Issue Date: 8/29/2015
Issue Time: 9:35 AM

NO FLOOD THREAT IS FORECAST.

A great Saturday is in store for Colorado as most locations will stay mostly sunny and dry thanks to an upper-level ridge over the region. The ridge axis will shift east, so locations along and east of the I-25 corridor will see temperatures climb a few degrees over yesterday, and temperatures to the west will be fairly similar, perhaps a degree or two warmer, to yesterday’s readings. A few isolated thunderstorms over the mountains, mainly along and south of I-70, will be all the atmosphere can muster in this overall environment. Heavy rainfall and flash flooding is not expected with precipitable water values well below 1 inch at all four normal reporting stations (Boulder, Grand Junction, Pueblo, and Shriever AFB). For more details, including rain rates and timing, be sure and check out 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

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

Sunny and warm, with highs ranging from the 80s to the 90s. Steering winds will be northerly again today, so mountain thunderstorms will remain mountain thunderstorms and no threat to adjacent lower elevations. A few more clouds will be present near the mountains compared to the abundant sunshine further east.

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

Mostly sunny becoming partly sunny during the afternoon as daytime heating and terrain play on the available moisture. Isolated-to-widely scattered thunderstorms will develop during the afternoon and evening hours, roughly along and south of I-70. The best coverage will occur across the Central Mountains and San Juan Mountains. The main threats will be gusty winds and lightning, with brief moderate rainfall. Rain rates will generally be 0.25-0.5 inches/hour, maximizing around 0.4-0.8 inches/hour.

Timing: Noon – 9 PM

Issue Date: 8/28/2015
Issue Time: 9:20 AM

NO FLOOD THREAT IS FORECAST.

Following behind yesterday’s disturbance/trough is a high pressure ridge that will build across the southwestern US today. The water vapor imagery below depicts the situation well, and I have annotated the image for clarity. With the building ridge will come decreasing moisture, though there will be enough remaining moisture for isolated thunderstorms mainly over the Continental Divide, San Juan Mountains, Southeast Mountains, and Front Range mountains/foothills. With northerly flow aloft expected, the adjacent Urban Corridor and western portions of the Southeast Plains should stay dry as storm motions will keep them from moving overhead.

Further east over the Northeast Plains, Southeast Plains, Palmer Ridge, and Raton Ridge, a mostly sunny and dry day is expected, with temperatures climbing slightly above average for this date. An isolated thunderstorm may develop over the Cheyenne Ridge and Raton Ridge during the late afternoon/evening hours, but probability of occurrence is ~10%

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

Isolated showers and thunderstorms are expected, mainly over the Continental Divide, San Juan Mountains, Southeast Mountains, and Front Range mountains/foothills. With moisture decreasing, heavy rainfall is not expected, nor is flash flooding a threat. Maximum rain rates will be 0.5-0.9 inches/hour, with most rain rates a more modest 0.2-0.5 inches/hour.

Timing: Noon – 9 PM

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

Mostly sunny and dry for most areas, with only a slight chance (~10%) for a thunderstorm to develop over the Cheyenne Ridge and Raton Ridge. With northerly flow aloft, it will be very difficult for the Urban Corridor/western extents of the Southeast Plains to experience the common “thunderstorms drifting off the mountains.” A bit more cloud cover than other areas to the east will be about all the environment can muster.

Timing: 2 – 8 PM

Issue Date: 8/27/2015
Issue Time: 9:05 AM

NO FLOOD THREAT IS FORECAST.

The IPW graph below certainly shows that there is plenty of moisture available to showers/thunderstorms. Grand Junction, after peaking above 1.35 inches yesterday evening, has come back down slightly to just over 1.2 inches. As the monsoonal disturbance has rotated across Colorado, IPW has climbed at the other three of the normal reporting stations (Boulder, Pueblo, and Shriever AFB), as well. Moisture is definitely out there.

Fortunately, that amount of water is not coupled with proper timing of the mid-level trough currently moving across the Central/Northern Rockies. The trough axis sits roughly along the Continental Divide at this time, with the best forcing for showers/thunderstorms already across Eastern Colorado. The trough/associated forcing will continue to push east, targeting central NE/KS and points further east for stronger thunderstorms this afternoon/evening. Behind that trough is a weakly subsident airmass, designed to try and suppress strong convection. This will limit what storms can do along and north of I-70, with a more neutral environment south of I-70.

The best opportunity for storms will be over the higher terrain, favoring the Continental Divide and mountains to the east, near the mountains along the Urban Corridor and western extents of the Northeast/Southeast Plains, and wherever sunshine can produce enough daytime heating to break the cap across the Palmer Ridge, Northeast Plains, Southeast Plains, and Raton Ridge. Rain rates are not expected to be great enough to warrant a flood threat, but street/field ponding are expected under isolated, stronger thunderstorms. For more information on timing and rain rates, please jump below the map for zone-specific discussions.

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

Early morning showers and thunderstorms continue to wind down across the area at this time. By this afternoon, expect coverage to uptick again, with the main focus of scattered showers/thunderstorms along and east of the Continental Divide. There will be isolated showers/thunderstorms elsewhere with partly sunny skies. Instability will be lacking, which will limit rain rates, thus no flood threat is warranted. Maximum rain rates will be 0.3-0.7 inches/hour for locations west of the Continental Divide, and 0.6-0.9 inches/hour along and east of the Continental Divide.

Timing: 11 AM – 9 PM, with a few weak showers lingering into the overnight hours.

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

Isolated-to-scattered showers and thunderstorms expected, with the best coverage along the Urban Corridor, Palmer Ridge, and western extents of the Northeast and Southeast Plains. Main storm threats will be lightning and gusty winds, but locally moderate-to-heavy rainfall will be attend stronger storms. Under the stronger storms, street/field ponding will likely occur, but nothing to warrant a flood threat, as storm motions will limit the impact. Maximum rain rates are as follows:

Urban Corridor and Palmer Ridge: 0.7-1.4 inches/hour
Northeast Plains and Southeast Plains: 0.8-1.2 inches/hour
Raton Ridge: 0.4-0.8 inches/hour

Timing: 2 PM – Midnight, with a few lingering showers/thunderstorms over the Southeast Plains into the early morning hours.

Issue Date: 8/26/2015
Issue Time: 9:10 AM

A MODERATE FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SAN JUAN MOUNTAINS, SOUTHWEST SLOPE, GRAND VALLEY, NORTHWEST SLOPE, AND NORTHERN MOUNTAINS.

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SAN JUAN MOUNTAINS, SOUTHWEST SLOPE, GRAND VALLEY, CENTRAL MOUNTAINS, FRONT RANGE, NORTHERN MOUNTAINS, AND NORTHWEST SLOPE.

An increase in shower/thunderstorm coverage today is expected over the mountains as a monsoon disturbance rotates across Colorado from the west/southwest. Ahead of this disturbance, moisture has increased fairly significantly over western Colorado, with IPW readings greater than 1 inch at Grand Junction. Widespread showers and thunderstorms are expected over the High Country and adjacent low valleys, with areas east of the mountains looking to remain mainly dry and hot today.

With the increase in moisture and support from the monsoonal disturbance, showers and thunderstorms will produce rainfall fairly efficiently today, resulting in locally moderate-to-heavy rainfall. Instability will not be great due to extensive cloud cover and storm motions will be decent; both of which will help to limit the flood threat somewhat.

Along the Urban Corridor, as well as western portions of the Northeast Plains, Southeast Plains, Raton Ridge, and Palmer Ridge, gusty outflow winds from mountain convection will be the main impact today, but a very low chance of isolated thunderstorms will exist during the afternoon and evening hours. Further east, mostly sunny skies and hot temperatures will be the name of the game.

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, and Southeast Mountains:

Isolated-to-scattered showers and thunderstorms expected, though moisture will be slightly less than the deep moisture found over points to the west. Brief, localized, moderate rainfall is possible, with rain rates breaking down like this:

Front Range: 0.5-1.0 inches/hour
Southeast Mountains: 0.3-0.6 inches/hour

Timing: Noon – 10 PM, with a few showers/weak thunderstorms lingering into the early morning hours tomorrow

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

Aside from low chances of an isolated storm or two across the Urban Corridor, and western portions of the other regions, these zones will stay mostly sunny and hot today. The mixing out of moisture will keep rain rates low, with maximum rates of 0.25-0.6 inches/hour from any thunderstorms.

Timing: 2 PM – 8 PM, with a shower/weak thunderstorm or two during the early morning hours as the disturbance rotates across eastern Colorado.

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

Widespread showers and thunderstorms expected today and tonight, with locally heavy rainfall possible. Widespread cloud cover will keep temperatures much cooler than yesterday. Maximum rain rates will be 1.0-1.5 inches/hour, 1.8-2.2 inches/3 hours. The best chances of realizing these rates will be over the San Juan Mountains, Southwest Slope, Grand Valley, Northwest Slope, and Northern Mountains regions.

Timing: 11 AM – 11 PM will be the prime time, but scattered showers/thunderstorms will continue into tomorrow morning. For that reason, the flash flood threat areas will be valid through 11 AM tomorrow.