What you can actually observe

Depth against a curb and a tire, flowing against ponded, a rain gauge read to hundredths, and the difference between a flooded street and a flash flood.

What the NWS actually asks for

The Weather Spotter's Field Guide asks five questions about flash flooding, and they are worth reading as a checklist rather than as prose, because that is how they are meant to be used:

Report flooded roadways, rivers and streams, giving approximate water
depth. Does the flooding consist of standing water or is it flowing?
Is the water level continuing to rise, staying steady or falling? Is
the flooding occurring in a known flood prone area? Any damage from
the flooding or mud slides?

NWS Weather Spotter's Field Guide, flash flooding items. Quoted in full.

Local offices add their own wording, and it is worth reading yours. One coastal office asks for two things this way: "Flooding: Report any flooding of roads, homes, or businesses", and separately, under heavy rain, "Rainfall reports are always welcome, including rainfall totals. Rain totals of more than 1 inch in an hour are also helpful."

Depth, against something

Depth is the number the field guide asks for first, and you will be estimating it rather than measuring it, because measuring it means getting into it. So estimate it against objects whose size you already know. The three from lesson 4 are the ones to use, for the good reason that they are also the danger thresholds.

The directive gives two depths that matter for whether an event is filed at all: "Six inches or more of swiftly moving water flowed over a road or bridge, posing a threat to life or property" and "Three feet or more of ponded water that poses a threat to life or property". Note that both clauses end the same way. Depth alone does not make an event; depth plus a threat does.

Flash, or just flooded

A flooded street is not automatically a flash flood, and the distinction is one a spotter can make from what is in front of them. Instruction 10-1605 says events are categorized by their impacts, not by the infrastructure that contributed to them, so urban effects and poor drainage do not by themselves upgrade an event.

Poor-drainage flooding

Water collecting in the usual low spots, rising slowly, not moving much, draining away within an hour of the rain stopping. It is a Flood, and a genuinely useful report, because it tells the office the rain is beating the drains.

Flash flooding

A rapid rise, water moving with enough force to carry things, water in places that are normally dry, and continuing after the rain stops. Rate of rise is the discriminator a spotter can actually see: watch a fixed object for two minutes.

You are not required to make that call in your report, and you should not force it. Describe what you can see and let the office classify it. Rate of rise, whether it is moving, and how deep, in that order, gives them everything they need to decide.

Rainfall, to hundredths

Rainfall is the one flood observation you can measure properly, and the standard instrument is the four-inch CoCoRaHS gauge. Its funnel collects over the full four-inch mouth and delivers into a narrow inner tube, which magnifies the reading by a factor of 10. That is why observers report to the nearest 0.01 inch rather than guessing at a quarter of an inch: the instrument resolves it.

1. Read the inner tube first. It holds 1.00 inch. Read to the hundredth, with your eye level with the meniscus.
2. If it has overflowed, the excess is in the outer cylinder. Pour it into the inner tube and read it again, repeating until it is empty, and add the readings up.
3. Record the time you read it, not just the total. A 3.00 inch total means very different things over ninety minutes and over nine hours.
4. Report short-duration amounts separately where you have them. An hourly rate is what connects your gauge to a rainfall rate product.
5. Say whether it was measured or estimated. Always. A measured 2.14 and an estimated "about two inches" are different observations and a forecaster treats them differently.

What a complete flood report contains

• Where the water is, not where you are. A landmark, a cross street, a named crossing. This is the mistake people make most often on this hazard, because the observer is usually somewhere safe and the water is not.
• When it happened, rather than when you are calling.
• How deep, against a named reference: over the curb, to the middle of the tires.
• Flowing or standing, and whether it is rising, steady or falling.
• Whether it is a known flood-prone spot, which turns your report into a comparison with every previous event there.
• Any damage, including washouts, mud and road surface gone.
• Measured or estimated, for any number in the report.

Check yourself

Water is flowing across a road, just over the top of the curb, and has risen visibly in the two minutes you have watched it. What is the most useful single addition to "flooding on Highway 90"?

1. That it is flowing, roughly six inches deep, and still rising
2. That it is probably a flash flood rather than areal flooding
3. Your GPS coordinates at the time of the call
4. An estimate of how much rain has fallen so far

Sources

1. NWS Weather Spotter's Field Guide (YPA-201154, June 2011)
2. NWS Mobile, submit a storm report
3. NWS Instruction 10-1605, Storm Data preparation (current edition 26 July 2021)
4. CoCoRaHS, measuring precipitation
5. NWS, measuring rainfall (public education material)