FTO 08-11-2022: Break In Heavy Rainfall Chances This Weekend With the Flood Threat Likely Returning Next Week

Issue Date: Thursday, August 11th, 2022
Issue Time: 2:45PM MDT
Valid Dates: 8/12 – 8/26

There has been a welcome break in heavy rainfall over the last couple of days, which should continue through early this weekend before more widespread rainfall and the chance for heavy rain returns to the forecast. That also means that the state should continue its rain-day streak through this next FTO period with plenty of afternoon and evening rainfall forecast with the subtropical High migrating across the western US. For Event #1, the center of the High will move slightly west of Colorado before rebuilding back over the Great Basin next week (Event #2). This summer ridge should also help to keep hot temperatures in the forecast until a cold front drops through Colorado early next week and brings some relief from the heat starting on Tuesday. Northwesterly flow aloft is then forecast through the end of next week, which will likely allow for shortwaves to move through the state and help increase the chance for rainfall over the adjacent plains.

While there’s a nice cap and drying over eastern Colorado today, PW values should have a quick upturn behind the passage of the cold front early next week with values returning to much above normal for this time of year. A High flood threat has been issued for Tuesday with an Elevated threat issued Monday and through the end of next week. There is a chance for Wednesday’s threat to be upgraded in Monday’s FTO, but at this time, there is concern that continuous rainfall and cloud cover may inhibit instability to build, and thus limit the flood threat. Over western Colorado, PW values look to remain elevated over the next week keeping rainfall in the forecast each day with the best chance for higher accumulations central/south (right, below). Daily surface moisture and small disturbances will need to be tracked in the FTB with on and off flood low-end flood threats likely to be issued for the area. Overall, this long monsoon season looks to stay active with a peak in the flood threat anticipated early to mid-next week.

Event #1: Friday – Sunday (8/12 – 8/14)

No Apparent flood threat for scattered high terrain storms due to lack of surface moisture.

Although there is a slight increase in storm coverage anticipated over the mountains and western high terrains this weekend, storm cores should be relatively small and lack rainfall rate intensities to cause a flood response. Max 30-min/1-hour rain rates up 0.7/0.9 inches will be possible along with strong outflow winds and dangerous lightning under the more intense storms that develop. Best chance for a minor flood threat day would be on Sunday due to an increase of mid-level lift and moisture moving in from the west. This would most likely occur over the northern and central mountain areas.

Event #2: Monday – Thursday (8/15 – 8/18)

Longer duration precipitation along a front and post-frontal moisture will cause a High/Elevated flood threat to be issued next week.

The calm weather won’t last long as the High finally breaks down and allows a cold front to push through the state early next week. There is an increased likelihood that PW values will return to well above average, especially east, and some heavier rainfall may fall over the adjacent plains in a 3 to 6 hour period. The forecast becomes more complicated on Wednesday in a post-frontal upslope flow regime with ongoing convection and cloud cover potentially limiting instability, and thus the flood threat. Still, with PW well above average, some longer duration rainfall may be able to exceed the 1.75 inch/3-hour flood threat threshold. The Elevated flood threat issued on Wednesday and Thursday is for the southern mountains.

Event #3: Friday – Monday (8/19 – 8/22)

Another monsoon surge looks likely over the weekend, causing an Elevated flood threat to be issued.

It is still a bit far out for many details about this third event, but long-term guidance hints at some mid-level energy and favorable monsoon moisture returning the state for next weekend. If the northwest flow sets up overhead as anticipated, it is likely that a couple weak disturbances could move through the flow and help expand precipitation into the eastern plains. More updated and accurate details about this event will be available in Monday’s FTO.

FTO 08-08-2022: A Brief Lull, Before Heavy Rainfall Potential Returns

Issue Date: Monday, August 8th, 2022
Issue Time: 3PM MDT
Valid Dates: 8/9 – 8/23

We are now approaching 60 days of precipitation somewhere across Colorado, as this busy monsoon season has taken few breaks thus far. Tomorrow looked to be the best chance for a dry day statewide, but at least isolated storms are now looking more probable. Thus, we will likely eclipse 60 straight days with rainfall!

In this Outlook, we see a battle between some drier air (a rarity since mid-June!) and the restart of more monsoonal air being imported from the south, as shown in the water vapor image, below. With little in the way of large-scale dynamics, all eyes will be on the position of the monsoonal ridge. In the short-term, the ridge will strengthen overhead, temporarily blocking the import of fresh moisture. Thus, we expect reduced storm chances through Wednesday, though at least isolated activity is still expected over the higher terrain. But after this brief “lull”, at temporary isolated heavy rainfall will return by later this week (Event #2) warranting an Elevated Flood Threat. With weak steering flow, storms are likely to have a hard time making it off the higher terrain, thus only limited rainfall along with hot temperatures (3-5F above normal) are expected for most of eastern Colorado through this weekend.

As shown in the GEFS PW plumes, below, the battle between dry and moist air appears to favor the latter by early next week. With plenty of above normal moisture around most of western North America by that point, it appears to be the start of another prolonged busy period of heavy rainfall for our state. In addition to the usual monsoonal dynamics of southerly moisture transport, a couple of cool front disturbances could swing down from Canada, providing boundary layer support to fuel storms. There is significant model uncertainty at this time, but enough consensus exists to identify a fourth precipitation event beginning next Monday with an Elevated flood threat.

The identified precipitation events are described in more detail below.

Event #1: Tuesday – Wednesday (August 9 – August 10)

Isolated To Widely Scattered Higher Terrain Storms; No Apparent Flood Threat

Isolated to widely scattered storms are expected mainly over the southern and central higher terrain during the afternoon and evening hours. Max 30-min rainfall up to 0.4 inches looks possible on Tuesday, increasing to 0.6 inches by Wednesday. However, there is no apparent flood threat at this time.

Event #2: Thursday – Friday (August 11 – August 12)

Scattered Higher Terrain Storms With Low-end Elevated Flood Threat

Fresh monsoonal moisture will increase storm coverage and maximum rain rates by Thursday. The best chances of storms will be over the central and northern higher terrain. Max 30-min/60-min rain rates of 0.8 and 1.1 inches, respectively, could support a minor flood threat. Daily Bulletins will have a better look at these features as the depth of moisture recovery is determined.

Event #3: Saturday – Sunday (August 13 – August 14)

Isolated To Widely Scattered Mainly Higher Terrain Storms; No Apparent Flood Threat

Another period of dry air intrusion from the east should reduce storm coverage for all but the highest terrain. Isolated activity still looks possible for the entire higher terrain from the WY to NM borders. However, max 30-min rain rates of up to 0.5 inches suggest flooding is not expected at this time.

Event #4: Monday – Thursday (August 15 – August 18)

Elevated Flood Threat At Least For Higher Terrain With Some Spillover Likely Onto Plains

Things turn interesting by Monday, when a fresh pulse of monsoonal moisture looks to increase PW to above 1 inch for a prolonged duration. Additionally, one or more cool front could drop south out of Canada, further enhancing storm coverage.

Although there is significant uncertainty, it currently appears that mainly higher terrain storm on Monday will give way to more widespread storms by Tuesday and Wednesday. Any cool front presence will allow storms to move off the higher terrain and sustain a 3-6 hour duration threat of heavy rainfall. At this time, given the very long lead time, an Elevated flood threat looks appropriate. However, more details will be available in the next Outlook, which could support an increase to a High threat.

FTO 08-04-2022: High Flood Threat This Weekend Before A Break In Rainfall Activity

Issue Date: Thursday, August 4th, 2022
Issue Time: 3PM MDT
Valid Dates: 8/5 – 8/19

Below is a quick recap of precipitation from July using PRISM data. On the left is the estimated total precipitation for the month (inches), and on the right is the percentile of precipitation. Outside of portions of the western central and southern valleys, precipitation was generally above normal to much above normal statewide, and for portions of the eastern plains, July was the wettest on record. The one exception for the eastern plains is Sedgwick and Phillips Counties, which are still experiencing Extreme Drought conditions and recorded much below normal precipitation. As we’ve been monitoring in the SPM and FTB, it was a very wet month for the southern Southeast and San Juan Mountains with maximum estimated precipitation coming in at over 6 inches. A CoCoRaHS station outside of Pagosa Springs measured 5.18 inches for the month. As expected, local rivers are running higher than usual for this area with increased runoff from saturated soils, and gauges are showing well-defined spikes in streamflow associated with each day’s rainfall. Head on over to the DWR site to check them out.

Heading into this next Outlook, it will remain very active on the rainfall front through this weekend (Event #1) before a nice break in precipitation and heavy rainfall chances next week (Event #2). Driving this weekend’s precipitation event will be an incoming trough that should displace the ridge axis eastward. In turn, this and a Low off the west coast will help to pull the subtropical moisture plume and mid-level energy marked below northwards into western Colorado first, and then into eastern Colorado later this weekend. Following Event #1, drying is expected for a couple days, although residual moisture, south, will likely continue to produce diurnally driven storms over the high terrain. Guidance is hinting at another monsoon surge after that (Event #3), so we aren’t quite done with this long rainy season yet. Here are some quick stats about Colorado’s heavy rainfall threat. Since mid-June, there have been 37 threats issued, and the current FTB threat streak is at 13 days with additional threats likely continuing over the next 3 days.

A High threat has been issued for Saturday and Sunday with above average PW values forecast statewide. The drying trend early next week can be seen by the downward trend in the middle of the PW plots below. Although moisture does look to decrease, PW values are likely to remain right around average. Although some capping is expected next week, the isolated storms that are able to pop over the high terrains (most likely south), may still be able to produce a flood threat with steering flows. Be sure to tune back into the FTO on Monday as there will likely be more clarity as to how much of a break in heavy rainfall there will actually be.

Event #1: Friday – Sunday (8/5 – 8/7)

Above average moisture, increased dynamics and saturated soils will cause a High flood threat to be issued for this weekend.

This event will first begin over western Colorado and shift into eastern Colorado with the passage of a trough to the north. The flood threat on Friday should continue to be in the 30-minute to 1-hour range from storms, but some longer duration, moderate rainfall may also be possible this weekend with the passage of a front. Depending on the timing of the frontal passage, a couple of severe storms may be possible with the main threat being strong outflow winds. As for potential flood impacts, saturated soils further south, may continue to cause rises on local rivers and creeks and there is an increased threat for mud flows and debris slides over steeper terrains. Over the lower elevations and valleys, road flooding and field ponding will be possible.

Event #2: Monday – Thursday (8/8 – 8/11)

No Apparent flood as PW decreases and rising heights help to suppress precipitation chances.

Widely scattered to scattered storms will be likely over the high terrain during this period with the best coverage over central western Colorado and the southern mountains. With the High building over western Colorado, increased subsidence and capping should keep the adjacent plains and valleys dry. Max 30-minute rain rates up to 0.5 inches will be possible, so at this time, there is No Apparent flood threat.

Event #3: Friday – Monday (8/12 – 8/15)

Guidance is hinting at a return of monsoon moisture, so an Elevated flood threat has been issued with highest precipitation amounts forecast over the high terrain.

The monsoon season looks to continue with another monsoon moisture surge possible over the western Colorado and the mountains at the end of next week. Rain rates could easily reach the 1 inch in 1 hour threshold with PW anomalies likely increasing to above average values. Tune back into the FTO on Monday as the details of this next event will inevitably evolve.

FTO 08-01-2022: Moisture Abounds; Flood Threat Continues

Issue Date: Monday, August 1st, 2022
Issue Time: 3PM MDT
Valid Dates: 8/2-8/16

As mentioned in the July 25th Outlook, there were 44 straight days of rain somewhere across Colorado. Well, fast forward one week and we can safely bump that number up to 51 straight days. And fast forwarding at least 7-10 more days within this Outlook, that streak will easily continue to play out.

It has certainly been a memorable monsoon already, and it has been many years since we have seen this kind of consistency in the flow of above normal moisture into our state. As shown in the water vapor image, below, this Outlook is a status quo of the past 45 days or so. The circulation of the monsoonal ridge will continue to wobble across the western North America. But, it will be centered primarily to the south and east of Colorado. With plenty of moisture available over the eastern tropical Pacific, moisture here at home will continue to be above normal to, at times, near record level (see forecast PW plumes, below). As an interesting aside, tropical cyclone activity over the eastern Tropical Pacific is currently almost twice the normal value (compared to well below normal over the western Pacific), possibly helping explain our abundance of moisture here.

To start this Outlook, we see a relative lull in the flood threat as moisture will dip just a bit (Event #1), which combined with quicker steering flow will promote at least scattered storms but limited heavy rainfall potential. But by later this week, a monsoonal surge will re-ignite the heavy rainfall mainly over the higher terrain. An Elevated threat is warranted over a prolonged stretch (Event #2). The passage of a Pacific cool front (currently located over the Gulf of Alaska) looks to enhance the heavy rainfall ingredients so that a High flood threat looks possible though the exact timing is uncertain. With plenty of antecedent precipitation, soils will be particularly vulnerable not only to enhanced runoff rates, but also mud flows and debris slides. This provides extra confidence in the issuance of a High flood threat.

The pulse of monsoonal moisture responsible for Event #2 looks to carry further north. However, in its wake, above normal moisture should continue through mid next-week, leaving an Elevated flood threat for the higher terrain. At this time, the threat looks to shift further west towards the Utah border as more subsidence and relatively drier should curtail the threat over eastern Colorado.

Throughout this Outlook, temperature anomalies will persist in a similar fashion as the past month or so: most of western Colorado should be cooler than normal due to the presence of storms and frequent cloudiness. More variations in temperature is likely for eastern Colorado as very hot days are mixed with more seasonable days following cool front passages, and also when storms are able to make it off the higher terrain.

The identified precipitation events are described in more detail below.

Event #1: Tuesday – Wednesday (August 2 – August 3)

Scattered High Terrain Storms, But No Apparent Flood Threat

Widely scattered to scattered showers and storms are expected daily during the afternoon and evening. The best coverage will be over the western and central higher terrain. Max 30-min rainfall up to 0.5 inches is currently expected. However, outside of fire burn areas, flooding looks unlikely at this time.

Event #2: Thursday – Sunday (August 4 – August 7)

Mainly Higher Terrain Elevated/High Flood Threat; Uncertainty For Lower Elevations

An increase in storm coverage should begin on Thursday over the southern higher terrain, then spread northward by Friday. During this period, the flood threat appears to mainly reside in the 30-60 minute duration where 1.0 inch and 1.5 inch intensity is possible, respectively.

By Saturday, a frontal passage from the north is expected. Depending on the exact setup with timing, moisture availability and steering flow, a 36-48 hour period of enhanced threat will be possible over the Front Range southward through the Southeast Mountains. Heavy rainfall in the 3-6 hour duration looks possible, with max 6-hour accumulation exceeding 3 inches possible. Additionally, precipitation could extend well into the evening and overnight hours, adding to the impact. Given enough heavy rainfall coverage, along with the growing importance of wet soils, this could translate into a riverine flood threat.

The flood threat for the eastern Plains looks limited at this time.

The next Outlook should have more details on this Event.

Event #3: Monday – Thursday (August 8 – August 11)

Plenty Of Storms Expected Daily For Higher Terrain; Elevated Flood Threat

With moisture dialing back a bit, afternoon and evening storms are likely to drop in coverage but still be of at least the scattered variety. The best coverage looks to be over the western and central high terrain. Maximum intensity of 0.8 inch/30 minutes and 1.2 inch/60 minutes supports an Elevated flood threat for the risk of isolated flash flooding, debris slides and mud flows.