Cold snap

Another bout of precipitation is forecast for the south coast Friday and Saturday. Then things turn cold.

Over the past week a broad ridge has established itself over the eastern Pacific. Meanwhile a deep arctic vortex has been anchored over the Hudson Bay area, setting up a northwesterly flow in the upper levels across BC.



 The above image is a time-longitude chart from 1 December to 12 February of this year. The field plotted is the 300 hPa height averaged from 46-53N and the longitudinal band covers east Asia to central Canada. Notice the very strong ridge/trough dipole highlighted for the last week. One can see the position of the ridge retrogressed (as forecast last week) to lie over the eastern Pacific. These charts offer a good picture of the past and present state of the atmosphere but as a prognostic tool it needs to be viewed in conjunction with numerical weather prediction models.


A disturbance brewing over the Gulf of Alaska has managed to disturb the ridge. This system will make its way across the south coast Saturday and head into Washington Sunday. In its wake the ridge will rebound with more authority and create a cold northerly flow over the province.

There is some uncertainty with regard to how long we feel the effects of the pacific system. Forecast guidance suggests that it will intensify as it carves out a path across southern BC. Under these scenarios, disturbances tend to hang around just a little longer than expected.



 Update - This is forecast issued Thursday and valid Sunday shows a sharp trough over southern BC. The models have since indicated that the trough will be slightly weaker and shall move out of the area on Sunday.

In any event, the end of the weekend will see dramatically colder conditions pervade BC. Coldest conditions will likely occur Monday before temperatures begin to moderate by mid week.


The above image indicates the probability of the minimum temperature falling below -5C over the next week. The highest probability is shown to be on Monday.

The bottom line – rain with wet snow over higher terrain is forecast as the Gulf of Alaska system crosses the south coast Saturday. Precipitation may linger into the morning hours of Sunday before we see a colder drier airmass envelop the region.


Comments

  1. David JonesFebruary 16, 2018 at 10:13 AM

    Hey Big Weather, I like this post. Coupla suggestions:
    1) Put Vancouver on the Hovmuller
    2) Make the font on the images bigger and more readable...try white or black or either with the other as an outline or a veil behind.
    3) Put the Day of the Week on the 500mb image

    cheers

    THE AUDIENCE

    ReplyDelete

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