FTB 07-31-2015: Monsoon Feed Will Support Heavy Rainfall For Some Areas

Issue Date: July 31, 2015
Issue Time: 10:20AM

— Moderate flood threat for San Juans and Southeast Plains
— Low flood threat for Palmer Ridge, Southwest Slope, Southeast Mountains, San Luis Valley and Raton Ridge

This morning’s water vapor image, below, shows the continuation of a subtropical moisture feed that has elevated precipitable water (PWs) values to well above an inch in the Four Corners region. In addition to this feature, a strong jet is noted just northeast of Colorado. The presence of this jet allowed for very strong wind gusts to be transported from the upper atmosphere down to the surface yesterday as intense thunderstorms crossed the Northeast Plains. Surface dew points continue to run high, in the 50s to lower 60s across most of the state, with lower values in the northwest. PWs ranged from 0.6 (and climbing) inches in Grand Junction to near 1 inch in Boulder to 1.3 inches in Pueblo. Thus, there is ample moisture for a heavy rainfall threat.

watervapor_20150731For today’s weather evolution, we expect mostly sunny skies this morning to lead to thunderstorm activity by early afternoon. Most action today will be focused on the higher terrain, south of I-70 with some spillover onto the Palmer Ridge. High moisture content and adequate instability will support heavy rain rates capable of flash flooding. A Low flood threat has been posted for much of the higher terrain across southern Colorado. A Moderate threat is in place for the San Juans due to higher storm coverage. While most activity should subside by evening, a complex of storms may be ongoing across northeast New Mexico that could clip southeast Colorado. Very heavy rainfall, up to 2.8 inches per hour will be possible with these storms, warranting a Moderate flood threat for that region as well.

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.

07312015_MFT
Flood Threat Legend

Zone-Specific Forecasts

San Juans, Southwest Slope, Southeast Mountains, San Luis Valley, Palmer Ridge, Raton Ridge:

Mostly sunny early with scattered to widespread thunderstorms forming by early afternoon. Highest coverage will be in the southwest. Max 1-hour rain rates up to 1.3 inches will be possible, leading to isolated flash flooding, mud flows and debris slides. In addition, gusty winds up to 50 mph will be possible with the strongest storms. Activity will subside shortly after sunset. Due to higher coverage, a Moderate flood threat is in place for the San Juans, with a Low threat elsewhere.

Primetime: 1PM to 8PM

Southeast Plains:

Sunny early, then scattered thunderstorms developing by mid afternoon. Closest to the mountains, these storms will be capable of rain rates up to 1.3 inches per hour. Isolated flash flooding may be possible, warranting a Low flood threat. In the southeast corner, ongoing storm activity will likely spill over from New Mexico by early evening, and continuing through around midnight. With PW values as high as 1.6 inches, these storms could produce rain rates up to 2.8 inches per hour, and 4 inches per 3 hours. A Moderate flood threat is in place.

Primetime: 1PM to 8PM near the mountains, 3PM to 1AM in the far southeast

Northeast Plains, Front Range, Urban Corridor:

Mostly sunny early, then partly cloudy with isolated thunderstorms possible. Max 1-hour rain rates up to 0.7 inches are possible, along with gusty winds up to 45 mph. Flooding is not expected today.

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

Mostly sunny early, then partly cloudy by mid afternoon. An isolated thunderstorm cannot be ruled out, especially farther south in the Central Mountains. However, max 1-hour rain rates will be limited to about 0.6 inches. No flooding is expected. Gusty winds up to 45 mph may accompany the strongest storms.

STP 07-31-2015: Some Of The Season’s Heaviest Rainfall Seen Over The Northeast Plains

Issue Date: July 31, 2015
Issue Time: 9:00AM

Summary

Though storm coverage was limited on Thursday, the storms that did form were quite impressive. With subtropical moisture feeding in from western Mexico, storms were able to produce extremely efficient rainfall. The most noteworthy storm of the day was observed over Yuma County during the mid and late afternoon hours. An initial thunderstorm eventually led to the development of a small complex of slow moving storms. During their peak intensity, these storms were able to produce about 2.8 inches of rain in 30 minutes! We believe that this is the heaviest rainfall rate observed anywhere across the state thus far this season. With this kind of rain rate, it was not surprising to see flooding reports: 3 reports across Yuma County noted up to a foot of water covering roadways. It is interesting to note that these storms were so confined (and moved over relatively rural land) that the highest observed CoCoRaHS report was a 24-hour total of only 2.1 inches. In addition to the rainfall, very strong winds up to 80 mph accompanied this storm complex, knocking down trees up to one foot in diameter.

Elsewhere, heavy rainfall was observed in far southeast Colorado, with a report in Baca County of right around 2 inches. Rain also fell in the higher terrain of the Southeast Mountains and San Juan Mountains. There, reports were limited to the 0.4 – 0.75 inch range.

For an estimate of 24-hour rainfall that occurred in your area, please check out our radar based map below.

07312015_STPImage
Storm Total Precip Legend

FTO 07-30-2015: Elevated Flood Threat To Persist As Monsoon Spigot Turns On

Issue Date: 7/30/2015
Issue Time: 2:30PM
Valid dates: 7/31 – 8/14

Overview threat_timeline.20150730

Moisture. That is quite possibly the single most important variable in the equation for heavy rainfall across Colorado during the summer. If it is present, even benign conditions, such as strong solar heating, can cause thunderstorm activity. If it is not present, even a strong atmospheric disturbance can end up producing storms with gusty winds, but only a few raindrops. This summer, moisture has seemingly been in never-ending supply. A few weeks ago, we published this Twitter post showing that it has been the most humid summer in Denver since at least 1961:


Fast forward to today, and we have continued to see a very humid summer in Colorado, with almost daily thunderstorm activity at least somewhere across the state.

For this Flood Threat Outlook, all indications point to a continuation of active weather across Colorado. In particular, we anticipate two separate heavy rainfall periods over the next 15 days. Now on to the details. The water vapor image, below, shows a disturbance positioned off the California coast. Instead of following a typical eastward movement, this disturbance will actually move south, or “dig” deeper into the subtropics. To its east, we see a return flow of subtropical monsoonal moisture that is currently providing active weather from Southern California all the way to New Mexico.

watervapor_20150730

This latest push of subtropical moisture is some of the highest of the season, as is seen by looking at the Precipitable Water chart for Albuquerque, NM since May 1st. Note that values have recently been in the 1.3 to 1.4 inch range, which is in the 98th percentile based on the region’s climatology.ABQ_ipwAs the aforementioned trough digs deeper, moisture will continue to trickle into the four corners region. This will support a prolonged 4 day period of wet weather, mainly across southwest Colorado (Event #1). After Event #1, there will be a lull, as the monsoon connection is severed for a few days. But by late next week, another period of unsettled weather will start as a disturbance currently in the west Pacific migrates towards Colorado (Event #2). It appears that this event may be focused east of the Divide. Looking out past Event #2, there are indications of more active weather. However, at this time, we cannot confidently pinpoint the timeframe, or region most under threat. We will reinvestigate these details during the next update of the Flood Threat Outlook on Monday, 8/3. Stay tuned.

Now we look at the details of the two flood threat Events:

Event #1: Friday (7/31) – Monday (8/3)

An Elevated flood threat as a stubborn plume of subtropical moisture impacts mainly southwest CO

The digging disturbance off the California coast will send moisture into southwest Colorado, resulting in daily afternoon and evening thunderstorm activity. The region under the highest threat is the San Juan mountains, where upslope flow will result in heavy rainfall threat every day. Over the entire period, over 2 inches of rainfall can be anticipated for localized areas of the San Juans, with lesser totals to the north and even as far eastward as the Southeast Plains. In addition to an elevated flash flooding risk, isolated mud flows and debris slides can be anticipated with this event.


Legend

Event #2: Thursday (8/6) through Saturday (8/8)

An Elevated flood threat, mainly east of the Divide

A disturbance will trek all the way across the Pacific Ocean (see water vapor image) and impact Colorado by later next week. As this disturbance will likely be coming in from the west/northwest, it appears that the monsoonal connection may be limited. We anticipate that the Northeast Plains will be under the highest threat for heavy rainfall as a cool front makes its way down the east face of the Rockies, promoting easterly upslope flow. In additional to the rainfall threat, since this disturbance will be accompanied by jet stream dynamics, hail and strong winds may be possible. At this time it appears that the disturbance will keep moving east. However, it is possible, that the cool front will sag all the way into southeast Colorado, which will increase rainfall chances there too. We anticipate to have a much better grasp on this setup in the Flood Threat Outlook, on Monday 8/3.

Legend

FTB 07-30-2015: Moist Subtropical Flow Continues, Heavy Rain Threat Increasing

Issue Date: 7/30/2015
Issue Time: 10:15 AM

A MODERATE FLOOD THREAT IS FORECAST FOR PORTIONS OF THE SOUTHEAST PLAINS, RATON RIDGE, SOUTHEAST MOUNTAINS, SAN JUAN MOUNTAINS, AND SOUTHWEST SLOPE.

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, NORTHEAST PLAINS, SOUTHEAST PLAINS, RATON RIDGE, SOUTHEAST MOUNTAINS, CENTRAL MOUNTAINS, SAN JUAN MOUNTAINS, AND SOUTHWEST SLOPE.

The broad high-pressure ridge aloft will remain anchored to the south-central US, building slightly west and north throughout the day today. This will continue streaming subtropical moisture into Colorado, mainly south and east of a line from Logan County through Montrose County. North and west of that line, drier, westerly flow will continue and keep precipitable water values below average. This line that divides the moisture breaks the forecast into two distinct forecasts.

07302015_WV

To the north and west of the line, mostly sunny skies and warm temperatures will rule the day, with only a few isolated showers/weak thunderstorms possible over the higher terrain of the Front Range, Central Mountains, and Northern Mountains, with one or two moving over the northern portions of the Urban Corridor and Northeast Plains. Gusty winds and lightning will be the main threats from these storms, with modest rain rates (0.4-0.8 inches/hour).

The main action today and tonight will occur to the south and east of the line mentioned above. Deeper moisture, a passing disturbance, and decent instability will combine to bring a heavy rain threat to a relatively widespread area. That is not to say there will be widespread storms, just a widespread area where the heavy rain will be possible with any thunderstorm. Coverage is expected to be isolated-to-widely scattered across the lower elevations of eastern Colorado, with more scattered coverage across the higher terrain of southern Colorado and near the CO/NM border. Storm motions will be fairly slow, especially across mountain locations. A few showers and thunderstorms will linger into the early morning hours 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.
FTB_snapshot_20150730

Zone-Specific Forecasts

Northeast Plains south of I-76, Urban Corridor and Front Range south of I-70, Raton Ridge, Palmer Ridge, and Southeast Plains:

Isolated-to-widely scattered showers/thunderstorms expected, with coverage increasing from north to south. Heavy rain will be a threat, as deep moisture will be present, allowing for efficient rainfall production. Fairly slow storm motions will also be a factor, increasing the flood threat potential over locations that remain under a stationary, or near-stationary, storm. Maximum rain rates will be 2.0-3.0 inches/hour under the strongest storms, with 1.0-2.0 inches/hour being within reach for most storms.

Timing: 11 AM – Midnight, with a few showers and thunderstorms continuing into the early morning hours, mainly across southern zones.

Northwest Slope, Northern Mountains, Central Mountains, Grand Valley, Northeast Plains north of I-76, and Urban Corridor and Front Range north of I-70:

Mostly sunny and warm, with a few isolated thunderstorms possible. Drier air will be present, keeping rain rates relatively low (0.2-0.5 inches/hour over the mountains, 0.4-0.8 inches/hour over the Urban Corridor and Northeast Plains). Temperatures will be a few degrees warmer than yesterday.

Timing: 11 AM – 8 PM

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

Scattered showers and thunderstorms are expected, with fairly slow storm motions being a concern. The strongest storms will be capable of producing 1.2-1.8 inches/hour, or, with respect to slow-moving/stationary storms, up to 2.0-3.0 inches in 3 hours will be possible.

Timing: Primetime will be 11 AM – 10 PM, but showers and thunderstorms are expected to continue into the morning hours tomorrow.