FTB 06-30-2015: June To End Hot And Mostly Dry

Issue Date: 6/30/2015
Issue Time: 9:45AM

Flooding is NOT expected today.

The active month of June will end with a twist as generally dry, and very hot weather is expected today. This is all courtesy of an impressive, though temporary, ridge of high pressure parked just west of Colorado, as shown by the water vapor image below. The ridge will promote very warm mid and upper-level temperatures: for example, the freezing level today will be around 20,000 feet. Because surface moisture still exists, with morning dewpoints in the 40s to low 50s, there will be some isolated to widely scattered thunderstorms. However, with precipitable water falling from 0.6-0.8 inches this morning to about 0.5 inches by late afternoon, we expect rainfall to stay below 0.5 inches. Hikers: do not forget that thunderstorms can produce lightning even without rainfall!

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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_20150630

Zone-Specific Forecasts

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

Mostly sunny and very hot today with high temperatures up to 105F in the lowest elevations. An isolated thunderstorms coming from the higher terrain is not impossible, but will produce more of a gusty wind and lightning threat that any rainfall.

Central Mountains, Northern Mountains, Front Range, Southeast Mountains, San Juans:

Mostly sunny early, then partly cloudy and warm with isolated to widely scattered thunderstorms forming by mid afternoon. Hourly rain rates are expected to stay below 0.5 inches, and no flooding is expected.

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

Mostly sunny early with a few clouds forming by early afternoon. An isolated thunderstorm cannot be ruled out, especially by the Wyoming border. However, rainfall will stay below 0.25 inches and no flooding is expected.

STP 06-30-2015: Very Limited Storm Coverage

Issue Date: 6/30/2015
Issue Time: 9:00AM

Drying from above was a formidable obstacle for thunderstorm development on Monday. Storms that were able to form looked very impressive for one or two radar scans (radar scans are about 6 minutes apart), before their dry downdrafts evaporated the storm structure from within. Only limited rainfall actually made it to the ground. The highest official 24-hour rainfall totals came from the San Juan region with a few reports around 0.25 inches. Otherwise, our radar estimated rainfall map, below, shows one storm in eastern Lincoln and western Kit Carson counties that may have delivered over 0.5 inches to mainly rural areas.

Switching topics a bit, in all of Colorado only one stream flow gage officially remains in Minor flood stage: the Arkansas River at Avondale (just downstream of Pueblo). Flows jumped up just a bit overnight due to releases from the nearly full Pueblo Reservoir. All in all, it seems we can finally close the book on the impressive 2015 snow melt season here in Colorado.

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FTO 06-29-2015: RIDGE WILL CONTINUE TO DOMINATE, BUT NOT WITHOUT SOME RAINFALL

Issue Date: 6/29/2015
Issue Time: 1:50PM

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Over the past week or two, it has been both fascinating and impressive to watch the atmosphere generate some noteworthy thunderstorms given unfavorable conditions in the upper atmosphere (courtesy of the famous upper-level high pressure system). Fortunately, all of these storms have been very localized in both space and time, resulting in little to no flooding across the state. So why has the atmosphere been “surprisingly” active? The best explanation is that the May rains have resulted in a deep water source in our soils that has maintained above average near-surface moisture levels for the greater part of 7 weeks now. This increases in the odds of thunderstorms, even when upper-level conditions would suggest otherwise.

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Over this Outlook period, which covers through July 14th, we expect similar conditions to continue. The water vapor image, above, shows the upper-level ridge is back in full force this morning. It is causing near record hot temperatures across the Pacific northwest. The ridge’s intensity will pulse up and down over the next two weeks, while its position will slightly wobble. However, in general it will preclude any kind of organized flood threat across Colorado. Meanwhile, the Tropical Pacific remains active with daily thunderstorm activity sending moisture northeastward into Mexico and the southwest U.S. However, this moisture has, thus far, not made it all the way into Colorado.

Collectively, two particular features are identified in this Outlook that may produce more widespread rainfall, which are described below. At this time, the 4th of July weekend looks to be mostly dry, though there may still be isolated thunderstorms across the higher elevations east of the Continental Divide.

  • Event #1: Thursday (7/2) – Friday (6/3)
    • No apparent flood threat as a disturbance and weak cool front combine to increase shower and thunderstorm chances across mainly southeast Colorado
  • Event #2: Tuesday (7/7) through Thursday (7/9)
    • No apparent flood threat as the high pressure ridge scoots east and allows for monsoonal flow to help thunderstorms form west of the Divide

Event 1: Thursday (7/2) – Friday (7/3)

No apparent flood threat

A wave currently over the Pacific, will drift “over the top” of the ridge and combine with a Pacific cool front to increase shower and thunderstorm activity. The greatest chances and coverage will be over the Southeast Plains. While rain totals may approach 2 inches over isolated regions, this will be spread out over a two day period and will occur over mostly flat terrain that can accommodate more rainfall before runoff becomes a problem. Thus, we do not anticipate a flood threat.

Legend

 

Event 2: Tuesday (7/7) – Thursday (7/9)

No apparent flood threat

After Event #1, the high pressure ridge will intensify briefly, but then scoot to the east of Colorado. This will open up the gates for some monsoonal moisture to make its way into the southwest part of the state. At this time, no apparent threat is visible, but rainfall amounts up to 1.5 inches may occur over a multi-day period in the climatologically favored San Juan mountains.

Legend

Longer-range outlook

We have repeatedly made reference to Eastern Pacific weather conditions this season, with all eyes being focused on the strengthening El Nino. Today, instead of looking at sea-surface temperatures, we show the impact of El Nino through the atmosphere. The three maps below show anomalies of outgoing radiation, as seen from above the earth by numerous satellites. The top two maps show the latest daily anomaly and weekly anomalies, while the bottom map is the monthly anomaly. The key thing to note is that blue and purple colors denote where below normal radiation is leaving our planet (and vice versa for yellow/red colors). This generally implies above normal thunderstorm activity, as storm clouds are much colder than the atmosphere would otherwise be in the absence. Note that all three maps show very similar patterns! Notably, all suggest a plume of above average thunderstorm activity emanating from the deep Tropics and stretching eastward into the southwest United States and even into our state. While Colorado day-to-day weather is not exclusively tied to this, we are undoubtedly under the influence of a pattern that consistently promotes above average storm activity. And that has certainly been the case thus far this summer.

OLR

FTB 06-29-2015: Another Day of Isolated-to-Scattered Showers and Thunderstorms

Issue Date: 6/29/2015
Issue Time: 10:22 AM

A LOW FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, CENTRAL MOUNTAINS AND SAN JUAN MOUNTAINS.

The forecast has been fairly consistent over the last few days as the upper-level ridge continues to be anchored across the Western US, centered over Utah. This is has left Colorado under the influence of northerly flow aloft, creating mean storm motions from north to south, and leaving the bulk of shower/thunderstorm activity over the High Country. Moisture has not only remained, but maintained, near-average to above-average, and today is no different; precipitable water is above average for this date, running between 0.75 and 1.0 inches at the normal four reporting stations shown in the graph below (Boulder – blue, Grand Junction – green, Pueblo – pink, and Shriever AFB – red).

IPW_06292015

Scattered showers and thunderstorms are expected across the High Country, following the diurnal pattern of beginning around lunchtime, maximizing coverage during the late afternoon and early evening, before diminishing just after sunset as daytime heating ends and radiational cooling begins. Just as yesterday, a few isolated thunderstorms are expected over the adjacent lower elevations of the Urban Corridor, Palmer Ridge, Northeast Plains, and Southeast Plains, with the relative best chances occurring near the preferred terrain of the Front Range, Palmer Ridge, and Southeast Mountains. Adding an additional complexity to today’s forecast is a weak surface boundary that will scrape across the far Northeast and Southeast Plains, providing support for isolated afternoon/evening thunderstorms. Primary concerns with most thunderstorm activity will be gusty outflow winds, lightning, small hail, and brief moderate-to-heavy rain. For more specifics regarding rain rates, check out the zone-specific forecasts 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.
FTB_snapshot_20150629

Zone-Specific Forecasts

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

Mostly sunny and hot will be the main story today, as high temperatures remain in the mid-80s to low-90s across the regions. Isolated thunderstorms are expected this afternoon and evening, thanks to the conditions outlined above. The main threats will be lightning, strong outflow winds, hail (up to 1.5 inches in diameter), and brief moderate-to-heavy rain. Rain rates will typically be 0.4-0.8 inches/hour, but a couple strong/marginally severe storms could produce rain up to 0.9-1.3 inches/hour.

Timing: Noon – 10 PM

Front Range and Northern Mountains:

Isolated showers and thunderstorms expected, with the main threats being gusty outflow winds, small hail, and lightning. The Front Range south of I-70 will hold the best chance for bouts with brief heavy rain, with rain rates in the 0.5-0.8 inches/hour range. With most of the moisture confined to the mid-levels, and relatively shallow low-level moisture, dry air beneath the cloud base will eat away at precipitation before reaching the surface. However, an outflow boundary from another storm interacting with a thunderstorm over the southern Front Range could push maximum rain rates to 0.8-1.2 inches/hour. This is a relatively low probability, but cannot be ruled out, thus the inclusion in the outlined Low Flood Threat area.

Timing: Noon – 8 PM

Northwest Slope and Grand Valley:

Mostly sunny and hot will be the main story of the day, with an isolated thunderstorm or two possible over the higher terrain near the Central Mountain and Southwest Slope regions. The main threats from any isolated thunderstorms will be lightning and gusty outflow winds, as rain rates will stay below 0.25 inches/hour.

Timing: Noon – 6 PM

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

Scattered showers and thunderstorms expected, mainly over the higher terrain. With mean storm motions from north to south, activity will move over lower elevations and valleys with time. Rain rates will typically be in the 0.3-0.6 inches/hour range, but brief heavy rain over the San Juan Mountains (0.9-1.3 inches/hour) and Central Mountains (0.8-1.2 inches/hour) is possible. The main threats from thunderstorms will be gusty outflow winds, lightning, and small hail. There have been numerous reported injuries in the mountains from lightning over the last few weeks, so please be aware of changing weather conditions and seek shelter, if necessary.

Timing: 11 AM – 9 PM