FTB 07-29-2016: Another Round of Thunderstorms Expected

Issue Date: 7/29/2016
Issue Time: 10:08 AM

A LOW FLOOD THREAT IS FORECAST TODAY FOR PORTIONS OF THE URBAN CORRIDOR, PALMER RIDGE, NORTHEAST PLAINS, SOUTHEAST PLAINS, AND RATON RIDGE.

Northwest flow aloft will continue across Colorado today, as shown by the black line/arrows in the water vapor image below. Embedded within the northwest flow is a mid-level disturbance (yellow line), which will serve as a trigger for another afternoon and evening of isolated-to-widely scattered thunderstorms along and east of the Front Range/Southeast Mountains. The support provided by the disturbance is fairly weak, so the number of storms will be less than yesterday.

Low-level moisture is good to the east of the mountains, where widespread dewpoints in the 50s and low 60s can be found. This amount of moisture combined with sufficient daytime heating will keep a low flood threat in the forecast. A few strong/severe storms are likely, especially along the Front Range and Urban Corridor where terrain influences will enhance the local storm environment. The main threats from these storms will be large hail, gusty winds, and locally heavy rainfall. Rain rates will be mentioned in the zone-specific discussions.

WV_20160729

West of the Front Range/Southeast Mountains, drier air will result in very few isolated showers/thunderstorms, if any at all. The water vapor image above shows the drier air overtop Colorado, especially as compared to the moist plumes (brighter white colors) over Arizona, New Mexico, and Utah. Low-level moisture is of utmost importance to storms in this type of environment, and there just isn’t much to be found west of the Front Range/Southeast Mountains. Overall, the locations with the best chance to see an isolated, high-based shower/storm or two are the higher terrain of the Central Mountains, Southwest Slope, and San Juan Mountains. The main impact will be gusty outflow winds, lightning, and virga, with very little rainfall reaching the surface.

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.
20160729_LFT

Flood Threat Legend

Zone-Specific Forecasts

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

Isolated showers/thunderstorms are expected this afternoon and evening, with a couple continuing over the Northeast Plains and Southeast Plains until around midnight or so. Due to cooling provided by yesterday’s thunderstorms, activity will likely be delayed until the mid-afternoon, waiting on the disturbance and ample daytime heating able to break through the mid-level cap.

The first storms of the day will go up over the Front Range/Southeast Mountains, generally moving to the east-southeast across the lower elevations with the mean flow. A couple of the storms will be strong/severe, producing large hail, gusty winds, lightning, and locally heavy rainfall. Maximum rain rates are as follows:

Front Range and Southeast Mountains: 0.6-0.8 inches/hour
Urban Corridor and Palmer Ridge: 1.5-2.0 inches/hour
Northeast Plains and Southeast Plains: 1.5-2.5 inches/hour
Raton Ridge: 0.8-1.2 inches/hour

Timing: Noon – 11 PM for the mountains, 2 PM – Midnight for the Urban Corridor, 3 PM – 3 AM for the Palmer Ridge, Northeast Plains, and Southeast Plains, and 1 PM – 10 PM for the Raton Ridge

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

Mostly sunny and hot will be the main weather story today, minus a couple isolated, high-based showers/thunderstorms over the higher terrain. Very little rainfall, if any, will reach the surface, so gusty outflow winds and lightning will be the main threats.

Timing: Noon – 10 PM

STP 07-29-2016: Hot and Mostly Dry over the High Country, Strong/Severe Thunderstorms East

Issue Date: Friday, July 29th, 2016
Issue Time: 9:00 AM MDT

Summary:

The High Country and Western Slope remained mainly dry yesterday as the atmosphere could only an isolated shower/weak thunderstorm or two. Virga was noted under a few showers/thunderstorms; very little rain, if any, reached the surface, as gusty winds were the main result. Due to nearby shower/thunderstorm activity, an outflow wind gust of 39 mph was recorded at the Grand Junction Airport, and a gust of 47 mph was recorded at the La Plata County Airport.

The big weather story of the day were the isolated-to-scattered strong/severe thunderstorms that rumbled across portions of the Urban Corridor, Front Range, Palmer Ridge, Southeast Plains, and Northeast Plains. Heavy rain caused standing water on roadways in Limon, flooded roads in Security-Widefield (where a water rescue was needed), street flooding 2 miles NW of Peterson AFB, and likely caused street/field flooding/ponding underneath the majority of strong/severe thunderstorms due to a combination of heavy rain and hail. The largest reported hail stones were 2.5 inches in diameter: 2 miles S of Peterson AFB (El Paso County), 4 miles WSW of Falcon (El Paso County).

For a complete look at 24-hour precipitation totals, please take a look at the Storm Total Precipitation map below. Keep in mind that hail contamination has likely inflated rainfall totals as estimated by the radar.


Storm Total Precip Legend

FTO 07-28-2016: Potential for the First High Threat of 2016 Shows up for Event #3

Issue Date: 7/28/2016
Issue Time: 1:30 PM

Threat_Timeline_20160728

Three events await Colorado over the next fifteen days, with two main culprits at which to point. The first culprit is the upper-level trough denoted as Event #1 in the water vapor image below. Within this trough, there are multiple low pressure circulations, and this will allow for the more poleward circulation to split from the main trough and become embedded in the main flow channel aloft. This will flatten the ridge, provide a shortwave disturbance to focus Event #1’s flood threat on Friday, and then exit to the east in a generally quick fashion. The more equatorward disturbance will instead dig along the West Coast and set the stage for Event #2, the early stages of the next monsoon surge. Event #2 receives no flood threat designation at this time, as there isn’t enough indication of a widespread heavy rain event. That doesn’t mean a flood threat will not present itself during the FTB, so stay tuned to daily updates.

WV_20160728

Event #2 can be described as the precursory event to the main monsoon surge associated with Event #3 – the big show in this Flood Threat Outlook period. An influx of deep moisture from the south/southwest looks increasingly likely, providing plenty of fuel for multiple days of showers and thunderstorms capable of heavy rainfall. This is a fairly typical monsoon setup for August, and we have our eyes on it. We will continue to monitor this period as we move forward in time. After Event #3, the monsoon will slowly subside, leading to generally less coverage throughout the rest of the period. Temperatures will heat up during this time frame, likely above average for many locations, but such is August weather in Colorado.

Event #1: Friday (07-29-2016)

Elevated Flood Threat as Upslope Flow Reinforces Moisture

The upper-level high will get pushed eastward as the upper-level low digs along the West Coast, which will help a little extra mid-level moisture work its way into Colorado. Meanwhile, at the surface, upslope flow behind a cool front will reinforce moisture across eastern Colorado. This moisture influx will set the stage for a shortwave aloft to kick off scattered showers/thunderstorms across the Front Range/Southeast Mountains, working across the adjacent lower elevation. A moderately capped environment will exist across the far eastern plains, so expect less coverage there. Out west, it will remain dry and hot.

Legend

Event #2: Sunday (07-31-2016) through Wednesday (08-03-2016)

No Apparent Flood Threat as we Await More Details

Monsoon moisture will begin to increase across the state underneath the building ridge aloft, while a mid-/upper-level low spins over/near the Pacific NW. The moisture tap will not quite get unleashed, remaining steady or only slightly increasing each day. Pinning down the exact location of any disturbance aloft will be key in identifying any developing flood threats, but, at this time, there isn’t enough indication of a widespread heavy rain event to require a flood threat. We will watch this time frame closely in case moisture, instability, or upper-level support are better than currently advertised.

Legend

Event #3: Thursday (08-04-2016) through Tuesday (08-09-2016)

Elevated Flood Threat as Monsoon Tap Opens Up

The monsoon moisture tap will really open up during this time frame, with deep south-southwesterly flow aloft across the region. This influx in deep moisture will look to bring about a widespread heavy rain threat for multiple days. The longer the moisture tap remains open, the more concerning this period may potentially become due to antecedent conditions. This period may require an upgrade to high threat as details become clearer over the next few days. Stay tuned.

Legend

FTB 07-28-2016: Hot and Dry West, a Few Strong/Severe Storms East

Issue Date: 7/28/2016
Issue Time: 9:56 AM

A LOW FLOOD THREAT IS FORECAST TODAY FOR PORTIONS OF THE URBAN CORRIDOR, PALMER RIDGE, NORTHEAST PLAINS, AND SOUTHEAST PLAINS.

A fairly complicated flood threat situation for eastern Colorado awaits us today as different atmospheric components will work against one another. On one hand, the moisture profile that exists currently is top heavy, meaning the most of the moisture lies in the mid-levels. On the other hand, outflow boundaries from earlier convection have moistened the boundary layer over the plains, making it more conducive to heavy rain. How far westward this moistening can go, while fighting dry westerly winds from the mountains, is a big question mark with respect to any storms over the Urban Corridor producing heavy rainfall.

Additionally, the outflow boundaries will have a negative effect on potential for thunderstorms – the cooling associated with them, underneath warm mid-level temperatures, will act to “cap” the environment, effectively limiting thunderstorm potential. And on the other side of that coin, the convergence associated with outflow boundaries could overcome the cap, and strong instability will await with steep mid-level lapse rates. All in all, isolated-to-widely scattered thunderstorms are expected across eastern Colorado, with coverage increasing from west to east. The low flood threat is issued for the potential of heavy rainfall due to increasingly moistened low-levels by outflow boundaries. Storm motions will also be moving fairly briskly to the southeast, so storm motions will help mitigate the flood threat somewhat.

IPW_20160728

For areas along the Continental Divide and westward, only a couple isolated, high-based showers/thunderstorms are expected. The atmosphere is drier over western Colorado, and nearly all of the moisture resides approximately 4-5.5 km above the surface. This means that any storms that develop will produce gusty winds and lightning, but very little, if any, rainfall at the surface. Virga will likely be the best that any activity can do. Otherwise, it will be a hot day across the region, with lower valleys reaching into the 90s and above 100 in the Colorado River valley near Grand Junction.

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.
20160728_LFT

Flood Threat Legend

Zone-Specific Forecasts

Urban Corridor, Northeast Plains, Palmer Ridge, Raton Ridge, and Southeast Plains:

Isolated-to-widely scattered showers/thunderstorms expected throughout today and tonight, with a couple lingering into the morning hours tomorrow. The scenario is a bit jumbled, as described at length above, but the overall result will be a few strong/severe thunderstorms, mainly across the Northeast Plains, northeastern sections of the Southeast Plains, and eastern portions of the Palmer Ridge. These storms will be capable of producing large hail (up to 2-2.5 inches in diameter), strong winds (gusts up to 70 mph), and an isolated tornado or two.

Further west, any strong/severe development will be capable of producing hail up to 1.25 inches in diameter, wind gusts up to 60 mph, and an isolated, weak tornado cannot be ruled out. Storm motions will help mitigate the flood threat, making today a low-end, low flood threat. Maximum rain rates are as follows:

Urban Corridor: 0.8-1.2 inches/hour
Palmer Ridge: 0.8-1.4 inches/hour
Northeast Plains and Southeast Plains: 1.5-2.0 inches/hour

Timing: Noon – 6 AM

Front Range and Southeast Mountains:

A few isolated showers/thunderstorms are expected throughout today and into tonight. Stronger storms will produce gusty winds and small hail, as well as brief bouts with moderate rainfall. There is a low chance (~15%) for redevelopment after midnight tonight as outflow boundaries from plains thunderstorms move into the foothills. Maximum rain rates will be 0.6-0.8 inches/hour.

Timing: 1 PM – 11 PM, with potential redevelopment between 11 PM and 3 AM.

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

A couple isolated, high-based showers/thunderstorms will be all the environment can muster, mainly over the higher terrain of the San Juan Mountains and Southwest Slope. Any activity will produce mainly gusty winds and lightning, with very little, if any, rainfall reaching the surface. Maximum rain rates are less than 0.10 inches/hour. Temperatures will be hot and run about 10 degrees above normal everywhere.

Timing: Noon – 8 PM