Issue Date: Thursday, June 15th, 2017
Issue Time: 2:50PM MDT
Valid Dates: 6/16 – 6/30

This afternoon’s North Pacific Ocean water vapor imagery, below, continues to show a zonally oriented, strong jet stream extending nearly to the North American coastline. While there has been a lot of variability in weather across the northwest U.S., our weather in Colorado has been quieter, in large part due to the substantial amount of dry air that has been entrained from the southwest. This large reservoir of dry air is easily seen west of the Gulf of California and will continue to affect our state over the next 7-10 day period. However, areas west of the Continental Divide will be disproportionately impacted, because eastern areas will see episodes of moister air drawn in from the Gulf of Mexico.

While we have identified two precipitation events for this 15-day Flood Threat Outlook, it is becoming more likely that hot and dry weather will continue to dominate headlines, especially for western Colorado. A look at the precipitable water (PW) plumes from the GFS ensembles highlights this story. Note at Denver, PW is expected to briefly peak just under 1 inch and then retreat back to its seasonal normal range. On the contrary, at Grand Junction, after a brief spike to 0.7 inches (though this will be dominated by mid-level moisture that will carry little odds of precipitation), values retreat to below seasonal normal level through 6/23, and even thereafter. In all, due to the combination of above normal temperatures, below normal precipitation over the past 2 weeks and little to no precipitation over the next 10-15 days, it is likely that short-term drought conditions will develop west of the Continental Divide.

Below we describe each of the two identified precipitation events in more detail.

Event #1: Saturday (6/17) and Sunday (6/18)

Isolated, short-term heavy rainfall possible but No Anticipated Flood Threat

A Pacific-origin cool front will come in from the north, being supported by a shortwave feature embedded within a seasonably strong jet stream. For Colorado, the main chance of precipitation will arise due to the low-level forcing as aloft, a weak ridge will remain the status quo. A modest amount of moisture will be available for isolated to scattered shower and storm activity both days. Using climatology as guidance, the highest chances for precipitation will be in the foothills above 6,000 feet east of the Continental Divide. Additionally, some storms may develop along the frontal feature towards the Kansas border. However, we do not expect widespread precipitation exceeding 1.0 inch, and thus, a flood threat is currently not warranted. Stay tuned to daily Flood Threat Bulletins for a more detailed assessment each morning.

Event #2: Thursday (6/22) and Saturday (6/24)

Isolated, short-term heavy rainfall but No Anticipated Flood Threat as subsidence from ridge aloft negates low-level moisture

There will not be much change in the atmospheric pattern from Event #1 to #2 as a strong jet will continue to stay north of Colorado, with a subtropical jet continuing the feed of dry air from the southwest. A ripple in the jet stream is expected to send a cool front and weak upper-level dynamics across eastern Colorado starting Thursday and Friday, with residual moisture supporting more activity on Saturday. The nature of the activity will be similar to Event #1. On Thursday and Friday, expect isolated to scattered showers and weak thunderstorms mainly in northeast Colorado as well as the higher elevations of south-central Colorado. On Saturday, expect more isolated shower and storm activity mainly in the Southeast Mountains and Southeast Plains. In all, we do not foresee widespread amounts above 1 inch. While there will likely be short-term heavy rainfall, it will be too brief and limited in coverage to warrant a flood threat.

Issue Date: Monday, June 12th, 2017
Issue Time: 3PM MDT
Valid Dates: 6/13-6/27

Colorado’s weather pattern has certainly taken a big step towards the quiet side, compared to the active period experienced towards the end of May and early June. The last 7 days, see below, have seen below normal rainfall (especially west of the Continental Divide [CD]) and slightly-to-much above normal temperatures statewide. While there have been many days with precipitation, the rainfall has been very streaky – arising from individual thunderstorm cells that do not cover much real estate.

Looking at the water vapor image, below, we expect similar conditions over the next 15 day period that is covered by this Flood Threat Outlook. Over the Pacific, an elongated and unusually strong (as measured by wind speed) jet stream is stretched out, nearly reaching the coast of North America. A strong low pressure is located over the Great Basin, but its effects on Colorado will be mainly limited to gusty winds, and elevated fire threat conditions in western areas. Precipitation will be limited to non-existent as dry air takes over the entire state by tomorrow (Tuesday, 6/13). In fact, this pool of dry air will be a staple over an area from the Gulf of California, northward through southwest Canada. This will keep western Colorado very dry over the next 15 days. In fact, it is possible that little to no measurable rain falls west of the Continental Divide. Although water supply will remain in good shape, simply from it being so early in the summer, we continue to expect the fire danger to stay elevated. With snow melt concluding for most areas over the next few days, it is highly likely that more fires will occur especially over the next week.

The next precipitation event on the horizon is currently located in the far northwest Pacific but will race into North America’s west coast by the end of this week. However, with little to no return moisture and quick movement, we expect only isolated and short-lasting heavy rainfall with this event. Thereafter, another disturbance that is currently not even shown on the water vapor image, will affect our weather. It will also likely come in from the northwest and its rapid movement will prevent organized heavy rainfall activity, though isolated downpours will certainly be possible especially across the eastern border.

Overall, the Precipitable Water (PW) forecast plumes for the GFS ensembles, shown below, capture the essence of the next 7-10 days. For Denver, note that PW is expected to recover to near seasonal normal, though there is a very large spread in guidance. However, for Grand Junction, PW will remain suppressed through nearly the entire 15-day period. Thus, any precipitation chances will be limited to eastern Colorado, and there a flood threat is not warranted at this time.

Below we describe each of the two identified precipitation events in more detail.

Event #1: Saturday (6/17) and Sunday (6/18)

Isolated, short-term heavy rainfall possible but No Anticipated Flood Threat

The disturbance responsible for Event #1 will move across Colorado, but likely stay to the north. It is likely that there will be frontal features associated with the upper-level dynamics. Thus, isolated thunderstorm activity appears likely both in the foothills and higher elevations, as well as east towards the Kansas border. However, at this time, we do not anticipate total rainfall to exceed 1 inch, and thus a flood threat is not foreseen.

Event #2: Thursday (6/22) and Friday (6/23)

No Anticipated Flood Threat as ridge to the northwest means downsloping flow

After a break of a few days that will feature a ridge reasserting dominance over Colorado, another disturbance will graze the northern part of the state by the end of the following week. There may be more moisture available with this disturbance. However, its motion will be swift and organized thunderstorm activity does not appear likely. Spotty, short-duration heavy rainfall will be possible, especially favoring foothills and higher elevations of central and southern Colorado. However, just like with Event #1, we do not expect more than 1 inch of rainfall as the disturbance moves through.

Issue Date: Thursday, June 8th, 2017
Issue Time: 9AM MDT

Abnormally moist air has been a staple in Colorado’s weather for the past 96 hours, causing numerous rounds of daily showers and storms despite otherwise unimpressive atmospheric conditions. This will abruptly end as a strong disturbance enters the west coast of North America, being aided by a powerful jet stream and very dry conditions especially in the lower levels of the atmosphere. This will cause a multi-day period of very warm weather statewide with high temperatures of 10-15F above normal. An elevated fire danger is likely from Friday (6/9) through Sunday (6/11) especially west of the Continental Divide due to the combination of low relative humidity, prolonged warm weather and gusty winds.

Despite some moisture return at the tail end of Event #1 (see precipitable water forecast for Denver, shown below), it now appears that there simply will not be enough moisture to generate heavy rainfall. Its main sensible impact will instead be a return to cooler conditions. The next disturbance on the horizon is currently over the western Pacific Ocean. It will arrive in the northern branch of the jet stream. Its impact will largely be conditioned on the low-level moisture supply. At this time, it appears that precipitable water will stay below about 0.7 inches, even in far eastern areas of the state. Thus, no flood threat is currently expected.

Finally, we are making one final mention of the state’s snowpack, as the majority of snow melt will be concluding this weekend. While there are some creeks that are at elevated levels (e.g. Cache La Poudre in the Front Range or the Gunnison River), without any substantial precipitation we expect snow-melt dominated rivers to begin to subside starting next week.

Below we describe each of the two identified precipitation events in more detail.

Event #1: Monday (6/12) and Tuesday (6/13)

Event downgraded to No Anticipated Flood Threat as low-level moisture will be absent

Expect some scattered showers and perhaps a weak thunderstorm, first in the Northern Mountains on Monday, and then spreading to eastern Colorado on Tuesday. Precipitation amounts are likely to stay below 0.5 inches.

Event #2: Friday (6/16) and Saturday (6/17)

No Anticipated Flood Threat as moisture recovery is uncertain

The elongated Pacific jet stream will relatively quickly bring another disturbance into North America. However, with more substantial drying expected after Event #1, it does not appear that there will be enough moisture available to warrant a flood threat. Nonetheless, daily showers and weak thunderstorms are expected on Friday and Saturday mainly along the foothills east of the Continental Divide, and also farther east towards the NE/KS border. At this time, it appears that up to an inch of rainfall may occur especially in eastern areas over the two day period. It is possible that this event will be upgraded so stay tuned to Monday’s Flood Threat Outlook.

Issue Date: Monday, June 5th, 2017
Issue Time: 3:00PM MDT
Valid Dates: 6/6 – 6/20

This afternoon’s water vapor imagery, shown below, continues to place an elongated and strong Pacific jet stream that stretches almost to the west coast of North America. At the far eastern end of the jet, the flow becomes more amplified as can be seen by a strong low in the Gulf of Alaska and modestly strong ridge currently centered over the Great Plains. These two features will be the main players impacting Colorado’s weather in this edition of the 15-day Flood Threat Outlook.

The first precipitation event will be provided courtesy of the return flow being brought into Colorado by the high-pressure ridge. This is well depicted by the Precipitable Water (PW) forecast for Denver from the Global Forecast System Ensembles, shown below. The long-term mean value for central Colorado is about 0.55 to 0.65 inches (rising slowly through the period). Note that the next 72 to 84 hours will have substantially above normal PW, approaching or exceeding 1.0 inch. PW is a good predictor of heavy rainfall likelihood in Colorado (not true elsewhere, especially in the eastern United States), warranting an Elevated flood threat for Event #1 for isolated flash flooding.

The PW forecast chart also shows a large drop in moisture level in anticipation of Event #2 as it approaches the state by Sunday 6/11. This will occur due to west-southwesterly flow advecting dry air from the subtropical eastern Pacific Ocean. Thus, although the disturbance accompanying Event #2 will bring strong dynamics, it will be handicapped by a lack of moisture, at least initially. Instead, a 24-48 hour period of elevated fire risk will likely result due to the combination of hot temperatures, gusty winds and low relative humidity. As the disturbance rotates across Colorado, it will begin to draw more moisture from the Gulf of Mexico and at least 1 day of heavy rainfall will be possible, warranting an Elevated flood threat.

Finally, we are closely monitoring the state’s snow pack. With a multi-day period of warm to hot weather, we expect that snow melt will slowly come to an end by the end of this week. At this time, no flood-related impacts are anticipated statewide.

Below we describe each of the two identified precipitation events in more detail.

Event #1: Tuesday (6/6) and Wednesday (6/8)

Elevated Flood Threat as isolated heavy rainfall is likely due to high moisture content

Even though a ridge of high pressure will be overhead, plenty of residual moisture will continue to fuel daily rounds of showers and thunderstorms east of the Continental Divide. The vast majority of the rainfall will be of light to moderate intensity, in the 0.25 to 0.5 inch range. However, in parts of southeastern Colorado, the combination of plenty of instability and slow storm motion will result in isolated pockets of rainfall approaching 1.5 inches per hour. Isolated flash flooding will be possible, especially in the foothills of the Southeast Mountains and Raton Ridge, where 1 inch or less of rainfall per hour is enough to cause runoff capable of flooding. It is possible that this threat will continue into Thursday, though the storm coverage looks to be to minimal to include in the threat presently.

Event #2: Monday (6/12) and Tuesday (6/13)

Elevated Flood Threat especially in eastern areas as moisture returns after a dry period; severe weather is a possibility

As mentioned earlier in the discussion, the dynamics responsible for Event #2 are expected to move overhead by Sunday, 6/11. However, moisture levels will be too low to support a flood threat (or even precipitation chances). By Monday, return moisture is expected to make it into far eastern Colorado, which supports the chance of isolated heavy rainfall exceeding 1.5 inches per hour. Severe weather, including large hail and possibly tornadoes is a possibility with this system. By Tuesday, higher moisture should find its way farther west and support another afternoon of heavy rainfall in the Front Range, Palmer Divide and areas to the north and east. Heavy rainfall approaching 1 inch per hour appears possible, and severe weather may accompany the strongest cells. Collectively, however, the disturbance should move quickly enough to limit anything beyond isolated flash flooding.