In an effort to cut down on confusion that has been stirred up in recent days, I'm publishing this post to give an idea of my thoughts for the rest of the month.
To stay as accurate as possible, we'll start off with the weather overseas.
The image above shows forecasted 500mb height anomalies over the West Pacific. In this image, valid on December 6th, we see a deep trough over Japan. Those of you who have followed us for a while know that stormy weather over Japan correlates to stormy weather here about 6-10 days later. This graphic tells us that the weather will likely be colder than normal on a December 12-16th timeframe, possibly with an accompanying storm system (but I wouldn't hold my breath for it).
By December 10th, the trough has moved out and a ridge has filled in. This ridge won't be of a spectacular magnitude, but it looks strong enough to indicate quiet and warm weather here in the United States in a December 16th-20th time period.
The last image I'll show you with relations to Japan is above, valid on December 13th. Looking at East Asia, notice an elongated trough to the north of the island nation, inducing below-normal height anomalies in the country. Using the Typhoon Rule, we can approximate a cold/stormy period following the aforementioned ridge, namely in a December 19-23 period, if not for longer. It should be noted that this trough doesn't "dig" south into Japan, raising concerns that a zonal flow (average to slightly warm) pattern may evolve instead of this cold weather. I will issue additional updates as needed.
Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast
A quick refresher on the PNA and NAO...
The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.
The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.
Glancing over this four-panel forecast, we find that the PNA is expected to stay positive throughout the entire forecast period. This will allow for high pressure to reside in the West, which would usually give way to a cold period in the Central and East US. However, on the next panel, the positive NAO projection tells us this ridge will be allowed to bleed east into those aforementioned areas, keeping most of the country warmer than normal.
The positive WPO and EPO, both Pacific teleconnections, are good for cold weather prospects in the Central and East US, but until that positive NAO ends, we're likely stuck with a warm period.
Wait... so you're predicting some good cold shots, but also a warm period for the same timeframe?
The idea I'm getting right now is that the next 10-15 days of December are looking predominantly warm. However, in between those warm spells, some cold air outbreaks (not intense, but still noticeable) may be expected. This is a pretty turbulent forecast period, since I'm experiencing a good deal of uncertainty.
* For those of you who have been following on Facebook and were confused by my analog postings last night, please disregard those postings and go with this article as your guide to my expectations.
To summarize:
- A cold spell may be expected in a December 12-16 period.
- Warmth will then follow in December 16-20th, roughly.
- Around December 19-23 and beyond, the risk of below-normal temperatures may return, but uncertainty is high.
- The overall pattern remains generally unfavorable for sustained cold weather.
Andrew
To stay as accurate as possible, we'll start off with the weather overseas.
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| Tropical Tidbits |
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| Tropical Tidbits |
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| Tropical Tidbits |
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| ESRL |
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast
A quick refresher on the PNA and NAO...
The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.
The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.
Glancing over this four-panel forecast, we find that the PNA is expected to stay positive throughout the entire forecast period. This will allow for high pressure to reside in the West, which would usually give way to a cold period in the Central and East US. However, on the next panel, the positive NAO projection tells us this ridge will be allowed to bleed east into those aforementioned areas, keeping most of the country warmer than normal.
The positive WPO and EPO, both Pacific teleconnections, are good for cold weather prospects in the Central and East US, but until that positive NAO ends, we're likely stuck with a warm period.
Wait... so you're predicting some good cold shots, but also a warm period for the same timeframe?
The idea I'm getting right now is that the next 10-15 days of December are looking predominantly warm. However, in between those warm spells, some cold air outbreaks (not intense, but still noticeable) may be expected. This is a pretty turbulent forecast period, since I'm experiencing a good deal of uncertainty.
* For those of you who have been following on Facebook and were confused by my analog postings last night, please disregard those postings and go with this article as your guide to my expectations.
To summarize:
- A cold spell may be expected in a December 12-16 period.
- Warmth will then follow in December 16-20th, roughly.
- Around December 19-23 and beyond, the risk of below-normal temperatures may return, but uncertainty is high.
- The overall pattern remains generally unfavorable for sustained cold weather.
Andrew
8 comments:
I live in the East (North of Boston). I don't understand why there is all this talk about warm temperatures! It's freezing! I do love the cold weather but I am confused about postings, not just on your site, but on our local weather broadcasts about having a mild December. I must say they are now backing away from saying it will be mild. So the east will continue to be somewhat cold? Thank you!
Does January still look like a good shot at cold, potentially more sustained as well? I see the NAO is fading towards the 20th as well.
Andrew past analogs do not support your Dec. view of milder temperatures.
We will see.
Thank you Andrew. Question. When you speak of what to expect weather wise, what part of the country are you speaking of? East coast, mid west, plains, mountains or west coast? I don't know where you are based so perhaps you can break it out for different parts of the country.
There is going to be a NE storm next week. Check out Joe Bastardi's weather !!
Boo. I was hoping to hear talk of a pattern change to kick off 2015. We need the AO/NAO to go negative at some point. What about all the SSW?
Anonymous at 4:44: The month of December is still looking to be warm for a period of time; just what it ends up as in terms of average is up in the air.
Anonymous at 5:02: The month of January should be colder than normal.
Anonymous at 8:11: My analogs do support a warm December, as do the analogs of other knowledgeable weather folk.
Anonymous at 1:31: These indices usually work best for the Central into the East US, unless mentioned otherwise.
Anonymous at 1:45: An SSW is most likely this January.
The Euro control ensembles for the 12/14 0Zrun skirt the system along the southern states than to the Mid Atlantic. GFS model runs had shown inverted trough giving snow to N.Missouri and IL but 6Z GFS backing down to more of a Euro look. In conclusion, may have some snow for the OV and Mid Atlantic but Western Lakes will have to wait till Xmas for a shot at a clipper. Northern stream energy will merge with system to south for East Coast storm. Chicago area looking to turn in paltry snow amts for the month of December. Only a 1/3 of winters with this little snow at this point went on to have normal or above normal seasonal totals.
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