Flood Threat Bulletin

FTB 08-15-2017: Moisture Hangs On Over The Northeast Supporting More Heavy Rainfall

August 15, 2017 by Dima Smirnov ·

Issue Date: Tuesday, August 15, 2017
Issue Time: 10:55AM MDT

—LOW flood threat for Urban Corridor, Palmer Ridge, Northeast Plains
—Threat will continue into the overnight hours
—Afternoon forecast update is possible today

Despite general drying, especially across central and western Colorado, above normal low-level moisture continues to hang on in the northeast quadrant of our state. As shown in the water vapor image, below, a large-scale trough is positioned over the west coast. It is currently too far west to have a direct influence on our weather. However, it is supporting a surface low pressure system over southeast Colorado. This will maintain its intensity through this afternoon, promoting the influx of moist air into northeast Colorado. Although morning Precipitable Water (PW) values at the two main sounding sites, Grand Junction and Denver, are a relatively unimpressive 0.69 and 0.67 inches, respectively, much higher moisture exists just to the east. For example, the North Platte sounding’s PW was 1.28 inches, which is in the 90+ percentile for this time of year. This moist air will be readily available for import into Colorado this afternoon and evening.

For today, we expect several rounds of showers and thunderstorms across mainly the northern half of Colorado as well as the far southern areas over the Raton Ridge. The highest rain coverage and intensity will be in the Northeast Plains. Afternoon and evening thunderstorms will have the potential to cause isolated flash flooding. In addition, large-scale outflow boundaries are expected to arrive into northeast Colorado from convection farther north in the late evening hours. Thus, a secondary round of isolated heavy rainfall will be possible south and east of the South Platte River into the early overnight hours. A Low flood threat has been posted for today/tonight for isolated flash flooding, street flooding and small stream flooding. Heavy rainfall coverage is not expected to be wide enough for major rivering flooding. Large hail, up to 1.75 inches, will be possible with the strongest cells this afternoon in the Northeast Plains.

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, Palmer Ridge, Northeast Plains, Southeast Mountains, Raton Ridge, Southeast Plains:

Scattered showers and thunderstorms developing by early afternoon. Highest coverage will be over the Palmer Ridge and Northeast Plains. Max 1-hour rainfall up to 2.9 inches (east) and 1.0 inches (west). Max 3-hour rainfall up to 3.8 inches across eastern areas. A Low flood threat has been posted for isolated flash flooding, street flooding small stream flooding (for higher elevations). A secondary round of storms is possible after sunset and the Low flood threat persists into the early overnight hours.

Primetime: 1PM to 3AM

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

Partly cloudy early with isolated to scattered showers and weak thunderstorms developing by afternoon. Highest coverage will be over northern areas (especially north of I-70). Max 1-hour rainfall up to 0.6 inches is possible. Gusty winds up to 50 mph and small hail could accompany the strongest cells. Activity will quickly subside after sunset.

Primetime: 12PM to 9PM

Filed Under: Flood Threat Bulletin ·

FTB 08-14-2017: Garden Variety Storms

August 14, 2017 by Dana McGlone ·

Issue Date: Monday, August 14, 2017
Issue Time: 09:40AM MDT

— There is NO flood threat today

The 500mb ridge will build both north and east of Colorado today putting us under weaker and more westerly flow with a bit of a southerly component. This will cut off the influx of shortwaves that have helped enhanced thunderstorm activity over the last few days. There is some patchy fog this morning, which can be seen on the surface observations. This should quickly burn off and skies should be sunny most of the morning throughout the state. Dew points over the far eastern plains are up to 60F, and rapidly decrease to the west. Precipitable Water (PW) at Denver this morning was 0.71 inches and 0.88 inches at Grand Junction. The decrease in moisture continues, which in turn decreases the flood threat. This decrease in available moisture is not uncommon for August. There is a climatological decrease in PW that occurs at the beginning of this month.

Today, scattered showers will form over the higher terrain just after noon. As these storms move off into the adjacent plains later in the afternoon, minimal atmospheric moisture will produce gusty winds and limit heavy precipitation. These more isolated storms will favor the higher terrains of the Palmer and Raton Ridge, but the many outflow boundaries may help trigger additional storms along the adjacent plains. These garden variety storms will move east, but are not expect to become severe today since the combination of instability and shear is negligible. Surface S/SE flow may return some moisture to far eastern plains, but the higher PW values will likely stay east of Colorado as will 1-hour rain rates exceeding 1.5 inches. Storms over the mountains will quickly end after sunset, while storms over the eastern plains may linger until midnight. There is no 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, Palmer Ridge, Northeast Plains, Southeast Mountains, Raton Ridge, Southeast Plains:

Mostly sunny this morning becoming partly cloudy this afternoon as scatter storms form over the mountains. More isolated storms over the eastern plains this afternoon with the main threat being strong winds and possibly small hail. 1-hour rain rates up to 1.5 (east) and .75 (west).

Primetime: 12PM to 12AM

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

Sunny this morning becoming partly cloudy this afternoon with scattered showers forming over the higher terrain and Northwest Slope. Max 1-hr rainfall up to 0.6 inches. There is a no flood threat for today.

Primetime: 12PM to 8PM

Filed Under: Flood Threat Bulletin ·

FTB 08-13-2017: Heavy Rain and Severe Weather for Southeast Plains

August 13, 2017 by Dana McGlone ·

Issue Date: Sunday, August 13, 2017
Issue Time: 10:30AM MDT

—LOW flood threat Northeast Plains, Palmer Ridge, Southeast Plains

Less low-level clouds over Colorado again this morning making for a pleasant start to the day. A trough located over the Pacific NW continues to release multiple mid-level waves through Colorado along the 500mb flow. Another wave is expected this afternoon over southern Colorado that will enhance daily thunderstorm activity over the southern Front Range and Southeast Mountains. A weak 500mb ridge will build over Wyoming today and the associated W/NW mid-level flow will continue to steer storm motion to the E/SE as the thunderstorms move into the adjacent plains. At the surface, a lee trough is expected to set up over and with this boundary, low-level moisture is expected to increase to the east, while the west remains drier. The best instability and shear will also be over the Southeast Plains this afternoon and evening, which could help produce gusty winds and hail on top of heavy, local rainfall. Some storms are expected to move into the Northeast Plains that form over the Cheyenne Ridge. Higher moisture with dew points in the mid-50s to 60F, may support 1-hour rain rates up to 1.8 inches and severe storms will also have large hail and gusty winds.

Diurnal heating patterns will allow scattered storms to begin to form over the higher terrain after 1PM. Drier air should limit coverage over the San Juan Mountains and a mid-level wave will enhance storms that form along the Southeast Mountains. Downsloping winds associated with the lee trough will limit thunderstorm formation over the Front Range. Storms that do form over the mountains will have lower atmospheric moisture and will produce gusty winds rather than heavy rainfall. As the storms move off the eastern mountains in the afternoon, they will favor the higher moisture and terrains on the Palmer and Raton Ridge. Severe thunderstorms are possible with winds up to 60mph and 2 inch hail as the storms move east into the plains. A cluster of storms form in the evening and moves SE exiting the state around midnight. With elevated flows along the Arkansas River basin and widespread precipitation, the Low flood threat will include the Arkansas River from La Junta to the Kansas border for minor flooding.

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, Palmer Ridge, Northeast Plains, Southeast Mountains, Raton Ridge, Southeast Plains:

Mostly sunny this morning becoming partly cloudy this afternoon. Some severe storms are possible today over the eastern plains with the main threats being damaging winds and large hail (2 in) as well as arroyo, field, road flooding. Max 1-hour rainfall up to 2.5 inches (east), 0.8 inches (west). Minor riverine flooding along the Arkansas River may be possible from La Junta to the Kansas border along more narrow portions of the river. A Low flood threat has been issued today.

Primetime: 2PM to 12AM

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

Sunny this morning with more showers over the Northern and central Mountains. Max 1-hr rainfall up to 0.7 inches with storm totals up to 1 inch. There is a no flood threat for today.

Primetime: 1PM to 8PM

Filed Under: Flood Threat Bulletin ·

FTB 08-12-2017: Severe Storms for the Northeast Plains

August 12, 2017 by Dana McGlone ·

Issue Date: Saturday, August 12, 2017
Issue Time: 10:00AM MDT

— LOW flood threat for portions of the Palmer Ridge and Northeast Plains

Lots of low level clouds and fog to start off the day over the eastern plains. Otherwise, it is sunny across the state this morning with highs today about average for August. Precipitable Water (PW) this morning at the Denver sounding showed 0.79 inches with some drying in the mid-levels. PW was a bit higher in Grand Junction at 0.83 inches. The higher PW values are once again located over the eastern plains and should stay confined to this area this afternoon and evening. With drying in the mid-levels, this should limit widespread heavy precipitation and promote damaging winds.

500mb flow will bring moderate NW flow over the state. A shortwave trough makes its way down from NE/WY this evening, which will help support thunderstorm activity as it moves over the Northeast Plains. This area also has the best instability and shear, so this may support some severe storms with large hail and gusty winds. The storms merge into a line of convection and move E/SE in the evening hours. The clearer skies throughout the day will also help form more instability over the mountains and allow more widespread coverage of thunderstorms development over the northern portion of the state today. Expect storms to start firing over the high country this afternoon and favor the Northeast Plains and Palmer Ridge as they move off into the adjacent plains. The flood threat should end a couple hours after sunset.

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, Palmer Ridge, Northeast Plains, Southeast Mountains, Raton Ridge, Southeast Plains:

Fog burning off becoming mostly sunny this morning and partly cloudy this afternoon. Some severe storms are possible today over the NE Plains with the main threats being damaging winds, large hail (2 in) and field and road flooding. Max 1-hour rainfall up to 2 inches (east), 1 inch (west). Riverine flooding along the Arkansas River is not expected today. A Low flood threat has been issued today.

Primetime: 2PM to 11PM

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

Sunny this morning with some showers over the Northern and San Juan mountains this afternoon. Max 1-hr rainfall up to 0.8 inches. There is a no flood threat for today.

Primetime: 12PM to 8PM

Filed Under: Flood Threat Bulletin ·