That’s what meteorologists do quite a bit each day while forecasting. New information comes in and you have to decided if the new data is enough to change your initial forecast numbers. I frequently like to use a saying I got from Randy Querin. He said we are "constantly refining & improving the forecast". That’s much more positive than saying we have a bad forecast and need to change it.
Anyway, a few thoughts on our little upcoming cold snap:
1. Within 48 hours there will be a big cold trough sitting right overhead. The first strong shortwave is west of Juneau this evening. That’s the one that drops straight south, carving out a nice curved trough.
2. No organized cold front steady rain with this system. Just a deep marine layer tomorrow-Thursday AM, then a destabilizing atmosphere as cold air arrives Thursday (and continuing into Friday). So I think we’ll see showery weather, not steady rain. Mainly Thursday & Friday afternoons and each morning in the usual upslope areas (my house). Of course thunder is possible too during this pattern.
3. Not a ton of snow in the mountains, and probably not much below 5000′ either.
4. The big trough kicks out Friday evening and we wait until Monday for another wave to come in not from the north but from the west.
The "tweak" I’ve made to the forecast is that the low kicking out is followed by briefly strong ridging ahead of the next system Monday. 18z GFS fell into line this afternoon with the 12z ECMWF. They both show nice offshore flow Sunday ahead of that next wave. 75-80 is the result in mid September with an 850mb temp near 10 or so in the afternoon. So we have a nice weekend coming up after a chilly Thursday-Friday…Mark