STP 05-28-2015: Storms Battle Dry Air To Stay Alive; Snow Melt Begins In Earnest

Issue Date: May 28th, 2015
Issue Time: 8:55AM MDT

Summary:

Thundershowers had a tough time battling the drier upper-level air in place on Wednesday. This is quite a contrast from the past two to three weeks where the upper-level atmosphere was very humid and conducive to storm formation. Regardless, quite a number of mostly weak thunderstorms formed across the state by early afternoon. Storm motion was enough to keep one hour rain rates in the 0.2 to 0.5 inch range. Overall, the highest 24-hour rain total came in from Trinidad in Las Animas county at 1.23 inches.

The strongest cells of the day were found along the Kansas border, where moisture and instability were in more abundance. There, hail up to 2 inches was observed, along with a tornado sighting in Yuma.

As can be expected this time of year, the warmer temperatures have begun to work on the high elevation snowpack. An inspection of several SNOTEL sites reveals that up to 2 inches of snow water equivalent melted or sublimated (from snow straight to water vapor) on Wednesday alone. For many locations, this represents 10% or more of the entire snowpack. Interestingly, this shows that while snow takes many months to accumulate, it can be gone in a matter of a week or two given warm weather.

No flash flooding was reported yesterday. However, parts of the Arkansas and South Platte Rivers continued to be in minor flood stage.

Please check the map below for estimated rainfall in your area. Note that the rainfall in Yuma county may be overestimated due to hail.

05282015_STPImage
Storm Total Precip Legend

FTB 05-27-2015: Fewer Showers and Thunderstorms Overall, Severe Storms Possible Near CO/KS Border

Issue Date: 5/27/2015
Issue Time: 10:33 AM

A LOW FLOOD THREAT CONTINUES FOR PORTIONS OF THE CACHE LA POUDRE/SOUTH PLATTE RIVER, AND PORTIONS OF THE ARKANSAS RIVER.

The broad-scale picture that the water vapor imagery (below) shows will help to define the forecast for today. The upper-level low (marked by the red “L”) will continue to dig to the south-southeast, placing Colorado under west-southwest flow for today. The water vapor imagery shows that the best mid-level moisture will remain to the south and north of Colorado, leading to a decrease in precipitable water values. Additionally, the westerly flow will mix down to the surface, eroding surface dewpoints and limiting the coverage/intensity of thunderstorms over most of the state. Expect the number of showers and thunderstorms to be slightly less than yesterday over and near the mountains.

WV_05272015

The exception to all of this, of course, will be near the CO/KS border, where dewpoints just east of a developing dryline will remain in the upper 40s/low 50s. Furthermore, the dryline will provide support for strong-to-severe thunderstorms to develop, and this is where the heaviest rain of the day will be found, along with the possibility for large hail (> 1 inch), damaging wind gusts (> 60 mph), and an isolated tornado or two.

Finally, the last feature in the water vapor imagery to be discussed is a remnant upper-level low (marked by the purple line). This remnant low will transition into a shortwave trough and slide into western Colorado overnight tonight. This will likely keep a few showers/weak thunderstorms going until after midnight over the mountains.

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.
FTB_snapshot_20150527

Zone-Specific Forecasts

Front Range and Urban Corridor:

Isolated-to-scattered showers and thunderstorms expected this afternoon and evening, producing mainly gusty winds and light-to-moderate rain. Rain rates will generally be in the 0.2-0.4 inches/hour range, but a stronger thunderstorm will hold the potential for 0.5-1.0 inches/hour. However, drying of the lower levels will discourage those maximum rates from being realized at the surface.

Timing: Noon – 8 PM

Palmer Ridge, Southeast Plains and Northeast Plains:

These are the regions of interest today, mainly east of the 104th meridian, particularly for strong-to-severe thunderstorms developing this afternoon and evening. West of the 104th meridian, showers and thunderstorms will be weak and isolated; east of the 104th meridian and near the CO/KS border, the environment will be favorable for the development of a few strong-to-severe thunderstorms. Maximum rain rates under these storms will be 1.5-2.5 inches/hour, but storm motions will limit any flash flooding potential.

Timing: 2 PM – 11 PM

Southeast Mountains and Raton Ridge:

Isolated showers and thunderstorms are on tap, resulting in rain rates in the 0.2-0.4 inch/hour range. High temperatures will continue their trend upward towards “normal” for this time of year.

Timing: 1 PM – 7 PM

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

Scattered showers and thunderstorms, slightly fewer than yesterday, are expected across these regions. Rain rates will generally be in the 0.2-0.4 inches/hour range, but a stronger thunderstorm over the San Juan Mountains will hold the potential to produce 0.3-0.6 inches/hour.

Timing: 11 AM – 9 PM, with a few showers and thunderstorms continuing until after midnight.

STP 05-27-2015: How Many Different Ways Can One Say “Scattered Showers and Thunderstorms?”

Issue Date: Wednesday, May 27th, 2015
Issue Time: 9:00 AM MDT

Summary:

FLOOD WARNINGS CONTINUE FOR PORTIONS OF THE CACHE LA POUDRE, SOUTH PLATTE, AND ARKANSAS RIVERS.

Weak ridging moved overhead yesterday and led to a slight downtick in the number of showers and thunderstorms across the state. However, an embedded weak disturbance still provided a trigger mechanism for scattered storms to develop. Moisture on Tuesday was up a bit from Monday, so a few storms produced heavier rain. A local storm report from 4 miles NNE of Colorado Springs observed 0.5 inches of rainfall in 10 minutes, equal to a 1-hour rain rate of 3 inches/hour! Storm motions kept flash flooding from being realized, so that is a positive note. Rainfall totals from CoCoRaHS observers show that most locations that received rain reported less than 0.25 inches, but the big winners were:

El Paso County: 0.63 inches
Elbert County: 0.61 inches
Otero County: 0.59 inches
Park County: 0.59 inches
Douglas County: 0.56 inches
Kiowa County: 0.50 inches

No flash flooding was reported yesterday. Be sure to check out the radar-estimated rainfall map below.

STP_snapshot_20150527

FTB 05-26-2015: Showers and Thunderstorms are Still in the Mix

Issue Date: 5/26/2015
Issue Time: 10:02 AM

A LOW FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, SOUTHEAST PLAINS, AND SOUTHEAST MOUNTAINS. A LOW FLOOD THREAT CONTINUES FOR PORTIONS OF THE CACHE LA POUDRE/SOUTH PLATTE RIVER, AND PORTIONS OF THE ARKANSAS RIVER.

As the upper-level trough slides to the east of Colorado today, weak mid-/upper-level ridging will move in to take its place. This weak ridging will add a bit of subsidence to the mix, bringing a downtick in showers and thunderstorms for this period. However, this is not to say that showers and thunderstorms won’t occur (they will), there will just be slightly fewer than previous days. Providing the fuel for today’s activity will be residual moisture (slightly greater than yesterday, see IPW chart below), marginal instability, and orographic influences, as well as a weak disturbance that will move west-to-east across Colorado. This weak disturbance is currently located over central Utah and, as it moves eastward, will be the focal point for kicking off today’s activity.

IPW_05262015

The greatest coverage of showers and thunderstorms will be over the higher terrain where orographic influences provide the best forcing on a day marked by a relative lack of upper-level support. Further east, the lower elevations will rely on storms moving off of the higher terrain and marginal instability to develop/maintain showers and thunderstorms. A stronger thunderstorm or two will be possible along the Front Range, Urban Corridor, Palmer Ridge, and western portions of the Northeast/Southeast Plains, with hail, lightning, and brief heavy rain being the main threats. A low flood threat will accompany this due to saturated soils lowering the maximum rain rate needed to produce any flash flooding/mudslide/rock slide issues.

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.

Flood Threat Legend

Zone-Specific Forecasts

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

Warmer and drier overall as compared to yesterday, but isolated-to-scattered showers and thunderstorms will make an appearance again this afternoon and evening. A stronger thunderstorm or two is likely, and will produce hail, gusty winds, lightning, and brief heavy rain. Storm motions aren’t particularly quick; couple this with areas that have saturated soils from previous days of rain, and we have reason to include a low flood threat area. Maximum rain rates from the stronger thunderstorms will be in the 1.0-1.5 inches/hour range.

Timing: 11 AM – 10 PM, with a few showers/thunderstorms lingering until 2-3 AM near the CO/KS border.

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

Scattered showers and thunderstorms expected again today, mainly over the higher terrain. Even so, storm motions will allow for storms to drift over lower elevations with time. Rain rates will mainly be in the 0.2-0.4 inch/hour range, but a few thunderstorms will produce maximum rain rates in the 0.4-0.7 inch/hour range.

Timing: 11 AM – 9 PM, with a few showers lingering after midnight and into the early morning hours on Wednesday.