April Forecast Thread

[BrickFielder]

Nobody has started ones of these yet so I thought we might get the ball rolling.

The stratospheric vortex looks pretty weak to me and looks like fading away much earlier this year. For me this means Sea Surface temperatures and more importantly la nina will be in control of the weather pattern this spring. This I think will mean a high pressure in the central Pacific leading to a trough through the central plains of the US. Cold air will meet warm to give probably an active tornado season there although the QBO changing westerly may well shift things further north. For us I think the jet stream off the US will likely tank north on occasions with cut off lows crossing mid atlantic towards spain. Some cooler periods mixed in with very warm, so very changeable with models struggling a little with the pattern changes.

[Glacier Point]

Even though we're moving towards the mid point of Spring, April for me has the look of the same parameters defined by the Winter, particularly as the polar vortex is showing signs of significant weakening during a period when climatology suggests its eventual break up.

La Nina has showed a recent (seasonally consistent) trend to weaken as stenghtening westerlies across the Pacific Equator develop inhibiting the upwelling of cold subsurface waters. It is important to note however that this is not unexpected and ENSO regions 3.4 and 4 in the western Pacific remain at values between -0.5 and -1. Composite reanalysis of Aprils with such a pattern suggest that a weak to moderate strength Nina is still sufficiently powerful enough to influence global weather patterns, thus extending one of the winter's pattern drivers.

http://www.bom.gov.au/climate/enso/indices.shtml

http://iri.columbia.edu/climate/ENSO/curre.../technical.html

The latest IRI ENSO assessment suggests Nina conditions will continue into May and surface easterly winds in the central/western Pacific combined with deep cold water anomalies give weight to this conclusion.

For weak to moderate strength cold ENSO events (La Nina), the reanalysis suggests:

• Large Pacific High ridging into Alaska with central Plains trough and SE States ridge;
  
• Blocking over NE Canada and Hudsons;
  
• Weak -NAO pattern with upper low off Newfoundland.
  

Warm water surface anomalies exist in the western Indo-Pacific region and the tropical Atlantic. These are associated with bands of convective activity and westerly wind development.

The area of -ve OLR shown in the Indian Ocean and western Pacific is the most significant as this has been the main area for tropical forcing throughout the Winter. The development of strong sub-Arctic ridges may be linked to convective waves (imagine shock waves or ripples after a pebble is thrown into a lake) in this area. Tropical forecasts suggest that shower activity will persist in the area, most likely within the central / east Indian Ocean and western Pacific around the Philipines:

http://www.cdc.noaa.gov/MJO/Forecasts/imag...orbar_large.gif

The MJO signal has become indeterminate of late although the key signals from the tropics should be of weather patterns forced by phase 3 to 5 type, which teleconnect to further ridge development in the NE Atlantic and Scandinavia.

Global angular momentum is consistent with the overall Nina pattern of SSTAs being low base state. An upward movement in GLAAM stemming from increased westerly flow over warm equatorial waters and increased tropical convection has recently occured. This is likely to add westerly winds in the mid and higher latitudes in due course.

Perhaps most fundamentally, the polar stratosphere has undergone a series of well documented warming events during February and March. These have now penetrated the upper troposphere to obvious effect with both height rises and -ve zonal wind anomalies streaking down from the upper atmosphere. The time lag for warming events and the duration of the original warming suggest that high latitude blocking will continue to occur for the next 2-3 weeks.

http://www.cpc.ncep.noaa.gov/products/stra...JFM_NH_2008.gif

http://strat-www.met.fu-berlin.de/cgi-bin/...xes&alert=1

Of note here is the very low stratospheric zonal wind speeds with recent reanalysis data suggesting a great swathe of -ve wind anomalies (consistent with -NAO) over the Atlantic and forecasts for this to continue into April.

A recent cooling in the stratosphere is likely to impact the last week of the month however but the overriding theme will likely be for the polar vortex to remain weak and for an early seasonal break up sending much colder air south in a potentially volatile mix in close proximity to tropical air masses.

Putting some 'weather detail' on these thoughts.....

The impacts of northern blocking will make the first 10 days feel pretty raw with some snow potential. As the month progresses, northern blocking will stay in evidence but a shifting of low pressure to our west will tend to draw up slightly milder air but still close to average temperatures for a traditional April with emphasis on falling pressure to our west and over the UK during the month with above average rainfall totals so feeling paticularly unsettled, cold and wet. The last week of the month holds some promise for a welcome rise in temperatures as the AO might trend back towards positive but generally a total contrast to last year's rampant warmth.

Finally, not a forecast but perhaps a cautionary note to raise our awareness. It's 10 years since the big Easter floods that affected much of the Midlands. The overall pattern suggests the potential for the same volatile mix of polar and tropical air within a slow moving and unsettled theme. We should therefore just be on the lookout for extreme weather events within the next 4 weeks with the Midlands, Yorkshire and western fringes of the UK.

Edited March 31, 2008 by Glacier Point

[Glacier Point]

8 days in and the initial thoughts pretty much remain valid. Tropical convection has dropped off a little allowing a strong La Nina base state allied to northern blocking to dominate the pattern. Recent NWP is highly consistent with composite reanalysis for April weak to moderate strength La Ninas combined with stronger high latitude blocking. The latest 8-10 day ECM anomalies are a spitting image for the composite La Ninas for April:

http://www.meteo.psu.edu/~gadomski/ECMWF_12z/hgtcomp.html

Of particular interest at the moment is the upper atmosphere.

Latest reanalysis data identifies large high level anticyclones which cross the tropopause into the middle layers driving the tropospheric circulation. Also of note is an easterly wind anomaly across the equatorial Pacific.

Easterly 30 mb zonal wind anomalies extend across the Atlantic. These are undoubtedly related to the stratospheric warmings during Feb and March. The point about these are that such anomalies are slow to change. Recent coolings in the stratosphere will feed through, but only slowly - perhaps towards the end of the month into May but for the time being, more conditions for northern blocking until the polar vortex breaks up.

ECM forecasts show the relationship between 50 mb geopotential height and zonal winds. High pressure centres forecast to continue and zonal wind anomalies remaining largely unchanged, possibly suggesting more of the same as zonal wind anomalies sink through the atmosphere.

With no suggestion of a strong area of organised tropical convection to shift ridges, I would suggest that we will follow a pattern dominated by La Nina and northern blocking. See the above post for the forecast detail for this although this does not mean cold throughout as lows attempt to undercut from the SW.

[summer blizzard]

Brickfielder and Glacier Point, am i correct in thinking that as we see the stratospheric Jet increase in responce to the QBO now being westerly, that the convection over the Tropical Atlantic will migrate toward Africa, with higher pressure over Mid-Lattitude Europe and possibly the UK as a response as we enter May??

[BrickFielder]

The Honest answer is that I am not sure.

The direction of the QBO has affects on planetary wave breaking and on rossby wave breaking with differing affects on Wave 1 and 2 during spring. It is expected that convection over the Tropical Atlantic will migrate toward Africa, but my undestanding is that this is related to La Nina and warmer waters off the coast of Africa than normal. This is likely to affect the hurricane season and I guess the Azores high.

[Glacier Point]

Final update for the month before the new challenges presented by May. By and large, this month has gone to plan with the final week promising a welcome warm up. Long range model guidance, has, up until 00z and 12z ECM, suggested a return of northern blocking and cooler conditions for the UK.

I am however slightly sceptical because to get this we need both a -NAO and -AO to fall into place. The first is likely. The second, well, I'm not so sure.

A band of tropical convection has become organised over the Indian Ocean with a further band to the west. Westerly winds are moving across these areas with downstream anticylones likely to develop interfering with the underlying La Nina signal.

http://www.cdc.noaa.gov/map/images/olr/olr.anom.gif

This is likely to shift the extra-tropical ridges around from where they are now. Reanalysis of phase spaces associated with the regions showing anomalous tropical convection (MJO phase 3 through 6) suggests a variable pattern but with bias towards a more tropical maritime and mid latitude ridge type pattern.

http://raleighwx.easternuswx.com/MJO.html

Latest MJO plots show the wave to be indeterminate although I suspect that it will come to life in the next week through the Indian Ocean.

http://www.bom.gov.au/bmrc/clfor/cfstaff/m...Last40days.html

ECM forecasts demonstrate nicely the presence of a strong jetstream between 20N and 50N consistent with both La Nina forcing and the legacy of climatic breakdown of the polar vortex and stratospheric warmings. This is a continued signal for a -NAO, particularly with the developing SSTA in the Atlantic. More cut off lows are to be expected to develop off the Gulf Coast.

http://strat-www.met.fu-berlin.de/cgi-bin/...=f192&var=u

Up until now, northern blocking has been remarkably persistent. However, as mooted before, the impacts of the stratosphere cooling in April are now likely being felt with negative height anomalies and colder air starting to move down through the stratosphere towards the tropopause.

http://www.cpc.ncep.noaa.gov/products/stra...AMJ_NH_2008.gif

http://strat-www.met.fu-berlin.de/cgi-bin/...mps&alert=1

http://www.cpc.ncep.noaa.gov/products/prec...hgt.ao.cdas.gif

There are however no indications of increasing zonal winds over the stratosphere and no clear cut signal for a strongly +ve AO are presented although the reorganisation of cold pooling over the polar field is likely driving the AO more towards the neutral / positive territory.

This leaves us with a potentially interesting transition period last week April into May, one for me more likely characterised by a neutral / positive AO and -ve NAO. Weatherwise, feeling much more warmer with troughing in the Atlantic and mid latitude highs developing towards the UK with just enough polar jet flow to our north to prevent the formation of mid-Atlantic ridge / Greenland unions so plenty of warm spring sunshine likely to become evident but also bands of rain moving west-east across the UK due to the variability driven by tropical forcing.

Edited April 21, 2008 by Glacier Point