If the plume did reach at least 30 miles (or higher, who knows?) it could have been seen by the Hebrews Keep in mind, from the Nile delta (which was probably farther south than it is now), only the part of the plume which was 30 miles high or higher would have been visible above the horizon. It was probably more 'blip' than plume.
Just roughly, and doing the math in my head, a plume sixty miles high would only have stuck up 3 or 4 degrees.
If the prevailing wind was blowing towards Egypt the plume needn't have been so high to be visible.
Had it been the high altitude winds the bulk of the ash wouldn't be carried along, so there wouldn't be as heavy an ash fall, and the plume would be illuminated by the sun well after local sunset, and before local sunrise.
Anyone know the typical jet stream patterns over the Med?