FTO 08-31-2017: Summer To Leave With A Few Storms And Lots Of Heat; Fall Cool Front To Bring Relief

Issue Date: Thursday, August 31, 2017
Issue Time: 12:20PM MDT
Valid Dates: 9/1-9/15

The afternoon’s water vapor imagery, below, shows a disturbance has finally penetrated the previously impressive upper-level ridge. This disturbance will cross Colorado on Friday, bringing widespread shower and storm activity statewide. This is identified as Event #1 in today’s 15-day Flood Outlook. However, the ridge will not cede its ground that easily, and will return with a vengeance over the weekend. High temperatures up to 15F above normal are likely statewide with a few lower elevation locations likely to top 100F. For reference, here are the latest dates that long-term Colorado weather stations have recorded a 95F reading:

  • Alamosa – July 5 (only happened once),
  • Colorado Springs – September 17,
  • Denver – September 19,
  • Pueblo – September 25 (though 94F has been observed as late as mid-October), and
  • Grand Junction – September 18.

The upper-level ridge re-establishment will be temporary as a strong fall cold front will race southward out of Canada starting Monday. The frontal passage, identified as Event #2, will cause in increase in shower and storm coverage. However, as shown in the forecasted Precipitable Water plumes, below, we do not foresee a heavy rainfall threat due to limited moisture and quick storm motion. Thereafter, a slow decrease in available moisture will limit heavy rainfall chances, as well as precipitation in general.

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

Event #1: Friday (9/1)

Low-end Elevated Flood Threat as shortwave moves across Colorado

A shortwave will continue moving across Colorado on Friday, temporarily increasing rainfall coverage and intensity. Afternoon and evening showers and thunderstorms are expected. Highest coverage will be in the foothills east of the Continental Divide, though some activity will make it farther east especially in the Southeast Plains. With the upper-level support, scattered thunderstorms are also expected across the eastern plains. A severe threat, mainly for large hail, will accompany the isolated heavy rainfall. Max 1-hour rainfall up to 1.1 inch (west) and 1.5 inch (east) can be expected. These rates are right at flood threat intensity and a low-end flood threat is expected mainly for isolated flash flooding. The most vulnerable areas are likely to be burn scars in the Southeast Mountains (Junkins, Beulah Hill, Hayden Pass).

Legend

Event #2: Monday (9/4) through Wednesday (9/6)

No Apparent Flood Threat as front passage likely, but return moisture limited

Guidance is in excellent agreement today regarding the passage of a strong fall-like cool front starting on Monday, 9/4. An increase in rainfall coverage is expected mainly east of the Continental Divide. However, moisture will be limited, and showers and the main precipitation type. Isolated thunderstorms remain a possibility and it is not impossible that a low-end flood threat will develop as the amount of moisture becomes more certain. The best chances of precipitation exceeding 0.5 inches will be in south-central Colorado.

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FTO 08-28-2017: Two Precipitation Events Over Next 7 Days, But Headline Is Very Warm & Mainly Dry Conditions Statewide

Issue Date: Monday, August 28, 2017
Issue Time: 2PM MDT
Valid Dates: 8/29-9/12

Calmer weather has settled into Colorado, followed a prolonged, very active stretch during most of July and early August. Daily rounds of isolated to widely scattered showers and weak thunderstorms have still persisted especially in climatologically favored regions of the central and southern part of the state. However, heavy rainfall has been isolated and brief, and flooding has not been an issue.

As shown in the water vapor image, below, relatively quiet weather is expected through the entirety of this 15-day Outlook. A seasonably strong ridge is currently positioned over the Great Basin. This is maintaining dry and well above normal temperatures west of the Continental Divide and a few isolated storms in the picture across eastern Colorado. Off the west coast, a shortwave embedded in the jet stream is expected to temporarily flatten the ridge and support an increase in precipitation coverage across mainly eastern Colorado; this is identified as Event #1.

As shown in the forecasted Precipitation Water plumes, below, the shortwave passage will cause a brief increase in moisture, especially east of the Continental Divide. Short-term heavy rainfall will be possible, along with isolated severe weather in far eastern Colorado. However, rainfall rates are expected to stay just below flood threat level. After a rapid warm up following Event #1, a strong high pressure is expected to move south out of Canada, driving a fall cool front across mainly eastern Colorado. This is currently identified as Event #2. Although precipitation coverage and intensity are expected to increase, heavy rainfall is expected to be limited, and flooding is not expected at this time.

Finally, it is interesting to note that the lull in monsoon moisture continues across the western slope. Although monsoon surges can occur well into September, chances of heavy rainfall drop off drastically starting in early September. Climatologically speaking, it is much too soon to suggest that the monsoon may be finished for western Colorado, but when we look back several weeks from now, this could end up being the truth.

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

Event #1: Thursday (8/31) and Friday (9/1)

No Anticipated Flood Threat as shortwave treks across Colorado; upgrade possible

A shortwave will move across Colorado on Thursday and Friday, temporarily increasing rainfall coverage and intensity. Highest coverage will be east of the Continental Divide. Afternoon and evening showers and weak thunderstorms are expected across the High Country of central and southern Colorado. With the upper-level support, scattered thunderstorms are also expected across the eastern plains. A severe threat, mainly for large hail, will accompany the isolated heavy rainfall. Max 1-hour rainfall up to 1 inch (west) and 1.5 inch (east) can be expected. These rates are just below flood threat intensity. However, if slightly more moisture return occurs, an elevated threat could occur on Thursday.

By Friday, precipitation chances will scoot further south towards the Southeast Mountains and plains. However, moisture levels are expected to drop and rainfall rates should be limited to 1 inch an hour or less. Flooding is not expected at this time.

Legend

Event #2: Monday (9/4) through Tuesday (9/5)

No Apparent Flood Threat as front passage likely, but moisture appears to low for heavy rainfall

Guidance is in reasonable agreement today regarding the passage of a strong fall-like cool front starting on Monday, 9/4. An increase in rainfall coverage is expected mainly east of the Continental Divide. However, moisture return appears to be limited, and it is not clear if there will be enough instability to even generate thunderstorms. At this time, precipitation amounts in the 0.25 – 0.5 inch range are foreseen, and thus a precipitation map is not provided.

FTO 08-24-2017: Several Rainfall Events, One With A Brief Low-End Flood Threat

Issue Date: Thursday, August 24, 2017
Issue Time: 2PM MDT
Valid Dates: 8/25-9/8

This afternoon’s water vapor imagery, below, shows a rather busy atmospheric pattern over the eastern North Pacific Ocean and western North American continent. A weak upper-level ridge was positioned over Alberta with an undercutting jet stream to the south. Within this jet, several weak disturbances were noted, causing large-scale upward motion. As we move into the next 72-96 hours, these features will trek across Colorado, providing enhanced periods of generally light to moderate rainfall. The exception will be on Sunday, as a surface high pressure system is expected to drop south out of Canada, helping drive a cool front southward across eastern Colorado. A brief period of heavier rainfall will be possible, warranting an Elevated flood threat on Sunday afternoon and evening.

After Event #1, drier conditions are expected statewide as an upper-level ridge rebuilds to the northwest of Colorado. With a maintenance of average to above average Precipitation Water (PW; see GFS ensemble plume forecasts below), isolated showers and weak thunderstorms will be possible during the afternoons but the coverage and intensity appear to be too marginal to designate this as an individual precipitation “Event”. However, by Saturday, 9/2, guidance is in reasonable agreement that a strong surface high pressure will move southward into the Great Plains out of Canada. This late in the season, it is uncertain whether there will be enough low-level moisture available for heavy rainfall, with even more uncertainty due to the presence of Hurricane Harvey in the Gulf of Mexico. However, we have labeled this as Event #2 due to an expected increase in precipitation coverage.

As shown in the forecasted PW plumes, above, most of the precipitation “action” during the Outlook is expected to stay east of the Continental Divide as monsoon moisture should remain bottled up in Arizona and New Mexico.

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

Event #1: Friday (8/25) through Monday (8/28)

Low-end Elevated flood threat on Sunday in between a period of light/moderate rainfall

Several upper-level disturbances are expected to trek eastward across Colorado through next Monday. Most of these will be strictly upper-level driven, and with only marginal boundary layer moisture/convergence are not expected to produce heavy rainfall. Instead, light to moderate rainfall up to 0.75 inches will be possible during Friday and Saturday afternoons east of the Continental Divide. On Sunday, a frontal passage is expected across eastern Colorado. With PW expected to be at or above 1 inch across eastern Colorado, localized heavy rainfall rates up to 1.5 inches per hour will be possible. An accompanying isolated flash flood and small stream flood risk will be present with the strongest thunderstorms. However, riverine flooding is not expected. Marginally severe thunderstorms (main impact: large hail) will be possible especially eastward towards the KS border.

By Monday, drier air will overtake most of the state though showers and weak thunderstorms will be possible over southeast Colorado.

Legend

Event #2: Sunday (9/2) through Monday (9/4)

No Apparent Flood Threat as front passage likely, but moisture availability uncertain

Guidance is in good agreement that a relatively strong surface high pressure will move southward out of Canada, driving a cool front passage across eastern Colorado. A period of enhanced rainfall coverage is expected starting Saturday, 9/2. At this time, rainfall rates are not expected to exceed 0.5 inches per day, thus no precipitation map is provided. Depending on what transpires in the Gulf of Mexico with Hurricane Harvey, it is possible that more moisture will be available to draw into Colorado, which in turn could elevate the flood threat. Check back to Monday’s Outlook for more information.

FTO 08-21-2017: Moisture Plume Returns and Increases the Flood Threat

Issue Date: Monday, August 21, 2017
Issue Time: 3PM MDT
Valid Dates: 8/22-9/5

As seen on the water vapor image below, a strong upper-level ridge resides over the Pacific Northwest. At 500mb there is also a Low pressure system off the coast of southern California. With a broad area of higher pressure aloft over Texas, the gradient between the Low and High pressure has advected a plume of monsoon moisture into the state. With slow and variable steering winds aloft under the ridge, the increase in moisture has brought a High Flood Threat to Colorado tomorrow and an Elevated Flood Threat through Saturday.

Below are the forecasted Precipitable Water (PW) plumes for Denver and Grand Junction. The PW began to increase Sunday afternoon, but is expected to peak Tuesday evening. PW will be higher than the long-term normal at both locations and could reach as high as 1.3 inches over SE Colorado. This setup is similar to the active period at the end of July, except no mid-level high pressure is well-defined over Texas during the full period. This may limit the intensity of the moisture advection and confine the higher moisture to southern Colorado and the eastern plains. The surge of moisture, and antecedent atmospheric moisture, hangs around through Sunday evening. The extended period of active weather has been labeled as Event #1.

On Thursday, the upper-level low pressure system begins to move eastward. With the elevated moisture in the atmosphere, this extra energy could support stronger storms and widespread rainfall over the eastern plains. There is enough confidence to identify this period as an elevated threat, although the full details of this event are still developing. Please check back to the daily Flood Threat Bulletin for more detail.

Below we describe Event #1 in more detail.

Event #1: Tuesday (8/22) through Sunday (8/27)

High/Elevated Flood Threat as monsoon moisture plume creates daily rounds of heavy rainfall.

A plume of monsoon moisture continues to make its way into Colorado peaking Tuesday evening. A mid-level Low/High pressure gradient has created a strong southerly component to the flow that is advecting moisture into Colorado. Short-term heavy rainfall will be likely over the climatologically favored regions of southwestern Colorado, Southeast Mountains and Palmer Ridge daily throughout this week. On Thursday, the mid-level Low pressure system will move through Colorado and is expected to produce another round of heavy precipitation that could last into the overnight hours. Max 1-hour rainfall rates greater than 1.5 inches (east) and 1.0 (west) are expected tomorrow afternoon and evening. With slow steering winds under the ridge and elevated moisture, it is likely max 1-hour rain rates could exceed those same thresholds throughout the week. Threats include flash flooding as well as debris slides and mud flows over the higher terrains and susceptible burn scars. Multiple rounds of rainfall will also increase these threats as the soil becomes saturated towards the end of the week.

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