The More Things Change, the More They Revert to The SameApril 7th, 2014 at 9:50 pm by Don Paul under 4 Warn Weather
This week will again present us with ups and downs, and a fairly typical early spring lack of consistency. After a Very Wet Monday evening, drier conditions will set in for the midweek with below average temperatures giving way to above average on Thursday, slipping a few degrees on Friday, and coming back above average during the weekend. One feature this spring which has not yet plagued us it the “cutoff low” in a blocking pattern. These tend to become more common in climatology as we move a little closer to mid-spring, but there are no signs yet of such a development in the next 14-16 days. So, there’s THAT–at least. Of course, in a block, you could also get under the ‘sweet spot’, the blocking ridge. THAT would be a good deal. No signs of that either.
Ensemble means (I’m relying on them more than operational models) continue to show those ups and downs into next week. By April 17-19, a flatter zonal flow shows up in the means. However, some of that flatter, low amplitude flow shows up because of the huge spread between ensemble members farther out in time. That is, the members and models cannot see short waves, their amplitudes and their paths so far in advance. When the spread between ensemble members is very large at the 500mb height level, the mean tends to be flatter. It’s a statistical compromise which has little to do with actual predictions for amplitude and teleconnections. What’s been happening so far this spring is a tendency to return to ups and downs the closer we get to the ensemble means prediction of a flat, zonal flow…as soon as the members and the models are able to “see” the short waves and the wavelengths between ridges and troughs. We’re certainly out of the extremes of winter, but for the umpteenth blog thread in a row (or so it seems), I still see no sign of a lasting pattern shift to a western trough/eastern ridge.
There are smaller uncertainties about this weekend and the timing for showers either Sunday night or Monday or, if the GFS 18z run were to be believed, showers during Sunday. There will be a transition back to Below Average temps for several days next week. Showers will again cross the region by Thursday night, exiting by early Friday. The weekend is not a lock for Sunday staying dry, but temps both days should be above average…unless 18z GFS (with the GFS’ inherent fast bias) wins out on Sunday and gets those showers and a trailing cold front in here and beats the slower ECMWF.