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12Z Analysis for 11-15 June 2008


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Posted
  • Location: Lincoln, Lincolnshire
  • Weather Preferences: Sunshine, convective precipitation, snow, thunderstorms, "episodic" months.
  • Location: Lincoln, Lincolnshire

    Inspired by some posts from John Holmes and Paul B in the model discussion thread, and for a bit of light relief, I thought I'd do a brief daily update on how things are progressing with the upcoming northerly. The main reason is by analogy with winter northerlies- if the northerly came off as progged today, we'd be in for a prolonged cold spell with many snow showers in the north and east, but in practice, often the northerly gets shortened to a 36-48 hour event and shortwaves cause pools of warmer air to coincide with troughs and cut off the coldest air. Furthermore, what the models show today reminds me strongly of projections for various winter northerlies that got downgraded nearer the time.

    Will the equivalent thing happen for the current progged northerly?

    I will only consider the three main models (UKMO, ECMWF, GFS) and will increasingly be restricted to just using GFS once the northerly gets significantly inside T+72, but it's no different for any winter northerly.

    8 June 2008- T+72 through T+168

    GFS:

    Northerly flow sets in on 11 June accompanied by strong, 1025-1030mb block from the mid Atlantic up to Greenland. 11-12 June has a weak northerly flow with pressure of 1015-1020mb over Britain and 850s of around 0C in the south, -2C in the north.

    13 June has a weak shortwave develop south of Greenland, with embedded pool of >5C 850hPa air, but this comes to nothing, while Britain is entirely covered by <0C 850hPa air. Low pressure winds itself up over Scandinavia bringing more of a cyclonic bias.

    14-15 June has a toppling low develop over Greenland and head south-eastwards to Iceland, but in the meantime Britain stays in a slackening northerly flow with low pressure out in the North Sea and 850hPa temperatures slightly less cold, at around 0C for all parts.

    ECMWF:

    Much the same as GFS for 11 June, northerly flow getting underway, high pressure close by. However, it develops a shortwave south of Greenland on 12 June, and this shortwave temporarily cuts off the polar flow on 13 June, giving a slightly less cold NW'ly (this is distinctly familiar!)

    Low pressure then sinks south to the east of Britain on 14 and 15 June re-introducing the cold polar flow, with more of a cyclonic bias and plenty of showers.

    UKMO:

    Block holds strong to the W and NW throughout the run with no shortwaves. Northerly flow sets in from 11 June, becoming increasingly cyclonic in nature on 12 and 13 June as low pressure extends SW from Scandinavia, while Britain stays in the polar flow. 14 June has a weak low over NE Scotland and a cyclonic NW'ly regime.

    This isn't the start of regular 12Z model updates but I thought, for my own amusement as much as anyone else's, that I wanted to track the upcoming northerly.

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    Posted
  • Location: Norton, Stockton-on-Tees
  • Weather Preferences: Snow and cold in winter, warm and sunny in summer
  • Location: Norton, Stockton-on-Tees

    Very good TWS, but you know exactly what will happen - as determined by Murphy's Law.....

    This northerly, because it is happening in summer, will be upgraded and will become more potent and long-lasting!!

    ;)

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    Posted
  • Location: Lincoln, Lincolnshire
  • Weather Preferences: Sunshine, convective precipitation, snow, thunderstorms, "episodic" months.
  • Location: Lincoln, Lincolnshire

    Well, no significant evidence of downgrades on today's GFS, but ECMWF and UKMO tone down the northerly:

    9 June- T+48 through T+144

    GFS:

    The onset of the cold northerlies is delayed by about 12 hours, with the northerly airflow sweeping south at 12Z on 11 June- yesterday it was shown as already established across central and northern Britain by then. Greenland/Atlantic block at 1030-1035mb.

    12 and 13 June have a northerly flow with 850hPa temps ranging from -2C in the north to just above 0C in the south. Pressure is around 1020mb on the 12th, but falls on the 13th as a trough comes out of Scandinavia and heads south towards Britain, pepping up shower activity.

    14 and 15 June have a low slowly filling out in the North Sea maintaining a cold, showery cyclonic northerly flow. The "toppling" low again develops over Greenland on the 14th but only moves slowly south on the 15th, reaching a position just north of Iceland, and not influencing the British Isles.

    If this was winter, the run would probably be neither an upgrade nor a downgrade- a little bit of both, but generally much the same.

    ECMWF:

    The northerly is already well established on 12 June, with pressure of 1015-1020mb and a strong Atlantic block.

    13 and 14 June have no shortwaves coming out of Greenland/Iceland (unlike yesterday's run), but also have the Scandinavian Low somewhat further east, only just influencing Britain- although we keep the cold airflow, we stay in relatively high pressure at 1015-1020mb, thus implying limited shower activity. On 15 June, Scotland keeps a slack northerly flow and some showers, but England & Wales see rain sweep east from the SW as a southerly tracking low moves in.

    I reckon that if this was winter this would be seen as a slight downgrade on yesterday's ECMWF as the northerly flow is shortened to just four days' worth as opposed to yesterday's week-long event, and while there are no shortwaves, the proximity of high pressure suggests limited precipitation.

    UKMO:

    Northerly flow sweeps south on 11 June, becoming fully established on 12 June with pressure around 1020mb.

    13 and 14 June see high pressure building from the west, with the Scandinavian Low not really coming to much, though the high is always slack, so at this time of year it means we could still have showers, but in winter, it would mean clear and sunny everywhere.

    15 June's output looks distinctly implausible with a huge, shallow low pressure area moving south across the British Isles introducing a slack NE'ly to the north.

    Again, this looks like it would be classed as a downgrade if it happened in winter, due to higher pressure, and the northerly flow being cut off already by 14 June limiting it to a 2-3 day northerly, though it tentatively tries to return on 15 June.

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    tks Ian, interesting that you rate the output, overall, as a slight downgrade, re how it would be viewed in the winter. Especially noting that the time its predicted to onset and last is shortening.

    I do hope some of you remember this when the nashing and wailing occurs next winter -'it never does this in summer' type comment!

    book mark it for reference I would suggest.

    On a more serious note I wonder why GFS and, it seems, the other 2 models, have a tendency to do this?

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    Posted
  • Location: Lincoln, Lincolnshire
  • Weather Preferences: Sunshine, convective precipitation, snow, thunderstorms, "episodic" months.
  • Location: Lincoln, Lincolnshire

    As an aside, I'm noting that the 850hPa average temperatures at this time of year range from around 4C in northeast Scotland to 7C in southern England. Thus, GFS projections are about 6C below the average.

    It's difficult to do a direct 850hPa comparison with an equivalent winter northerly because of the Britain-Arctic temperature differential being much greater in the winter half-year, complicated further by greater airmass modification over the north Atlantic. However, my best guess would be 850hPa values about 8C lower, this would translate to -8C 850hPa over most of the country based on the GFS, with the -10 isotherm periodically flirting with northern Scotland. I have to say that, comparing with past June northerlies, the projected 850hPa values are far from being exceptional for the time of year.

    Re John's question above, I often point to underestimation of Icelandic cyclogenesis etc. as a key factor in the handling of winter northerlies, but I have no idea why the models may also behave similarly for summer northerlies. However, shortwaves are certainly one thing that models tend to be poor at showing at long range- I remember countless occasions in the 1990s when I looked forward to a spell of convective sunshine and showers, only for a shortwave to turn up and bring cloudier, drier weather with a spell of steady rain from the main frontal system.

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    Posted
  • Location: Lincoln, Lincolnshire
  • Weather Preferences: Sunshine, convective precipitation, snow, thunderstorms, "episodic" months.
  • Location: Lincoln, Lincolnshire

    10 June- T+24 through T+120

    Surprisingly, a general upgrade to the northerly on the UKMO and ECM today, falling into line with what the GFS was showing.

    GFS:

    Northerly setting in on 11 June, again sweeping south during the day like shown yesterday. 12 and 13 June have a moderate northerly with 850hPa values of 0 to -2 in the north, 0 to +2 in the south, and limited shower activity, probable downgrade in snow-equivalent terms for the 13th as the Scandinavian Low takes longer to move in.

    14 and 15 June have a weak low settling over the North Sea bringing a cyclonic/northerly regime and plenty of showers. If this was winter, we'd probably have a fair number of snow showers for northern and eastern areas, but mainly dry in the west and south. 850s remain close to 0.

    ECMWF:

    11-12 June are unavailable. 13 June has a slack northerly flow with the Scandinavian Low only just approaching the NE, but 14 and 15 June see the low extend its influence further west and south. More of a NW'ly develops on the 15th, so in winter, that would mean snow showers developing more widely in the west and south, and becoming fewer in the east. The Atlantic low that was progged on yesterday's run for 14/15 June is still there, but further south, so it doesn't affect Britain's weather. Thus, if it was winter, this would be a significant upgrade.

    UKMO:

    Much the same as ECMWF. 13 June has a slack northerly flow, then the low pressure settles in the North Sea on 14 and 15 June, though UKMO has the low much slacker than ECM on 15 June, so fewer showers. An upgrade on yesterday's UKMO.

    In summary, this northerly looks to be shaping up in much the way that the March 2006 northerly did, and looks like it may well continue GFS's excellent track record with northerlies since around mid-March, contrasting with its distinctly poor record earlier in the year.

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    Posted
  • Location: Rochester, Kent
  • Location: Rochester, Kent
    On a more serious note I wonder why GFS and, it seems, the other 2 models, have a tendency to do this?

    It's likely to be an assumption that all initial conditions are equally important at the offset which magnifies an effect downstream. The way to mitigate this effect is, if you can, discern from the ensembles - I suspect at least one of them is very close to the truth.

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    ah but how does one pick the right one?

    remembering that hindsight is not available at the time one needs to do it!

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    Posted
  • Location: Lincoln, Lincolnshire
  • Weather Preferences: Sunshine, convective precipitation, snow, thunderstorms, "episodic" months.
  • Location: Lincoln, Lincolnshire

    Well, it seems that the models haven't done the downgrade I expected, they are sticking with a typical "spring northerly" type incursion, and hats off to GFS for keeping consistent with the same pattern while ECM/UKMO chopped and changed.

    10 June- T+0 through T+120

    GFS:

    Today was progged pretty well. A moderate northerly is progged for 12 and 13 June with 850's of 0 to -2C in the north, 0 to +2C in the south. Shower activity is limited but on the 13th, signs of lowering pressure from the NE.

    For 14 and 15 June low pressure travels slowly south through the North Sea, bringing a continuation of the northerly flow. We see the re-introduction of the low developing over Greenland and moving south to Iceland on 14 and 15 June, but not affecting Britain until after 15 June. 15 June's 850s, however, are downgraded slightly with 0 to +2 in the north and +2 to +4 in the south.

    ECMWF:

    Also continuing in the same vein for 14 and 15 June, slackening low moving slowly south to the north-east giving a cyclonic, NW or N flow.

    UKMO:

    Again similar. The charts for 13 June have a low moving slowly south out of Scandinavia, settling to the E/NE of Britain on 14 and 15 June but with a slackening N flow for 15 June.

    So some differences, but I don't think this is a classic downgrade scenario.

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    Posted
  • Location: Lincoln, Lincolnshire
  • Weather Preferences: Sunshine, convective precipitation, snow, thunderstorms, "episodic" months.
  • Location: Lincoln, Lincolnshire

    Unless we get any last minute changes I think the analysis is pretty much done and dusted as today's outputs for the period are identical to yesterday's. Decent handling by UKMO and ECM but with a wobble on the second day, excellent handling by GFS.

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