Issue Date: 6/6/2015
Issue Time: 10:40 AM

A MODERATE FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE SAN JUAN MOUNTAINS AND SOUTHWEST SLOPE.

A LOW FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE SOUTHWEST SLOPE, SAN JUAN MOUNTAINS, CENTRAL MOUNTAINS, AND GRAND VALLEY.

A LOW FLOOD THREAT CONTINUES FOR THE WEST SALT CREEK LANDSLIDE AREA.

A LOW FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, SOUTHEAST PLAINS, SOUTHEAST MOUNTAINS, AND RATON RIDGE.

A LOW FLOOD THREAT CONTINUES DUE TO ACTION-TO-MINOR STAGE FLOODNG FOR PORTIONS OF THE ARKANSAS RIVER, SOUTH PLATTE RIVER, AND CACHE LA POUDRE RIVER.

The slow-moving upper-level low continues to spin over the Southwestern US, currently centered over southern NV (as seen on the water vapor image below). It will continue its track slowly to the ENE today, continuing the stream of moisture-rich air over Colorado. The best moisture/lift will stay over western Colorado today as the upper-level Jet noses across the 4-corners region. This will provide upper-level support to areas already influenced by favorable orographic lift (specifically the San Juan Mountains and Central Mountains regions), producing widespread showers and thunderstorms throughout today and tonight. Storm motions will be fairly quick (ESE at 25-35 mph), and this will help lessen the flash flood threat for today. However, soils are nearly saturated after bouts with moderate-to-heavy rain yesterday and showers ongoing this morning, and burn scars are of particular concern for Flash Flooding today. For this reason, the moderate flood threat has been hoisted for the San Juan Mountains.

For areas east of the Continental Divide, showers and thunderstorms will not be nearly as widespread, and a bit less-intense as compared to yesterday. This is due, in part, to the lessening easterly flow thanks to the developing surface low over the northern plains. This will bring surface dewpoints and instability lower than yesterday, as well as lessen the low-level shear that brought yesterday’s tornado threat. While an isolated tornado or two cannot be ruled out over the Palmer Ridge and Northeastern Plains, the chances are much less than the previous two days. Hail, strong winds, and locally heavy rainfall will be the main threats of stronger storms. Storm motions will be fairly quick today (E at 15-25 mph), and this will lessen the potential flooding threat. With that said, though, previous days rainfall and burn scars will warrant a low flood threat across portions of the Front Range, Urban Corridor, Palmer Ridge, Southeast Plains, Southeast Mountains and Raton Ridge.

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

Scattered showers and thunderstorms, a few will be strong-to-severe, are expected today and into the overnight hours. After the activity of this afternoon and evening clears out to the east, some redevelopment during the overnight hours is expected as a weak cool front pushes in from the north at the same time that upper-level lift increases from the northeastward moving upper-level low.

Maximum rain rates will be in the 1.2-2.0 inches/hour range during the afternoon and evening hours, lessening somewhat to 0.8-1.5 inches/hour during the overnight hours.

Timing: Noon – 5 AM

Northern Mountains and Northwest Slope:

Scattered showers and thunderstorms are expected, though generally less than locations to the south. Rain rates will also be slightly less in these regions, with maximum rain rates in the 0.6-0.8 inches/hour range.

Timing: 11 AM – 9 PM, with a few weak showers continuing overnight and into tomorrow morning.

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

Widespread showers and thunderstorms, a few will be strong-to-marginally severe over the Southwest Slope and San Juan Mountains. In fact, an isolated tornado cannot be ruled out over the lower elevations in far SW Colorado, where favorable shear and instability will be present this afternoon and evening.

Maximum rain rates will be in the 0.7-1.4 inches/hour range, with the highest likelihood across the San Juan Mountains and Central Mountains due to favorable orographic lift from NW-to-SE oriented mountain ranges.

Timing: 11 AM – 10 PM, with a few showers and weak thunderstorms continuing into the early morning hours.

Issue Date: 6/05/2015
Issue Time: 10:58 AM (Updated 2:50PM)

— Moderate flood threat for Urban Corridor, Palmer Divide, Southeast Plains and parts of Raton Ridge
— Low flood threat for San Juan Mountains
— Low flood threat for parts of Grand Valley for potential landslide

Afternoon update: Thunderstorms continue to fire, in association with small-scale upper-level disturbances entering the state from the southwest. Individual storms will be capable of producing very heavy rainfall, up to 1.8 inches per hour, such as is being reported in the Denver Metro area, as well as over the Raton Ridge. However, widespread flooding is NOT anticipated. Isolated tornadoes and very large hail, up to 2.5 inches, will accompany the strongest storms.

An impressive persistence in very moist conditions east of the Divide is now enveloping most of the state. This is associated with a dual combination of an approaching upper-level cyclone over California and a connection to subtropical moisture from the eastern Pacific. The upper-level cyclone is in no hurry to move, and despite very slowly weakening, it will impact our weather today and for the next two days (see a more detailed discussion in our Flood Threat Outlook from Thursday, 6/4). The water vapor image below shows the aforementioned features as well as two specific disturbances: one crossing over Colorado currently, the other approaching the Four Corners and impacting our state this afternoon.

Meanwhile, a chart of the precipitable water, below, shows a roller coaster ride over the last several days. Note that Pueblo now sits over 1 inch, with Boulder fast approaching that. Grand Junction has seen a tremendous spike in moisture with values of 0.2 inches yesterday, but up to 0.7 inches this morning. The atmosphere is getting primed for showers and thunderstorms.

Shower and thunderstorm activity is already ongoing both east and west of the Continental Divide. With limited instability, this activity is only mustering rain rates of up to 0.4 inches per hour. However, as partial sunshine and the atmosphere’s dynamics work in tandem today, we expect repeated rounds of thunderstorms over many parts of the state. The extreme southeast and northwest corners will be generally quieter. Hourly rain rates up to 1.8 inches, and 3-hourly rain rates up to 3.00 inches warrant a Moderate flood threat across a large swath of eastern Colorado. The San Juan region is under a Low threat for persistent light to moderate rainfall that will fall on top of the remaining snow pack.

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

Partly to mostly cloudy skies with repeated rounds of shower and thunderstorm activity. Hourly rain rates up to 1.8 inches possible, with 3-hr rates up to 3.0 inches. Short-term heavy rainfall rates, up to 0.5 inches in 5-10 minutes are possible. Hail, up to 2 inches cannot be ruled out in the afternoon, along with a slight tornado threat. A Moderate flood threat is in place for a large part of the region, with a Low threat elsewhere. The extreme southeast will likely see some drier air infiltrate, limiting rainfall chances.

Primetime: 2pm through 11pm

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

Partly to mostly cloudy skies with intermittent rain showers and weak thunderstorms. Coverage and intensity will increase throughout the day. Hourly rain rates up to 0.6 inches possible. A Low flood threat is in effect for parts of the Grand Valley for a potential landslide from additional rainfall. Otherwise, no other flooding is expected.

San Juan Mountains and Southwest Slope:

Partly to mostly cloudy with repeated rounds of showers and thunderstorms for a majority of the day and into the night. Hourly rainfall rates up to 0.8 inches possible, with 3-hourly rainfall up to 1.4 inches warranting a Low flood threat. Rainfall will fall over the remaining snowpack and may cause localized minor flooding in streams.

Issue Date: 6/04/2015
Issue Time: 10:10 PM

HIGH FLOOD THREAT IS ONGOING ACROSS PORTIONS OF THE FRONT RANGE AND URBAN CORRIDOR. A MODERATE FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE URBAN CORRIDOR, FRONT RANGE, AND NORTHEAST PLAINS. A LOW FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, AND NORTHEASTERN PLAINS.

This special update is due to the unfolding situation across portions of the Front Range, Urban Corridor, and Palmer Ridge. There are two big culprits responsible for this result:

1. Easterly, upslope, low-level flow continues to stream moisture-rich air back into Front Range and Urban Corridor.

2. The picture below shows that a ribbon of 300-mb divergence (produces lift) has rotated across Colorado ahead of a weak shortwave, providing upper-level support that wasn’t present up until late this evening when these strong thunderstorms producing heavy rain developed.

Showers and thunderstorms capable of producing heavy rain will continue across the Front Range, Urban Corridor, Palmer Ridge, and western portions of the Northeastern Plains until 3-5 AM, before continuing to the east across the rest of the Northeastern Plains and exiting the state around 8-9 AM tomorrow morning.

Issue Date: 6/4/2015
Issue Time: 10:26 AM

A LOW FLOOD THREAT IS FORECAST FOR THE WEST SALT CREEK LANDSLIDE AND SURROUNDING LOW LYING AREAS IN THE GRAND VALLEY, CENTRAL MOUNTAINS, AND NORTHERN MOUNTAINS REGIONS. A LOW FLOOD THREAT CONTINUES FOR PORTIONS OF THE CACHE LA POUDRE AND SOUTH PLATTE RIVERS.

As is the case on most days, today’s water vapor image tells the forecast story fairly well. First things first, the black line in the image denotes yesterday’s disturbance that aided in the development of storms across NE Colorado. It continues to track eastward, producing showers and thunderstorms along the way. Behind this disturbance is the maroon line, depicting the location of the high pressure ridge in its wake. As the low pressure trough deepens over the CA coast, the high pressure ridge will build across Colorado, helping to suppress the number of thunderstorms that Colorado will see today. Also noted on the water vapor image is the presence of the mid-level dry air being pulled across Colorado from the southwest; this will help to keep areas west of the Continental Divide dry today.

East of the Continental Divide, low-level moisture remains in place as easterly flow continues to stream moisture into eastern Colorado. The culprit behind this moist flow is a slow-moving surface Low, currently positioned over far SW KS. This moisture, combined with daytime heating, will lead to sufficient instability this afternoon and evening. This instability will fight the high pressure ridging above and produce a few isolated showers and thunderstorms this afternoon and evening over the Front Range, Urban Corridor, Palmer Ridge, Northeast Plains, Southeast Plains, and Raton Ridge. The number of thunderstorms will be less than yesterday as only a few storms will be able to break through.

As the day progresses, the easterly flow will become more southerly, which means that the most likely locations for storms will be across the Front Range, Urban Corridor, Palmer Ridge, and Northeast Plains. This southerly flow will also allow for the Denver Convergence Zone to develop across Douglas and Arapahoe counties. IF a storm can develop in this region, this will support an isolated strong storm capable of producing small hail, brief moderate-to-heavy rain, and an isolated landspout tornado.

By early tomorrow morning, expect a few showers and perhaps a thunderstorm or two to push in across the SW corner of the state. Tropical moisture from Hurricane Andres is being pulled northeastward from the deepening low pressure trough, and will set the stage for tomorrow’s FTB.

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

A few isolated showers and thunderstorms are expected to develop this afternoon and evening, capable of producing brief moderate-to-heavy rain, small hail, and strong, gusty winds. Relatively quick storm motions and lack of upper-level support do not warrant a flood threat to be issued.

Maximum rain rates will be in the 1.2-1.8 inches/hour range, but storm motions will limit the amount any one location will receive. Additionally, storms will have a hard time sustaining themselves with the presence of upper-level subsidence.

Timing: 2 PM – Midnight, with one or two showers/weak thunderstorms continuing into the early morning hours.

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

One more day of dry southwesterly flow will lead to sunny skies and warm temperatures. Changes will come late Tuesday night as tropical moisture begins to encroach on the SW corner of the state, producing showers and a few weak thunderstorms. This moisture push will set the stage for tomorrow, but will not warrant any flash flood threat through the end of this forecast period at 11 AM tomorrow.

Timing: 1 AM – 11 AM