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JANUARY 2018 - FINAL GUIDANCE
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Issued: Friday 29th December 2017
Contact: simon@weatherweb.net

HEADLINES
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+ Stormy at times, especially first half
+ Changeable temperatures at first, large variations north to south
+ Colder second half

FACTORS AFFECTING JANAUARY 2018 PRELIMINARY GUIDANCE
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+ High pressure forecast to develop over Scandinavia, introducing easterly winds later
+ Models in disagreement, presently influenced heavily by westerly flow
+ Fine line between being milder and colder weather becoming established
+ La Nina is expected to be weak throughout
+ Water west and north of UK generally warmer than average, coler close to UK & Ire shores

UNCERTAINTIES - WHAT COULD GO WRONG?
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+ Models may have correctly established more westerly regime throughout the month
+ Warmer Atlantic could lead to stronger westerly
+ La Nina may weaken impacting teleconnections

OVERVIEW
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Since our Preliminary Guidance for January issued on 18th December models have tended to lean towards a more mobile, westerly weather pattern during the month. This would bring heavy rain, severe gales at times and very mild temperatures.
However, we do not see any reason to change our initial assessments of a westerly start to the month with unsettled weather, before changes take place in the second half of the month as conditions turn colder.
It is worth noting that there is a very fine balance between which weather regime will win through.
Higher pressure may be building to the north in that second half of the month with low pressure passing through southern areas. This increases the risk of wintry weather and some heavy rain together with localised flooding to the south.
Note the low confidence now in the forecast as so many factors are at play in determining the outcome.

BREAKDOWN
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1st to 7th
Jet stream over and to the south of the UK.
Cold air intrusions into Scotland bringing snow to higher ground.
Strong winds for all. Rain, mostly in the wes and to the south of the UK.
Temperature varying day to day but mainly cooler in Scotland, nearer average elsewhere.

8th to 14th
Ridge builds to east. Jet stream remains close or over the UK.
Staying unsettled overall.
Probably more showery with the showers wintry, even to the south coast on certain days.
Generally cool overall, possibly cold in Scotland.
Risk of strong winds at times.

15th to 21st
Indications are that the week starts unsettled.
Westerly flow dominates with the jet stream staying strong.
Further rain and strong winds, near average temperatures.
Later in the week it may be that the flow weakens as a ridge builds to the south and east.
Conditions become drier for a time.

22nd to 31st
There are indications that pressure may build to the north during this week.
The flow turns more to the south or southeast and cooler weather may arrive in the north and east.
Remaining unsettled in the west and south for a while with rain at times.
A tendency towards cooler weather for all as the month closes.
Risk of some more persistent rain or sleet and snow due to low pressure passing to the south.

PROBABILITIES
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Temperature
- Very warm 10%
- Warm 20%
- Average 35%
- Cool 20%
- Cold 15%

Rainfall
- Very Wet 15%
- Wet 25%
- Average 30%
- Dry 20%
- Very Dry 10%

CONFIDENCE
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Confidence is low to moderate overall.

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