek burn area today, and max 1-hour rain rates up to 0.50 inches are possible. So, if a storm forms over the burn area, flash flooding, mud flows, and debris slides will be possible. The always active Palmer Ridge and eastern Raton Ridge could see rain totals just over 1 inch/hour, which could cause urban flooding issues such as road flooding and sharp rises in local streams. A Low flood threat has been issued for these two forecast regions.

As storms move east, they are expected to expand in coverage and intensity. Max 1-hour rain rates up to 1.75 inch/hour will be possible over the eastern plains, so a Low flood threat has been issued. Best instability, lift, and moisture should reside over the Northeast Plains, and severe thunderstorms that form in the area will be capable of producing large hail (2-inch diameter) and damaging winds, and very heavy rainfall. Max 1-hour rain rates up to 1.9 inches and morning totals just over 2 inches will be possible. Storms should exit out of the state just before midnight.

Primetime: 11:30AM to 11PM

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

Residual moisture will allow for another round of afternoon convection to form over the mountains and ridges. Coverage will be more spotty north of the San Juans, and the best chance for storms over the Central Mountains will be along and near the Continental Divide. Max 1-hour rain rates up to 0.25 inch (north) and 0.40 inches (south) will be possible. The main threat from storms today will be strong outflow winds in the 40 to 50 mph range and lightning. The interior San Luis Valley is expected to stay dry today, but had some nice rainfall accumulation yesterday. High temperatures should be in the 90Fs across the lower elevations with high-70Fs for the mountains.

Primetime: Noon to 9PM

Issue Date: Saturday, July 18th, 2020
Issue Time: 9:30AM MDT

— A LOW flood threat has also been issued for the Spring Creek burn area

The 500mb High has start its migration east, which has shifted the axis of moisture and mid-level energy with it (orange dashed line). PW values over Grand Junction were still measured at 0.80 inches with slightly higher values south. As the High shifts and flow becomes more northwesterly with the passing trough to our north, there should be a downtick in moisture from northwest to southeast across western Colorado. There will still be plenty of moisture across the southern border, so widespread storms are still anticipated to fire over the southern high terrains this afternoon and favor the San Juan Mountains for development. With PW values decreasing over the Central and Northern Mountains, rain rates and coverage of storms should decrease as well.

There has been a good amount of rain that has fallen over the southern San Juan Mountains the last three days, but gages in the area are not indicating large increases in runoff or high flows. So, with better mid-level energy to the east, decreasing rain rates, and small rainfall cores, no flood threat will be issued. As for the 416 burn area, rain rates in the area are forecast to remain under our flood threat threshold of 0.75 inches/hour and coverage of storms near the area are expected to decrease. So, no flood threat will be issued for the area today. Of course, if a storm forms directly over a burn area, there is always a chance of flash flooding, so beware and follow NWS Grand Junction for the latest on warnings.

Over eastern Colorado, a dry surface layer will cause storms to produce more wind than rain as they move off the high terrains. A little better moisture will be south, associated with the shifting plume, so best coverage of storms is expected over the Southern Front Range, Southeast Mountains, and western Raton Ridge. With the plume and associated mid-level energy moving east, max 1-hour rain rates around 0.50 inches will be possible near the Spring Creek burn area. Thus, a Low flood threat has been issued. A cold front will cross the northern border by early this evening, which may spark a few thunderstorms over the far northeast corner of the state, but flooding is not anticipated from the storms.

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, Grand Valley, Southwest Slope, Northern Mountains, Southeast Mountains, Front Range, Raton Ridge, & San Luis Valley:

Widespread afternoon showers and thunderstorms are anticipated again this afternoon for the San Juan Mountains, southern Front Range and Southeast Mountains, and the highest totals should be near the southern border. With rain rates up to 0.5 inches possible (east and south), a Low flood threat has been issued for the Spring Creek burn area. Storms could sit with the weak flow aloft, so slightly higher totals could cause flash flooding of local stream, mud flows, or debris slides. Back over the San Juan Mountains, isolated, max 1-hour rain rates up to 0.60 inches will be possible. Further north over the Central Mountains and higher terrains, max 1-hour rain rates under stronger storms will be 0.30 inches, and some gusty outflow winds will be possible.

Primetime: 11:30AM to 11PM

Northwest Slope, Southeast Plains, Palmer Ridge, Northeast Plains, & Urban Corridor:

These regions should stay mostly dry today with high temperatures similar to yesterday. As storms move off the mountains/Cheyenne Ridge, expect more gusty wind than rainfall with totals up to 0.15 inches possible. A cold front will move through the eastern plains tonight, and there is a chance that it may spark an isolated thunderstorm or two that could track over the northeast corner of the state. Max 1-hour rain rates up to 1 inch with be possible along with small hail and gusty winds if storms track into the area. There may also be a chance for some light, overnight rainfall over the Southeast Plains as mid-level energy mixes with the increased surface moisture from the frontal passage. Flooding is not forecast.

Primetime: 2PM to Midnight

Issue Date: Friday, July 17th, 2020
Issue Time: 9:15AM MDT

— A MODERATE flood threat has been issued for the 416 burn area
— A LOW flood threat has also been issued for the San Juan Mountains, Central Mountains, and portions of the Front Range, Northern Mountains, Southwest Slope, and Grand Valley. This includes the Decker and Lake Christine burn areas.

It’s going to be a rinse and repeat pattern for today with only a couple small changes to the details. The 500mb high is still over Texas, which is continuing the subtropical moisture stream from the south. This, along with some mid-level energy moving through the flow, will return widespread showers and thunderstorms to the mountains this afternoon and evening. One change to the forecast will be that the High starts to migrate to the east and a weak shortwave to our northwest will move overhead. This will change flow aloft to more westerly over the state, which will cause some drying over the adjacent eastern plains and northern border. So as storms move off the eastern mountains, more gusty winds are expected and rainfall totals should decrease when compared to yesterday.

PW in the Grand Junction sounding this morning was measured at 0.88 inches with slightly higher values over the southwest corner. This PW value is in the ~80th percentile for this time of year, which is plenty high for meaningful rainfall accumulation. With clearing already starting to happen over the San Juan Mountains, expecting higher CAPE values to build again this afternoon, which will lead to widespread thunderstorm coverage. Storm motion will also slow down a bit when compared to yesterday, and rain efficiency should increase with the boundary layer a little moister. Thus, a Low flood threat has been issued again for the San Juan Mountains and will include the Central Mountains along with high terrains near the Continental Divide.

The 416 burn area yesterday received between 0.10 and 0.25 inches. With thunderstorms likely again this afternoon, and increasing rain rates expected, a Moderate flood threat has been issued for the burn area. The Low flood threat includes the Lake Christine and Decker burn areas. While thunderstorm coverage will decrease further north, rain rates will likely be high enough to meet flood threat criteria for those burn areas. If a storm tracks overhead this afternoon flash flooding of local streams, mud flows, and debris slides may be possible.

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, Grand Valley, Southwest Slope, Northern Mountains, Southeast Mountains, Front Range, Northwest Slope, & San Luis Valley:

Widespread afternoon showers and thunderstorms are anticipated again this afternoon. Best coverage and highest rain rates will be over the San Juan Mountains. A Low flood threat has been issued with a Moderate threat for the 416 burn area. Max 1-hour rain rates up to 0.75 inches will be possible with totals up to 1.1 inches by morning. Some rainfall is also forecast for the San Luis Valley with totals along the edges and southern end of the valley up to 0.20 inches. Similar to yesterday, expect storms to linger overnight with the extra mid-level energy in the area.

Further north, coverage will decrease, but max 1-hour rain rates up to 0.5 inches will still be possible. This could cause flooding issues if storms track over the Decker or Lake Christine burn area, so they have been included in the Low flood threat. Higher rainfall amounts north will occur along and near the Continental Divide. Over the Southeast Mountains, Northern Mountains, and northern Front Range max 1-hour rain rates up to 0.30 inches will be possible.

Primetime: 12:30PM to 2AM

Southeast Plains, Palmer Ridge, Raton Ridge, Northeast Plains, & Urban Corridor:

It’s going to be warm today with temperatures increasing a few degrees. There is a decreased chance in rainfall compared to yesterday. A few storms may survive as they move off the eastern mountains over the Palmer and Raton Ridge. However, max 1-hour rain rates should decrease to 0.50 inches, so flooding is not expected. Additional storms may fire along the southern border along a west to east boundary, but the main threat from those storms today will be gusty outflow winds with a drier surface layer. Overall, it should be a pleasant day.

Primetime: 1PM to Midnight