Issue Date: Monday, September 21, 2017
Issue Time: 1PM MDT
Valid Dates: 9/22 – 10/6

This afternoon’s water vapor image of the eastern Pacific and western North America shows a high amplitude pattern over and to the west of Colorado. A high amplitude pattern sometimes implies that disturbances will be slow to move eastward, and such is the case today. The strong trough that has now entered the western US coast will very slowly trek eastward and provide a long duration precipitation event (Event #1) for most of Colorado. In addition, a particularly moist airmass has developed over the US Central Plains. Precipitable Water (PW) values up to 2 inches will stream northward into OK, KS and NE. While we certainly do not expect these kind of values to make it into Colorado (which would be record highs even during the summer), some of this moisture will be available to draw from.

Thus, we expect a very active period over the next 5 days or so. Initially, there will be an elevated fire danger across central CO due to high winds and low relative humidity. By Saturday, colder air will slowly over take the state from the northwest. Friday and Saturday (and possibly Sunday) will feature elevated chances of severe weather across the far eastern Plains, and possibly as far west as the Palmer Ridge. By Sunday, much cooler/colder air will overtake most of the state, changing the dominant precipitation to stratiform, as opposed to convective, rain and snow showers.

In terms of the flood threat, the forecasted PW plumes at Denver and Grand Junction, below, show a very large moisture gradient developing by this weekend. For example, by late Saturday, Denver’s PW could be as high as 0.9 inches while Grand Junction is expected to remain below 0.6 inches.

In fact, just these two sites do not tell the entire story. The forecasted PW at Lamar (below) shows values as high as 1.4 inches. While there is no long-term record of PW here, data from Goodland, KS (which is almost always moister than Lamar) shows that during late September, the 90^th percentile of PW is about 1.1 inches. Thus, it is safe to say that the forecast values are well above average and likely close to record values. In addition, due to the slow movement of the trough, we expect a high potential of thunderstorm training. Although the higher flood threat remains just to the east of CO into KS and OK, an elevated flood threat will exist for southeast CO on Friday, Saturday and possibly Sunday for isolated flash flooding in southeast Colorado. Check back for our daily Flood Threat Bulletins for the most updated information.

Below we describe the identified precipitation event in more detail.

Event #1: Friday (9/22) through Wednesday (9/27)

Early Elevated Flood Threat for Southeast Plains transitioning to scattered rain and snow later in the event

A prolonged period of active weather is expected across Colorado starting on Friday. Multiple rounds of severe thunderstorms are expected in the Southeast Plains (and possibly Palmer Ridge) on Friday and Saturday, with decreasing chances by Sunday. Gusty straight-line winds and large hail (up to 2.5 inches) are expected to be the main threat. Isolated heavy rainfall will be likely in mainly far southeast Colorado on Friday and Saturday. Max 1-hour rain rates up to 2.5 inches will be possible, with max 3-hour amounts up to 4 inches. Training of storm cells could lead to a prolonged threat of heavy rain. Isolated field and road flooding appears likely, with isolated flash flooding also possible though only localized impacts are expected. Riverine flooding on the Arkansas River basin is NOT currently expected, but will be likely farther east in OK and KS.

By Sunday, cooler air will overtake most of the state leading to a sharp reduction in precipitation rates. Mountain snow levels will drop to valley floors especially north of I-70. Several inches of snow accumulation is expected, mainly north of I-70. Scattered rain and snow showers are expected to continue through Wednesday as the upper-level disturbance sticks around the state.

Issue Date: Monday, September 18, 2017
Issue Time: 3:50PM MDT
Valid Dates: 9/19-10/3

A much drier and cooler pattern arrived to Colorado toward the end of last week. As the cold front moved through the state, the streak of September heat was brought to an end. While temperatures will continue to rebound today and tomorrow, another cold front is forecasted to bring more climatologically normal temperatures Wednesday to eastern Colorado. There is one precipitation event that has been identified for this Outlook. In the water vapor imagery below, this event can be seen over the Pacific Northwest. The strong trough is forecasted to dig south, west of Colorado, and produce an unsettled weather pattern over the state beginning later this week. After this trough moves through, a quieter, more zonal pattern sets up for the remainder of next week. While models hint at another trough moving through the area at the end of this Outlook, confidence in details are not high enough to issue any threat at this time.

Below average Precipitable Water (PW) is forecasted for Denver and Grand Junction through Thursday night. As the trough digs south, winds are expected to increase in speed and become more southwesterly at the surface. The low relative humidity and warmer temperatures will create extreme fire danger from Tuesday night through Friday morning. Please avoid outdoor burning as increased wind speeds can quickly spread fires. Towards the end of the week, PW values increase over the state with the eastern portion of the state remaining above average through the weekend. The increase in moisture occurs the same time the west coast trough begins releasing shortwaves into the state. The upper level dynamics and increased moisture could bring heavy rain and severe weather to the eastern plains and snow to the high country on Saturday.

Below we describe the identified precipitation event in more detail.

Event #1: Friday (9/22) through Tuesday (9/26)

Elevated/No Apparent Threat as increased moisture pairs with upper level dynamics across Colorado

Moisture returns to southwest Colorado and the far eastern plains later this week. Increasing SW and SSW surface winds will draw in moisture as the trough digs south, west of Colorado. On Friday, the main upper level trough will begin to eject shortwaves over the state that will cause enhanced lift and unsettled weather for southwest Colorado. Saturday, a cold front associated with the west coast trough is expected to move through the state and drop temperatures enough over the mountains that a couple inches of snow may fall at elevations above 9,500 feet. Over the plains, the increased dynamics paired with higher moisture is expected to create favorable conditions for severe weather and prolonged rainfall. This main trough moves northeast through Colorado overnight on Sunday. An Elevated Flood threat has been issued for Friday and Saturday with no apparent threat on Sunday.

A second, weaker trough then develops southwest of Colorado on Monday and Tuesday. Southwest flow aloft will allow the mid-level energy to move across the state during this period. PW is expected to decrease quite a bit after Sunday, but there should be enough residual moisture for daily rounds of rainfall. At this time, there is no apparent flood threat on Monday and Tuesday.

Issue Date: Thursday, September 14, 2017
Issue Time: 12:15PM MDT
Valid Dates: 9/15-9/29

Although it is already mid-September, it has hardly felt like it statewide with month-to-date temperature anomalies up to +10F across the western parts of the state (see left panel below). In fact, even when we extend to the past 30 days, a similar pattern is seen with a slightly lower magnitude of anomalous warmth (right panel below). However, it seems that summer’s run may have ended as in this Outlook, we see evidence of an imminent pattern change to much cooler/colder conditions.

As shown in this afternoon’s water vapor image, below, the long standing ridge that dominated Colorado’s weather has finally been tempered down by an incoming large-scale disturbance. The upper-level pattern in western North America features a split flow in the jet stream: a northern “polar” branch, and a southern “subtropical” branch. There are disturbances in both branches, roughly in phase with one another that will combine to generate the first precipitation event. This will mainly be manifested as rain and snow showers across the western slope, with some embedded weak thunderstorms. Thereafter, this Event will carve out a strong upper-level trough across the Rocky Mountain states, which will provide much cooler conditions and upward motion for another precipitation event. The impulse that will support Event #2 is still far off in the western Pacific but will rapidly move eastward within the strong Pacific jet stream.

The forecasted Precipitable Water (PW) plumes from the GFS ensembles, below, show initially above normal moisture at both Grand Junction and Denver. However, PW values, along with boundary layer moisture, will remain below the necessary level to support heavy rainfall. Instead, both events will bring about the season’s first and second rounds of widespread precipitation coverage. With an influx of cold air from the northwest, both events will have snowfall. The Gore Range should receive a widespread coverage of at least 1 inch of precipitation (combined) with isolated locations receiving over 2 inches and perhaps more. The other big story is that much cooler conditions will be on the way statewide, but especially west of the Continental Divide where days with precipitation will be accompanied by temperatures up to 25F below normal. After Event #2, there will be a rebound in temperatures, though given that it is already mid-September, at this time it appears that summer-like heat may be on its way out for good.

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

Event #1: Friday (9/15) through Sunday (9/17)

No Apparent Flood Threat but multiple rounds of showers, weak storms and high elevation snowfall expected

This disturbance will provide enhanced precipitation coverage over the Western Slope on Friday and Saturday, with coverage decreasing by Sunday. The most common precipitation type will be showers with embedded weak storms possible. Up to 1.5 inches of precipitation will be possible over a 24 hour period, with the greatest chances of meeting this intensity being in the higher elevations of the Gore Range, Roan Cliffs and Uncompahgre Plateau. Snow levels could drop as low as 9,500 feet overnight Saturday, especially near the Wyoming border. Flooding is not anticipated at this time, though a low-end flood threat could emerge for longer duration precipitation. Stay tuned to daily Flood Threat Bulletins for more specific, and up-to-date information.

Event #2: Tuesday (9/19) through Saturday (9/23)

No Apparent Flood Threat but precipitation coverage to increase again

Another disturbances is expected to arrive into the soon to be developed upper-level trough early next week. This disturbance will have similar features as Event #1, though with several important differences. First, instability is expected to be limited to non-existent, implying that precipitation will fall in the form of rain and snow bands void of thunderstorms. Second, this disturbance will have much colder air to work with, implying snow levels could drop to 6,500 feet towards the Wyoming border. There will be an initial band of enhanced mid-level moisture that will support up to 1.5 inches of rain and snow on Tuesday. Wednesday and Thursday will see drying conditions with more scattered precipitation. A secondary surge of moisture could provide another round of precipitation on Friday and Saturday. In all, a widespread precipitation event is expected for the Western Slope with over 2 inches of precipitation likely for the western facing slopes of the Gore Range. East of the Continental Divide, precipitation chances looks much less favorable and it is uncertain whether there will be over 0.5 inches anywhere. This represents a big difference from the last Outlook where it appeared tropical moisture may become involved. Currently, the tropical moisture plume is expected to stay far enough east to not impact Colorado. Flooding is not expected with this event.

Issue Date: Monday, September 11, 2017
Issue Time: 1:45PM MDT
Valid Dates: 9/12-9/26

While chances of heavy rainfall in Colorado typically wane quickly during September, it is always prudent to appreciate that the state’s unique climate can support late season flooding such as the Front Range 2013 event. While we do not see anything particularly impressive for this 15-day Outlook, we do foresee an abnormally active period both in terms of rainfall coverage and intensity. As shown in the water vapor image, below, Colorado is currently under a weak upper-level ridge, sandwiched between the remnants of Hurricane Irma to the east and a cut-off (synonymous with “stubborn”) upper-level trough off the California coast. The latter feature will finally eject eastward over the next 48-96 hours, providing Colorado with nice rainfall coverage and generally limited rainfall intensity. We have labeled this feature as Event #1.

As shown in the GFS ensemble forecasted Precipitable Water (PW) plumes, below, PW will gradually decrease statewide as the disturbance supporting Event #1 approaches. Nonetheless, PW will remain above average and support up to 1.5 inches of rainfall, locally, in the climatogically favored San Juan Mountains and Southeast Mountains. Other locations will see less rainfall, but most locations should see at least a tenth of an inch over the course of this event.

The other interesting note about the forecasted PW plumes is the enhanced uncertainty by early next week.

This enhanced uncertainty is arising from a spread in guidance due to the possible interaction between a large-scale trough expected to enter the North American west coast and tropical moisture. There is reasonable confidence regarding the upper-level trough, which by itself will increase rainfall coverage and intensity. However, this afternoon’s GFS model guidance, below, shows a fetch of deep tropical moisture (from a recurving tropical cyclone) into the south/central United States. This could provide Colorado with a potential for longer duration heavy rainfall. At this time, it would be the southeast quadrant of the state that would be most vulnerable, but with plenty of uncertainty, stay tuned for this Thursday’s Outlook for more information.

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

Event #1: Tuesday (9/12) through Saturday (9/16)

No Apparent Flood Threat but increased rain coverage likely, especially over High Country

Precipitable Water values will remain above long-term average, combining with favorable upper-level dynamics to support daily rounds of moderate intensity thunderstorms and showers. The best coverage will be over the High Country (especially southern Colorado). Hourly rainfall rates up to 0.75 inches are expected over central and western Colorado with up to 1.25 inches for eastern areas. Over the course of the Event, up to 1.5 inches of rainfall could accumulate in isolated regions. At this time, we do not expect a flood threat, however, stay tuned to daily Flood Threat Bulletins as Tuesday and Wednesday could end up generating a Low flood threat if moisture levels are only slightly higher than currently anticipated.

Event #2: Tuesday (9/19) through Friday (9/22)

Elevated Flood Threat as dynamics and moisture could combine favorably

Another large-scale upper-level trough is likely to enter the North American west coast early next week. This feature, by itself, will cause an increase in rainfall coverage an intensity statewide but especially across the southern half of Colorado. Isolated heavy rainfall amounts exceeding thresholds for isolated flash flooding could occur across the San Juan, Southeast Mountains and Raton Ridge. If tropical moisture does end up becoming involved, a longer duration of heavy rainfall (preliminarily, in the 3-6 hour duration) can be expected over the southeast quadrant of the state. There is enough confidence to assign an Elevated flood threat for this event for isolated flash flooding. However, the placement of heavy rainfall is uncertain. Moreover, the possible impacts of longer-duration heavy rainfall are also currently very uncertain and not incorporated in the precipitation map below. Stay tuned to Thursday’s Outlook for a fresh look at this event.