Synopsis:
- Anomalous weather patterns in late April through Mid-May have lead to our current unsettled C. Plains weather
- Expect moderation, up and down likely
- Near average temperatures to the east
Unbutton your belts, this is a long read…
Discussion:
500 mb height anomalies 1st half of May showed huge -NAO anomalies west of Greenland. Southern Canada had strong low pressure anomalies. Out towards the west coast, a split flow pattern was evident with HP anomalies off the coast and LP anomalies extending from the E. Pac around 30N into the SW US.
Which of course has lead to higher precipitation in SW US into the South Central U.S.
The second half of the month (below) displayed continue high pressure anomalies over Greenland, this time also over Canada and the Northern high latitudes well into the Arctic. Low pressure anomalies dominated the west coast. There is currently a huge upper level low out there creating more severe weather in the Plains (see today’s disco for more).
There are two areas of interest, one is in the South Central Pacific nearer Hawaii that has strengthened HP anomalies, and the other is over the Pacific Northwest where HP anomalies have been wiped clean.
Heading further north, we have seen a strong disruption to the Arctic Oscillation over the entire month of May. There is very strong anomalous HP sitting over the North Pole. After that we see wave number 5 pattern (initially wave number 2 pattern). This suggests more active weather, we have more longwave troughs available to create problems.
Into The Tropics…
MJO has been rounding through phases 4-5-6 late April causing that wetter weather to spread into the U.S. It has spent most of May in phases 6-7-8. These are generally volatile and transitional phases.
Here is a composite anomaly map for phase 6 in April, setting the table for where we were headed. And as you can see, we have west coast lower anomalies and much lower into W. Canada. The US is actually overall neutral with the E. US having HP anomalies. It isn’t a straight up exact match but there are similarities and this is just a general composite.
Meanwhile in the Atlantic…
Cooler waters in the N. Atlantic have likely helped with ensuring HP dominated that area for the month. Cooler air = sinking air= high pressure
Up In The Stratosphere
Huge warming event helped to shift the Arctic Oscillation. Prior to this the Arctic was dominated by the Polar Vortex. Yes THEE polar vortex (oh the horror!). Except it is supposed to be there! And then poof, it got warm real fast and the polar vortex was vanquished. This happens every year in the spring. It’s called the “Final Warming”. And after it, we see high pressure take over for a bit. And from the maps above, especially image #3 where the viewpoint is from the Arctic, you can see HP anomalies presiding this month.
Which has us going from a 500mb map where the vortex is only subdivided into 3 lobes and is concentrated over the poles
To something like this…
Elongated longwave troughs, huge HP anomaly extending into the Arctic. This is just a single day anomaly chart but you get the picture.
In Summary
- Volatile wavenumber pattern
- MJO wave propogation
- Stratosphere warming
- Anomalous troughing leading to wet and stormy conditions
Where We Are Headed
Currently out in the Pacific we have quite an active pattern. We will have to wrestle with this for the next 2 weeks. That means back and forth weather, for both the W. US and E. US is highly likely.
The Climate Prediction Center has a wide range of neutral and moderate chances for above average in the W. US and SE US for the first week of June.
This is in conjunction with HP over the E. Pacific sliding in as the current W. US trough lifts out. It’s a straighforward forecast. In the east, we will be battling rainier conditions and likely average temperatures.
The -NAO is expected to weaken, eventually beaten up by the departing troughs exiting the E. US and lifting in the N. Atlantic. We are headed for zonal weather after that.
CFS forecasts week 1 and 2 show the high pressure anomalies fading over Greenland. Low Pressure anomalies which have dominated the SW US also disappear. But you can also see advancing LP anomalies week 2 into the Pac NW. Note, these images are also anomalies, not mean SLP.
The Tropics…
MJO filtered anomalies at 200 show impressive wave in the Americas. Weeks 1 and 2, this area fades. In the Indian Ocean another wave develops. This will be important for the 2nd half of June.
Indian Ocean activity can once again reignite monsoon activity in the E. Pacific into SW US. If this MJO wave develops out of the Indiean Ocean during the end of the 1st week of June, we could be entering a similar pattern end of June into July. The MJO wave is a slow progressing tropical oscillation that typically moves over the span of 30-45 days. Enhanced activity is associated with rainfall into the tropics and mid-latitudes (via feedback mechanisms). However, the MJO wave itself needs to actually remain intact in order to contribute to significant impacts.
After the first week of June and the -NAO subsides, the pattern will shift more zonal to the east, but the other low pressure areas in the Pacific will need to roll through. That will be weeks 2 and 3. The southeast ridge will be able to poke its head between low pressures and cold front. There will be enough N. Atlantic low pressure activity to force some high pressure anomalies in E. Canada, so it is unlikely that the NAO will flip to high positives. Most likely we are looking at NAO neutral.
That being said, the sudden warming event over the Arctic has subsided, but it is summer and unlikely the polar vortex dominates like it should during the winter. Temperature anomalies will be closer to neutral for June.
Our major movers for the first week…
Into the Second Week We Go..
The 3rd week of June will feature our Pacific storms (really, beginning the week prior, at the end of the 2nd week of June).
So again we will see LP track into the W. US, this time likely also the Pac NW getting in on the action. The E. US will be stuck between HP in the C. US slipping out to the SW US. And also below the neutral NAO with departing stormy weather from week 1. Very likely Norther New England experiences below average temps in this time frame.
We will keep an eye on the MJO in this time. If a wave holds together it will likely be heading into phases 4-5 in the 3rd week of June, which can trigger the E. Pacific jet. However, heading into week 4, the W. US will clear out and the E. US will have to deal with the rain and seasonal temps. I think the S/SE US will have a quasi-stationary high pressure throughout the month of June with periods of relief as the Pacific storms move through weeks 1-3. This will create a more zonal pattern, especially weeks 3-4.
End of June will likely experience quieter conditions overall in the US. I think the heat may be building again in the east after the stormy days come to an end.
Conclusion:
- Volatile weeks 1-2, transitioning to warmer period out west as June moves along
- Parade of Pacific storms weeks 1-3 for US, high pressure building in SW US and also poking its head in the SC Plains/SE US
- Cooler than average temperatures with higher confidence in Northern New England middle of the month
- Week 4 our summer regime starts to take hold, likely warmer in the SE US and W US. Mid-Atlantic near normal.
- Likely up and down for the country as a whole regarding temps/precip
Caveman Drawings
Week 1 (models like the idea of a cut-off in the SW) – makes sense with more zonal flow but high energy remains behind
Expanding the view out for Week 2, next storm incoming in the W. US
Week 3
Week 4 as the last of our parade of storms makes it through the NE. Back to just U.S map
Philadelphia Weather Authority 
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