STP 07-30-2015: Subtropical Moisture Increased, but the Effects were Limited

Issue Date: Thursday, July 30th, 2015
Issue Time: 9:00 AM MDT

Summary:

Yesterday was another pleasant day overall, providing for a great mid-week bridge to the downhill side of the work week. While subtropical moisture streamed overhead, it was again the general lack of instability and mid-/upper-level dynamics that kept showers/thunderstorms on the weak side. Mountain locations along the Front Range, Central Mountains, and Southeast Mountains experienced the most coverage of thunderstorms where orographic effects helped make up for the lack of other support. Also, the Raton Ridge and Southeast Plains along the CO/NM border received a few pockets of rainfall during the afternoon and evening hours, as stronger storms graced northern NM.

Rain totals and, for that matter, rain rates, were generally low. As of 7 AM, the only totals reported greater than 0.1 inches by CoCoRaHS observers were Chaffee County (0.77 inches) and Teller County (0.36 inches).

There weren’t any severe storm reports from yesterday, and no flash flooding was observed.

STP_snapshot_20150730

FTB 07-29-2015: Fairly Stable Atmosphere Will Limit the Effect of Monsoonal Moisture

Issue Date: 7/29/2015
Issue Time: 10:38 AM

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SOUTHEAST MOUNTAINS, RATON RIDGE, AND SOUTHEAST PLAINS.

Today is certainly a day where the water vapor imagery is useful in the sense that it shows the locations where monsoonal moisture is present. With the mid-/upper-level high pressure system still anchored across the south-central US, southwesterly flow aloft is dragging monsoonal moisture across southern and eastern portions of Colorado. Observations show that precipitable water values are above average at Shriever AFB (~0.9 inches) and Pueblo (~1.2 inches).

WV_07292015

This monsoonal moisture plume (denoted by the green arrow) will continue to work across southern and eastern portions of the state today, with fair amounts of moisture available along the Continental Divide and Central/Southwestern Mountains areas. The moisture, along with weak instability generated by daytime heating, will create isolated-to-scattered showers and weak thunderstorms across the Front Range, Central Mountains, and San Juan Mountains. Instability will be a bit better to the south and east, allowing for better coverage/intensity of storms across the Southeast Mountains, Raton Ridge, and portions of the Southeast Plains (both near the mountains and along the CO/NM border).

Dry air (noted on the water vapor image) will keep the Northern Mountains, Northwest Slope, Grand Valley, and Southwest Slope mostly sunny through the day. The Northeast Plains, Palmer Ridge, and areas of the Southeast Plains not mentioned above will remain dry today, as well; the atmosphere will remain stable and unsupportive of thunderstorm development. Far western portions of the Palmer Ridge and Northeast Plains could see an isolated shower/weak thunderstorm move overhead from the mountains before quickly dissipating in the stable air.

Much like yesterday, the Southeast Plains near the CO/NM border and Raton Ridge will hold the potential for thunderstorms to continue overnight, as more widespread storms are expected to develop over New Mexico and move to the northeast. For this reason, the low flood threat will continue overnight and into tomorrow.

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.

07302015_LFT
Flood Threat Legend

Zone-Specific Forecasts

Urban Corridor, Northeast Plains, and Palmer Ridge:

Mostly sunny and warm will be the main story today for the Northeast Plains and Palmer Ridge as the atmosphere remains stable overhead. The Urban Corridor, and western portions of the Northeast Plains and Palmer Ridge will likely see a couple isolated showers/thunderstorms move off the mountains overhead this afternoon and evening, before dissipating in the stable air.

Rain rates will be rather modest at 0.4-0.8 inches/hour.

Timing: 2 PM – 8 PM

Grand Valley, Northern Mountains, Northwest Slope, and Southwest Slope:

Mostly sunny for the Grand Valley, Northern Mountains, and Northwest Slope regions, becoming partly sunny across the Southwest Slope where slightly better mid-level moisture resides. No precipitation is expected, except for a weak shower or two across the higher terrain of the Southwest Slope. Rain rates will be 0.2-0.4 inches/hour.

Timing: 11 AM – 9 PM

Front Range, Southeast Plains, Southeast Mountains, and Raton Ridge:

These zones will see the most thunderstorm coverage today, as well as the best rainfall intensities. Moisture values are higher to the south and east, and the rain rates below will reflect this.

Front Range: 0.5-0.8 inches/hour
Southeast Plains and Raton Ridge: 1.2-2.2 inches/hour
Southeast Mountains: 0.8-1.2 inches/hour

Timing: Noon – 10 PM for the Front Range, 11 AM – 5 AM for the Raton Ridge, 11 AM – 11 PM for the Southeast Mountains, and 1 PM – 6 AM for the Southeast Plains

Central Mountains, San Luis Valley, and San Juan Mountains:

Isolated-to-scattered showers/weak thunderstorms expected, with coverage increasing from north to south. Generally light rain will fall early, but as moisture increases during the afternoon and evening hours, rain rates will increase a bit.

Max rain rates will be 0.5-0.8 inches/hour.

Timing: 11 AM – 9 PM

STP 07-29-2015: Mostly Sunny and Cooler, with a Few Showers and Weak Thunderstorms

Issue Date: Wednesday, July 29th, 2015
Issue Time: 9:00 AM MDT

Summary:

After the passage of a weak cool front, Colorado received a cool down on high temperatures as compared to previous days. Breezy conditions accompanied this cooler air at times, and it was an overall pleasant day across the state. A few showers/weak thunderstorms were able to develop (mainly during the evening/overnight hours) over central/southern mountains and lower elevations of eastern Colorado, but nothing of note. The best dynamic forcing associated with the disturbance mentioned in yesterday’s FTB existed to the south and east of Colorado, causing the best rainfall to occur across northeast NM, southwest KS, and the OK pandhandle. As it slid by, a prolonged period of light rain impacted eastern Baca County.

There weren’t any severe storm reports from yesterday, and no flash flooding was observed.

07292015_STPImage
Storm Total Precip Legend

FTB 07-28-2015: Late Developing Low Flood Threat

Issue Date: 7/28/2015
Issue Time: 9:47 AM

A LOW FLOOD THREAT IS FORECAST FOR THE EVENING/OVERNIGHT PERIOD ACROSS PORTIONS OF THE RATON RIDGE, SOUTHEAST MOUNTAINS, AND SOUTHEAST PLAINS.

Much of today will be marked by mostly sunny and stable conditions thanks to a few main players:

  1. Upper-level low tracking across south-central Canada (red “L”)
  2. Upper-level high anchored over the south-central US (blue “H”)
  3. A weak, cool front that passed through this morning

The upper-level low and high will promote the continued advection of dry air into Colorado, thanks to west-southwesterly flow aloft, and the weak cool front has brought drier and cooler conditions to the lower levels. Altogether, these main players will lead to a stable and dry start to the afternoon for most locations across the state.

WV_07282015

As the day wears on, isolated, weak thunderstorms will develop this afternoon over the higher terrain; mainly the San Juan Mountains, Central Mountains, Front Range south of I-70, and Southeast Mountains. The main impacts from these storms will be gusty winds and light rain – there simply will not be much moisture to aid precipitation processes. One or two weak storms will move over adjacent locations of the Urban Corridor (south of I-70), Southeast Plains, and Raton Ridge, but will struggle to produce good rainfall.

The main event will come during the evening and overnight hours, as a weak disturbance will rotate across New Mexico and into southeastern Colorado, leading to scattered showers and thunderstorms capable of producing heavy rain. Moisture will hold on best for these southeastern locations throughout the day as upslope flow reinforces surface moisture, and the plume of modest subtropical moisture continues overhead (green arrow). Please note the timing of the flood threat in the map/zone-specific discussions below.

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.

07282015_LFT
Flood Threat Legend

Zone-Specific Forecasts

Front Range north of I-70, Urban Corridor North of I-70, Northeast Plains, Grand Valley, Northern Mountains, Northwest Slope, and Southwest Slope:

Mostly sunny and warm, with temperatures a few degrees cooler than yesterday. A few fair weather clouds will develop, but no precipitation is expected.

Front Range south of I-70, Urban Corridor south of I-70, Palmer Ridge, Southeast Plains, Southeast Mountains, and Raton Ridge:

Isolated weak thunderstorms over the higher terrain and adjacent lower elevations are expected to develop after noon, and continue through the late afternoon/evening hours. The main event, as described above, will begin later in the evening as a disturbance moves across southeastern Colorado. The flood threat will start around 8 PM, and continue into the early morning hours. Maximum rain rates will break down as follows:

Front Range/Urban Corridor south of I-70 and Palmer Ridge: 0.4-0.8 inches/hour
Southeast Mountains: 0.7-1.0 inches/hour
Raton Ridge: 1.2-1.8 inches/hour
Southeast Plains: 1.5-2.5 inches/hour

Central Mountains, San Luis Valley, and San Juan Mountains:

A few isolated, weak thunderstorms are expected over the higher terrain later this afternoon and into the evening hours. A couple of lingering showers will continue into the early morning tomorrow. Rain rates will be low, less than 0.2 inches/hour.