February 19, 2019

Waves of Storms Set to Bring Flooding Rains to the Southeast This Week

Every day is mildew season in the southeastern United States these days since it's apparently never going to stop raining. That interminable wet streak will continue with five or more inches of rain anticipated over the next seven days across an enormous swath of the south. It's uncommon to see such a big slug of heavy rain in this part of the country without a tropical system involved.

The latest precipitation forecast from the Weather Prediction Center shows the opportunity for five or more inches of rain centered on the mid-South. The precipitation will come in several major waves. The first storm system will develop tonight and last through Thursday morning, bringing heavy rain and thunderstorms to the southeast. Some of the thunderstorms could even be severe with gusty winds, large hail, and a few tornadoes. The next chances for heavy rain will begin Friday and last through the weekend. More rain is expected over the affected areas next week.
Source: Tropical Tidbits


It looks like a safe bet that the heaviest rain will continue moving over the same areas because the upper-levels seem to be on pause for the time being. The above animation shows height anomalies at the 500 millibar level between Tuesday morning and Sunday morning. The trough over the western United States and the ridge over the southwestern Atlantic don't move much longitudinally through early next week. The southeast is in the unlucky position to get pinched between the trough and the ridge, forcing that highway of precipitation to keep moving over the same areas.

Not everyone will see all of the rain in the forecast. Some communities will only see an inch or two of rain and others could see half a foot or more. It depends on the tracks of thunderstorms and batches of heavy rain as they traverse the area.
Maps: WPC


Rain this heavy could lead to flooding, especially along streams and rivers. Sustained heavy rain could lead to street flooding. I'm sure we'll see video of someone trying to ford a flooded roadway like they're the protagonist in Oregon Trail. Here's a hint for you: you won't get dysentery, but try to cross a flooded road and you could wash away and die. It's nearly impossible to judge the depth of water crossing a roadway and you can't be completely sure that there's even a road under the water. It can take just a foot of moving water to wash a vehicle downstream.

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February 14, 2019

Flash Flooding Is Likely In Central And Southern California On Thursday



Flash flooding is likely across parts of California and Arizona on Thursday as an approaching storm system threatens to bring several inches of rain in a short period of time. Flood watches stretch from northern California to southeastern Arizona in anticipation of the heavy rain. It doesn't take much heavy rain in this part of the country to cause significant flooding problems.

A potent upper-level trough moving toward California this hour is responsible for the low-pressure system that will cause all of the headaches on Thursday. The latest forecast from the Weather Prediction Center calls for several inches of rain across most of the state, save for some of the rain shadows and the southern half of the Central Valley.



Accordingly, flash flood watches (that light green Mr. Yuk color) and flood watches (dark green) are in effect for much of California and parts of Arizona. Urban flooding, overflowing waterways, and flash flooding in rough terrain is likely during the heaviest rain. Mudslides are also possible on burn scars.

There's a ton of moisture in the atmosphere for the developing showers and thunderstorms to work with. Southwesterly flow around the southern end of the low is dragging deep tropical moisture north toward California. This ribbon of moisture, known as an "atmospheric river," is usually what's responsible for the intense rain events we see on the West Coast.

Source: CIMSS


You can see the atmospheric river approaching southern California in the image above, which shows precipitable water (PWAT) across the eastern Pacific. Precipitable water is the amount of rain that would fall if you condensed all the water vapor in a column of the atmosphere. If the PWAT over your house is 1.15", it means that you'd get about 1.15" of rain if you condensed all of the water vapor in the atmosphere above your town. PWAT isn't the whole story, of course, but higher PWAT values indicate a better opportunity for heavy rain—it's a deeper reservoir of moisture for showers and thunderstorms to tap into.

It's not just the heavy rain you have to look out for. Some of those thunderstorms could be on the stronger side. The Storm Prediction Center says there's a marginal risk of severe thunderstorms in the Central Valley between Sacramento and Fresno. The agency's late-night forecast on Wednesday said that any thunderstorm that can get strong enough could produce gusty winds, small hall, or even a brief tornado. Tornadoes aren't all that uncommon in California, and this area is exactly where you'd expect to see them develop.

Source: NOHRSC


Also: snow. Lots of it. Not only is this good for ski resorts, but this kind of snowpack will help replenish bodies of water downstream once it all starts to melt during the warmer months. The above analysis from NOHRSC shows seasonal snowfall accumulations between 10 and 20 feet near the mountain peaks, with some areas seeing seasonal totals closer to 30 feet. The latest snowfall forecasts easily add 3+ feet to those totals through this weekend.


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February 8, 2019

The Southeast Just Saw An Exceptionally Warm First Week Of February



This has been an exceptionally warm week for parts of the southern and eastern United States, where record highs fell on Thursday—some in resounding fashion—as daytime high temperatures climbed 30°F or more above normal in spots. The spring preview came to a dramatic end on Friday as a potent cold front returned temperatures to what they should feel like at the beginning of February.

It's something else to see such a stark contrast on a nationwide temperature map. That's about as sharp of a difference between north and south as you can get. The temperature on Thursday afternoon at 2100 UTC—which is 4:00 PM Eastern—was in the 80s across parts of the Carolinas and Georgia while northeastern Montana was mired in the -20s. Temperatures dropped as much as 40 degrees in the Mississippi River Valley when the cold front passed through.

Here's what that setup looks like in the upper levels:

Source: Tropical Tidbits


That upper-level trough is the force behind those bitterly cold temperatures in the north-central part of the United States. It made it up to a whopping -16°F on Thursday afternoon in Glasgow, Montana, while Rapid City, South Dakota, saw a balmy high of -1°F. While that high in Glasgow was a record low maximum (the lowest high temperature) for the date, there have been multiple days in February that didn't get out of the -20s since the station's records began in the late 1940s.

Farther south, that strong ridge over the southeastern states, along with warm, humid winds blowing some evaporated paradise straight from the Gulf and Caribbean, allowed temperatures to soar into record territory today, the climax of an exceptionally warm week for the beginning of February.

The temperature at the airport near my tiny North Carolina town hit 81°F on Thursday. The official co-op station down the street from me recorded 79°F, which was a record high for the date. In fact, if you include today's warmth, 11 of the 29 record highs for the month of February for the nearby observing station were recorded in 2017, 2018, or 2019.

Some more records across the Southeast:
  • Danville, VA, saw a high of 80°F, the second-warmest on record for February.
  • Greensboro, N.C., reached 79°F, beating the previous daily record of 73°F.
  • Raleigh, N.C., reached 79°F, beating the previous daily record of 76°F.
  • Charlotte, N.C., also reached 79°F (see a pattern?), 5°F higher than the old record.
  • Fayetteville, N.C., managed to get up to 81°F, also a daily record.
  • Wilmington, N.C., reached 83°F, tying February's second-highest temperature.
  • Columbia, S.C., hit 83°F, falling 1°F shy of tying the all-time record for February.
  • Charleston, S.C., set a daily high temperature record of 80°F today.
  • Greenville, S.C., saw 77°F today, 7°F higher than the previous daily record.
  • Augusta, GA, reached 85°F (!), the daily record and second-highest for February.
  • Macon, GA, saw a high of 81°F, two degrees higher than the previous daily record.
(Note: I compiled the highs from NOAA's obs pages and the records from xmACIS2.)

It's worth remembering that this is the first week of February—it wouldn't be too out of the ordinary to see temperatures like this on February 28, especially farther south, but that's three weeks from now. Three weeks is an eternity when it comes to the changing seasons.

This exceptional warm-up came directly on the heels of a piece of the much-ballyhooed polar vortex breaking off and slumping down over the Upper Midwest. Raw air temperatures dipped below -40°F for several days in Minnesota and Wisconsin, breaking some impressive records in many locations.

Above-normal temperatures will likely persist in the southeastern United States over the next two weeks, though not by nearly as much of an extreme as we saw this week.

It's malpractice to talk about the warmth without addressing climate change. With each winter cold snap comes an avalanche of climate change jokes.  Har har, climate change is a hoax because I threw a snowball in the Senate. We could use some of that global "waming" when it's this cold out! And on, and on.

Source: NOAA/NCEI


When the temperature is 30°F above normal on February 7, though, the folks who make those jokes don't make a peep. Aside from the intellectual dishonesty involved, a large part of the silence in the face of abnormal warmth is the fact that we've grown so used to warmth that we hardly pay any mind to a day that's double-digits above average. We only seem to notice it when it's 80°F at the beginning of February and there are wasps flying around. Unusually cool days stick out like an especially sore thumb because they're getting outnumbered.

I wrote an article for Outside last week explaining how weather is not climate. Climate is the average of weather over a period of time. It can get bitterly cold for a week or two and we can still come in warmer-than-average on the whole. 2018 was the world's fourth-warmest year on record, coming in behind 2016, 2017, and 2015. 2018 was warmer-than-average in 41 of the 48 contiguous states, placed in the top-ten warmest years in 14 states, saw the most precipitation ever recorded in nine states, and saw lots of above-average low temperatures across the country, which is one of the hallmarks of climate change

It may be tempting to question climate change in the face of daytime temperatures far below zero in the northern Plains, but the world is much bigger than our backyards.



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Seattle Could (Briefly) Wind Up With Three Times As Much Snow As Boston By Next Week



The Pacific Northwest has two decent chances at seeing wintry weather over the next couple of days. The first storm could produce up to half a foot of snow at the lower levels in northwestern Washington, while the next storm, which arrives by the middle of next week, could bring the region another shot at accumulating snow. Seattle, which has already seen more snow (2.7") than Boston (2.3") so far this winter, could lap the Massachusetts capital several times before they have a chance to see some snow of their own next week.

Several inches of snow are possible along the I-5 corridor in Washington and Oregon over the next couple of days. The snow will start in northwestern Washington during the day on Friday and last through Saturday morning. The current NWS forecast calls for 4-6 inches of snow across the interior lowlands, including Seattle, Tacoma, and Everett, with higher totals at higher elevations.

The snow on Friday and Saturday won't be too big of a deal down near Portland and Vancouver, but an inch or two of snow can still lead to slick roadways and slippery sidewalks. The NWS forecast for the Portland area actually shows the best chance of accumulating snow on Sunday night, where a couple of inches is possible.

Much like we see in cities back east, it's hard to get a big thump of snow at lower elevations in the Pacific Northwest, especially near the ocean where the water has a moderating influence on temperatures. Nevertheless, it still does snow in cities like Seattle and Portland. Seattle averages a couple of inches of snow every year. Seasonal snowfall totals in Seattle can reach the double-digits, but big snows are far less common here now than they were in the 1950s through 1970s.

Another storm is possible toward the middle of next week, but it's too early for details. Again, a lot has to go right to get a good snowstorm in the lower elevations in this part of the country, so be wary of specifics too far ahead of time.

This has been an interesting winter for the United States so far. If you need more convincing, look no further than the social media posts of jilted snow lovers in Boston, Massachusetts. Despite interior parts of the Northeast getting walloped by snowstorms in recent months, the snow has largely missed the immediate coast. Boston has only seen 2.3" of snow so far this year, and there's not much of anything on the horizon until next week. There's a chance that next week will begin with Seattle's seasonal snowfall total three times higher than what Boston has seen all winter. Isn't that something? (Sorry, snow-loving Bostonians.)


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