Those who live here- the forecast predictions are dependable, they are usually accurate.
However who is to say when the 80% chance of rain will occure and for how long.
Temperatures are often warmer than predicted during the day, but the lows seem to also be very accurate.
None can say for sure.
THEREFORE, this event cannot be planned nor restricted according to the rain.
INCASE OF RAIN- pull on your rain boots and wear a coat with a hood, bring a change of clothes if you wanna warm up at the end of it all. There are lockers at Sirkeci to lock up belongings if you need, which is a short walk or train ride from our ending point, SULTANAHMET.

4 people and we can confirm.

We will meet at SIRKECI TREN ISTASYON at 1000.
We will end at SULTANAHMET BLUE MOSQUE at 1600
UPDATE:

We must bypass YerALTI Camii- I will explain this before we start our excursion.

We will enter my favorite: EMINONU YENI CAMII,

we will follow this by a walk up to SULEMANIYE CAMII

and we will take some food and tea/coffee there (the specialty is Fasulye) OR bring some picnic foods to share,

we will then wind in and out of old city streets and hike the hills of Istanbul to get to
FATIH CAMII.

On our way we will pass by the
Aquaducts
for a nice photo taking opportunity.

From there we will walk down and be in Aksaray, from here we will travel to
"Pertevniyal Valide Sultan Mosque",

from here we will go to our final destination-
SULTANAHMET Blue Mosque.

http://www.mydestination.com/istanbul/attractions/117018/yeralti-camii--underground-mosque

http://en.wikipedia.org/wiki/Pertevniyal_Valide_Sultan_Mosque

http://en.wikipedia.org/wiki/Fatih_Mosque,_Istanbul

http://en.wikipedia.org/wiki/New_Mosque_(Istanbul)

At some point in between we will stop and eat again, of course we will! and maybe we will do it in true Turkish style and stop more than once to eat and drink our tea!

Please think of this as a nice guided tour , as this is what I am doing for a living next month.
I have already had to walk this route. I did not get lost, nor did I lose any who joined :)

We will be learning the history of each of these mosques and the significance before entering.

There are a lot of photo opportunities that are destined while
strolling through Old City so bring cameras if you like to.