Issue Date: Friday, September 7th, 2018
Issue Time: 08:35AM MDT

— Flooding is NOT expected today

Finally expecting some movement with the upper-level trough. Throughout today and early tomorrow the trough will migrate eastwards across Colorado. Behind the trough, drier, stable air will start to work its way into the state as seen with the clearer skies over western CO and Utah. This morning there is some dense fog over the eastern plains and northern Urban Corridor with added cloud cover over the Southeast Plains. Expecting the fog to last until about 9AM this morning. Cloud cover will also begin to burn off with the sunrise, but between the two, cooler high temperatures and minimal instability is forecast over eastern Colorado today. A few breaks in the clouds near the Palmer Ridge is likely as drier air moves in from the east. This may allow for some instability to build for a few showers and weaker storms this afternoon.

A bit more instability will be able to build over San Juan Mountains, Front Range and Southeast Mountains and mix with some minimal low-level moisture for some isolated afternoon showers. The 12Z sounding at Denver shows quite the dry layer at 700mb, so the threat for heavy rainfall will decrease when compared to the last couple of days. This is especially true as the drier air will start to move into the area today. This also means storms will be much more isolated in nature. Also forecasting stronger steering winds (20mph from the north), so this will keep the storms moving and prevent too much rainfall over one area. Over the Southeast Mountains, moisture will be slower to mix out behind the trough, so the higher rainfall amounts are expected in this area along the CO/NM border. Over western Colorado, PW values have already dropped to 0.66 inches at Grand Junction. This shows the trough has passed through and a drier, more stable air mass is beginning to establish itself. A little better moisture remains over the San Juan Mountains, so some isolated storms will likely form this afternoon. With the drier air mass and decent steering winds, flooding is not likely today even over recent burn areas.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

Zone-Specific Forecasts:

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

Moisture will start to decrease this afternoon as the trough passes through and a more stable, dry air mass moves into these regions. So not expecting any heavy rainfall today. There is a possibility for some breaks in cloud cover that would allow some instability to build over the Palmer Ridge this afternoon. Low-level convergence may initiate some showers and weak storms over this area. Max 1-hr rain rates up to 0.6 inches are possible. More isolated storms are possible over the Front Range and Southeast Mountains/Raton Ridge. With north to south movement, not expecting storms to track into the adjacent plains. Max 1-hr rain rates up to 0.2 inches (north) and 0.6 inches (south) are possible. Flooding is not expected today.

Primetime: 2PM – 9PM

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

The trough has passed through, which will allow a more stable and dry air mass to fill in behind it. Moisture will be slower to mix out over the San Juan Mountains, so expecting some rainfall, and the highest totals, to be over the southern-most, east-west oriented ridges today. Cloud cover will still increase over the Central and Northern Mountains this afternoon, though measureable rainfall is not likely. Gusty outflow winds are also possible. Swift steering winds and drier air should keep rainfall rates below the flood threat criteria for Friday.

Primetime: 1PM – 8PM

Issue Date: Thursday, September 6th, 2018
Issue Time: 09:30AM MDT

— A LOW flood threat has been issued for portions of the Front Range, Urban Corridor, Palmer Ridge, Southeast Mountains, Southeast Plains, Raton Ridge

— A MODERATE flood threat has been issued for the Spring Creek, Hayden Pass and Junkins burn areas

Upper-level clouds are blanketing the majority of the state with some light rain and fog mixed in to start the morning. Mostly clear skies over the Southwest Slope with some breaks in the cloud cover over western Colorado as some drier air has been entrained from the west. Should be a similar pattern to yesterday with a few minor differences. Current temperatures are in the 50Fs over the lower elevations with upper 30Fs to mid-40Fs over the higher terrains. Warmer temperatures over CO/UT border as the front stalled out before it reached this area.

PW this morning was measured at 0.82 inches and 0.85 inches at Denver and Grand Junction, respectively. So not much change in low-level moisture though the water vapor imagery shows some pockets of drier air have moved into western CO. This should cause storms today to be a bit more isolated when compared to yesterday. Also, expecting some gusty outflow winds with storms today (Central/Northern Mountains) as well as there is quite a bit of spread between the temperature and dew points. The greatest coverage will likely be over the San Juan Mountains with more patchy coverage and lower rainfall intensities over the Central and Northern Mountains.

To the east, rinse and repeat pattern of yesterday. Storm motion may look to increase a little, which will mean storms are less likely to produce as much rain as yesterday. However, 15 mph steering winds to the SE will still allow another day of some heavy rainfall. The SW to NE oriented upper-level jet and shortwave marked over the NW corner of the state will help support storms as they move off the mountains. So, storm coverage will likely increase over the adjacent plains this afternoon when compared to yesterday. Storms will decrease north to south this evening starting around sundown with some storms likely over the Raton Ridge/Southeast Plains intersect through midnight.

Burn scars over the SE Mountains will be susceptible to flooding again today, so storms that track near these areas should be monitored closely this afternoon. There is high confidence storms today will have rain rates greater than 0.5 inches/hour, so a Moderate threat has been issued for the Southeast Mountain burn areas. A Low flood threat has been issued  for eastern CO again today as heavy rainfall is expected once again and decent rainfall over the mountains yesterday will likely increase runoff this afternoon.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.

 

Zone-Specific Forecasts:

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

Rinse and repeat pattern of yesterday with some more action in the adjacent plains this afternoon into tonight. Instability along the Urban Corridor is not expected to be as high today, so not expecting as much hail as yesterday. However, a quick 0.5 inches in a half hour could cause some minor flooding issues in low-lying roads. Over the mountains, small hail and snow above 13,000 feet will be possible. A Low flood threat has been issued as moisture and slow steering winds combine to create pockets of heavy rainfall this afternoon. Saturated soils from yesterday will also increase runoff. A Moderate threat has been issued for the Spring Creek, Hayden Pass and Junkins burn areas. A Low threat for Weston Pass should suffice. If a storm tracks over a burn area, mud flows, debris slides and local stream flooding are possible. 1-hr rain rates up to 1 inch are possible over the higher terrains with max 1-hr rain rates up to 1.2 inches over the Front Range/Urban Corridor intersect.

Primetime: 1PM – 12AM

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

Some pockets of drier air have been entrained from the west, so storm coverage will be more isolated this afternoon when compared to yesterday. Low-level moisture will still allow some decent rain to reach the surface with the most action over the San Juan Mountains. Gusty winds will be possible with the stronger storms to the north. Max 1-hr rain rates up to 0.75 inches (south) and 0.5 inches (north) are possible. Storms should end a couple of hours after the sun sets.

Primetime: 12PM – 10PM

Issue Date: Wednesday, September 5th, 2018
Issue Time: 10:55AM MDT

—A MODERATE flood threat has been issued for portions of the Front Range, Urban Corridor, Palmer Ridge and Southeast Plains. Also for the Spring Creek and Junkins burn areas.

—A LOW flood threat has been issued for the Front Range, Urban Corridor, Palmer Ridge, Southeast Mountains, Raton Ridge and Southeast Plains. Also for the Lake Christine, Hayden Pass and Weston Pass burn areas

A large, upper-level trough situated over the western United States is creating substantial atmospheric lift throughout Colorado. Meanwhile, a surface high pressure system located over the western Dakotas is pushing humid air from the Great Plains and into the eastern regions of Colorado with low-level southeasterly flow. Areas in the Urban Corridor and Front Range received heavy rainfall overnight, which shows the strong low-level moisture resurgence behind a cold front passage. Low-level moisture is expected to remain high today with dew points already in the low 50s along the Front Range. This humid air is present in atmospheric soundings as well: precipitable water at Denver was measured at 0.83 inches this morning, with these values expected to rise a bit more throughout the day along the Urban Corridor and Palmer Ridge areas.

Upslope flow is expected to initiate showers and thunderstorms over the Front Range and Southeast Mountains this afternoon. With the vorticity max moving north throughout the day, coverage should also increase making storms this afternoon more widespread in nature. Extra convergence along the Palmer Divide will allow storms to remain active as they slowly move off the mountains this afternoon and evening. Storms are not expected to survive too far east of I-25 as the setting sun will decrease instability. It is possible for a few lighter showers to linger over the mountains tonight. Cloud cover this morning may hinder some afternoon convection over certain areas, but skies are already starting to clear. This should allow some moderate instability to build for storms this afternoon. Upper-level steering winds are weak, so slow thunderstorm movement will lead to heavy, localized rainfall totals. Convection may also pose a hail threat, which may exacerbate flooding in urban areas. A Moderate and Low flood threat have been issued. A Moderate threat has also been issued for the Spring Creek and Junkin burn areas with a Low threat for the Weston and Hayden Pass burn areas. With nearly stationary storms and possible back-building, local stream flash flooding, mud flows and debris slides are possible with storms that track over these areas.

For western CO, Moderate rainfall is being reported over portions of the San Juan and Central Mountains with heavy fog in the mountain valleys. There’s also some thunder just west of Pueblo over the Southeast Mountains. Storms are expected to pick up in coverage this afternoon into this evening from south to north. Storms should be widespread in nature with the vorticity max moving north, so expecting the lower valleys to get some rainfall as well. The cloud cover will likely limit instability some this afternoon, so between that and higher-based storms, max 1-hr rain rates should remain flood threat criteria. However, 2-3 hour localized totals around 1 inch are possible. Due to the slow moving nature of the storms, a Low flood threat has been issued for the Lake Christine burn area. If a nearly stationary storm forms over the burn area, flash flooding, mud flows or debris slides may be possible.

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.

Moderate Flood Threat Today

 

Zone-Specific Forecasts:

 

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

Showers and thunderstorms are anticipated to build over the Front Range and Southeast Mountains late this morning/early afternoon and over into the Urban Corridor, Palmer Ridge, Raton Ridge and the Southeast Plains during the afternoon. High low-level moisture along with the slow-moving nature of the storms will produce heavy rainfall. Max 1-hour rainfall rates up to between 1 inch (west) and 1.4 inches (east) should be expected. Localized 24-hour totals near 2.5 inches are possible over the Moderate threat area. The Northeast plains will likely stay capped today after the frontal passage, but some rain is possible over the Southeast Plains. Unlike your typical pattern where storms move east of the mountains into the Southeast Plains, today some showers and thunderstorms could move in from the east. Max 1-hr rain rates up to 1.1 inches/hour possible.

Primetime: 12PM to 2AM

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

Morning rain and cloud cover over most of this region will continue into the afternoon, which should limit instability and 1-hour rainfall intensities this afternoon.  With higher-based storms, gusty winds will come into play underneath stronger storms. Rainfall rates are expected to remain generally below 0.5 inches/hour, though isolated max 1-hr totals up to 0.9 inches are possible. Due to the nearly stationary rainfall this afternoon, a Low flood threat has been issued for the Lake Christine burn area as there is high confidence storms in the area will exceed 0.5 inches in 1 hour.

Primetime: 11AM to 11PM

Issue Date: Tuesday, September 4th, 2018
Issue Time: 08:40AM MDT

—A LOW flood threat has been issued for the Spring Creek, Junkins and Hayden Pass burn areas

Colorado will again be under the influence of the upper trough today. Similar to yesterday, expecting some shortwave action associated with this feature to help spur better coverage of showers and thunderstorms this afternoon over the higher terrains in tandem with upslope flow. Currently, there is a cold front over WY that will continue to drop south today. Behind the front northerly flow is expected, which will set up a convergence boundary over plains this afternoon. This means that over the far eastern plains, there may be a few thunderstorms that initiate along this line. Though, the main action will be a bit to our east. Deeper moisture is also located east of Colorado today, so the heaviest rainfall will likely be over Kansas. The passage of the front may help return some more low-level moisture over the Front Range and Palmer Ridge as the post-frontal upslope pattern sets up. Therefore, there will likely be an uptick in coverage of storms this afternoon over this area.

Precipitable Water (PW) this morning was measured at 0.67 inches at Denver and 0.69 inches at Grand Junction. While these values may increase slightly today, still forecasting the higher moisture to be over far eastern plains this afternoon. Both soundings show quite a bit of dry air, so thunderstorms may produce some gusty winds this afternoon. As the storms move into the adjacent plains, they will likely dissipate before making it too far east of I-25. Further south, they have a better chance of making their way eastward. Storms over western Colorado will be a rinse and repeat pattern to yesterday with the most abundant action over San Juans and south Central Mountains. Further north, storms will be more isolated in nature and likely produce some gustier outflow winds. Most storms east and west of the Divide will dissipate a couple of hours after the sunset. However, guidance is showing a few storms reforming over the high terrains tonight.

Not expecting widespread flooding over the higher terrains this afternoon. Dry soils (especially north) with moderate max 1-hour rain rates will keep the flood threat minimal. However, with max 1-hour rain rates up to 0.9 inches possible over the Southeast Mountains, recent burn scars will be at risk for flash flooding should a storm track over a scar. A Low flood threat has been issued for the Junkins, Hayden Pass and Spring Creek burn scars.

Today’s Flood Threat Map

For more information on today’s flood threat, see the map below. For Zone-Specific forecasts, scroll below the map.