The phenomenon described in II Kings 3:20-23 is well known in the area. Rain can fall in the higher areas such as Edom (now Jordan) or in the Judean hills while down in the Jordan valley on either side of the river, the sky is clear and sunny. When this happens, the wadis (gulleys) that drain the uplands erupt suddenly in flash floods that endanger unsuspecting hikers in the lowlands. The weather service in Israel issued a warning about this yesterday. That is, a flash flood occurs, but in the lowlands you don't hear wind and you don't see any rain, as stated in verse 17.
The miracle in this case is that the rain in the uplands of Edom and the resulting flood of water occurred out of season. The result was that the Moabites, who did not see the rain, could not believe that what they were seeing was water. The reflected light of the morning sun as seen looking east south east gave the impression that what they were seeing must be rivulets of blood. The Israelites, who were closer to the water in any event, having approached Moab from the high ground of Edom south east of Moab as stated in verse 8 saw it looking west north west, away from the sun.
The path of the allied forces was, from Jerusalem, south on the King's highway to Bethlehem, Hebron, Carmel, Mt. Amasa, descending to Arad, descending to the Dead Sea, skirting the Dead Sea to the south of the salt marshes, across the Jordan valley and up the east ascent to the south of Moab and then attacking from the high ground to the south east of Moab. [Lever]
Now the question to ask is why such a round-about way of attack was chosen, given that in any event it was impossible to hide the movement of such a large army, as hinted in verse 21, so that surprise per se was not possible.