FTB 05-31-2015: A Few Afternoon Storms Possible, But More Bark Than Bite

Issue date: Sunday, May 31st, 2015
Issue time: 9:10AM MDT

For the second day in a row, Colorado remains under the influence of an upper-level ridge, as shown in the water vapor image below. True to its nature, the high pressure is inducing downward motion resulting in a fairly dry atmosphere. Though dewpoint temperatures this morning are in the 40s and even 50s, the moisture is very shallow. Just a few thousand feet off the ground, the relative humidity values are only 10-20%. There is, however, a weak disturbance noted on the water vapor image. As this disturbances treks southeastward, it may help organize scattered thunderstorms in eastern Colorado. The limited moisture will keep rainfall rates below 1 inch per hour, and no flood threat is anticipated today.

watervapor_20150531

The Arkansas River continued to slowly recede yesterday, and only one official gauge, at La Junta, remains in Minor flood status. We have removed our Low flood threat for the Arkansas because the hot and dry weather is expected to further promote the river’s lowering.

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.

Flood Threat Legend

Zone-Specific Forecasts

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

Mostly sunny and very warm with temperatures rising into the low to mid 90s in the lowest elevations. No flood threat is expected today.

Northern Mountains, Central Mountains and San Juan Mountains:

Mostly sunny to partly cloudy with isolated to scattered thunderstorms possible by early afternoon. Hourly rain rates will be 0.3 inches or less, so no flooding is expected.

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

Mostly sunny early, then becoming partly cloudy with isolated thunderstorms developing by early afternoon. Maximum hourly rain rates will be 0.4 inches close to the mountains, increasing to 0.8 inches  farther east. However, the strongest storms will be very isolated in nature, and no flooding is expected.

STP 05-31-2015: A Handful Of Storms Beat The Odds, But Little Rainfall

Issue Date: Sunday, May 31th, 2015
Issue Time: 9:05 AM MDT

Summary:

With a ridge overhead, thunderstorm activity was effectively squashed statewide on Saturday. By late afternoon, a few rogue storms were able to form over the Northwest Slope, Front Range and Southeast Mountains. However, the storms looked more impressive that they actually were: highest observed rainfall was only 0.05 inches. No flooding was reported yesterday. See our map below for the radar estimated rainfall in your area.

We are taking this lull in action to begin to appreciate just how active May was; not so much from the flooding aspect, but certainly from the total precipitation aspect. One tool we commonly use for our Flood Threat Bulletin forecasts is the precipitable water (PW). This measures the amount of water vapor (not liquid, only gaseous water!) in the overhead column of atmosphere. In Colorado, when PW exceeds about 0.7 inches, this may be the first indication of a heavier rainfall threat. When values exceed 1 inch, there is a very high likelihood of action somewhere in the state. The charts below, courtesy of NOAA’s Earth System Research Lab, show the PW from this past May. The 0.7 inch line is marked by a thick black line. Impressively, during almost every day except for a handful, PW exceeded 0.7 inches in at least one of the three main stations we use: Pueblo (purple), Grand Junction (green) and Boulder (blue).

IPW_historicalSo how does this compare to normal? Well, we also included a chart of 2014, a more typical year. Note that only 7-10 days exceeded the 0.7 inch threshold. Meanwhile, also shown is 2012, the year of an intense spring/summer drought. Note that during that year only a couple of days exceeded PWs of 0.7 inches, but Grand Junction did not exceed 0.7 inches for the entire month! This is a quick way to get a perspective on just how unusual this May has been.
20150531_STPImage
Storm Total Precip Legend

FTB 05-30-2015: After A Long Active Stretch, Finally A Quieter Day In Store

Issue date: May 30th, 2015
Issue time: 9:10AM MDT

— LOW flood threat for Arkansas River from Pueblo through La Junta

Today’s water vapor image, below, shows something that we have almost forgot exists: a ridge of high pressure scooting eastward towards Colorado. This ridge will promote downward motion across our state, resulting in lower moisture content and a reduced chance of rainfall. This kind of pattern has certainly been a commodity this spring.

watervapor_20150530

This morning, we see most of the state covered in sunshine with the exception of some low-clouds in the Northeast and Southeast Plains. These clouds will quickly evaporate in a few hours once the atmosphere begins mixing in dry air from above. We expect sunshine to rule the land this morning. By early afternoon, scattered clouds will develop mainly over the higher terrain. Scattered weak thunderstorms will develop by mid-afternoon mainly across the higher terrain east of the Divide. Precipitable water values are in the 0.4 to 0.6 inch range, while dewpoints are expected to fall into the 20s to near 40F later this afternoon. Thus, one-hour rainfall from these storms will be 0.5 inches or less, and no flood threat is required.

A Low flood threat remains in place for the Arkansas from Pueblo trough La Junta, but we expect levels to recede slowly today. The river may final subside below flood status by tomorrow.

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.

Flood Threat Legend

Zone-Specific Forecasts

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

Sunny early with scattered clouds developing by early afternoon. Very warm with temperatures topping out in the low 90s in the lowest elevations. An isolated thunderstorm cannot be ruled out, however, little if any rainfall will reach the ground. No flooding is expected today.

Northern Mountains, Central Mountains and San Juan Mountains:

Sunny early, then partly cloudy with isolated showers and storms developing by early afternoon. Maximum rain rates will be 0.3 inches per hour. No flooding is expected.

Front Range, Urban Corridor, Northeast Plains:

Sunny early with scattered clouds developing by early afternoon. An isolated storm cannot be ruled out but rainfall will be limited to 0.2 inches per hour.

Palmer Divide, Southeast Plains, Southeast Mountains, Raton Ridge:

Sunny early, then scattered showers and thunderstorms developing by early afternoon. Hourly rain rates up to 0.5 inches are possible, along with some small hail and gusty winds. No flash flooding is expected. A Low flood threat continues to be in effect for the Arkansas River from Pueblo through La Junta.

STP 05-30-2015: Storms Coverage Decreased, But Still Several Heavy Rain Makers

Issue Date: Saturday, May 30th, 2015
Issue Time: 9:00 AM MDT

Summary:

The tail end of a multi-day disturbance brought scattered showers and thunderstorms across Colorado on Friday. While most storms looked more intimidating than they actually were, several storms over the Palmer Ridge and Southeast Plains produced very heavy rainfall. Fortunately the rainfall was brief and did not cause any flooding problems. For example, an observer in El Paso county noted 0.25 inches fell in 6 minutes (the equivalent of 2.50 inches per hour). These storms were also responsible for several hail reports, up to 1 inch in diameter. In all, about a handful of 1+ inch rainfall amounts were noted across El Paso and Pueblo counties. Meanwhile, several rounds of mostly weak storms amounted to 0.5 to 0.9 inches of rainfall across Weld and Larimer counties. Aside from this action, most everyone else experienced less than 0.5 inches of rainfall.

The South Platte River continued to subside below Minor flood stage. However, parts of the Arkansas River remained in Minor flood stage, receiving a short-term boost from yesterday’s rainfall in the upstream regions.

Snowmelt was ongoing with the warm conditions. Interestingly, though, an inspection of SNOTEL data across each of the eight major drainage basins revealed that seven of the eight had an above average snowpack for this time of year. The one exception was the Yampa / White River basin, but even there conditions were very close to what is expected for late May. This is certainly quite a welcome change from a month ago when the western parts of the state were experience a serious snowpack “drought”.

No flash flooding was reported yesterday. Please check the map below for estimated rainfall in your area.

STP_snapshot_20150530