This is a discussion about some of the weather patterns and their relation to climatology. Long range forecasting has a low accuracy rate, but we continue to improve that the more weather data history grows and computer simulations. This post will looks at some of the parameters that go into long range forecast in the winter months. Temperature trend patterns are easier to identify while snowfall is nearly impossible in longer range. Advance snowfall forecasting can be done if you correlate above/below precipitation patterns along with warm/cold patterns. Then you could conclude if there is above normal precipitation and colder than normal temperatures expected, above normal snowfall would be a likely outcome. However, typical snowfall forecasts cannot be completed with any degree of accuracy until 48 hours out from the event with early indications possible in the 7-14 day range.
Enjoy the read and let's see what happens :)
Ah yes...that time of the year again :)
First off, Happy Halloween!
Now let's get to the fun stuff.
First off...a look at last winter's report:
Our forecast for winter temperature departures...
Actual result (excluding December due to range limits):
We had a VERY warm December which really skewed the numbers.
We ended up with a temperature departure of: 5.4° ABOVE normal
We had the right zones for general warmer/cooler, just was too cool with forecast.
Our forecast for snowfall departures:
Actual result (very challenging to find national maps for this):
We had the battle zone of warm/cold nailed for impact. We weren't high enough on snowfall for N Plains/Rockies and too high for S. Plains.
We ended up with 14.9" of snow for the winter. This put us at 2.4" ABOVE normal.
Snowfall forecast locally was right on.
OVERALL: The winter got off to a slow start as expected. We were aiming for a January "flip". That kicked in January 16th. We remained in an active pattern through February. One of the more interesting events was the January 22nd snowstorm that slammed southern KY especially:
The forecast was a decent one with the main thorn in our side being the very warm December. We knew it was be warm, but +12° was just insane! It ended up being Louisville's 2nd warmest December on record!
To start with any forecast...you have to look at what drivers are at play. And there are MANY when it comes to winter forecasting. We usually end up learning post-winter that one or two really became the ones in the driver's seat. The goal each year is to try to find any patterns either recently or over multiple years that could match with the current trends/thinking. The problem with this idea/theory (besides the obvious low predictability of such) is that new items can enter the equation once winter gets started. I will address those toward the end. If you wish to skip the next part and go right to the forecast, scroll toward the bottom.
I will start with each parameter and list its potential normal impact on OUR weather in regards to that indicator alone.
ARCTIC SEA ICE:
I have found some good truth to this parameter when it comes to the cold that can be available for a lower 48 (United States) attack. The overall extent is in decline over the past half century. However, it still builds up each fall/winter enough to keep that freezer full. The lowest it has been was during that VERY hot 2012 year. We were in the 100s that summer. The ice extent to our north melted to its lowest levels on record. That is the benchmark on the low-end. There is also an average from the 1960s through the start of our decade that is a good high-end benchmark to use. The winters of '13-'14 and '14-'15 certainly "hugged" those levels. For '15-'16...a lower extent in comparison was noted.
The chart below shows that for the most part, the extent in 2016 has been on par with 2015. It rapidly increased in September/early October but the most recent data shows it has slowed down its extend dramatically. Which is very interesting.
Arctic blasts would be significant colder with lower temperatures in the source region to pull from if the extent increases.
PACIFIC DECADAL OSCILLATION :
The PDO is a great indicator to look at for winter patterns. We are most interested in the Eastern Pacific Ocean section of this indicator. History has shown that warm water temperatures along the North American west coast have lead to cold shots into land areas of the eastern United States.
The two indices:
If the water is WARMER along the coast of the western United States up through Alaska: PDO is POSITIVE
If the water is COLDER along the coast of the western United States up through Alaska: PDO is NEGATIVE
If you want a cold pattern for us, you want the PDO to be POSITIVE.
You can see on the chart below that we were negative during the 2010-2012 period when we have record warmth and very little snow.
Then...the switch to positive took place in 2013 and the pattern speaks for itself as a result.
However, during the summer...the waters have shown a cooling signal. Especially as the surface. You look at the chart below in terms of depth of the water. Notice how WARM it was in the higher latitudes of the Pacific especially. But then it faded out.
Having said that, the last frame of this chart does show another warming period starting to take shape which you can see more on the 2nd frame of the chart.
The model runs over the past several months have been too quick to go negative ...but the reality (solid black line) was more on the positive side. The last update is coming in with a near neutral/negative signal for the winter. But are the models now too positive with the forecast?
There is even a difference in how the models are looking this the ocean temperatures ahead. NMME has a more positive PDO along the coast. CFS....not so much.
There is no doubt this will be a major driver this winter. However, confidence in this being another positive PDO winter is low for now. The ocean temperature trends will need to be watching over the next 30 days especially to fully gauge this since the outlooks are just too on the fence right now.
This is the cooling of the central/eastern equatorial Pacific Ocean waters. It was a hot topic at the end of our last winter. The forecast then was for a strong La Nina by the summer into the fall. However, it became clearer over the summer months that the data was too aggressive. We trended more neutral to only a slight/weak La Nina. Here is how the CFS model has handled the La Nina forecast over the past year. Once again, look at its forecast (red) and what actually happened (black).
We quickly dropped to neutral but then slowed....and have nearly held steady since.
What happens with a La Nina pattern in the winter?
More of a dominate polar jet and a quiet sub-tropical jet to the south. This can allow for some interesting winter storm setups, but it can put us at risk for dry and cold as well. All depends on any potential phasing...if any.
La Nina winters can be significant for wintry weather in our area ...depending on its strength. The current forecast continues to hold at a weak La Nina and/or (perhaps more likely) neutral conditions. The result? Not a strong driver for the winter at this point as other indicators could influence the pattern more so.
NORTH ATLANTIC OSCILLATION:
The NAO is certainly a driver to watch out for in the winter. Unfortunately, we cannot get data on this until we are say a week/two weeks out. So it is hard to use this indicator in a winter outlook. The main driver with this indicator is pressure. The low pressure over Iceland plays a balancing act with the high pressure over the Azores. Then the pressure difference in between the two increases, storm system over the United States move quickly. Very little time for significant impact. However, when the pressure difference is low (-NAO)...the storm track slows down. And we enter what we refer to often as a "blocking pattern".
It is another significant driver for our winters. However, it is more a short-term tool that we will just have to monitor carefully. Just remember a NEGATIVE NAO would mean stormier/colder setup; POSITIVE NAO would feature less impactful systems and likely milder/water.
This indicator has been long thought to be a driver in the cooling/warming patterns on the globe. The 11 year cycle of the solar activity is what we look at. We are currently on the downward trend of our 24th cycle. And the peak of this one was much lower than previous solar cycles in the past. And yes, we have had some cold winters as a result.
It certainly aids in the cold air source regions being well...cold. Not a huge driver for us as it does not influence our entire story. You can have the cold...but if it never funnels into our area....it will not matter to us.
This has been a growing tool for meteorologists when it comes to what many call "signal" or "organic" forecasting.
Rossby waves are high latitude waves that rotate around the globe. Models now are tracking them to try to pick up on a pattern that will likely repeat again later in time. Or maybe not? It is intriguing to say the least.
I did pull the data from the end of September on how it looks for our area through March. I posted the temperature profiles below for each month. We are looking for the lines to tighten up close together for a "strong" signal of that event.
Already seeing signs of a significant cool-down toward mid-month on the models today. So to see this outlook from 3 weeks ago is impressive. Notice the signal for a potential warm-up before Thanksgiving with perhaps a storm system shortly after that.
Starting off with a storm system to track early month. But it is later in December that catches my eye. I have seen this signal show up for several updates now. This would be the period of Christmas to New Years. Could this be an impacting winter storm then? Perhaps. However, if this timing is right...we would just barely miss out on a White Christmas. Barely. Signs of a warm-up just before it. Which makes sense for deep troughs/low pressures. Certainly an interesting piece of data.
Another up and down month on this one. A cold attack early/mid month. Another later month. But also some decent signals of warm surges.
Could we make it 3 years in a row of wintry weather near Valentines Day? This indicator certainly hints at that. But when I dove into the data more, I will say it doesn't look like all snow if the setup pans out as shown. Snow ....Ice.... Rain to/from? We shall see.
Another March with potential cold attacks fairly late in the season.
I do think this is a fascinating one. And while I think the dips are interesting where they are placed, I do think it shows more of the pattern being warmer than average with some cold attacks to prepare for. Remember, this type of forecasting is to be used as a tool...and not verbatim.
This is one that I still am trying to find good data when correlating to winter patterns. You get a natural "high pressure" that forms over widespread severe drought areas. And it is tough to shake those patterns up.
And this year, we have a zone very close to us.
We are already seeing the impact of this with fronts/troughs that make a run at us and basically bounce off and move into the New England. Could we see this hold true for the winter? It is certainly possible. And a weak La Nina would actually aid in that.
A potential significant one. But not enough data to have high confidence. If realized, it would be a warm/dry signal for us.
FORECAST THOUGHTS: As you can tell, there is A LOT to consider when putting any long range outlook together. Especially one in the winter. Our overall thinking is that unlike the past few winters, there is less support of a mid-winter "flip" that would lead to a core of winter cold/snow. Instead, the temperature pattern looks to average near to slightly above normal from December into March. Significant cold attacks are still expected, however. This could easily lead to sub-freezing days at some point during the winter. Having said that, the period of normal/above would balance those cold attacks out to give us the general average posted below in regards to temperatures. So why is the snowfall forecast higher than last year? With the core of the cold attacks focusing on the Great Lakes/Northeast, this will put WAVE Country in the zone for a risk of warm air advection winter events. Which would mean larger snow events are possible. It also means that we will find ourselves more on the fence than previous years. I know many of you are aware of the dreaded rain/snow/ice line that divides our viewing area every year. I think we are in for one heck of headache this year with several of those setups. It is nearly impossible to say how many of events and what types. We won't know that until days...or hours before they happen. We just see the lack of deep intrusion of the cold putting us more at risk for such. I do like the signal analysis for a cold wave later in November and perhaps a significant wintry event between Christmas and New Years. We will need to see where we stand at that point to get an idea how the rest of the winter will hold up. We plan to do a winter forecast update near Christmas to provide such information. As far as severe thunderstorms, being on the edge of these cold attacks would certainly raise eyebrows to the potential for thunderstorms. The concern here is the dry air to our south may limit the moisture content for such. Something we will be monitoring.
WAVE Country Winter Outlook with Louisville as the main point of reference:
1-4° ABOVE Average
We were 5.4° ABOVE last winter...although the 12.1° ABOVE in December '15 really pushed that value up.
Average is 12.5". Our forecast would put us 1-5 inches above normal range. We reached 14.9" last year.
Things to watch out for this season for adjustments:
*PDO turns positive and holds
*NAO turns negative for a period of time
*High pressure ridge over the southeast remains strong through December
*USA Snowcover southern edge by New Years. The more north, the less impacting cold shots will be.
*Dry air to our south "chokes" winter storms with significant dry slots
Kevin Harned will have national maps to go along with this forecast along with more details...coming up Thursday evening on WAVE 3 News.
Thanks for reading!
Bring On The Snow (BOTS)!
Credits: NOAA, www.organicforecasting.com, Knox College, NWS Louisville