Reading the Signs

Read the signs and you will learn about your weather.

For the weatherwise, storms send out signs. Today was one of those days. Cirrostratus (Cs) is often a storm’s calling card. This is especially true if cirrostratus is widespread and increasing across the sky. In the photo below Cs is covering more of the sky than it appears. Even the blue sky in this photo is covered by cirrostratus. Cs is usually thin, sometimes almost invisible. Except for the “thicker” Cs on the left and bottom of this photo, the Cs might be entirely missed, especially at night. Look closely when you look at the sky. You may see very thin filaments of Cs.

The thing is, in this photo the Cs is extensive and it is thickening to the west (left) and north. The sky should be reported as overcast with thin Cs. Patchy Cs usually does not herald a storm. But if Cs is increasing in thickness and steadily overspreading the sky it is worth watching. If a storm is coming the Cs will likely thicken and overspread the sky before lower clouds appear.

Cirrostratus 3-17-2020, Photo by Craig Johnson

The Cs in the next photo is more prominent. It is spreading over the sky from right to left. The increasing coverage and thickness indicates there may be a storm approaching.

Ciirrostratus increasing from right to left. Photo by Craig Johnson 3-17-2020.

About 6 hours after the Cs first appeared, the clouds thickened and lowered. The photo below shows a middle cloud layer that appeared underneath the Cs. This cloud deck is altocumulus (Ac). The lumpy nature of the clouds indicates a layer of instability overhead caused by warmer moist layer aloft being lifted. It is also a sign of an approaching storm.

Altocumulus approaching from the west. Photo by Craig Johnson 3-17-2020.

Next in the progression is the altostratus below. The cloud streaks indicate precipitation falling aloft. Notice the streaks below the cloud layer - a classic sign of precipitation. Because the air below the cloud base is very dry (surface dew points were in the upper 20s) it will take 6 to 8 hours for the precipitation to saturate the air and allow rain to reach the ground. By then surface temperatures will be above freezing so freezing rain is not expected.

Altostratus, Precipitation is falling aloft but not reading the ground, Photo by Craig Johnson, 3-17-2020.