Hot and humid weather here would need a southern or even better a southwestern airstream, so that humid warm airmasses from subtropical regions like the Mediterrenean or the subtropical Atlantic can be transported to us.
Since i could explain it much better by inserting a weather model, i will use the weather conditions from June 17 to June 21st 2013 as an example. During that time a "Spanish Plume" made its way to us, bringing warm humid air and leading to dewpoints of up to 23°C and temperatures up to 30°C here in Northern Germany.
Here's how it started..
June 17th
On the bottom you can already see the hot desert air from the Sahara being pushed up north towards Eastern Spain and the Baleares. The South of Germany gets a first touch of the hot humid air with temperatures rising to about 30°C and dewpoints mainly in the upper teens (C), while the North was still cooler and drier with about 20°C and a dewpoint of 8-10°C
June 18th
The hot air pushes further north with the 20°C isotherme reaching the Alps and Southern Germany.
Temperatures here climbed above 30°C in many places, even reaching up to 35°C in some spots.
With the hot air traversing over the mediterrenean sea it also picks up moisture, which resulted in dewpoints raising above 20°C in Southern Germany already, while it was still a little drier up north with dewpoints of about 15°C, but temperatures also raised up to 28°C here.
June 19th.
Now the hottest air almost made it to the north, but we are still sweating at temperaturss of about 16-18°C in the 850 mbar layer. That day had the highest dewpoints in the Northern and Eastern parts of Germany, reaching 20°C or above even up to the coast, while the South got a little drier with dewpoints of about 13-15°C.
On that evening a large MCS formed over larger parts of Northern and Northeastern Germany with the large anvil shield even reaching up to Southern Sweden.
That lead to a long lasting thunderstorm of over 4 hours, which brought up to 40 mm (1.5 inches) of rain in some parts of town, many streets and cellars were flooded that evening.
This was a rather unusual event for us, since Bremerhaven is mostly "sitting between two chairs", so one thunderstorm line moves north of us and the other one south, but this time we were sitting quite in the middle.
June 20th. Now almost all of Germany is wrapped in a hot humid layer, dewpoints were 16-24°C in the afternoon all over the country. Day remained sunny here until about 4 PM, after that a heavy thunderstorm passed brought 18 mm (0.8 inches) of rain. Afternoon high was 29°C, but after the storm it remained a little below 20°C but with almost 100% humidity.
June 21st. Another thunderstorm passed during the nighttime hours and as you can see during the day
the humid warm air retreats, giving way to cooler drier air. Dewpoint went back overnight from close to 20°C, to the mid teens by afternoon.
Hope this helps!
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