FI Charts ( T120+ onwards) Winter 2020 **READ MOD NOTE IN FIRST POST**

Longing

Very disappointed after looking at the ECM and GFS was hoping the downgrades would continue but instead it has prolonged and increased the cold spell with some major events in the works. The only glimmer of hope I have is the later runs this evening. bring it on I mean ahhh westerly's and southerly's.

bazlers

Longing wrote: »

Very disappointment after looking at the ECM and GFS was hoping the downgrades would continue but instead it has prolonged and increased the cold spell with some major events in the works. The only glimmer of hope I have is the later runs this evening. bring it on I mean ahhh westerly's and southerly's.

Mind games, i like it!

TheMilkyPirate

Minor differences but cold arriving quicker on the 06z

Reckless Abandonment

6z a slight upgrade early on. That low at 102 is further south too.

sean555

Rebelbrowser wrote: »

On the chat thread (which I read first) talk is of the south potentially missing out. That's what MT is saying too but having just looked at the 0z runs I really don't see that? Looks to me like there are -8 s everywhere (save maybe the Dingle peninsula) for a good two days and in fact locally on the very south coast (which includes two of our cities obviously) the -8s arrive Sunday and stay til, what looks like a massive snow storm, arrives Wednesday night! That's what I took from the charts anyway, was over the moon when I saw them, so surprised by what I'm reading now....

And on the 06z even the Blaskets have -8 uppers.

Kermit.de.frog

6z GFS is an upgrade to 144 hrs, a tad colder and a bit more snow shower potential for first half of week.

TheMilkyPirate

Definite upgrade at +144 on the 6z. Cold still firmly entrenched on Wednesday and that low much further west and not as deep.





Compared to the 00z

There’s a definite trend for that low to head towards France or stall. . It’s not there yet though.

bazlers

Kermit.de.frog wrote: »

6z GFS is an upgrade to 144 hrs, a tad colder and a bit more snow shower potential for first half of week.

Kermit, are you not seeing the same stremer potential compared with other easterlis in the past?

TheMilkyPirate

That Atlantic ridge even more developed on the parallel at +120

TonyMaloney

The GFS is seeing a lot of ways to prolong the cold, and quite few to end it

Stevieluvsye

What in the name of blue jesus is that above?

TonyMaloney

Stevieluvsye wrote: »

What in the name of blue jesus is that above?

Each column is an ensamble member from the GFS

The values are 850hPA temps. The darker the better.

Kermit.de.frog

bazlers wrote: »

Kermit, are you not seeing the same stremer potential compared with other easterlis in the past?

Not that severe for now. Showers, yes but more moderate.

That will change either way though as we get closer.

Synoptics --> Temperature --> Precipitation

Two of the above firmly in place. We just need to see upgrading of the third going forward.

Calibos

Kermit.de.frog wrote: »

Not that severe for now. Showers, yes but more moderate.

That will change either way though as we get closer.

Synoptics --> Temperature --> Precipitation

Two of the above firmly in place. We just need to see upgrading of the third going forward.

It looks like we hit the magic 20ºc temp differential. ie -12 uppers and +8 SST's. What else is required to generate the intense Snow Streamers? Remember to explain it like I'm 5 because I'm one of the regulars that unlike others hasn't learnt a thing in the last 10 years and I still don't know WTF I'm looking at when I see a chart. :o

lucernarian

Calibos wrote: »

It looks like we hit the magic 20ºc temp differential. ie -12 uppers and +8 SST's. What else is required to generate the intense Snow Streamers? Remember to explain it like I'm 5 because I'm one of the regulars that unlike others hasn't learnt a thing in the last 10 years and I still don't know WTF I'm looking at when I see a chart. :o

-12 uppers are not really forecast though. It's -10 more like. Still should provide convection even if not thundery.

Calibos

lucernarian wrote: »

-12 uppers are not really forecast though. It's -10 more like. Still should provide convection even if not thundery.

I must be behind on the latest output or I saw a cherry picked outlier.

Also, from the other thread Someone posted charts showing South Easterlies instead of the ENE I thought we were getting which is great for the south coast as long as they get the right uppers but is a big nothing burger for large swathes of the East Coast which would be in the Wales Snow Shadow in a SE'rly.

So I wonder is it Wind Direction that needs to be nailed down before forecasting precipitation that Kermit is talking about or is it something else too? Whats this 'Disturbed Flow' that I've seen mentioned. Is that needed to actually generate the convection and precipitation or is it simply related to how spread out the streamers are by the time they hit the East Coast. ie. Smooth flow means the Convection cells slipstream each other and stay in a tight line as they cross the Sea and whoever is under the bullseye hits the jackpot but theres nothing for anyone North or South of that spot, but in a disturbed flow the cells have spread out a bit by the time they reach the east coast and more of the coast gets in on the action?

Rebelbrowser

Calibos wrote: »

It looks like we hit the magic 20ºc temp differential. ie -12 uppers and +8 SST's. What else is required to generate the intense Snow Streamers? Remember to explain it like I'm 5 because I'm one of the regulars that unlike others hasn't learnt a thing in the last 10 years and I still don't know WTF I'm looking at when I see a chart. :o

I think thats for thunder snow. For streamers / lake effect snow you just need a 13c differential with -8 uppers I think

Rebelbrowser

Actually, just for interest, this is from wikipedia...

"...Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer lake water. The lower layer of air, heated up by the lake water, picks up water vapor from the lake and rises up through the colder air above; the vapor then freezes and is deposited on the leeward (downwind) shores.[1]

The same effect also occurs over bodies of salt water, when it is termed ocean-effect or bay-effect snow. The effect is enhanced when the moving air mass is uplifted by the orographic influence of higher elevations on the downwind shores. This uplifting can produce narrow but very intense bands of precipitation, which deposit at a rate of many inches of snow each hour, often resulting in a large amount of total snowfall.

The areas affected by lake-effect snow are called snowbelts. These include areas east of the Great Lakes, the west coasts of northern Japan, the Kamchatka Peninsula in Russia, and areas near the Great Salt Lake, Black Sea, Caspian Sea, Baltic Sea, Adriatic Sea, and North Sea.

Lake-effect blizzards are the blizzard-like conditions resulting from lake-effect snow. Under certain conditions, strong winds can accompany lake-effect snows creating blizzard-like conditions; however the duration of the event is often slightly less than that required for a blizzard warning [2] in both the US and Canada.

If the air temperature is low enough to keep the precipitation frozen, it falls as lake-effect snow. If not, then it falls as lake-effect rain. For lake-effect rain or snow to form, the air moving across the lake must be significantly cooler than the surface air (which is likely to be near the temperature of the water surface). Specifically, the air temperature at an altitude where the air pressure is 850 millibars (85 kPa) (roughly 1.5 kilometers or 0.93 miles vertically) should be 13 °C (23 °F) lower than the temperature of the air at the surface. Lake-effect occurring when the air at 850 millibars (85 kPa) is much colder than the water surface can produce thundersnow, snow showers accompanied by lightning and thunder (caused by larger amounts of energy available from the increased instability)...."

typhoony

interesting change in the models overnight and this morning, the trajectory of the Atlantic Low and speed has changed giving us a real prospect of a battleground scenario. still a week away but you'd think the models should have a handle on it by tomorrow evening giving us a better picture what will happen next Wednesday evening and into Thursday

Kermit.de.frog

Calibos wrote: »

Remember to explain it like I'm 5 because I'm one of the regulars that unlike others hasn't learnt a thing in the last 10 years and I still don't know WTF I'm looking at when I see a chart. :o

To cut it down to your criteria, consider the chart above, basically the bluer the shade the greater the sea based convective potential and precipitation potential generally will be.

Nqp15hhu

For convective weather you need to look at the air pressure too. It can be blue and cold but produce no convection.

For the above, the air pressure is 1010mb-1010mb, so the 850hPa level will be raised to 1650 metres. The Lifted Index will be around +12 throughout the cold spell so not very unstable.

The precipitation over southern England on Sunday is caused by a low.

igCorcaigh

Kermit.de.frog wrote: »

To cut it down to your criteria, consider the chart above, basically the bluer the shade the greater the sea based convective potential and precipitation potential generally will be.

Oh... So the blue means potential for precipitation, not cold...?

Calibos

Rebelbrowser wrote: »

The same effect also occurs over bodies of salt water, when it is termed ocean-effect or bay-effect snow. The effect is enhanced when the moving air mass is uplifted by the orographic influence of higher elevations on the downwind shores. This uplifting can produce narrow but very intense bands of precipitation, which deposit at a rate of many inches of snow each hour, often resulting in a large amount of total snowfall.

...intense bands of precipitation on the lee side of the mountains or the upwind side??

So in terms of Bray nestled into the Wicklow and Dublin Mountains, does the orographic lift give the intense snow to the mountains...or...Kildare/Carlow. Or does it give the intensity to the upwind side of the mountains which would be Bray. Is the orographic friction slowing down the cells causing them to dump more on the upwind side??

It must be the former I assume because it feels like Bray always gets less than most areas not more even under a Streamer....or....is it the case that Bray should be in prime position in an ENE with a long Sea Track to Morecambe Bay in front of it and the Wicklow Mountains behind our backs for the Orographic lift.....but the problem is we hardly ever get an ENE wind and are instead usually in an Angelsey or IOM Shadow??

Nqp15hhu

igCorcaigh wrote: »

Oh... So the blue means potential for precipitation, not cold...?

No it refers to the Geopotential heights at 500mb. So the altitude of the 500mb height.

It also provides the 1000-500mb differences in distance. In order to get the true value, if above 1000, you need to subtract the guide i.e 1015 at 540 to 15 is 525 decametres.

In the above example, Dublin is 522 decametres. This just means that the air column is thinner and denser, with the cold air being closer to the surface.

Kermit.de.frog

igCorcaigh wrote: »

Oh... So the blue means potential for precipitation, not cold...?

It's connected to cold in the sense that the lower the heights (darker shade of blue) the lower the height at which the 850hpa isotherm applies (usually it's around 1.5km). This is through the atmospheric layers, not just at that level.

But, no, it does not represent cold. It's a better representation of instability.

typhoony

the only issue with a big Low pressure to our South is that the cirrus clouds stretch northwards and have a blocking effect on any convective showers

Nqp15hhu

The 850hPa level is higher with increased air pressure. Not lower.

Gaoth Laidir

Nqp15hhu wrote: »

For convective weather you need to look at the air pressure too. It can be blue and cold but produce no convection.

For the above, the air pressure is 1010mb-1010mb, so the 850hPa level will be raised to 1650 metres. The Lifted Index will be around +12 throughout the cold spell so not very unstable.

The precipitation over southern England on Sunday is caused by a low.

That's not true. The 850 level will not be raised at all.

For the Internation Standard Atmosphere (ISA), sea-level pressure is 1013 hPa and 850 hPa is at 1450 m amsl. This actual height varies greatly day-to-day, depending on both surface pressure but also mean airmass temperature. The colder the airmass the lower 850 hPa will be for the same surface pressure. For that time next Tuesday the GFS has 850 hPa at 1300 m in the south to 1400 m in the north.

Lifted index is meaningless in this case as convection will be limited to below 500 hPa. There will be plenty of low-topped convection (up to around 700-600 hPa) and steep low-lelvel lapse rates (around -18 °C at 700 hPa). Graupel and snow, but still some residual moisture at the very lowest levels could mean some melting at times.

pad199207

Calibos wrote: »

...intense bands of precipitation on the lee side of the mountains or the upwind side??

So in terms of Bray nestled into the Wicklow and Dublin Mountains, does the orographic lift give the intense snow to the mountains...or...Kildare/Carlow. Or does it give the intensity to the upwind side of the mountains which would be Bray. Is the orographic friction slowing down the cells causing them to dump more on the upwind side??

It must be the former I assume because it feels like Bray always gets less than most areas not more even under a Streamer....or....is it the case that Bray should be in prime position in an ENE with a long Sea Track to Morecambe Bay in front of it and the Wicklow Mountains behind our backs for the Orographic lift.....but the problem is we hardly ever get an ENE wind and are instead usually in an Angelsey or IOM Shadow??

They usually get a bump of energy over the south Dublin hills over towards Rathcoole and then dump in E Kildare going by the last two beasterlies