Wednesday, April 4, 2018

Winter 2018's Last, Best Spring Stand: Reposting 3 Capital Weather Gang Entries on Impending Bracing April U.S. Cold Wave and Possible (Not Probable) Mid-Atlantic Saturday Snow Event

Cherry blossoms in a spring snow in New York City's Central Park, April 2, 2018; source here.

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This entry contains some extended weather talk about this early April's unusually cold temperatures across much of the central and eastern United States, as well as the possibility of accumulating snow for the Mid-Atlantic region to include the Metro D.C. area over the upcoming weekend (especially on Saturday).

Spring snowy morning, walkway in New York City's Central Park, April 2, 2018; photo by Todd Hofacker.

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I've opted to repost in their entirety three Capital Weather Gang entries by Jason Samenow that were posted earlier today. The images are also from those images. In nearly all the cases, I've rewritten the image captions to be what I consider clear and/or more detailed.

Another snowy spring morning view in New York's Central Park, April 2, 2018.

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I am also posting some pictures from this CWG entry about the heavy, wet snowfall in New York City on Monday, April 2nd, 2018, as well as some linked from the Central Park NYC twitter feed (with links given).

Officially, 5.5 inches fell in Central Park (KNYC), as did LaGuardia Airport (KLGA), while Newark Airport (KEWR) recorded 5.0 inches and JFK Airport (KJFK) picked up 3.7 inches. All four of these were daily records. It was also the most snow in New York City since 9.6 inches fell on April 6, 1982.

Spring snow and a little dachshund making his way along the sloppy sidewalk, Brooklyn, New York, April 2, 2018; photo by Michael Heiman (see above-linked CWG entry).

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About that April 1982 snowstorm, I was 12 years old and living with my dad in New Jersey at the time (at his house-in-the-woods in Upper Freehold Township) while my paternal grandparents were in Long Branch.

As a backstory, my mom and Ray and I had recently -- the previous November -- moved from Texas back to Maryland, but in the intermediate time, because the situation was so horrendous at Ray's parents' house in Baltimore, she sent me to live with my dad until she and Ray found their own apartment in Glen Burnie. (I would move back with them in time for the start of the 1982 - '83 school year, although I continued to visit New Jersey in the summers until 1988 -- going to Trade Winds Beach Club in Sea Bright.)

Snowy spring scene in New York's Central Park; photo by Cathie Reid.

Anyway, I remember that April snowstorm quite well. Not surprisingly, it did not snow in D.C. -- or at least, it did not accumulate. It was mostly a (cold) rain event here.

OK, below are the reposted three CWG entries ...

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Unusual, record-breaking April cold to crash into eastern U.S. Friday through the weekend

By Jason Samenow
April 4, 2018 at 12:53 PM EDT
Source here

Departure from normal temperature [in Fahrenheit] for portions of North America for the period 8PM EDT Thursday, April 5th through 8PM EDT Sunday, April 8th, 2018 in 24-hour time steps (animated) based upon the 12Z April 4, 2018 operational GFS, as prettied up by the CWG / Washington Post.

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On Thursday, the polar vortex will unleash a winterlike blast of cold into Montana and the Northern Plains. By Saturday, abnormally cold weather will extend from Bismarck to Baltimore.

This “ridiculous late season arctic outbreak” is likely to threaten many cold records, tweeted Michael Palmer, a meteorologist for the Weather Company.

The harshest of the cold weather is set to target the north central United States on Friday, when temperatures may fall 30 degrees below normal. Wind chill temperatures are forecast to plunge below zero early Friday from Minneapolis to Billings and to the north. The actual air temperatures are predicted to remain well below freezing even through the afternoon in these areas.

These are almost like midwinter conditions.

NWS NDFD 2-meter high temperature forecast [in Fahrenheit] for Friday, April 6, 2018 for the North Central sector of the United States as prettied up by WeatherBell.com; Forecast issued at 15Z April 4, 2018; Temperatures within a degree of daily and/or monthly record low high temperatures are circled.

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The coldest April day ever recorded is forecast for Minneapolis on Friday, when the high is predicted to reach only 21 degrees. On Thursday, when highs are expected to reach the mid-30s, the Minnesota Twins could play their coldest home opener on record, noted Star Tribune meteorologist Paul Douglas.

On Saturday morning, most locations in Minnesota and North Dakota will see lows plummet into the single digits, which is record territory. Minneapolis is expected to fall to 5 degrees, which would mark its second-lowest April temperature on record.

NWS NDFD 2-meter low temperature forecast [in Fahrenheit] for Saturday, April 7, 2018 for the North Central sector of the United States as prettied up by WeatherBell.com; Forecast issued at 15Z April 4, 2018; Temperatures within a degree of daily and/or monthly record low temperatures are circled.

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This frigid cold heads into the Twin Cities after nine inches of snow fell Tuesday, its eighth-biggest April snowfall on record.

The Arctic air will moderate as it heads south and east. But it is still expected to be cold enough to support rare April snow in the Mid-Atlantic and Northeast on Saturday, from Washington to Boston, where the temperature will be 10 to 30 degrees below normal (depending on location and time of day).

Precipitation type and intensity at 2PM EDT Saturday, April 7, 2018 for a portion of the eastern United States including New England, the Mid-Atlantic, the Ohio River Valley, and the Eastern Great Lakes region based upon the 12Z April 4, 2018 operational GFS, as prettied up by the CWG/Washington Post.

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By Sunday morning, half the nation is predicted to have freezing low temperatures, including areas as far south as Oklahoma, Arkansas and Tennessee.

NWS NDFD 2-meter daily low temperature forecast [in Fahrenheit] for Sunday, April 8, 2018 for the Lower 48 United States as prettied up by WeatherBell.com; Forecast issued at 16Z April 4, 2018.

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As cold as it is expected to be over the eastern United States over the next five days, temperatures across the remainder of the northern hemisphere are forecast to be generally warmer than normal. Temperatures in the Arctic are forecast to be about 30 degrees above normal, offsetting the cold in eastern North America.

In other words, this pocket of cold in no way refutes or invalidates the planet's long-term warming trend.

5-day forecast averaged temperature departure from average [in Fahrenheit] for the period April 4th - April 9th, 2018 for the Northern Hemisphere based upon the 12Z April 4, 2018 GFS using quarter degree by quarter degree spatial resolution as created by ClimateReanalyzer.org (Climate Change Institute / University of Maine); Average based upon the 1979 - 2000 CFSR baseline.

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Some rare April snow still seems likely Saturday, but the accumulation forecast is a mess

By Jason Samenow April 4, 2018 at 2:53 PM EDT
Source here

6-hour quantitative precipitation forecast (QPF) [in inches] and dominate precipitation type for the period 12Z - 18Z April 7, 2018 based upon the 12Z April 4, 2018 operational ECMWF for a large portion of the eastern third of the United States, as prettied up by WeatherBell.com.

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Snow is still in the forecast Saturday, and computer models, across the board, predict we'll see flakes. But the nagging question is: How much will actually fall? And that’s one that's almost impossible to answer, at least right now. Accumulating snow of more than an inch, which would be the most in April since 1924, is possible but not necessarily likely.

Rain is forecast to develop predawn Saturday or a little later, just hours after temperatures leap well into the 60s Friday afternoon. But an unusually strong Arctic front for the time of year will be seeping south, while a dynamic storm system develops along it. The cold air will follow the front, likely allowing the rain to change to snow Saturday. But when this transition occurs will be critical in determining accumulation chances.

As we discussed Tuesday, the snow will have to overcome many obstacles to accumulate. The storm has to be intense, and its track and timing just right.

"To get accumulating snow in April requires a perfect storm track, a high-pressure system to the north to feed in cold air and the zone of most intense precipitation to track right over your area," said Wes Junker, Capital Weather Gang's winter-weather expert. "The models are suggesting all three of those factors have a chance to come together. Even then, the high sun angle in April will try to fight accumulations. During any lulls in the precipitation during the late morning and afternoon, the snow will tend to melt because of the sun and the warmth stored in the ground."

The way for Washington to see an accumulating and even historic April snowstorm would be for rain to quickly change to heavy snow in the predawn hours Saturday and then fall hard through the early and midmorning hours before the sun gets too high.

The big question is: Will such a snowy scenario play out? That's what some weather apps have advertised, but reality is a lot messier. It will be very difficult for all of the ingredients to gel, especially in April, one day after highs in the 60s.

We see three possible scenarios, with the first being somewhat more likely than the other two. Based on the time of year, and recent shifts in the models, we do not see the snow jackpot scenario in Washington as most likely, but rather in locations to the north.

Scenario 1: Heaviest snow falls north of D.C. (45 percent chance)

The cold air is a bit slow to arrive. Rain begins predawn Saturday and becomes heavy. It turns to wet snow from northwest to southeast during the day Saturday, but temperatures mostly remain above freezing near Washington, limiting snow accumulation. However, it’s colder to the north and northwest, where more significant accumulation would be possible. The NAM model supports this scenario.

Forecasted radar showing precipitation type and intensity for the Mid-Atlantic and adjacent regions for the period 2AM EDT through 8PM EDT Saturday, April 7th, 2018 in 3-hour time steps (animated) from the 12Z April 4, 2018 NAM, as prettied up by the CWG/Washington Post.

Accumulation potential: Under an inch, mainly on grassy areas around Washington. Colder areas to the north and northwest could see a few sloppy inches.

Scenario 2: Washington is in the snow sweet spot (30 percent chance)

Moisture and cold air perfectly converge over Washington. Rain quickly changes to snow predawn Saturday before the sun comes up and falls heavily Saturday before tapering off in the afternoon. Temperatures fall below freezing as the snow falls. The American model supports this scenario.

Forecasted radar showing precipitation type and intensity for the Mid-Atlantic and adjacent regions for the period 2AM EDT through 8PM EDT Saturday, April 7th, 2018 in 6-hour time steps (animated) from the 12Z April 4, 2018 operational GFS, as prettied up by CWG/Washington Post.

Accumulation potential: At least a few inches, maybe more. A historic April snowfall.

Scenario 3: Heaviest snow is southwest of D.C. (25 percent chance)

Rain breaks out early Saturday and changes to snow from northwest to southeast. However, the zone of heaviest precipitation sets up southwest of Washington. While it snows around the area, it’s not heavy enough to cool the air down to freezing, so accumulation is light and limited mostly to grassy areas. The possible exception to this is in Washington's far western areas and in a localized band of heavy snow to the southwest. The Canadian model supports this scenario.

Forecasted radar showing precipitation type and intensity for the Mid-Atlantic and adjacent regions at 2PM EDT Saturday, April 7th, 2018 based upon the 12Z April 4, 2018 operational GDPS (Canadian) model, as prettied up by the CWG/Washington Post.

Accumulation potential: Less than an inch, mainly on grassy areas around Washington. Colder areas to the west and southwest could see a few sloppy inches.

Note: The European model shows a blend of the scenarios, with the heaviest snow falling to the north, west and southwest of Washington (at higher elevations) because temperatures do not cool enough, but it still predicts snow.

We'll try to narrow down these scenarios in our next comprehensive update Thursday. But this forecast is going to be a tricky one down to the wire.

Some fun April snow trivia
  • The last three measurable April snowfalls on record in D.C. have all occurred on the 7th (in 2007, 1990 and 1972), which is this Saturday (hat tip: Mike Thomas, Fox 5).
  • The year of the heaviest April snow on record in Washington, 1924, and the last time at least an inch fell during the month, was the last time Washington won a baseball championship. The Senators won the World Series that year (hat tip: Peter Mullinax).
Washington's 20 biggest April snow events

Top 20 April daily snow events in April ranked [including ties] for Washington, D.C., for the period July 1, 1884 - April 2, 2018; Snowfall measurements have been done at National Airport (KDCA) since 1945; prior to that, they were taken at other locations in the District of Columbia.

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Some notes about snow accumulation in the spring (adapted from article posted Tuesday)
  • Accumulation, if any, depends heavily on snowfall intensity and time of day.
    • If the snow is light and intermittent, it will simply melt on most surfaces, day or night.
    • Snow that falls steadily at night and in the early morning can accumulate, especially if moderate to heavy.
    • Snow that falls between midmorning and late afternoon will have a hard time accumulating, especially on pavement, unless it is very heavy. Typically, daytime April snow melts and/or compacts faster than it can accumulate.
  • Any snow accumulation will tend to be very elevation-dependent, simply because temperatures cool with altitude. Typically, during spring snowstorms, temperatures are above freezing at low elevations, which limits accumulation potential. However, snow can still pile up as you head toward the mountains.

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Flake news: Please disregard snow total forecasts on your weather apps

Yoshino (Japanese) cherry blossom buds in the snow along the Tidal Basin, Washington, D.C., March 21, 2018 (Kevin Ambrose)

By Jason Samenow April 4, 2018 at 10:48 AM EDT
Source here

Hysteria hit Washington on Wednesday morning as popular weather apps on mobile devices were predicting 5 to 8 or even 8 to 12 inches of snow Saturday. Yes, it may snow Saturday, but no person or computer has enough information to provide a reliable snow forecast right now.

By and large, weather apps are generated by computers (some have manual override, if the app-provider takes the initiative to alter the computer-generated forecast). They mostly do not take into account critical factors that can limit how much snow will fall in Washington in early April, including:

*The warm ground. Temperatures may hit 67 degrees Friday, the day before the snow. Even if snow starts to pile up Saturday, the rate of accumulation will be lessened by melting from below.

*The warm air. Assuming snow materializes, temperatures will be above freezing when it begins and during any lulls in the storm, resulting in melting from above.

*The sun angle. A significant amount of the snow is predicted to fall when the strong April sun is high in the sky Saturday. You won’t be able to see the sun, but its energy will ooze through the clouds and make it hard for snow to stick, unless it is falling very heavily.

These apps also do not communicate uncertainty in the storm track or take into account how deviations could decrease snow amounts. Right now, weather models have the D.C. area in the snow sweet spot for this storm. But if the storm track shifts, and the snow is not heavy or mixes with rain, it will not accumulate much. Your app is not telling you that.


These apps also do not communicate uncertainty in the storm track or take into account how deviations could decrease snow amounts. Right now, weather models have the D.C. area in the snow sweet spot for this storm. But if the storm track shifts, and the snow is not heavy or mixes with rain, it will not accumulate much. Your app is not telling you that.

In other words, accumulating snow in April is very rare. The amounts these apps are predicting would be exceptional and historic.

We probably won't know until Thursday or Friday whether a historic event could materialize. The amount of snow being forecast by these mobile apps is not out of the question, but -- at this point -- there is a very low probability.


Because these apps mostly lack human input and experience, and fail to communicate uncertainty, my advice is to ignore their snow amount forecasts. Full stop.

These apps are fine for guiding you when the forecast is relatively straightforward. But their pitfalls are massive once the forecast becomes complex.

In times like these, it is well worth your while to find a trusted human source of weather information who can walk you through the complexities of the forecast and describe the full range of possibilities.

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Honestly, I'm not expecting any meaningful -- or, indeed, measureable -- snow in D.C. proper. More like this -- ESPECIALLY if Cantore shows up again ...


It's just way too late in the year for that and the Sun is too strong and the odds that everything do not align just perfectly too high. That being the case, I don't expect to make a big fuss about the impending weather event. I just hope we get some decent rainfall -- that's what we really need in the Baltimore/Washington region.

OK, that's al for now. For tonight, it is a non-gym night. (I went on Monday and Tuesday nights.) Instead, I'm going to stop at Proper 21 by Metro Center -- which has a really nice salmon burger sandwich -- and then Trade and/or No. 9.

My next planned entry will be late tomorrow (Thursday) night or early Friday morning.

--Regulus

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