September's outlook is not as clear-cut as prior months, with more disparity amongst guidance. NMME is once again going all in on a warmer/drier month. The ECMWF Weeklies, CFS Monthly & CPC are also hinting at a warmer than normal month however precipitation forecasts are a bit murkier. The ECMWF Weeklies & NMME call for below normal precipitation, however the CFS Monthly is forecasting above normal precipitation. CPC is non-committal and is calling for, "equal chances". Beginning with the first ten days of the month, there is fairly high confidence that a robust ridge will set up over the western US with a strong -NAO signal. With a setup such as this, I am expecting above normal heights/zonal flow for New England, which should result in seasonal/slightly above average temperatures. Looking further, the general pattern for the month shows a ridging pattern across the western US and troughing pattern across the eastern US. If a pattern such as this were to verify, seasonal/above average temperatures could occur along with increased chances of precipitation, which New England desperately needs. Having said all that, I am expecting temperatures to be slightly above normal for the month with average to just slightly below average precipitation. Although the tropics have been quiet thus far, September is climatologically the most active month so it'll be worth monitoring what transpires because it just takes one storm or remnants of one to move inland or along the coast to give New England a good rainfall event. Much of Southern New England is in a severe or extreme drought so any rainfall would be welcomed.
VERIFICATION New England finished slightly warmer than normal for the month as expected with ridging to the west and -NAO which resulted in overall zonal flow across the region. New England also finished wetter than normal which was largely due to a very moist, slow moving frontal system in the area on September 5th-6th and then another one September 18th-20th.
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The time has come; the long awaited winter outlook. Before we get into the weeds, it should be known that this outlook will likely change from time to time as newer information is received and forecast confidence increases. The structure of this outlook is going to go through each teleconnection separately, and then at the end tie it all together. So let's begin: El Nino Southern Oscillation (ENSO) La Nina conditions will likely continue through the rest of the year, albeit a weakening trend the rest of the year. Once the calendar flips to 2023, I am anticipating La Nina to be weak, if not neutral. Most guidance suggests a weak (borderline moderate) La Nina to carry through the rest of the year before making a push towards neutral. A common question that may come up is, "will the progression of the MJO or CCKWs disrupt this?". While some may argue that it could, I personally don't think it'll be enough to generate enough upwelling of warmer, subsurface water that is currently over the Western Pacific to eradicate La Nina. It may weaken it some, which models are suggesting, but I don't think it'll be persistent enough to flip the script. Looking at the current EPS & GEFS Velocity Potential Anomaly Control plots, there appears to be some MJO/CCKW progression across the Maritime Continent mid-end September which could have a ripple effect in the current ENSO forecast if that were to come to fruition but until then, I wouldn't put much stock into it at the moment since it is still over a month out, however it is certainly worth monitoring. Indian Ocean Dipole (IOD) The IOD is currently negative as indicated by the strong westerly winds across the western Indian Ocean. It is forecast to remain negative through at least the end of the year, similar to La Nina. The -IOD helps reinforce La Nina, which is another reason why it may be difficult to wash out La Nina before the end of the year. The IOD climatologically falls back to earth around January, so it could leave La Nina more vulnerable to a transition around that timeframe. Quasi-biennial Oscillation (QBO) The QBO just became westerly and will likely remain in said phase through at least early next year. A +QBO usually means a stronger jet stream. As a result, an uptick in winter storms could be favored, however the chance of Arctic intrusions are lower. There is still a lot of research being done on the QBO and its exact affect on winter weather so just keep that in mind when reading any forecast that integrates the QBO. Pacific Decadal Oscillation (PDO) The PDO has been negative since late 2019, which also interacts with ENSO. When the PDO and ENSO are in phase with one another (like they currently are), ENSO tends to be "stronger". Having said that, I also expect this index to remain negative through at least the winter season. Atlantic Multidecadal Oscillation (AMO) The AMO continues to remain positive, and will continue through this winter season. A +AMO favors the frequency of a -NAO which is an important piece to netting big winter storms. October Siberian Snow Cover Extent Still too early in the game for this but will be revisited when that time comes. Siberian snow cover extent in the fall helps strengthen the Siberian High via surface cooling. As a result, poleward heat flux increases, weakening the stratospheric polar vortex which can result in a -AO in the troposphere. Methodology I sorted each aforementioned dataset by picking out the years in which I think will closest match my forecast values for each index. Starting with ENSO, I selected years where the Nino 3.4 ONI for DJF & JFM was between -0.9 and 0C since I think we will be looking at a weak La Nina by the end of the year, possibly becoming neutral in early 2023. From there I filtered that down to years where the ONI reduced between DJF & JFM. For the IOD, I selected years that it had a negative index in any regard (note: -0.4 to 0.4 = neutral IOD) concurrently with the filtered La Nina years I chose. For the PDO, I selected years that it was in a negative phase concurrently with the filtered La Nina years I chose. For the QBO, I used the same process for when it was in a westerly phase. Lastly for the AMO, I selected years that it was in a positive phase concurrently with the filtered La Nina years I chose. Once I ran my filters and looked everything over, the IOD years I had selected were not negative or positive enough (i.e. neutral) to give it much weight in my forecast. At the end, I had eight sets of years to pick apart further. The years that matched the best to what I'm expecting were 1964-1965 & 1971-1972. Breaking those two sets of years down, 1964-1965 had a weak La Nina in 1964, becoming neutral in early 1965. It also had a -PDO & negatively neutral IOD between December-March. 1971-1972 had a weak La Nina in 1971, becoming neutral in early 1972. It also had a +QBO between December-March. The biggest difference between these two datasets was that 1964-1965 had a negatively neutral IOD (1971-1972 had a positively neutral IOD), and 1971-1972 had a moderately +QBO (1964-1965 had a weak, -QBO). When running the analogs of these years separately, they were nearly identical, with the exception that the Central Plains showed negative temperature anomalies in 1964-1965. New England also showed negative precipitation anomalies that year. Below are 1964-1965 & 1971-1972 blended composites. I had three other sets of years that had the second best analog scores which I took into consideration (1983-1984, 2005-2006 & 2011-2012). Aside from having a weak La Nina either becoming neutral or staying weak by the early part of the following year, they all matched up with either a -PDO, +AMO or negatively neutral IOD index so I decided they were worth including. The last three sets of years (1954-1955, 1995-1996 & 2000-2001) did not match up with any of the indices I was tele-connecting, so I decided to omit them. Forecast
Bringing everything together, the following are the composites that were generated. Reading this verbatim, the NW US and central-northern Rockies will see below normal temperatures. Analyzing the precipitation graphic, most of the country will receive average precipitation with a couple exceptions. The Pacific Northwest will see above normal precipitation and coastal California/Desert SW will see below normal precipitation. The 500mb composite supports the cold outlook fairly well, with deep troughing extending from the western to central US, with neutral heights over the Pacific and Atlantic coasts. My "adjustments" to the following composites are as follows:
Not shown, but I've also looked at the SEAS-5, CFSv2 & NMME seasonal forecasts. This blog post is already long enough so I won't dive into extensive details here, but I think SEAS-5 and NMME are running too warm, however I do like the look of the CFSv2 from a temperature standpoint, which has more of a classic Nina look to it for DJF & JFM; definitely a model I'll be following along with over the next few months. If you've made it this far, thanks for reading and stay tuned for additional updates as more information is received. QBO has now entered its westerly phase (Figure 1) which could have some implications for this winter. Generally, a western QBO = stronger jet stream which could mean an increase in winter storms, however Arctic intrusions aren't as favored (although still possible). Coupled with La Nina which is a small sample size (1975, 1985, 2008), 500mb gph anomaly plot doesn't appear too favorable for the blocking needed for "productive" winters in Mid-Atlantic/New England, with a +EPO/+NAO setup shown in Figure 2. Dec-Mar anomalies for these 3 years show cooler than normal temperatures for the northern & southeast US (Figure 3), with average precipitation for most of New England and below average for Mid-Atlantic (Figure 4). Disregarding ENSO phase, deep troughing is still seen over western/central North America however we do see a weak -NAO signature (Figure 5). Temperature/precipitation anomalies are still below normal for the aforementioned regions, however the western US shows a noticeable uptick in precipitation (Figures 6 & 7). SEAS-5 (Figures 8 & 9) & NMME (not shown) forecasts are pretty much opposite so it will be interesting to see how this winter pans out. Still lots of research is being done on QBO and its exact affect on tropical/winter weather phenomena so any seasonal forecast w/ QBO involved should be taken cautiously.
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