Two storms carried by a powerful Pacific jet stream will play havoc with the lives of millions of Americans during the first full weekend of 2017. We can expect some of the heaviest wind-whipped rain and snow in years to strike parts of central California this weekend. In the Southeast, a significant belt of snow and/or ice will stretch from Atlanta to Norfolk, accompanied by high winds and bitter cold. The setups in both the East and West have the potential to produce localized record-setting amounts of rain and snow. Power outages in the Southeast could affect many thousands, while floods and avalanches
will become a growing concern in California.Figure 1.
Winds at the jet-stream level (250 mb, or roughly 33,000 feet) were howling across the southern U.S. at speeds topping 120 mph in spots on Friday, January 6, 2017. The energy was undercutting a massive ridge of high pressure extending north into Alaska and eastern Siberia, with a weaker jet arcing above the ridge (top left). Image credit: Climate Reanalyzer/University of Maine
The atmospheric river predicted for days to strike the U.S. West Coast is still on track to arrive within about 100 miles of San Francisco Bay and proceed headlong into the Sierra Nevada over the weekend. Carrying vast amounts of moisture--more than four standard deviations above the mean--this onslaught of strong southwest flow should deliver widespread rains on the order of 4” or more from late Saturday into Monday across much of central California, including the cities of San Francisco and Sacramento. Single-day rainfall records include 5.59” for San Francisco International Airport (January 4, 1982; records go back to 1945) and 3.77” at Sacramento Executive Airport (October 13, 1962; records go back to 1941).
Where the flow impinges on mountains, the lower-elevation rain and higher-elevation snow will be phenomenal. As of Thursday, the new year (Sunday to Thursday) had already delivered 70” of snow to the Mt. Rose ski area
, with 40” - 84” at Mammoth Mountain. A moderate avalanche risk
will continue into at least Saturday, according to the Sierra Avalanche Center. Even with the snow level rising as warm, moist air is pumped in, the highest elevations of the Sierra will likely rack up several more feet of snow in just a day or two.
After the weekend system abates, at least two more significant winter storms could sweep across California during the following week, which will raise the potential for serious flooding. The National Weather Service in Sacramento is already noting that flood impacts in the area this weekend into early next week could be the worst since at least December 2005
. The office warned in a tweet
:“Precipitation totals are forecast to be:
• once in a 10-25 year storm for areas SOUTH of I-80
• once in a 5-10 year storm for areas near and NORTH of I-80”Figure 2.
Precipitation totals predicted as of Friday morning, January 6, 2017, for the period from Saturday to Monday, Jan. 7-9. Image credit: NWS/Sacramento
.Complex winter storm across the Southeast
Ahead of California’s atmospheric river, another segment of the jet stream pushed Pacific moisture atop an Arctic air mass, producing heavy snow along Colorado’s Front Range. Boulder recorded 13.8” on Thursday, making it the city’s snowiest January day since 1962
. The upper-level impulse continued racing across the Texas Panhandle and much of Oklahoma late Thursday, with several inches of fluffy snow
on the ground from Amarillo to Oklahoma City by mid-morning and a second round of snow moving through the area.
A new low-level storm center organizing over the Gulf of Mexico on Friday will help wrap in moisture from the Gulf and Atlantic. The result will be a messy mix of wintry precipitation across much of the Southeast. Winter weather advisories for potential sleet and freezing rain extended to parts of southeast Louisiana and southern Mississippi, while the Atlanta area braced for 4” - 6” of snow expected late Friday. Even heavier snow could fall if the atmosphere chills down quickly enough, while a nose of warm air will favor sleet and cold rain closer to the Southeast coast.
The storm (dubbed Helena by The Weather Channel
) will pack additional punch from the western and northern Carolinas into southeast Virginia. Widespread snowfall of 4” to 12” is expected across the region on Saturday, accompanied by strong winds and temperatures that will drop into the single digits by Monday morning. Experimental snowfall potential maps
on Friday morning showed the risk of snow amounts as high as 13” north of Raleigh, NC, with up to a foot expected
across the Hampton Roads area of southeast Virginia and the southern Delmarva. Even the northern Outer Banks of NC could pick up 3” or more; a winter storm warning
was in effect for all of Dare County.
See the weather.com article on Helena
for continuously updated state-by-state summaries from the Southeast. We'll be back with a new post on Monday. Stay safe this weekend, especially if you're in one of the storm-affected regions!