Friday, October 4, 2019

Temperature Extremes Across the U.S. This Week

My blog on UW Censoring of Social Media is Here.
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Politics is not the only place in which extremes can exist in our country--- the weather is showing that it is no slouch in this domain.

During the last week, there has been an extraordinary contrast of temperatures across the nation, with temperatures hitting 100F or more in the Southeast, but lows in the teens/twenties and heavy snow in portions of the Northwest.

To illustrate, here are the mean temperatures for October 1-2 (top), with the difference from normal (anomalies) for the same period on the bottom.  Huge warm anomalies over the eastern U.S., reaching nearly 20F.   And nearly equal cold anomalies over the upper Plains and the West down to the Mexican border.


Many records have fallen on both sides of the country:  cold records in the west, warm records in the east.  For example, here are daytime records on October 1.  Wow.  There are not only daily high temperature records, but monthly records as well.  THAT is impressive.


On the other hand, the same morning produced low-temperature records
over the west, including breaking daily records, and in some locations, monthly ones.  Several locations, such as Spokane, broke daily/monthly snowfall records.


So why the simultaneous extremes on differing sides of the nation?  It has to do with a highly perturbed and anomalous upper level wave pattern, which caused a large undulation in the jet stream.

To illustrate, here is the upper level (500 hPa--about 18,000 ft) analysis for 5 AM PDT September 29th (Sunday).  Blue and purple indicate much lower than normal heights (a trough), and orange/red are higher than normal heights (ridge).
You can see a very disturbed, undulating pattern, with a ridge over the Northeast Pacific, a very deep trough over the west, and a ridge over the eastern U.S.

Troughs are associated with cold, low-level air, ridges with warm air.


Why?  Such a pattern moves warm air into the western side of ridges (southerly flow) and ridges have sinking (which warms air by compression).  Thus ridges are warm.   Just the opposite for troughs.

The same chart on October 1, the day of the records shown above, is very similar:

 
The pattern today has the same general idea, but a bit weaker.


But I have some news.  It looks like ANOTHER major push of cold air will surge into the Northwest on Tuesday, with more snow at higher elevations (see map below for Tuesday at 5 PM PDT).


If we don't shake this pattern, November could bring snow to the lowlands.  But don't worry...this weekend should be decent, and Sunday will be perfect.  Sunny, no precipitation, and mid-60s in the western lowlands.

11 comments:

  1. so cliff... you are saying if this pattern stays like this we could get snow in november??

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  2. Hey Cliff - enjoy your blog immensely. We live in Calgary and a lot of your weather has a tremendous influence on ours. Just wondering when you'll be putting out a winter forecast. As skiers, we obviously hope this weather pattern holds pretty much forever. :>)

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  3. Snow in the low lands is always going to be a "Seeing is believing" event. Still 2010 gave a double whammy on 23 November. A snow dump and THEN a windstorm. So maybe take a look at the data from months preceding that and see if there are any correlations.

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  4. Can you explain further why ridges (which on land are high points) which gets warmed air by sinking and troughs, which typically are low lying, at least on land, have cooling??

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  5. Seattle WeatherBlog did do this and in other neutral years (1985, 2003 and one other) were the years we got dumped on in November. Stakes are high.

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  6. Perhaps this could be another binary winter, below average temps and snow until January, then the pattern flips.

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  7. The September lowland rain is greening up the shrub step hills around the Methow Valley. More new snow in the East side North Cascades mountains has me thinking about snow layers for the upcoming Backcountry ski season. Early snow that lingers can and usually does become a weak base layer, especially when associated with cold clear weather. This occurred last season and I reported that weakness here late November of last year.

    As the snow pack increased in early December, that weakness become apparent as we experienced several climax (to the ground) avalanches above the Hwy 20 hairpin, near Washinton pass, that sent snow down to the hwy with several large debris piles. The hwy was of course closed by then.

    I had a discussion with a local guide who had been skiing that area prior those climax Avalanches. I was concerned because a guide group was up checking out the snowpack in early December just after a major snowstorm.

    The guides had just crossed the Spire Gully avalanche run out zone when a large natural avalanche had run almost to their skin track.

    Unfortunately the guide took this as a positive sign that they had made the right call after all as that avalanche did not hit their skin track. They had surmised that there was not sufficient Snow to endanger them that day and they believed they were right to be there.

    In my text conversation with that guide, I detailed several other historical near-miss Avalanche incidents in our area where local guides were involved and though they could mitigate the hazard.

    The guide went on to explain that Spire Gully would not be a problem until late in the season as the snowpack depth increased. I told the guide not to underestimate Spire Gulley's capability to produce an avalanche capable of reaching the road.

    So the very next storm produces a very large avalanche that reaches the road.

    My guess is that the guide had underestimated and or was not considering the effect of the snow depth build up in the snow deposition zones where snow sheds off the upper steep rock walls and collects (deposits) below those walls and the effects of wind driven snow contributing to deeper snow slabs.

    Add the weak base layer to the equation and the resulting climax Avalanche that hit the road had an observed estimated 8 foot slab crown line in the deposition start zone.

    It's a reminder to Mountain Travelers to consider what is above them.

    That unlearned lesson manifested itself later in the season on a local commercially guided trip during a snowstorm. The guide had guided his group through Avalanche Terrain run out zones on a day when NWAC advised that it wasn't safe to travel in avalanche terrain that day.

    Two of his clients were hit from above by a natural avalanche originating from Steep rock walls with one client buried and pinned against a tree with lost skis and injuries requiring a sled evacuation.

    The lesson; unwary clients have to understand that commercial guides will take your money and will take you into hazardous situations because they think they can mitigate the hazard.

    Money, prestige and desire can have a strange effect on influencing decision making.


    Chris H.
    Heli-free North Cascades

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  8. I really hope we get snow soon(ish)... Looks like the odds are, we will?

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  9. Also I saw that the temperature got down to 35 F a few days ago, in Kirkland.

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  10. The last time any particularly cold weather occurred in NW Bellingham during November was that of 2015. I measured a maximum monthly temperature of 57-58F and a monthly minimum temperature of 21F. November's monthly average maximum temperature at KBLI is 50F and the average monthly minimum temperature is 37F.

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  11. Cliff, Hope you're out in the sunshine today!

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