#Reigate fresh January cold snap courtesy of Arctic outbreak

January 5, 2016 — Leave a comment
2016-01-05_20-38-55

GEFS cool dip mid January

A cold snap looks more likely next week from around 13 January. Nothing extreme, just a long-overdue “normal” wintry feel is on the cards.

2016-01-07_07-06-57

12Z gefs and ecm ensembles show more distinct dip in temps

 

2016-01-05_20-52-09

ECMWF upper air goes cold

Temperatures are due to take a dip below the seasonal norm.  It’s still a way off so details will change but here’s the current idea moving ahead.

2016-01-05_21-19-27

The current run of wet Atlantic westerlies (above) that has brought flooding to many parts of the country, is due to weaken as pressure rises over the Atlantic and further north over the Pole.   As the persistent Atlantic LOW pressure gradually moves East this week it will bring more rain across the UK.  As it moves further east over the weekend it is forecast to draw down cooler northerly winds from the Arctic next week, at least for a while. (see below). (update: “Atlantic block” noted on chart below is probably overstating it a bit … as HIGH is likely to give way fairly promptly)

2016-01-05_21-20-37

2016-01-05_21-04-00

Arctic Oscillation goes negative january 2016

The cause of this Arctic outbreak is indicated by the Arctic Oscillation (AO) going negative.  The AO is a measure of air pressure over the Pole relative to mid-latitudes.  It has been positive for most of the autumn and winter so far and this usually means a strong jetstream and mild wet westerlies for the UK.

When the AO goes negative it indicates building pressure over the Pole and more likelihood of Arctic air “leaking” into mid-latitudes.  The chart below shows the 500mb mean heights for 8-10 days time.  Spot the anomalously high pressure over the Pole relative to the mid-latitudes.

2016-01-05_20-44-49

8-10 day mean 500mb heights (ecm and gfs models)

The negative AO has been predicted by many expert long range forecasters for a long time partly because of a lack of sea ice in the Kara Sea (part of the Arctic Ocean near Siberia).  This is a long-term indicator for potential pressure rises in this region.

2016-01-05_21-50-05

In addition, the recent enormous pump of warm air, courtesy of the Storm Frank, will have encouraged tropospheric height rises over the Pole.  The result is an inflating balloon of relatively cold air waiting to pop into the populated mid-latitudes!

2016-01-05_20-59-38

surface temperature anomalies Jan 14 2016

As well as a push of cooler Arctic air, pressure is also due to rise over the UK.  With increased pressure we can thankfully expect a drier period.

2016-01-05_20-42-36

wintry pressure rise

A classic winter high might be expected to bring dry, bright clear days with cold nights with views of the stars.  On the other hand, frost, fog and icy conditions might also be expected too.  In the SE huge dumps of snow look most unlikely next week from current model runs but lows can move south in the Arctic flow and cause unexpected events. The duration of the cold snap doesn’t look long** as the Atlantic HIGH is swept away by more westerlies.  However, this is uncertain so stay tuned for more regular updates on twitter.  Of course, seek professional forecasts for decision making purposes.

update 06/01 **looking potentially more prolonged now. cold snap turning into a spell.2016-01-05_20-55-31

January 2016 cold snap

2015-16 winter forecasts have long considered the possibility of cold weather in the latter stages from Jan thru to feb. This was explored in a post here

https://rgsweather.com/2015/12/29/winter-is-nigh/

No Comments

Be the first to start the conversation!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s