Sunday, November 18, 2012

November 27-29 Potential Winter Storm

There is potential arising for a major storm system to hit the Midwest, Plains, Ohio Valley and Northeast. Let's look at how this could all go down.

This is hour 228 of the 12z GFS, valid on the 27th of November. There is precipitation falling across the nation, with rain and thunderstorms hitting the Ohio Valley, Northeast and Midwest, while a snowstorm rages in the Plains. This comes from a 989mb storm system- quite strong for a system on land. At this point, the storm has not yet pulled out of the mountains and has not reached its full potential. Despite this, it is clear that this system will cause troubles as it is producing major precipitation across a wide swath of the nation.

Moving ahead 12 hours to the morning of November 28th, we now see this system has emerged from the mountains and is now located in the Upper Midwest. The storm's pressure rests at 988mb, ever so slightly weaker than what it was in the mountains. The GFS model now indicates the storm system has developed a strong cold front, as the frigid air wrapping around to the western side of the system would indicate. Thanks to this front, heavy precipitation has emerged ahead of the frontal boundary, with heavy rains down both sides of Lake Michigan into Arkansas, and as far east as the Michigan/Canada border. Because of the cold air now wrapping around the system, a larger portion of the Plains looks to be under threat from a snowstorm. Blizzard conditions appear to be in the cards for Minnesota and the Dakotas, the three states closest to the low pressure system yet still north of the blue rain/snow line.

Fast forward another 12 hours and we find this system has intensified quite a bit. The system is now down to 978mb, a whole 10mb lower than the morning of November 28th (the second image of this post). This strong storm system is now into south central Canada, where it is battering the region with blizzard conditions. Due to the intense cold air wrap-around from the cold front, the blue rain/snow line has now caught up with the heavy precipitation, meaning that states like Iowa, Wisconsin, Illinois and Missouri would be seeing snowflakes fly- possibly at an accumulating rate. To the east of the rain/snow line, what could be a linear thunderstorm formation is now progressing east, and hitting Indiana, Ohio, Kentucky and Tennessee with its wrath. As is the situation in the aforementioned Midwest states, these heavy storms would likely switch to snow, more likely in the northern states. Still under snowstorm conditions is Minnesota, which, should this forecast verify, would emerge with several inches of snow under its belt.

Another 12 hours later and we are on the morning of November 29th. The strong storm system has now moved off to the northeast, well off into Canada. Blizzard conditions have now ceased in the Upper Midwest and lower Midwest states, but now the Ohio Valley is north of the blue rain/snow line. Judging by the amount of precipitation accumulated in this forecast, some accumulation could be found, especially in Ohio and western Pennsylvania. Cold air would follow this strong storm system, which has now strengthened further to 965mb- another 10mb dropped in a 12 hour period. Heavy rain could be ongoing in New England before the cold front races through.

I know what you're thinking: "Why should I get excited over a forecast over 10 days away? It'll never happen- the model is too far out!" But there is substantial evidence that this isn't just another 'fantasy forecast' (a forecast that looks great but doesn't happen).

This was the observed surface map for November 13th, just 5 days ago. This surface map is for the northern Pacific and Bering Strait. As you can see, there is a very strong storm system swirling around in the waters just off the end of the Aleutian Islands on this date.

Why is this significant? There has been a connection found between strong Gulf of Alaska (GOA) storm systems, and a strong storm system hitting the Lower 48 around 17-21 days later. If we take November 13 and apply that 2.5-3 week gap, we find that this strong storm system would hit...

...between November 30 and December 4. It just so happens that the storm system forecasted above falls in this timeframe.

Is this a coincidence? I doubt it. While the GFS may be over-exaggerating some aspects of the storm (i.e. strength, precipitation, air temperature), I have a feeling that the GFS is catching on to this Bering Sea Rule, as it's called.

I can't say I am willing to pinpoint the exact track and effects of this storm system because the models are all over the place. One cannot figure all those factors out just by seeing a strong GOA system, either- all you can determine is that a strong storm system will be around the Lower 48 roughly 17-21 days later.


Forecast for November 19, 2012

The forecast for Monday, November 19 calls for a pretty quiet day as a whole across the nation. A low pressure system looks to pose chances for showers and storms in Missouri, Arkansas and Kansas in the morning and early afternoon, before this threat for precipitation shifts to the northeast into states such as Iowa and Minnesota. No significant rainfall is expected. A strong low pressure system will be making its way onshore in the Northwest, bringing with it a threat for heavy rains and some heavy mountain snowfall. Travel delays are likely. High pressure will keep the Northern Plains relatively warm on Monday, and the same goes for the Southwest. An offshore system will spin up some showers and thunderstorms along the immediate East Coast, especially in the Carolinas and Virginia. The heaviest precipitation will be offshore.

Temperatures for Monday will stay in the low 50s in the Northwest as that strong storm system comes onshore. The Southwest and Southern Plains will stay in the 60s and 70s, a comfortable temperature at this time of the year. The Midwest and Ohio Valley will stay in the upper 40s and lower 50s, with the Ohio Valley most likely to reach into the mid 50s in the early afternoon. The Northeast will stay in the mid to upper 40s, while the Gulf Coast and Southeast will get treated to 60s and 70s. Mid 70s are possible in the southernmost regions of Florida.

3-Year NAO Correlation Suggests Exuberant Winter Ahead

I have found that there is a significant connection between observed conditions in the North Atlantic Oscillation index (NAO) and observed winter conditions across North America approximately 3 years later.

DO NOT use the blue line's values for Nov. and Dec. Data
has not been gathered for those months this year yet.
In this article from the AGU website, it is noted that there is a very strong correlation between what the NAO does in a certain year, and the winter observed in North America 3 years later. This would mean that one could find the general idea of the oncoming winter by using this 3 year rule. Above is a chart showing observed NAO values in the year 2009 and the year 2012 (I know it says 2009-2010, but that was put there for the purposes of relating this back to winter).

What we want to focus on in this chart is the purple line of observed NAO conditions in 2009. As you can see, the NAO began the year with pretty neutral values before doing a quick positive run in May and dipping into negative territory as summer approached. The NAO went back to a positive phase in September 2009 before fluctuating and ending up in strong negative territory for winter.

If we do a quick comparison between the two years, we can see that they are actually pretty similar. They both began fairly neutral/slightly positive in the late winter months before dropping into negative territory towards summer. June and July both included very negative values for both years. Both years' NAO went positive into Fall and took a slight dip in October. If we follow the purple line and compare it to what we could see this year, one would think that this winter could have a pretty strong negative NAO.

Now, this is not an EXACT correlation- the chances of two years' NAO values matching up exactly is next to impossible. However, if we get a feel for the winter of 2009-2010 and use the 3 year correlation on the upcoming winter of 2012-2013, I think that the NAO will stay pretty negative, providing a strong base for an active Northeast winter.