Wednesday, 27 March 2013

Summer 2013 Outlook

Key points

  • The core of the heat will be centered across the eastern rockies and central plains states in june before shifting northwards as we head into july and august
  •  Drought conditions will persist across the central part of the CONUS and intensify across the intermountain west and southern prairies
  • Normal precipitation is expected across the great lakes and northeast with an active severe weather season likely across the ohio valley, great lakes and ontario. 
  • Much warmer then normal temperatures will occur in the western great lakes with near to above normal temperatures in the northeast.
  • Drier then normal conditions will continue across the southeast in june before much needed rains become more common in the second half of summer
The current IRI forecast predicts a warm biased la nada as we head into the summer. I think that this is likely given the current warmer then normal sub surface temperature anomalies in the pacific. A full blown el nino is unlikely with a -PDO allowing warm water to drift southwards. SOI values have been variable lately with changeable SST's. Important to note is the warmest SST's are focused across eastern ENSO regions which would help intensify the summer heat. This would also allow more Sub Tropical energy to send moisture into the southeast and gulf coast states.
CPC forecast:
Current SST's consist of a warm AMO tripole with east based warmth in the tropical pacific.


For my summer analogs I focused on years with a +AMO, -PDO and neutral ENSO state coming after a weak la nina. For my analogs I used 1932,1936, 1937, 1952 and 2012. Here are the temperature departures from the blend of years.


The temperature trend on these years is very similar to my summer thoughts. Notice how the heat is centered across the eastern rockies, plains states and extending northeast towards the great lakes. Cooler weather is likely along the west coast in the -PDO regime but the intermountain west may end up hotter and drier then the analogs show. An active monsoon season is present across the SW causing near to slightly below normal temperatures. I am also expecting wet conditions across the southeast in mid to late summer which is showing up in the analog package and european monthlies.

I will break down each analog year individually.

This year had a moderate la nina in the winter season before a transition to la nada by the early summer. The warmest water temperature anomalies were centered in nino 1+2 which helped cause a hot summer across the central and eastern u.s. Dry conditions expanded northeast towards the great lakes region.
June to August SST's
Palmer Drought Index

Temperature Departure
1936 was a weak la nina with a warm AMO and cold PDO horseshoe. This was a very hot summer across the western great lakes and plains states. I don't think this year will end up as hot as 1936 due to the neutral ENSO state predicted as opposed to a la nina which would enhance ridging in the mid latitudes.
Temperature Departures

Palmer Drought Index

1952 had a downwelling easterly QBO with a neutral ENSO state coming after an el nino winter. The heat was centered further Southeast then I am expecting.

I added last summer to my analog set because the ENSO trend was very similar along with the PDO and AMO states. I do expect a wetter and cooler summer across the northeast and eastern great lakes compared to last year. Notice the cooler anomalies across the southeast.

This year came after three la nina years in a row which is very similar to our current ENSO state. Widespread warmth occurred similar to much of the 1930's and 1950's.

Tropical Forcing

This summer I am expecting negative OLR anomalies to develop across the indian ocean and maritime continent. This should work together with cooler SST's near the dateline with warmth off the coast of south america. The CFSv2 is pointing towards a northwards momentum transport between the equator and the tropic of cancer which favours more southern branch moisture and ridging in the central u.s. Phases 2-5 of the MJO/GWO will be common this summer in my opinion.

My Temperature Forecast

Precipitation Forecast:

Monday, 25 March 2013

Long Range Outlook

So far this march we have seen a tongue of colder then normal anomalies develop underneath a powerful block in the arctic. Here are the temperature departures for the month so far.
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I tried to match up some years so that we can get a better idea of what april will look like. Here is what march looked like in my analog years which is pretty close to what we have seen so far. Notice the warmer then normal anomalies across the bering sea with the tongue of cold extending from NW to SE.
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I will break down each year in my analog set.

1965 had a very cold march with strong blocking west of greenland. The core of the cold was focused towards the northern plains, prairies and western great lakes. We also saw a transition to neutral ENSO conditions in the spring. In april of 1965 we saw cold shots focused across quebec, ontario and the great lakes with warmth building up across the southern tier of the CONUS.
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In 2002 we saw very similar temperature anomalies in march with a slow transition out of cold bias la nada conditions. In april cold shots were common across the north as the PV drops south at times.
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In spring 2011 a moderate to strong la nina began to weaken into a la nada state. It had a warm AMO and -PDO as well which matched nicely.
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2009 is one of the top analogs in my set with a cold start and a warm finish. The ENSO signal was also very similar with a +AMO and -PDO. The first half of april is on the left and second half on the right.

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In 1996 we had a very cold march with a similar tongue of cold showing up. Notice the strong blocking over greenland. ENSO trend also coming out of a weak/moderate la nina.
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Based on analogs, modelling and indices it looks like the first half of april will start off cool across the prairies, great lakes and quebec as pieces of the PV drop south. A weak -NAO and ridge along the west coast will allow cold shots to move down with warmer temperatures across the southern u.s. Towards the end of the month the weak -NAO/AO couplet should begin to relax and the -PNA trough begin to re-establish itself along the west coast. This will allow the heat from the south to begin to move northwards towards the ohio valley, great lakes and northeast. I still think that moisture stays above normal in and around the GL's with northern branch features dropping down, especially in the first half of the month. I do see a drier trend in the great lakes towards the end of the month with the SE ridge strengthening and less blocking across greenland. With features moving into the ridge expect some severe weather to break out, mostly south of the canadian border but a shift northwards is likely in may. Once we get out of this cold pattern, it looks like a quick snap to above normal conditions is likely.