Cold, Active Pattern to Continue, but for how Long?

 

A current look at the weather map as of 8:34pm EDT Wednesday shows two weather systems affecting the country. There is a low pressure scooting through the eastern US, bringing severe weather on the south side and some snow on the northern side. This system will get out of the picture by Thursday night. Another system is moving into the west, and this will slide east this weekend.

 

A look at the flow 18,000 feet up shows that the jet stream is active, coming strait out of the Pacific supplying a moist supply of systems to move across the central US. Note how there is a large upper low over southeastern Canada causing a cool flow of airmass into much of the northern US east of the Rockies, which will set the stage for more snow/ice events, mainly from I-80 north.

 

A look at the European model (ECMWF) for Saturday morning shows the first piece of energy ejecting out of the west. Note in the upper left panel how there is an upper level block over central Canada and another one of Greenland (called a –NAO) that is holding the large upper low over southeastern Canada, which will continue to pump cold air into the northern US.

The first piece of energy to eject out of the west early this weekend will be fairly weak, note how there is only a small bend in the 500mb height lines (upper left) and a little vorticity (red fill in upper left). However, this energy will be enough to cause some overrunning precipitation in a swath from the Plains east through the mid Mississippi Valley, Ohio Valley, and eventually the Mid Atlantic, as warm moist air is lifted over the cold air in the lower levels. This will produce a swath of snow, with a swath of 2-4” of snow appearing possible. Note how the hydrometeorlogical prediction center is showing a slight risk of heavy (4”+) snow through Saturday evening over parts of the Plains:

 

The snow will slowly dampen out as it heads east due to the large upper low over SE Canada shearing out the piece of energy and suppresses it to the south.

 

Note how by Sunday morning the European model shows some modest mid level moisture with the energy leftover over the lower Mid Atlantic, which may produce a swath of light snow. Nothing significant.

By Sunday however another piece of energy is getting ready to eject out of the Rockies, shown in the above European model image. As was discussed above and yesterday we will still have blocking highs over Greenland and central Canada holding the deep upper low in place over southeast Canada which will keep cold air in place over much of the northern US.

If the energy fails to phase (discussed yesterday) with a lobe of northern branch energy dropping down over the upper Plains, the storm will be flat and farther south, however if phasing occurs a more significant storm could occur with a swath of heavy snow.

 

By Monday the ECMWF model does not show phasing, with a fairly flat wave of low pressure moving over the southern US. The model is printing out some light rain/snow over the Plains, however nothing significant.

 

By Tuesday the model shows the storm amplifying a little bit, however it is racing east out to sea. This would be a light to moderate snow event over parts of the Plains, Mid Mississippi Valley and points east to the lower mid Atlantic. However, the model shows little to no phasing and no real major storm.

 

The Canadian model (GGEM) is similar to today’s European model run, showing the energy moving east but not really phasing, resulting in a light swath of snow from the Plains east.

 

Note how by Tuesday morning the GGEM is also racing the storm east out to sea, with only light precipitation in the cold air over the Mid Atlantic.

 

The GFS (shown above, valid Tuesday morning) is about 12 hours slower than the Canadian and European models, however is very similar with an unphased, flat storm.

So, the models today have come into much better agreement on the early week threat over the central and eastern US, with no models showing a real solid phase and showing a flatter storm. Like I said yesterday whether or not the energy ejecting out of the Rockies phases with energy dropping down over the upper Plains is key, as no phasing will mean a flat storm with such a large upper low over southeastern Canada.

Today the models have trended away from a phase, supporting a flat storm with light snow. This seems like a reasonable solution as it is sometimes hard to get two pieces of energy to phase perfectly, however we will continue monitoring the situation. The Plains, mid Mississippi Valley, Ohio Valley, and parts of the Mid Atlantic (especially lower) seem to stand the best chance for a light accumulating snow event, although we are still several days away and things can change.

 

As we look ahead towards mid week next week, the European model ensemble means show what would be a significant pattern shift occurring. The ensembles are showing the blocking high pressure over Greenland weakening, allowing the upper low over southeastern Canada to lift northeast towards Greenland, which would allow upper level heights to rise over the central and eastern US and allow warmer weather to occur.

At the same time, the ensembles are showing a very active Pacific jet stream; note the tightly packed height lines off the west coast of N. America, indicating a very strong jet. This would suggest a continued stormy pattern, however instead of being suppressed to the south the storms would cut north into the Plains, which would cause severe weather over the Plains and a warming trend over the eastern US starting early April.

 

When we look at the Madden/ Julian Oscillation (MJO), which indicates where convection is occurring over the tropics and how strong it is, we see that there is currently a developing MJO pulse between the western Pacific and eastern Indian Oceans. This is significant because convection in the tropics alters jet stream patterns in the mid latitudes. Currently we are in a La Nina, which means cooler than normal waters over the tropical Pacific, which often makes it hard for MJO pulses to push east over the Pacific. However, this pulse is strong and models show it attempting to move east into the western Pacific before it potentially collapses in 2 weeks or so. The Global Forecast System Ensembles (GEFS) MJO forecast shows this pulse moving out into the Pacific before collapsing:

 

 

Note how the MJO forecast above shows a phase 5 MJO next week, into phase 6 the following week.

During March, courtesy of this graphic produced by Allan Huffman, a phase 5 MJO supports a trough over the west coast of the US (strong Pacific jet) and ridging over the central/eastern US, which supports the pattern change the European ensembles are showing.

 

So, the next week to 10 days will be cold over much of the central/eastern US with a couple shots of some snow possible, especially over the Plains, Mid Mississippi Valley, Ohio Valley, and portions of the Mid Atlantic, however it appears possible that the pattern will break by next weekend, allowing the eastern/central US to slowly moderate, and potentially setting up some severe threats in the Plains by the end of next week.

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