FTO 07-17-2017: Persistent Elevated/High Flood Threat Over The Next Week

Issue Date: Monday, July 17, 2017
Issue Time: 2:10PM MDT
Valid Dates: 7/18 – 8/1

Thus far, July has seen a rather persistent weather pattern marked by a wobbling upper-level ridge anomaly centered north of Colorado. The left map below shows the composite month-to-date 500-mb height anomaly and it is seen that the bullseye of the anomaly is located on the border of Alberta and Montana. The right map below shows month-to-date precipitation anomaly, as a percentage of normal. Generally speaking, wetter than normal conditions have been favored southeast of Colorado with more of both sides of the coin seen for our state. However, a notable exception to this is in southeast Colorado where it has been very wet (up to 250% of normal). One factor that helps explain this is the above normal moisture that has been available for thunderstorm formation, especially in the past 5-7 days.

As shown in the current water vapor image, below, things have changed compared to the maps above. The upper-level ridge has flattened a bit and is now centered south of Colorado. Dry air is found far to the west across California and offshore. Meanwhile, moist air, characterized by Precipitable Water values exceeding 1 inch, continues to hold its fort across eastern, southern and southwestern part of Colorado. Over the next 15-day period covered by this Flood Threat Outlook, the key to our weather pattern will once again come down to determining how the upper-level ridge will be positioned. Current guidance suggests the ridge will first move east of the next 3-5 days, followed by a gradual retraction back to the northwest, to be in a position reminiscent of that observed during the first half of July. Importantly, the temporary shift of the ridge to the east will allow for an effective return of both moisture and upper-level dynamics into Colorado.

We have identified two organized precipitation events, which together span the first 6 days of this Outlook. The first event will be the result of elevated moisture levels that will produce isolated heavy rainfall mainly over the higher elevations. The second event is a more organized stretch of heavy rainfall potential as a monsoon moisture surge moves north/northeast into Colorado, along with a cool front expected to cross the eastern half of the state. As shown in the forecasted PW plumes for Denver and Grand Junction, both locations will experience generally above or even significantly above moisture content through early next week, which is conducive to heavy rainfall in Colorado. Below each event is described in more detail.

Event #1: Tuesday (7/18) and Wednesday (7/19)

Elevated Flood Threat for isolated higher-elevation heavy rainfall

Above average moisture will continue to maintain a presence over most of Colorado for this event with Precipitable Water values in the 0.75 to 1 inch range statewide. With clear skies in the morning, expect early shower and thunderstorm activity to form across the entire high country, with the highest intensity and coverage over southern areas. Brief heavy rainfall, up to 0.5 inches in 30 minutes will be common, and some locations will experience several rounds of such storms from the later morning through the early evening hours. By later in the afternoon, the merging of storm outflow boundaries will maintain the potential for heavier rainfall, up to 1.2 inches in 1 hour. This will be capable of causing isolated mud flows, debris slides and flash flooding especially over steeper terrain where less rain is needed to cause problems.

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Event #2: Thursday (7/20) through Monday (7/24)

Prolonged Elevated/High Flood Threat statewide with monsoon surge (west) and cool front (east) acting as forcing

As hinted by the forecasted PW plumes (see above), higher moisture is expected to arrive into Colorado from the southwest by Thursday, 7/20. PW values are expected to exceed 1.0 inch across most of the state with localized readings up to 1.2 inches (west) and 1.5 inches (east). On Thursday and Friday, scattered to numerous thunderstorm activity is expected for the western slope and Continental Divide. These storms will be capable of producing 0.8 inches of rain in 30 minutes. However, up to 2.0 inches will be possible over a 24 hour period. Mud flows and debris slides will become a bigger impact by Friday, especially over regions that experience consecutive days of heavy rainfall.

By Saturday and Sunday, attention turns east of the Divide as an expected cool front will provide a focal point for storm activity. With relatively weak wind shear expected, relatively brief heavy rainfall is expected. However, with plenty of moisture available, up to 1.8 inches in 30 minutes and 2.7 inches in 1 hour will be possible. Isolated flash flooding appears likely, especially over the sensitive newer burn scars. By later in the event, antecedent wet soils will also contribute to an increased runoff risk. It is possible that event will be upgraded to a High threat by the next Outlook. The day by day features will be described in much more detail in the Flood Threat Bulletins, so stay tuned.

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FTO 07-13-2017: Copious Moisture Supports Early High Flood Threat

Issue Date: Thursday, July 13, 2017
Issue Time: 3PM MDT
Valid Dates: 7/14-7/28

For the Flood Threat Outlook, one of our staple images is the North Pacific water vapor map. The rationale for this is that this map is a meaningful snapshot of current conditions and also a reasonable first guess at what the future may bring. For today’s Outlook, however, that map is just not very informative. Instead, the key to deciphering the weather pattern over the next 15 days is to look much closer to home. The three panel image, below, shows the forecasted mean 500-mb height anomaly from the GFS ensembles for day 1, day 5 and day 13 (clockwise). The take-away message is that, in the upper-levels of the atmosphere, there will be surprisingly little change in the pattern. A weak-to-moderate upper-level ridge is expected to maintain an influence over western North America.

Unfortunately, during this time of year, the upper-level atmosphere provides surprising little translation into precipitation intensity especially across our state. However, the fact that the anomalous ridge will be north of Colorado for the duration of the Outlook is important, because such a position is typically associated with above normal Precipitable Water (PW) across most of Colorado. [Some early research has suggested that this is due to an increased frequency of surface high pressures moving southward out of Canada, providing fresh surges of upslope, moist air from the southeast.] Indeed, as seen in the forecasted PW values in the GFS ensembles, above normal values are expected throughout almost the entirety of the Outlook period. In fact, “above normal” may not be a strong enough word, since over the 7/14-7/16 period, significantly above normal PW (and though not shown here, low-level moisture as well) is expected to affect Colorado. Although PW is a strong predictor of precipitation potential, one mitigating factor to the rainfall threat will be a lack of upper-level dynamics. At this point in the summer, this is not much of a constraint, as we saw with the widespread heavy rainfall on Wednesday 7/12 despite rather weak upper-level dynamics. However, the weak steering winds tend to favor more isolated nature of the heavy rainfall threat, rather than a widespread/evolving storm cluster-type threat that is associated with shortwave disturbances.

We have identified two organized precipitation events for this 15-day Outlook. Below each event is described in more detail.

Event #1: Friday (7/14) through Monday (7/17)

High Flood Threat as very high moisture content supports at least isolated heavy rainfall

Surface high pressure will quickly move southward out of Canada, providing “return” southeasterly flow into Colorado. Strong moisture transport will result in PW values potentially exceeding 1.5 inches across eastern parts of the state – this is an extremely high reading that is very unusual even for eastern Colorado. With a lack of upper-level forcing, the main forcing mechanisms for this event will be (i) frontal dynamics on Friday and Saturday, and then (ii) climatological diurnal anabatic (i.e. upslope) flow from quick morning heating. Expect daily rounds of showers and thunderstorms, first focused on northeast Colorado on Friday, then east-central and southeast Colorado later in the event. With the high moisture content, max 1-hour rainfall up to 2.7 inches will be possible across eastern areas with up to 1.9 inches closer to the mountains and up to 1.1 inches across the western slope. Repeated storms moving a given locale will be of concern and max 3-hour rainfall up to 3.5 inches looks like a possibility on Friday – Sunday. Isolated flash flooding will be likely, along with debris slides and mud flows as wet soils become an increasing concern due to repeated rainfall. Rainfall intensities are expected to drop somewhat by Monday as higher moisture leaves the state. Please stay tuned to daily Flood Threat Bulletins for a much more detailed look at this event as it transpires.

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Event #2: Wednesday (7/19) through Saturday (7/22)

Elevated Flood Threat as cool front provides focus for heavy rainfall

After a brief return to heat following Event #1, another surface high pressure is expected to bring in a fresh cool front starting Wednesday, 7/19. Scattered to numerous showers and thunderstorms will accompany the front’s passage across northeast Colorado. Max 1-hour rainfall up to 2.3 inches (east) and 1.7 inches (central) will contribute to an isolated flash flooding threat. Across the western slope, moisture content is uncertain, and the flood threat looks to be subdued at the moment. With current guidance again suggesting very high moisture availability across eastern Colorado for this event, it is possible that an upgrade to a High threat will be required during the next Outlook.

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FTO 07-10-2017: Incredibly Persistent Pattern Continues To Favor Heat; Rainfall, Not So Much

Issue Date: Monday, July 10, 2017
Issue Time: 3PM MDT
Valid Dates: 7/11-7/25

Once again, the upper-level ridge that is typically a staple over western North America during the summer months, continues to maintain a firm grasp on Colorado’s weather. Although it is common to see the ridge centered somewhere in close proximity to Colorado, the past 7-14 days have seen remarkably little variation in the ridge position and intensity. Furthermore, there has been a near absence of any upper-level disturbances. Interestingly, it appears this pattern will persist for nearly the entire period covered by this 15-day Flood Threat Outlook. Although the upper-level ridge is expected to wobble around a bit (see water vapor image, below), it will remain centered very close to Colorado. Such a pattern is simply not conducive for any kind of organized rainfall in our state. That is not to say there will not be heavy rainfall, but such activity is expected to remain of a very isolated nature.

We have identified two precipitation events for this Outlook. The first event will be associated with a surge of monsoonal moisture traversing the state over the next 72 hours. An elevated flood threat is anticipated for isolated flash flooding. Notably, the western slope will get much needed rainfall. After a break in action, another event is identified later in the Outlook. Current guidance suggests influences from both the main jet stream as well as possible monsoonal moisture (and inverted trough from the south). Unfortunately, it is too soon to determine if a flood threat will accompany the second event.

What is much more certain is the incredibly persistent above average temperatures expected for at least a portion of the state over nearly the entire Outlook (see image below). This follows on the heels of a very warm June, almost setting in stone above average July temperatures, at least on a statewide level.

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

Event #1: Tuesday (7/11) through Thursday (7/13)

Elevated Flood Threat as monsoonal surge treks across the state

A surge of monsoon moisture will move across Colorado starting on Tuesday, supporting heavier rainfall both west and east of the Continental Divide. First on Tuesday, scattered to numerous showers and storms are expected over the San Juans, Central Mountains and Northern Mountains. With little shear and negligible upper-level forcing, only relatively brief heavy rainfall is expected. Max 1-hour rain rates could approach 1 inch per hour in favored parts of the western Central Mountains and San Juans. Isolated flash flooding, mud flows and debris slides will be possible, especially given the parched soil that will promote initial rapid runoff. Meanwhile, out east, a round of showers and storms is expected to develop mainly over the Palmer Ridge and track eastward. With plenty of moisture in place, quick bursts of heavy rainfall, to the tune of 1.25 – 1.5 inches per hour will accompany the strongest cells. Gusty winds will be possible in cells that are light on moisture content as rain-cooled downdrafts descend to the surface. Scattered shower and storm activity is expected for Wednesday though with lower rainfall rates. By Thursday, a cool front will graze eastern Colorado and support higher rainfall rates, up to 1.5 inches per hour. Isolated flash flooding will be possible.

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Event #2: Wednesday (7/19) through Sunday (7/23)

No Apparent Flood Threat as a combination of disturbances approach Colorado

A persistent ridge will reform after Event #1, causing above average temperatures statewide (especially out west). The first signs of a possible pattern shift may be on the horizon as a disturbance currently situated across the central Pacific begins to move eastward. At the same time, the upper-level ridge is expected to move east allowing for some easterly transport of relatively moist area into Colorado. A prolonged period of unsettled weather may be on tap for both eastern and western Colorado. However, currently, there is only marginal confidence for widespread rainfall, and even less confidence of heavy rainfall. The precipitation map shows the possibility of rainfall east and west of the Divide but amounts are expected to remain below 1.0 inch. Thus, flooding is not expected at this time. Stay tuned to Thursday’s Flood Threat Outlook for an update on this event.

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FTO 07-06-2017: First Monsoon Surge On The Horizon; Heat Continues Statewide

Issue Date: Thursday, July 6th, 2017
Issue Time: 3PM MDT
Valid Dates: 7/7-7/21

Just like the last Flood Threat Outlook, we do not have to look far away from home to foresee what is in store for Colorado’s weather during the next 15-day period. Shown in the water vapor image, below, we see a strong upper-level high pressure ridge currently positioned just west of our state. With strong subsidence underneath and to the east of it, very hot temperatures of 5-15F above normal have been the rule over the past few days. Today’s guidance is in remarkably good agreement that a relatively persistent pattern will continue over at least the next 7-10 days and perhaps even through the entire 15-day period. This pattern will be marked by the above average intensity upper-level ridge over central and western North America and a trough of low pressure off the west coast. Despite the pattern’s persistence, subtle shifts in the ridge’s position will have sizeable implications on Colorado’s precipitation chances.

There are two precipitation events that have been identified for this Outlook. The first event will be courtesy of a surface high pressure and attendant cool front that will sweep across mainly eastern Colorado on Friday. This will increase shower and storm chances out east for Friday, with decreasing coverage on Saturday and Sunday. With average to above average Precipitable water expected, short-term heavy rainfall will be possible but flooding is not expected. The second event will be the season’s first organized surge of monsoon moisture from the southwest. This will begin by early next week but peak by next Wednesday or Thursday, as the ridge wobbles far enough east to promote southerly moist flow statewide (see the forecasted 500-mb height anomaly pattern below). Although isolated heavy rainfall will be likely during Event #2, the lack of confidence in the moisture availability (especially regarding near-surface moisture) does not support an elevated flood threat at this time.

After Event #2 guidance continues to show a persistent high pressure ridge expected over or north of Colorado, supporting above normal temperatures for the tail end of the 15-day period. This can be seen below using the Saint Louis University’s CIPS analog guidance, which shows probabilities of temperature being above or below normal. A noteworthy impact of this would be a continued risk of elevated fire danger, which research has shown is dictated by prolonged periods of above normal temperatures.

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

Event #1: Friday (7/7) through Sunday (7/9)

No Apparent Flood Threat as cool front traverses across eastern Colorado

A cool front will move southward across eastern Colorado on Friday, providing an enhanced focus for shower and storm activity. Short-term heavy rainfall will be possible mainly across eastern Colorado, though the higher elevations could also experience isolated heavy rainfall producing storms. Up to 0.6 inches in 30 minutes will be possible except for far eastern areas where up to 1.5 inches per hour will be possible. These intensities are below flash flood levels, and flooding is not expected. As the front losing momentum in southern Colorado, the diurnal upslope flow will allow for better storm coverage across the Southeast Mountains, Raton Ridge and Southeast Plains on Saturday and Sunday. However, with no upper-level support, thunderstorm activity is expected to remain of the pulse variety with only 15-30 minutes of heavy rainfall under any given storm. Up to 0.7 inches of rainfall in 30 minutes will be possible.

West of the Continental Divide, increasing low-level moisture will continue to support isolated to scattered storm activity. Up to 0.6 inches per hour is possible here. With these values being well below flood-intensity thresholds, flooding is currently not expected.

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Event #2: Monday (7/10) through Thursday (7/13)

No Apparent Flood Threat at this time as moisture and precipitation chances finally increase for western slope

The first organized surge of monsoon moisture is expected to affect Colorado starting early next week. Precipitable water values will increase to near 1 inch along the Utah border. Daily rounds of afternoon/evening shower and storm activity will increase in coverage on Monday and Tuesday, peaking on Wednesday and Thursday. With only weak wind shear, only short-term heavy rainfall is expected. Max 30-minute rates up to 0.6 inches will be possible. However, with the prolonged duration of the event, the rainfall will gradually add up. Over 1.5 inches of total event rainfall will be possible along the climatologically favored southern slopes of the San Juans and perhaps the Central Mountains. At this time, there is no apparent flood threat. However, it is possible that this event will be upgraded to an elevated flood threat by Monday’s Outlook. Stay tuned.

 

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