WaterWatch PRO

A National System Built From Multiple Data Sources

Each hour, NOAA integrates several complementary data streams to create a high-resolution (~1 km) rainfall map:

Dual-polarization Doppler radar from ~180 overlapping NWS sites
More than 20,000 rain gauges providing ground-truth measurements
Satellite rainfall estimates updated every 15 minutes
Surface observations from airports and automated stations
Numerical weather models and climatoligy
High-resolution topographic adjustments
Hourly bias correction against regional stations

These inputs work together to generate a precise hourly rainfall grid — a digital map with millions of evenly

spaced data points representing actual rainfall reaching the ground. See more detail here: qpeOnePager.pdf

Why Gauge-Adjustment Matters

While radar excels at detecting precipitation aloft, it cannot directly measure what falls at the surface. Because the radar beam sits above the ground, it may overshoot low-level rain or underestimate intensity beneath the beam.

NOAA corrects this automatically by blending in rain-gauge observations each hour. This calibration step adjusts radar-based rainfall up or down and dramatically improves accuracy. The result is a rainfall value that reflects

true ground-level precipitation, not just radar estimates.

How the Hourly Grid Works

The final NWS rainfall product is a seamless grid where every square — roughly 1 kilometer on each side - contains its own hourly rainfall value.

WaterWatch PRO identifies the exact grid cell containing your GPS location and extracts that value directly:

No interpolation
No averaging with nearby sensors
No smoothing or adjustments beyond NOAA’s own process

This ensures the rainfall amount you see is the same value NOAA assigns to that precise location in their national dataset.

Expected Differences With A Gauge

The digital NOAA data described above will almost always differ slightly from an individual sited gauge, perhaps by as much as ±0.05" to ±0.20", depending on microclimate, elevation, and the spatial footprint of a storm. It is important to understand that these are two different measurement methods. This is expected and actually designed behavior. Here are factors leading to the differences.

Environmental Factors

Wind is the biggest source of under-measurement in rain gauges.

When wind flows across the top of a gauge, it creates turbulence and updrafts that deflect raindrops away from the opening. This is especially true when:

Wind speeds exceed 10–15 mph
Raindrops are small (as in drizzle or light stratiform rain)
The mounted gauge is elevated, as is the case for nearly all airports
Storms occur with frontal passage, when winds increase sharply

Digital NOAA data is immune to this issue because it is derived from radar reflectivity volumes rather than physical collection.

Microclimate Variability

Climatological wind circulations and topography could result in a gauge:

Sitting under a temporary lull in a shower.
Lying in a rain shadow depending on storm angle.
Receiving more or less rainfall than neighboring areas.

Once again, the digital NOAA data integrates radar from all quadrants around the site, smoothing out these micro-scale inconsistencies.

Light, patchy rain

Radar sees rainfall aloft, even if droplets evaporate before reaching the gauge.
This is especially likely if the air is dry, winds are strong or warm air is moving into the area.
Light mist or drizzle can register regionally but miss a single gauge.

Heavy Downbursts

While a heavy burst may pass directly over a gauge, the digital NOAA data blends radar data with information from surrounding sensors, leading to a slight smoothing of maximum values.

Mechanical Limitations

A gauge can misread rainfall if:

Drops hit the rim and bounce out.
High winds blow rain across the opening.
Insects or debris partially block the funnel.
The tipping bucket mechanism undercounts during very high intensities.

Traditional Methods vs. WaterWatch PRO

Traditional stormwater monitoring often relies on isolated gauges and manual logs, which may miss highly localized bursts or fail to capture regional variability.

In contrast, WaterWatch PRO uses NOAA’s continuous gridded coverage, delivering consistent rainfall measurement everywhere — even where no gauges exist. This approach captures short-duration, high-intensity events that often trigger compliance actions, inspections, and BMP responses.

Why You Should Trust This Data

Traditional Methods vs. WaterWatch PRO

NOAA’s digital rainfall map is the same system used by:

The National Weather Service for flash-flood forecasting
The U.S. Geological Survey and Army Corps of Engineers
Emergency management and hydrology operations nationwide

The data stream itself earned NOAA’s Silver Medal

for scientific excellence.

WaterWatch PRO simply extracts this authoritative data and aligns it precisely to your site.

How WaterWatch PRO Uses Official NOAA Digital Rainfall Data

A National System Built From Multiple Data Sources

Why Gauge-Adjustment Matters

How the Hourly Grid Works

Comparison Between Digital Data and Gauges

Expected Differences With A Gauge

Environmental Factors

Microclimate Variability

Light, patchy rain

Heavy Downbursts

Mechanical Limitations

Traditional Methods vs. WaterWatch PRO

Traditional Methods vs. WaterWatch PRO

Traditional Methods vs. WaterWatch PRO

Why You Should Trust This Data

Traditional Methods vs. WaterWatch PRO

Traditional Methods vs. WaterWatch PRO

What This Means for Your Project or Facility

How WaterWatch PRO Uses Official NOAA Digital Rainfall Data

A National System Built From Multiple Data Sources

Why Gauge-Adjustment Matters

How the Hourly Grid Works

Comparison Between Digital Data and Gauges

Expected Differences With A Gauge

Environmental Factors

Microclimate Variability

Light, patchy rain

Heavy Downbursts

Mechanical Limitations

Traditional Methods vs. WaterWatch PRO

Traditional Methods vs. WaterWatch PRO

Traditional Methods vs. WaterWatch PRO

Why You Should Trust This Data

Traditional Methods vs. WaterWatch PRO

Traditional Methods vs. WaterWatch PRO

What This Means for Your Project or Facility

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