We have some GVN (greenland vortex northerly) scenarios on the GFS6Z.
I've coined this term because I'm not sure if this phenomenon has been given a name before, perhaps it has and if anyone has any research on this I'd love to see it. Anyway a GVN is an exceptionally rare event that can cause brutally cold, if short, northerly feeds. The mechanism is this.
- Tropospheric polar vortex forms a wave-1 pattern while being fairly strong. Note this is actually pretty rare; and is most likely outside of winter e.g. late autumn or spring.
- The TPV moves over greenland enhancing the deep cold core low. Low baroclinicity near the surface makes this much stronger at higher latitudes, and near the surface there could even be a shallow high.
- The TPV starts to break down with a deep HP moving in from canada towards greenland with WAA. This pushes the deep cold core low east over the greenland sea
- The deep high from canada rapidly strengthens and expands over the greenland, launching the core core low and basically the entire TPV south towards the UK
- In a race against time it hits the UK
before significant cyclogenesis occurs over the surface.
If this happens the upper air temps can be brutally cold. The last time this happened was in April 2021 where -15C T850s hit the northern isles, but it was an indirect hit; some of the ensembles showed 850s as low as -28C hitting shetland. That is not a typo. I have these charts saved somewhere if you don't beleive me.
Anyway GVN showing up on some of the forecasts. Could we see a repeat of April 2021?
2023/2024 Snow days (approx 850hpa temp):
29/11 (-6), 30/11 (-6), 02/12 (-5), 03/12 (-5), 04/12 (-3), 16/01 (-3), 18/01 (-8), 08/02 (-5)
Total: 8 days with snow/sleet falling.
2022/2023 Snow days (approx 850hpa temp):
18/12 (-1), 06/03 (-6), 08/03 (-8), 09/03 (-6), 10/03 (-8), 11/03 (-5), 14/03 (-6)
Total: 7 days with snow/sleet falling.
2021/2022 Snow days (approx 850hpa temp):
26/11 (-5), 27/11 (-7), 28/11 (-6), 02/12 (-6), 06/01 (-5), 07/01 (-6), 06/02 (-5), 19/02 (-5), 24/02 (-7), 30/03 (-7), 31/03 (-8), 01/04 (-8)
Total: 12 days with snow/sleet falling.