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FI CHARTS AUTUMN/WINTER 2014/15 **Mod Note Post #1**
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Glad some got snow (you must be in the west?)My kids got to build a snowman this year, first time in their lives we had enough snow for it, glad you got it so wrong!
Also winter 13-14 gave us a storm factory which we have not seen at all this year.
No, I'm not one of these "if-it-doesn't-snow-in-Dublin-its-not-a-real-winter" types.
I didn't get it "so" wrong actually, this winter is completely dominated by the Azores and it was also a major player last winter.
http://www.wetterzentrale.de/archive/2014/brack/bracka20140123.gif
By the way I usually say in my posts - "I hope I'm completely wrong" and we may get something decent in Feb other than topplers by the look of the charts, all still in FI of course. Signs of the Azores ridging to Greenland at long last hopefully?
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icesnowfrost wrote: »this day week looking nice and the build up to it on the GFS
and 2 days later still good. gfs. GOOD NORTHERLY
GEM sees it to
Look at the thickness's on those though(especially gfs,as the Gem isn't really good for Ireland much as a model) the coldest air is off over Britain and we are too near high pressure for much precip
Certainly I'd expect none in Leinster/Munster
That low over the north sea would need to go due south but instead it's heading SE dragging most of the useful cold air with it
It's also c0ck blocking a north easterly if it does that So honestly Not a good northerly, too much maratime mix around Ireland, with hill snow mostly where you do get showers
Plenty of possibilities/potential though for mother nature to tweak the finished product this day week and early Feb towards a more western cold flow
There's plenty potential coming up for more snow similar to two weeks ago in selected western areas especially northern facing parts and obviously west central Ulster again
We are in a ground hog day mood in weather terms0 -
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I think I see where you are coming from however the jet stream is buckled, weak and amplified over the Atlantic at 228 on the ECM and thus the shallow trough just southeast of Greenland is another so called trigger low because the orientation of the jet stream is broadly north/south (as opposed to east/west which is needed to collapse the mid Atlantic ridge). Indeed if you look off the east coast of the US and Canada you can see the heights being pushed northward again building toward GL. This is best seen on the pressure chart for the 12z at 240 hrs as opposed to 228 (which still incidentally continues the northerly flow over Europe).
But it is even more noticeable when you consider the air mass this developing "sliding low" in lay mans terms is both coming up against and will propagate west again once it clears south (classic snow chart for Ireland btw). The 528 dam line straddles the northeast of the country - this too will come west on the northern flank of this secondary depression.
That is going north/south which means the jet is north/south which means it's a reload basically. There is no indication the jet stream is firing up on an east/west axis there - at least not at that point.
Academic anyway as it is in cuckoo land but that's the broad trust of how I read that particular scenario. DAM lines are not altogether that meaningful in a synoptic sense.0 -
At this range, 500 hPa maps are better indicators, as the surface pressure will follow these. It's notable how the 12Z shores up the PV, which is not a good sign if it continues.
ECM 850 hPa temp anomalies for the next 10 days are on the cool side though...
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Someone with more knowledge on criteria for snowfall;
850's on the 12z are actually pretty mediocre tbh. GEM for example, uppers never go below -4/-5 in the main interest period (wednesday/thurs). Is this being overhyped due to people jumping on appearance of the charts?? Or are the other parameters much more favourable?
And regards shower potential, surely instability won't be that bad with only -5/-6/-7 850's moving over the sea, won't you want a bigger gap between 850's and sea temps to drive the big beefy showers??
It's a lot more than just 850 hPa temperatures. The height of this pressure level is important too. This week's snow will come with a 850 hPa level around 1250 m, a lot lower than the more normal 1400-1500 m, therefore we can get away with an extra degree or two of warming.
Instability is important, and it needs to be fairly deep (with saturated levels at -10 °C or lower to allow ice crystals to form. Sea-effect showers form if the 850 hPa temperature is 13 degrees or more colder than the sea surface, therefore this will not be a problem.
Just to note that the warm seclusion of the low on Tuesday will back down during Thursday, bring a marked rise in freezing levels and a more mixed situation.
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And another word of caution; the GFS - cold and all as it is - has 950 hPa levels at around 400 m and around 0 °C while 975 hPa are at around 210 m and about +3 °C. That is a strong sign of modification, despite the 50 knot winds. Surface cooling may minimise that warm layer, but it is something to watch.
Blue= -5 - 0 °C, First green= 0 - 5 °C
950 hPa (18Z Wed)
975 hPa (18Z Wed)
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Kermit.de.frog wrote: »I referred to the heights in terms of the dynamic of the situation shown on the chart - the trough southeast of GL, how it is a secondary depression or a trigger low and the height rises behind. That chart like every chart is only a snapshot so a bit of intuition is needed. It's hard to explain, I try to run it forward in my own mind based on the other variables we actually know - the jet stream in this case is the most important. We don't have actual jet stream analysis from the ECM model but we do know it's weak and we know it's north/south because that is the direction the center of the secondary low takes. There may be a firing up of the jet stream coming out of north America pointing at us like a big vindictive finger but that's not as easy to discern from just pressure and 500 hPa charts.
All i'm saying is that feature prolongs the cold snap that little bit more. How long? I don't know - could be one, could be three days. It's impossible to know.
That's my opinion of the situation presented. It's only speculation on my part. Again though as you know it's all academic because that won't be the actual situation anyway.
Of course one snapshot is nothing to go by, I was just posting it to make a point. It all about the patterns in motion, which can sometimes get lost with people.
This is the ECM 300 hPa wind, showing a strengthening zonal jet out of the eastern US to New Foundland.
The southerly tip towards Greenland and the associated ridge advect too far eastwards beyond 240. See the animation here.0 -
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