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Best synoptic conditions for frost in September/October?


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What our good setup scenarios for it and what are some notable ones especially west of the Cascades?  Any top picks?

 

You want the same setup that would deliver an Arctic blast during the winter. GOA ridge with a trough over us and cold northerly flow. Clear skies and calm winds as well. Mid-Septembers of 1965 and 1970 are excellent examples of early freezes along the I-5 corridor, although it has happened as early as August 18, 1973 (33 in Olympia and 31 in Toledo, WA). 

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Interesting to note how the lows over the east side really start to separate from the west side of Cascades this time of the year. Up until a couple weeks ago the lows weren't all that far apart. Now its the time of the year where freezes will be more common out here.

Bend, OR

Elevation: 3550'

 

Snow History:

Nov: 1"

Dec: .5"

Jan: 1.9"

Feb: 12.7"

Mar: 1.0"

Total: 17.1"

 

2016/2017: 70"

2015/2016: 34"

Average: ~25"

 

2017/2018 Winter Temps

Lowest Min: 1F on 2/23

Lowest Max: 23F on 12/24, 2/22

Lows <32: 87

Highs <32: 13

 

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Clippers are another thing that works really well.  They bring cold air, but very little moisture so we end up with low dp to go along with the cold air mass left in the clipper's wake.  October 2006 was probably the most over performing October cold snaps on record for my area and it was not a classic Arctic blast setup.  All time October record lows were set at some stations.  September 1949 was another example...widespread frost on Sept 12 with a very unimpressive upper level setup.  That one had everything to do with surface pressure gradients.

Death To Warm Anomalies!

 

Winter 2023-24 stats

 

Total Snowfall = 1.0"

Day with 1" or more snow depth = 1

Total Hail = 0.0

Total Ice = 0.2

Coldest Low = 13

Lows 32 or below = 45

Highs 32 or below = 3

Lows 20 or below = 3

Highs 40 or below = 9

 

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Clippers are another thing that works really well.  They bring cold air, but very little moisture so we end up with low dp to go along with the cold air mass left in the clipper's wake.  October 2006 was probably the most over performing October cold snaps on record for my area and it was not a classic Arctic blast setup.  All time October record lows were set at some stations.  September 1949 was another example...widespread frost on Sept 12 with a very unimpressive upper level setup.  That one had everything to do with surface pressure gradients.

 

It was still a classic cold pattern. Upper level low dropping south from BC with cold, dry northerly flow. Granted, the offshore ridge wasn't very impressive but it was still there.

 

Halloween 2006 was more of a classic clipper. That cold snap really overachieved considering the lackluster pattern. 

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Here is more imagery.  Here is September 17th 1965 which pegged 28F. It says 30F but when you click on it the records tab says 28F.  https://www.wunderground.com/history/airport/KSLE/1965/9/17/DailyHistory.html

 

Here is the Metociel imagery for that date. http://www.meteociel.fr/modeles/archives/archives.php?day=17&month=9&year=1965&hour=12&type=ncep&map=6&type=ncep&region=&mode=0 I set it to Temperature et flux 850Pa. Hem.Nord for that one imagery.  It's there also in the Toutes Les Cartes which means all imagery.   Europe first then North America last.

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Clippers are another thing that works really well.  They bring cold air, but very little moisture so we end up with low dp to go along with the cold air mass left in the clipper's wake.  October 2006 was probably the most over performing October cold snaps on record for my area and it was not a classic Arctic blast setup.  All time October record lows were set at some stations.  September 1949 was another example...widespread frost on Sept 12 with a very unimpressive upper level setup.  That one had everything to do with surface pressure gradients.

Was  September 1965 a clipper then?  That whole year seems a lot of chilly lows.

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