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Post by deneb78 on Jan 20, 2021 19:25:42 GMT -5
I was wondering why the San Francisco Bay area never gets Arctic outflow winds like the Pacific Northwest gets sometimes in winter. Of course I understand that being a lower latitude with warmer temperatures, it would have less effect but it seems absent altogether. For example, an Arctic outflow event in Vancouver would produce a high of -2°C (28°F) and a lot of snow. One would expect the same type of event in San Francisco would produce a high of around 6°C (42°F) and rain showers but I don't think I have ever seen a forecast like that for San Francisco. Why is that the case?
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Post by 🖕🏿Mörön🖕🏿 on Jan 20, 2021 19:46:40 GMT -5
Even Redding barely sees the effects of cold continental air.
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Post by deneb78 on Jan 20, 2021 20:28:21 GMT -5
Even Redding barely sees the effects of cold continental air. Why is that though? |
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Post by 🖕🏿Mörön🖕🏿 on Jan 20, 2021 21:23:53 GMT -5
Even Redding barely sees the effects of cold continental air. Why is that though? Geography, rarity of backdoor blasts going that far south, etc. Even in December 2013 (which was a very notable backdoor blast for Oregon) which provided a -10F low for Eugene, Redding only got down to 20F. Coldest high was 39F. The Siskiyou mountains are very good at blocking cold northerly air. That same month, SF's coldest high and low were 47F and 34F. Check for yourself: sercc.com/nowdatamap |
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Post by deneb78 on Jan 20, 2021 22:01:41 GMT -5
Geography, rarity of backdoor blasts going that far south, etc. Even in December 2013 (which was a very notable backdoor blast for Oregon) which provided a -10F low for Eugene, Redding only got down to 20F. Coldest high was 39F. The Siskiyou mountains are very good at blocking cold northerly air. That same month, SF's coldest high and low were 47F and 34F. Check for yourself: sercc.com/nowdatamapWhat makes the Siskiyou mountains better at blocking cold northerly air compared to the Cascades? Cold continental air can certainly make it that far south. I mean look at Reno, NV on the other side of the mountains. It averages 2.5 nights a year below -12°C (10°F) and 5.1 days a year where the average high doesn't go above freezing. There is clearly influence from arctic outflow there that can't cross the mountains for some reason. |
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Post by 🖕🏿Mörön🖕🏿 on Jan 20, 2021 22:03:33 GMT -5
Geography, rarity of backdoor blasts going that far south, etc. Even in December 2013 (which was a very notable backdoor blast for Oregon) which provided a -10F low for Eugene, Redding only got down to 20F. Coldest high was 39F. The Siskiyou mountains are very good at blocking cold northerly air. That same month, SF's coldest high and low were 47F and 34F. Check for yourself: sercc.com/nowdatamapWhat makes the Siskiyou mountains better at blocking cold northerly air compared to the Cascades? Cold continental air can certainly make it that far south. I mean look at Reno, NV on the other side of the mountains. It averages 2.5 nights a year below -12°C (10°F) and 5.1 days a year where the average high doesn't go above freezing. There is clearly influence from arctic outflow there that can't cross the mountains for some reason. Reno is farther east and at a higher elevation. It's just hard for that cold air to make its way westward. |
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Post by omegaraptor on Jan 20, 2021 23:32:21 GMT -5
NE California high plateaus are still very much capable of intense cold east of the Cascade Crest. Burney, Alturas, Susanville, Portola, etc all have record lows below -25F.
The Siskiyous run west-east, rather than north-south like the coast range and Cascades, forming a wall for northerly blasts west of the Cascade Crest, if they didn't already die at the Calapooyas anyway. From the east there are very few if any places that allow easy cold air drainage from the east side to the Central Valley. By the time Arctic air reaches down to 40-42N it just doesn't have the steam to break past the south Cascades and Sierra.
That of course doesn't mean it outright doesn't happen - it's just quite rare. In 2009 and 2012 Redding, CA got to 16F, and in the same 2009 blast Chico got to 15F. Honestly, since 1970, Portland's potential for cold extrema has not really been excessively higher than that of Redding or Chico. Those freak events in 1950 and 1969 have not been even approached since then.
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Post by deneb78 on Jan 21, 2021 10:47:07 GMT -5
NE California high plateaus are still very much capable of intense cold east of the Cascade Crest. Burney, Alturas, Susanville, Portola, etc all have record lows below -25F. The Siskiyous run west-east, rather than north-south like the coast range and Cascades, forming a wall for northerly blasts west of the Cascade Crest, if they didn't already die at the Calapooyas anyway. From the east there are very few if any places that allow easy cold air drainage from the east side to the Central Valley. By the time Arctic air reaches down to 40-42N it just doesn't have the steam to break past the south Cascades and Sierra. That of course doesn't mean it outright doesn't happen - it's just quite rare. In 2009 and 2012 Redding, CA got to 16F, and in the same 2009 blast Chico got to 15F. Honestly, since 1970, Portland's potential for cold extrema has not really been excessively higher than that of Redding or Chico. Those freak events in 1950 and 1969 have not been even approached since then. Cool! I didn't know the Siskiyous ran east to west. That explains a lot actually. Still, it looks like the central valley can get cold sometimes. 15F is pretty chilly. |
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Post by psychedamike24 on Jan 21, 2021 14:19:03 GMT -5
San Francisco is also relatively far south (38 N), around the same latitude as Washington DC or Athens. It's shielded from Arctic blasts by the Rocky Mountains, the Sierra Nevada, and as other posters mentioned the Siskyous.
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Post by nei on Jan 21, 2021 18:11:13 GMT -5
Geography, rarity of backdoor blasts going that far south, etc. Even in December 2013 (which was a very notable backdoor blast for Oregon) which provided a -10F low for Eugene, Redding only got down to 20F. Coldest high was 39F. The Siskiyou mountains are very good at blocking cold northerly air. That same month, SF's coldest high and low were 47F and 34F. Check for yourself: sercc.com/nowdatamaphere's the coldest day of the December 2015 cold snap, much colder to the north of San Francisco, coldest air had trouble reaching San Francisco 850 mb  surface  |
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Post by deneb78 on Jan 22, 2021 15:05:28 GMT -5
Geography, rarity of backdoor blasts going that far south, etc. Even in December 2013 (which was a very notable backdoor blast for Oregon) which provided a -10F low for Eugene, Redding only got down to 20F. Coldest high was 39F. The Siskiyou mountains are very good at blocking cold northerly air. That same month, SF's coldest high and low were 47F and 34F. Check for yourself: sercc.com/nowdatamaphere's the coldest day of the December 2015 cold snap, much colder to the north of San Francisco, coldest air had trouble reaching San Francisco 850 mb surface Very interesting information. I also had a look at the data for Redding and further north in Eugene. What a sharp contrast and how effective the Siskiyous are at blocking cold air. Even on the days where Redding had a 16F low, the high was in the upper 40s. That's quite some diurnal range. San Francisco's would be less but still remarkable that arctic blasts never reach there at 38 latitude. |
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Post by 🖕🏿Mörön🖕🏿 on Jan 22, 2021 17:10:05 GMT -5
here's the coldest day of the December 2015 cold snap, much colder to the north of San Francisco, coldest air had trouble reaching San Francisco 850 mb surface Very interesting information. I also had a look at the data for Redding and further north in Eugene. What a sharp contrast and how effective the Siskiyous are at blocking cold air. Even on the days where Redding had a 16F low, the high was in the upper 40s. That's quite some diurnal range. San Francisco's would be less but still remarkable that arctic blasts never reach there at 38 latitude. Even if SF was an east coast climate (like eastern China/US) the geography would still make it difficult for cold air to reach it. The fact that it's on the west coast makes it all the less likely that actual cold air reaches SF. |
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Post by deneb78 on Jan 22, 2021 18:25:15 GMT -5
Very interesting information. I also had a look at the data for Redding and further north in Eugene. What a sharp contrast and how effective the Siskiyous are at blocking cold air. Even on the days where Redding had a 16F low, the high was in the upper 40s. That's quite some diurnal range. San Francisco's would be less but still remarkable that arctic blasts never reach there at 38 latitude. Even if SF was an east coast climate (like eastern China/US) the geography would still make it difficult for cold air to reach it. The fact that it's on the west coast makes it all the less likely that actual cold air reaches SF. Not really true though. Richmond, VA which is on the same latitude as San Francisco on the East coast is hardiness zone 7b and averages 6.0 days a year where the high doesn't reach freezing. That shows that it's clearly not difficult for cold air to reach 37.5 latitude with the right set up. |
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Post by 🖕🏿Mörön🖕🏿 on Jan 22, 2021 18:27:35 GMT -5
Even if SF was an east coast climate (like eastern China/US) the geography would still make it difficult for cold air to reach it. The fact that it's on the west coast makes it all the less likely that actual cold air reaches SF. Not really true though. Richmond, VA which is on the same latitude as San Francisco on the East coast is hardiness zone 7b and averages 6.0 days a year where the high doesn't reach freezing. That shows that it's clearly not difficult for cold air to reach 37.5 latitude with the right set up. No I'm saying without the interference of the mountains, SF would get much colder if it was an east coast climate. Not talking about the eastern US but the Appalachians provide little interference compared to the various mountain ranges in the west. |
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