Issue Date: 7/24/2015
Issue Time: 10:00 AM

NO FLOOD THREAT IS FORECAST.

Not much has changed since yesterday, as upper-level high pressure remains the big player in our weather today. Mid-/upper-level disturbances continue to track across the northern US (black dashed lines), but remain unable to break into that strong high pressure ridge sitting across Colorado.

Southwest flow aloft will continue through today, bringing an increase in mid- and upper-level moisture to much of the state, while the Northwest Slope and Northern Mountains regions remain under drier flow aloft. This is denoted well in the water vapor image below; the green line separates the dry air from more moist air, and the green and orange arrows show the separate feeds supporting the resultant conditions. This has led to an increase in cloud cover this morning for areas under the moist flow, which will have limiting effects on the strength of potential thunderstorms this afternoon.

Scattered thunderstorms are expected across the High Country today and tonight, with the best coverage across the San Juan Mountains, Central Mountains, and Southeast Mountains (where the best mid-level moisture resides). The Northwest Slope will stay dry, while the Northern Mountains and Grand Valley will hold a slight chance for an isolated thunderstorm or two this afternoon/evening. The Front Range will see a couple isolated thunderstorms, mainly south of I-70.

As far as eastern Colorado is concerned, most locations will stay dry today, but isolated-to-scattered thunderstorms are expected across the Northeast Plains, Palmer Ridge, Southeast Plains, and Raton Ridge. A surface convergence zone will lay approximately along a line from Sterling to Limon to Trinidad, and this will help aid thunderstorm development. Any thunderstorms that develop will produce rainfall fairly efficiently compared to yesterday, but dry air in the lowest 10,000 feet will evaporate a fair amount of the precipitation before reaching the surface. The main threats from these storms will be gusty outflow winds, lightning, and hail. Storm motions will generally be to the east/northeast with the mean flow.

The Urban Corridor will see an isolated thunderstorm or two move off of the Front Range, but the main threat will be gusty winds and lightning – dry air near the surface will really cut into the heavy rain potential.

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.

Zone-Specific Forecasts

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

Isolated-to-scattered thunderstorms expected this afternoon and evening, with the biggest threats being strong outflow winds and dangerous lightning. Large hail will be possible along and east of the surface convergence zone mentioned above. Maximum rain rates will vary, so they are as follows:

Urban Corridor: 0.4-0.7 inches/hour
Raton Ridge: 0.7-1.0 inches/hour
Palmer Ridge, Northeast Plains, and Southeast Plains: 1.0-1.5 inches/hour.

Timing: 1 PM – 9 PM, with a few isolated thunderstorms continuing until around midnight across far eastern Colorado

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

Isolated-to-scattered thunderstorms expected today and tonight, with the best coverage occurring across the higher terrain of the Central Mountains, San Juan Mountains, and Southeast Mountains. The Northwest Slope will stay dry, and the Grand Valley/Northern Mountains regions will be mainly dry with only a couple isolated storms over the higher terrain. The Front Range will see isolated thunderstorms mainly south of I-70. Storms will generally move to the northeast, so lower elevations downstream will likely see thunderstorms move overhead. Rain rates will not be particularly high, as the moisture is mainly confined above 550 mb; maximum rates will be 0.4-0.6 inches/hour.

Timing: 11 AM – 8 PM, with a couple showers/weak thunderstorms lingering until about midnight across southern areas.

Issue Date: 7/23/2015
Issue Time: 9:41 AM

NO FLOOD THREAT IS FORECAST.

The overall pattern governing Colorado’s weather today will not change much over yesterday, but subtle shifts are apparent and will have effects on today’s forecast. If you take a look at the water vapor image below, and remember back to yesterday’s image, you’ll be able to pick out the features in the broad scale pattern that remain basically the same; the upper-level ridge across the central US, flanked by upper-level troughs on either coast.

The high pressure center denoted by the blue circle/arrows and blue “H” will build to the west/northwest, becoming centered over the TX/OK panhandles. This will keep west/southwest flow aloft in place across Colorado, which will continue to pump in drier, mid-level air for northern and northwest portions of Colorado, and reinforce the mid-level moisture across southern and southeastern Colorado.

Low-level moisture is lacking statewide, so the result of any mid-level moisture will be high-based thunderstorms, confined to over, and near, the higher terrain along and south of I-70. Heavy rain is not expected, so the main impacts will be dangerous cloud-to-ground lightning and gusty winds. Further east over the plains, today will be hot and dry as slight warming takes places aloft and caps off the environment. The exception to this will be along the CO/KS border; details are in the zone-specific forecast below. High temperatures will be a few degrees warmer than yesterday statewide.

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.

Zone-Specific Forecasts

Front Range north of I-70, Grand Valley, Northern Mountains, and Northwest Slope:

Mostly sunny and hot will be the main story today, as temperatures climb a few degrees over yesterday. A few fair weather clouds will develop this afternoon, but due to the dry air in place, no precipitation is expected.

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

Mostly sunny and hot, with temperatures climbing into the upper 80s/low 90s across the Urban Corridor, Palmer Ridge, and Raton Ridge, with widespread mid-90s temperatures across the Plains. The Arkansas River Valley across the Southeast Plains will be the hot spot today, with temperatures reaching into the low 100s.

A thunderstorm or two will be possible near the interface with the Front Range south of I-70 and Southeast Mountains for the Urban Corridor, Southeast Plains and Raton Ridge regions. Westerly flow aloft will try and push an isolated, weak thunderstorm overhead. Gusty winds and lightning will be the main result, with very little precipitation expected. Rain rates will be less than 0.4 inches/hour.

Along the CO/KS border, there will be a slight opportunity for an isolated thunderstorm or two to develop this afternoon/evening. A surface trough will be present along the border, with sufficient low-level moisture pumping in from the southeast. It is entirely possible that the activity stays on the Kansas side of the border, but there remains a small chance that it occurs just inside the Colorado border. For that reason, it gets a mention in today’s forecast. Maximum rain rates for these storms will be 0.6-1.2 inches/hour.

Timing: 2 PM – 9 PM

Front Range south of I-70, Central Mountains, Southeast Mountains, San Luis Valley, Southwest Slope, and San Juan Mountains:

Isolated, high-based showers and weak thunderstorms will develop this afternoon and evening as strong daytime heating works with the available mid-level moisture. Due to lacking low-level moisture, precipitation will generally be light, and the main threats will be dangerous cloud-to-ground lightning and gusty winds. The best coverage will occur across the San Juan and Central Mountains. Maximum rain rates will be 0.15-0.3 inches/hour.

Timing: Noon – 8 PM

Issue Date: 7/22/2015
Issue Time: 9:55AM

No Flood Threat Today

Due to the wavy nature of the atmosphere, the day after a disturbance passage is often times accompanied by drier weather as subsidence takes hold. Such is the case today. As shown in the water vapor image, below, yesterday’s disturbance is now a gradually weakening mass of clouds currently over central Kansas. Much drier conditions are noted over Colorado, although interestingly this is not yet reflected by surface dewpoints. Morning dewpoints range from the in the 40s to as high as low 60s statewide. However, this moisture is shallow, with Precipitable Water (PW) values in the 0.6 to 0.8 range. Yesterday’s incredibly “juicy” PWs have fallen by 30-40% into a more typical range for Colorado in mid-July.

For today, we anticipate mostly sunny skies this morning to fill in with puffy cumulus clouds by early afternoon. Scattered high-elevation thunderstorms will once again dot the landscape, mostly north of I-70. By later in the afternoon, a few isolated to widely scattered storms will be possible in the Northeast Plains as well. However, due to the strengthening ridge to the south, increasing subsidence will cap rainfall rates at around 0.4 inches over the higher terrain and around 0.7 inches in the far eastern Plains. Thus, no flooding is expected 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, jump below the map.

Zone-Specific Forecasts

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

Sunny early with a few cumulus clouds developing by early afternoon. Scattered high-elevation storms will occur over, mainly north of I-70 from the early afternoon through early evening hours. Max 1-hr rainfall will be 0.4 inches. Isolated storms will also be possible over the Northeast Plains, with max 1-hr rain rates up to 0.7 inches. No flooding is expected today.

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

Sunny to partly cloudy early with scattered afternoon storms possible across the higher elevations, mainly north of I-70. Max 1-hr rain rates will be 0.4 inches so no flooding is expected today.

Issue Date: 7/21/2015
Issue Time: 10:42 AM

A MODERATE FLOOD THREAT IS FORECAST FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, NORTHEAST PLAINS, AND SOUTHEAST PLAINS.

A LOW FLOOD THREAT IS FORECAST FOR PORTIONS OF THE GRAND VALLEY, NORTHWEST SLOPE, NORTHERN MOUNTAINS, CENTRAL MOUNTAINS, FRONT RANGE, URBAN CORRIDOR, NORTHEAST PLAINS, SOUTHEAST PLAINS, PALMER RIDGE, SOUTHEAST MOUNTAINS, AND RATON RIDGE.

There are quite a number of factors at play for today’s forecast, and it leads to a forecast full of caveats. Let’s start by taking a look at the water vapor imagery below; the black line represents the location of a mid-level disturbance, the green line/arrows represent moist, east/southeasterly surface flow, and the “dry air”/white arrow indicate the presence, and motion, of the dry air working in from the southwest. Not shown on the water vapor imagery, but every bit as important, is cloud cover. Timing will mean everything today.

The timing of the shortwave is a concern today, as it is currently sliding across northern Colorado, kicking off scattered showers and thunderstorms. This has also allowed for widespread cloud cover to develop along and east of the Mountains, and this will limit atmospheric instability for those areas underneath the clouds this morning. This will not eliminate the heavy rain threat because moisture values are still sufficiently high. Instead, this will lead to bands of thunderstorms, rather than discrete supercells. This brings in a separate concern for portions of the Urban Corridor, Palmer Ridge, and adjacent portions of the Northeast and Southeast Plains: training/slow-moving thunderstorms.

A surface convergence line will set itself up across those areas this afternoon, providing a focus for thunderstorm development and consistent motion. Due to the limited instability from early cloud cover, rain rates will not be as high as they could be, but training/slow-moving storms will be a concern, and thus, a moderate flood threat has been hoisted for those areas. Additionally, if the shortwave moves through quick enough and allows for clearing skies over the mountains early this afternoon, sunshine will create a corridor of greater instability, which will bring the threat of strong, semi-discrete thunderstorms into play across the Front Range foothills, Urban Corridor, and Palmer Ridge, which would then grow upscale into a mesoscale convective system as it moves east-southeast. With the amount of moisture available, storms that develop in that environment and with that morphology would produce high rainfall rates.

The shortwave will be more favorably timed for afternoon thunderstorms further east over the plains. Combined with greater instability, rain rates will be sufficiently high to warrant a widespread low flood threat. Field and street flooding will be the main concern under strong thunderstorms, especially for areas that received heavy rainfall over the past few days. Low-lying road crossings and streams will need to be monitored for flash flooding issues.

Training thunderstorms are ongoing across the I-70 corridor of western Colorado, producing rainfall at approximately 0.75 inches/hour. As scattered thunderstorms develop this afternoon, this swath will need to be monitored for unstable soil conditions and flash flooding issues. If you weren’t tired of caveats, here is one more for you: the dry air working in from the southwest will begin to push moisture values down across western Colorado this afternoon. Moisture from the north is fighting back against it for the time being, but by later this afternoon/early evening, the dry air will win out. It’s a race against the clock for thunderstorms, and due to this limiting factor, only a low flood threat is warranted.

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.

Zone-Specific Forecasts

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

With a long discussion above, I will focus on rain rates and timing in this section. Maximum rain rates, if all ingredients come together correctly, break down as follows:

Front Range and Southeast Mountains: 1.2-2.0 inches/hour
Urban Corridor, Palmer Ridge, Northeast Plains, Southeast Plains, and Raton Ridge: 2.0-3.0 inches/hour

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

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

Scattered showers and thunderstorms are ongoing across the I-70 corridor, and will continue through the next few hours. Additional scattered development is likely to begin early this afternoon. Maximum rain rates will be 0.8-1.2 inches/hour.

Timing: 11 AM – 8 PM, with a few scattered showers/weak thunderstorms continuing over the higher terrain into the early morning hours.

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

Isolated showers and thunderstorms are expected as sunshine increases this afternoon, but rain rates will be lowest across these regions as dry air continues to move in from the southwest. Maximum rain rates will be 0.25-0.6 inches/hour.

Timing: 11 AM – 6 PM