FTO 08-29-2016: Roller Coaster Early, Coasting Late

Issue Date: 8/29/2016
Issue Time: 2:10 PM

Threat_Timeline_20160829

This 15-day period will open up active, with two elevated flood threats in the first 8 days, before quieting down with no events expected for days 9 through 15. If you take a look at the water vapor image below, the first thing you may notice is how it looks like a roller coaster; an apt description of this FTO period. Event #1 is a blend of a weakening/exiting upper-level low (black line), and the southwest flow aloft that will take shape behind the ridging (easternmost blue-dashed line) forecast to take its place. Plenty of moisture will remain behind the exiting upper-level low to bridge the gap between it and the return of southwesterly flow aloft (subtropical moisture). This moisture will keep generally unsettled weather in the forecast, but not for the same areas each day. With the transitioning upper-level pattern, different areas of the state will be favored on different days – so be sure and check back with the daily FTB forecasts.

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Between Event #1 and Event #2, the upper-ridge will win out, keeping the state mostly dry during those two days. A few showers/thunderstorms will still be able to develop, mainly over/near the mountains, working off of residual moisture and daytime heating. Otherwise, mostly sunny skies and generally dry conditions are expected. Event #2 refers to the next upper-level trough in the roller coaster sequence, marked by the purple line and “#2” in the water vapor image. The upper trough will be hanging around the west coast through the week and into the weekend, finally sliding across the Rockies on Monday and Tuesday (9/5 – 9/6). At this time, it appears that Gulf moisture will be transported into eastern Colorado ahead of the trough passage, setting the stage for showers/thunderstorms capable of heavy rainfall. Upslope flow in the low-levels, combined with favorable dynamics aloft, will require close watching of this event, especially for the Front Range/Southeast Mountains and adjacent lower elevations.

After Event #2, stronger ridging is expected to build across the region, bringing a much drier second half of the period. Isolated mountain showers/weak thunderstorms more capable of gusty winds than rain will be the norm of any activity, while the majority of the state dries out and warms up. We’ll keep watching this time frame for any smaller scale disturbances that may upset the dry period. Stay tuned to Thursday’s FTO.

Event #1: Tuesday (08-30-2016) through Friday (09-02-2016)

Elevated Flood Threat as Moisture Remains Through Transition

Event #1 is a blend of a weakening/exiting upper-level low and the return of southwest flow aloft. Plenty of moisture will be retained across Colorado, keeping generally unsettled weather in the forecast, but not for the same areas each day. With the transitioning upper-level pattern, different areas of the state will be favored on different days – so be sure and check back with the daily FTB forecasts.

Legend

Event #2: Monday (09-05-2016) and Tuesday (09-06-2016)

Elevated Flood Threat as Upper-Level Trough Passes through the Rockies

The upper-level trough will begin shifting eastward late in the weekend, bringing its broad-scale support for showers/thunderstorms to Colorado for Monday and Tuesday. Meanwhile, east-southeasterly low-level flow will transport Gulf moisture into eastern Colorado, providing fuel for heavy rainfall. As stated above, upslope flow combined with favorable dynamics aloft will require close watching of this event. Stay tuned.

Legend

FTB 08-29-2016: Upper-Level Low Will Lift Slowly Across Colorado

Issue Date: 8/29/2016
Issue Time: 10:09 AM

A MODERATE FLOOD THREAT IS FORECAST FOR PORTIONS OF THE URBAN CORRIDOR, FRONT RANGE, PALMER RIDGE, SOUTHEAST PLAINS, SOUTHEAST MOUNTAINS, AND RATON RIDGE.

A LOW FLOOD THREAT IS FORECAST FOR SURROUNDING AREAS AND PORTIONS OF THE NORTHEAST PLAINS, CENTRAL MOUNTAINS, SAN JUAN MOUNTAINS, AND SOUTHWEST SLOPE.

The upper-level low (shown in the water vapor image below) that was discussed in yesterday’s forecast is not in a hurry to move, with its circulation center located across northwestern NM/southwestern CO. The low will lift slowly towards the northeast through this forecast period, providing favorable dynamics for the development of scattered showers/thunderstorms along/east of the Continental Divide. The best coverage of storms will be over/near the higher terrain south of I-70, and along the preferred terrain of the Palmer Ridge and Raton Ridge. Instability values are fairly low and wind shear is weak, but favorable dynamics aloft will overcome these limiting factors to produce a few stronger thunderstorms capable of heavy rainfall. Exacerbating any flooding concerns are saturated soils from previous days’ rainfall, and this is accounted for in the moderate flood threat area. Urban areas and burn scars, such as the Waldo Canyon and Hayden Pass fire burn scars, will need to be monitored closely.

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To the west, daytime heating and orographic effects will be relied upon to produce scattered activity, as the best dynamics will be situated to the east. Cloud cover will be a limiting factor on the amount and intensity of any showers/thunderstorms that develop, due to daytime heating being of primary importance. In general, expect isolated coverage for northern regions, with more scattered coverage across central and southern regions. For more information regarding timing and rain rates, please see the zone-specific forecast 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.
FTB_20160830

Flood Threat Legend

Zone-Specific Forecasts

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

Scattered-to-numerous showers and thunderstorms are expected along/near the higher terrain south of I-70, and along the preferred terrain of the Palmer Ridge and Raton Ridge. Elsewhere, more isolated-to-scattered coverage is expected. Heavy rainfall is expected within a few stronger thunderstorms, and flash flooding concerns are exacerbated due to previous days’ rainfall (especially last night’s heavy rain). Maximum rain rates are as follows:

Front Range and Southeast Mountains: 1.0-1.5 inches/hour
Urban Corridor: 1.2-1.6 inches/hour
Southeast Plains and Northeast Plains: 2.0-2.5 inches/hour
Palmer Ridge: 1.5-2.0 inches/hour

Timing: 11 AM – Midnight, with a few showers/thunderstorms lingering into the morning hours

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

Isolated-to-scattered showers/thunderstorms expected, with the best coverage across the Central Mountains, Southwest Slope, and San Juan Mountains. Instability will be fairly meager, as will wind shear, so storm organization/maintenance will be a limiting factor. Most activity will be garden variety, with only a couple isolated thunderstorms capable of brief heavy rainfall. Maximum rain rates are as follows:

Central Mountains, Southwest Slope, and San Juan Mountains: 0.8-1.1 inches/hour
Grand Valley: 0.4-0.6 inches/hour
San Luis Valley: 0.5-0.8 inches/hour
Northwest Slope and Northern Mountains: 0.2-0.5 inches/hour

Timing: 11 AM – Midnight

STP 08-29-2016: Stronger Thunderstorms Produced Bouts with Heavy Rain

Issue Date: Monday, August 29th, 2016
Issue Time: 9:00 AM MDT

Summary:

As the upper-level low tracked across the 4-corners region and into northern New Mexico/southern CO, favorable dynamics aloft overspread Colorado, and combined with sufficient moisture/instability to produce isolated-to-scattered showers/thunderstorms through the afternoon and early evening hours. As the evening turned to nighttime, activity diminished across the western slope, but picked up along the Front Range Urban Corridor and adjacent plains. A few stronger thunderstorms produced heavy rain, likely resulting in minor street/field flooding and ponding.

Heavy rain observations (greater than 1 inch) reported to local National Weather Service offices include:

5 miles S of Lindon (Washington): 2.31 inches
8 miles NW of Pinon (Pueblo): 1.49 inches
6 miles SSE of Air Force Academy (El Paso): 1.40 inches
6 miles NW of Peterson AFB (El Paso): 1.10 inches
2 miles NNE of Colorado Springs (El Paso): 1.06 inches
5 miles SSE of Air Force Academy (El Paso): 1.03 inches

For a statewide look at 24-hour precipitation totals, please take a look at the Storm Total Precipitation map below.

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Storm Total Precip Legend

FTB 08-28-2016: Upper-Level Low Brings Another Unsettled Period

Issue Date: 8/28/2016
Issue Time: 10:15 AM

A LOW FLOOD THREAT IS FORECAST TODAY FOR PORTIONS OF THE NORTHWEST SLOPE, NORTHERN MOUNTAINS, CENTRAL MOUNTAINS, GRAND VALLEY, SOUTHWEST SLOPE, SAN JUAN MOUNTAINS, FRONT RANGE, SOUTHEAST MOUNTAINS, URBAN CORRIDOR, PALMER RIDGE, RATON RIDGE, NORTHEAST PLAINS, AND SOUTHEAST PLAINS.

Another unsettled period is expected as an upper-level low shifts eastward out of the 4-corners region and into northern New Mexico. This will provide favorable dynamics for the production of isolated-to-scattered showers/storms across much of Colorado. Much like yesterday, the higher terrain and western slope will see the best coverage, while areas east of the mountains remain in the isolated-to-widely scattered category. A slight increase in moisture (shown well in the chart below), as well as an uptick in instability, will provide a bit more fuel for today’s thunderstorms, keeping the threat of heavy rain in the forecast for areas south of I-70. Even so, the flood threat remains on the low end of the scale, thanks to weak wind shear keeping storm organization/maintenance limited. Most activity will come to an end around 10-11 PM, with a few showers/storms continuing into the overnight and early morning hours thanks to favorable dynamics aloft.

IPW_20160828

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.
20160828_LFT

Flood Threat Legend

Zone-Specific Forecasts

Northeast Plains, Urban Corridor, Palmer Ridge, Southeast Plains, and Raton Ridge:

Isolated-to-widely scattered showers/thunderstorms expected, with the best coverage near the mountains, along the preferred terrain of the Palmer Ridge and Raton Ridge, and across the far Southeast Plains. Most will be garden variety, but a couple will become strong-to-marginally severe, producing bouts with heavy rain, small hail, and strong winds. Maximum rain rates are as follows:

Northeast Plains: 1.5-2.0 inches/hour
Southeast Plains: 1.8-2.5 inches/hour
Urban Corridor, Palmer Ridge, and Raton Ridge: 0.8-1.2 inches/hour

Timing: 1 PM – 11 PM, with a few lingering showers/storms into the early morning hours

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

Scattered showers and thunderstorms are expected, with most being garden variety. Better daytime heating today, as compared to yesterday, will bring about a couple more thunderstorms capable of locally heavy rainfall. Maximum rain rates will be:

Northwest Slope, Northern Mountains, and Central Mountains: 0.6-1.0 inches/hour
Grand Valley, Southwest Slope, San Juan Mountains, San Luis Valley, Front Range, and Southeast Mountains: 0.8-1.2 inches/hour

Timing: 11 AM – 9 PM, with a few lingering showers/weak thunderstorms into the early morning hours