Pleasant May Weather

MODIS Imagery of the Southeastern US. Fair weather here is set to head eastward towards Bermuda into the weekend.

After yesterday morning’s cold front with associated strong thunderstorms that left over an inch (and a half – in some places!) of rain across Bermuda, high pressure is building in from the west. As the front pulled away and high pressure began to build last night and today, a stiff northerly breeze kicked in blowing in cooler (below normal), less humid air from the Maritime Canadian Provinces and New England. As high pressure settles in, winds should shift to the west and south west allowing temperatures to return to near normal levels.

As a characteristic of high pressure, this system should bring fair and settled weather to the island while in control. Expect this area of high pressure to stay in control through Saturday when a front approaches from the north and northwest. At this time, it is uncertain whether this front will push through Bermuda on Sunday, but it appears that the front could be close enough, and the area of high pressure weakens enough by then for there to be a chance for some showers.

An interesting trough in the Western Atlantic

A disorganized area of troughiness has brought inclement and unsettled weather to Bermuda for the past few days. Today, however, it appears to have split from the dying low pressure system that dragged it into the western Atlantic, leaving behind a distinct trough north of Puerto Rico.

Although very slim, there is a chance for this trough to develop into a sub-tropical cyclone in the next day or so. It is surrounded by dry mid-upper level air, but it is tucked underneath an upper level low and so is relatively safe from high wind shear for now. This set up is characteristic of these precursor sub-tropical systems. This system has a lot going against its development, but is interesting enough to keep an eye on. This should serve as a reminder that the 2013 Atlantic Hurricane Season is just around the corner and early preparation should commence soon.

For now, this feature is nearly stationary. But in the next day or so, it should begin a slow westward to northwestward drift – a track that doesn’t threaten any land areas in the near future.

Severe April Extra-tropical Storm

A cold front’s approach was slowed on Saturday, April 6th as a wave of low pressure began to form along it. a section of the front kinked leaving an east-west oriented segment to the north of Bermuda, and a north-south oriented segment to the west (As seen on Bermuda Weather Service Radar Imagery to the left). Both segments approached the island acting as a cold front. The east-west oriented segment lingered to the near north of the island for hours keeping southerly winds, temperatures in the low 70s, overcast skies, and some mist and drizzle. That same segment dipped southwards and crossed Bermuda. Winds shifted to the east/east-northeast and temperatures fell into the mid-60s.

Behind that section of the front, light rain continued until after mid-night when the north-south oriented segment of the cold front passed to the near south of the island. However, instead of shifting winds to the west-northwest as cold fronts with that orientation do, winds shifted to the south – a clear indication of the flow around the deepening low pressure approaching from the west-southwest. The center of this low passed very near or over the island and light winds shifted from a southerly to a northerly direction. As light rain associated with the occluded front moved in (pictured to the right as seen on Bermuda Weather Service Radar Imagery at 4:53am April 7th 2013), the winds quickly ramped up to gale force with storm to hurricane force wind gusts. This resulted in some tree damage and led to power outages.

Winds quickly fell below gale force as the low passed to the northeast and weakened on the 7th. A map with peak gusts from around the island according to ASOS from the Bermuda Weather Service:

 

End of March Cold

The end of March saw a pattern in which a massive area of low pressure took over the flow across the North Atlantic, extending from Newfoundland and Labrador all the way across into Western Europe.  The westerly and northwesterly flow that this low pressure system induced to its southwest in the western Atlantic brought a very unseasonably cold  air mass into the Eastern United States and the Western Atlantic. Indeed, a series of troughs that rotated around the main large area of low pressure centered near Iceland brought near gales and showers to Bermuda. Some of these showers were intense enough to produce small hail for Good Friday.

Although the cold commenced almost a week earlier, the 28th and 29th of March saw record low temperatures at 53.6F and 52.5F, just below the daily record lows for both dates which were 54F set in 1951 and 53F in 1959 respectively*. This impressive cold snap follows a slightly below normal January and February, and made March 2013 substantially below normal in terms of temperatures. A stormy pattern before the cold snap associated with the frequent showers and record lows pushed March to being wetter than normal by over an inch of rain – good news for agricultural and hydrological purposes, keeping us out of short term drought for another month.

* These records aren’t updated with data from 2000-2012.

2 Rounds of Severe Thunderstorms

The first full day of spring brought with it an approaching storm system. The associated cold front developed into a severe squall line with a few discrete supercell thunderstorms out ahead. While the super cell thunderstorms missed the island, the squall line crossed the island around noon on the 21st bringing isolated gusts approaching 70mph and much needed heavy rains. 65mph in Commissioner’s Point, 63mph at Fort Prospect, 68mph at St. David’s Lighthouse, and 59mph at the airport.

As the cold front continued off to the east of Bermuda, there was a marked cool down with a brief period of westerly gales and post frontal showers that remained on the light side. A part of this front stalled to the south of the island as another system approached.

That same front lifted northwards as a warm front a few days later and easterly winds ensued in advance of this warm front. Patchy generally light to moderate rains changed to steady moderate rain as the warm front crossed the island. Winds shifted from their easterly direction to a southerly/southwesterly direction as the warm front passed. Stalling to our near north, rain continued with the warm front despite its passage. These warm advecting southerly winds increased to gale force as a trough approached from the west and temperatures increased to near 70F. Showers and thunderstorms associated with the trough began before midnight and brought further isolated gusts that surpassed severe limits. 66mph at Commissioner’s Point and 63mph at Fort Prospect.

The island is currently is an expansive cold sector behind this system and has been effected by a series of weak troughs that are keeping a strong westerly flow with widely scattered showers in the picture and temperatures 5-8 degrees below average. These conditions are set to continue through Easter. Combined, these two systems have brought over two inches of rain to Bermuda. Although these recent rains have brought the year-to-date precipitation to near normal levels, long term hydrological impacts from previous dry years will continue.

Gale Complex

A squally cold front (BWS radar image to the left) crossed the island in the wee hours of Thursday morning bringing gusts to 54mph to the airport and about a third of an inch of still much needed rain.  The front ushered in a very unsettled airmass with scattered, mainly light, showers and persistent gales.

The storm responsible for this weather is the same storm that brought Chicago’s heaviest snow of the season thus far, and at one point threatened to dump heavy snow on Washington, DC. Temperatures in the latter region remained safely above freezing as the snow fell, so snow totals didn’t quite reach their potential. Currently, a band of mainly moderate snow with some heavier patches lingers over New England.  This band is a ‘troughy’ extension to the winter storm’s low center that is stalled north of Bermuda.

As this storm (NOAA GOES recent satellite image to the right) interacts with an upper atmospheric short wave, this ‘troughy’ extension will try to wrap around the west side of the surface low’s center and head towards Bermuda. So after gales dropped to near gale status for a while this afternoon, they should pick back up sharply Saturday morning as the trough axis passes. This should come with a slight wind shift, from the present westerly gales, to a northwesterly flow. Additionally, the upper atmospheric disturbance carries with it colder air aloft and so an increase in shower and potentially thunderstorm activity should be expected – hail is a definite possibility with these showers. This, however, should be the storm’s last breath for Bermuda and winds will begin to wind down Saturday night, and should be below gale force by midday Sunday.

Heavy February Rainfall

Bermuda Weather Service Radar showing moderate to heavy rain stalled over Bermuda. Friday, February 15th, 2013 12:13pm local time.

In what was perhaps the wettest single February day on record, 3.29″ of rain fell at the Bermuda Weather Service office at the airport on Friday the 15th of February. Certainly in the last decade, no single day’s precipitation totalled more than 3″ in February.

Rainfall totals of this magnitude are typically reserved for Spring, Summer, and Autumn when deep tropical maritime air masses interact with slow moving fronts or tropical cyclones pass nearby. Another unusual characteristic of this event was that it largely occurred along a slow moving warm front so rainfall rates weren’t that extreme for a prolonged period of time. Despite this, there were some bursts that did generally reach 2-4″/hr.

Few reports of flooding came in. They were limited to ponding on the roadways in low lying areas. The bulk of the heavy rain fell as tides were receding which may have helped in keeping flooding to a minimum. Of course, the tides combined with the relative drought – at this point Bermuda was in a year to date deficit of about 4.5″ (an entire month’s worth). This departure from normal was almost entirely erased by this rainfall event. Although we are no longer in a short term meteorological drought, longer term hydrological and agricultural implications may still exist from the drought of the last few years.  However, this continues the trend of the bulk of the precipitation falling all at once. This is something that I will look into in further detail in a future post.

My Personal Weather Station observed 3.10″ of rain. Similar rainfall totals likely occurred across the island – this event was more regional than local because of its origins along a slow moving front, and the precipitation being derived more from regional scale mechanical lifting along that front, than convective lifting which can be more chaotic.