The series of hot days beginning last Saturday and ending last Tuesday was very unusual for this early in the season. Intense heating of the land relative to the chilly ocean (still about 60°F at the surface) typically supports a strong sea-breeze circulation, which serves to draw relatively cool air from Boston Harbor into Cambridge. During this heat wave, however, large-scale westerly winds generally did not allow the sea-breeze circulation to penetrate beyond the immediate coast line. The result was very hot afternoons, including as estimated high of 99°F (37°C) in Cambridge last Tuesday.
Earlier this week, a slow-moving storm system dumped extremely heavy rain over a wide arc from northeast Texas to southern Indiana. Accumulated rainfall totals of over 6 inches (15 cm) in a 36 hour span were commonplace in this area, with scattered reports of up to 12 inches (30 cm) of rain. Some rivers with localized drainage basins in southern Missouri and northern Arkansas have experienced the highest water levels since record keeping began. Even portions of main-stem rivers like the Ohio and Mississippi are expected to see major flood conditions this weekend, as the rainwater continues to flow downhill toward the Gulf of Mexico.
Logan airport has recorded almost 8 inches (20 cm) of rain and melted snow this month, making it the wettest February on record in Boston. However, only a small portion of this precipitation was snow and ice, such that the accumulated frozen precipitation depth for the month is just 15 inches (38 cm). This brings our seasonal frozen precipitation total to 50 inches (127 cm), still above average due to the near-record snowfall total in December.
The deadliest tornado outbreak in the United States since 1985 occurred this past Tuesday and Wednesday, killing at least 50 people in a region spanning 5 southern states. While the extensive loss of life cannot be attributed to meteorological factors alone, the rare phenomenon of long-track supercell thunderstorms certainty did play a major role.
Although not exactly ideal, weather conditions today and tomorrow should not cause major problems for those departing Cambridge for the holiday. A warm front moving in from the west will make for a dreary day today, bringing cloudy skies and light rain or wet (non-accumulating) snow. Tonight will be chilly and damp, but then the temperature will rise all the way through Thanksgiving afternoon, in response to southerly flow induced by a low pressure area developing well to our southwest. Overcast skies will predominate during this interlude of warmth, with the clouds perhaps squeezing out a few showers on Thanksgiving.
With the passage of the autumnal equinox early Sunday morning, the summer season will soon draw to a close. Nevertheless, rather summer-like conditions will prevail through the middle of next week, as a ridge of high pressure stations itself over the eastern portion of the country. For the weekend, the wind will switch to offshore from the recently prevailing onshore direction, allowing daytime temperatures to rise above 80°F (27°C). The wind direction on Monday is uncertain, but if it stays offshore, the temperature will be warmer than currently expected.
The rapid growth of the wildfires in Southern California early this week was primarily facilitated by the prevailing weather conditions. In particular, there was an intense northeasterly wind and the relative humidity was extremely low, both characteristics of the Santa Ana flow regime. The Santa Ana wind is a regional example of the more general phenomenon of downslope flow. As the name implies, downslope flow occurs when wind is directed down a gradient of surface elevation. In the case of the Santa Ana, the wind blows from the plateau of the Mojave desert (elevation of roughly 1,000 meters) towards the Pacific coast. As the air travels along such a path, it descends and is consequentially compressed, as it adjusts to the higher environmental pressure at lower elevation. The compression results in heating of the air, 18 degrees (10°C) for every 1000 meters of descent. While the temperature of the descending air increases, its water vapor content remains unchanged, such that the relative (to temperature) humidity decreases. Hence, the downslope Santa Ana winds are necessarily accompanied by low relative humidity, setting the stage for explosive wildfire growth.
A cold front sweeping through the region may provide the necessary trigger for some showers today, but will ultimately make way for a spectacular Labor Day weekend. Sunny skies, a comfortable late summer diurnal temperature range, and low humidity will prevail from Saturday through Monday. Such conditions are not surprising for early September, which on average is the sunniest time of year in Cambridge.
On every day this month, the mean temperature has been lower than the climatological average. This cold spell looks almost certain to continue through at least the middle of next week, as we will not get the southwesterly flow necessary for warm conditions this time of year. Instead, after a couple days of chilly northwesterly winds, we will have a potent Nor'easter to ride out. All of the long-range numerical weather prediction models develop the storm just off the mid-Atlantic coast on Sunday, and then park it somewhere off the south coast of New England on Patriot's Day. This means we will likely see a prolonged period of stiff easterly winds and heavy rain from Sunday night through Monday, with the timing of the heaviest rain and strongest winds dependent on the exact trajectory of the storm. Anyone participating in the Boston Marathon or planning to go out and cheer on the runners should keep a close eye on this late-season Nor'easter as it develops.
Fairly tranquil weather is in store for the first weekend of the spring season in Cambridge. We will be under the influence of a high pressure system today and tomorrow, keeping skies clear and temperatures seasonal. A developing low pressure system will skirt to our south on Saturday night and Sunday, potentially close enough to give us some light snow or rain showers. However, it is most likely that we will just see an increase in cloud cover.
The bitterly cold President's Day earlier this week appears to be the final gasp of the cold weather regime we had been stuck in for almost five weeks. During that period the mean temperature in Boston was 6.6°F (3.7°C) below the climatological average. This cold regime was preceded by a warm regime of just over five weeks, in which the mean temperature was 10.0°F (5.6°C) above the climatological average. Now, it appears that we are settling into a flow pattern conducive to temperature variations about the climatological average, rather than persistent warmth or cold.