FTB 08-30-2017: Storm Coverage To Increase, But Still No Flood Threat

Issue Date: Wednesday, August 30, 2017
Issue Time: 9:48AM MDT

— Flooding is NOT expected today

As shown in the water vapor image, below, a few changes are noted in this morning’s atmospheric analysis over western North America. First, the strong upper-level ridge that has been around for the last 48-72 hours is slowly weakening and will continue to weaken throughout the day. Meanwhile, a modest shortwave disturbance is entering the California coast. Weak lift and moisture convergence is enhancing mid-level moisture over the northern Great Basin. Some of this moisture will make it into Colorado by later this afternoon. Precipitable water has increased to 0.64 inches (from 0.5 inches 24 hours ago) at Grand Junction and 0.65 inches at Denver. However, most of this moisture is in the mid-levels and despite plenty of morning sunshine, instability will once again be limited today.

Wrapping everything together, we expect an increase in shower and weak thunderstorm coverage today, especially over the higher terrain as the increase in mid-level moisture will help storms stay intact. However, with subsidence still being prevalent and only weak instability, expect a continuation of light to moderate rainfall rates. Thus, flooding is once again not expected today.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below (hover over threat areas for more details). For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

Front Range, Urban Corridor, Palmer Ridge, Northeast Plains, Southeast Mountains, Raton Ridge, Southeast Plains:

Mostly sunny this morning, then partly cloudy with scattered showers and weak thunderstorms possible. Highest coverage will be over the higher terrain east of the Continental Divide as well as the Cheyenne, Palmer and Raton ridges. Max 1-hour rain rates up to 0.8 inches closer to the foothills and 1.2 inches across the eastern plains. Gusty winds could accompany the strongest storms. Flooding is not expected today.

Primetime: 1PM to 9PM

Northwest Slope, Grand Valley, Southwest Slope, Northern Mountains, Central Mountains, San Juan Mountains, San Luis Valley:

Sunny early, then partly to mostly cloudy with scattered showers and thunderstorms likely this afternoon. Highest coverage will be over the San Juans, Central Mountains and Northern Mountains. Max 1-hour rainfall up to 0.6 inches is possible, with max 24-hour rainfall up to 1 inch especially over higher elevation locations. Gusty winds will accompany the strongest cells. Flooding is not expected today.

Primetime: 12PM to 9PM

SPM 08-29-2017: Isolated Storms Continue Out East, While Heat Stays Put West

Issue Date: Tuesday, August 29, 2017
Issue Time: 11AM MDT

Summary:

The Continental Divide continued to be a clear demarcation of weather regimes on Monday. To the west, hot and mainly dry weather was the norm. Grand Junction reached 96F, about 9F above its average high temperature for late August. Even higher elevation locations such as Copper Mountain reached the low 70s F. Isolated showers and storms were observed over the southern San Juans, though only a few tenths of an inch of rainfall, at best, were reported.

To the east of the Divide, higher boundary layer moisture allowed for slightly stronger storms. A cell in Chaffee County produced 0.69 inches (CoCoRaHS) and even prompted a brief Areal Flood Advisory during the late afternoon. Meanwhile, several rounds of rainfall up to 0.5-0.75 inches per hour moved across the far southern Sangre de Cristos. In all, radar estimates suggest up to 1.5 inches occurred. However, flooding was not reported.

For rainfall estimates in your area, check out our State Precipitation Map below.

Click Here For Map Overview

The map below shows radar-estimated, rainfall gage-adjusted Quantitative Precipitation Estimates (QPE) across Colorado. The map is updated daily during the operational season (May 1 – Sep 30) by 11AM. The following six layers are currently available: 24-hour, 48-hour and 72-hour total precipitation, as well as maximum 1-hour, 2-hour and 6-hour precipitation over the past 24 hour period (to estimate where flash flooding may have occurred). The accumulation ending time is 6AM of the date shown in the bottom right corner. Also shown optionally are fire burn areas since 2012. The home button in the top left corner resets the map to the original zoom.

Note: We have identified a possible underestimation in QPE over the southwest part of the state. We are working to on this issue, and will provide an update as soon as possible.

FTB 08-29-2017: Persistent Pattern: Hot Out West, Isolated T-Storms East

Issue Date: Tuesday, August 29, 2017
Issue Time: 9:40AM MDT

— Flooding is NOT expected today

This morning’s water vapor image, below, shows a very persistent atmospheric pattern across the central and western US. The remnants of Hurricane Harvey continue to be a very serious threat across the Gulf Coast states. However, drying is noted on the west side of the storm’s center, and Harvey’s moisture will never make it to Colorado. Instead, subsidence will continue to be the main player today, supporting mainly clear skies and hot weather west of the Continental Divide with isolated storms possible over the higher elevations and foothills to the east. Morning Precipitable Water (PW) values were 0.62 and 0.47 inches at Denver and Grand Junction, respectively. This is down 10-20% from yesterday. PW is expected to stay steady today. With plenty of morning sunshine, expect weak instability and upslope flow to generate isolated to scattered storms mainly over the higher elevations and foothills east of the Continental Divide. A few storms could make it farther eastward off the Cheyenne, Palmer and Raton ridges. However, only short-term moderate rainfall is expected. Flooding is not expected today.

The main headline today will continue to be above average temperatures statewide, but especially west of the Continental Divide where high temperatures of up to 15F above normal are expected.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below (hover over threat areas for more details). For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

Front Range, Urban Corridor, Palmer Ridge, Northeast Plains, Southeast Mountains, Raton Ridge, Southeast Plains:

Mostly sunny this morning, then partly cloudy with isolated to scattered showers and weak thunderstorms possible. Highest coverage will be over the higher terrain east of the Continental Divide as well as the Cheyenne, Palmer and Raton ridges. Max 1-hour rain rates up to 0.75 inches (below 7,000 feet) and 0.5 inches (above 7,000 feet), though most everyone will stay dry. Gusty winds will accompany the strongest storms. Flooding is not expected today.

Primetime: 1PM to 8PM

Northwest Slope, Grand Valley, Southwest Slope, Northern Mountains, Central Mountains, San Juan Mountains, San Luis Valley:

Sunny early with isolated showers and a weak thunderstorm possible this afternoon. Highest coverage will be over the San Juans. Max 1-hour rainfall up to 0.4 inches is possible. Gusty winds will accompany the strongest cells. Flooding is not expected today.

Primetime: 12PM to 8PM

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.