Saturday, 19 January 2013

Preliminary 2013 Spring Forecast

Key Points:

  • A quick transition to spring is expected in the southeast and along the east coast as an active southeast ridge is in place
  • The northwest will be colder then normal with frequent troughiness and wet conditions especially in march and april
  • Drought will continue to intensify across the southern plains states and florida with a strengthening ridge developing as we head into april and may
  • The bulk of the severe weather this spring will be focused across the tennessee, ohio valleys and great lakes region with below normal activity in the plains states and eastern rockies
  • Wetter then normal conditions are present across the great lakes, northern ohio valley and southern canadian prairies
  • Dryness continues to increase across the southwest and gulf coast states as a weakening of the sub tropical jet stream takes place

The state of the ENSO this spring will play a big part in my overall forecast. Right now we have a neutral ENSO state but the pattern is acting very la nina like due to the cooler then normal anomalies showing up in the eastern nino regions. Trade winds are dominating the equatorial pacific right now with positive SOI readings.

Despite all this we are not necessarily heading into a la nina. Right now we are seeing several conflicting signals with the MJO propagating westwards in the pacific. This is helping to move the warm water east from the sub surface. Some of the cold anomalies may shed away in nino 3.4  in the short term with an enhancement of sub tropical jet disturbances.
Moving into spring I don't think that this warming of SST's is going to last and the -PDO regime will help support the la nina. I do expect the MJO to return to phases 2 to 5 in the spring which favour a southeast ridge. I think that we will see a continuation of neutral ENSO conditions this spring although biased cold with a la nina look to the pattern. The MJO is the wild card here and if it stays in phases 2 to 5 we will definitely see the la nina pattern dominate with a ridge in the east and dryness in the south. My ENSO forecast is in line with the consensus:
The UKMET for MAM shows the dryness prevailing across the south and more moisture across the northern tier with an active northern jet. Notice the MJO phase 3-6 look to the pattern.

Monthly Breakdown


A quick transition from winter to spring will be in store to start of the month as ridging continues to strengthen in the southeast. This will be driven by the strengthening la nina pattern in the equatorial pacific and -PDO regime which favours increasing dryness across the plains states and a more active SE ridge. I am expecting warmer then normal temperatures across the southern plains, great lakes, southeast and ohio valley. Cooler then normal temperatures will dominate in the pacific northwest with a parade of storms diving into the area. This is caused by the very active northern branch of the jet stream which is driven by the cooling sea surface temperatures in the pacific. The negative PDO regime favors a -PNA pattern which causes troughiness in the northwest and warmth across the eastern half of the country. A relaxation of the -NAO/ AO combination is likely as the effects of the SSW ware off and the relaxation of the jet stream allows the -PNA to take over. The severe weather battle zone will set up in the middle across the Ohio/Tennessee valley's and eastern great lakes where a battle will take place between the warmth and the cold. This area will experience wetter then normal conditions in march. Below are my march analogs. I think ridging will be stronger and centred further southeast but this is very close to my current pattern ideas. Ridging in the bering sea will be a key feature this month with a strong trough in the gulf of alaska

This is in line with the latest SST's. Important to note is the strengthening -PDO supporting a gulf of alaska trough and ridge centred in the southeast u.s

April and May

Warmth will continue to intensify in the east with a strengthening ridge across the plains states. Little drought relief will be felt with warmer and drier then normal temperatures prevailing over much of the southern plains, midwest, southeast and east coast. Departures may be cut down at times in the eastern great lakes and interior new england with northern branch features moving along in the flow but warmth will prevail overall. The bulk of the severe weather will shift NE away from the heat ridge developing in the southern plains. I expect this area to set up across the eastern ohio/tennessee valley's and lower great lakes. Wet and cool conditions will continue along the west coast with some surface highs building down which may promote some cool nights in the southwest and pacific northwest. These wet conditions will begin to relax as we head into may with more seasonal values across the area. May can end up very close to last year in the east with early heat showing what may come later in the summer. I am expecting may to be warmer then normal across the eastern two thirds of the nation with severe weather quite active across the great lakes, ohio valley and interior northeast. Severe weather should shift north as the upper ridge in the plains steers  away systems from the northern branch.

Current U.S drought monitor shows the current drought conditions which should continue to intensify in the plains and florida common in -PDO years.

Here is my analog package for april at 500mb. I think the ridge will end up centered further SE than this. Heights may end up lower in northern new england with a +NAO overall and some northern branch systems moving through. I like the -PNA, and +EPO look to the pattern.

The CFSv2 is in line with my thoughts with dryness across the south and an active northern jet supplying some moisture to the ohio valley, pacific northwest and great lakes region.

Here is the CFSv2 upper pattern for MAM

My forecast for March, April and May




Feel free to post any questions or comments you may have!

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