Open the forecast and each day has a number from 0 to 100, a label, and a short note on what’s driving it. It’s easy to look at a number like that and wonder where it came from.

So here’s what goes into it, and why. We’ll be honest about one thing up front: there is almost no research on weather and MCAS specifically. We chose these ingredients from what’s known about mast cell triggers in general, from conditions that overlap with MCAS (allergies and dysautonomia), and from what people living with MCAS tell us. They’re informed starting points, not proven facts. The best test of any of them is you.

How the score works

Every day starts at 100. Each ingredient can take some points off, up to its own share, depending on how much of a trigger it looks like that day. A calm, clear day loses almost nothing. A smoky, stormy one loses a lot.

And when something is truly extreme, it can lose more than its share. Hazardous air, a huge pressure swing, or a severe geomagnetic storm can take off up to twice what that ingredient normally can, so it can dip below zero and pull the rest of the day down with it. We think a day like that deserves to look as bad as it is.

Not every ingredient counts equally. We give the most weight to the ones with the strongest reasons behind them, and the least to the ones that are more of a hunch. Here they are, roughly from best supported to least.

Air quality

Smoke, ozone, and fine particles are well known to make allergic and breathing problems worse, and irritants and smoke show up on nearly every MCAS trigger list. Particles can activate mast cells in laboratory studies. We use the day’s highest reading of the US Air Quality Index.

It’s the most direct exposure, so it counts the most. It’s also where the below-zero part matters most: on a hazardous smoke day it can cost double, and sink the whole score on its own.

Pollen

Mast cells are at the center of allergic reactions, and plenty of people with MCAS have allergies too. The link is direct and well understood. We look at tree, grass, and weed pollen separately and add them together, since exposure to several allergens at once tends to add up. One high type matters, and so does a little of all three. Even a “low” count costs a few points, because sensitive people can react at low levels.

Pressure changes

This is the most complicated ingredient, and anecdotally it may be the most important one. That is a hunch from experience, not something proven, but it is the one we would watch first.

Many people with MCAS, and with related conditions like dysautonomia, POTS, and Ehlers-Danlos, say they feel worse when the barometric pressure moves, and that includes rises as well as drops. The research is mixed, and none of it is about MCAS. The physical effect on the body is tiny, so if it matters, it may work through the inner ear, blood vessels, or sinuses, or it may be standing in for other changes in the weather. We call it plausible and personal, not established.

The part we’re careful about is how we measure it. We look at the swings inside each day, from midnight to 11 PM: every rise and fall, with a change of direction starting a new one. The biggest swing counts most, and any others add something on top, so a day with a sharp rise followed by a drop later on scores worse than a day with just one. Those can be the hardest days, and a single reading or an average would smooth them away.

Pressure also has a small built-in daily rhythm that is a different size in different places. We learn your local rhythm from the days around it and take it out, so an ordinary day’s wobble isn’t counted as weather. What’s left is the weather.

Cloud cover

We include clouds because gray, overcast days are reliably harder days for the people we built this for. There’s no known mast cell reason for it. At best it’s a stand-in for weather systems, low light, and mood, and it overlaps with pressure. So it counts, but modestly.

Solar activity

Some studies connect geomagnetic storms to lower heart rate variability and heart events. MCAS often overlaps with dysautonomia, where the nervous system is already unsteady, which is why we pay attention. But the effects found are small, repeat studies have been weak, and none are about MCAS. It’s the most speculative ingredient, so it counts the least. If there’s an effect, it may also arrive hours or a day after a storm, which a daily reading can miss.

Shown, but not scored

A few things appear on the cards without changing the score: the day’s UV (handy if you’re thinking about vitamin D), wind, chance of rain, humidity, and the moon. We think they’re useful for planning, but we don’t have good enough reasons to call them triggers.

Don’t take our word for it

None of this replaces what you know about your own body. If clouds never bother you but pressure always does, that’s your answer, and it matters more than any weight we picked.

That’s what the Health Diary is for. It puts each day’s rating next to its forecast, so over weeks you can see which patterns hold up for you and which don’t. One match means little, since a smoky or stormy day comes up often. A pattern that keeps showing up is worth noticing, and worth mentioning to your doctor.

And since the score is a forecast of known and suspected triggers, not a prediction of how you’ll feel, a rough forecast doesn’t mean you’ll have a rough day, and a perfect one doesn’t promise you won’t. It’s a planning aid, built from patterns. It isn’t medical advice.

We’d love to hear what you notice. Get in touch.