FTO 09-14-2017: Significant Pattern Change Could Mean The End Of Summer

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.


FTO 09-11-2017: Active Period Expected, Including Possible Rare Infusion Of Tropical Moisture

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.


FTO 09-07-2017: Above Normal Moisture Hangs Around, Supports Brief Elevated Flood Threat

Issue Date: Thursday, September 7, 2017
Issue Time: 12PM MDT
Valid Dates: 9/8 – 9/22

A high-amplitude jet stream is seen this morning across western North American with a large upper-level ridge over the Great Plains and a trough off the coastline. Within the trough are two embedded disturbances, or shortwaves, that will be the main features controlling Colorado’s weather over the next 7-10 days. Two precipitation events have been identified for the 15-day Outlook, corresponding to each shortwave. The first event will be mainly light to moderate precipitation as low-level moisture remains marginal. The second event, while more uncertain because the shortwave is expected to cut off from the main flow, has more potential to cause heavier rainfall due to a supply of higher moisture content available for it to draw from.

As shown in the forecasted Precipitable Water plumes, below, moisture content is expected to remain at or above seasonal normal through the majority of the next 10 day period. However, seasonal “normal” is entering a period of rapid decline. For example, at Denver, normal PW decreases by about 5% every week during September and October. Thus, above normal does not have the same impact as it did during July and August. Nonetheless, note that during Event #2, there is a potential to approach PW of 1 inch, which combined with other factors, can cause heavy rainfall east of the Continental Divide. To the west, it is looking more likely that the Southwest monsoon may be waning, at least for Colorado. Monsoon surges continue in Arizona, but moisture is not expected to make it far enough north to support heavy rainfall across the Western Slope, even over the climatologically favored San Juans.

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

Event #1: Friday (9/8) through Sunday (9/10)

No Apparent Flood Threat but increase in PM showers and weak storms likely for San Juans

A weak shortwave will approach Colorado from the west, causing a slight increase in rainfall coverage mainly across the San Juan mountains. Daily round of storms could produce up to 0.5 inches per hour of rainfall, locally, with up to 1 inch could fall over the entire period of the Event. However, flooding is not expected. Furthermore, storm coverage outside of the San Juans is expected to be rather limited.


Event #2: Wednesday (9/13) through Friday (9/15)

Brief Elevated Flood Threat as cut-off low finally moves east

Despite a considerable amount of uncertainty remaining regarding timing, it does appear that the expected cut-off low will move eastward out of the eastern Pacific early next week. The system will move slow enough to allow for moisture return across most of Colorado. An increase in afternoon and evening shower and storm activity is expected over the San Juans, Southeast Mountains, Raton Ridge and Palmer Ridge. Currently, it appears that Thursday could have a brief Elevated flood threat as the main disturbance moves overhead. With Precipitable Water values as high as 1 – 1.1 inches east of the Continental Divide, isolated heavy rainfall up to 1.7 inches per hour (east) and 1.2 inches per hour (west) could be possible. Fire scars in the Southeast Mountains will need to be watched since even 0.5 inches of rain per hour could cause excessive runoff.


FTO 09-04-2017: A More Active Period Is Expected Statewide, Starting With The Cool Front Passage

Issue Date: Monday, September 4, 2017
Issue Time: 12:25PM MDT
Valid Dates: 9/5-9/19

After a brief lull, active weather is expected to return to Colorado this week, though active weather does not necessarily translate to heavy rainfall this time of year. As shown in the water vapor image below, a split flow pattern is currently noted over the eastern North Pacific Ocean. Closer to home, a strong fall cold front is being accompanied southward by a surface high pressure ridge across the northern Great Plains. The frontal passage will result in up to a 30F drop in afternoon high temperatures from Monday to Tuesday for parts of eastern Colorado (no such relief will occur west of the Continental Divide). The aforementioned split flow pattern will be key to deciphering precipitation chances for this Labor Day edition of the 15-day Flood Threat Outlook. In particular the feature of most interest is the trough currently positioned west of the North American coast. This feature is expected to remain nearly stationary over the next 5-7 days, while ejecting disturbances both to the north and south. The implications are interesting for Colorado because a south/southwesterly flow aloft is conducive for transporting monsoon moisture and precipitation for the western slope. Meanwhile, embedded shortwaves in the northern jet stream are expected to provide at least slight increases in precipitation coverage and intensity east of the Continental Divide.

In all, we foresee three precipitation events for this Outlook, though there will be relatively little demarcation between each event. Event #1 will be rainfall activity associated with the frontal passage, expected to last through Wednesday with No Apparent Flood Threat. Event #2 will be the approach of the eastern Pacific trough that may provide isolated heavy rainfall east of the Continental Divide as well as across Southwest Colorado. Event #3 will be due to the presence of residual moisture that will provide scattered diurnal storms across the High Country, with a possible eastward extension into the Plains. In all, the forecast Precipitable Water (PW) plumes, below, show that moisture at Denver and Grand Junction is expected to stay above to well-above climatological levels for early September. In turn, this suggests above normal precipitation chances, though confidence in heavy rainfall is weaker.

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

Event #1: Tuesday (9/5) and Wednesday (9/6)

No Apparent Flood Threat as front passage leaves residual moisture in southern and southwestern areas.

Daily rounds of isolated to widely scattered showers and thunderstorms are expected in the Southeast Mountains and San Juans on Tuesday and Wednesday. Instability will be limited and moisture will remain marginal so daily rainfall amounts of up to 0.5 inches are expected. The best coverage will be in the San Juans. Elsewhere, northeast Colorado will see a tremendous cool-down on Tuesday, followed by a rapid warm-up on Wednesday. Flooding is not expected with this event.

Event #2: Thursday (9/7) through Saturday (9/9)

Low-end Elevated Flood Threat for mainly isolated heavy rainfall and flash flooding

As the trough approaches Colorado, an increase in precipitation chances is expected statewide. Precipitable water is expected to increase to 0.8 – 1.0 inch out west and up to 1.2 inches out east. At a minimum, daily upslope flow is expected to produce scattered to numerous showers and thunderstorms mainly in central and southern Colorado. However, some storms could make it out onto the eastern Plains given even slight upper-level support. Maximum rainfall rates are expected to remain right at flood threat thresholds: roughly 1 inch per hour out west and up to 1.7 inches per hour out east. Thus, an isolated flash flooding risk will be possible, with mud flows and debris slides also possible over the Foothills. The Southeast Mountains’ fire burn areas will be of particular concern given that precipitation chances will be high in this region.


Event #3: Sunday (9/10) through Tuesday (9/12)

No Apparent Flood Threat as residual moisture and trough position in question

A weak ridge is expected to re-establish following Event #2. However, residual moisture is expected to remain implying isolated to scattered shower activity in the higher terrain. At this time, it does not appear that heavy rainfall will occur and precipitation rates are expected to remain below 0.5 inches per hour out west and 0.75 inches per hour east. It is possible that this event will be upgraded given what transpires farther southwest with the monsoon.