Frosty Recipe
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Twin Cities NWS Weather Story highlights potential frost areas Wednesday morning.
When you're going to bake a cake, you need the right ingredients.
It turns out you need the right recipe for frost too.
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There's a particular set of atmospheric ingredients forecasters look at to make a frosty forecast. If any one is not in the batter, then frost may not form.
1) The "inner isobar." Is the forecast area inside the inner isobar (line of equal pressure) that surrounds the high pressure center at the surface? If you're near the center of the high that usually guarantees light winds, which allow maximum radiational cooling.
2) Dry air mass. Is the dew point expected to be less than 32 degrees? The rule is, the temperature can't drop below the dew point. Forecasters look for dew points in the low 30s or upper 20s at minimum temperature time around sunrise.
3) Clear skies, calm air. Any cloud cover can act as a blanket and reradiate warmth back to the ground which limits frost in temperature critical situations. Even slight breezes can stir up the air enough to prevent frost. Forecasters look for these conditions right around sunrise.
So what about frost tonight?
It looks like the center of the Canadian high pressure will be just east of the metro by sunrise Wednesday. We should see clear skies, but there may be just a slight breeze in the Twin Cities.
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NAM model shows high pressure ridge just east of the metro by 7am Wednesday.
Dew points look to be in the lower 30s, bottoming out at 31 in the metro according to one forecast model.
The bottom line is, it looks like the best chance of frost will be just north and east of the Twin Cities into northwest Wisconsin. If you're a gambler you may not need to cover those plants in the heart of the metro. But if you want to play it safe, cover up.
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Enjoy the sunshine and lighter winds and somewhat milder temps today and Wednesday. It looks like a big soaker is on tap starting Thursday. More on that this afternoon.
Stay tuned!
PH
