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04/25/2021 06:13AM  
With canoe season rapidly approaching (even with record cold this AM in the Arrowhead- Hibbing 15F - Ely 18F at 6am observations) figured I would start this thread and share some knowledge I have gleaned over the years in predicting weather with little help. Maybe that of a weather radio or handheld weather station (mainly to measure pressure).

First up is types of clouds and today's cloud is ACC- or altocumulus castellanus.

Probably one of the most tried and true methods of predicting afternoon strong/severe thunderstorms is the presence of altocumulus castellanus clouds in the morning, say between 8am-10am. Translated from latin, meaning castle,the castellanus species of cloud is uncommon, but can be expected to be seen on days when the atmosphere is unstable. Altocumulus castellanus clouds are a particular favorite kind of cloud amongst storm chasers because they usually point to unstable air, which can mean for thunderstorm development sometime during the day. Castellanus clouds in the morning indicate stormy weather in the afternoon. Basically they are showing that mid to upper levels of the atmosphere (at least for thunderstorm development) from about 7000' to around 20,000' are all ready unstable and all it takes is basically heating of the day and connecting to surface instability and BOOM- Thunderstorms.
Altocumulus castellanus 1
ACC 2
probably the best pic I've seen on the net is that with ACC 2 link on wiki- blow that pic up as it's an excellent pic of ACC clouds.
Here are several other good examples- this link (don't be concerned about the top- just the pics) shows a progression of ACC from what many would consider not much. However- you can add in this pic at tail end of those 4 and this is what happens most times when you have ACC in the mid morning hours around 2-3pm
a full blown TS is not far off.
Winds from a southerly direction - especially SE, with a falling barometer along with ACC would be as much a slam dunk for PM thunder as you can get.
 
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04/25/2021 02:40PM  
This poster was quite common in the fire halls during my wildland firefighting career.
 
04/28/2021 09:39AM  
Very interesting! I have always struggled with cloud identification. After searching the web for more photos of altocumulus castellanus, I found that there were some YouTube videos of them that really helped me see the formation and upward motion.
 
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