FTB 09-27-2017: Widespread Showers for Colorado as Upper Level Low Increases Moisture and Lift

Issue Date: Wednesday, September 27, 2017
Issue Time: 10:45 AM MDT

LOW flood threat for the Hayden Pass and Junkins burn scars

As expected, the upper level Low over AZ has drawn in a lot of low level moisture over Colorado as seen in the water vapor (WV) imagery below. The increase in low level moisture allowed showers and snow (at the higher elevations) to persist overnight and into the morning along the Front Range and Southeast Mountains. There is also an overall increase in cloud cover across the eastern portion of the state and the mountains, which kept low temperatures overnight warmer than normal. Precipitable Water (PW) this morning at Denver was measured at 0.73 inches, and in Grand Junction it was 0.4 inches. With southwest surface winds becoming more southerly throughout the day, moisture will continue to increase with the highest moisture expected over the southern portion of the state.

The upper level low is expected to slowly track north today and will be positioned over Utah by tonight. As the low tracks north, increased shortwave activity and upper air support from the jet stream will bring multiple rounds of showers and snow. Snow should be confined to elevations above 11,000 feet today, but as temperatures decrease tonight, the snow line is expected to drop to 10,000 feet. The initial wave of showers occurring now will be confined to the higher terrains, but by this afternoon, showers will increase in intensity and spread into the adjacent plains and valleys. Thunderstorm activity is expected to be marginal and more isolated due to cloud cover limiting instability.

The highest accumulations for the next 24-hour period are expected to be over the Southeast/San Juan Mountains and southern Central/Front Range Mountains. Over the highest peaks, greater than 6 inches of snow is possible, and a Winter Weather Advisory has been issued for elevations about 11,000 feet. With limited instability, max 1-hour rain rates are expected to be under 1 inch today. However, 3-hour rain rates up to 0.9 inches are possible. There is enough confidence that multiple areas over the Southeast Mountains will have 24-hour totals exceeding 2 inches, so a Low Flood Threat has been issued for the Hayden Pass and Junkins burn scars. Threats include road and small stream flooding, mud flows and debris slides. Shower activity is expected to decrease after midnight over the San Juan Mountains, but rain will continue into the morning over the other southern mountains.

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.

Flood Threat Legend

Zone-Specific Forecasts:

San Juan Mountains, Southwest Slope, San Luis Valley, Southeast Mountains, Raton Ridge, Central Mountains, Southeast Plains:

Cloudy this morning with showers over the high terrains. More widespread showers expected this afternoon. Showers are also expected to increase in intensity throughout the day. Max 1-hr rainfall rates up to 0.6 inches and max 3-hour rainfall rates up to 0.9 inches possible. 24-hour amounts up to 2.75 inches are possible over the Southeast and San Juan Mountains. Elevations greater than 11,000 feet will likely see all snow all day with the snow line dropping to 10,000 feet overnight. Snow totals greater than 6 inches are likely at the higher elevations. A Low flood threat has been issued for the Hayden Pass and Junkins burn scars for sufficient confidence 24-hour totals will exceed 2 inches over multiple areas. Threats include mud flows, debris slides and small stream and road flooding.

Primetime: 11AM to 7AM

Grand Valley, Northwest Slope, Northern Mountains, Front Range, Urban Corridor, Northeast Plains, Palmer Ridge:

Cloudy this morning over the high terrains and east of the Continental Divide. Currently, showers are confined to the higher terrains, but this afternoon are expected to increase in intensity and spread into the adjacent plains and valleys. Over the higher terrains, max 3-hour rain rates up to 0.8 inches with 24-hour totals up to 1.75 inches (south) and up to 1 inch (north) are possible. 24-hour amounts up to 0.8 inches are possible over the adjacent plains and valleys further south.

Primetime: 12PM to 12AM

SPM 09-26-2017: Precipitation Coverage Decreased, But Clouds Persisted For Some

Issue Date: Tuesday, September 26, 2017
Issue Time: 11AM MDT

Summary:

The past weekend’s storm system slowly departed from Colorado on Monday, and precipitation coverage and intensity took a big downward step. Nonetheless, scattered showers and weak thunderstorms rumbled in two separate areas of the state: one area in the Front Range and Palmer Ridge and a separate area in the Southeast Plains. With limited instability, rainfall rates were subdued, especially compared to Saturday and Sunday. Highest 24-hour totals of 0.5 – 0.75 inches were reported in El Paso and Baca counties, falling over a 2-3 hour period. In other regions, most totals were limited to 0.25 inches or less.

For areas west of the Continental Divide, plenty of sunshine resulted in a boost in temperatures to just below seasonal normal. However, for areas to the east, a pesky low cloud deck insulated the cold air near the surface. High temperatures continued to run up to 20F below normal on Monday.

With the storm system moving away, we can now assess the total precipitation over its roughly 72 hour duration (see map below). Widespread amounts exceeding 1 inch were reported across the eastern third of the state. Of particular note were areas in southeast Colorado that received over 2 inches (with local maximums up to 4 inches). A separate maximum was observed in the Front Range where up to 2 inches of rainfall occurred in Larimer and Weld counties. Finally, another area of enhanced amounts was over the Central Mountains and Grand Valley where between 1 and 1.5 inches of precipitation (both rain and snow) fell.

Flooding was not reported on Monday. 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 09-26-2017: Lull In Between Systems

Issue Date: Tuesday, September 26, 2017
Issue Time: 10:40AM MDT

— Flooding is NOT expected today

As shown in the water vapor image, below, Colorado currently lies between two strong disturbances embedded in the larger-scale western North American upper-level trough. Over the northern Great Plains, we can see the current position of the system that affected our weather over the past 72 hours. That system is now a memory, though its footprint still remains across eastern Colorado in the form of a patchy low-level cloud deck. With subsiding flow in its wake, Precipitable Water (PW) values are bottoming out, with readings of only 0.47 and 0.30 inches at Denver and Grand Junction, respectively. However, just to our east, Dodge City, KS, has a PW of 1.00 inch, implying that this moisture has just barely been pushed east of our state.

Looking farther southwest, we see the makings of the next active weather producer. It is currently a shortwave with an axis roughly along the UT/NV border. As this feature “digs” south-southeast over the next 24 hours, it will cut-off from the main flow: a guaranteed headache for meteorologists! By the end of the day, we will begin to see some low and mid-level moisture return into southern Colorado. With only patchy clouds this morning, sunshine is expected to increase statewide (especially east of the Continental Divide). We expect some instability to be generated in the southern third of the state. Scattered showers and a few weak thunderstorms will be possible over the San Juans, San Luis Valley, Southeast Mountains and Raton Ridge. However, rainfall rates are expected to remain below flood threat intensity. Thus, flooding is not expected today.

Looking further ahead, the cut-off low will likely produce a period of very active weather for our state beginning tomorrow. Please check out the Flood Threat Outlook for a detailed overview of what we can expect, and stay tuned to daily Bulletins for updates on heavy rainfall and flooding chances. Remember that although we are in late September, the ingredients on the weather map suggest a continued possibility of heavy rainfall in the coming days.

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 Mountains, Raton Ridge, Southeast Plains, San Juan Mountains, San Luis Valley:

Patchy early low clouds, then sunshine returning by early afternoon. Isolated to scattered showers and weak thunderstorms will develop by mid-afternoon especially over higher terrain. Max 1-hour rainfall up to 0.4 inches with max 3-hour rainfall up to 0.7 inches. Activity could persist into the overnight hours, especially along the NM border. Flooding is not expected today.

Primetime: 2PM to 1AM

Front Range, Urban Corridor, Palmer Ridge, Northeast Plains, Northwest Slope, Grand Valley, Southwest Slope, Northern Mountains, Central Mountains:

Mostly sunny this morning (though with some patchy low clouds east of the Cont. Divide) and turning warmer this afternoon. An isolated shower or weak thunderstorm is possible during the afternoon hours in the southern Front Range and Central Mountains. Max 1-hour rainfall up to 0.25 inches. Flooding is not expected today.

Primetime: 2PM to 7PM

FTO 09-25-2017: Unsettled Weather Pattern Continues with a Cutoff Low Over the Desert Southwest

Issue Date: Monday, September 25, 2017
Issue Time: 3PM MDT
Valid Dates: 9/26 – 10/10

The slow moving, high-amplitude weather pattern that has influenced Colorado’s weather since late last week will continue to impact the state for the first portion of this forecast period. Today and tonight the first trough, that has driven the unsettled weather pattern over Colorado since late last week, will finally start to lift to the northeast. Another trough begins to dig south Tuesday and a closed Low forms over AZ/UT by Wednesday morning. This closed Low spins and remains nearly stationary over Utah until Thursday afternoon when it begins to lift northeast (Event #1). By Saturday morning, more westerly flow will replace the southwesterly flow aloft bringing warmer temperatures and should confine showers to the high terrains. A weak trough forms over the Pacific Northwest Saturday, and by Sunday afternoon/evening the disturbance makes its way into Colorado for Event #2. By mid-week, more zonal flow is expected, which will decrease the chances for heavy rainfall.

Currently average to below average Precipitable Water (PW) is present in Denver and Grand Junction. Beginning Tuesday, PW is forecasted to increase rapidly for locations east and west of the Continental Divide. The upturn in PW is expected to last through Sunday morning. This intensification of low level moisture occurs due to the counterclockwise rotation of the surface Low over the desert southwest. To the east of the Low (Colorado) there will be an increase in south/southeast surface winds, which will draw in moisture in from the Gulf of Mexico. The GEFS has PW values over 1 inch on Thursday over the Southeast Plains, which is 30-40% above average for this time of year. The uptick in moisture occurs in tandem with multiple shortwaves from the cutoff low. This will create an Elevated Flood Threat for Thursday and Friday. Although Event #2 occurs directly after Event #1, the atmosphere is expected to dry out quite a bit. This decreases the chances for heavy rainfall, and at this time there is No Apparent Flood Threat.

Below we describe the identified precipitation event in more detail.

Event #1: Wednesday (9/27) – Saturday (9/30)

Elevated Flood Threat with highest accumulations expected over the southern mountains and eastern plains

The high amplitude system over the western US continues to drop multiple troughs to our west. The next trough is expected to dig south Tuesday, and a cutoff Low forms over the desert southwest by Wednesday morning. This pattern is known for producing heavy rain in southern Colorado and along the Front Range. As is expected with a closed Low system, little to no movement occurs as it draws in moisture on its east side from the Gulf. Moisture is expected to increase rapidly both east and west of the Continental Divide starting Tuesday and reach its maximum Wednesday and Thursday. At the same time, multiple shortwaves and an upper level jet stream over the state are expected to increase lift. Wednesday, showers are expected to be strongest over the Central and San Juan Mountains. By Thursday showers and thunderstorms will spread to the east over the Southeast Mountains, Palmer Ridge, Raton Ridge and Southeast Plains. With high PW values, especially over the Southeast Plains, there is an Elevated Flood Threat for Thursday and Friday. Severe thunderstorms may be possible both days over the Southeast Plains should instability be able to pair with the available shear. The Low lifts to the north/northwest overnight Friday. Showers are likely again Saturday with residual moisture in the atmosphere, but accumulations will be confined to the higher terrains.

Legend

Event #2: Sunday (10/1) – Monday (10/2)

No Apparent Flood Threat as an upper trough develops over Montana and moves into the state

A second, weaker trough then develops Saturday following Event #1 over the Pacific Northwest. By Sunday afternoon, the trough moves into Colorado with the jet stream positioned over the northwest corner. Increased shortwave activity and upper level support will bring another round of showers to the state Sunday afternoon and Monday. Drier air works its way in from the desert southwest, so precipitation should be confined east of the Continental Divide. Flood is not expected at this time, but please check back to Thursday’s FTO to see if a special FTB will issued Monday.

Legend