Cold front moved through this morning dropping temps back to their seasonal norms for this time of year. Temps will now be in the 30's & 40's through the weekend with quiet weather.
We have plenty of time today with the quiet weather so I would like to explain my errors from earlier in the month and discuss what is going on in the pattern. I will be using some acronyms but they are clickable links so that you can look at what I'm referencing.
Up until the second week of January we saw a consistent pattern of a negative NAO and a negative PNA. In a La Nina we do not have much of a Southern jetstream because of the colder waters along the equator that do not support it. So our preferred pattern is a negative PNA and a positive NAO.
The negative NAO can be "negative" for us because it locks in cold over the East that drops out of Canada. That can block the ridge in the West from progressing and keep the storm door closed for us. This can be enhanced by the AO being negative which displaces even more cold air from the Arctic into the U.S. That can be blamed this winter for the record cold and snowcover this winter in the Central and Eastern U.S. What has countered this pattern for us in November and December was blocking that formed in the North Pacific which can happen during a negative PNA pattern. That suppressed the jetstream Southward and displaced and ridging along the West Coast and brought us lots of snow.
Going into January there was no blocking and the PNA had gone a little positive. A positive PNA pumps a ridge over the West and keeps the storm track up into Western Canada and Alaska. That is why I predicted 2 weeks of quiet weather to start the month. What I saw though was the PNA trending back negative as well as the NAO and AO trending positive by mid-month. That led me to believe that the storm door would re-open around mid-month. We did see the storm track get close to us last weekend with the light snow showers we saw.
The second week of January the forecast for the PNA was for it to go Positive again around mid-month which would mean a ridge in the West and the PNA pattern will normally overwhelm any other pattern for CA. But at the same time the MJO began to strengthen and was forecasted to move into the Western Pacific by the third week of January. That is normally the one card that would trump the Positive PNA as it would act as a psuedo El Nino and bring a break through of the Westerlies. Therefore I have been optimistic that we would see this occur by the end of the month.
That brings us to where we currently sit. Unfortunately during the last La Nina this strong back in 1988 I was still in school and the one before that in 1973 I was not alive. So my experience with MJO's and a La Nina this strong are not very great and I would guess the same for most meteorologists. Over the years it has been pretty much a gimme that with an MJO, where the one we have now is, we would see a period of strong storms that follows. So far though with this MJO episode the cold La Nina waters are inhibiting convection and the pattern is not being affected yet nor is it forecasted to over the next 2 weeks. There is still a little time left but not much.
If we take the MJO out of the picture we are left with a postive PNA and a NAO & AO that are going negative again. That spells dry through the end of the month. Looking at the long-range forecast the PNA is forecasted to stay positive and the NAO & AO are forecasted to go postive by the end of the month. That means only half of what we need will happen but the most important part, the PNA going negative, will not.
It is much more common to have a negative PNA during a La Nina and a Positive PNA during an El Nino. Unlike in La Nina years, during El Nino years there is actually a Southern jetstream so it will just come under the ridge. The good news for us is that since it is more rare to have a positive PNA during La Nina it should not last that long. The other good news is that for right now with the Negative AO there is cold air pouring into the country and the ridge over the West is more offshore over the next week, so the temps will stay somewhat cool.
For now we can hope that the MJO still has a chance to work its magic and if not we have to wait and watch for the PNA to trend back negative. Until then I am not expecting any big storms or storm series. Enjoy the nice weather and still great snowpack. Stay tuned for updates.....BA