FTB 08-31-2016: Heavy Rainfall Threat Still Present, Shifts South

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.

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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.
FTB_20160831

Flood Threat Legend

Zone-Specific Forecasts:

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

Sunny early then scattered thunderstorms developing by early afternoon. Coverage will be highest in the foothills as well as the Northeast and Southeast Plains. Max 30-minute rainfall up to 1.2 inches is possible, with max 1-hr rates up to 2.2 inches especially later in the day in the southern parts of the area. A Low flood threat has been posted for southern parts of the area for isolated flash flooding. A localized Moderate threat is posted for higher terrain where late afternoon/early evening storms could pose a threat of mud flows and debris slides. Hail up to 1.25 inches is possible under the strongest cells especially earlier in the afternoon.

Primetime: 2PM to midnight, especially for southern areas

San Juan Mountains, Southwest Slope:

Mostly sunny this morning then scattered to showers and thunderstorms developing by early afternoon. Coverage will be highest in the San Juans. Max 1-hr rain rates up to 0.6 inches possible. Hail up to 0.5 inches could accompany the strongest cells. Graupel is possible above 12,500 feet. Flooding is not expected today.

Primetime: 1PM to 7:30PM

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

Mostly sunny early then isolated showers and a weak storm possible through the early evening hours. Max 1-hour rain rates up to 0.3 inches. Flooding is not expected today.

STP 08-31-2016: Very Heavy Rainfall For Denver Metro, Also Farther East

Issue Date: Wednesday, August 31st, 2016
Issue Time: 9:25AM MDT

Summary

Scattered showers and thunderstorms dotted the landscape across large swaths of northeast Colorado. Activity started early, around noon, along the eastern fringes of the Palmer Ridge as well as farther north. These thunderstorms had narrow updrafts and produced only short-term heavy rainfall to the tune of 0.5 inches in 30 minutes. As the afternoon wore on, the smaller updrafts combined to form more threatening storms over the Northeast Plains. Hourly rainfall rates up to 1.5 inches were observed in mostly rural parts of Weld, Washington, Kit Carson and Cheyenne counties. An areal flood advisory was issued for western parts of Cheyenne County for field and roadway flooding.

Later in the afternoon and into the evening, a secondary surge of activity began around 6PM as a push of very moist air, to the tune of dewpoints near 60F and precipitable water around 1.1 inches, moved westward. Intense thunderstorms formed over the northeast part of the Denver metro area, then sent outflow boundaries southward and westward sparking more activity over the next 3-4 hours. With rainfall falling over the most densely gaged area in the state, some very high rainfall rates were recorded. These include: numerous gages exceeding 0.5 inches in 10 minutes, 1.8 inches in 30 minutes near Cherry Creek Reservoir, and close to 3 inches per hour just west of Boulder. Needless to say, roadway and street flooding was common across the entire metro area. Fortunately, the limited area of the heavy rainfall prevented significant riverine flooding, although some impressive spikes were seen in area stream hydrographs. For example, below is the spike on Clear Creek near the South Platte River confluence: 5 to 1000 c.f.s. in a matter of 30 minutes (provisional data).

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For rainfall estimates specific to your area, check out our Storm Total Precipitation map below.

STP_20160831
Storm Total Precip Legend

FTB 08-30-2016: More Heavy Rainfall Possible Today

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.

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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.
FTB_20160830

Flood Threat Legend

Zone-Specific Forecasts:

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

Sunny early then scattered to widespread thunderstorms developing by early afternoon. Coverage will be highest in the foothills as well as the Northeast Plains. Max 30-minute rainfall up to 1.1 inches is possible, with max 1-hr rates up to 2.1 inches. Isolated flash flooding, debris slides and mud flows are possible, especially in areas that have received previous rainfall. Hail up to 0.75 inches is possible under the strongest cells. A secondary flare up of storms is possible in the late evening hours, warranted a Moderate flood threat for parts of the region. Otherwise, a Low flood threat is in place.

Primetime: 2PM to 2AM

San Juan Mountains, Southwest Slope:

Mostly sunny this morning then isolated to scattered to showers and thunderstorms developing by early afternoon. Coverage will be highest in the San Juans. Max 1-hr rain rates up to 0.6 inches possible. Hail up to 0.5 inches could accompany the strongest cells. Graupel is possible above 12,500 feet. Flooding is not expected today.

Primetime: 12PM to 7:30PM

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

Mostly sunny early then isolated showers and a weak storm possible through the early evening hours. Max 1-hour rain rates up to 0.3 inches. Flooding is not expected today.

STP 08-30-2016: Same Bullseye Two Days In A Row

Issue Date: Tuesday, August 30th, 2016
Issue Time: 9:05AM MDT

Summary

A slow moving disturbance was in a position to provide favorable dynamics, moisture and upslope flow that resulted in scattered to widespread showers and thunderstorms mainly east of the Continental Divide. Activity started by late morning with northwestward-moving showers over parts of the Southeast Mountains and Raton Ridge. Shortly after noon, stronger thunderstorms began to form over the Palmer Ridge, especially El Paso County, which was hit hard with heavy rainfall, deep accumulations of hail and flooding. Highest 24-hour totals were around 2.3 inches, with most of that falling over a 1-2 hour period. In fact, many stations received over 1.5 inches of rain, which contributed to very high runoff rates. This was compounded by the antecedent rainfall from the previous day that also targeted the area, causing saturated soils and likely maximized runoff. For example, the time series below is from a USGS precipitation gage that measured 3.5 inches over a 36-hour period. Warnings products issued yesterday afternoon included areal flood advisories, flash flood warnings, flood warnings for Fountain Creek and a severe thunderstorm warning for hail. Street flooding was exacerbated by hail accumulation, leading to a few unlucky folks having to be rescued from cars.

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The Palmer Ridge was not the only hot-spot yesterday, as other storms were found farther north and east. Isolated rainfall amounts up to 1.5 inches were found with the strongest storms, prompting a short-term areal flood advisory. Elsewhere, the San Juans continued a string of wet days with up to about 0.75 inches of rainfall falling over favored locations. Above about 12,500 feet, graupel showers were seen that coated the higher peaks with a layer of the white stuff.

For rainfall estimates specific to your area, check out our Storm Total Precipitation map below.

STP_20160830
Storm Total Precip Legend