June 12, 2019

Parts Of Southern Georgia Saw 7 Inches Of Rain In Just Three Hours On Tuesday

A large cluster of thunderstorms that popped up in southern Georgia on Tuesday evening produced more than 7.00 inches of rain in just a couple of hours, prompting a flash flood warning as local waterways and storm sewers were inundated by the abrupt surge of water. The sudden nature of the storms highlights the flooding risk that summertime thunderstorms can pose in the moisture-laden southeastern United States.

Folks who live in the southeast are no strangers to a drenching afternoon thunderstorm. It's not uncommon for a thunderstorm to pop up and drop a quick inch or two of rain before moving on an hour later. The storms north of Valdosta, Georgia, however, are an example of how quickly things can get serious when your run-of-the-mill summertime thunderstorms sit in the same spot for too long.

The cluster of thunderstorms that put down the torrential rain were the result of converging outflow boundaries. An outflow boundary is the rush of cool air that descends out and away from a thunderstorm. Outflow boundaries often act like little cold fronts that scoop up unstable air ahead of them and trigger the development of more thunderstorms as the afternoon wears on. This domino effect can continue until the unstable air is exhausted—usually around sunset.

Outflow boundaries were responsible for the flooding rains over southern Georgia on Tuesday. Imagery from the Valdosta radar showed multiple outflow boundaries colliding almost head-on across the counties north of Valdosta. A cluster of thunderstorms bloomed when the boundaries collided and the unstable air had nowhere to go but straight up.
Radar-estimated rainfall amounts on Tuesday evening. Source: GREarth/AllisonHouse

Since there weren't any prevailing boundaries or strong steering currents to drive the storms out of the area, they just sat and poured over the same communities for several hours at a time as they very slowly drifted toward the south. A weather spotter near Weber, Georgia, reported 5.81 inches of rain between 5:54 PM and 7:54 PM. NWS Tallahassee reported on Twitter that one community—possibly that same weather spotter—saw more than 7.00 inches of rain by the time the storm wound down. Radar estimates indicate that several counties saw 5-7 inches of rain during the storm.

The Weather Prediction Center warns that there's a chance for more flash flooding across coastal sections of Georgia, South Carolina, and southeastern North Carolina on Wednesday. It's hard to say who will see the heaviest rains, but any thunderstorms that pop up in the region have the potential to produce lots of heavy rain in a short period of time.

[Radar Imagery: GR2A/Gibson Ridge]

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June 11, 2019

Typically-Comfy San Francisco Hits 100°F As West Coast Heat Wave Continues This Week

Hoowee. It hit 100°F at San Francisco International Airport on Tuesday, the warmest temperature ever recorded so early in the year. The heat wave baking the West Coast with unusually toasty temperatures for early June will continue through the end of the week, forcing some residents to deal with almost unbearable indoor temperatures.

Temperatures easily soared into the 100s as far north as northern California's Central Valley on Monday and Tuesday, with 90s at lower elevations through eastern Washington. Temperatures at 4:00 PM PDT on Monday are shown in the map at the top of this post. Even downtown San Francisco, which is typically rather cool given the influence of the chilly Pacific waters, made it all the way up to 97°F on Monday, breaking the station's record for June 10 by one degree.

Things didn't cool down much on Tuesday. Portland, Oregon, hit 95°F at 3:00 PM PDT on Tuesday, and temperatures were right up around 100°F again in the San Francisco Bay area. 

San Francisco's high on Monday was one of only seven times SFO Airport's temperature reached 100°F or warmer, and the earliest it's ever done so. Every other triple-digit reading at the city's airport occurred during the month of September, according to data pulled from xmACIS2 and professionally compiled on the lovely PowerPoint chart above.

Other record highs on Monday include 113°F in Thermal, CA; 105°F at Salinas Airport in Monterrey County, CA; 105°F in Stockton, CA; 104°F in El Cajon, CA; and 101°F in Redwood City, CA; and 101°F in Santa Rosa, CA. Many of the records broken in California, especially around the San Francisco area, have stood since at least 1994.

500mb height anomalies on Monday afternoon. Source: Tropical Tidbits

A large ridge of high pressure parked over western North America is responsible for the prolonged heat wave. Ridges tend to foster calm, hot weather—stronger and more anomalous ridges can bring about stronger and more anomalous heat waves. Models show the ridge sticking around for at least a couple more days, which means temperatures will be slow to cool down through the end of the week.

An animated loop of expected high temperatures across the western U.S. between Tuesday, June 11, and Friday, June 14.
The National Weather Service's forecast on Tuesday afternoon called for high temperatures at or near 100°F to persist across most of California's Central Valley through Friday. Things will progressively start to cool down at the coast in places like San Francisco and Los Angeles—the high in downtown San Francisco will only hit the mid-60s by the end of the week, but even inland the warmth won't be anywhere near as brutal as we saw on Monday.

It's bad enough to have to deal with hot temperatures when you're not used to them, but many homes and businesses in the western United States—especially near the coast—aren't equipped with air conditioning, which makes a days-long heat wave an uncomfortable and potentially dangerous prospect.

Heat is a compounding hazard. Enduring a successive period of extremely warm days and nights prevents the indoor temperature (and, back east, the humidity) from rebounding to a livable level. The longer a heat wave lasts, the more unlivable it becomes indoors. That's why so many people fall ill or die during long heat waves in low-income communities or climates where air conditioning isn't a standard in homes and businesses.

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June 7, 2019

A Rare Tornado Hit A Small Town In Northern Canada, One Of Only 4 Recorded So Far North

A tornado struck a tiny town in northern Canada’s vast wilderness on June 2. The tornado damaged homes and businesses in Fort Smith, Northwest Territories, leaving residents shocked by the storm that just hit them. Tornadoes are rare at such a high latitude, and it’s even more rare that the tornado managed to hit such an isolated community.

Environment Canada confirmed that an EF-1 tornado touched down in Fort Smith on the afternoon of June 2, causing some structural damage and bringing down trees and power lines. Photos obtained and posted by CBC North show significant tree damage, crushed vehicles, and what appears to be a metal shed that was tossed and smashed in someone’s yard.

Residents in this part of the country have no reliable way to know a tornado is coming unless they see it for themselves. Environment Canada only has 31 weather radar sites set up across the country, centered on population centers near the southern border and in parts of the tornado-prone Prairie provinces. The nearest weather radar to Fort Smith is more than 350 miles away—that’s like using the radar at Washington’s Dulles Airport to see a storm over Providence, Rhode Island. This leaves folks up north to rely on satellite imagery or old-fashioned sky watching to stay ahead of an approaching thunderstorm.

Folks in Fort Smith probably never thought they'd see a tornado there. Tornadoes are extremely rare this far north. This is reportedly only the fourth tornado on record to strike Northwest Territories. It’s possible there are more tornadoes than we realize in interior and far-northern Canada, but communities are so few and far between that it takes a direct strike like we saw in Fort Smith for a tornado confirmation.

Tornado data maintained by Environment Canada shows more than 1,800 confirmed tornadoes across the country between 1980 and 2009, mostly focused around populated areas where people are actually around to witness tornadoes. Most tornadoes in Canada are relatively weak, though some tornadoes on the Prairies and in southern Ontario have been quite strong. The strongest tornado in Canadian history was an EF-5 that hit Elie, Manitoba, on June 22, 2007.

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June 5, 2019

Tropical Downpours Could Lead To Flash Flooding In Parts Of The Southeast This Week

Heavy downpours could lead to localized flash flooding across parts of the southeastern United States this week as a deep plume of tropical moisture spreads over the region. Forecasts on Tuesday night called for a widespread drenching across the southeast through early next week, with some areas potentially seeing more than five inches of rain by the end of the period.

Last week, we started watching a tropical disturbance in the Bay of Campeche for signs of tropical development. The National Hurricane Center had given the system a 60 percent chance of developing into a tropical system at its beefiest, but the disturbance was never able to take root and grow. The disturbance ran out of room to develop on Tuesday as it approached eastern Mexico.

Source: Tropical Tidbits

Nothing ever really goes away in the weather, of course. Even though the soon-to-be-erstwhile disturbance is no longer a thing of interest on weather maps, the remnant moisture from the system will continue spreading across the southeast. The above chart from Tuesday night's run of the GFS model shows precipitable water (PWAT) values through the weekend.

Precipitable water is a great way to visualize how much moisture shower and thunderstorms can work with. PWAT tells us how much rain would fall if you could wring all the moisture out of that part of the atmosphere. Higher PWAT values indicate a greater potential for showers and thunderstorms to produce heavy downpours that could lead to flooding. A PWAT value over 2.00" is considered delightfully soupy and tropical, a ripe environment for drenching rains.

As a result of all that evaporated paradise moving over land, it won't be hard for a hefty thunderstorm to put down a quick inch or two of rain if it sits over one spot for too long. It's important to note that not everybody covered under, say, the five-inch rainfall contour in the Weather Prediction Center's forecast will definitely see five inches of rain. Storms are hit-or-miss during the summer. Most everyone will see rain, but some could see a whole lot more than others.

Stay alert for flash flood watches and warnings over the next couple of days. It's always wise to memorize or program multiple safe routes to get home, to work, or wherever you need to go, just in case your normal route is covered in water and you need to turn around. It only takes a few inches of moving water to pick up a vehicle and carry it away, and it's impossible to tell how much water is covering a roadway—or if the road is even still there under the water.

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