SPM 09-30-2017: Rain And High Elevation Snow Mainly Across Western Colorado

Issue Date: Saturday, September 30, 2017
Issue Time: 10:45AM MDT

Summary:

Unsettled weather continued for Colorado as a large-scale trough continued to envelop the western United States. Over the last week, temperatures have ranged anywhere from 2 to 10F below normal across the state (see below), one indicator of frequent cloud cover that has limited heating.

Despite the active weather, rainfall rates continued to be limited due to weak instability and limited boundary layer moisture. Isolated twenty four rainfall amounts up to 1 inch occurred over the higher terrain of southwestern Colorado on Friday. However, hourly rain rates were limited to less than about 0.4 inches. Despite the limited instability, a few reports of small hail were reported along the counties by the New Mexico border. This was probably more of a testament to low freezing levels than atmospheric instability.

Flooding was not reported on Saturday. 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 09-30-2017: Widespread Showers & Weak Storms For Higher Terrain, But Rain Rates Limited

Issue Date: Saturday, September 30, 2017
Issue Time: 10:30AM MDT

— Flooding is NOT expected today

NOTE: This is the last scheduled Flood Threat Bulletin of the 2017 season. However, additional Bulletins may be necessary next week to cover the heavy rainfall threat anticipated in southern/eastern Colorado.

A broad upper-level trough continues to be positioned over the western half of the United States, as shown in this morning’s water vapor image, below. Despite the many, rather disorganized smaller-scale features, it is apparent that plenty of upward motion is currently occurring east of the trough’s axis in UT, western CO and northern New Mexico. In fact, showers and isolated weak thunderstorms are already occurring in the San Juan Mountains, showing that the system is bringing plenty of dynamical support. From the standpoint of heavy rainfall, the main limiting threat will once again be moisture, along with fast steering winds. This morning’s Precipitable Water (PW) measurements at Grand Junction and Denver are a rather subdued 0.54 and 0.48 inches, respectively. Higher values approaching 1 inch are found just east of the Colorado border into KS and NE. However, with only a weak easterly component of low-level flow, PW is expected to remain generally steady across the state.

With plenty of dynamical support, we expect a round of numerous to widespread shower and storm activity coinciding with peak heating this afternoon. Highest coverage will be over the higher terrain, mainly west of the Continental Divide. Rain rates are expected to remain well below flood threat thresholds over most of the state. Isolated higher rain rates are possible with storms that come off the Palmer and Cheyenne ridges. However, we expect that even these will stay just below flood threat intensity. Thus, flooding is not expected today. Nonetheless, widespread precipitation totals above 0.25 inches are expected over the central and western part of the state.

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:

Northeast Plains, Southeast Plains:

Mostly sunny this morning then becoming partly cloudy and breezy with isolated to scattered showers and thunderstorms this afternoon and evening. Highest coverage will be over western areas, towards the foothills. Max 1-hour rain rates up to 1.2 inches possible with the strongest storm cells, along with gusty winds up to 55 mph. Flooding is not expected today.

Primetime: 2PM to 9PM

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

Mostly cloudy and cool with numerous shower and weak thunderstorms increasing in coverage this afternoon. Max 1-hour rainfall up to 0.4 inches (north) and 0.7 inches (far south, along NM border). Snow levels 10,000 feet (north) and 11,500 (south). Gusty winds and small hail could accompany the strongest storms. Flooding is not expected today.

Primetime: 11AM to 10PM, with light rain and snow showers continuing overnight

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

Mostly sunny this morning, then increasing clouds with scattered to numerous showers and weak thunderstorms developing by early afternoon. Highest coverage will be over the foothills and higher terrain. Max 1-hour rain rates up to 0.75 inches (below 6,000 feet) and 0.6 inches (above 6,000 feet). Snow level will fluctuate around 11,000 feet, lowering later tonight. Gusty winds up to 50 mph and small hail could accompany the strongest cells. Flooding is not expected today.

Primetime: 2PM to 9PM, with light rain and snow showers continuing overnight

SPM 09-29-2017: Ongoing Showers with Northward Movement of the Upper Level Low

Issue Date: Friday, September 29, 2017
Issue Time: 10:40AM MDT

Summary:

The upper level low to the west of Colorado continued to promote rain and cloudiness across the majority of the state again yesterday. The morning began with showers over the high terrain sustained by weak upslope flow and high moisture. Heavier rain was occurring over the eastern plains, which was supported by the upper level jet as the Low tracked north. By mid-afternoon, the showers over the eastern plains had worked their way over the KS border, and the dry slot had moved into western Colorado. The drier air to the west limited widespread showers and heavy rainfall, though some storms still dropped a decent amount of precipitation over the valleys and Northern Mountains. Overnight showers and snow lingered in the mountains with the highest accumulations over the Southeast Mountains/Raton Ridge. The showers over the south were enhanced by a shortwave rotating around the Low, which helped increase 24-hour rainfall totals. By early this morning, the showers had drifted into the Southeast Plains and have sustained themselves with lift from the jet stream and the shortwave. The isolated showers will continue to gradually move to the northeast throughout the morning.

Southwest flow over western Colorado helped limit rainfall totals when compared to Wednesday. A bit more moisture managed to maintain itself over the eastern San Juan Mountains. CoCoRaHS stations near Pagosa Springs recorded totals from 0.15 to 0.3 inches. A bit of instability was able to build over the western valleys and afternoon thunderstorms produced hail and storm totals of 0.5 inches. Along the Front Range, the Eldora SNOTEL station recorded 0.9 inches. Radar rainfall estimates were up to 1 inch in this area. The highest accumulations were again over the Southeast Mountains and Raton Ridge. The heavier rain fell in ungagged areas, but there was a report of 1 inch in Kim, Colorado. Radar rain estimates are up to 1.5 inches with 2-hour totals up to 1 inch.

There were no reports of flooding Thursday. To see how much rain fell in your area, scroll down to the 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 09-29-2017: Isolated Showers and Thunderstorms over the Higher Terrains and Adjacent Plains

Issue Date: Friday, September 29, 2017
Issue Time: 10:30 AM MDT

— Flooding is NOT expected today

As seen yesterday morning, showers over the eastern plains are being supported by the jet stream and mid-level energy rotating around the Low to our northwest. These showers will continue to move to the northeast and exit the state by late this morning. There is quite a bit of fog again this morning in the mountains and Urban Corridors. This should quickly burn off due to breaks in the clouds being more numerous today. Precipitable Water (PW) in Grand Junction was measured at 0.75 inches and has dropped to 0.65 inches in Denver. With more southwesterly flow today, decreases in moisture are expected today. This should make showers and thunderstorms more isolated in nature and limit accumulations.

The Low pressure is currently over the UT/WY border and will continue to move north throughout the day. Shortwave activity associated with the Low will provide enhanced lift one last time for another round of showers and thunderstorms this afternoon. Moisture will continue to retreat east throughout the day, but dew points over the southeast plains and southern mountains will be high enough for moderate rainfall further south. Breaks in cloud cover will allow some instability to build this afternoon, which will increase the possibility for thunderstorms. Isolated showers and thunderstorms are expected to form over the mountains by later this afternoon and move into the adjacent plains this evening. The storms are expected to dissipate as they move off the higher terrains, but brief heavy rain with gusty winds and thunder are possible. Overnight shower activity is not expected, but a few showers may linger over the southern mountains with weak upslope flow. Flooding is 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:

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

Partly cloudy with some showers over the Northwest Slope. Drier air has allowed breaks in cloud cover over the region. Enough residual moisture and instability is expected for another round of showers and thunderstorms over the higher terrains this afternoon. Highest accumulations are expected over the southern San Juan Mountains; especially further east where moisture hangs on. Max 1-hr rain rates up to 0.5 inches (south) and 0.3 inches (north) are possible with 3-hr totals up to 0.8 inches (south). Flooding is not expected today.

Primetime: 1PM to 9PM

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

Cloudy and foggy this morning with rain over the eastern plains. Showers and thunderstorms likely over the higher terrains this afternoon, although they will be more isolated than the last couple of days. Highest accumulations are expected over the southern Front Range and Southeast Mountains. Max 3-hr rain rates up to 0.8 inches are possible. Showers and thunderstorms will move NE off the higher terrains this afternoon and may briefly produce heavy rain and gusty winds on the adjacent plains before dissipating. Max 1-hr rain rates up to 0.7 inches are possible.

Primetime: 2PM to 11PM