Several days of potent early-season storminess in the northeast Pacific, just off the U.S. West Coast, culminated on Sunday with an unusually intense center of low pressure that developed in “bomb cyclone” fashion. A bomb cyclone is one that intensifies by at least 24 mb in 24 hours at 60° latitude, with lesser intensification rates needed at lower latitudes. At 40° latitude, close to the location of Sunday’s storm, the criterion for a bomb cyclone is an 18-mb drop in pressure in 24 hours.
Centered a few hundred miles west of Washington and Oregon, the storm bottomed-out with the lowest central pressure (942.5 mb) ever recorded across a large swath of the Northeast Pacific. (See embedded tweet below; note that the latitude range was later corrected from 20-40 degrees north to 30-50°N).
Data on such cyclones is not comprehensive prior to the advent of satellites in the mid-20th century. Still, there is no question this is one of the most intense storms of modern times off the U.S. West Coast, especially since the center happened to pass very near a buoy that confirmed the central pressure.
Some of the heaviest rains ever seen in October drenched northern and central California over the weekend. Flash flooding was widespread in urban areas and near burn scars as the heaviest rain unfolded. River flooding was relatively minor, as the region’s drained aquifers and reservoirs and its parched landscape took big drinks.
Totals of 4-10” were reported at many Bay Area locations from San Francisco northward and eastward, extending well into the Central Valley and Sierra Nevada.
A couple of the most notable records:
– At the downtown station in San Francisco, Sunday was the wettest calendar day for any October by far – and the fourth wettest day for any month – in nearly 173 years of recordkeeping. Sunday’s 4.02” was topped only by storms in 1866, 1881, and 1994.
– At the downtown station in Sacramento, Sunday was the wettest calendar day of any month in 165 years of recordkeeping, with 5.44” breaking the old record of 5.28” set on April 20, 1880. The nearby Sacramento Executive Airport totaled 5.41” from early Sunday to early Monday; that’s more than 80% of its entire water year total for 2020-21 (6.61”) in just a single 24-hour period. (The downtown Sacramento record discussed in the tweet below, which is also their all-time calendar day record, is not their all-time 24-hour rainfall record; as noted by weather historian Christopher Burt, downtown Sacramento received 7.24″ in a 22-hour period straddling April 20-21, 1880.)
The record-setting rains in Sacramento arrived just a few days after the end of the city’s longest dry spell on record (212 days without measurable rain, from March 20 to October 16). It’s a particularly striking example of the exacerbation of precipitation extremes that a warming climate is likely to continue producing in an area naturally prone to weather whiplash, as documented by Daniel Swain and coauthors in a 2018 paper in Nature Climate Change, “Increasing precipitation volatility in twenty-first century California.”
Autumn severe weather rakes the central United States
A “second season” tornado outbreak pummeled Missouri and southwest Illinois on Sunday afternoon and evening, thrashing several towns near the Mississippi River. After filtering out duplicates, the NOAA/National Weather Service Storm Prediction Center listed 15 tornado reports from Sunday, including 10 from Missouri, four from Illinois, and one from Kansas.
One powerful storm dropped at least one tornado (possibly two) around 8:30 p.m. CDT. The storm mangled parts of the small towns of St. Mary, Missouri, and Chester, Illinois, separated by fewer than 10 miles on either side of the Mississippi. As the storm ripped through the area, an unusually strong tornado debris signature was detected on radar.
Another tornadic storm just south of the St. Mary–Chester cell produced damage in Fredericktown, Missouri, that was assigned a preliminary rating of at least EF2 (see update below) on the Enhanced Fujita Scale by the NWS office in St. Louis.
Storm survey teams were continuing to evaluate damage on Monday. No deaths or major injuries had been reported from Sunday’s tornadoes as of early Monday. Update (6:45 p.m EDT): The NWS/St. Louis office has assigned preliminary ratings of EF3 to the Fredericktown tornado and EF2 to the St. Mary/Chester tornado.
More storminess was lurking on the horizon, though. SPC placed parts of the Carolinas and mid-Atlantic under a slight risk of severe weather for Monday, mainly for a threat of non-tornadic high winds. The second-day outlook for Tuesday included an enhanced risk across the Southern and Central Plains. Supercell thunderstorms with large hail and tornadoes will be possible Tuesday afternoon before the storms organize into a squall line packing high winds. Additional severe weather could occur on Wednesday along and near the northwest Gulf Coast.
Damaging coastal storm to affect northeastern U.S. Tuesday and Wednesday
Fueled by some of the jet-stream energy that plowed into the West Coast this past weekend, a powerful low-pressure system (a nor’easter) is predicted to take shape off the mid-Atlantic/Northeast U.S. coast on Tuesday morning and rapidly intensify, bottoming-out with a central pressure near 975 mb at 2 a.m. EDT Wednesday, according to the 12Z Monday runs of the GFS and NAM models. The storm’s intensification rate will be close to 24 mb in 24 hours, which would qualify it as a bomb cyclone (see definition above)
The low will bring torrential rains, damaging winds, and coastal flooding (but no snow) to a large swath of the coast, from New Jersey to Maine. The most damaging hazard is likely to be wind; with many trees still in full leaf, tree damage could well be widespread, causing considerable power outages, and a high wind watch is in effect for much of the region. The 12Z Monday run of the HRRR model (Figure 1) predicted that wind gusts near hurricane-force could affect coastal New York, Rhode Island, and Massachusetts.
Heavy rains of three to five inches are expected in coastal New Jersey, New York, Connecticut, Rhode Island, Massachusetts, and New Hampshire, and flash flood watches are posted for portions of this region. A storm surge of two to three feet is expected along the coast from New Jersey to New Hampshire, but only minor coastal flood is predicted, as the moon’s waning gibbous phase will bring some of the lowest astronomical tide of the month.
October has been relatively dry across much of the Northeast, but the expected rains from the nor’easter and a second rainmaker late this week will add to the region’s autumn totals (going back to September 1). If Boston notches 7.45″ by October 31, it would be the wettest September-October period on record. New York’s Central Park would need 9.40″ to achieve the same feat.
The low is predicted to move generally eastward, away from the U.S., Wednesday through Saturday. This motion will carry the low over waters along the northern border of the warm Gulf Stream current, and these warm waters will potentially allow the low to acquire tropical or subtropical characteristics. The highest chances for tropical development may occur over the weekend, when the system is expected to turn more to the southeast. In its 8 a.m. EDT Monday Tropical Weather Outlook, the National Hurricane Center gave this future low 2-day and 5-day odds of tropical cyclone development of 0% and 40%, respectively. The next (and last) name on the Atlantic list of storms is Wanda. Update: As of 8 a.m. EDT Wednesday, October 27, the 2- and 5-day odds of developments are 10% and 40%, respectively. Any development would most likely be well east of the U.S. coast.
Category 2 Hurricane Rick hits Mexico with 105 mph winds
Hurricane Rick made landfall at 6 a.m. EDT Monday, October 25, as a category 2 storm with 105 mph winds about 15 miles east of Lázaro Cárdenas, Mexico (population 180,000), putting the city on the storm’s weaker left-hand flank. Rick was a small hurricane at landfall, with hurricane-force winds that extended out 25 miles from the center and tropical storm-force winds that extended out 80 miles. Though Lázaro Cárdenas is just 15 miles from where the center of Rick made landfall, the city’s peak winds were only 34 mph, gusting to 48 mph, recorded at 9:15 a.m. EDT Monday. According to storm chaser Josh Morgerman (iCyclone.com), Rick blocked roads with fallen trees and shattered windows in Ixtapa, which experienced the strong right eyewall of the hurricane.
The most serious impact of Rick is likely to be flooding from its rains of five to 10 inches. Isolated amounts of up to 20 inches are predicted.
At 11 a.m. EDT Monday, Rick was about 40 miles inland from the coast of southwestern Mexico, headed north at 9 mph, with maximum sustained winds of 80 mph winds and a central pressure of 985 mb. Rick is expected to dissipate over the high mountains of southwestern Mexico by Tuesday morning.
As detailed in yesterday’s post at this site, Rick is the fifth named storm and fourth hurricane to hit Mexico this year from the Pacific. It is unusual to see a landfalling Pacific hurricane in Mexico this late in the season when La Niña conditions are present.
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