October 31, 2019

Dangerous T'storms Will Roll Through The Mid-Atlantic Just In Time For Trick-Or-Treating

A round of severe thunderstorms will move across the southeast and Mid-Atlantic states through the afternoon and evening hours on Thursday, bringing the potential for widespread damaging wind gusts and possibly even some tornadoes. The greatest threat for storms along and east of the Appalachians will coincide with kids trick-or-treating for Halloween in some populated areas. Please stay mindful of approaching storms and avoid going out tonight if you're under a risk for severe weather.

Broken lines of thunderstorms are already developing along the Appalachian Mountains as a strong cold front pushes into a warm, muggy airmass parked over the eastern United States. Conditions are downright tropical right now east of the front. Dew points were all the way up in the 70s as far north as Washington D.C. by 2:00 PM, which is a level of tropical humidity more common in August than the last day of October.

The thunderstorms will coalesce and strengthen as they move over the Appalachians and start ingesting all that unstable air. Severe thunderstorms are possible from central Alabama to southern Quebec, but the greatest risk for severe thunderstorms today exists across the Mid-Atlantic states. The Storm Prediction Center has issued an enhanced risk for severe weather, or a 3 out of 5 on the scale used to measure the threat for severe thunderstorm, from Greensboro/Raleigh in North Carolina up to the Scranton area in northeastern Pennsylvania. 

The main threat will be damaging winds and tornadoes. Winds are ripping just a few thousand feet above the ground. Winds at the surface are pretty strong as well—I can hear the gusts thunking against the window as I write this near Greensboro. It won't take much of a thunderstorm to shove those winds down to the surface and produce wind gusts of 60+ MPH.

The strong southerly winds giving us those warm, muggy conditions will also provide the low-level shear necessary for some storms to produce tornadoes. The greatest risk for tornadoes will exist in and around the area under the enhanced risk. The threat appears maximized up around the D.C. area, but the threat will exist anywhere along the squall lines that form today.

Tornadoes in squall lines can happen quickly—often with reduced tornado warning lead time—but duration doesn't relate to intensity. Any tornado is dangerous if it hits you...I know it sounds simplistic to say that, but even an EF-0 can send debris flying around at lethal speeds.

Please reconsider taking kids out trick-or-treating tonight if you're under a risk for severe weather. These storms will move fast and you may not have enough time to get to safety if the storms approach while you're out walking the neighborhood.

I look at it this way: if parents are willing to sit there and check each individual piece of candy to make sure nobody tampered with it, then they should be willing to say "no go" when there's the threat of dangerous thunderstorms on the evening of Halloween. You're much more likely to get hurt by lightning/falling trees than you are to get a piece of candy with a pin or poison in it. Kids will get over the disappointment, and many communities may even push back trick-or-treating to this weekend.

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October 27, 2019

An Extreme Wildfire Danger Will Develop In Northern California On Sunday

Dangerous fire weather conditions will develop across much of northern California on Sunday. A period of powerful winds and bone-dry humidity could allow even a tiny spark to grow into an out-of-control fire in short order. Authorities have issued a mandatory evacuation order across a significant portion of Sonoma County as worsening weather conditions could allow the Kincade Fire to rapidly grow and spread toward the Pacific Ocean.

The Storm Prediction Center's fire weather forecast for Sunday highlights an area of extremely critical fire weather conditions north of the Bay Area—including Santa Rosa, Napa, and Davis—with critical fire weather conditions existing along a swath from Fresno County to Shasta County. The agency used the term "potentially historic" to describe the fire weather threat in this area on Sunday, language they don't use lightly.

Forecasters also expect critical fire weather conditions to develop in southern California along the Transverse Ranges near Los Angeles.

High winds are likely across California on Sunday as a tight pressure gradient develops over the state. The strongest winds are likely in northern California, where we could see a widespread area of sustained winds of 40+ MPH with gusts reaching 60 MPH or higher beginning Sunday morning and lasting through the evening hours.

Winds descending on the leeward side of the mountains will warm and dry as they reach lower elevations, leading to the warm temperatures and extremely low humidity that will contribute to favorable conditions for rapid fire growth on Sunday.

Sonoma County's evacuation map around 11:00 PM PT on October 26, 2019. Areas in purple are under a mandatory evacuation. Areas in blue are under a voluntary evacuation. The area in red is the extent of the Kincade Fire.

This is a dangerous situation. Authorities have issued mandatory evacuation orders to everyone near and downwind of the Kincade Fire, stretching all the way to the coast. The situation on the ground in the evacuation area is otherworldly, with air raid sirens blaring across a moonlit horizon darkened by PG&E's blackout.

More than a million people are without power tonight as part of PG&E's Public Safety Power Shutoff program, and more power outages are likely as the winds pick up on Sunday. The goal of the intentional blackouts is to prevent downed lines or equipment failures from igniting fires on windy days; however, this may not have been enough to prevent the Kincade Fire, which reportedly started near a set of powered transmission lines during Wednesday night's blackout.

Weather conditions in northern California should become less favorable for major fire development by Monday morning as winds begin to die down.

[Top Image: Smoke from the Kincade Fire on October 24, 2019 via NOAA]

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October 24, 2019

Extreme Fire Weather Conditions Will Exist Across Parts Of California On Thursday

Strong winds and low humidity will create favorable conditions for wildfires across portions of California on Thursday. An intense fire had already broken out in Sonoma County on Wednesday night, where winds are gusting as high as 76 MPH. More fires could ignite before weather conditions slowly begin improve on Thursday night.

The combination of strong winds, low humidity, and dry vegetation will allow dangerous fire weather conditions to persist through the day on Thursday across southern California, with the the greatest risk existing in the Transverse Ranges surrounding the Los Angeles area. Critical fire weather conditions will also exist on Thursday in portions of northern California, including the northern Coast Ranges and the Chico area.
An intense fire broke out Wednesday night near Geyserville in Sonoma County, about 20 miles north of Santa Rosa on Highway 101. The Kincade Fire had consumed 1,000 acres in just a couple of hours, with no containment as of the publication of this post around midnight Pacific Time. Several folks published webcam snapshots of the fire on Twitter, showing a sea of flames on the nearby hillsides.

As we see so often during the autumn months, the greatest fire risk today will come from a Santa Ana wind event in southern California. Santa Anas develop when northeasterly winds blow across southern California. Winds cresting the mountains speed up as they race downslope toward the coast. Since sinking air compresses as it descends, these winds grow very hot and very dry by the time they reach lower elevations. This leads to an extended period of strong winds—often in excess of 40-50 MPH—hot temperatures, and bone-dry humidity levels, all three conditions necessary for explosive fire development.

More than 180,000 people in northern California were without power late Wednesday night as part of PG&E's Public Safety Power Shutoff program, designed to prevent wildfires if strong winds damage power equipment. Investigators found that PG&E's power lines sparked the November 2018 Camp Fire, which killed 85 people and destroyed most of the town of Paradise, California.

SoCal Edison warns that 308,000 customers could lose power as a safety precaution when the Santa Ana winds crank up on Thursday.

Conditions should improve on Thursday evening, though critical fire weather conditions could extend into Friday across parts of southern California.

The Storm Prediction Center issues fire weather outlooks much as they issue severe weather outlooks. The three categories—elevated, critical, and extremely critical—convey how favorable conditions are for the development and spread of wildfires. Fires can spark more easily and grow more quickly as you get into "critical" and "extremely critical" territory.

Forecasters issue the "extremely critical" category when sustained winds of 30+ MPH, very low humidity, and warm temperatures are all expected to coincide for three or more hours across an area with dry vegetation. The SPC has an extended rubric on its website explaining the criteria necessary for a forecaster to issue each fire weather category.

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October 19, 2019

Tropical Storm Nestor Set To Hit Florida Saturday With Heavy Rain, Gusty Winds

Tropical Storm Nestor is hauling tail toward Florida's Gulf Coast this evening. The lopsided storm should make landfall near Apalachicola, Florida, sometime early Saturday morning, bringing much-needed rain to the southeast over the next couple of days. The storm's winds could lead to power outages and potentially a life-threatening storm surge along parts of the Florida coast.
Nestor around midnight on October 19, 2019. || College of DuPage

Nestor isn't exactly a "classic" tropical storm. The system is lopsided, for one, and it's right on the line between tropical and extratropical, or the more common type of low-pressure system that's powered by upper-level lift. This is one of those cases where we have to focus on the storm's impacts rather than the storm itself.


We won't see a ton of rain from Nestor, but the southeastern United States is mired in a growing drought and any little bit of rain will help.

The latest forecast from the Weather Prediction Center shows the potential for a couple of inches of rain across most of the southeast over the next week, with the heaviest totals expected around Tallahassee where Nestor's core comes ashore. Some areas in the Florida Panhandle could see 5 or more inches of rain, which could lead to flash flooding in vulnerable areas.

Storm Surge

The unique shape and composition of the coast along Florida's Big Bend can expose coastal communities to a potentially life-threatening storm surge as Tropical Storm Nestor comes ashore. If the storm remains at its predicted strength on its predicted track into Florida, the latest advisory from the National Hurricane Center shows the potential for:

—a storm surge of 3 to 5 feet above ground level is possible from Indian Pass to Chassahowitzka
—a storm surge of 2 to 4 feet above ground level between Chassahowitzka to Clearwater Beach
—a storm surge of 1 to 3 feet above ground level in Tampa Bay.

I've highlighted the NHC's maximum surge potential on the map above, even though we all obviously know where Chassahowitzka is, right?


Power outages are likely along and to the east of Nestor's point of landfall as strong winds knock down trees and power lines. Sustained winds of 60 MPH winds don't sound like much, but keep in mind that severe thunderstorm warnings are issued for thunderstorm wind gusts of 60 MPH.

Tropical storm watches and warnings don't extend inland since the storm is expected to lose its tropical characteristics not long after landfall. However, wind advisories are in place for much of Georgia as gusts as high as 50 MPH could accompany the storm as it moves through on Saturday.

Make sure you keep your phone on the charger tonight, and keep a flashlight—a real flashlight, not your cell phone's flashlight—handy in case you lose power in the middle of the night.


Radar image of a tornado northeast of Tampa, Florida, on October 18, 2019. || Gibson Ridge

Tornadoes are in progress across the Florida peninsula as I publish this article. Conditions are usually favorable for tornadoes to develop in thunderstorms embedded on the eastern side of landfalling tropical systems. Some of the tornadoes can be rather strong; we've already seen one classic supercellular tornado between Tampa and Orlando, complete with a well-defined hook echo and strong debris signature on radar (shown above).

The threat for tornadoes will continue across Florida through Nestor's landfall on Saturday morning, with the threat following the storm into Georgia and South Carolina through Saturday.

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October 16, 2019

A Strong Nor'easter Will Rapidly Strengthen Over New England On Wednesday Night

A potent but quick-hitting nor’easter will move into New England through Thursday afternoon, bringing a period of rough weather to the northeastern states that could result in some flooding issues and power outages. Aside from potential issues caused by heavy rain and gusty winds, much of the hullabaloo surrounding this storm stems from the term “bomb cyclone,” a hypetastic phrase that appears in just about every news article about the storm. Here’s a quick look at how the storm will strengthen so quickly.

Gusty Winds and Heavy Rain

Wind advisories are in effect from New Jersey to Maine ahead of tonight’s storm. Winds are gusting as high as 50 MPH in spots as the storm moves through the area; the combination of wet soil and full foliage will stress trees to their tipping point, potentially leading to power outages, home damage, and blocked roads. Don’t forget to stick your phone on your charger before going to bed tonight, and be mindful of large limbs or trees looming over your home.

Several inches of rain could fall during the storm, which could lead to flooding issues in low-lying areas. Roads that are normally fine during heavy rain at other times of the year could see standing water or outright flooding due to fallen leaves clogging up drains and sewers.

Bomb Cyclone

This storm is a “bomb cyclone.” The term is everywhere. Always. We can’t escape it. It’s like “polar vortex” and “wedge tornado.” It’s just there and we’ll have it forever and it’ll be used to get clicks until the internet dissolves in the fiery inferno of the Sun’s…wait, what was I talking about?



A low-pressure system undergoes bombogenesis when its minimum central pressure drops 24 mb in 24 hours. The resulting storm—a “bomb cyclone,” if you will—is usually pretty impressive in both its effects and its appearance on satellite imagery
Record low air pressure records for the month of October. | Source: NOAA/WPC
This storm’s minimum pressure dropped in a hurry. The low had a minimum central pressure of 998 mb as it passed over the Delmarva Peninsula at 2:00 PM on Wednesday. Its pressure had dropped to 988 mb six hours later as it approached New York City. Most weather models have the system’s minimum pressure falling below 975 mb as the storm moves into interior New England during the day on Thursday. A pressure that low would set some all-time minimum pressure records at some weather stations in New England; air pressure records for the month of October are shown above.

How does a storm strengthen that quickly, anyway? Divergence.

Divergence describes winds fanning out in the upper levels of the atmosphere. Winds tend to spread apart as they leave the base of a trough or as they enter and exit stronger parts of the jet stream, known as jet streaks. Air has to rush upward to fill the void left behind by the diverging winds, leaving less air—a center of low pressure—at the surface.

An analysis of the jet stream at 8:00 PM on Wednesday. | Image: Tropical Tidbits, with my annotations

We have three sources of lift working on tonight's nor'easter as it revs up off the coast of New Jersey. The above model image shows an analysis of the jet stream at 8:00 PM on Wednesday.

A strong trough, combined with two different jet streaks, are all working together to lift massive amounts of air from the surface. If multiple sources of divergence align with each other like we're seeing right now, air has to rush upward from the surface really fast to compensate for the void left by the winds spreading out. This leads to the rapid strengthening of a low-pressure system at the surface. The storm will begin to weaken once it starts to lose that lift from above. In this case, the storm will slowly lose steam as it meanders toward Atlantic Canada on Friday.

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October 10, 2019

Two Feet Of Snow Is Possible In North Dakota As The Season's First Cold Air Arrives

A major winter storm could bring several feet of snow to the northern Plains over the next couple of days, bringing the first widespread shot of winter to much of the continental United States. Temperatures are falling like a rock behind the developing storm, dropping as much as 30°F in just a couple of hours. The chill will continue to sag south through the weekend, eventually reaching the Mississippi River before it starts to taper off.

A strong upper-level low swinging across the northern Rockies is to blame for this early-season taste of winter. A cold front moving across the Rockies on Wednesday sent temperatures falling off a cliff through the afternoon hours. Take a look at these temperature drops on the Front Range as the cold front swept through:

Chart: me | Source: Iowa State/IEM
Denver fell from 82°F to 47°F in just four hours on Wednesday afternoon. Cheyenne, Wyoming, saw an equally impressive drop, falling from 70°F to 38°F between 1:00 PM and 5:00 PM on Wednesday. The cold front sent temperatures plummeting from 80°F to 45°F in just three hours in Fort Collins, Colorado.

The cold air will continue to spread across the Plains on Thursday and Friday, eventually crossing the Mississippi River this weekend as a weaker (but still much cooler) burst of fall chilliness. The animation above shows the National Weather Service's forecast low temperatures between Thursday and next Monday. Low temperatures could reach the upper 30s as far south and east as the Tennessee River Valley.

Such an intense blast of early-season cold is almost always followed by snow somewhere, and it's going to get cranking in a hurry.

Above is the National Weather Service's snowfall forecast as of Wednesday evening, stretching through Saturday night. These forecasts were generated by individual NWS offices, so some of the borders and gradients may look a little bit wonky.

Overall, the latest forecasts show more than a foot of snow falling on a significant portion of North Dakota and South Dakota, with the greatest totals likely throughout central North Dakota where totals could easily exceed two feet. The highest totals are likely closer to the Canadian border.

Snow is already underway along the Front Range as I write this post late Wednesday night—including reports of thundersnow!—and the snow will continue spreading toward the Dakotas through the day on Thursday. Snow should slow down in North Dakota on Friday night, coming to an end on Saturday morning as the storm system pulls away from the area.

Now, about Canada. I know quite a few of you live in Canada or have family and friends up there. Every time we have a major weather event near the northern border, I get emails/tweets asking me why we discriminate against our polite neighbors.

There are a few reasons most American weather maps stop at the border. It's a sovereignty issue, for one, given that official American government forecasts could clash with an official Canadian government forecasts. The second is that NWS offices can only forecast for their area of responsibility—their forecasts come to a hard stop at a county border, let alone the dividing line between the U.S. and Canada.

Thankfully, snow is one of those events where we can look at products issued by NOAA's Weather Prediction Center, which actually do transcend international borders. The forecast is a little different from how local NWS offices arrive at their numbers, but it's usually close enough to give us a good regional overview.

The two-foot snow totals will extend deep into southern Manitoba, not too far from the Winnipeg metro area, which is home to about 800,000 people. The city could see more than a foot of snow if the forecasts verify. That much snow is more than enough to mess things up for a day since it's the first snow of the season and, no matter how used to it you are, that much snow is tough to deal with.

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October 4, 2019

Historic Heat Wave Produces Some Of The Hottest Temps Ever Recorded In October

Temperatures soared into the triple digits across parts of the southeastern United States this week at the peak of a historic late-late-season heat wave. Cities from the Gulf to the Great Lakes shattered all-time October high temperature records on Wednesday and Thursday, and many of those spots recorded temperatures hotter than any they saw during the summer.

It's been hot. Really hot. It's easy to forget that it's the beginning of October with temperatures this darn hot. It's funny...abnormally warm weather and shattered high temperature records are so common nowadays that we hardly notice them anymore. (Compare that to, say, the frenzied reaction to the major snowstorm that hit Montana and Alberta earlier this week.)

This major heat wave closes out what's been a long and dry summer in much of the eastern part of the country. Some cities saw their earliest and latest 100+°F readings this year, beginning with a major heat wave in May and ending with this nonsense here in October.

It's been a daunting task to keep up with all the records over the last couple of days; not just daily record highs, mind you, but all-time monthly high temperature records (which appear as pink dots on the following maps). Those are the hottest temperatures ever recorded at those stations during the month of October, with records stretching back more than 100 years in many locations.

Here's a look at where high temperature records fell on Wednesday:
Source: CoolWx.com

...and a look at Thursday's heat:

Source: CoolWx.com

Dozens of reporting stations broke their all-time high temperature for the month of October on Wednesday, and many of those stations re-broke their all-time monthly high on Thursday. Some of those records weren't close calls, either.
  • Huntsville, Alabama, hit 100°F on both Wednesday and Thursday, which is the latest triple-digit readings—and the hottest October days—on record there.
  • Raleigh, N.C., hit 100°F on Thursday, which is the latest triple-digit reading on record there.
  • Birmingham, Alabama, broke its record high on each of the 9 days between September 26 and October 4.
  • Charlotte, N.C., broke its record high on 7 out of the last 8 days, which, according to Brad Panovich, is the city's longest streak of broken high temperature records in 140 years of recordkeeping.
  • The summer began and ended with the earliest and latest 100°F readings on record in Augusta, Georgia. The city saw its earliest 100°F back at the end of May, and their latest 100°F occurred on Thursday. Columbia, S.C., also saw its earliest 100°F on record back in May, but missed its latest triple-digit reading by just two days this year.
The heat will wane through the weekend as the first major cold front of fall continues to push through the Mid-Atlantic and southern states. Seasonable temperatures could reach as far as the northern Gulf Coast by early next week—and, really, hoping for average highs shouldn't be such a reach in October.

500 mb normalized height anomalies. Source: Tropical Tidbits

A significant upper-level ridge parked over the eastern half of the United States is responsible for the excessive heat. Sinking air beneath a ridge of high pressure compresses as it reaches the surface, leading to the unpleasant warmth we've felt for the last couple of days. That sinking air also serves to put a lid on any convection or clouds, which is why you hardly get any relief from a passing cloud or afternoon shower.

Extremes beget extremes; the intense ridge over the eastern United States was directly related to the sharp trough over the Rockies earlier this week that brought historic early-season snows to parts of Montana and Alberta. Some spots saw as much as three feet of snow to begin the month of October, which is among the highest snowfall totals ever recorded during the month in those communities.

Speaking of extremes, it's important to address the elephant in the room. Climate change increases the probability of seeing heat waves like this in the future. It's hard to link directly link any one specific event to climate change; climate consists of patterns and trends, after all. Abnormal heat is certainly a trend. The five hottest years on record were 2016, 2017, 2015, 2018, and 2014. It's likely that 2019 is going to rank high on the list at the rate things are going. Climate change exacerbates the extremes, meaning we're likely going to see longer and more intense heat waves as the atmosphere warms and it begins to affect weather patterns.

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