Central U.S. Dousing on Tap; Heavy Spring Snow in CO, WY

Soon to be abandoned by the jet stream, a strong upper-level low will park near the Four Corners this weekend, pulling in a rich moisture feed from Texas to Nebraska that will dump near-record April rain on the High Plains and tree-challenging wet snow along the Colorado/Wyoming Front Range. The upper low will be marooned for several days near Colorado, which will prolong the precipitation event and lead to some truly impressive totals.

As of midday Friday, winter storm warnings encompassed most of the Colorado high country as well as the urban corridor from near Colorado Springs, CO, to Cheyenne, WY. Snowfall totals predicted by the NWS through Sunday range from 8” - 14” in Denver and 6" - 12” in Cheyenne to a whopping two to four feet in the mountains and foothills just west of Denver, including areas within a few miles of Boulder. The biggest impact may be on local trees, many of which are now flowering and leafing out. The heavy, protracted snowfall is likely to break countless limbs and bring down many power lines. Streets and soil are quite warm around Denver after temperatures soared into the 70s on Thursday; between this and high-angle sunlight filtering through, there may be enough melting to keep snowfall on the ground and on highways well below the totals gleaned from snow measuring boards. Even so, a dense, slushy mess is like to impede travel thoughout the weekend.


Figure 1. WunderMap of projected flow at 300 mb (about 30,000 feet) from the 06Z Friday run of the GFS model, valid for 00Z Sunday (8:00 pm EDT) on April 17, 2016, shows the highly meridional (north-south) configuration of the jet stream, with the upper low near Four Corners largely cut off from the jet.


Figure 2. Precipitable water—the amount of moisture in the atmosphere available for producing rain or snow—will be at extremely high values for the location and season over the central High Plains by 18Z (2:00 pm EDT) Saturday, April 16, 2016. Areas of blue represent precipitable water 4 to 5 standard deviations above average. Also shown are surface highs and lows and winds at the 850-mb level, about a mile above sea level. This projection is from the 06Z Friday run of the GFS model. Image credit: Levi Cowan, tropicaltidbits.com.

An April snow to remember
Cut-off upper lows near the Four Corners are the most common set-up for heavy snow in the Denver area, especially in autumn and spring. The biggest snows in Boulder are several times more likely during El Niño than La Niña. However, a weak La Niña was in place 95 years ago this weekend, when a similar but stronger cut-off low led to the heaviest official 24-hour snowfall in U.S. records: 75.8”, recorded on April 14-15, 1921, at Silver Lake, about 30 miles west of Boulder at an elevation near 10,000 feet. The observer on site reported a total of 95 inches in just 48 hours, according to a Monthly Weather Review analysis cited in a Wasatch Weather Weenies blog post. (Another contender for top 24-hour snowfall is the 78” recorded at Mile 47 Camp, Alaska, on Feb. 7, 1963, according to WU weather historian Christopher Burt).


Figure 3. The snowstorm of March 17 - 19, 2003, was the most intense to strike the Denver area in over 80 years. In some parts of the Rocky Mountain foothills west of Boulder, more than 80 inches (two meters) of snow fell. This photo was taken near Nederland. Image credit: Carlye Calvin/ UCAR Image and Multimedia Gallery.

High Plains drencher
Not to be outdone by the Colorado snow, a strip from western Nebraska to northern Texas will see torrential rain over the weekend. The amount of precipitable water available for rain or snow will be close to record values for so early in the year across the central High Plains (above 1” at North Platte, NE, for example). The NOAA/NWS Weather Prediction Center is calling for widespread rains topping 4-5” by early next week from northern Nebraska to the Texas coast between Friday and Wednesday (see Figure 4). The projected maximum of about 7.5” close to North Platte would break the April precipitation record of 7.10” set in 1915. It would also be more than a third of the city’s typical yearly precipitation (20.19”).

While not as dramatic next to climatology, similar amounts could fall in localized areas toward western Oklahoma and northern Texas. Flood watches are in effect for parts of this area, but overall the moisture should be quite welcome. The CO/WY/NE area is already running well above average in precipitation for the year thus far; in contrast, much of the Southern Plains has seen less than half of its annual average to date (see Figure 5). This is part of a large dry swath--unusual for an El Niño spring--that extends westward all the way to southern California. As of Thursday, Los Angeles had seen less than half of its average precipitation for April (0.24”, vs. average of 0.51”) and little more than half for the water year since October 1 (5.80”, vs. average of 9.86”). Some parts of the area have done better, but overall the 2015-16 El Niño has been a major disappointment for the southern third of California.



Figure 4. Projected 5-day precipitation totals (rainfall and melted snow equivalent) from 12Z (8:00 am EDT) Friday, April 15, 2016, through Wednesday, April 20, 2016. Image credit: NOAA/NWS Weather Prediction Center.


Figure 5. Precipitation for 2016 through April 14 as a percentage of the annual average up to that point. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service.

Severe weather threat over the Southern Plains this weekend
NOAA’s Storm Prediction Center has an enhanced-risk area extending across the central and southern High Plains on Friday, with a slight-risk zone over West Texas on Saturday and South Texas on Sunday. The bulk of the threat on Friday will be large hail, although tornadic storms may develop if the moisture surge from the south is especially robust. After a busy start to the 2016 tornado season in February and March, the pace has slackened in recent weeks, bringing the annual total closer to the long-term average (Figure 6).

Stay safe and have a great weekend, everyone!

Bob


Figure 6. Cumulative tornado counts for 2016 (black) against the distribution of high and low tornado counts for various other years. On each given day, the red curve represents the highest annual total up to that point in records going back to 1950, with the magenta curve showing the lowest total and the green curve showing the median value. The totals have been “inflation-adjusted” to remove the effects of increased tornado observing in recent decades. Image credit: NOAA/NWS Storm Prediction Center.