The end of this month could be shaping up to bring very unusual weather to interior and northern Alaska, judging from recent computer model forecasts; but there is extreme uncertainty as to how it might play out. In fact I don't recall when I last saw such pronounced disagreement among the leading models, as illustrated below in the 7-10 day 500mb height forecast from the ECMWF, GFS, and Canadian models (click for a larger version):
The ECMWF - widely considered the best model on this time scale - is showing a strong trough and cold anomaly over Alaska, but the Canadian (CMC) model shows a huge high-pressure block. These are ensemble mean forecasts, so typically when they show a large anomaly at this lead time, it is quite likely to occur - and therefore it's very rare to see such strong disagreement. The GFS is taking the middle of the road, although recent runs have been flipping back and forth. The very latest GFS run (more recent than shown above) shows cold air becoming entrenched over the state by the end of the month.
It will be fun to see which model wins out - or if they're all wrong. In any case, it seems quite likely that there will be an interesting outcome with the potential to break records. Those with outdoor plans towards the end of the month should pay attention, as the cold scenario would probably bring snow to the hills in many areas.
Update Aug 23: 48 hours later, and it looks like the ECMWF will be nearer to the mark. No surprise there. Here's the latest 7-10 day forecast:
Here's a time-height cross-section of temperature above Fairbanks from the latest GFS deterministic (not ensemble) forecast: pretty chilly by next weekend. It could still be wrong, of course; 5-7 days is a long time in Alaska weather forecasting.
Fall and spring are fairly variable as compared to winter and summer. So I'm not too suprised at some discrepancy. But the differences between the models are rather striking, especially when 2 of the 3 usually line up rather well. How does the few other models compare?
ReplyDeleteThis also made me think: does the orientation of the trough or ridge axis have an effect on persistence? An east-west ridge might be harder to dislodge with a east-west system since there is more to move and the target is smaller.
Eric, the CMC forecast shows a classic rex block pattern, but I don't know if this is more or less stable than other blocking patterns. I expect there are some systematic differences in stability and persistence.
DeleteNot sure about other models - these 3 are the leading medium-range models and I don't have quick access to others.
It's about time for ex-Tropical depressions to come scooting NE towards Alaska. They can affect us and their course ultimately determines our weather during their presence. Their course can bring cool/wet or warm dry depending on your location in Alaska.
ReplyDeleteGary
Good comment, Gary. Super-typhoon Atsani is about to recurve and I suspect the Alaska forecast hinges on how that goes, i.e. where does the energy/circulation go and how does the capturing trough evolve.
DeleteFollow typhoon progress: http://www.usno.navy.mil/JTWC/
DeleteGary
Reblogged on "Climate Change Chronicle" @ paper.li
ReplyDeleteThis is interesting, because it does show the variability in the models quite strikingly. Of course the climate is a chaotic object, and those models are mere attempts at extrapolation from measured data. They would do better if they referred to known historical repeating patterns. Models based on repeating patterns, and known behaviours of certain blocking patterns, within the Jetstream are most likely to be sucessful, at least in the short to mid-term. Long-term nobody can really be certain, though the balance of probabilities is that we are heading for a cold spell.
I say we, and mean the Worldly "We", but history has shown, that during persistent blocking patterns, which cool Canada, and the Eastern Seaboard of The USA, and Continental Europe, Northern China, and Russia, perversely this has led to a slight overall warming of the Alaska peninsula, as warm air currents from the Equatorial regions enter the Arctic circulation at that point.
"Global" cooling can mean "warming", at least in Alaska. So then there is an opportunity for specimen plant growers to increase World Records for Cabbages, and many other vegetables. Alaska already hold a surprising number of World records, and of course this is due to the increased day length in the Summer growing season. With increased temperatures, this can only be good news for Alaskans, and not just with specimen vegetables either. Think what this could mean for Alaskan agriculture in the short to mid-term !
I should have stated that the clipping appeared in :
DeleteFriday, Aug. 21, 2015 Edition,
Which had some good videos about Alaska Ice Roads, and Dog Sleds, and other ice travel videos too. Friday's edition had a lot of stuff about ice. Saturday's edition will be .... who knows.
Find Friday, Aug. 21, 2015 Edition in the "Archives" if you missed it. Find loads more interesting News, Papers, and Opinion, with a daily selection of the best Climate related videos, from the Web, and a variety of graphs, charts, images, and cartoons as well.
I shall certainly look in to your blog again, now that I know of it's existence. It is refreshing to see the data presented, without some green hokum attached, or "woe is me" type propaganda.
Thank You !
Thanks for reading. I won't get into the climate change debate, but would point out that short-range deterministic weather forecast models are unquestionably better than statistical models when it comes to forecasting broad circulation patterns and weather features. Statistical predictors can begin to add value as the so-called butterfly effect takes over in the medium-range forecast, although the dynamical models are still very useful all the way out to the seasonal timescale and perhaps beyond.
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