The title pretty much sums it up, we’ve had no official snowfall in Portland or Salem this winter so far and I don’t see any in the next week or more. A couple of days ago it was looking like we’d see our first snow dusting in weeks down below 1,000′ next Tuesday. But now models have less moisture, slightly warmer atmosphere and the dreaded “split flow”.
What is that? In the case of next Monday-Tuesday, a cold system moves south out of the Bering Sea (between Alaska and Siberia) then dives SE towards the west coast of North America. It’s looking increasingly likely that system will be headed mainly to our south, California and southern Oregon. This is never good if you want decent precipitation here. We just get the “leftovers” or light precipitation/showers.
Here is the 12z GFS ensemble 850mb chart, lots of lines! (Click for a clear view)
An explanation for the newbies since I use these charts quite a bit…
Time runs from left to right. Left side is this morning, far right side is the end of the model run, 16 days away or March 1st. Note that all weather maps are in Universal Time, which means the new world “day” begins when midnight arrives in England. The next calendar day begins at 4pm each afternoon here on the West Coast. The “21 FEB” line is right at 4pm on the 20th, the 19 FEB line is 4pm on the 18th etc…
Temperature is plotted in celsius at the 850mb level, which is right around 4,000′ or so. So you could think of the zero line going across the middle as roughly Government Camp
- THICK BLUE LINE: The actual high-resolution GFS run, also called the operational run
- BLACK LINES: The 21 lower resolution “ensemble” runs, basically 21 other runs with slightly different initial starting conditions
- THICK RED LINE: The average of all the ensembles, or the “ensemble mean”
- GREEN LINE: The average for this time of year, that’s why it doesn’t move much
What can you learn from this chart today? Quite a bit!
1. The average temp at 4,000′ in mid February over Portland is around 33 degrees
2. On the operational (blue) run, from Saturday afternoon the 16th onward about 10 days into the future the temperature is forecast to remain below normal…a cooler than average period ahead
3. We have high confidence in the trend through the next 7-8 days (through the 20th) since all the ensemble lines are clustered quite close together.
4. In the 12-16 day forecast timeframe, the operational run is quite a bit warmer than many ensemble members, you can see it way above many of them. So confidence is quite low that the maps we are seeing (the operational run) will end up correct in that period. This is common in the long-range, huge variability from model run to model run each day.
5. Wide variation beyond about day 8; low confidence beyond that period.
6. We need to see 850mb temps down around -7 in late February to even think about lowland snow. That possibility is just about gone in the next 8-9 days. Sure, one or two ensemble members are cold enough, but is it just cool and dry? Could be.
7. There are still quite a few ensemble members wanting to bring those chilly temps back the last 4-5 days of February. However, a week or so ago it was looking very cold for Valentine’s Day, snow levels down around or below 1,000′. Then models backed off on the chilly air.
A few important caveats:
This model runs 4 times a day…that means a new chart comes out every 6 hours!
This is only one model, there are several others.
There is a huge amount of model forecast data out there nowadays, we really look for trends or grouping of good model data, not just one particular model.
I’m not putting a “Fork in Winter” yet, but it’s getting very close. The chance of a big arctic blast, freezing rain, flooding, or significant snowstorm is fading away quickly.
Will we get “sleddable snow” (1″ or more) in Portland the rest of the winter? I’d put that chance at about 40% right now. Yes, yes, I know last year we had our latest ever snowfall in Portland on March 21st, but there is a reason it was the latest ever, it’s very rare!
Chief Meteorologist Mark Nelsen