El Niño-Related Rains Take Aim on California--and Arkansas

The Pacific wave train set to bring a much-anticipated storm to the West Coast this weekend looks like it has a second destination in mind. Parts of the south-central U.S., especially Arkansas, are in line for what could be some of their heaviest March rains on record next week, once the Pacific storm cuts off and settles in for a spell.

Lower-elevation rains and mountain snows will push inland on Friday and Saturday from Washington to central California as the powerful Pacific jet stream slams into the coast. Initially, snow levels will be on the high side across the Sierra Nevada--above 7000 feet on Saturday--but as colder air filters in, the snow level will drop to the 3500-5000 foot range, which is good news for replenishing the critical Sierra snowpack. Many parts of the Sierra will receive one to two feet of snow by Monday, with even more across the higher peaks. As the jet stream continues plowing inland, a second wave at its base should goose the rains and mountain snows across southern California, which has missed out on many of this winter’s wet storms. Coastal SoCal can expect widespread 1-2” amounts, with 2-4” possible in the San Francisco Bay area.


Figure 1. The latest weekly U.S. Drought Monitor shows that as of March 1, more than 38% of California remains in exceptional drought (D4, the worst category). This is only marginally better than the 40% coverage from one year ago this week. Some improvement can be expected over the next week or two. Image credit: National Drought Mitigation Center.


The Los Angeles area needs more than 6” of rain just to catch up to what an average wet season would have produced by now, much less a strong El Niño winter. In a blog post late Thursday, Daniel Swain (California Weather Blog) was cautiously optimistic: “…it appears increasingly likely that March will at least be able to make a dent--even though it’s quite clear that California’s multi-year drought will persist through the summer.”

The follow-up storm for late next week that we discussed in our Wednesday post isn’t looking quite as potent in recent model runs, but as Swain noted, “Present model solutions still suggest a storm that would be quite impressive by the low standards set during recent drought winters.”


Figure 2. Rainfall projected for the 7-day period ending at 7:00 am EST Friday, March 11, 2016. Image credit: NOAA/NWS Weather Prediction Center

Next stop: Southern Plains
In classic El Niño fashion, the Pacific jet stream will dip to uncommonly low latitudes as it move inland. By midweek, the jet will have carved out an upper-level low in northern Mexico (see Figure 3 below). At that point, its progress will be halted by strong upper ridging over the eastern U.S. In between, this will allow for extremely rich tropical moisture to flow northward from the Caribbean and Gulf of Mexico into the lower Mississippi Valley. The amount of precipitable water in this air mass is projected to surpass record levels for March in some areas.

A stationary front expected to lie from Texas toward Illinois will become the focus of multiple rounds of heavy showers and thunderstorms over several days. Some severe weather could emerge, including tornadoes, although the set-up looks less like a prolific twister producer and more like a torrential rainmaker. The focal point is likely to be Arkansas, where the stationary front should be aligned near or just east of the Ozarks. This would help enhance uplift and set the stage for what could eventually pose a serious flooding threat. The 00Z Friday run of the GFS model dumped 8” to 12” of rain over most of the southeast half of Arkansas over the ten days ending on Sunday, March 13. A secondary peak of 8" or more could develop in parts of Louisiana. Such a forecast is quite plausible, given the very moist air mass expected and given the fact that upper-level lows often move even more slowly than expected. March is the most common month for river floods across the southeast U.S., and heavy spring rains are a hallmark of El Niño in this region.

In records going back to 1876, Little Rock, AR, has never seen a March with more than 10.43” of rain (set in 1897). If the ingredients come together as models suggest, Little Rock could approach that record before the month is even half over.


Figure 3. WunderMap depiction of upper-level flow at the 200-mb level (around 34,000 feet) from the 0Z Friday GFS model projection valid at 10:00 am EST Thursday, March 10. Wind speeds are shown in knots; multiply by 1.15 for mph.


Mexico could also see unusual weather next week as the strong upper low dives into place. An upper-level trough extended far into Mexico during mid-December 1997, near the peak of the “super” El Niño of 1997-98. That month brought Guadalajara its first snowfall since 1881 and Monterrey its first snow in 30 years. Next week’s upper-level low may be even stronger over northern Mexico than the one in December 1997. Lower-level temperatures won’t be as cold with this event, now that it’s early March, but at least some dabs of higher-elevation snow are possible.

Bob Henson

PS from Jeff and Bob: Last call for blog-name suggestions!
We’ve had great fun sifting through more than 100 potential new names for this blog submitted in the comments section for last Friday’s post. As we explained, we’re renaming the blog to better reflect our joint authorship. The ideal would be a cool, pithy name that reflects the spirit of the blog in covering both weather and climate, with particular emphases on tropical meteorology, severe weather, and climate change. Our full names will serve as a subtitle, so they needn’t be part of the new blog name. Please feel free to chime in with your suggestions in the comments section of this post, or in last Friday’s post. If you’re a WU member, you can drop us a line via WU Mail. All suggestions made by March 10 will be considered. Thanks again for your creativity and enthusiasm--it is much appreciated!


Figure 4. A word cloud produced by using the titles of all 3200-plus entries posted to this blog since it was launched in 2005. Image credit: Dr. Jeff Masters.