November 7, 2009
What is instability? I often mention instability as the cause of post-frontal showers after the cold front of a storm system has passed in the PacNW, but what does it really mean for the atmosphere to be unstable? It means that the atmosphere has a steep temperature gradient. Our surface air is relatively mild; it comes straight off the Pacific Ocean in most cases after the passage of a cold front. However, the upper air levels vary considerably from warm sector to cold sector. In the warm sector (after the warm front and before the cold front), tropical air aloft and the surface (although moderated by the Pacific) dominates. In the cold sector (after the cold front), the air aloft is VERY cold but is moderated at the surface by the Pacific. This creates a steep temperature gradient which allows for these showers to pop up.
One thing that shows how the temperatures aloft can vary much more than those at the surface is the snow level in the warm and cold sectors. It is not uncommon for snow levels to shoot up to 7,000 feet during the warm sector, even if the temperature in Seattle is 51 degrees. The following day, after the front has passed, the temperature in Seattle could by 49 degrees, only two degrees cooler, but the snow level may be as low as 3,000 feet. And, the heavier showers associated with instability can often bring the snow level even lower both because they release cold air from the atmosphere in their downdraft and the larger, heavier snowflakes have less time to melt.
The above illustration demostrates how clouds are formed. When updrafts bring a parcel of warm air into the cooler air above it, the parcel expands, cools, and condenses, since atmospheric pressure at higher elevations is lower. As it rises, the air around it becomes saturated with water vapor, as cooler temperatures cannot hold as much water vapor (100% humidity at the North Pole and 100% humidity at Bangkok are NOT the same humidity!). When the air cannot hold any more vapor, the cloud as we see it is formed because the water vapor condenses into droplets. If you have a steeper gradient, you have more clouds and condensation, and therefore more showers.
The reason why there are so many showers and sunbreaks is because wherever air is rising it must also be sinking. When air sinks, it warms and dries, creating clear skies. These clouds are almost exclusively cumulus, cumulus congustus (towering cumulus), or cumulonimbus (raining cumulus, often including thunder).
Hopefully that clears some stuff up for you. The reason the East Coast doesn’t get these in the winter is because they have cold ground and cold air aloft, resulting in less of a pressure gradient, and therefore less convection currents (warm air rises) and less showers. However, they do have the Gulf of Mexico and arctic air both at upper levels and the surface that surges down from Canada in the in the spring months, creating mammoth thunderstorms and tornados with winds over 300 miles per hour! I’ll talk about that some other time. We get nothing like that, although check out the picture of a funnel cloud over Enumclaw Friday afternoon taken by Tyson Gamblin! That is an impressive-looking funnel cloud. Usually you see wimpy ones over here, not ones like that!
Regarding the forecast for the next couple days, we will see a continued threat of showers tonight and Sunday, but the threat will decrease somewhat. Monday will see a short period of heavy rain as a front passes through. Similar fronts will pass through Tuesday night and Friday. With these fronts, rainfall rates will be rather heavy but the fronts will be fast and not stick around for long. After the fronts we will see more showers. Some of these could be quite powerful. It’s hard to predict the strength of these showers days in advance but I’ll let you know if we are looking at anything serious as far as a threat of thunderstorms goes.
Or maybe you’ll hear it instead. 🙂