The European leaders gathering in Brussels should come prepared for all weathers.
BBC European Affairs Correspondent in Brussels
The political climate is stormy, but patches of sunshine are also predicted. Spain is the country making the weather.
France, Germany, and Britain have agreed a list of common EU goals
The election defeat of its conservative leader, Jose Maria Aznar, soon after the massive bomb attacks in Madrid on 11 March, has brought a "wind of change" over Europe.
Leaders of the 15 present European Union states and the 10 countries joining on 1 May are meeting with three hot issues to face:
- Countering the increased threat of terrorism in Europe following the Madrid train bombings
- Re-launching top-level talks on a controversial new EU constitution, so as to agree a treaty giving the Union a stronger, supranational framework of laws
- Breathing new life into the EU's flagging ambition to making Europe the world's most dynamic high-tech economy
On counter-terrorism, European foreign and interior ministers have proposed a new Action Plan. The main elements are to be:
- Swift implementation of measures previously agreed but not acted on in some states, such as a pan-European arrest warrant system and harmonising rules on defining and punishing terrorism.
- Appointing a new Security Co-ordinator, to push all EU states to carry out common and effective policies.
Better sharing of sensitive terrorist-related intelligence among all EU countries.
This Action Plan will be adopted at the summit. The Madrid bombs have forced the 25 leaders to stand together.
But there are important internal divisions over how to respond to terror attacks blamed on Islamic militants as well as how to treat the growing population of Muslims in Europe.
A wider dispute about the proper basis for a common EU foreign policy also threatens to break out, after a promise of a major switch in Spain's foreign policy by the newly elected Jose Luis Rodriguez Zapatero.
The Spanish Socialists' leader promised to renew Spain's "magnificent" relationship with the EU's traditional leaders, France and Germany. And he denounced the US, Britain and Jose Maria Aznar's outgoing Spanish government for their actions in Iraq, calling the war and occupation a "disaster".
Mr Zapatero also repeated his election promise to withdraw Spain's peacekeeping troops from Iraq, unless a new UN-led structure is put in place quickly.
An old fault-line in Europe over relations with America is being re-opened. British Foreign Secretary Jack Straw says it would be a mistake for any nation to think it could take itself off the "terrorists' target list" by a policy of non-engagement.
Backroom talks about Iraq may be more significant than anything in the summit communique.
Zapatero promises changes in Spanish foreign policy
The government heads may well shiver when they re-enter the stark marble and glass summit building in Brussels. It was there last December that a political squall capsized the EU's project to revise 50 years of European treaties and agree on a grandly-titled Constitution for the European Union.
A confrontation over the EU's delicate internal power balance, between France and Germany on one hand and Poland and Spain on the other, blocked all progress. Everyone went home rather dazed.
In the last two weeks the prospects for an accord on the constitution have risen sharply, thanks to the election victory of the Spanish Socialists on 14 March. Mr Zapatero, now predicts an early agreement.
And the Irish Prime Minister Bertie Ahern, who will chair the summit, thinks agreement can be reached within three months.
But talk of a new deadline may provoke groups of countries again to flag up their "red lines" - points on which they will refuse to compromise in the planned new EU structures for sharing power.
In several EU states a large majority of voters oppose the whole idea.
The liberal economic reform agenda, the original point of the summit, has been overshadowed by more urgent matters.
The sun came out briefly last month, when Germany, France and Britain formally agreed a list of common EU goals - like more flexible labour markets, and the need for an EU "Supercommissioner" to take charge of all aspects of economic performance.
But the clouds are still there: most of mainland Europe is still stuck with high unemployment and low growth. Some have not practised what they preached.
What will it take for the sun to shine on all the EU leaders at the same time? Maybe a miracle.