Here is a general tip for those of you who want to know more than a few days in advance (when we have the great high res models) whether snow showers (lake effect) will likely hit your area. This is what you should do.
- Firstly establish that the airmass is cold enough, it should be at least 13C colder at 850hpa than it is at the surface. This requirement is almost always met for almost always any cold outbreak.
- Secondly do the isobars point towards you from the coast? For example if your nearest coast is facing NW then NWerly winds are your safest bet. The stronger the winds the more inland penetration.
- Is there any directional/speed wind shear? The easiest way to establish this is look at your standard 500hpa height/SLP charts. Do the isobars roughly align with the height contours. If they do not, then showers are unlikely I'm afraid.
- Is there sufficient moisture? The answer to this question should be yes if the fetch is good, and the above criteria are met upstream. But you can check by looking at something like 700hpa humidity. You want it to be high (>70%)
- Does friction/orography play a major role? If showers rain out over the sea then there is something called absolute instability , what this means is that showers are likely to remain in their cluster type formats which some slight intensification over coastal regions followed by weakening as they move further inland. However if there is a thin stable layer somewhere that stops showers from raining out over sea you can get continuous precip (though it tends to be lighter) when friction/orography provides that little bit of lift needed. The shape of the coast also matters. If the wind hits convex or concave bits of land you ramp up the convergence even more; this tends to focus convection into streams and dries out the surroundings.
Twitter: @QuantumOverlord (general), @MedicaneWatch (medicane/TC stuff)
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.