I think it goes without saying that this has been a strange winter. Before I touch base about what lies ahead, I did want to take a second to explain what has taken place.
The big question many of you have been asking is...why no snowstorms? Easy answer: the pattern is just too fast moving. Look at the storm tracks since October. I marked "x" where you would not want a storm track to get a cold pattern for us here. You can see the problem.
When most of our put out our winter forecasts in the fall...we were highly betting on the idea of a strong influence of moisture from La Nina and a "blocking pattern" that would develop. The blocking idea made sense given that we have seen in increase in blocking patterns over the past several winters. This year however...the blocking (particularly the Greenland Block) has been absent thus far.
You need that block to slow down areas of low pressure....and deepen low pressures.
So far, we have only been able to maintain a north wind for say 24-48 hours before it changes direction again back toward the south. The cold air is up there. And it has been extremely cold. We just have not been able yet to slow the storms down to allow for a strong northerly flow into the Ohio Valley.
The one indicator we use alot to see when that is possible, is the NAO. The more negative....the more blocking that develops. However, a positive PNA can also help drive the jet stream up the east coast for a storm or two, but doesn't really help with a sustained blocking.
Here are the latest outlooks for both.
NAO: neutral and slightly positive
PNA: moves back into positive
So those are the two indicators that control the dips of the jet stream. If we were to get strongly forecast negative NAO and positive PNA--- yup...we'd be all about letting you all know that a cold pattern is coming.
That then leads to the next question. How cold would it get? That is where the AO comes in play. If negative...the cold air that has been trapped up north moves south into the United States. If positive...it stays locked up.
Latest outlook of the AO: All over the place..but mostly negative.
Another topic that has come up alot...is the stratosphere. It is the layer just above the troposphere (where the weather take place). It is a fairly calm layer...which is why planes and even some birds fly barely into it to avoid turbulence. It is however a gaseous layer where lots of chemical reactions take place. One of those involves the temperature. We have learned that when this layer warms...it causes the troposphere below to "thin out". That thinning out leads to a lowering in temperature. There is a lag in that reaction however. Sometimes up to 10 days.
There has been warming of this last over the past few days. And the long term still does show more warming.
So if we went with that theory alone, February would see the most reaction of this in terms of colder temperatures to work with.
That means we would then have to watch out for blocking patterns...or that cold that we could tap into...would be wasted elsewhere across North America. And that can still happen.
See how complicated this can all get? Forecasting long range is a challenge as we can use the above tools to help us gauge the pattern ahead...but what those tools don't tell us are specifics about each storm. So while they may show a warm pattern---we still could get a snow event. And if they showed a cold pattern--that doesn't mean we wouldn't see a brief warm spell. So they are just tools, not a hard core forecast. That is why we have our 7 day outlook :)
MY THOUGHTS ON FEBRUARY:
I think blocking will show up more often than it has in recent months...but I am not sure how well it can be maintained. I do think we still run a risk for some very cold outbreaks and big snow events. LA Nina is still with us...and helping with the active storm tracks. Severe thunderstorms are concerning with the pattern...but if we do get the blocking, the storm risk will be limited to the Gulf Coast/FL region. This will be the month of the most wintry weather we will see of the season. Considering we have only had 1.8" so far, that isn't saying much. Do I think we will get my season total of 28"---doubting it more and more...but I am not ready to lower it just yet. I want to see how well the atmosphere reacts to the signs we have been getting lately. If they start to match up well, I will have more confidence in that total. If they don't...obviously I will lower it. March does looks to be an extension of the colder pattern by the way. If you think our pattern has been weird so far, just wait. The next 2 months are going to continue to look that way.
So either I am going to look like a complete idiot...or a genius with this forecast. LoL. Most of you I am sure will go with idiot :)
Just for fun, here is the Farmer's Almanac forecast they just put out for the month of FEB for our area:
FEBRUARY 2012: temperature 26° (6° below avg.); precipitation 3.5" (2" above avg. east, 1" below west); Feb 1-5: Snow showers, cold; Feb 6-9: Showers, mild; Feb 10-16: Snow, then snow showers, cold; Feb 17-20: Snow, very cold; Feb 21-25: Sunny, turning mild; Feb 26-29: Heavy rain to snow, turning cold.
Don't give up yet snow lovers. I know it is very difficult to...esp with so many talking about sun angle changes and such----but the weather is much more complex that such a simple thought as that. We have many challenges ahead...and likely some surprises to face.
BACK TO NOW:
Rain obviously our main headline this evening ...but did want to mention the chance for it to end as a few snowflakes in the morning. Especially across IN where the thermal profiles will be easier to cool to snowflake potential. Rain/snow line well to our south in the morning. Either way, it doesn't look to be anything significant as of this post.
There appears to be 2 clippers this weekend. One Saturday...one Sunday. Both will struggle with moisture.
SATURDAY:
Moves down from the north. Temps at ground level may be warm intially to keep it as a rain/snow mix before that cold layer can lower in the afternoon. Best chance across IN to see all snow...the mix risk comes in once you get into the Louisville Metro.
SUNDAY:
Next fast moving clipper rolls in. It looks to have even less moisture to work with...but temp profiles are certainly cold for this one.
Both system have the chance for maybe a minor accumulation...with the best chance for that accumulation on Sunday than Saturday I think.
NAM Snow Map
GFS Snow Map
Next week looks like another decent warming with rain. Highs in the 50s.
It is late next week and next weekend that the models start to go nuts again.
EURO is showing cold air over much of the country...and dry by next weekend. That looks is a close blocking pattern.
GFS: It is going with a negative NAO blocking idea...where our flow gets stuck out of the NW/N. That would mean light snow events...no major snows...but cold.
Canadian: Not as deep with the cold air, but that does also allow for a storm to develop across the SE USA. That would be a potential rain to snow-maker for us.
NOGAPS: Similar idea...with a low developing over the Carolinas...cold enough for snow locally.
So once again, the models are not exactly agreeing on how things will play out...but it is 7 days away. We'll trend it.
BOTS!
Thanks Brian:-)
Posted by: Michelle Swindall | January 26, 2012 at 04:25 PM
You just gave me more respect for what you do!!! AWESOME explanation!!!!
Posted by: Phil G | January 27, 2012 at 10:50 AM