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Moisture at Last for California; Severe Storms, Heavy Rain for TX/LA/AR/MS

By: Bob Henson 6:57 PM GMT on March 07, 2016

Rain lovers across central and northern California--and snow lovers across the Sierra Nevada--were equally pleased at the storminess that enveloped the region over the last several days. Multiday rainfall totals of 1” to 4” were widespread across the Bay Area, with larger amounts toward the north and at higher elevations. Several points along the Coast Range racked up 10” to 12” totals. Snowfall was also generous across the Sierra Nevada mountains to the east (see below). All told, the weekend storms gave a big boost to a region that had gotten only scant precipitation during February--usually one of the wettest months of the year, and often a prodigiously stormy month during strong El Niño events, such as the one now in place. The precipitation wasn’t as abundant toward Southern California, but even this drought-scarred area got an encouraging dose of precipitation during the weekend and into Monday morning.

Figure 1. Webcam photo of snow atop Frazier Mountain in far northern Ventura County, CA, on Monday morning, March 7, 2016. Image credit: NWS/Los Angeles.

Figure 2. WunderMap interactive radar shows a line of intense thunderstorms moves across the San Diego/Tijuana area at around 9:15 am PST on Monday, March 7, 2016.

Thunderstorms raged off the southern California coast on Sunday night, producing ample lightning and paving the way for a dark and stormy Monday morning over coastal Southern California. Several potent lines of thunderstorms moved across the region’s coastal metropolitan areas, bringing lightning, hail, and brief heavy rain. Severe thunderstorm warnings were in effect for most of San Diego County as one line approached.

Sierra snowpack clawing its way back toward average
The crucial Sierra Nevada snowpack, which stores about a third of the water used by Californians, has improved over the last few days. As of Sunday, March 6, the Sierra Nevada had pushed back above its long-term average values for accumulated precipitation in the water year to date, starting July 1 (see Figure 3). A good portion of that moisture has run off or melted off, helping to replenish downstream reservoirs and aquifers. The actual amount of water held in the snowpack--the snow water equivalent--is between about 75% and 85% of average for the wet season to date, according to daily analysis from the California Department of Water Resources. With several storms still in the pipeline over the next few days, that percentage is likely to climb.

Figure 3. Precipitation totals for the three regions of the Sierra Nevada from July 1 through March 6 (green bars), compared to the last four years (tan, grey, yellow, and red bars) and to average long-term values for the same period. Image credit: Jan Null, Golden Gate Weather Services.

L.A. still lacking; Seattle still soaked
Even after this weekend’s storms, Southern California remains on the low side when it comes to 2015-16 winter precipitation. As of midnight Sunday night, Los Angeles had racked up 0.64” for the month, with another half inch or so on Monday morning. Yet the total through Sunday night of 5.64” remained far below the average wet-season total to date (October 1 – March 6) of 11.61”. As of midnight Sunday, San Diego had garnered just 0.33” of rain this month. That left its total since October 1 (3.91”) almost 7” below the average rainfall received by this point in the year. Additional rain on Monday will help, but it appears that the bounty of heavy rain over the next week-plus will be aimed primarily north of the LA-San Diego area, with the possible exception of another weekend storm around Friday/Saturday. On the plus side, there remains a chance of additional rain heading toward the region over at least the following week or so, as more impulses travel along the subtropical Pacific jet stream.

Most folks in Seattle would probably be happy to see the El Niño rain belt focusing further south. The winter of 2015-16 is already the wettest in Seattle’s 123-year weather history--an utterly unexpected result during a strong El Niño year--and the downpours show no sign of abating. As of Sunday night, Sea-Tac Airport had racked up 39.44” of moisture in the wet season that began on October 1, 2015. Measurable rain has occurred at Sea-Tac on each of the first seven days of March. The only time March began in Seattle with a longer streak of rain was March 1-8, 1919. That record appears likely to tumble, as there is rain in Seattle’s forecast every day this week.

Figure 4. The 500-mb forecast from the 12Z Monday run of the GFS model, valid at 72 hours (12Z or 7:00 am EST Thursday, March 10), shows a deep, slow-moving upper low centered over southern Texas, with a strong, warm ridge over the northeast U.S. Image credit: Levi Cowan, tropicaltidbits.com.

Severe weather, heavy rain still heading for south-central U.S.
The powerful upper jet plowing across California this weekend will soon carve out an unusually deep upper low extending far south into Mexico. Near-record low temperatures (especially cool daytime highs) can be expected across northern Mexico, with snow falling atop some of the higher Sierra Madre peaks. The upper low will help channel copious amounts of tropical moisture northward into the south-central U.S., setting the stage for a multi-day period of torrential rain and severe weather. With the atmosphere so moist at all levels, and with upper- and lower-level winds all blowing more or less from the south, this doesn’t look like a classic spring outbreak with isolated supercell thunderstorms. Instead, we’re likely to see many clusters and lines of intense storms dumping extremely heavy rain, with some severe weather embedded in the mix. Short-lived circulations and brief tornadoes may crop up as the storms multiply, making for a tough week for local forecasters. As of midday Monday, the NOAA Storm Prediction Center was not highlighting any areas of significant severe weather for the next several days, another sign that most of the storms are unlikely to be especially destructive. For most areas, SPC is dubbing the event a “long duration-low probability tornado risk.”

Figure 5. Convective outlooks issued by NOAA’s Storm Prediction Center as of midday Monday, valid for the 24 hours until 6:00 am CST on (left to right) Tuesday, March 8; Wednesday, March 9; and Thursday, March 10.

The very moist air mass expected to flow into the south-central states will approach or exceed precipitable water records for March at some locations. Together with the powerful upper low, this means the potential for extreme rainfall, gradually spreading from Texas and Arkansas into Louisiana and Mississippi as the week unfolds. The details will depend on how each night’s showers and thunderstorms evolve; it’s likely that the zones of highest rainfall will get shunted southeastward each day as rain-cooled air pushes a surface front in that direction. When all is said and done by week’s end, we may see totals of more than a foot of rain in some locations, with widespread 6-12” amounts.

Bob Henson

Figure 6. 7-day precipitation forecast for the period from 12Z (7:00 am EST) Monday, March 7, though Monday, March 14, 2016. Image credit: NOAA/NWS Weather Prediction Center.

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