Issue Date: Thursday, August 19th, 2021
Issue Time: 1:30PM MDT
Valid Dates: 8/20-9/3

With the calendar progressing into late August, it is common knowledge that the monsoon season begins to quiet down. But what is less well known is how quickly this transition occurs. The charts below show average probability of precipitation in Colorado Springs (which surprisingly to many lies in an area that experiences one of the most pronounced monsoon spikes in the state), as well as Durango, which lies near Colorado’s monsoon hotspot of the San Juan Mountains. Although both locations are at a relatively low elevation of ~6,000 feet and miss out on more frequent higher-elevation summer rainfall events, both data records are superb in their length (100+ years) and quality. The thin green line shows that peak daily precipitation probabilities occur in mid-July, and reach the 60-65% range at Colorado Springs (very impressive by Colorado standards) and just above 50% at Durango. However, by early September, precipitation probabilities drop to below 40% on their eventual decline into the 25-30% range by the fall dry season. The reason why this data is relevant today is that hereafter, the probability of widespread monsoon-related rain, as is occurring today, drops off rapidly so savor it!

Looking at this afternoon’s water vapor imagery, below, we see that the overhead vigorous disturbance is actually the last remaining semblance of significant moisture over the southwest United States. A growing reservoir of dry air has overtaken the eastern Pacific Ocean and although relatively far from Colorado, it will be a tough obstacle to overcome over the foreseeable future. This leads to an Outlook where No Apparent Flood Threat is anticipated over the next 15 days. Fortunately, precipitation chances will not disappear entirely. A trough over southwest Canada is expected to merge with the briefly cut-off disturbance currently over Colorado, and provide isolated to widely scattered rain (and very high-elevation snow!) showers on Saturday and Sunday, Event #1. Thereafter, an upper-level ridge will re-emerge over the central and western United States. The westward extension of this ridge will determine rainfall coverage over Colorado. A further west ridge extent will imply increasing amounts of dry air transport from the aforementioned dry pool. At this time, as shown in the forecasted Precipitable Water (PW) plumes, below, it appears just enough moisture will return by the middle of next week for widely scattered showers and weak storms over the favored southern and central higher terrain (Event #2). For everyone else, however, a relatively dry and seasonably warm period is in store.

Before moving on to discuss the precipitation events, it is interesting to note that, while the monsoon has been relatively active since late June, the Water Year total precipitation anomalies (% of normal) are still well below normal for almost all of western Colorado. It is quite surprising to see this is true for even large parts of AZ, which have experienced significant flooding issues this summer. But alas, the monsoon is only one relatively small portion of Colorado’s entire hydrologic cycle.

The two identified precipitation events are described in more detail below.

Event #1: Saturday – Sunday (August 21 – August 22)

No Apparent Flood Threat as widely scattered showers possible for mainly northern CO

Cold air aloft will make up for a relative lack of moisture to spark showers and weak storms across mainly northern Colorado. Max 1-hour precipitation up to 0.4 inches will be possible both Saturday and Sunday. Scattered coverage will be seen on Saturday, but only isolated coverage is expected by Sunday.

Event #2: Wednesday – Sunday (August 25 – August 29)

Precipitation Chances To Increase But No Apparent Flood Threat Still At This Time

Guidance continues to show a rebound in moisture courtesy of a cool front passage by early next week, followed by a more uncertain contribution of monsoonal origin from the south. On Wednesday, light to moderate precipitation is expected over the Northeast Plains and Southeast Plains where max 1-hour rainfall up to 0.6 inches looks possible. Beginning Thursday and onwards, isolated to widely scattered shower and storm activity is looking more likely for the climatologically favored San Juan Mountains, Southeast Mountains and Central Mountains. Max 1-hour rainfall up to 0.5 inches appears possible, but rainfall coverage looks isolated to widely scattered at best. Over the course of the entire 5-day period, however, lucky high-elevation spots in the San Juans could pick up a little over 1 inch. But there is currently no expected flood threat from this event.

Issue Date: Monday, August 16th, 2021
Issue Time: 1:15PM MDT
Valid Dates: 8/17-8/31

Fortunately, the ingredients for a widespread rainfall event look to be coming together for Colorado’s Western Slope. This is excellent news for the region, which is currently experiencing the worst drought conditions in the entire state, by far. Unfortunately, there will be an Elevated flood threat with this event, in addition to the chance of severe weather. Let’s get into the details.

As shown in the water vapor image, below, Colorado remains on the eastern periphery of a weakening upper-level ridge. However, to the northwest, an incoming, initially fast-moving trough (Event #1) will set the stage for very active weather mainly for western Colorado over the next 48-72 hours. As the trough moves SE, it will tap into some sub-tropical (monsoonal) and even tropical moisture that currently exists along the Mexican west coast. Fortunately, the previous Outlook’s concern of a deep tropical moisture feed is unlikely to occur. Nonetheless, a 48 hour period of very high moisture will overspread the entire state. After some isolated showers and storms Tuesday, things quickly pick up by Wednesday as multiple rounds of moderate to heavy rainfall are expected starting in the afternoon and lasting well into the evening, and even into the overnight hours. The prime region on Wednesday will be far western areas, right along the entire Utah border. The trough will actually cross the state on Thursday afternoon and the heavy rainfall threat will move slightly east over the entire high terrain along the Continental Divide. Conditions will quickly improve late Thursday as drier air races into the state.

Severe weather looks like a strong possibility with this system, as deep wind shear will combine with at least moderate instability to support the chance of damaging winds, large hail and even tornadoes on Wednesday and Thursday across the Western Slope. There is also a chance of severe weather over the far northern parts of the Northeast Plains, though most of the action is expected to stay in NE and WY.

As shown in the forecasted Precipitable Water (PW) plumes, below, moisture quickly drops to at or slightly below seasonal normal level by Friday with isolated weak storms still possible mainly over northern Colorado even into Saturday. By next week, there is significant disagreement in guidance but currently it appears there are a few different pathways to bring rainfall back into the state (Event #2). This includes both monsoonal and non-monsoonal rainfall. Due to the uncertainty, there is currently No Apparent Flood Threat with this event.

The two identified precipitation events are described in more detail below.

Event #1: Tuesday – Saturday (August 17 – August 21)

Elevated Flood Threat On Wednesday And Thursday Across Entire Western Slope

Isolated showers and weak storms are possible over the higher terrain of the Western Slope on Tuesday, but Wednesday is when the much heavier and more widespread precipitation moves in. We expect a few rounds of showers and thunderstorms Wednesday afternoon and evening, especially over far western parts of the Western Slope. Max 1-hour rainfall up to 1.5 inches looks likely, which will support isolated flash flooding, debris slides and mud flows. On Thursday, there is some uncertainty regarding the amount of instability available, which will dictate the type of rainfall. With higher instability, max 1-hour rain rates up to 1.5 inches once again look possible. With lower instability, it will be more of the 3-6 hour rainfall up to 2 inches that could trigger debris slides and mud flows, especially with already saturated soils expected after Wednesday. Overnight moderate to possibly heavy rainfall could also occur both Wednesday and Thursday nights. Overall, the main factor precluding a High threat for this event is the fast steering flow, which will tend to suppress point rainfall totals and keep high rainfall intensity relatively isolated in space.

With strong downsloping winds, rainfall chances are not looking promising for the lower terrain east of the Continental Divide. The best chances of heavy rainfall here will be over the far northern portions of the Northeast Plains where showers and thunderstorms will be possible on Thursday. There will be a chance of severe weather also as strong wind shear and instability will co-exist. However, rainfall over 1.5 inches, which is the rough threshold for flooding, is looking unlikely at this time as storm motion will be rapid and storms will quickly move out of CO into WY/NE.

Conditions will turn much calmer by Friday as significant drying will take place, but with cold temperatures aloft and some retrogression of the upper-level trough, a few showers and weak storms could pop over the northwest quadrant of Colorado. These are unlikely to pose a flood threat. Similar conditions are expected on Saturday, as the trough of low pressure slowly weakens.

Event #2: Tuesday – Wednesday (August 24 – August 25)

Precipitation Chances To Increase; Flood Threat Highly Uncertain So No Apparent Threat At This Time

At this time, some guidance is suggesting a monsoonal moisture plume that will mainly affect western Colorado. Other guidance is showing frontal-type precipitation that will affect mainly eastern Colorado. Regardless of the eventual outcome, an increase in rainfall chances looks likely, but more details will follow in Thursday’s Outlook.

Issue Date: Thursday, August 12th, 2021
Issue Time: 11:45PM MDT
Valid Dates: 8/13-8/27

After a busy stretch of monsoonal rainfall through early August, Colorado went through a dry and warm period over the past 7 days or so. Lucky locations mainly over central and eastern Colorado picked up a few tenths of an inch of rainfall over this time, while very hot temperatures (up to 6F above normal over the Plains) and smoky conditions were the main headlines.

Fortunately, we are still in mid-August, and while average Precipitable Water (PW) has begun its initial seasonal decline from monsoon season, climatology suggests that there is still plenty of chances for rainfall before the drier fall season begins. As shown in the water vapor image, below, this does indeed appear to be the case for the first ~5 days of this Outlook. Moisture, both of monsoonal origin as well as from the Great Plains, has begun to infiltrate into Colorado, and will continue to do so over the next few days. Although the dynamical support for rainfall remains rather subtle, the ample moisture will be enough to support a 4-day period with an Elevated flood threat (Event #1). Scattered to regionally widespread showers and thunderstorms are expected over the course of this time, initially over central and eastern Colorado on Friday, then shifting towards the southwest and southern higher terrain by Saturday through Monday.

As shown in the forecasted PW plumes, below, moisture will increase even further by the middle of next week, this time statewide. The driving force behind this will be a large-scale trough, currently entering the Gulf of Alaska, that is forecasted to strengthen and move into the Pacific Northwest by late Monday and further east-southeast by Wednesday. Deep southerly flow is expected to develop in advance of the trough passage, which will support a strong moisture surge for Colorado. PW forecasts suggest values as high as 1.2 inches in Denver, which is noteworthy for this time of year. The good news is that widespread rainfall is looking more and more likely for a large part of Colorado with this trough, Event #2.

From the perspective of the flood threat, there are two major wildcards with this feature. First, as seen in the water vapor image, the eastern subtropical Pacific currently has an extreme moisture gradient, with dry air to the east of Hawaii but significant moisture closer to Mexico. If the dry air is pushed far enough east, there will be a strong atmospheric river-type tropical connection for Event #2. This could support moderate to heavy rainfall for longer durations than our typical 1 hour period, say 6-12 hour. The second wildcard is that it is unclear if there will be enough instability to support anything beyond moderate rainfall intensity. This will depend on the exact timing of the trough passage. For example, an early morning passage would constrain instability and lead to widespread moderate rainfall but not much of a flood threat. At this time, as a blend of these possibilities, we are posting an Elevated threat for Event #2. Much more certainty will exist by Monday’s updated Outlook.

After the passage of Event #2, a significant cooldown is expected across the entire state, with temperatures at least a few degrees below normal and limited precipitation chances.

Each of the two identified precipitation events are described in more detail below.

Event #1: Friday – Monday (August 13 – August 16)

Elevated Flood Threat Mainly For Both Higher Terrain And Plains

On Friday, scattered showers and thunderstorms, capable of moderate to heavy rainfall, are expected both over the higher terrain mainly along and east of the Continental Divide, as well as the adjoining foothills and plains to the east. Up to one inch per hour of rainfall intensity is expected over the higher terrain. Isolated heavy rainfall will also remain a possibility further east, especially along the KS border, where intensity up to 2 inches per hour will be possible. However, the coverage over the Plains is uncertain, but currently appears to remain on the isolated side.

By Saturday, and through Monday, most of the activity will shift over the central, southern and southwest higher terrain where scattered to possibly numerous showers and thunderstorms will be likely each afternoon and evening. Isolated rainfall intensity over 1 inch per hour looks like a good bet at this time, which supports and Elevated flood threat. However, with only modest instability available, it does not appear that the threat will be constrained to be only isolated in nature.

Event #2: Tuesday – Wednesday (August 17 – August 18)

Widespread Rainfall Anticipated, But Intensity Uncertain; Elevated Flood Threat Currently Warranted

Widespread rainfall is looking more and more likely for drought hampered parts of the Western Slope, as well as central and southern Colorado. The big uncertainty at this time is whether there will be enough instability to present a flood threat, or whether the rainfall will be more stratiform and lower intensity. At this time, it appears that at least on Tuesday, sufficient instability will exist to support 1 inch per hour intensity over the entire higher terrain. There is more uncertainty on Wednesday, but it is still early enough in August, where climatology does support heavy rainfall likelihood, hence an Elevated threat is extended through Wednesday.

Overall, up to 1.5 inches of rainfall is currently looking possible over the course of this event, likely for areas of the Central and Northern Mountains. This is excellent news for both drought, as well as suppressing any late season wildfire activity.

Issue Date: Monday, August 9th, 2021
Issue Time: 3:05PM MDT
Valid Dates: 8/10 – 8/24

It has been pretty much bone dry, hot and smoky since last week, after quite a prolonged monsoon event. More of the heat and smoke is on tap for the first part of this week before our next shot at rainfall. This will occur as the High pressure begins to build over the western US again behind the trough marked below. An elongated and strong ridge, oriented from northwest to southeast, will begin to pull subtropical moisture northward into the state by Friday and help produce some hot temperatures this week. Then another incoming shortwave (Friday into Saturday) will help to shift the ridge eastward, putting Colorado in a favorable position for a stronger moisture surge by mid-weekend.

Once again we will see dry conditions both east and west of the Continental Divide as shown by PW values remaining well-below average. Then, there’s a strong upturn in moisture to more seasonable values by Thursday night and a likely surge to above average PW values by this weekend. The mid-level energy moving through the flow in a moisture-rich environment will help to spark more numerous storms over the mountains and increase the threat for a couple of severe thunderstorms with the main threat being strong outflow winds. Steering flows through early next week look to be on the lower-end, which will aid in increasing the flood threat. Therefore, an Elevated flood threat has been issued. Keep scrolling below to read more details about the events for this FTO.

Below is the Colorado average reservoir storage (%) separated by river basin through July 2021. There has been a steady decrease in usable storage over the Gunnison, Colorado and San Juan River Basins throughout the warm season. Storage has been steady over Yampa/White River Basin the last two months with increases in average storage over the South Platte and Upper Rio Grande. Statewide, we are still at 80% of average usable storage. Below that image is the state precipitation ranks for July. As you can see over the Southwest US, it has been an active North American Monsoon (NAM) with most states receiving much above average rainfall. The NAM helped Colorado reach near average precipitation. The southwest, central and southeast mountains received above average precipitation for July, and below average precipitation was recorded over the eastern plains and portions of the Northwest Slope and Northern Mountains. We’re not quite done with the NAM yet, so looking forward to getting rainfall back in the forecast this weekend.

Event #1: Friday – Wednesday (8/13 – 8/18)

Elevated flood threat as the next monsoon moisture surge moves into the state.

Monsoon moisture should reach the state by late Thursday into Friday. Except more scattered storms first across the San Juan, Central and Front Range Mountains. Then as the ridge axis gets pushed eastward and flow becomes more southwesterly on Sunday, expect showers pick up in intensity and coverage over the San Juan, Central and Southeast Mountains. Extra mid-level energy and a weak frontal passage will help to spark more numerous storms. There perhaps could be some heavy rainfall activity over the Southeast Plains earlier in the weekend. Monday into the middle of next week, the state will be sandwiched between a trough to our northwest and strong High pressure center over the Midwest. This should keep moisture streaming across the state, and storms should become more numerous over western Colorado during this period. It’s likely that as the next trough passes to our north sometime at the end of next week, the moisture will be cut off and we’ll get another break in rainfall. Due to the slower steering flows, high moisture and extra mid-level energy for this event an Elevated flood threat has been issued. Expect a peak in heavy rainfall activity from Sunday into early next week for this event, and tune back into the FTO on Thursday for the latest.