FTO 05-25-2017: Two Elevated Flood Threat Events, Then Snow Melt Begins In Earnest Later Next Week

Issue Date: Thursday, May 25th, 2017
Issue Time: 3:00PM
Valid Dates: 5/26 – 6/9

In today’s eastern Pacific water vapor imagery, shown below, we get a glimpse at many of the features that will be affecting Colorado’s weather over the next 15 days. The first feature is the disturbance located over southern Canada with a trough draped southwest across Colorado. This will provide a few days of elevated flood threat conditions mainly for isolated flash flooding (along with severe weather) in eastern Colorado. Looking slightly farther west we see a ridge of modest strength located just offshore of North America. This ridge will attempt to bulge into Colorado but should be help back from doing so over the next week or so, in part due to the disturbance that will be causing our second precipitation event (Event #2) next week. Thereafter, the ridge will appear to regain the upper hand which will provide an extended warm period for most of the state. With a healthy snowpack still in place, we have labeled Event #3 as a string of days where substantial melt is expected.

Below we describe each Event in more detail.

Event #1: Friday (5/26) through Sunday (5/28)

Elevated Flood Threat through Saturday for eastern Colorado, followed by more steady rain

The upper-level trough responsible for today’s (Thursday) shower and storm activity will actually generate a daily low-level circulation east of the Continental Divide. Friday’s system will have the best chances of generating widespread heavy rainfall. We anticipate 1-2 waves of organized shower and storm activity beginning late morning over the higher terrain, then moving east into the Plains. Very heavy rainfall will be possible especially towards the Kansas border, with max 1-hour rain rates up to 2.4 inches and 3-hour rain rates up to 2.9 inches. Isolated flash flooding will accompany the strongest storms, and street flooding will be likely for low-lying municipal areas. In addition to the heavy rainfall, severe weather in the form of large hail (up to 2.5 inches), damaging winds (up to 75mph) and isolated tornadoes will be possible. Heading into Saturday, cooler, more stable air will slowly overtake northern Colorado, but instability will still be present in the southeast. At least one round of showers and storms will develop across the Southeast Plains with 1-hour rain rates up to 1.6 inches, causing isolated flash flooding. By Sunday, cooler air will overtake most of the state, but residual moisture will allow for upslope shower and maybe a weak thunderstorm. However, flooding is not expected on Sunday.

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Event #2: Wednesday (5/31) through Thursday (6/1)

Elevated Flood Threat as disturbance approaches Colorado from the southwest

After a short break following Event #1, a southern stream disturbance will penetrate through the west coast ridge and will affect mainly southern Colorado beginning Wednesday 5/31. The strength of the atmospheric dynamics are uncertain right now, but it does appear that there will be enough moisture for at least isolated thunderstorms capable of producing heavy rainfall. The areas most likely to be impacted are the San Juans, stretching east into southeast Colorado. Up to 1.5 inches of rainfall will be possible, based on current guidance.

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Event #3: Sunday (6/4) through Wednesday (6/7)

Snow melt to accelerate but No Anticipated Flood Threat at this time

A high-pressure ridge will begin to control the western North American circulation starting later next week. High temperatures will climb to above normal levels for a multi-day period, allowing for high-elevation snow melt to begin in earnest. Locations below 9,000 feet have already generally melted out (with some exceptions due to the recent late season snowstorm). However, a solid snowpack remains above 9,000 feet. At this time, only modestly above average temperatures are expected, thus we do not expect major flood-related issues. However, this will need to be monitored. The basins that are most prone to snow-melt related high flows will be the South Platte and San Juan region, as shown in the SWE charts below. Nonetheless, other isolated locations with a high snowpack may be vulnerable to high flows as well. Time to book those rafting trips, if you have not already done so!

FTO 05-22-2017: High Flood Threat Anticipated Later This Week

Issue Date: Monday, May 22nd, 2017
Issue Time: 2:45PM MDT
Valid Dates: 5/23 – 6/6

Last week’s snowstorm has finally departed, but its impacts have not. The low-pressure that was responsible for the late spring snows along the Front Range has helped carve out a strong, large-scale trough centered over northern Minnesota as of this afternoon (see water vapor image below). The main sensible features from this trough have been a string of cooler than normal days across most of the state and frequent (but generally light intensity) precipitation. In turn, the cooler weather has helped refrigerate a sizeable snowpack across the higher terrain.

Looking forward over the next 15 days, we anticipate the first seven days of the period to continue to be impacted by the Central Plains trough. In fact, an interesting situation appears to be in the making as another strong disturbance, currently in the Gulf of Alaska, enters the west coast over the next 48-72 hours. During that time period, there is enough consensus that return flow will have time to re-establish itself east of the Continental Divide. With a return to near-seasonal temperatures by Thursday, there will be sufficient instability for a multi-day flood threat across eastern Colorado. At this time, it appears that over Thursday (5/25) through Saturday (5/27), each day will have a possibility of at least isolated flooding. However, an especially strong consensus in guidance and historical analogs pegs Friday as the day with the highest chances of widespread heavy rainfall and flash flooding.

The three maps below show the probability of Convective Available Potential Energy exceeding 500 J/kg (a rough guideline for heavy rainfall potential) for the past 3 forecast cycles of the GFS ensembles. A widespread swath of probabilities exceeding 50% is found across eastern Colorado for Friday afternoon. Analysis of moisture fields shows that Precipitable Water values will approach or exceed 1 inch in far eastern areas, and a long fetch of moisture advection will be likely. The only possible complication is if cool, stable air overtakes the entire area, which is looking very improbable right now. Although it is difficult to ascertain the severity of potential flooding, it does appear likely that at least isolated flash flooding will occur; hence the High flood threat.

After Event 1’s trough moves eastward, expect a return to quieter and warmer weather statewide. There is enough consensus to identify another precipitation event, Event #2, currently located in the far western Pacific Ocean. However, as this disturbance will likely be battling a high-pressure ridge, it does not currently warrant any flood threat, though that could change.

In addition to the precipitation events, consistently warmer weather is slated to begin after the passage of Event #1. With the recently rejuvenated snowpack, snow melt-driven high river and stream flows will need to be monitored. We should have a better grasp of the degree of warm weather in the next update of the Flood Threat Outlook, scheduled for this Thursday (5/25).

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

Event #1: Thursday (5/25) through Saturday (5/27)

Elevated/High Flood Threat likely for eastern Colorado

Thursday will mark the first day of the flood threat as showers are expected in scattered to numerous coverage statewide, with thunderstorms mainly east of the Continental Divide. Maximum hourly rain rates up to 1.6 inches will be possible, with max 24-hour rainfall exceeding 2 inches. Hail will be possible with the strong storms, and a tornado threat appears possible towards the Kansas border. For Friday, it is likely that some cooler, less unstable air will begin to affect northern Colorado. In this region, light to moderate rainfall will occur, perhaps amounting to 1 inch. Farther south and east, instability will be adequate enough to continue to fuel multiple rounds of showers and thunderstorms. Up to 2 inches of rainfall will be possible. With increasingly wet soils, it appears runoff will be particularly efficient and thus both flash flooding and possible riverine flooding could occur. It is difficult to anticipate whether the riverine flooding will be limited to tributaries or the main rivers. However, the downstream portions of the South Platte and Arkansas rivers could both end up in at least Minor flood stage. Hail could also accompany some of the stronger storms on Friday, though this could be mitigate if convection is organized into larger segments (which would be more conducive to a flood threat). By Saturday, the cool, stable air mass will likely overtake most of Colorado except for parts of the Southeast Plains. There, continued storm activity is likely to cause isolated heavy rainfall that could fall over already wet soils. At this time, it appears that the Front Range and Urban Corridor will not have sufficient instability for widespread, long-duration heavy rainfall. However, this is not certain and will be monitored over the next few days.

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Event #2: Wednesday (5/31) through Thursday (6/1)

No Anticipated Flood Threat as disturbance approaches Colorado from the southwest

After a respite following Event #1, warmer weather will overtake most of Colorado. A disturbance is forecasted to approach the west coast by the middle of next week. Should it make it to Colorado, there will likely be a flood threat as moisture levels appear to be high enough to generate a heavy rainfall threat. However, at this time, the precipitation map below captures the regions that are most likely to be affected by rainfall. Stay tuned to updates on this event in ensuing Flood Threat Outlooks.

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FTO 05-18-2017: Very Active, Cool Pattern But Flood Threat Subdued

Issue Date: May 18, 2017
Issue Time: 1PM MDT
Valid Dates: May 19th – June 2

The North Pacific/North American water vapor image, below, shows a very active jet stream across the entire region. During the next 15 days, there are four precipitation events that have been identified as capable of producing widespread precipitation. Event #1 is ongoing, courtesy of a cut-off low that is currently swinging through Colorado, providing a late spring snowstorm for the higher terrain and rain for the lower elevations. Most of the precipitation associated with this storm will be finished by later this evening. However, some additional precipitation will be possible for eastern parts of the state on Friday. Thereafter, the ridge currently off the west coast will shift slightly eastward, intensify and aid in the development of an impressive trough across the central United States. This kind of pattern will promote a slew of disturbances, referred to here as Event #2 and Event #3, originating off the Canadian and Montana Rockies and quickly moving southeastward across eastern Colorado. While precipitation chances with each disturbance are high, no flood threat is currently warranted.

The period from May 27 – May 30 currently looks to be an in-between period where the west coast ridge will temporarily bulge into Colorado. A few warm days are likely during this stretch. Thereafter, a breakdown of the southern periphery of the ridge is expected, which will provide access for disturbances to affect Colorado from the west and southwest. Once again, no flood threat is warranted at this time, and future Outlooks will have a better grasp of this event.

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

Event #1: Friday (5/19)

Elevated Flood Threat as cut-off low pressure exits the region

An impressive late spring snowstorm will depart Colorado early Friday. As it does so, light to moderate rainfall, possibly mixed with morning snow above 6,000 feet will continue through Friday afternoon. The rainfall intensity will not be enough to cause flooding concerns. However, the large amount of antecedent rainfall will mean that many creeks and streams of the Front Range will be swollen. Some of this will be mitigated by low snow levels, which will greatly temper the runoff rates. Nonetheless, nuisance stream-side flooding will be common in the foothills. This is expected to subside by Saturday. Please pay attention to National Weather Service warnings and advisories for key problem areas.

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Event #2: Sunday (5/21) through Tuesday (5/23)

Daily precipitation chances but No Anticipated Flood Threat

A large-scale ridge-trough combination will setup by Sunday across western and central North America, respectively. Several “northwest flow” disturbances will quickly trek across mainly eastern Colorado from Sunday through Tuesday. The amount of instability through this time period appears to be low as return (easterly/southeasterly) flow will not have time to develop. Although many areas will see light precipitation, flooding is not expected. Nonetheless, many higher elevation regions east of the Continental Divide should pick up over 0.5 inches of beneficial rain and snowfall during this time period.

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Event #3: Thursday (5/25) through Friday (5/26)

No Anticipated Flood Threat at this time, but upgrade possible

Although the large-scale pattern from Event #2 is expected to continue as Event #3 occurs, there may be more moisture available for heavier precipitation amounts. The favored regions are expected to be northeast Colorado and into the Palmer Ridge. At this time, there is not enough confidence to warrant a precipitation map.

Event #4: Wednesday (5/31) through Friday (6/2)

No Anticipated Flood Threat at this time

After Event #3, a high pressure ridge will bulge eastward causing a string of warm and mainly dry days. Thereafter, reasonable consensus exists that the flow along the southern periphery of the ridge will break down and allow for a return of disturbances from the west and southwest. Such a pattern would be conducive for moderate to heavy rainfall in the San Juans as well as most of eastern Colorado, east of the Continental Divide. At this time, there is not enough confidence that the ridge will break down. Thus, a precipitation map is not provided.

FTO 05-15-2017: Active Period Ahead

Issue Date: 5/15/2017
Issue Time: 1:00 PM

An active period is in store for Colorado over the next ten days with three events lined up to swing through the region. The water vapor image below highlights the three disturbances associated with each: Event #1 is the upper-trough currently over the western US, Event #2 is an upper-level low that will dig in immediately behind #1, and Event #3 is an upper-level trough/low gathering near Alaska. Event #1 is a quick-hitting event, with the upper-trough pushing through the region Tuesday and exiting Tuesday night ahead of the second event. Moisture isn’t expected to be deep enough to warrant widespread flooding concerns, but close attention will be paid during tomorrow’s FTB. Make sure you check back in for that updated information.

Event #2 will present an elevated flood threat to Colorado, thanks in part to its expected slow movement, broad upper-level support, the return of deeper moisture from the subtropics, and surface features including a cold front and upslope flow. At this time, it appears that Wednesday and Thursday will be the days with the greatest flood threat during this event. Event #3, at the moment, is expected to move into the region by Monday of next week, but the flood threat is far from certain. Questions about the depth of moisture and locations of any surface boundaries will need to be answered before a determination can be made. However, using climatology as a tool, one can infer that there will likely be a chance for strong-to-severe thunderstorms across eastern Colorado during a setup such as the one expected, and with that usually comes a flood threat. With that in mind, stay tuned to the update in Thursday’s FTB.

Event #1: Tuesday (05-16-2017)

No Apparent Threat as Stage Gets Set for Following Event

Event #1 is a quick-hitting event, but it will “set the state,” so to speak, for Event #2. Decent lapse rates and ascent will provide the support for scattered showers and thunderstorms across the state. Far eastern Colorado will have a slim chance for a strong-to-severe thunderstorm or two, but it looks like the majority of the state will be under the risk of garden-variety showers/storms. The depth of moisture, at this time, doesn’t appear to be enough to warrant a flood threat. Pay attention to the FTB tomorrow for any updates to the situation. No precipitation map will be drawn due to expected totals below 0.5 inches.

Event #2: Wednesday (05-17-2017) through Friday (05-19-2017)

Elevated Flood Threat as Upper-Level Low Swings Through

A fairly slow-moving upper-level low, combined with intensifying surface low pressure and the presence of a frontal boundary, will set Colorado up for an unsettled 3-day period of showers/thunderstorms. Scattered-to-widespread activity is expected, with swaths of greater than 1 inch of rain expected across portions of the Grand Valley, Central Mountains, San Juan Mountains, Front Range, Southeast Mountains, Urban Corridor, Palmer Ridge, Northeast Plains and Southeast Plains. Stay tuned to daily FTB’s for more information.

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Event #3: Monday (05-22-2017) through Wednesday (05-24-2017)

No Apparent Threat Due to Uncertainty, But Climatology is Important

Uncertainty will keep this period from obtaining any flood threat designation, but climatology points toward the possibility with the upper-level pattern expected. Eastern Colorado will be under the greatest concern for strong-to-severe thunderstorms and heavy rainfall, but the presence of easterly low-level flow could push this back into the mountains and foothills, as well. No precipitation map will be drawn until more details come into focus. Stay tuned.