One of the inherent risks of medium range forecasting is its uncertainty. And so it is with our first potential heat wave of the season.
More than a week ago it became clear that the extremely cool weather we’ve seen for much of the spring was about to come to an end. Last Friday’s high of 72 with some light rain in the morning would no doubt punctuate the El Nino enhanced rainy season and prove to be the end of an era. This past Memorial Day weekend was a trailer for coming attractions of hot summertime weather just ahead. I remember highlighting the impending changes quite strongly all last week. Then came Sunday and Monday with highs in the 90s; hot, but not too hot for a summer holiday weekend.
But as is often the case with wholesale shifts in a weather pattern, the details became fuzzy. Computer models now routinely make predictions 384 hours into the future (that’s 16 days, half a month). When the primary ensemble keeps forecasting roughly the same idea for model run to model run, meteorologists are reasonably confident that the expected weather will come to pass. These models are run every 6 hours, at 5 and 11 O’Clock Pacific Time, AM and PM. There are primary model runs (at 5 AM and PM) and secondary, or intermediate runs (11 AM and PM). We look for consistency. What is a week in the future needs to show the same meteorological texture as the target date approaches.
With the well advertized coming heat wave there have been problems. At first, models displayed a large summer-like ridge overspreading the southwestern US for late this week. But then the ridge appeared to expand farther to the east and the polar jet stream was portrayed much closer to central California. And thus the forecast changed for more of a moderation in the future- not as hot. Then came another kink in the mind of our medium range computer models. The ridge would expand but a fast moving wave was seen swooping down from the northwest bringing cooler weather to the region before the hot ridge had a chance to pump our temperatures up. Sometimes things like Tropical Storm Agatha through a monkey wrench into the works, essentially ingesting a huge area of unexpected warm and humid air into the general mid-latitude flow. When that happens, the computer solution changes. But finally, after some back and forth, all models are coming to a consensus.
The general idea is that we will warm up to above normal temperatures for early June and that it will be felt this weekend, most dramatically on Sunday. But the high heat probably won’t persist as long as first thought. No rain is expected through the period; it will be dry with only some high clouds from time to time. The south valley and desert portions of the valley will see close to 100-degree weather for 2 or 3 days and then it will cool off incrementally thanks to a trough. The Hot Times Contest from last month, in which participants guessed when the first triple digit minute would come to Bakersfield, may well have a winner within a week. Get ready for hot weather regardless of whether it hits 100 or not. The cool spring is definitely over!