The global models that show what the weather COULD be like 1, 2, and even 3 weeks from now, are honing in on a couple of storm threats that I think need to be looked at more closely. The notable dates are:
December 28th - December 29th
December 31st - January 1st
January 3rd - January 6th
Our weather pattern has been driven by the presence of Pacific blocking, also known as the WPO (west-Pacific oscillation) or EPO (east-Pacific oscillation) which have been in predominately negative phases dating back all the way to the fall. This has allowed ridging, or positive heights, to extend into Alaska and at times the western Arctic region, which in turn forces the arctic air southward into the CONUS. This has created a cross-polar flow effect which takes cold air from the west U.S. to the east U.S. Another element to note from our pattern up to this point is the lack of blocking (+NAO) has allowed the southeast ridge to amplify and draw warm temperatures up the eastern seaboard at times, which is evident by the couple of ice storms we have seen already since extremes usually occur when polar air collides with tropical air.
However, the pattern is changing and the concentration of this blog will be on the PNA, AO, and NAO. I have already mentioned the NAO has been positive so far this Meteorological winter. The AO has also been positive despite the impressive cold shots we have seen at times this month. The PNA has been negative, which supports trough's in the western U.S. (but I have already disclosed the -EPO has helped us offset warmer temperatures from getting into the northeast).
There is evidence that all those signals do a flip once we approach the New Year. The AO is heading negative, the PNA positive, and the NAO negative. I'm more confident on the first 2 panning out than the latter, but we'll look at things more closely and you can draw some of your own conclusions as well.
The above image from the GFS shows the AO going from a positive stage to a negative one. Once we look at the h5 maps, you will be able to see why this is the case with not just the AO, but all the signals.
Also from the GFS, the PNA is rising into positive territory.
Lastly, the NAO on the GFS is shown to all be going negative. In case you did not know already, a -NAO/-AO/+PNA strongly supports colder than normal and stormier than normal weather in the eastern U.S.
The following 500 mb maps are taken from the 00z EURO run from last night.
The first storm threat we have to deal with is the one for this weekend. You can see from the broader pressures around Texas that energy is building and a low pressure system is trying to form. Since the cold air has retreated north, positive heights off the Atlantic are getting into the northeast. At this time, the PNA is still trying to get positive (shown to be neutral on this image), the tanking of the AO is still in its early stages, and the NAO is positive. So, if a storm does run up the coast this weekend, it supports a rain storm. There is a slight chance the energy in the south holds back a bit, allowing cold air get into the northeast first, but this does not seem likely at this time.
The next time period I mentioned is December 31st to January 1st. This will not be a big storm if it materializes, but at this time the pattern is setting itself up for a potential bigger storm around the 4th. Some models are showing a clipper rotating around the Polar Vortex and potentially amplifying once it reaches the coast. This is similar to the clipper storm in mid-December that bombed out off our coast and brought decent snow to parts of the area and southern New England.
These type of systems favor New England the most, but there is still time for a south trend just like the other system. I do feel someone could see a measurable snowfall out of this storm. Plenty of cold air around at this time as well, with a nice High Pressure system to the north. Another thing to note from the 500 mb map I posted for this time period is the AO is now negative and the NAO is as well, evident by the positive height anomalies into Greenland. Look at what is happening in the north Pacific, the -EPO ridge is connecting with another ridge in the western Arctic, forming a mega-block. The formation of that block coupled with the -NAO, is forcing the Polar Vortex south to the Hudson Bay area and calling for the -AO regime to form.
The last time period I wanted to briefly talk about is the January 3rd-6th time period. The above 500 mb image from the 00z EURO is as extreme as it gets (*pay attention to the general idea, not the extreme-like solution*). The general idea from the long range EURO shows the north Pac ridge shifting into the western U.S. (+PNA) with crazy blocking consisting of positive heights still present in the Arctic region. The NAO is even more negative and the Polar Vortex is still situated around the Hudson Bay. It is unlikely the PV gets as far south as the EURO is showing here, which actually phases the PV with the sub-tropical jet stream and forms a massive storm system. Again, unlikely to happen.
To me, what is more likely to happen is the PV remains north around the Hudson and the +PNA ridge in the west comes to fruition. I think the blocking is modeled to be too extreme right now, which is usually the case, and it is causing the models to spit out extreme storm solutions. The general idea though is for a -NAO/-AO couplet to develop in conjunction with the +PNA. This should lead to interesting times down the road after we get by the next 2 storm systems I talked about earlier.
This type of pattern has support from most models, it is not just from the EURO. The ensembles agree as does the GFS OP.
One reason why we have seen moisture-laden storms this year is due to the active STJ, or sub-tropical jet stream. The STJ around the 3rd-6th time period is also projected to amplify with knots in excess of 150 along the eastern CONUS. Put together a signal package I mentioned above along with the STJ, and the pieces are all there for a big storm to potentially come together. Whether it does or not remains to be seen, but the pattern is turning ripe and the time period needs to be monitored.
Have a great Christmas and a safe and happy New Years.
Francesco Paparatto
very neat stuff.......I would love to chat more sometime when you have a chance
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