FTB 05-29-2015: Upper-Level Low + Cool Front Bringing Low Flood Threat

Issue Date: 5/29/2015
Issue Time: 10:32 AM

A LOW FLOOD THREAT HAS BEEN ISSUED FOR PORTIONS OF THE FRONT RANGE, URBAN CORRIDOR, PALMER RIDGE, SOUTHEAST PLAINS, SOUTHEAST MOUNTAINS, AND RATON RIDGE. A LOW FLOOD THREAT CONTINUES FOR A PORTION OF THE ARKANSAS RIVER.

As is the case on most days, the water vapor imagery paints a good picture for today’s forecast. It doesn’t tell the whole story, but it is a good place to start. A relatively weak upper-level trough will move to the east-southeast across the state today, providing support for another day of isolated-to-scattered showers and thunderstorms. West of the trough axis (black line), the large-scale environment will generally be unsupportive of thunderstorm development; however, a bit of moisture embedded in the northwesterly flow (red arrow) will combine with orographic support for isolated-to-scattered showers and thunderstorms west of the Continental Divide. The number of storms will be noticeably less than yesterday west of the Divide, especially for southern areas. The most activity will occur across the Northern and Central Mountains.

05292015_WV

East of the Continental Divide, another factor will be in play – a passing cool front. This cool front will do a few things:

1.  Provide upslope flow after passage, reinforcing moisture against the mountains, albeit a small net increase over yesterday
2.  Provide a forcing mechanism for thunderstorms to develop along the front
3.  The wind shear due to easterly flow behind the front with northwesterly flow aloft will support a few strong-to-severe thunderstorms, mainly across the Palmer Ridge, southern reaches of the Urban Corridor, Southeast Plains, Raton Ridge, and Southeast Mountains. The probability of occurrence will increase the further southeast you go.

Due to these factors at play, scattered showers and thunderstorms are expected east of the Continental Divide, with heavy rain, hail, and gusty winds being the main threats. For more details on timing and maximum rain rates, be sure to jump down to the Zone-Specific forecasts.

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_20150530

Zone-Specific Forecasts

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

Scattered showers and thunderstorms are expected today, and a few will be strong-to-severe. Rain rates will generally be in the 0.5-0.8 inches/hour range, but the strongest storms will produce rainfall at rates of 1.2-2.0 inches/hour. The likeliest location for heavy rainfall at that rate will be across the Southeast Plains and Raton Ridge, but strong storms across the Urban Corridor and Southeast Mountains will push rain rates near 1.0-1.5 inches/hour. Burn scars like Waldo Canyon and Black Forest will need to be watched, as well as areas that are still saturated.

Timing: 11 AM – 9 PM, a few thunderstorms continuing over the Southeast Plains until midnight. Storms will develop from north-to-south with the passage of the cool front, and will diminish from north-to-south this evening.

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

Considerably fewer showers and thunderstorms are expected today, with the most activity occurring across the Northern and Central Mountains. Instability will be fairly low, so any showers/thunderstorms will rely on orographic influences to develop/maintain themselves. This means that the higher terrain will see the majority of all precipitation, with lower valleys scraping by on the remainders as the storms move off of the mountains. Above 10,000 feet, it is likely that snow will fall, producing light accumulations mainly across the Northern and Central Mountains. Don’t expect heavy rain to cause any problems, with storms only being able to muster maximum rain rates of 0.2-0.4 inches/hour.

Timing: 11 Am – 8 PM, diminishing quickly thereafter.

STP 05-29-2015: Scattered Thunderstorms Rumbled Across Colorado

Issue Date: Friday, May 29th, 2015
Issue Time: 9:00 AM MDT

Summary:

An upper-level disturbance aided thunderstorm development yesterday, producing scattered thunderstorms statewide. Moisture near the surface was lessened compared to the previous day for most locations, resulting in less heavy rain at the surface and no flash flooding issues. The Northeast Plains, Southeast Plains, and Palmer Ridge retained better low-level moisture, and provided quite the show in the way of severe thunderstorms. The following are severe storm reports from the National Weather Service:

1.5 inch hail: NNW of Iliff (Logan County), Walsh (Baca County)
1.0 inch hail: 1 mile N of Elizabeth (Elbert County), 6 miles E of Denver (Arapahoe County)
Tornado: 15 miles S of Vilas (Baca County), 6 miles S Kim (Las Animas County)
Funnel Cloud: 8 miles SE of Kim (Las Animas County)

As far as rainfall totals are concerned, most locations received less than 0.25 inches of rainfall. CoCoRaHS observations from the following counties are yesterday’s “big winners:”

Las Animas: 1.68 inches
Baca: 0.78 inches
Yuma: 0.77 inches
Kit Carson: 0.60 inches
Cheyenne: 0.55 inches

No flash flooding was reported.

STP_snapshot_20150530

FTO 5-28-2015: TRANSITION TO SUMMER UNDERWAY; NO CLEAR FLOOD THREAT

Issue Date: May 28th, 2015
Issue Time: 3:00PM MDT
Valid Time: May 29th through June 12th

threat_timeline_20150528

For Colorado, the transition to summer means lots of hiking and camping, melting snow and hot afternoons with thunderstorms. From the standpoint of weather forecasting, however, summer brings about a challenging season where a given day’s weather can strongly impact the next day’s. This is the conundrum at hand. The next two weeks will see a classic duel between the hot southwest United States “ridge” and disturbances attempting to break into the heart of that ridge. Only such successful disturbances will bring about an organized flood threat. No such threats are apparent through the June 12th end date of this Flood Threat Outlook.

Today’s weather circulation, shown below, is crowded with many players. First off is the disturbance currently traversing across Colorado. This is bringing about several rounds of shower and thunderstorm activity, but the flood threat is limited to the Kansas border where low-level moisture is better. Next, we see the high-pressure ridge over western Canada. This has been establishing a roadblock to disturbances: forcing them to go either north of south of it. In the past two to three weeks disturbances have managed to find a way south of it and into Colorado. However, presently, there is an additional ridge building to its south that will provide a further obstacle in the flow. In fact, it is the development of the southern ridge that will prevent the copious amounts of subtropical moisture found across the eastern Pacific from making it into Colorado (note three active tropical disturbances on water vapor). Finally, we see a large cut-off cyclone in the middle of the Pacific Ocean. Though this feature will take it’s time, it could affect our weather pattern over the next 5-7 days.

watervapor_20150528_smallSo how do we see the next two weeks evolving? The current disturbance will slowly leave the state by late Friday, initiating another round of thunderstorm activity on Friday. Rainfall amounts are expected to stay under one inch, except for very isolated areas. See our Flood Threat Bulletin discussion tomorrow for further details. Beginning on Saturday and lasting through most of next week, Colorado will begin a period of drier and warmer weather. That is not to say that it will not rain: it will, with almost each afternoon resulting in higher elevations showers and weak thunderstorms. However, this will not amount to much rainfall, generally less than half an inch on any given day at any given locale. Furthermore, this stretch of warmer weather will cause a substantial decrease in the state’s remaining snowpack, the status of which is shown below. As the snowpack is already only a fraction of its seasonal maximum, we do not anticipate widespread flooding concerns. However, there may be isolated flooding issues in the South Platte and Arkansas basins due to the recent two to three weeks’ worth of heavy snowfall.

current_SWE

By next weekend and thereafter, our weather will be strongly impact by the fate of the low-pressure system over the Central Pacific. Current guidance and intuition shows a very large spread in possible outcomes. At this time, there is not enough confidence to determine when an “event” could occur, and event if an event occurs, how much rainfall could fall.

Finally, as the tropical Pacific weather appears to be an integral part of this summer’s precipitation outlook over Colorado, we encourage readers to check out our El Nino discussion from Monday’s (5/25) Flood Threat Outlook.

FTB 05-28-2015: Disturbance To Aid Thunderstorms, But Limited Flood Threat

Issue Date: May 28th, 2015
Issue Time: 10:10AM MDT

— LOW flood threat for parts of Northeast Plains, Palmer Ridge and Southeast Plains
— LOW flood threat for South Platte River from Greeley through Fort Morgan
— LOW flood threat for Arkansas River from Pueblo through La Junta

A disturbance is moving into Colorado from the northwest, shown by the water vapor image below. The disturbance’s passage will coincide with the maximum heating of the day, which will increase thunderstorm activity compared to yesterday. However, despite favorable upper-level dynamics, one important missing piece to heavy rainfall today is low-level moisture. Although morning dewpoints are in the 40s and 50s, precipitable water values are only around 0.6 inches. More importantly, since the disturbance is arriving from the northwest, this will allow dry air to mix down the eastern side of the Divide. The end result is that only eastern Colorado will have enough low-level moisture to warrant a low flood threat. There, in addition to heavy rainfall, large hail up to 1.75 inches and wind gusts up to 60 mph are all possible with stronger storms. Away from the threat area, expect scattered thunderstorms with one hour rain rates in the 0.2 to 0.5 inch range, along with small hail up to 0.5 inches.

watervapor_20150528

In addition to the flash flood threat today, the Arkansas and South Platte Rivers continue to run in minor flood stage. Today’s activity is very unlikely to significantly impact either river. Nonetheless, we expect both rivers to stay elevated due to the abundant upstream runoff.

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.

05282015_LFT
Flood Threat Legend

 

Zone Specific Forecasts

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

Partly to mostly cloudy with scattered showers and thunderstorms. Hourly rainfall up to 0.4 inches will be possible, along with small hail up to 0.5 inches and gusty winds up to 60 mph. No flooding is expected today.

Northern Mountains, Central Mountains and San Juan Mountains:

Scattered showers and thunderstorms developing by early afternoon. Hourly rainfall up to 0.6 inches will be possible, along with small hail and gusty winds. No flooding is expected today.

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

Becoming partly cloudy with thunderstorms developing by early afternoon. Closer to the mountains, hourly rain rates up to 0.7 inches will be possible. Farther east, stronger storms will be capable of up to 1.8 inches per hour of rainfall. A Low flood threat has been posted for the eastern part of the Palmer Ridge and the entire Kansas border. The strongest storms will be capable of 1.75 inch hail, and winds up to 60 mph.

Prime Time: 2pm through 9pm

Southeast Mountains, Raton Ridge:

Mostly sunny early, then becoming partly cloudy. An isolated high based thunderstorm cannot be ruled out. However, the passage of a dryline will lower dewpoints into the 20s and 30s preventing heavy rainfall. Gusty winds up to 35 mph will be likely in the afternoon, but the fire danger is not of concern due to wet soils. No flooding is expected today.