You need to blend it properly. It would be quite hard to do it with the Lasso Tool (L) and you would loose the aura arround the flash. Because of that you'll change Blending Mode instead. Change it from Normal to Screen. You'll get something as on the following picture.

As you can see there is the lighter rectangle still visible. To hide it add new adjustment layer Levels on the top of all layers and set the Input Levels on 62; 1,00; 255. The rectangle dissapears.

To adjust only BIG FLASH and not the whole image create clipping mask from this adjustment layer.
After this step your image should look like the one below.

Step 21 - Small Flash
According to me the picture would look more balanced if there is one more smaller flash near the woman.
Right click on the BIG FLASH and select the option Duplicate Layer... . Name this new layer SMALL FLASH. Press Ctrl + T on your keyboard and make flash smaller. Move it on the same place as you can see on the preview picture.
To blend the layer properly add new adjustment layer Levels above it and set the Input Levels on 62; 1,00; 255 as in the previous step. Create clipping mask too.
You don't want to this flash look completely the same as the big one. It would be weird because in nature you won't see the exactly same things. To add some variability you'll flip it and then use Warp to change its shape.
To flip the SMALL FLASH go to Edit > Transform > Flip Horizontal.
Then go to Edit > Transform > Warp. The gridd will appear. Adjust it to change the shape of flash and then press Enter on your keyboard to apply these changes.

On the following picture you can see how the manipulation looks so far.

Step 22 - Adding Rain
In this step you'll finally add the last missing element - rain. Download the rain brushes and install them into Photoshop.
Create a new layer on the top of all layers and name it RAIN. Grab the Gradient Tool (G) and fill the layer with solid black color. Change its Blending Mode on Screen. It's not necessary to do that, you can paint rain directly on empty layer. But if you fill it with black you can switch from Screen to Normal Blending Mode to see exactly where you paint the rain and be able to correct mistakes easily.
To get realistic result don't paint the rain over the whole image. Below you can see where I painted it to get some inspiration. (The layer has Normal Blending Mode.)

Another very important thing to get realistic result - you should have all the parts of the rain in the same direction.
After this step your image should be similar to the following one.

Step 23 - Adjusting the Woman
Right now the woman is bit more darker and has warmer tones than the rest of the image. Because of that it doesn't fit to the rest of the scene well. You'll fix it in this step.
Let's start with lightening. Add new adjustment layer Levels on the top of all layers and set the Output Levels on 12; 244.

You want to adjust only the woman not the rest of the image. Because of that go to Select > Load Selection and as Channel select Woman. Tick the option Invert and press OK.

Make sure that the layer mask of Levels adjustment layer is active (just click on it) and then go to Edit > Fill and use black color.

Now let's adjust the color palette. As I said at the beginning of this step it's bit warmer than the rest of the image and because of that it doesn't fit to the scene properly.
Add new adjustment layer Color Balance on the top of all layers and set the window as on the following picture.

You want to adjust the woman only. Because of that do the same process as after adding Levels adjustment layer.
On the picture below you can see how these two adjustment affect the picture.

Step 24 - First Global Adjustment
As you can see the final picture has more cyan/green tones and clouds are little less contrasted. You'll do both in this step.
Add new adjustment layer Color Balance on the top of all layers and set it as on the following picture.

Now let's move on changing contrast. According to me some parts should be little lighter to get more balanced look. I highlighted these parts with red on the following picture.

Add new layer on the top of all layers and name it CONTRAST. Grab the Brush Tool (B). Select some soft round brush and lower its Opacity on about 15%. Press Alt on your keyboard to activate Eyedropper Tool and select some light blue color from sky. Now gently paint over the red areas from the previous picture.
Your picture should be similar to the one below.

Step 25 - Second Global Adjustment
You're almost finished with the tutorial. There are only few minor things to do left. One of them is adjusting the color tones with Gradient Map and making the whole picture bit darker. You'll do it in this step.
Add new adjustment layer Gradient Map on the top of all layers and set the gradient as shown on the picture below.

Now let's make the picture bit darker. To do that add new adjustment layer Levels on the top of all layers and set the Input Levels on 0; 0,75; 255.

You get the result similar to the picture below after this step.

Step 26 - Adding Vignette
I really like using gentle vignette because it helps to keep viewer's attention longer in the picture.
To add vignette create a new layer on the top of all layers and name it VIGNETTE. Grab the Gradient Tool (G) and fill the layer with solid white color.
Now go to Filter > Lens Correction... (the path may vary in different versions of Photoshop, if you can't find it simply press Shifht + Ctrl + R on your keyboard) and set the window as on the following picture.

To blend VIGNETTE properly with the rest of the image change its Blending Mode on Multiply.
- Jarka