1730 EDT 15 SEPT 2012
I suppose I probably should have posted on this upcoming pattern change a few days ago when it became apparent that the pattern was going to amplify and turn significantly colder as we came to the end of the month. But I need a few days to recover from the intensely long and very active summer and hurricane season.
Over the past few days as I see more posts and forecast talk about the upcoming pattern amplification …I have seen a lot of references to high latitude blocking developing with remarks such as ”LOOK at the -NAO that forms!. …” As we will see shortly with the graphics and discussion… If you are reading comment from some of weather enthusiastic or weenie or even a professional meteorologist in reference a possible g -NAO in the last week of September… then you are reading something from someone who doesn’t know what the hell they are talking about.
Over the past few days most of the model data has been showing the upcoming pattern amplification quite nicely. The model agreement between the Canadian the GFS and the European model have all been pretty good since about Monday Tuesday (Sept 10–11). At first some of the model data especially the European was developing a large cold (cold being a relative term in reference to mid and late September) HIGH over Northern Canada and dropping it towards Montana and the northern Rockies . But the European model saw the light and corrected itself back on Thursday and Friday ( yesterday)
Let us start by taking a look at the 12Z Friday operational European model with surface pressure and the 850 temps. We see two maps here but there both showing the same thing which is what the model says the day 10 surfacemao and 850 mb temps will look like on September 24. Now if this model depiction of the cold air mass coming south of the Midwest is correct this would bring about a killing freeze or frost for most of the Midwest on the morning of your September 24 or September 25.
This event is going to unfold essentially in three stages. The first stage hass to do with initial cold front which will be driving towards the Appalachian Mts and the East coast on Monday and Tuesday September 17 and 18. Most of the model data shows the trough itself over the Midwest developing a slight negative tilt ( the trough axis is aligned NNW to SSE) which allows for the cold front to slow down and to develop significant showers and thunderstorms with it as it nears the East coast.
As a result the front will see an increase in the amount of shower and thunderstorm activity. Right now it looks like the best thunderstorms and heaviest rains will probably be over portions of the eastern Great Lakes… northeast OH western and central PA… western and central NY state …then up into western or northwestern New England. For the Middle Atlantic region the cold front looks to be a fairly decent event but not a huge event. There will be a lot of moisture flowing up the front from the large western Atlantic Ridge ( the ” Bermuda High”) but the really strong dynamics which brings about widespread severe weather will not be favorable for the Lower middle Atlantic regions.
This next map — the 12z GFS ensemble– shows quite nicely how this event is going to develop with 3 ” surges”. The first surge as I mentioned above has do with the establishment of the deep trough over the eastern half of the CONUS and the INITIAL cold front the moves into the East Coast September 24 (the map on the left). The SECOND surge of energy comes into the western Great Lakes and the upper Mississippi Valley SEPT 21 -22 but it still does not quite reach the East Coast and most of the cold air is still- (the Middle map.) And the 3rd SURGE comes into the Midwest and East coast and it is this third surge which finally reaches the East Coast on or about September 24 ( map on the right).
The 12z CMC ensemble mean is in strong agreement with the 12z GFS ensemble
Now if we take a look at the day 14 SEPT day 9 runs at 4 12Z of the European and the GFS side by side ….we see very strong model agreement. We can clearly see several important hemispheric patterns on and this map. Most prominent is the massive +PNA ridge which has formed over Western Canada into Alaska along with the very strong Aleutian low ( -EPO). And we can see the very strong Eastern Atlantic Southern Russia ridge which is often referred to in more popular terms as the Caspian Sea or Siberian central Asian Ridge. When the -EA/ WR Teleconnection is in the negative phase it can lead to +PNA development which is exactly the case here.
For those you that do NOT fully understand the Eastern Atlantic Western Russia pattern you can read about it here .
Now let’s talk about whether not this pattern has a negative phase of the NORTH ATLANTIC OSCILLATION. The basic answer that THIS map for DAY 9 does NOT show a -NAO. This is one of these patterns which confuses a lot of people including some professional meteorologist you should know better but for whatever reason to exaggerated and HYPE things which don’t actually exist.
The red shading over the which barely pushes into southern Greenland is not and can never be counted as a -NAO. Indeed if you look closely at this map you will see a deep 500 mb Low over Iceland… which by definition counts as a +NAO. Instead what we are looking at here is a strong ridge over the Northwest North Atlantic Ocean… buts that all.
In the updated version from this afternoon’s modes… SEPT 15 valid day 9… we see even more proof that this is not in anyway a pattern that features a -NAO pattern. We see a lot of dark red shading which represents strongly above normal 500 MB heights over the northwest Atlantic and the Canadian maritime provinces which barely touches southern Greenland. And both the European and the GFS have a deep closed 500 Low located over Iceland!!!! Which again by definition can only mean you have a +NAO .
Next you can see a series of plots from Allan Huffmans’s weater Model page. This first one shows the various weather models in the depiction of the coming strongly positive PNA pattern . And we can clearly see the all the models are in strong agreement.
Then we can see the next image which are plots of the EPO ( eastern Pacific oscillation or the Aleutian Low) and again there is strong model agreement that the EPO is negative.
Now is the the NAO Plots. And notice that none of the models are showing anything but neutral NAO… which makes a lot of sense given that we have someone above normal 500 MB heights over southern Greenland but also a closed 500 mb Low over a Iceland.
Now at this Point I am usually bombarded with questions about the CPC forecasts which have for the past several days been showing a distinct drop into the negative phase of the Arctic oscillation (AO) and the NAO . I am sure that over the next 200 days I am going to have to make this point many many times: the CPC plots / forecast for the AO PNA and NAO are essentially useless crap. The reason for this is that CPC uses criteria that defines what the AO is… what the PNA is .. what the NAO is… that nobody else uses.
There are so many cases of this happening that my computer hard drive sometimes cannot fit them all. For example there will instances over the next 200 days where the pattern on the West Coast is showing a ”split flow” but because of the criteria used by CPC the PNA forecast from CPC will show a strongly negative PNA or a PNA that turns strongly negative .
To see how this tendency often works we can look at the actual CPC forecast for the AO and the NAO and compare them to reality/ facts . In this image we can see the forecast for the AO / arctic oscillation on the right hand side with the top chart on the right hand side showing the actual forecast. As you can see as of September 15 it looks like the arctic oscillation is turning sharply negative. But we compare the CPC forecast of the AO at 7 Days… 10 Days… and 14 days.. The tendency for CPC forecasts to overdo the AO to the negative phase is VERY clearly.
Looking on the right hand side is chart we can see this Bias in the CPC/ GFS ensemble forecast very clearly.
This is also true with the NAO / north Atlantic oscillation pattern. As we move deeper towards November and the heart of the winter …the negative bias or anomalies that the CPC forecast shows now at day 7 day 10 and Day 14 … will become quite strong.
On last issue to discuss and tha has to do with the 12z sept 15 Run of the European model developing a fairly significant LOW pover the southeast US SEPT 23. Now as we discussed above the models from this morning and yesterday were not showing this feature of the new European model has a strong piece of energy in the southern jet stream over the lower Plains on the morning of September 23. ( again this feature was NOT there on the earlier or previous model runs. This feature merges with the new piece of energy — SURGE #3 as I discussed above— which I have highlighted with the capital “B” over South Central Canada. Some 48 hpurs later (Day 10)… the 2 systems Merge or ” phase” into one system over the East Coast …. Note surface Low tracks from SC to VA .
The 12Z SATURDAY Sept 15 run of the GFS has no such southern stream feature but given that it’s past 180 hours is often the case that the GFS simply does not detect energy in the southern jet stream until you get within 144 or 120 hrs.