UPDATE 11:30 PM: Wednesday will likely be very similar to Tuesday, partly cloud and maybe a click warmer with some highs topping 70. Showers arrive by Thursday ahead of the cold front. END UPDATE
It was nice to finally see the sun and some blue skies in the Roanoke Valley today after 3 days of low clouds, fog, drizzle and rain showers. Tuesday should provide even more sun, though it may start off similar to the last few days, with near normal to slightly above normal high temperatures in the mid 60s to low 70s.
The second half of October is taking on a chilly look. A cold front at midweek — it may trigger some showers, and cut highs back into the 60s with 30s-40s lows — and especially one over the weekend are the advance units in this invasion of cold air. But it will take several days for a significant pattern shift to fully take hold, featuring strong high pressure over the eastern Pacific/western North America (what we call the positive phase of the Pacific-North America pattern, or PNA+), a deep upper low of Arctic air sinking into eastern Canada, and significant high pressure blocking over the North Pole (the negative phase of the Arctic Oscillation, or AO-) and at least some level of blocking high pressure in the northern Atlantic near Greenland (the negative phase of the North Atlantic Oscillation, or NAO- — though this feature seems to be the most questionable of this list, currently). The last week of October looks like it may be unseasonably cold throughout much of the central and eastern U.S., as these factors combine to create a classic pattern to pull cold southward from the Arctic and trap it over the eastern half of North America. Some of the long-range model depictions are very impressive with the level of cold air forced southward, though of course, reality often ends up more moderate than these early model ensembles. That said, I think we are likely headed for frost on the pumpkin multiple mornings and perhaps our first mountain snow showers of the season between this weekend and Halloween.