Explaining the uncertainties with the upcoming snowstorm

Although yesterday saw pretty good agreement among most computer model guidance regarding a major snowstorm on Monday, we were still four days away from the storm, which is a pretty long time in the realm of computer modeling. In more plain terms, this means that despite this agreement, we were still susceptible to changes in modeling that would ultimately affect the outcome.

The current weather pattern is dealing with a strong storm system crashing into southwest California, as well as a strong Polar Vortex in Southern Canada, supplying cold air into much of the country. This storm system in southwest California is forecast to move eastward, and as it does, will be increasing the warmth and moisture out ahead of it as it interacts with the Gulf of Mexico. Additionally, the Polar Vortex is forecast to exert its muscles and intensity the cold around it and just to its south. This leads to a strong source of anomalously cold air in the north, and anomalously warm air to the south — creating a strong temperature gradient which should lead to widespread, heavy precipitation just north of this gradient. This part seems easy.

But it’s much more complicated than that. The Polar Vortex in a way is a double-edged sword. While it can supply the cold air to strengthen the gradient, it can also be so strong that it shears out the initial storm system. If the initial storm system gets sheared out, then its ability to generate warm air advection out ahead of it and spread northward is decreased, which means the thermal gradient weakens and gets pushed to the south.

Let’s take a look at some computer models. Today’s 18z NAM showed a major snowstorm of 10-15″, whereas today’s 18z GFS only shows a few inches. Below, we will see the 18z NAM valid for 10:00pm, Saturday night.

Today's 18z NAM at 500mb valid for 11:00pm Saturday night shows a favorable setup for major snow, due to the Polar Vortex becoming elongated, as opposed to compressed.

Today’s 18z NAM at 500mb valid for 10:00pm Saturday night shows a favorable setup for major snow, due to the Polar Vortex becoming elongated, as opposed to compressed.

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Significant winter weather event possible next week

Forecast models have come into much better agreement regarding a storm system which is forecast to impact the area early next week. Several days ago we mentioned in a blog post that the next significant precipitation event wouldn’t come until early this coming week — and that idea continues. A powerhouse upper level trough will slam into the West Coast over the next day or two, providing much needed rains. But more interestingly for us, the associated energy will de-amplify somewhat, but eject northeastward towards the Tennessee and Mississippi Valleys. As a result, a surface low pressure system will develop from the Arkaltex towards the Mid Atlantic coast.

To our north, a piece of the Polar Vortex will be meandering in Southeast Canada, maintaining an impressive low level cold air source. The result of the ejecting shortwave and cold air to the north will be an impressive thermal gradient and stalled frontal boundary, which will extend from the Central US towards the Mid Atlantic. Along this thermal gradient, a plume of moisture is forecast to develop in response to enhanced lift from the aforementioned disturbance. Accordingly, areas to the north of this thermal gradient (including much of our forecast area) will be in line for a potentially significant winter weather event.

GFS model showing a large-scale winter weather event impacting the Eastern United States early next week.

GFS model showing a large-scale winter weather event impacting the Eastern United States early next week.

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Technical Discussion: Snowstorm Early Next Week

The winter that never ends continues into March with a large snowstorm taking aim on our area once again. After taking a closer look at data this afternoon and getting a better understanding of how this storm will develop, confidence is increasing in the fact that another plowable snowstorm will impact the northeast Sunday into Tuesday morning (March 2nd-4th). The worst of the storm is expected to occur throughout the day on Monday.

One of the driving mechanisms bringing back the arctic air and the stormy weather is the MJO. This phenomena monitors convective storms that form around the world and is divided into eight phases. Phases 8-1-2 in the month of March, which is the current and foreseeable state of the MJO, bring colder than normal and stormy weather to the eastern U.S. This is one of the reasons why the drought-stricken state of California is finally able to see rain over the next few weeks.
The MJO is not the only signal jump starting our winter pattern. The EPO, as has been the case most of this winter, is back in a negative state and trending even further negative (near -4 value) by this weekend. This means blocking in the eastern Pacific, including Alaska and western Canada has formed. Additionally, the AO is also negative which does not surprise me given the extreme negative state of the EPO and the arctic air mass we are currently dealing with and will still have to deal with for the next 10 days.

Wednesday’s light snow gives way to near record cold

Light snow is falling throughout the area this morning and will continue to do so for the next several hours. The culprit is a weak mid level disturbance and associated elongated surface low, which in actuality is quite progressive and weak. But sufficient lift for precipitation and the presence of markedly colder air will set the table for a very light snow event through Wednesday evening. Accumulations will be quite light, thanks to the transient and light nature of precipitation and still relatively warm ground temperatures, ranging from a trace to 1″ in some isolated areas. The snow is expected to wind down by later Wednesday afternoon.

More notable than the light snow accumulations will be the impending shots of cold air, which will move from Canada and the Great Lakes into the Northeast on Thursday and Friday. A piece of the displaced Polar Vortex will shift southeastward toward Southeast Canada, and the resulting arctic front will drag anomalously cold air into the area behind Wednesday’s weak storm system and another clipper on Thursday. Temperatures aloft, at the 850mb level to be specific, will fall below -20 C in much of the area. Compare that to the +15 c temperatures at 850mb which reached the area last Friday, and you have yourself an idea as to how cold the airmass really is.

High resolution NAM model forecasting low temperatures in the single digits to near zero in the area on Friday morning.

High resolution NAM model forecasting low temperatures in the single digits to near zero in the area on Friday morning.

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