The weather for the first few days of April has been much different than March for much of the country. In the East and Midwest, cooler weather has been more prominent than the well above average temperatures of last month. In the far west, warmer weather has been prevalent for the past few days.
Focusing on the east, high-latitude blocking has been present for the better part of a week. The blocking pattern will likely continue for another few days. Before I go into the effects of blocking, I’ll explain what it is. Blocking can lead to a stagnation of weather patterns. If an area of high or low pressure becomes stationary, the entire flow of storms can be slowed or even stopped.
When this happens, any one place tends to see the same weather for a rather long duration of time. Take a look at the upper-air map from this afternoon, you’ll notice a large circulation over southeast Canada.
This large circulation has moved little over the past few days, and will move little in the next coming days. The main effect of this upper-level low will be to keep things cooler than normal in the east and warmer in the west (though the trough in the west may throw some clouds or showers onshore, which may negate the warmth and keep it cooler). The upper-level low will finally weaken this weekend and a ridge will build in the east while a trough builds in the west. You can see that trough off of California now.
What does next week and beyond hold? Well, that far out the models become less accurate so we turn to the climate indicators. These include El Nino, La Nina, the NAO, PNA, AO, etc. In the near-term, we tend to look at the NAO, AO, and PNA. These short-term climatic patterns can have a big influence on our weather.
We’ll take a look at the PNA first. The PNA (Pacific North American pattern) tends to influence the locations of ridges and troughs in North America. When the PNA is negative, the east tends to be under the influence of a ridge while the west is under a trough. The opposite is true for a positive PNA. More on the PNA: http://www.nc-climate.ncsu.edu/edu/k12/.PNA
Looking at the graph above, (focus on the first/top one) we can see that April has been under the influence of a positive PNA. Ensembles (lots of models) point to a more negative PNA by the end of the month. (even though the negative PNA may come mid-month, the effects may take a few days to take hold). This may lead to a warmer pattern in the east while the west slides into a cooler and wetter pattern.
This is only one factor in the climate realm. There are others that can trump the effects of the PNA and vice versa, or they may add to it. The NAO (North Atlantic Oscillation) also lends to the idea of a warming east with the west cooling down. The NAO is negative now, which favors high-latitude blocking and cooler weather in the east, which is what we are seeing. The NAO is slated to go positive next week and this tends to lead to a warmer east. The NAO doesn’t have as large of an influence in the west, where the PNA is more important. Since these two climatic indicators line up, that lends to the idea of a warming east and cooling west, at least in the mid-range.
So, in conclusion, we can expect the cooler than normal weather in the east to continue until the weekend. The trough breaks down later this week and warmer weather will move in this weekend and continue into next week. The west will remain warm until the effects of a negative PNA take hold, if they do. These effects may be delayed though, arriving later in the month. Also, for those in the lower Great Plains, the trough in the west may lead to a greater chance of showers and thunderstorms along with continued warmth. Only time will tell! This is not a day-to-day forecast, it is a general pattern forecast. While any one particular day may not be warm or cool, on average the aforementioned conditions will be the case.