FTB 09-30-2015: FTB Season Ending with Unsettled Weather

Issue Date: 9/30/2015
Issue Time: 9:03 AM

**THIS IS THE FINAL FTB OF 2015. THANK YOU ALL FOR READING, AND WE LOOK FORWARD TO ANOTHER SUCCESSFUL SEASON IN 2016.**

NO FLOOD THREAT IS FORECAST.

In a slightly poetic end to a drought-busting season, Mother Nature is sending the FTB into the offseason with isolated-to-scattered showers/thunderstorms. Apparent in the water vapor imagery below is the west-northwest flow aloft over Colorado, with good moisture and embedded disturbances earmarked for the state. The result will be another day of unsettled weather, but no flood threat is warranted.

WV_09302015

A general lack of instability across most of the state will keep rain rates well below flood threat thresholds. The only regions that will hold the threat of bouts with moderate-to-heavy rain will be the Raton Ridge and Southeast Plains regions, namely the counties of Las Animas, Baca, Bent, Prowers, and Otero Counties. In that corner of the state, a pocket of instability will coincide with favorable low-level moisture, weak surface convergence, and a slight upsloping wind across the Raton Mesa to produce isolated-to-widely scattered thunderstorms. A couple will become strong-to-severe, with the main threats being wind gusts of 40-60 mph, hail 0.75-1.5 inches in diameter, and brief periods with moderate-to-heavy rain. For more information on rain rates and timing, be sure and check out the zone-specific discussions below.

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, jump below the map.

Flood Threat Legend

Zone-Specific Forecasts

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

Isolated-to-scattered showers/weak thunderstorms are expected today, favoring the higher terrain. Steering winds will help push showers over adjacent valleys during the afternoon and evening hours. Rain rates will generally be low, with most showers producing less than 0.25 inches/hour. The exception will be over the San Juan Mountains, San Luis Valley, and Southeast Mountains, were maximum rain rates will be 0.4-0.75 inches/hour.

Timing: Currently, a few isolated showers are ongoing across the Northern Mountains, Northwest Slope, and Central Mountains regions. Overall, the timing of today’s activity will be 11 AM – 8 PM, with a few showers lingering until 10 PM.

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

Isolated showers/weak thunderstorms expected, with the best coverage over the Front Range, Southeast Mountains, and Palmer Ridge regions. Much of the Urban Corridor and Northeast Plains north of I-70 will be too stable today to support precipitation. Rain rates will be low, with most activity producing 0.15-0.3 inches/hour. Across the Palmer Ridge, a thunderstorm will hold the potential to produce 0.4-0.7 inches/hour rain rates.

Timing: Noon – 10 PM

Southeast Plains and Raton Ridge:

Isolated-to-widely scattered thunderstorms expected, with a couple becoming strong-to-severe. Damaging wind gusts (40-60 mph), hail (0.75-1.0 inches in diameter), and periods of moderate-to-heavy rain are the main threats. Maximum rain rates will be 1.2-1.6 inches/hour, but storm motions will mitigate the heavy rain threat to any one location.

Timing: 2 PM – 11 PM

STP 09-30-2015: Moisture and a Corridor of Instability Produced “End of FTB Season” Fireworks

Issue Date: Wednesday, September 30th, 2015
Issue Time: 9:00 AM MDT

Summary:

In the wake of Monday’s cool front, moisture was pooled across eastern Colorado, from the plains and back into the Front Range/Southeast Mountains. After the morning clouds cleared along and near the higher terrain, daytime heating was able to develop a corridor of instability along the Front Range, Southeast Mountains, and Urban Corridor. These two ingredients, plus a little help from a weak upper-level disturbance, promoted the development of isolated thunderstorms (a couple were severe). Severe storm reports from the Boulder and Pueblo National Weather Service offices are as follows:

1.0 inch hail: 1 mile W of Centennial (Arapahoe), 1 mile W of Lakewood (Jefferson), 9 miles N of Golden (Jefferson), 1 mile ENE of Florence (Fremont)
1.25 inch hail: 1 mile ENE of Highlands Ranch (Douglas)
1.50 inch hail: 2 miles WNW of Lakewood (Jefferson)

No flash flooding was reported/observed yesterday. Please see the STP map below for a look at precipitation totals from the last 24 hours.


Storm Total Precip Legend

FTB 09-29-2015: Another Day of Unsettled Weather

Issue Date: 9/29/2015
Issue Time: 9:00 AM

NO FLOOD THREAT IS FORECAST.

In the wake of yesterday’s cool front, plenty of low-level moisture has been transported into eastern Colorado with widespread dewpoints in the 40’s and 50’s from along the Front Range/Southeast Mountains and eastward. Meanwhile, in the mid-/upper-levels, good moisture is streaming across the state from the west. Combined, these moisture sources have led to climbing IPW values as noted in the chart below. This amount of moisture raises some concerns about heavy rain, but other ingredients tell the story as to why there is no flood threat being issued today.

IPW_09292015

First of all, the timing of the weak shortwave disturbance is not ideal, as the best lift is already over eastern Colorado (associated with the group of showers/thunderstorms ongoing across the Northeast Plains) and continuing to move east. Second, the amount of instability that will be present coincident with good moisture will be marginal, at best. The best corridor of instability will be along the Front Range/Southeast Mountains and foothills, where higher terrain offers some help. Extensive cloud cover is in place across the Front Range, and to the east, at this time, and will need to clear out in order for sunshine to do its job. This will be a tough sell. Further to the south along and near the Southeast Mountains, more sunshine will be present, making it more likely to see the instability come to fruition. Without good upper-level support, though, only quick pulses of strong-to-marginally severe storms will be possible, and not for a long enough timeframe to create flooding issues. Street/field ponding in poor drainage areas will be the most likely result from any bouts with moderate-to-heavy rainfall, so no flood threat is warranted.

To the west of the Front Range/Southeast Mountains, the higher terrain will see isolated-to-widely scattered showers/weak thunderstorms develop as sunshine/terrain circulations work on residual moisture. No heavy rain is expected. For more details, namely regarding timing and rain rates, please see the zone-specific discussions below.

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, jump below the map.

Flood Threat Legend

Zone-Specific Forecasts

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

Isolated-to-widely scattered showers/weak thunderstorms are expected across the higher terrain, with high temperatures still above normal for the date. Lower valleys will have a hard time seeing any rainfall as storms will likely stay anchored to the preferred terrain. Rain rates will be less than 0.25 inches/hour.

Timing: Noon – 8 PM. A couple of showers will linger into the nighttime hours.

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

Currently, a complex of showers/weak thunderstorms is working across portions of the Northeast Plains, Palmer Ridge, and northeastern extents of the Southeast Plains. This will move eastward and out of the state by later this morning.

For the rest of today, isolated-to-widely scattered showers and thunderstorms are expected, with the best coverage along and near the Front Range/Southeast Mountains where the corridor of instability will reside. Storms will spread to the south and east through the afternoon and evening, but will fade in intensity the further east they travel. Maximum rain rates will break down like this:

Front Range and Southeast Mountains: 0.6-0.8 inches/hour
Urban Corridor and Palmer Ridge: 0.6-1.0 inches/hour
Raton Ridge and Southeast Plains: 0.7-1.1 inches/hour
Northeast Plains: 0.4-0.7 inches/hour

Timing: 11 AM – 9 PM, with a few lingering showers/weak thunderstorms until midnight.

STP 09-29-2015: Welcomed Rain Re-Appeared Across Colorado

Issue Date: Tuesday, September 29th, 2015
Issue Time: 9:00 AM MDT

Summary:

Behind the front that passed through eastern Colorado yesterday existed a cooler, moister air mass, as upslope flow transported moisture into the region. At the mid- and upper-levels, moisture streamed in steadily from the west. Together, the two moisture sources pushed IPW values above average for the date, between 0.8 and 1.0 inches at the four normal reporting stations (Grand Junction, Pueblo, Shriever AFB, and Boulder). Sprinkle in a shortwave and marginal instability, and we had a recipe for isolated-to-scattered showers and weak thunderstorms, with a few ongoing at this time.

Rain rates were not particularly impressive, so no flash flooding was reported/observed yesterday. Please see the STP map below for a look at precipitation totals from the last 24 hours.

20150929_STPImage
Storm Total Precip Legend