Saturday, February 15, 2014

Long Range Discussion: Severe Weather, Snowstorms To Kick Off March

This long range discussion will focus on the expected weather to end February and kick off March.

The image above shows the latest 500mb height anomalies across the Northern Hemisphere on the left, with cloud cover, pressure contours and high/low pressure denotations on the right. If you've been with The Weather Centre for a while, you know how we use a rule created by Joe Renken that states a storm system in East Asia then results in a storm in the United States 6-10 days later. This situation is no different- today, we saw ridging/high pressure emerge in East Asia. If we extrapolate that out 6-10 days, we arrive at a warm-up in the February 20-24 timeframe. I'll discuss this more a little later down this post.

Lately, as the image above shows, the weather pattern has been dominated by west-northwest to northwest flow, thanks to deep troughing in the Gulf of Alaska and suppressed ridging in the Southwest producing a pattern favorable for East Coast snow events. This whole pattern is expected to flip in the medium range.

The changes begin as ensemble members foresee strong ridging shifting east into far eastern Russia, which will then shift the Gulf of Alaska storminess to the east, resulting in the loss of high pressure in the Southwest US. That Southwest ridging then propagates east into the contiguous United States, producing mainly zonal flow, which keeps anomalous warm or cold outbreaks out of the picture. However, with the ridging now gone, the storminess in the Gulf of Alaska can drop south into the Western US. The result is a stormy West US, and a very warm East US.

Model guidance (with the exception of the CMC model on the far right) is in agreement on a pattern change evolving, where we see the troughing drop south into the West US, as high pressure prevails yet again in the Gulf of Alaska. The storminess in the West US isn't as well defined on the ECMWF forecast (left) or GFS forecast (center), primarily because this image shows projected 500mb anomalies over the 8-10 day forecast period. This means the forecasts may seem a little less 'intense' than they end up being, because the anomalies are averaged out over a 2 day period, rather than a 6 hour period like most models print out.

Regardless, the pattern will involve high pressure setting up along the East Coast in response to troughing out West, and this sort of pattern just screams for not only a snow event, but a severe weather event. We will see anomalously warm and humid conditions present across the Central and Eastern US as the jet stream bends north to accommodate for the ridging out East. This ridge also tells me we should keep an eye out for a storm system cutting north through the western Great Lakes, something model guidance is already picking up on. I have a feeling such a storm cutting north would result in not only a northern Plains snowstorm, but also a severe weather event for the Central US, something I discussed in yesterday's post.

As we head into the last days of February, ensembles give the Central and East US another shot of cold weather, as shown by the GFS Ensembles' portrayal of the trough at the 500mb level on the left panel. Again, the pattern we discussed above would strongly support this solution. It wouldn't surprise me to see additional chances for storms, both snowy and severe, before the month of February ends, apart from that other storm system we discussed yesterday in the link above.

For the opening days of March, I wouldn't be surprised to see us start on a chilly note before rebounding to rather seasonal temperatures. Depending on how the sudden stratospheric warming event continues to evolve, we could be looking at a cool March, though the chance of that would be maximized in the north-central US (primarily the Plains), and minimized along the East Coast and West Coast, where ridging and warm weather would be expected.