Snow risk increases for late Thursday; mega-cold shot next week?

cwg_JunkerThe threat of snow is increasing for late Thursday into Friday. Several models are suggesting a significant snowstorm for the Northeast and possibly even parts of this area. There is still way too much uncertainty concerning details to make a definitive call about timing, amounts and impacts. Snow is no guarantee.

A second storm is possible early next week, but the precipitation type is uncertain.

Arctic air will follow each storm, likely the coldest or near coldest of the season, and perhaps the coldest in years.

The bottom line is the upcoming week looks unsettled and on the cold side. The Thursday storm is one that needs to be monitored closely as it will influence the severity of the cold outbreaks.

The Thursday-Friday storm

Most models as of last night were predicting that this storm would track into the Ohio Valley region before reforming along the East Coast just far enough north to really limit our chances for getting snow. However, this morning’s GFS, UKMET and European models are now predicting that a low will track far enough to our south to threaten us with snow, possibly significant snow if the European model is correct.

The GFS model simulation for Thursday and Thursday night (WeatherBell.com)

The GFS model simulation for Thursday and Thursday night (WeatherBell.com)

The European model simulation for the storm Thursday night and Friday morning (StormVistaWxModels.com)

The European model simulation for the storm Thursday night and Friday morning (StormVistaWxModels.com)

Last night’s models suggested the snowy simulations were outliers, not the most likely solution. However, with today’s higher resolution models shifting the storm track significantly to the south and offering a snowier look, it is hard not to take them seriously. However, there still is not a consensus about how this storm will evolve.

The two most likely scenarios are:

1) A storm tracks across the Ohio Valley before re-forming off the coast of New Jersey. The initial shot of precipitation passes to our north and then the westerly winds result in a dry slot over our region. Such a scenario would offer us little in the way of snow except maybe a few snow showers or flurries in the wake of the storm. Today’s Canadian and NAM are predicting this general scenario.

2) The jet stream dives far enough south to produce a low over the Gulf Coast states. Then, that low tracks to our south and deepens rapidly off North Carolina coast or Virginia Capes. Such an evolution would produce snow and could bring significant snowfall to portions of the area. It’s way too early to entertain how much. The GFS, UKMET, and European model have jumped to this scenario.

Which is right? We don’t know and may not for another day or two. But, without a doubt, the threat of snow has grown.

Friday-Saturday cold shot

Some of the coldest air of the season is expected to arrive Saturday and Sunday as potent high pressure system settles across the east behind the exiting storm.

The box and whiskers diagram below shows the European model ensemble guidance.

European temperature simulation over the next 10 days (WeatherBell.com)

European temperature simulation over the next 10 days (WeatherBell.com)

The vertical blue lines tell you how much uncertainty among the various ensemble members (simulations of the model with tweaked initial conditions), the thicker green/gray lines encompass one standard deviation from the mean (about 67% of the members are in that range). Note on the diagram below how the forecast temperatures plummet January 4. High temperatures may struggle to reach freezing and lows will probably be in the upper teens to low 20s in the District, and perhaps closer to 10 in the colder suburbs.

I’m fairly confident about it getting cold Friday and Saturday. Exactly how cold it gets at night is very much up in the air and depends on whether we get any snow or not. Snow cover really can lead to low nighttime temperatures because of radiational cooling. Snow cover could easily mean temperatures 5 or more degrees colder than stated above.

Sunday-Monday storm

The models are hinting at another storm January 5 or 6. I’m not bullish on the storm’s potential for snow as the pattern favors a storm track north of D.C. which would put us on the warm side of the storm. But I’ve been wrong before. If the low tracks to our north, temperatures could rise into the 50s.

The box and whisker diagram above does a great job of showing the uncertainty inherent in the temperature forecast towards the beginning of next week. The length of the vertical blue lines indicates there is little certainty about how the pattern will evolve beyond this weekend.

At least one ensemble member is forecasting temperatures into the 60s because its simulation tracks the low well to our north putting us in southerly flow. However, other members keep us quite a bit colder and might support winter weather despite my reservations about the pattern. The majority of the members simulate the maximum temperatures in the 30s to around 50.

The precipitation simulations also vary widely. Last night’s GEFS plume diagram (below) illustrates the uncertainty about the potential for precipitation from late on January 5 through the early morning on the 7th.

gfs-storm2

Remember the green lines indicate rain and the blue indicate snow while the slope of the lines and how steep they are tell you the intensity of the precipitation.

With one exception, the ensemble members with the heavier precipitation simulate rain rather than snow. Some members don’t even have a storm, or have it but keep us in a dry slot. Such uncertainty is normal for a storm so far in the future. Even though I favor a northern track which usually doesn’t favor snow, all you can confidently say about the period is there is an above normal chance of precipitation.

Possible mega-cold shot next Tuesday

Both the GEFS and European ensemble members are in good agreement that there will be a second arctic blast after the second possible storm. The operational European has been predicting crazy cold, with historically frigid air dropping below zero across the area. But its temperatures have often been running too cold this season in long-range simulations.

The ensemble mean shown above (in the box and whisker diagram) probably offers a more reasonable estimate of the temperatures. Still, if it’s right, we would end up with a day with the maximum temperatures staying in the 20s and the minimum temperatures dropping into the low-to-mid teens with even lower temperatures out towards the mountains. This has the potential to be the coldest outbreak in quite a while.

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