FTO 09-29-2016: Final FTO of the Season, 2 Events

Issue Date: 9/29/2016
Issue Time: 2:50 PM

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Well, the day is finally upon us to issue the final Flood Threat Outlook of the 2016 season. As always, it has been a pleasure to serve you and to provide these valuable products. Before jumping into the discussion of this FTO period, I wanted to take a look farther down the road and discuss how things look in the long-term. The image below is the Drought Monitor for the state of Colorado, released this morning (09-29-2016).

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While much of the state remains without drought, a large swath of abnormally dry conditions has taken hold across portions of Colorado, particularly for northern areas. In additional, moderate drought conditions are depicted along the Front Range/Urban Corridor, and across the Palmer Ridge region. A drought, while never a good thing, is not as alarming this time of year as compared to one developing in the spring time. This is due to the fact that, climatologically speaking, precipitation drops off sharply between August and September, and a bit more into October. If this drought were creeping up during the spring when Colorado typically sees its highest rain and snow, the cause for concern would be greater. This situation is something to keep an eye on moving forward, and will be important to monitor going into next spring, when its impacts could become more profound. Now, let’s jump back into the present time period and discuss this FTO period.

There are two events in the upcoming period, both occurring within the first 6 days. Event #1 is the continuation of an event already underway, associated with the weakening upper-level low drifting northward across the Great Basin. This upper-level low has brought a subtropical surge to western Colorado, highlighted by the green arrow in the water vapor image below. Eventually, the low will wash out in the mean flow, with the lingering moisture providing fuel for a few more days of lingering showers across the High Country and Western Slope. No flood threat is apparent with this event, as rain rates are expected to remain well below flash flood thresholds.

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Event #2 is depicted in the water vapor image as an upper-level trough currently positioned off the West Coast. This trough, and developing closed low, will shift into the Great Basin on Monday, bringing strong southwesterly winds aloft to the region. Concurrently, a surface low will develop, with an attendant cold front sweeping across the area. Along the front, a few storms may become strong over the High Country and Western Slope, owing to sufficient shear and instability. These storms alone are the cause for any flooding concerns, and this is the reason behind the gradient in the threat timeline at the top of the page. Any flood threat that may develop will be low-end, but is worth mentioning in this FTO, as there will not be an FTB produced due to it being out of season. Behind the front, on Tuesday and Wednesday, cooler air from the northwest will invade the state, dropping snow levels to around 6,000-7,000 feet across the Northwest Slope, Northern Mountains, Central Mountains, and Grand Valley regions. A potentially significant early season snow event may unfold across the higher terrain, so be sure and stay tuned to your local National Weather Service offices for more information.

After Event #2, high pressure is expected to build across the region for an extended period of time; at least through the end of this FTO period. This will lead to dry and pleasant conditions for the state by October standards. For more information on events #1 and #2, including precipitation maps, please jump below to the event-specific discussions.

Event #1: Friday (09-30-2016) through Sunday (10-02-2016)

No Apparent Threat as the Upper-Low Weakens/Washes Out

The weakening upper-level low will continue to drift northward across the Great Basin, eventually washing out into the mean upper-level flow. The subtropical moisture surge that it has brought to western Colorado, however, will continue, lingering across the region. This will provide the fuel for a couple more days of showers, and perhaps an isolated thunderstorm or two. Due to lacking upper-level support and instability, rain rates are expected to stay below flash flood thresholds.

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Event #2: Monday (10-03-2016) through Wednesday (10-05-2016)

Elevated Flood Threat for Monday, Followed by Snowfall

The upper-level trough, and developing closed low, will shift into the Great Basin on Monday. The dynamics associated with this trough will lead to the development of a surface low and attendant cold front. The cold front will sweep across the area on Monday, with showers/storms expected ahead of, and along, the front. A few storms may become strong over the High Country and Western Slope, owing to sufficient shear and instability. These stronger storms are the cause for any flooding concerns, and this is the reason behind the gradient in the threat timeline at the top of the page. On Tuesday/Wednesday, behind the front, cold air from the northwest will invade the state, dropping snow levels to around 6,000-7,000 feet, potentially leading to a significant early season snow event across the higher terrain. In the precipitation map below, both the rainfall and snowfall are depicted as a liquid equivalent. Not all of what is depicted will fall as rain, and snow totals may reach over a foot in higher elevations. Stay tuned to your local National Weather Service office for more details as this event approaches.

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FTO 09-26-2016: Full-Steam Ahead Into Fall

Issue Date: 9/26/2016
Issue Time: 10:55 AM

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Colorado has settled into the first week of Fall nicely, with chilly temperatures across the state ringing in the day this morning. With the transition to Autumn, however, comes a climatological increase in the frequency of low pressure systems moving across the Continental United States. Gone is the steady summer ridge, replaced by transient upper-level troughs/ridges. Early in this period (Tuesday/Wednesday), dry conditions will prevail as the upper-level ridge will keep stable conditions in place across Colorado. By Thursday, however, a disturbance (purple circle, #1) will move out of the desert southwest and across the region, bringing subtropical moisture back into the state. Precipitation will be focused west of the Continental Divide on Thursday, spreading eastward and including the rest of the High Country (including the Front Range/Southeast Mountains) and adjacent low elevations for Friday/Saturday. By Sunday, the slow-moving disturbance will be northeast of Colorado, with the next event gathering itself over the Pacific Northwest.

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Sunday through Tuesday (10/2 – 10/4) have been included in the time frame of Event #2 on the timeline, but little precipitation is expected during those days. Drier and cooler air behind the first disturbance will keep precipitation hindered, but southwesterly flow aloft and weak support from the approaching upper-level low (purple line, #2) will allow for isolated light showers over the High Country during the early week days. The main show, so to speak, is expected mid-week (Wednesday, 10/5), as the upper-level low brings broad-scale support aloft and a surface low over southeast Colorado provides upslope flow. There is uncertainty in the amount of moisture that will be available to this activity, as the location of the surface low will play a role in the resultant moisture return. Right now, the best moisture looks like it will exist across the far eastern plains and into Kansas, so no flood threat is apparent at this time. We will monitor this event closely, and have an update on the time frame in Thursday’s FTO.

Ensemble models hint at an Event #3 following closely behind Event #2, best characterized as a reinforcing wave, or a “double-dip” from the parent upper-level low over Canada. Event #3 also looks to take a more northern track, leaving Wyoming to deal with the heavier precipitation, relatively speaking, while Colorado deals with showers and High Country snow. Stay tuned.

Event #1: Thursday (09-29-2016) through Saturday (10-01-2016)

No Apparent Threat as Showers will be the Dominant Rain Type, Isolated Thunderstorms

A disturbance, currently positioned over the Desert Southwest/California Baja, will slide across the region, bringing subtropical moisture back into Colorado. Precipitation will be focused west of the Continental Divide on Thursday, spreading eastward and including the rest of the High Country (including the Front Range/Southeast Mountains) and adjacent low elevations for Friday/Saturday. No flood threat is apparent at this time, but some locations across far western Colorado may receive a total of 1.0 inches by the end of the three day event.

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Event #2: Sunday (10-02-2016) through Thursday (10-06-2016)

No Apparent Flood Threat as Moisture Likely Stays out of State

While Event #2 is categorized as a five-day event, the main show will occur during the mid-week, likely on Wednesday, October 5th. On that day, the best combination of upper-level support, upslope flow, and low-level moisture return will bring the threat of showers/thunderstorms to eastern Colorado. The best moisture appears to be withheld across Kansas, lessening the threat of heavy rain in Colorado. No flood threat is apparent at this time as we await clarification on the magnitude of moisture return.

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Event #3: Friday (10-07-2016) through Sunday (10-09-2016)

No Apparent Flood Threat as the Double-Dip Favors Lighter Precipitation

Ensemble models hint at an Event #3 following closely behind Event #2, but at this time it appears to favor a more northern track. This would leave Colorado with mainly showers and High Country snow, and not a flood threat. A more southerly track would change those results, but it is uncertain at this time. Due to this uncertainty, and the current forecast that no locations would receive more than 0.5 inches of rainfall, no precipitation map will be drawn.

FTO 09-22-2016: Mainly Dry, Warm End To September

Issue Date: Thursday, September 22nd, 2016
Issue Time: 1:30PM MDT
Valid Dates: September 23rd trough October 7th

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It has been a warm September for Colorado, with average temperatures running up to 4 F above normal east of the Continental Divide (see map below). Accompanying the warmth has been a lack of rainfall basically statewide. Fortunately, September is not a particularly important month from the standpoint of water supply since average precipitation in valley locations begins to decrease from its summer peak. Meanwhile, higher elevation precipitation does not really begin in earnest until October as the jet stream intensifies and moves south, providing a steady stream of moist Pacific air that hits our mountain ranges and condenses.

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As the water vapor image shows, below, apart from an early event that will yield light to moderate rainfall amounts mainly over the higher terrain west of the Divide, the rest of September is likely to follow what we have seen up to now: dry and warm. This will be courtesy of a sprawling upper-level ridge that is seen on the water vapor image.

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The high-pressure feature will move eastward and intensify, the net effect being a clockwise circulation that will promote southerly warm air to impinge on Colorado. In concert with this, as the current upper-level trough moves eastward, it will transport dry (perhaps very dry) air from the subtropical eastern Pacific Ocean. This will set the stage for a multi-day period of very warm temperatures, up to 15F above normal accompanied by relatively low moisture content later next week. Some readers may have already made the connection: there is likely to be an elevated wildfire threat. The most likely areas to experience the anomalous warmth and heat will be east of the Continental Divide. However, the entire state is expected to average normal to above normal, temperature wise, over the 15-day period covered by this Flood Threat Outlook. One minor exception to this rule could be late next week (9/30-10/2) when a new upper-level trough will likely enter the west coast. There are some indications that a moderate amount of subtropical moisture could be drawn into this trough, resulting in a round of showers and maybe a few storms west of the Divide (Event #2). However, at this time, it will be surprising if this can generate widespread rainfall exceeding 0.5 inches. Thus, both Events are designated as No Apparent Flood Threat.

Overall, today’s analog anomalies centered on October 3rd provide an excellent summary of what we can expect over the next 15 days. Note the 500-mb height ridge position by that time moves into eastern North America, while a weak semi-stationary trough holds firm over the Pacific northwest. On the low-level temperature map (we are using the 925-mb level), note the very strong warm anomalies over the Great Plains, including eastern Colorado.

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Below we describe the details of the two Events.

Event #1: Friday (9/23) and Saturday (9/24)

No Apparent Flood Threat as trough moves across Colorado

The large-scale trough that is currently affecting western Colorado will move across the state on Friday and Saturday. Despite the seemingly high moisture levels, most of this moisture is being held well above the boundary layer. Nonetheless, several rounds of showers and perhaps a weak thunderstorm will race across the state on Friday, mainly limited to the higher terrain. Up to 0.75 inches of rainfall will be possible over isolated locations, but flooding is not expected as this precipitation will be spread out over several hours. Rainfall amounts will decrease further on Saturday with up to 0.5 inches possible over isolated locations of northern and eastern Colorado.

As the trough moves overhead by late Friday, it will be accompanied by a 48-hour period of much cooler weather with snow levels coming down to 9,500 feet. However, accumulating snow will be hit-or-miss and only a few inches, at most, are expected in any given location. The one exception to this will be the San Juans where perhaps up to 6 inches could fall. Overall, this event is likely to be known more for its winds, which could gust up to 70 mph over the higher terrain, than its precipitation.


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Event #2: Saturday (10/1)

No Apparent Flood Threat as another trough approaches Colorado

After Event #1, dry and very warm weather will return across Colorado (especially eastern areas). The next weather feature will be another large-scale trough (identified as Event #2 in Brad’s previous post) that is forecast to enter the west coast late next week. At this time, it does not appear that this trough will have enough moisture to deliver much rainfall across the state. Instead, it could be another wind event much like Event #1. The only caveat is the potential entrainment of subtropical air courtesy of a recurving tropical storm. Many things will have to come together for this to happen, and the most probable outcome at this time appears to be a light rainfall event (less than 0.5 inches; hence no precipitation map). Worst-case scenario, this could turn into a low-end flood threat for southwest Colorado should enough subtropical moisture get involved. Stay tuned to Monday’s updated Outlook.

FTO 09-19-2016: Elevated Flood Threat to Start, Quiet to Finish

Issue Date: 9/19/2016
Issue Time: 1:20 PM

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The 2016 season is winding down as we transition from summer to fall, but Mother Nature isn’t ready to wind down the flood threats just yet. Event #1 has been given the “Elevated Threat” designation, thanks to the combination of an upper-level trough and an upper-level high situated over northern Mexico pulling some mid-level, subtropical moisture associated with Hurricane Paine (red circle) over the region. The initial shortwave (black dashed lines) will prime the atmosphere with the push of moisture, while the main trough (purple line) follows closely and provides broad-scale support for showers/thunderstorms. The best support, and heaviest rains, will remain over Utah, Wyoming, Arizona, and Idaho, but a few stronger showers/storms capable of heavy rainfall are possible over western Colorado.

East of the Continental Divide, low-level dry air and minimal upper-level support will rule the forecast, resulting in only isolated showers/thunderstorms during the afternoon and evening hours for Tuesday through Thursday. By Friday, a surface low (and associated cool frontal boundary) will move across the area, bringing a more active day to eastern Colorado, mainly the far eastern plains. A few stronger thunderstorms are possible, so this will be monitored closely in the appropriate FTB’s. The upper-level trough associated with Event #1 will then eject to the northeast of the area on Friday evening, resulting in much cooler temperatures for the end of the week and into the weekend. Snow levels on Friday will drop to around 9000 feet, and it will be hard to escape the thoughts that Winter is just around the corner.

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After Event #1, westerly flow aloft will return to the region as high pressure settles over northern Mexico and the Desert Southwest once again. This will result in a drier period through the next mid-week, with high temperatures gradually warming to seasonal marks. Ensemble forecasts are indicating a second event late next week (Sept. 28-30) as an upper-level trough pushes into the western US. Moisture return ahead of this trough doesn’t appear to be too great, so “No Apparent Threat” will be the designation at this time. We will continue to monitor this event, and will provide an update in Thursday’s FTO.

Event #1: Tuesday (09-20-2016) through Friday (09-23-2016)

Elevated Flood Threat West of the Continental Divide

The combination of an upper-level trough, upper-level high, and the remnants of Hurricane Paine will result in a multi-day wet period for western Colorado. The best support for heavy rainfall and potential flooding issues will remain in neighboring states, but there remains potential for a few stronger showers/storms across Western Colorado to produce periods of heavy rainfall. As the event wears on, cooler temperatures will drop snow levels below 10,000 feet, perhaps as low as 8,000 feet in some locations. East of the Continental Divide, isolated showers/storms will be about all Mother Nature can muster, with a conditional threat for stronger storms on Friday across the far eastern plains. The entire event will be monitored closely in the daily FTB, so please stay tuned.

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Event #2: Wednesday (09-28-2016) through Friday (09-30-2016)

No Apparent Flood Threat as Moisture Returns Appears Lackluster

Ensemble forecasts are indicating the return of an upper-level trough to the western US late next week, providing broad-scale support for showers and perhaps a few thunderstorms. The biggest question mark is just how much moisture will be available to this potential event. No precipitation map will be drawn due to the inherent uncertainty in this type of event, so stay tuned to Thursday’s FTO for an updated look at this time frame.