Issue Date: Tuesday, September 12, 2017
Issue Time: 11AM MDT

— LOW flood threat for Hayden Pass burn scar

This morning’s water vapor image, below, shows a relatively persistent upper-level pattern across the central and western United States. Colorado remains under the influence of a weak ridge with troughs of low pressure to the east and west. However, change is on the way. The ridge will weaken today as the western trough begins to move eastward. But overall, atmospheric dynamics are not a strong factor today.

Without dynamics, we turn to moisture and instability analyses to provide clues about heavy rainfall potential. Mainly clear skies this morning will allow for weak to moderate instability to quickly develop over the High Country. Morning Precipitable Water (PW) readings were 0.76 and 0.84 inches in Denver and Grand Junction, respectively. Little if any moisture advection is expected today so PW is expected to maintain its values. Low-level moisture remains above average for this time of year with surface dew point temperatures in the mid-40s to mid-50s. Some of this moisture will mix out in the afternoon, limiting storm inflow and capping rainfall intensity. Another notable factor is dry mid-level air coming into the state from Wyoming. This will lower the rainfall coverage compare to yesterday.

Putting it all together, we expect isolated to scattered afternoon and evening showers/storms mainly in the southern High Country. Max rainfall is expected to stay just below flash flood thresholds. The only exception is for burn scars. In particular, precipitation chances for the Hayden Pass scar are high today with a ~30% chance of exceeding 0.5 inches per hour. Thus, a Low flood threat has been posted for this scar. Elsewhere, flooding is not expected today.

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.

Zone-Specific Forecasts:

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

Sunny early, then turning partly cloudy with isolated to scattered showers and thunderstorms this afternoon and evening. Highest coverage will be over the southern High Country and adjacent foothills. Max 1-hour rainfall up to 1.1 inches (below 6,000 feet) and 0.9 inches (above 6,000 feet). A Low flood threat has been posted for the Hayden Pass fire scar for isolated flash flooding, mud flows and debris slides. Elsewhere, flooding is not expected today.

Primetime: 1PM to 9:30PM

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

Partly cloudy early then scattered showers and thunderstorms this afternoon. Highest coverage will be over the Central Mountains and especially the San Juans. Max 1-hour rain rates up to 0.7 inches with max 24-hour totals up to 1.1 inches. Flooding is not expected today.

Primetime: 1PM to 8PM

Issue Date: Monday, September 11, 2017
Issue Time: 10:05 AM MDT

— LOW flood threat for portions of the Front Range, Urban Corridor, Palmer Ridge, Southeast Mountains, Central Mountains

The persistent weather pattern continues over the state again today. Colorado will continue to sit under the ridge with light westerly flow aloft. To the east and west are two low pressure systems creating a bit of an omega block. The low pressure system over the coast of California continues to slowly usher in moisture and mid-level energy over the state creating daily rounds of precipitation. Precipitable Water (PW) continues to rise across the state. This morning’s sounding at Denver recorded 0.87 inches and Grand Junction was measured at 0.99 inches. Moderate mid-level drying and increasing dew points mean more rain will reach the surface than the last couple of days, although gusty winds are still likely. There also seems to be some added mid-level energy and surface instability this afternoon, which will help intensify the coverage and strength of the storms that initiate over the higher terrains.

Scattered storms over the high country are expected again today and will begin early this afternoon. The most numerous coverage is expected to be over the Central Mountains and Front Range. Slow steering winds aloft will increase rain totals as storms slowly track east. This may present problems for fresh burn scars, and confidence is sufficient enough that storms that do form in this area will have rain rates greater than 0.5 inches. Elsewhere in the higher terrains, max 1-hr rain rates up to 1.1 inches are possible, so a Low flood threat has been issued. Threats include debris slides, mud flows and small stream and road flooding. As storms move off the higher terrains, they are expected to favor the Palmer Ridge where dew points are forecasted to increase throughout the day. Gusty winds are likely with the stronger storms as well as max 1-hr rain rates up to 1.2 inches. Storms are also likely to drift off the higher terrain along the Urban Corridor and subsequent outflow boundaries may continue storm development. These storms should have max 1-hr rain rates under flood threat criteria. Most storms will end around midnight with some lingering showers overnight. There is a Low flood threat today.

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.

Zone-Specific Forecasts:

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

Partly cloudy this morning this morning with partial clearing. Becoming partly cloudy again this afternoon with thunderstorm development over the mountains. Slow storm movement with increased moisture and upper-level energy has created a Low flood threat. Threats include mud flows, debris slides and small stream and road flooding. Burn scars should be monitored closely. Max 1-hr rain rates up to 1.2 inches (east) and 1.1 inches (west).  A Low flood threat has been issued.

Primetime: 1:30PM to 12AM

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

Mostly sunny this morning becoming partly cloudy this afternoon over the higher terrains. Slow storm movement and increased moisture will increase totals from the prior two days. Max 1-hr rain rates up to 0.6 inches (south) and 0.8 inches (north) are possible. Flooding is not expected today.

Primetime: 12PM to 11PM

Issue Date: Sunday, September 10, 2017
Issue Time: 10:00 AM MDT

— Flooding is NOT expected today

The upper-level trough continues to sit off the west coast this morning. With the upper-level high pressure over northern Texas, more southwest and westerly flow will advect moisture into the state today. On the water vapor imagery below, you can see the increase in moisture over the western portion of the state. Precipitable Water (PW) values this morning in Grand Junction were measured at 1.02 inches and measured at 0.81 inches in Denver. With drying mid-levels, some of this moisture will mix out, but PW will be higher than climatology and PW yesterday. This should allow more numerous storms to form over the higher terrains this afternoon.

Diurnal flow will initiate showers and thunderstorms early this afternoon. Some extra mid-level energy from the trough will help enhance the upslope flow. Storms are expected to be more numerous over the western portion of the state than the last couple of days. Scattered showers are also expected over the Front Range and Southeast Mountains. Storm motion will be similar to yesterday with storms to the west having more of a northerly component, while storms to east will have more E/SE movement as they slowly move around the ridge. As storms move off the higher terrains to the east, they will favor the Cheyenne and Palmer Ridges. Storms will be elevated again today, which means more gusty winds and dangerous lightning than heavy rain. Some small hail may also be possible. Storm totals up to 0.7 inches are possible over the flash flood prone Flat Tops and Roan Plateau region, so stronger storms should be monitored closely. Most storms will end by 11PM with a few showers lingering over the higher terrains after midnight.

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.

Zone-Specific Forecasts:

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

Sunny this morning becoming partly cloudy this afternoon with thunderstorm development over the higher terrains. As storms move east, they should are expected to favor the Cheyenne and Palmer Ridges. Gusty winds, small hail and lightning will be the main threats today. Max 1-hr rain rates up to 0.8 inches (east) and 0.7 inches (west).  Flooding is not forecasted today.

Primetime: 1PM to 11PM

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

Partly cloudy this morning with cloud cover increasing this afternoon. More numerous coverage in showers and thunderstorms this afternoon with an increase in moisture and upper-level energy. Storms are expected to drift north throughout the day. Main threats are gusty winds, small hail and lightning. Max 1-hr rain rates up to 0.7 inches are possible. Stronger storms that form over the flood prone region of the Flat Tops and Roan Plateau should be monitored closely. Flooding is not expected today.

Primetime: 12PM to 11PM

Issue Date: Saturday, September 9, 2017
Issue Time: 09:50AM MDT

— Flooding is NOT expected today

More westerly flow across the state should give us a break from the haze excluding the Northeast Plains, which is expected to have more northwesterly flow. Mostly clear across the state this morning with exception of a few high clouds over the SW corner. A small amount of low-level moisture was able to make its way into the western portion of the state, especially the Four Corner region. Precipitable Water (PW) values over Grand Junction this morning were at 0.85 inches, while in Denver they were measured at 0.66 inches. With the eastward shift of the ridge, the upper-level trough over the southern CA coast will begin to pull moisture northward into the SW corner of the state today. This flow will also help usher in mid-level energy from the CA trough. Between the increase in moisture and mid-level vorticity, shower activity is expected to be more numerous across the mountains and valleys with rain totals increasing as well.

Showers will begin to initiate over the higher terrains around 1PM. In western Colorado, storm movement will be more northerly, while east, storm movement will be to the E/SE. PW values will be under 1 inch for the majority of the state with mid-level drying expected as well. This should produce high-based showers and thunderstorms with limited rainfall and gusty winds with the stronger storms. Showers may linger overnight with the increased moisture and mid-level energy, but most will dissipate by 11PM. There is no flooding expected today.

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.

Zone-Specific Forecasts:

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

Sunny this morning becoming partly cloudy this afternoon with thunderstorm and shower coverage increasing over the higher terrains. These showers could drift into the adjacent plains with 1-hr max rain rates up to 0.8 inches further south. Over the mountains, max 1-hr rain rates could be up to 0.6 inches. Reduction in smoke is expected statewide with the exception of the NE Plains. Flooding is not forecasted today.

Primetime: 1PM to 11PM

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

Partly cloudy over the SW corner this morning with increasing clouds over the higher terrains this afternoon. Scattered showers and thunderstorms likely this afternoon over the higher elevations, which are expected to drift north into the valleys. Max 1-hr rain rates up to 0.5 inches (north) and 0.7 inches (south). Some showers may linger overnight. Flooding is not expected today.

Primetime: 1PM to 11PM