Hi everyone!  Hope you’re still enjoying the exceptionally dry November, even if the east wind makes things feel a little chilly.  We’re two-thirds of the way through the month as of yesterday the 20th, with only 1.22″ of precip at the Portland Airport.  Normal for this 20-day period is 3.60″, so this is definitely not typical November weather.

Temps have been much warmer than normal in the mountains, but close to normal for many lowland spots due to seasonal inversions.  That part is perfectly normal by ‘dry November standards’.  It’s very common for valleys and basins in the Pacific Northwest to remain seasonably chilly in the fall/winter months, whenever warm and dry air is aloft.

We’re going to see a big change though, beginning this weekend.  Not so much to a wet/stormy pattern, as much as toward a colder pattern.  Here are the GFS ensembles from this morning.  Notice how much the blue curve dips below the red curve?  That’s a clear sign that we’re probably going to see relatively cold air in the middle to upper atmosphere, beginning this Sunday or Monday.  Models have been fighting over this for the past 3 or 4 days, but now they are coming into agreement on the general chilly pattern.

There’s also agreement that we turn somewhat wetter as the cold air is beginning to move in, but then dry out once the cold is entrenched over the region.  This is pretty typical for most of our arctic fronts in the winter months.  The bottom blue curve rises up (wetter) between 11/24 and 11/27, then tends back toward dry beginning on Thanksgiving Day.  Note that there ARE a few green members which try to bring moisture in on top of the cold air later on (like Black Friday); this is something to keep a closer eye on as the timeframe approaches.

Trouble is, forecasting snow is even harder than forecasting cold.  It’s always a bit of a mystery as to how much moisture will show up in these events, and exactly where the storm track decides to go.  The general rule is:  We can usually see cold patterns a full 7 days in advance, sometimes even longer.  But details on a winter storm are tough to pinpoint more than 3 days away, and even then there’s still a huge range of uncertainty. 

Here is the WRF-GFS map for total precipitation Sunday morning through early Wednesday morning.  Not a deluge, but some decent showers of rain in the valleys and snow in the Cascades:

(courtesy of UW WRF-GFS models)

But the total snowfall map through Wednesday morning, clearly shows that it won’t be cold enough for snow at the lowest elevations:

This is pretty typical for an incoming cold spell.  Light to moderate showers with falling snow levels, then it turns dry as the really cold air arrives.  By Thanksgiving morning, this cold/dry air is firmly in place across the Columbia Basin & Eastern Oregon:

The next question, obviously, is whether any moisture moves in on top of the cold air beyond that.  Again…we can’t make any judgment calls this far in advance.  It all depends on how the Pacific storm track evolves, and where the low-pressure centers go.  Generally if the storm moves by to our north, it will wipe out the cold air before much snow or ice develops in Portland.  If it moves further south, it reinforces the cold east wind, allowing a significant snow storm over the city and parts of the Willamette Valley.  We’ve seen a lot of south-tracking lows over the past several winter seasons, which is one reason why places south of Portland have had quite a few major snowstorms in recent history.

As always…stay tuned for details as the forecast evolves.  For now, plan on some sort of chilly weather during the Thanksgiving Week and Weekend, with at least a small chance of lowland snow & ice at some point.  That means daytime temps in the Portland area in the 35-45 degree range, not 50-55 degrees like we’ve been getting.  At night it will be in the 20s/low 30s.  East wind will likely make it feel even colder.  Prepare accordingly, but please don’t go into a “Kale Panic” like what happened last February!

Also…the Cascade mountains will likely see at least some snow the first half of the upcoming week.  I doubt that 8-15 inches will be enough to get Thanksgiving skiing into full force, but it’s a step in the right direction.  Colder airmasses and weak sun angle also ensure the mountain snow won’t melt away too quickly.

Karl