Early Morning Front, Heavy Rain Expected Monday


westatlantic-radar-2015-01-25-0523
Bermuda Weather Service Radar imagery at 5:23am this morning as a line of heavy showers along a cold front crossed the island.

An active cold front crossed the island in the pre-dawn hours this morning. Near-gale to gale force south-southwesterly winds with gusts near storm force in exposed/elevated areas dropped off and shifted to the west-northwest as the front crossed. Peak gusts at the airport were around 37kts (43mph), Hinson’s Island 47kts (54mph) and St. David’s Lighthouse measured a peak gust near 58kts (67mph). However, the embedded heavier showers along the front did not produce any severe weather on island – these storm force winds were in the warm humid flow ahead of the front. Heavy rain along the front totaled between 0.33 and 0.67″ in about half an hour as the front passed.

PWS Meteogram Jan 24-25
Measurements from my PWS as the front passed. Top row – Temperature in red, Dew Point in green, and relative humidity in cyan. 2nd from the top – 1-minute sustained wind in light blue, wind gust in dark blue, and wind direction in pink. 2nd from bottom – temperature in red, other lines are apparent temperatures. Bottom row – Pressure in grey, rainfall rate in dark blue, and rain accumulation in light blue.

 Conditions at my Personal Weather Station above show South-Southwesterly winds (peak gust 37mph) shifting sharply to west-northwest at near 5:15am along with heavy rain (rainfall rate peaking near 5.01″/hr), pressure bottoming out near 1006mb, temperatures dropping from 71ºF to 65ºF in the heavy rain, and dew points dropping through the day today starting at 68ºF, dropping sharply to 64ºF along the front, and gradually dropping from there to near 55ºF.  Interesting points – temperature rose through the night (along with the dew point) indicative of warm and humid air being blown in on those south-southwesterly winds ahead of the front in the “warm sector”, and the falling dew point through the day is related to the cooler, drier air being blown in on west-northwesterly winds behind the front in the “cold sector”.

Jan 26-27 Storm GFS 12z Jan 25
12z GFS run from Jan 25. Image on the left is from early Monday evening, and early Tuesday morning. Cold front is in blue, warm front in red, and trough in purple. Winds are shaded in knots with white streamlines. Model output visualization from Wunderground’s Wundermap.

Monday, west-northwesterly winds die off through the morning as a transient cell of high pressure approaches from the west and passes south of the island. Winds then increase out of the southeast Monday afternoon as the high moves away to the east. A rapidly intensifying area of low pressure lifts a trough towards Bermuda from the south, and pushes a cold front toward the island from the west. The trough brings scattered rain or showers to Bermuda early Monday evening and strong winds veer from southeast to south-southwest as it lifts from south to north across the island. Winds then increase from strong to near gale/gale force out of the southwest Monday evening with widespread scattered showers or thunderstorms ahead of the cold front.

Very heavy rain is expected Monday evening and night between the trough and cold front with rainfall totals of 1-3 inches possible. This could lead to isolated flooding in low-lying and poor drainage areas, especially at high tide and in heavier showers. The cold front then passes in the pre-dawn hours of Tuesday morning from west to east. Near gale/gale force south-southwesterly winds drop off to strong and veer to the west-southwest behind the front. West-southwest or southwesterly winds behind the cold front then increase briefly to near gale force on Tuesday.

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