Weather Rainfall Forecast Calculator
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31
Estimate monthly rainfall totals by simulating uncertain rainy day counts and rain depth per rainy day using Monte Carlo sampling.
NOTEHow to adapt this model
Edit p in Rainy Days in Month to match your typical chance of rain per day for the location/season (e.g., 0.15 in dry seasons, 0.50+ in wet seasons). Edit the gamma parameters for Rain per Rainy Day to match your local rainfall intensity on rainy days. If you have historical data, fit: mean = shape*scale and variance = shape*scale^2.
NOTEInterpretation note
This simple model assumes independent daily rain events and a single typical rain depth per rainy day. For more realism, you can extend it to simulate each day separately, include dry-day drizzle thresholds, or model storm clusters.
VARIABLERainy Days in Month
binomial
Number of days in a 30-day period with measurable rain. p is the average probability a given day is rainy (edit for your location/season).
rainy_days
VARIABLERain per Rainy Day mm
gamma(2, 5)
Rainfall depth (mm) on a rainy day. Gamma(shape=2, scale=5) has mean 10 mm and is right-skewed; adjust to match local intensity.
rain_mm_per_rainy_day
CONSTANTDays in Period
30
Length of the simulated period in days (used for documentation; rainy_days uses the same n).
days_in_period
FORMULATotal Rainfall in Period mm
rainy_days * rain_mm_per_rainy_day
Approximate total rainfall (mm) over the period as (number of rainy days) × (typical rain depth on a rainy day).
rainy_days
rain_mm_per_rainy_day
total_rain_mm
OUTPUTTotal Rainfall Distribution
Total rainfall in period (mm)
Simulated distribution of total rainfall over the period.
OUTPUTRainy Day Count Distribution
Rainy days in period (days)
Simulated distribution of how many days are rainy.
OUTPUTRain Intensity Distribution
Rain per rainy day (mm)
Simulated distribution of rainfall depth on a rainy day.
What is Carlo?
Carlo is a visual tool for Monte Carlo simulation. Model uncertainty by dragging probability distributions, connecting them visually, and running thousands of scenarios instantly.