FTB 07-04-2020: Widespread Thunderstorms Return to the State for the 4th of July

Issue Date: Saturday, July 4th, 2020
Issue Time: 9:15AM MDT

— A MODERATE flood threat has been issued for the Spring Creek and Decker burn areas

— A LOW flood threat has been issued for the Front Range, Southeast Mountains, San Juan Mountains, portions of the Central Mountains, Palmer Ridge, Raton Ridge, Southeast Plains, and southern Urban Corridor

The moisture plume has nicely settled over the state, which can be seen by the widespread cloud cover in the satellite image below. This will set the stage for more widespread thunderstorm activity statewide on this 4th of July with action kicking off a little earlier than usual. PW has increased to 0.78 inches over Grand Junction, and although the lower 3km of the atmosphere is still a little dry, the moisture present throughout the upper layers of the atmosphere should allow storms to produce some measurable rainfall over the western mountains this afternoon. High dew points are present east of the green dashed line below, and PW was measured at 0.76 inches in Denver. There will be some mixing out of this low-level moisture over the ridges, mountains, and northern Urban Corridor this afternoon, but easterly and southeasterly surface flow is expected to help keep the higher dew points intact over the eastern plains.

No real distinct shortwaves in the flow, and the earlier onset of storms over the mountains may limit the amount of instability that can build. This should lower the severe thunderstorm threat back west, although hail between 0.75 and 1 inch may still be possible under a storm or two over the southern I-25 corridor. As for the heavy rainfall threat, slow, counterclockwise motion around the High will allow storms to drop moderate to heavy rainfall today. Storms will be more pulse-like in nature over the mountains and immediate adjacent plains this afternoon, but lower rain rates over the mountains should allow more gradual, wetting rainfall. However, since it is the holiday weekend (increased outdoor recreation) and storms will sit for a while, a Low flood threat has been extended back west as storms will still be capable of producing 1-inch rain totals over an hour or two. A Moderate flood threat has been issued for the Spring Creek and Decker burn areas due to higher probability of a storm forming directly over a scar today, which could cause flash flooding of local streams, mud flows and debris slides. Dangerous cloud to ground lightning is also expected from the afternoon storms, so be sure to plan your hikes accordingly.

There will be an increasing chance for storms to become severe thunderstorms as they move into the eastern plains. Higher instability out east, once again, will allow the severe storms to produce large hail (1.5” to 2”) and damaging outflow winds (up to 60 mph) on top of heavy rainfall as dew points will be in the upper 50Fs to 60Fs. A Low flood threat has been issued for the Southeast Plains/Palmer Ridge, and 1-hour rain rates up to 1.75 inches will 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:

Southeast Mountains, Front Range, San Juan Mountains, & Central Mountains:

Storms are expected to be more widespread and kick off just before noon today. Dangerous cloud to ground lightning is expected, so plan your outdoor recreation carefully. Max 1-2 hour rain rates up to 1 inch will be possible, which could cause local flooding issues – especially if you are camped near water. There is also a chance that a storm will park itself over a burn area, which could cause mud flow and debris slides on top of high water in local streams/rivers. A Low flood threat has been issued through about 9PM with Moderate flood threats issued for the Spring Creek and Decker burn areas.

Primetime: 11:30AM to 9PM

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

As storms move off the mountains by early afternoon, they will encounter better instability along and south of the Palmer Ridge as well as over the eastern plains. A few severe storms will be possible over the eastern plains with the main threats being heavy rainfall, large hail and damaging winds. Smaller hail (up to 1 inch) will be possible along the southern half of the I-25 corridor under a couple of the storms. Slow steering winds and high moisture will allow moderate to heavy rainfall to drop over all or portions these regions, so a Low flood threat has been issued for this reason. Flood threats include arroyo flooding, small stream flooding, road flooding, flooding of low-lying areas, and field ponding. Use caution when driving at fast speeds as hydroplaning was reported in Kit Carson yesterday on I-70. Storms should come to an end just after midnight.

Primetime: 1:30PM to 2AM

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

Moisture isn’t quite as high over the Northern Mountains and Northwest Slope, but a few weak storms will still be possible along the elevated regions (mountains, Roan Plateau, etc). Expect more gusty outflow winds (45 mph range) than wetting rainfall. Some very light rainfall will also be possible over the valleys as storm move off the high terrains this afternoon. Again, more wind than wetting rain is expected. There is also the chance for some light rainfall over the southern San Luis Valley due to the proximity of the moisture plume and some weak mid-level energy rotating around the high. Totals are expected to remain under 0.10 inches. Flooding is not forecast.

FTB 07-03-2020: Heavy Rainfall Threat Returns to Eastern Colorado & the Spring Creek Burn Area

Issue Date: Friday, July 3rd, 2020
Issue Time: 9:45AM MDT

— A LOW flood threat has been issued for the Northeast Plains, Palmer Ridge, portions of the Southeast Plains, and the southern Urban Corridor

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

Growing high pressure over the state has successfully pulled the aforementioned subtropical moisture northward on its west side. This can be seen in the visible satellite imagery below (“plume”), and PW has increased to 0.55 inches over Grand Junction. Most of this moisture looks to be in the mid-levels, so storms that form over the San Juan and Central Mountains with the combination of the diurnal flow and weak mid-level energy today should produce more wind than wetting rainfall. However, scattered storms will return to the forecast favoring south and southwest facing slopes for accumulations.

Over eastern Colorado, PW will be on the rise (measured at 0.67 inches in the 12Z sounding) as southeasterly surface winds increase in intensity throughout the day with a developing lee trough. Some mixing out of the surface moisture from west to east is still expected, but it is not forecast to be quite as severe as yesterday. Additionally, moisture looks healthier throughout the different layers of the atmosphere, so more wetting rainfall is anticipated.

The best moisture will sit east of ~104W (Highway 71), so expecting storms to increase in intensity and coverage around this marker. Storms further west will likely produce some strong outflow winds this afternoon. Furthermore, the 500mb high will begin to build over New Mexico/Arizona, which will decrease the counter clockwise steering flows and allow storms to drop more rainfall over an area. Thus, a Low flood threat has been issued for a majority of eastern Colorado. Some severe thunderstorms should also be expected over the eastern plains, and these will be capable of producing large hail along with damaging outflow winds. The Low threat has also been extended back to the Palmer Ridge as a quick 0.50 inches in 30-minutes could cause some urban flooding issues. Lastly, multiple rounds of rainfall may cause some flooding issues over the Spring Creek burn area today. Totals by morning around 0.50 inches will be possible, so a Low flood threat has been issued for precaution and increased outdoor recreation around the area.

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

Afternoon and evening storms will return to the forecast for all of these zones. Best coverage of storms over the mountains will be over the southern Front Range and Southeast Mountains. Max 1-hour rain rates up to 0.45 inches will be possible with most storms producing around 0.30 inches. Most surface moisture will mix out over the immediate adjacent plains, so expect some gusty outflow winds this afternoon. Moisture hangs on a little better over the Palmer Ridge, and with increased convergence, a quick 0.50 inches in 30-min could cause some urban flooding issue. So, a Low flood threat has been issued.

Severe storms will be likely over the eastern plains again, and outflow boundaries from yesterday/today will likely produce widespread coverage of heavy rainfall producing thunderstorms. With slowing steering winds, a Low flood threat has been issued. Lastly, multiple rounds of rain over the Southeast Mountains are anticipated. This could cause issues over the Spring Creek burn area if multiple storms track overhead, so a Low flood threat has been issued. Be sure to tune into your local NWS office this afternoon for the latest on Flash Flood Warnings issued. Storms will linger a little later tonight with the increase in moisture and mid-level energy over the state.

Primetime: 12:30PM to 2AM

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

Best chance for some wetting rainfall will be over the San Juan Mountains with windier (gusts up to 45 mph), lighter rain possible over the Central Mountains. This is due to another day with large dew point depressions. Rainfall today will likely help moisten the boundary layer, which will allow rainfall to become more efficient tomorrow. Max 1-hour rain rates up to 0.25 inches will be possible over the San Juan Mountains with totals under 0.15 inches further north. Flooding is not expected today.

Primetime: 12:30PM to 1AM

 

FTB 07-02-2020: Low-Level Moisture Returns to Eastern Colorado Setting the Stage for Afternoon Thunderstorms

Issue Date: Thursday, July 2nd, 2020
Issue Time: 9:10AM MDT

— Flooding is NOT expected today

The image below shows very different air masses present east and west of the Continental Divide. Clear conditions over western Colorado indicate that a very dry air mass is still in place, which is confirmed by the Grand Junction sounding this morning that measured PW at 0.32 inches. This means outside of some scattered clouds over the mountains this afternoon, it will be too dry for any rainfall. So, expect increasing high temperatures as the ridge builds to the northwest without much relief from the heat.

Over eastern Colorado, southwest surface flow has returned low-level moisture to the area, which can be seen by the cloud cover and fog over the eastern plains. Currently, high dew points (> 50F) are to the east of the dashed, green line below. Weak southwesterly flow aloft this afternoon and a developing lee trough will help to mix out this superficial surface moisture from west to east. So, low-level moisture over the Front Range looks to be limited, but some weak, upslope storms will still be possible over the southern Southeast Mountains. The main hazard from these storms will be gusty outflow winds with only light rainfall forecast.

Over the eastern plains, a moisture boundary sets up, so some severe thunderstorms should develop along that dry line this afternoon. The best instability will quickly erode into Kansas and Nebraska, but the severe storms that initially develop will be capable of producing large hail with strong, damaging outflow winds being the main threat shortly thereafter. Due to the better moisture continuing to mix out to the east through the late afternoon/early evening (especially over the Southeast Plains), local, heavy rainfall will be limited in coverage. Therefore, there is no flood threat issued, but storms that form over the Northeast Plains border counties will still be capable of dropping up to 1.40 inches of rain in their cores – if the moisture can hang on.

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 Plains, Northeast Plains, & Palmer Ridge:

High temperatures will continue to rise a few degrees this afternoon. It will also feel relatively muggy with the high dew points in the area, which will increase the heat index. Storms are expected to develop along a moisture boundary this afternoon and move to the northeast. Some severe thunderstorms are possible. Initially, storms will be capable of producing very large hail with the wind threat increasing as moisture continues to mix out to the east. Local, heavy rainfall up to 1.40 inches will be possible over the Northeast Plains (border counties) if the surface moisture can hang on. The coverage of the heavy rainfall will be limited, so there is no flood threat issued. Further south, max 1-hour rain rates around 0.75 inches will be possible. Storms should exit the state by 7PM or so. Lingering, light showers may be possible over the Southeast Plains tonight.

Primetime: 1PM to 7PM

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

Since the moisture will be mixing out to the east today, only some weak storms are forecast for these mountains’ zones with the diurnal flow pattern. Moisture will be a little better further south and with some disturbances moving through the flow, some light rainfall will be possible. Max 1-hour rain rates up to 0.20 inches will be possible over the southern Southeast Mountains, but the main threat from storms will be gusty outflow winds. Totals will decrease over the Front Range and San Juan Mountains with max totals under 0.10 inches. Virga will cause some windy conditions as storms create outflow boundaries.

Primetime: 1PM to 8:30PM

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

A Red Flag Warning remains in place for portions of the Northwest Slope today. Southwest winds are expected in the 15 to 20 mph range with gusts up to 30 mph. With relative humidity in the teens, elevated fire danger is expected. High temperatures will also increase a couple of degrees this afternoon when compared to yesterday. With little to no cloud cover expected, be sure to stay hydrated with any outdoor activities.

 

FTB 07-01-2020: Dry Day Forecast with High Temperatures on the Rebound

Issue Date: Wednesday, July 1st, 2020
Issue Time: 9AM MDT

— Flooding is NOT expected today

Very dry air has worked it way into the state, which can be seen by the cloud free conditions in the visible satellite imagery below. PW values have dropped to 0.58 inches over Denver and 0.31 inches over Grand Junction with both sounds showing very dry mid-levels. Therefore, rainfall is not forecast this afternoon, but still expecting an increase in cloud cover over the central and eastern mountains this evening. Overall, it should be a gorgeous day with high temperatures starting their rebound as the ridge builds in from the east. Additionally, there is less fire danger today with the jet to our north; however, stronger surface winds are still expected over the Northwest Slope and eastern Colorado. Tune into your local NWS office for the latest on Red Flag Warnings/critical fire weather.

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:

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

Highs today over the lower elevations should reach the upper 80Fs over the valleys with 70Fs for the slightly higher elevations and mountain valleys. So, the cool down was short-lived. A Red Flag Warning has been issued for portions of the Northwest Slope that will still be slightly under the influence of the trough to our north. Southwest surface winds between 10 and 20 mph are forecast with gusts up to 30 mph possible. Use extreme caution with anything that could cause a spark as fuels remain very dry. Rainfall is not forecast.

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

High afternoon temperatures will also be on the uptick in these regions with the ridge building back over the state. Expect afternoon highs in the upper 80Fs and low 90Fs across the I-25 corridor with slightly higher temperatures for the eastern plains. In the mountains, 70Fs are forecast. Subsidence and dry air will keep rainfall chances away, but increasing cloud cover over the Front Range is likely this evening. A developing surface low over the Denver Metro area will increase surface winds by early this afternoon, so spotty, critical fire weather conditions are likely over the Palmer Ridge, Urban Corridor, and Northeast Plains. Flooding is not forecast.