Issue Date: 5/15/2015
Issue Time: 10:22 AM

A LOW FLOOD THREAT HAS BEEN ISSUED FOR THE CACHE LA POUDRE/SOUTH PLATTE RIVER FROM GREELEY THROUGH JULESBURG.

The upper-level low/digging trough that was mentioned in yesterday’s FTB is still hanging around near the West coast, but today it is on the move. It will slowly slide to the east-northeast, becoming centered over Utah by tomorrow morning. As it does this, a few things will happen…

1. Broad-scale lift and mid-/upper-level moisture will continue to overspread the state, as shown by the green arrow on the water vapor image below
2. The “dry slot” (marked by the yellow lines) will move over the top of much of the eastern plains
3. Cooler air will move in from the west, bringing snow levels along and west of the Continental Divide down to about 8500-9000 feet.
4. Surface low pressure will deepen/strengthen on the eastern plains, which will reinforce low-level moisture across the Northeastern Plains, while mixing out moisture/drying the low-levels elsewhere across the Urban Corridor, Palmer Ridge, and Southeastern Plains.

All of these ingredients together equal quite the variety of weather over Colorado, which is pretty typical for this time of year. West of the Continental Divide, snow will occur above 8500-9000 feet, with scattered showers and thunderstorms for lower elevations. The Front Range and Southeast Mountains will experience isolated-to-scattered showers and thunderstorms. The Urban Corridor (south of Castle Rock), Palmer Ridge and Southeast Plains will only see a few isolated thunderstorms, as low-level moisture gets eroded quickly by the developing surface low pressure. The Urban Corridor (north of Castle Rock) and Northeastern Plains will see isolated-to-scattered thunderstorms, with a few becoming strong-to-severe. The greatest risk for severe thunderstorms, including an isolated tornado, large hail, and wind gusts up to 65 mph will be north of a line from Cheyenne (WY) to Greeley to Fort Morgan to Wray.

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 (north of Castle Rock) and Northeast Plains:

Isolated-to-scattered thunderstorms will develop early this afternoon, with a few becoming strong-to-severe. As stated above, the greatest risk for severe thunderstorms will be north of a line from Cheyenne (WY) to Greeley to Fort Morgan to Wray.

Storms will begin to develop between Noon and 1 PM, racing quickly to the northeast at 15-30 mph. The main threats from storms will be heavy rain, hail, and lightning, with isolated tornadoes, large hail, and wind gusts up to 65 mph north of the line mentioned above. Heavy rain is not expected to cause flash flooding due to the quick storm motions. Showers and thunderstorms will diminish over the area between 8-10 PM, with a few showers continuing until midnight.

Urban Corridor (south of Castle Rock), Palmer Ridge, Raton Ridge and Southeast Plains:

A few isolated thunderstorms, mainly high-based, will work across the area this afternoon and evening. Due to a lack of moisture in the low-levels, and the previously mentioned “dry slot,” there won’t be much fuel for storms. Expect rain rates to stay in the 0.1-0.3 inch/hour range, with storms producing more wind and lightning than rain.

Timing: Noon – Midnight.

Front Range and Southeast Mountains:

Isolated-to-scattered showers and thunderstorms, with the Front Range holding the better chance of the two regions. The strongest storms over the Front Range will produce brief, heavy rain, but quick storm motions will eliminate any flash flood threat.

Timing: 11 AM – 6 PM, with a few remaining showers through 6 am tomorrow.

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

Snow above 8500-9000 feet, rain below. 5-9 inches of snow will accumulate over the high elevations of the San Juan Mountains, with 3-5 inches over high elevations elsewhere. Below 8500 feet, expect scattered showers and a few thunderstorms throughout today and overnight tonight. Max rain rates will be in the 0.25-0.5 inch/hour range.

Timing: 11 AM – 11 AM tomorrow

Issue Date: 5/14/2015
Issue Time: 10:14 AM

A LOW FLOOD THREAT HAS BEEN ISSUED FOR THE CACHE LA POUDRE/SOUTH PLATTE RIVER FROM GREELEY THROUGH JULESBURG.

Today’s weather is fairly well-outlined in the water vapor image below; the black arrow represents the digging trough over the West Coast and the green arrow represents the Pacific moisture feed. These two features are working hand-in-hand to send another wave of showers and thunderstorms to Colorado, while simultaneously working to diminish the risk of heavy rain from those thunderstorms. Also working against the threat of heavy rain? A relative lack of low-level moisture.

The broad-scale lift that is associated with the digging trough over the west coast will spread mostly cloudy skies across Colorado; this will limit instability. Limiting instability is always a good bet to limit maximum rain rates, but combine that with the fact that moisture will be lacking near the surface, and you have a recipe for showers and thunderstorms that produce more wind than rain. Additionally, limited instability means that the plains will see much less coverage of showers and thunderstorms than the mountains. For more details, jump down to the zone-specific forecasts below the threat map.

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, Palmer Ridge, Northeast Plains, and Southeast Plains:

Mostly sunny skies early will give way to partly sunny skies this afternoon as the broad-scale lift and mid-level moisture overspreads the area, limiting instability. Isolated showers and weak thunderstorms will begin to move off of the mountains and over the area between 1 and 2 PM, with isolated activity developing over the plains between 2 and 3 PM. Most showers/thunderstorms will end just after sunset, with a few lingering until 10-11 PM. Rain rates will generally be in the 0.3-0.6 inches/hour range, with the strongest storms (near the CO/KS border) producing rain in the 0.8-1.0 inch/hour range.

Front Range, Southeast Mountains, San Luis Valley, and Raton Ridge:

Isolated-to-scattered showers and thunderstorms expected, first forming over the higher terrain and then spreading over the valleys. Rain rates will generally be in the 0.25-0.5 inch/hour range, with storms producing more wind than rain (gusts up to 40 mph).

Timing: 11 AM – 10 PM, with a few showers lingering after midnight over the higher elevations of the Front Range and Southeast Mountains.

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

These will be the regions that see the most activity today, with scattered showers and thunderstorms expected. A few showers are currently ongoing across southwestern Colorado, as a weak boundary lifts across the area. Cloud cover that is expanding across these regions will hold back stronger thunderstorm activity from developing until around lunchtime. Most showers and thunderstorms will produce rainfall in the 0.25-0.6 inches/hour range, with the strongest storms this afternoon producing rain rates of 0.7-0.9 inches/hour. The most likely regions for stronger thunderstorms to develop will be over the Northwest Slope, higher elevations of Grand Valley, and Southwest Slope. These storms will have the potential to produce wind gusts of 45-60 MPH.

Timing: 11 AM – Midnight, with a few scattered showers and thunderstorms continuing through the morning tomorrow.

Issue Date: 5/13/2015
Issue Time: 10:18 AM

— LOW flood threat for the South Platte River between Greeley and Sterling

Today’s weather pattern is crowded with players. Of those, one interesting player that has persisted for the greater part of two weeks now is the subtropical jet. Today, it is seen just south of the U.S. and Mexico border, by California and Arizona. And what an impressive jet it is. In the heart of the subtropical jet, the maximum wind speeds about 30,000 feet above the ground are a whopping 150 mph. This is a very high value in any jet stream, but is especially impressive given that it is May and the jet stream is supposed to fade at this time of year. This jet stream appears to be a signature of a potentially potent El Nino that may continue to strengthen through the course of the summer. But, since this is the Flood Threat Bulletin, we will simply mention to readers to check out our Flood Threat Outlook for more information about this and the longer range implications.

So one may ask how is the subtropical jet relevant to our weather? To answer this, let’s focus again on the water vapor image. At the eastern extent of the subtropical jet, disturbances often form and can then propagate farther “downstream” or in this case, northeastward. This morning, a pronounced disturbance is situated across Colorado and is clearly visible on water vapor imagery and the visible satellite. Light rainfall, with pockets of moderate rainfall, is already noted mainly across eastern Colorado. As this disturbance continues to trek northeastward, the shower activity will move with it. Rainfall rates will be generally light, in the 0.2 to 0.5 inch per hour range. However, during the afternoon, the heating of the day will cause weak instability, and enough to produce thunderstorm activity. Since the timing of thunderstorm formation will not favorably coincide with the disturbance dynamics, we expect more bark than bite from these storms. Nonetheless, once storms form this afternoon across the Front Range, Palmer Divide and Southeast Plains, isolated rainfall amounts of 1 inch per hour cannot be ruled out. This is supported by precipitable water values ranging from 0.6 to 0.8 inches this morning. However, with the heavier rainfall being confined to mainly east of the mountains, these amounts are not expected to cause flooding, even with the saturated soils. In the disturbance’s wake, expect more sun the clouds to emerge over southwest Colorado and spread slowly north and eastward throughout the day.

With an absence of additional rainfall, river levels continue to recede from the past storm. Today, we remove the flood threat over the Arkansas River, and downgrade the flood threat over the South Platte river to a LOW status.

Finally, as our Flood Threat Outlook suggested on Monday, there is no end to the active weather in sight as another impressive disturbance begins to encroach upon Colorado on Thursday.

Please see below for area-specific flood threat and forecasts (click on “About the FTB” link above to view Colorado region names).

Zone-Specific Forecasts

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

Scattered showers this morning with scattered thunderstorms developing in the afternoon. Rainfall rates will generally be in the 0.2-0.5 inch per hour range, but an isolated storm capable of producing 1 inch per hour rainfall cannot be ruled out. Gusty winds to 50 mph and small hail may accompany the stronger cells. Flooding from rainfall is not expected, but a LOW flood threat has been posted for the South Platte River.

Primetime for thunderstorms is between 2PM and 7PM.

Central Mountains, San Juan Mountains, Southeast Mountains and Northern Mountains:

Partly cloudy with scattered showers this morning gradually clearing by afternoon. A stray thundershower cannot be ruled out, but little if any rain will result from it. No flood threat is expected.

Southwest Slope, Northwest Slope, Grand Valley and San Luis Valley:

Partly cloudy this morning with isolated sprinkles and flurries. Clearing out this afternoon as a disturbances moves away from the area. No flood threat is expected.

Issue Date: 5/12/2015
Issue Time: 10:05 AM

A MODERATE FLOOD THREAT HAS BEEN ISSUED FOR THE CACHE LA POUDRE/SOUTH PLATTE RIVER FROM GREELEY THROUGH JULESBURG, AND FOR THE ARKANSAS RIVER FROM PUEBLO THROUGH LA JUNTA.

Showers and thunderstorms will make a return to parts of Colorado as the upper-level ridge shifts eastward. Take a look at the black-dashed line on the water vapor imagery below (represents the ridge axis). To the right of the axis, northwest flow aloft, and to the left, southwest flow. Southwest flow aloft means a bit more moisture will be present in the mid- and upper-levels, providing some fuel for isolated-to-scattered showers and thunderstorms this afternoon and evening, mainly over the higher terrain (a couple of showers continuing overnight). The best chance of scattered storms will be over the San Juan Mountains and Central Mountains, with storms being more isolated across the Northern Mountains, Northwest Slope, Southwest Slope, Grand Valley, San Luis Valley, Front Range, and Southeast Mountains. Additionally, a few isolated storms will work over the Urban Corridor, Palmer Ridge, Northeast Plains, and Southeast Plains during the afternoon and evening hours.

Moisture will not be deep enough to cause flooding concerns from thunderstorm rainfall today with precipitable water values near, or below, 0.5 inches. Additionally, most of the moisture is contained to the mid- and upper-levels of the atmosphere, while the low-levels remain fairly dry. This all equals a day where the strongest thunderstorms will produce more gusty winds than rain.

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, Palmer Ridge, Northeast Plains, and Southeast Plains:

Mostly cloudy for the Urban Corridor and Palmer Ridge, and mostly sunny for the Northeast and Southeast Plains. A few isolated storms will move off of the higher terrain this afternoon, moving in a southwest-to-northeast direction. Max rain rates will be in the 0.4-0.8 inches/hour range, but drier air in the low-levels will work against that, making the actual rain rates at the surface more likely 0.3-0.5 inches/hour.

Timing of storms: 2 PM – 10 PM, with one or two showers continuing until midnight.

Central Mountains and San Juan Mountains:

Scattered showers and thunderstorms expected today, beginning between 11 AM and Noon, and continuing until 10 PM, with isolated showers continuing through the night and into tomorrow morning. Thunderstorms will result more in gusty winds than rain. Max rain rates will be in the 0.5-0.8 inch/hour range, but as is the case for the region above, drier air in the lower-levels will work against that, likely resulting in rain rates near 0.4-0.6 inches/hour.

Front Range, Northern Mountains, Northwest Slope, Southwest Slope, Southeast Mountains and Raton Ridge:

Isolated showers and thunderstorms will begin around Noon, and continue through the afternoon and evening hours. Storms will diminish from north to south this evening, with southern mountain locations experiencing isolated showers into tomorrow morning. Rain rates will be in the 0.25-0.45 inches/hour range.

Grand Valley and San Luis Valley:

Isolated showers and thunderstorms will drift off of the surrounding higher terrain and over the valleys this afternoon and evening, producing mainly gusty winds and light rain.

Timing: Noon – Midnight for the Grand Valley, Noon – 6 AM for the San Luis Valley