STP 05-26-2015: A Mostly Pleasant Memorial Day Thanks to a Downtick in Thunderstorm Activity

Issue Date: Tuesday, May 26th, 2015
Issue Time: 9:00 AM MDT



Yesterday featured a downtick in thunderstorm activity allowing for most people to get outside and enjoy the holiday. Afternoon/evening showers and thunderstorms provided some late day fireworks with lightning and some brief moderate-to-heavy rain, but nothing that caused flash flooding issues. One afternoon thunderstorm, 4 miles NE of Beulah (Pueblo County), provided the only severe report of the day with 1 inch hail. Sub-severe, quarter-inch hail fell in Cedaredge (Delta County), covering the ground 1.5 inches of accumulating hail.

The strongest thunderstorm of the day rumbled across central portions of the Urban Corridor during the evening, producing the highest observed rainfall totals of the day. From CoCoRaHS observers:

Arapahoe County: 1.31 inches
Jefferson County: 1.18 inches
Douglas County: 0.84 inches
Denver County: 0.84 inches
Boulder County: 0.55 inches

No flash flooding occurred yesterday. Be sure to check out the radar-estimated rainfall map below.

Storm Total Precip Legend


Issue Date: 5/25/2015
Issue Time: 12:00PM
Valid period: May 26th through June 9


The prolonged stretch of very active weather characterized by cloudy, cool, rainy and snowy conditions appears to be winding down. The water vapor image below shows this morning’s snapshot of the Northern Hemisphere circulation. Much like the past two to three weeks, the overall pattern consisted of a strong extratropical jet over the west Pacific Ocean. The flow associated with the jet then splits into two flows midway across the ocean, resulting in a strong high pressure ridge over western Canada and the famous “cut-off” lows off the California coast and the southwest U.S. It is of course the latter that has been the culprit of consistent wet days across Colorado.


In the near-term, the major change to the above pattern will be two fold. First, a general slow-down in the progression of features. Second, the gradual development of a high-pressure ridge across the southwest U.S resulting in much warmer temperatures over Colorado by next week.

Over the next two weeks, we have identified three events that may impact the flood threat. Only the first event has an Elevated flood threat. The second event is tied to the forecasted snow-melt from much warmer temperatures. The third event is a combination of warmer temperatures plus the potential of tropical moisture impacting the western part of the state. Below is a more detailed description of each event.

NOTE: After we describe the three events below, we provide an El Nino update to get a glimpse at the longer-range summer forecast. In short, think “wet”, but please be sure to check out the full discussion.

The three events that we have identified are:

  • Event #1: Friday (5/29) through Saturday (5/30)
    • An Elevated flood threat east of the Divide
  • Event #2: The following Wednesday (6/3) through Saturday (6/6)
    • Much warmer temperature will result in elevated streamflows over large snowpack areas of the higher terrain. No flood threat is anticipated at this time, but this will be a key feature to watch.
  • Event #3: Next week’s Saturday (6/6) through Sunday (6/7)
    • A tropical system is expected to come up the Central American coast and possibly bring moisture west of the Divide. No flood threat is currently anticipated, but the tropical feature will need to be closely monitored.

EVENT 1: Friday (5/29) through Saturday (5/30)

An Elevated flood threat for most of eastern Colorado

A stationary front is expected to move across eastern Colorado later this week. This will combine with an upper-level disturbance that is currently positioned over the northwest U.S. coast (see water vapor image above) to result in heavy rainfall. The best estimate of the highest rainfall totals, in the 1.5 to 2.0 inch range, is expected in east-central Colorado, but this will be substantially refined in the next Flood Threat Outlook.


EVENT 2: Wednesday (6/3) through Saturday (6/6)

No flood threat is anticipated at this time

The development and intensification of the southwest U.S. high-pressure ridge will result in much warmer temperature by next week. High temperatures may reach the mid-80s to mid-90s in the lowest elevations with 50s and 60s in the highest elevations. Given the recent amount of heavy, wet snowfall across the higher terrain, this will cause significant snow melt. While no flood threat is currently apparent, individual basins will be monitored to assess the amount of snowpack remaining along with forecasted temperatures.

EVENT 3: Saturday (6/6) through Sunday (6/7)

No flood threat is anticipated at this time

A tropical disturbance is forecasted to develop and progress up the Central American coast through the Baja of California by the middle of next week. Once it reaches the Baja, it is unclear if the disturbance will disintegrate and release substantial moisture into the southwest U.S. or continue off to the west and keep the moisture for itself. The former will result in a flood threat west of the Divide, while the latter will not impact the Colorado much. The next two Flood Threat Outlooks should have a much better grasp of this feature.


As a final note in today’s Flood Threat Outlook, we present a brief overview of what the ongoing El Nino may bring for Colorado. There is mounting evidence that the current weak El Nino will continue and likely intensify as we head into summer. So what does that imply for Colorado precipitation?

The first map shows the current Sea Surface Temperature anomalies (source: NOAA). Note the warm patch of ocean over the equator in the Pacific Ocean; these conditions are referred to as El Nino. These surface anomalies, up to 3 degrees Celsius are also accompanied by sub-surface anomalies of up to 5 degrees Celsius implying a very deep “bank” of warm ocean water to draw from.

sst_anomOne interesting question to pose is: historically, how do warm temperatures in the region outlined by the box in the map above relate to Colorado precipitation? The following map, derived from NOAA’s Earth System Research Laboratory climate page, shows the correlation between ocean temperatures in the equatorial Pacific with precipitation across the U.S over the June – August period. Impressively, a large “bullseye” appears east of the Continental Divide over Colorado. This implies that El Nino conditions tend to favor above average precipitation over most of Colorado, but especially east of the Divide.

ENSO_corrSo what about longer-range forecasts? The Climate Forecast System, operated by the National Centers for Environmental Prediction, provides 9-month forecasts across the world. Below is a map showing the expected precipitation anomaly over the upcoming June-August period. The key word is: wet! It is also interesting how the model forecast is similar to what we have historically observed shown in the map above.

cfs_forecastNote that anomaly refers to precipitation in relation to average. Just for reference, Colorado averages 2-4 inches of precipitation per month over that time period.

While no correlation or forecast is perfect, when we see key data line up, it allows us to build confidence in the upcoming summer forecast. It will be interesting to see how it evolves!

FTB 05-25-2015: Isolated-to-Scattered Showers and Thunderstorms on Tap, Snow for Higher Elevations

Issue Date: 5/25/2015
Issue Time: 10:33 AM


The mid-/upper-level low that moved across the state yesterday is centered over NE Colorado/W Nebraska this morning. As this continues to push to the east-northeast, Colorado will come under the influence of weak subsidence behind the low. Despite this, there will be enough residual moisture, instability, and orographic influences to produce another afternoon and evening of isolated-to-scattered showers and thunderstorms across Colorado.


The most numerous storms will occur over and near the higher terrain as orographics play a key role in development. Above 8,000-8,500 feet, snow/graupel will fall, with rain below those elevations. Further east over the Plains, storms will rely on relatively weak instability and residual moisture to develop/maintain isolated showers and thunderstorms. The low flood threat area is as much a result of saturated soils as it is rain rates; overall, rain rates will not be particularly impressive (most between 0.25-0.5 inches/hour), but it will not take much more than that over certain areas to create flash flooding issues. Rain rates, as well as timing, will be broken down in the zone-specific forecast discussions.

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

Front Range, Urban Corridor, Palmer Ridge:

Isolated-to-scattered showers and thunderstorms will develop once again around noon, continuing through the evening before diminishing after sunset. Southern portions of the regions will have the best chance for a storm or two to become more organized, producing rain rates of 0.7-1.1 inches/hour. A storm with rain rates in this range will be capable of producing flash flooding, especially considering the already saturated soils, thus the low flood threat.

Timing: Noon – 10 PM

Northeast Plains, Southeast Plains, and Raton Ridge:

Isolated showers and thunderstorms are on tap again this afternoon and evening, with the best relative chances near the interface with the higher terrain. Western portions of the Southeast Plains and the Raton Ridge will have the best chance for a strong thunderstorm to develop, producing rain rates in the 0.7-1.2 inches/hour range, which is the reason for the low flood threat in those areas. Otherwise, rain rates will mainly be in the 0.25-0.5 inches/hour range.

Timing: 1 PM – 9 PM

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

Scattered showers and weak thunderstorms are expected again today and tonight, producing snow/graupel above 8-8,500 feet and rain below those elevations. Rain rates will typically be in the 0.25-0.5 inches/hour range, but a stronger thunderstorm over the Southeast Mountains will hold the potential to produce rain at 0.7-1.1 inches/hour.

Timing: 11 AM – 9 PM, a few showers lingering until 11 PM.

STP 05-25-2015: Scattered Showers and Thunderstorms, a Few Severe, to End the Weekend

Issue Date: Monday, May 25th, 2015
Issue Time: 9:00 AM MDT


Another day, another bout with showers and thunderstorms across Colorado. East of the mountains, a few of those thunderstorms were severe, producing hail, strong winds and even a few tornado reports. Rain totals were generally light under showers, with most location reporting less than 0.15 inches of rain. However, under the stronger storms (mainly near the CO/KS border), more than 0.5 inch of rainfall was reported in a few locations. From CoCoRaHS observers, these counties had the big rainfall winners yesterday:

Yuma County: 1.35 inches
Kit Carson County: 1.33 inches
Phillips County: 0.80 inches
Bent County: 0.78 inches
Morgan County: 0.69 inches
Cheyenne County: 0.53 inches

As far as severe storm reports are concerned, hail greater than 1 inch in diameter was the main impact, however a few funnel clouds/tornadoes were reported in Baca County, El Paso County, Prowers County, and Otero County.

Flash flooding was reported in Swink (Otero County); the underpass on Highway 50 was flooded with about 12 inches of water.

Storm Total Precip Legend