Brought to you, in part, by typhoon Wipha, much of the eastern two-thirds of the U.S. is due for a possible taste of late autumn-like or even early winter-like temperatures in just over a week’s time.
Wipha has now transitioned from a typhoon into a mid-latitude storm in the north Pacific. Its presence is helping to build a big ridge of high pressure over the western North America, which – in turn – is going to gradually cause the jet stream to buckle and dive south over the eastern two-thirds of the U.S. This will open up the floodgates of Arctic Air.
The pattern change will take about a week to evolve, as the core of chilly air is initially (through early next week) steered more towards the Plains and Midwest (see frames for October 17-20 in above image) October before making its way to the East Coast (October 21-23).
“The key to the whole pattern is the development of the big LOW pressure area over the Great Lakes that moves up into southeastern Canada [see last two frames of above animation] and the Canadian maritime provinces,” writes Dave Tolleris, meteorologist for WxRisk out of Richmond, on his Facebook page. “If that LOW does not do that ….then the northwest winds will be significantly less and the heart of the cold air will NOT plunge deep into the eastern half of the country.”
It’s not really a question of whether the cool air will arrive, but rather how cool it will be. All three of the major computer models: the Canadian, the GFS, and European (and the majority of their ensembles) are in reasonable agreement about the gradual infiltration of the cooler weather.
How cold are we talking? Preliminary indications are that by late next week, highs may only be in the 50s, with lows in the 30s and the possibility of a frost or freezing conditions in D.C.’s north and west suburbs.
The models suggest that once the cooler air arrives, there may be reinforcements that help it stay around for a while – though this is a lower confidence projection.
NOAA’s Climate Prediction Center is buying into the model projections and favors cooler than temperatures in the 8-14 day period for the eastern two-thirds of the nation.
Note that models sometimes overdo cold air outbreaks and confidence in the forecast declines considerably beyond 5-7 days into the future.
Of course, temperatures are in decline, on average, at this time of year without any help from the overall pattern. Average highs fall from the upper 60s to the low-to-mid 60s over the next 14 days in the D.C. area.
Related: It’s All Connected (Ryan Hanrahan)