FTB 05-26-2017: Moderate Flood Threat Today, Accompanied By Severe Weather

Issue Date: Friday, May 26th, 2017
Issue Time: 10:30AM MDT

— MODERATE flood threat for Urban Corridor, Palmer Ridge and Northeast Plains
— LOW flood threat for Southeast Plains

As shown in the water vapor image, below, a strong disturbance was located over the northern Great Basin this morning. As the moves east-southeast today, it will aid the in the formation of a surface low in the lee side of the Colorado Rockies. Impressive pressure drops up to 8-12 millibars are expected by late afternoon. This surface circulation will advect higher moisture values into eastern Colorado and generate instability exceeding 1,000 J/kg. With upper-level forcing from the incoming disturbance and a jet streak located to the north, expect at least one round of showers and thunderstorms to develop east of the Continental Divide. Closer to the mountains, limited instability will keep rainfall rates below flood threat levels. Farther east, heavier rainfall will be possible and a Moderate flood threat has been posted for parts of east-central Colorado. A Low flood threat is posted for areas bordering the Moderate threat area for more isolated heavy rainfall chances. Isolated flash flooding is the main impact today, with field/road flooding likely under the strongest storms.

In addition to heavy rainfall, large hail (up to 1.5 inches in the Low threat region, but up to 2.5 inches towards the KS border) will be possible along with strong winds up to 75mph and a threat of an isolated tornado.

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:

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

Mostly clear this morning then increasing clouds with scattered showers and thunderstorms developing by early afternoon. Early activity will start in the higher elevations, then transition eastward into the plains. Max 1-hour rainfall up to 1.6 inches except up to 2.4 inches in farthest east areas. Max 3-hr rainfall up to 2.9 inches. Hail up to 2.5 inches and gusty winds up to 75 mph will be possible with the strongest storms, along with the threat of an isolated tornado. Isolated flash flooding will be possible under the strongest storms. Field and road flooding will be likely with the heaviest downpours.

Primetime: 1PM to 9PM (across far east), with isolated showers continuing overnight

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

Partly to mostly cloudy skies with scattered showers and perhaps a weak storm this afternoon. Weak thunderstorms are possible in the afternoon. Max 1-hour rainfall up to 0.4 inches. Small hail (less than 0.5 inches) will be possible. Snow level 11,000 feet early, lowering to 9,500 feet later in the day. Flooding is not expected today.

Primetime: 11AM to 8PM, with isolated showers continuing overnight

Raton Ridge, Southeast Mountains

Mostly sunny early, then increasing clouds with an isolated shower or weak thunderstorm possible in the early afternoon. Max 1-hour rain rates up to 0.4 inches. Small hail will be possible with the strongest storms. Gusty winds up to 50mph will accompany the strongest storms. Flooding is not expected today.

Primetime: 2PM to 7PM

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.

Legend

 

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.

Legend

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!

STP 05-25-2017: Much Warmer, and Mostly Dry on Wednesday

Issue Date: Thursday, May 25th, 2017
Issue Time: 10:15AM MDT

A high pressure ridge briefly setup shop across Colorado on Wednesday, providing plenty of sunshine along with some locally gusty winds. Many lower elevation locations approached the 90F mark, notably Pueblo and Grand Junction. A few light rain and snow showers moved into northwest Colorado overnight as a large disturbance approached the region. However, with a dry atmosphere only a few hundredths of an inch of precipitation was observed.

For precipitation estimates in your area, check out our Precipitation Map below. Flooding was not reported on Wednesday.


Storm Total Precip Legend

FTB 05-25-2017: Severe Weather Is The Headline, Along With Low Flood Threat

Issue Date: Thursday, May 25th, 2017
Issue Time: 10:00AM MDT

— LOW flood threat for portions of Northeast Plains

A strong disturbance moved into western North America over the past few days, and will influence our weather beginning today. The water vapor image, below, shows that the main circulation is located in southern Saskatchewan today. Draped to the southwest is a trough axis that extends into the Great Basin. As this disturbance approaches Colorado, it will cause large-scale lift and generate plenty of showers and thunderstorms. West of the Continental Divide, limited instability will keep rain rates below flood-prone intensity. East of the Continental Divide, a pocket of higher moisture exists in northeast CO today. Acting against this is a plume of very dry air being advected from the southwest by a strong subtropical jet. This will limit the heavy rainfall threat to areas along the Nebraska border. Also acting against the flood threat will be very swift storm motions, above 40mph. Nonetheless, a Low flood threat has been issued for parts of the Northeast Plains for isolated flash flooding.

Perhaps the bigger story today will be the potential for severe weather – most notably gusty winds up to 75mph and large hail, up to 1.5 inches. These hazards will occur with the strongest storms. However, overall, expect windy conditions across northern Colorado today as high jet stream speeds get mixed downward into the boundary layer courtesy of solar heating.

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:

Northeast Plains, Front Range, Urban Corridor:

Mostly clear this morning then increasing clouds with scattered showers and thunderstorms developing by early afternoon. Early activity will start in the higher elevations, then transition eastward into the plains. Max 1-hour rainfall up to 0.7 inches, except up to 1.6 inches in the Northeast Plains. Hail up to 1.5 inches and gusty winds up to 75 mph will be possible with the strongest storms. Isolated flash flooding will be possible. Field and road flooding will be likely with the heaviest downpours.

Primetime: 2PM to 9PM (across far east)

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

Partly to mostly cloudy skies with scattered showers increasing in coverage. Weak thunderstorms are possible in the afternoon. Max 1-hour rainfall up to 0.4 inches with max 24-hour rainfall up to 0.6 inches. Small hail (less than 0.5 inches) will be possible. Snow level 11,000 feet. Gusty winds up to 60mph are expected in the higher elevations today, especially along mountain passes. Flooding is not expected today.

Primetime: 11AM to 8PM

Southeast Plains, Palmer Ridge, Raton Ridge, Southeast Mountains

Mostly sunny early, then increasing clouds with a few showers and a weak thunderstorm possible in the early afternoon. Max 1-hour rain rates up to 0.4 inches. Small hail will be possible with the strongest storms. Gusty winds up to 50mph will accompany the strongest storms. Flooding is not expected today.

Primetime: 2PM to 7PM