Issue Date: Thursday, September 1, 2016
Issue Time: 10:50AM MDT

— LOW flood threat for San Juans, Central Mountains, Southeast Mountains

Major changes are on the way for Colorado’s weather, as can be seen by the water vapor image, below. A large-scale trough is entering the western coast of North America. This feature is also being supported by a strong subtropical jet streak centered over southern CA, NV and UT. This jet will slowly approach Colorado, and provide favorable outflow winds for any thunderstorm activity that develops. An associated surface low pressure system, centered over the northern Great Basin, will begin to transport higher moisture content west of the Divide. Thus, thunderstorm activity will increase today along the western slope, most notably over the climatologically favored San Juans. Farther east, a decaying disturbance has allowed a ridge axis to stretch into northeast CO from the northeast. This will provide light subsidence in the mid-levels and be generally unfavorable for storm activity. However, with the high residual moisture content in place (dewpoints > 55F, precipitable water 0.8 – 1.2 inches), a rogue storm cannot be ruled out. Today, we have a Low flood threat out for the southern parts of the state where dynamics and moisture will be juxtaposed best for thunderstorm formation.

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.

Issue Date: Wednesday, August 31st, 2016
Issue Time: 10:45AM MDT

— MODERATE flood threat for Palmer Ridge, Southeast Mountains
— LOW flood threat for Front Range, Southeast Plains, Raton Ridge

The morning weather map, see water vapor image below, shows many similar features as yesterday. Despite the presence of a large-scale ridge over the Rocky Mountain states, a disturbance remains across Colorado, seemingly lost from the main steering winds. This disturbance has been slowly weakening, as measured by 500 mb temperatures rising by several degrees every day including today. However, as we saw yesterday, it is still causing generally favorable conditions for scattered to widespread thunderstorm activity mainly east of the Continental Divide. We expect much more of the same today, though with an overall southward shift in the region of stronger storms. Early storms will form across the higher terrain as well as parts of the Northeast and Southeast Plains. These will be garden variety cells, likely incapable of flood-worthy rainfall. However, later in the afternoon, more organized storms are expected across mainly southeast CO. Isolated flash flooding will be possible with these cells, warranted a Low flood threat. A small area of the western Palmer Ridge and Wet Mountains is under a Moderate flood threat due to favorable upslope winds during the late afternoon and early evening hours. Hail up to 1.25 inches will be possible with the earlier storms especially farther north as dry air aloft slowly works its way into the area.

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.

Issue Date: Tuesday, August 30th, 2016
Issue Time: 10:45AM MDT

— MODERATE flood threat for Northeast Plains, Palmer Ridge, Urban Corridor
— LOW flood threat for Front Range, Southeast Plains, Southeast Mountains, Raton Ridge

The morning water vapor image, shown below, shows the key factors in a somewhat challenging forecast today. While most of the western US is under the influence of a weak upper-level ridge, the disturbance that has affected Colorado for the past 48 hours remains centered over the western part of the state. Despite slowly losing strength today, it will continue to be a factor by favoring weak upward motion east of the Continental Divide. Low-level moisture has increased over the eastern part of the state with precipitable water values running in the 0.9 – 1.2 inch range. High surface dewpoints, up to 63F, can be found this morning along the KS border. Sunshine is in abundance this morning for almost all. Collectively, it will not be a problem to generate scattered to widespread thunderstorms capable of heavy rainfall for many areas east of the Divide, especially in the Northeast Plains. A Low flood threat has been posted to account for this. The more uncertain aspect is centered around a plume of very moist low-level air that is forecasted to move westward into CO a few hours before sunset. With a relatively deep easterly flow, about 10,000 feet high, this could spark off an additional round of slow-moving storms capable of very heavy rainfall rates. Although this is not guaranteed, the severity of the rainfall rates warrants a Moderate flood threat for parts of northeast CO. High antecedent rainfall will also be a contributing factor for western areas of the Moderate threat region.

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.

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.

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.

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