Turning progressively colder through the week, Very cold with snow showers later

Mmmnnnn...interesting!;)

darkman2 wrote: »

The North Atlantic Oscillation is positive so that is mitigating against a proper retrogression of high pressure into Greenland.

The Polar Vortex, the excitable atmospheric conditions depicted by the deep blue and purples on the charts above, for a long cold period like we had earlier this season would have to go into Scandinavia and stay there and preferably move south then.

There is little sign yet of a big pressure rise over Greenland but we have seen this forcast change in the past much closer to the time. For the moment it looks like the much colder weather will be temporary 48hr thing. However that can change. And given recent events there is not much reason to believe it won't change. My own feeling is that the cold will last longer then currently forcast and will get deeper during the coming weekend. At least lasting into the early days of next week. That's just my own interpretation. It could pan out exactly as forcast but that's unlikely.


Keep an eye on the NAO forcast. It is still positive or neutral but is forcast to go more negative in the next few days which could have interesting consequences in the next few forcast model runs.

Interesting update Darkman 2.

A couple of points also if I may. Firstly, the NAO is actually currently in a weakly negative phase, as indicated by the charts you have posted above. This was confirmed by NOAA in their Extended Forecast Discussion yesterday. However, it is only weakly negative it has to be said, at least at the moment.

I would tend to agree with your own feeling on the longevity of any cold incursion. Or at least, maybe it is a case that a slight interlude may be reached only for blocking to return again. Each NWP Operational evolution is subsequently building stronger pressure over Greenland for the timeframe on the previous run - a clear example of this being the overnight UKMO Evolution.

Apart from the NAO & AO Signal, by far the most interesting at the moment is that of the 30mb Temperature Profile. Latest observations indicate that immediate downwelling, or penetration to the troposhere, of the warming event, is taking place. The general signature of such warming at the tropospheric level is for largescale blocking to develop.

Overnight multi-model guidance is certainly in favour of a rather potent variable Northerly flow during the second half of the coming week. Again, there is much emphasis upon a semi-retrogressive block with at least some real height rises over Greenland, associated with this HP Block centred in the High Northwest Atlantic. Forcing, generated by the segment of the Polar Vortex dropping into Scandinavia will aid in the Westward progression of this block.. Longer term signals are very mixed, with no clear pattern emerging. It is quite possible that we will remain in a Polar Maritime airflow, after a Polar flow later in the coming week. There is even no general trend beyond Day 7 at the moment, however most Guidance overnight models a gradual collapse of this Block, by the end of the week.

If the overnight 00Z Guidance from ECMWF & UKMO were to verify, then we would be looking at rather widespread and relatively severe frosts during the second half of the coming week, with the threat of sleet & snow showers mainly across Northern districts and in coastal areas, but also into Eastern areas if the general evolution were to occur as modelled on the overnight UKMO 00Z Evolution. A certain amount of cold troughing is likely across the continent in the latter part of the week, with deep cold air in place.

A cold incursion does look increasingly likely during the second half of the week, but is by no means a certainty at this time.

The 30mb Temperature is now quite some way above average, still rising almost vertically, at about an angle of 87 Degrees overall. If we are to see another 5 - 7 degress of warming, we could be very optimisitc for at least some influences at the tropospheric level later into February.



SA

Morning everyone,

Models tend to over-react & be unstable when we are dealing with marked developments at the 30mb Level.. Even on the ECMWF Operational evolution last night, there was a clear strong area of positive height anomolies centred to the far Northeast in the form of a Siberian High. Over the coming 5 Days, we should begin to a see a gradual area of positive height anomolies forming over the Pole, intereacting to form a largescale blocking High to the Northeast.

Subsequently, longwave pattern developments also support the notion of a strong area of positive height anomolies centred to the variable North, Northwest & Northeast.

The initial northerly flow later this week into the weekend, is in my opinion, a direct response to the immediate warming at the 30mb level, the downwelling from which is already in evidence:

Zonal Wind Anomoly Time Series



The 30mb Temperature has warmed significantly in the past 5 Days, as is clear from the latest 30mb Temperature profile chart. This is indeed a very significant warming & we can have confidence that at least some largescale blocking will develop as a by-product from this warming event. Further warming is indicated over the next 7 - 10 Days after a brief lull in the level of significant warming.



The exact position of any largescale blocking will be uncertain but I think that overall, the background signals used some 4 Days ago predicted the coming Northerly flow fairly well. It will be turning increasingly colder later in this week, with the threat of wintry showers at times, especially in exposed Northern and Northwestern areas. The main feature will be harsh, to perhaps severe frosts developing at night time, with minima of -5 possible in several areas.

Further ahead, overnight 00Z Guidance is exceptionally positive, indicating Northward Warm Air Advection, with a further Easterly/Northeasterly flow developing in the long term.

After the Northerly flow later this week, another Polar Maritime Northwesterly flow should then follow, followed by a gradual rise in pressure with conditions remaining cold rather than mild overall. There is the potential in the 10 Day timeframe for another renewed incursion, however that is some distance out yet.

ECMWF 00Z - T+240



CPC Medium Term 500mb Prognostic Chart

http://www.cpc.noaa.gov/products/predictions/814day/814day.03.gif

An interesting period of weather ahead, with lots of potential & a solid major warming has taken place at the 30mb level. This is a positive indeed, but does not, as always, 100% Guarantee a cold outbreak further ahead, but it does increase the chances markedly. The location of any subsequent largescale blocking will also be uncertain.

SA