STP 05-28-2016: Cool, rainy (and snowy) May continued for many

Issue Date: Saturday, May 28th, 2016
Issue Time: 9:05AM MDT

Compared to Thursday, Friday saw a downtick in rainfall intensity. However, rainfall coverage remained high with many areas both east and west of the Divide experiencing a shower or two. Farther east, where easterly upslope winds remained (associated with a weakening Kansas low), stronger storms were found. In particular, a sequence of strong storms passed through Kiowa and Cheyenne counties resulting in over 2 inches of rainfall across a wide area, hail up to 1 inch and several tornado reports near Towner. Aside from this region, many other locations statewide picked up anywhere between 0.25 and 0.6 inches of beneficial rain.

Of course, not be outdone, the higher terrain received measurable snow for a second straight day. Up to 3 inches was the norm for scattered locations above 9,000 feet. For many locations above 10,000 feet, over 12 inches of snow water equivalent (SWE) is still on the ground. This time of year, 1 inch of SWE is roughly equal to about 3-5 inches of actual snowdepth. For all you skiers, this morning’s snow update from Arapahoe Basin showed an impressive snowpack of just under 5 feet! It looks like June skiing will once again be had.

No official flooding reports were received yesterday, though it is likely that field flooding occurred in Kiowa County in association with the repeated strong thunderstorms moving through.

For specific rainfall estimates in your area, please check our STP map below.

STP_20160529
Storm Total Precip Legend

FTB 05-27-2016: A Low Flood Threat Remains In Place For Eastern Colorado

Issue Date: Friday, May 27th, 2016
Issue Time: 10:30 AM

— LOW flood threat today for parts of the Northeast Plains, Palmer Ridge and Southeast Plains
— LOW flood threat today for parts of Grand Valley (landslide possible)

In today’s water vapor imagery, shown below, we see the remains of yesterday’s low pressure system that generated all kinds of active weather across Colorado (check out our Storm Total Precipitation discussion). As is often the case with the day after the storm, a downtick in rainfall activity is expected today. However, some active will still remain across parts of the state.
wv_markup

Let’s discuss the details:
• the atmosphere’s dynamics are weakening with the departing and weakening low pressure; mid-level height rises of 30 – 60 meters are expected throughout the days favoring weak subsidence
• atmospheric instability will be limited for most of the state with less than 750 J/kg of instability for most; the only exception is the far eastern part of the state where instabilities up to 1,500 J/kg will be possible
• precipitable water values are in line with their seasonal averages, though overall drying is expected for the majority of the state (again with the exception of the far east)
• storm motions will be in the 30+ mph range which will limit the amount of time a location experiences heavy rainfall; again, the exception is the far east where southeasterly boundary layer winds will lower storm motions
• Yesterday’s rainfall in the South Platte basin will slowly drain into the river, but is not expected to raise river levels above the flood stage
• With the cool temperatures, snow continues to melt in an orderly fashion and is not expected to induce flooding at high-elevation streams.

The details above warrant a low flood threat for the far eastern part of the state where hourly rainfalls could exceed 1.5 inches per hour, with 3-hour totals approaching 2.4 inches. The rest of the state is not expected to experience flooding.

We also want to point out that a Flash Flood Watch has been issued by the Grand Junction NWS for the Salt Creek landslide area (Mesa County). We urge everyone in that area to keep up to date with local officials or the NWS.

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_20160527Zone-Specific Forecasts

Northeast Plains, Southeast Plains, Palmer Divide:

Partly sunny with scattered rain showers early with scattered thunderstorms developing by early afternoon. One hour rainfall up to 1.6 inches is possible with 3-hour rainfall up to 2.4 inches. A Low flood threat is in place for the far eastern parts of the region for localized road and field flooding.

Prime-time: 12PM to 8PM

Central Mountains, Northern Mountains, Southeast Mountains, San Juan Mountains, San Luis Valley, Front Range, Urban Corridor, Raton Ridge:

Mostly cloudy with scattered rain showers through the day, gradually decreasing in coverage later. A few weak thunderstorms are possible during the afternoon in the lower elevations below 7,000 feet. One hour rain rates up to 0.6 inches possible in the strongest storms. Up to 1.2 inches of total precipitation possible by tomorrow morning. The snow line will be 10,000 feet early, moving up to about 11,500 feet by later in the afternoon.

Prime-time: 12PM to 7PM

Grand Valley, Northwest Slope, Southwest Slope:

Scattered clouds with gradually clearing skies. A stray shower or even weak thunderstorm cannot be ruled out, but max 1-hr rain rates will stay below 0.4 inches. Flooding is not expected. However, folks should pay attention to local officials in the Mesa County area regarding the condition of the Salt Creek landslide.

STP 05-27-2016: Most Active Day Of The Young Season

Issue Date: Friday, May 27th, 2016
Issue Time: 9:10 AM MDT

Summary:

It did not take long before a variety of active weather developed across Colorado on Thursday. Most of the focus was east of the Continental Divide, where upslope flow, adequate moisture and impressive dynamics combined to produce several waves of thunderstorms. Heavy rainfall, hail, wind, tornadoes, and yes, even snow (up to several inches in terrain above 9,000 feet) was observed across a wide swath of the state.

The highest rainfall totals of the 2016 warm season were tallied yesterday. Fortunately, short-term rainfall rates were fairly limited, ranging from about 0.5 inches per 30 minutes in the Front Range area to perhaps 0.7 inches in 30 minutes farther south and east. However, many locations experienced several hours of rainfall. Thus, in the 24-hour period ending early this morning, many locations in Washington, Arapahoe, Douglas, Adams, Elbert and El Paso counties received over 2 inches of rainfall.

Many strong thunderstorms were also observed mainly across the Southeast Plains, Palmer Ridge and Northeast Plains. A half dozen tornado reports were received from Kit Carson and Cheyenne counties. Meanwhile, a whole slew of hail reports were also noted. Fortunately, most of the hail was about 1 inch or less in diameter, hopefully limiting the damage that it may have caused. Flooding-wise, no official reports were received but 3 areal flood advisories were issued by area NWS offices, suggesting at least some minor field flooding was present. The map below, from Iowa State University, summarizes all of the warnings and advisories issued yesterday (yellow = thunderstorm, red = tornado, green = flood).

NWS_warnings_advs

For specific rainfall estimates in your area, please check our STP map below.
STP_20160527

FTO 05-26-2016: 3 Events in the Next 15 Days

Issue Date: 5/26/2016
Issue Time: 1:05 PM

Threat_Timeline_20160526

The water vapor image below highlights the fairly active/transient pattern that will mark the next 15 days. Event #1 is a continuation from Monday’s FTO; the upper-level trough continues to behave as expected, moving east-northeastward across the central Rockies and into the Central US Plains. After Event #1, residual moisture will allow scattered thunderstorms to develop during the afternoon/evening on Saturday and Sunday. Event #2 will skirt across the Central/Northern Rockies, ejecting from the Pacific NW late Sunday (5/29), early Monday (5/30). The upper-level trough associated with Event #2 will not dig along the West Coast as the last couple have, allowing it to move eastward quickly. This will not only shorten the period between events #1 and #2, but it also will allow the low to move out of the region in fairly short order.

WV_20160526

Stronger ridging will build in behind Event #2, allowing for a brief drying/warming period. The main circulation associated with Event #3 will remain anchored to, or near, its current position. It will, however, eject a shortwave trough that will move across the western US and across Colorado late next week/over the weekend. Ensemble model solutions diverge on the exact timing, path, and evolution of this wave, so uncertainty is high. Updates will be given in the Monday FTO. After the wave of Event #3, ridging will build across the west, and the Summer heat will be on.

Event #1: Friday (05-27-2016)

Elevated Flood Threat Continues as Upper-Level Low Exits Region

The upper-level low/trough will continue to slide to the east, keeping broad-scale lift overhead. Surface moisture will remain across eastern Colorado, especially in the wake of today’s shower/thunderstorm activity. Storms will be ongoing during the morning hours tomorrow, then another round of showers and thunderstorms will fire up in the afternoon. Stay tuned to tomorrow’s FTB for an update.

Legend

Event #2: Monday (05-30-2016) and Tuesday (05-31-2016)

Elevated Flood Threat

This upper-level trough will begin to eject eastward late Sunday (5/29), early Monday (5/30). Colorado will see an uptick in showers/thunderstorms beginning on Monday. Leeside trough-ing will transport good surface moisture into eastern Colorado, setting the stage for showers and thunderstorms on Monday and Tuesday. Dewpoints in the 40s/50s will be enough moisture for the production of moderate-to-heavy rainfall.

Legend

Event #3: Friday (06-03-2016) through Sunday (06-05-2016)

No Apparent Flood Threat Due to Diverging Solutions

Ensemble model solutions diverge on the exact timing, path, and evolution of this wave (mentioned above). No precipitation map will be made due to considerable uncertainty. Stay tuned to future FTO’s for updates on this time period.