psuhoffman

EPO Ridge Snowfalls I will start with the EPO dominant storms since we have been talking a lot about the EPO lately. This explains an EPO ridge some The EPO ridge shown there is a central (or even west based) EPO. Those don't do us much good without some HL blocking. The ridge shown there would likely lead to the storm track in red on the map. IF...we had some HL blocking to suppress the flow and hold high pressure in then the track in purple is more likely. Without any HL blocking...the EPO actually is almost equally able to bully the pattern into a snowy one as much as the NAO...9 events to 10 with only those factors in favor...but it has to be a specific type of EPO ridge. Below is the loading pattern for each of the 9 EPO driven snowfall events absent atlantic help You can see we need the ridge to be centered into northwest Canada NOT out over the PAC. That configuration can create a pattern favorable for progressive waves that get under us. We actually want the PNA negative for this to work. A full latitude ridge with no atlantic help just is cold and dry. The western trough with an EPO ridge over the top leads to a pattern like this... If the EPO ridge is a west based EPO then we need at least SOME atlantic blocking help. These all feature a -PNA also because a western EPO ridge almost always leads to a -PNA. There were no examples of a west based EPO ridge without any blocking help. Since we are starting a central PAC ridge in the face right now on guidance I will include those analogs. The ridge north of Hawaii is really a WPO ridge more than a EPO although the two overlap some but a ridge centered there can only work with extreme NAO blocking. There were no examples of a snowstorm with a ridge there without NAO help. This is the composite from those snow events.

Snowfall Study 5"+ events at BWI since 1948 88 Events studied For my snowfall study I looked at every warning event (5"+) at BWI going back to 1948 (that is when the upper air data is available). I know some of those events happened over a longer period than 12 hours and might not technically be a warning event but I thought 5" for the whole event was good enough. I used BWI because I feel like it is a bit more indicative for the urban corridor than DCA. If you live on a slab of concrete on an island in the Potomac south of DC...I apologize to you...and I feel bad for you, but oh well. I also included 3 events that fell just shy (they were all 4.8 or 4.9" at BWI but were significant events for a large portion of our area. That might be cheating but its my study and I can do what I want. Perhaps one of these events didn't hit your exact location the same...but when a storm misses by 200 miles its the pattern...when it misses by 20 miles its bad luck. So the patterns that caused these storms show potential, even if a particular event wasn't as good at your house as BWI. I then looked at the upper level patterns. For each storm it took a bit of analysis because sometimes I had to go back over the week leading up to determine what the predominant pattern or feature that lead to the storm was. Sometimes the key to a storm is something that happened the week before, and if you just look at the day of the storm you can't tell why the pattern was right for a snowstorm at all. So some of this is a bit subjective...the index might have shows something different on the day of the event...but if you look at the week leading up to it the pattern was in a different state. Also some h5 features key to a snowfall here can be invisible on the indexes. A Hudson Bay ridge might now show up. An extremely west based NAO Block can be muted by a trough in the eastern NAO domain or a trough near the Azores. An east based epo can be hidden by a trough in the western epo domain. I wanted to identify what features at h5 really indicate snow potential. So with that out of the way... There were 88 warning level events going back to 1948 Looking at each index on its own The NAO was the most important single index for predicting snow 58 of the storms featured a -NAO 3 were neutral 27 were positive The AO was second most predictive 54 featured a -AO 9 neutral 26 positive The EPO was evenly split 42 -EPO 4 neutral 42 positive The PNA surprisingly had more events in the negative phase (however most of our really HUGE snowstorms did features a +PNA ridge) 44 negative PNA 5 neutral 39 positive This was the breakdown for the index combinations and snowstorms for each combo the order for the chart is EPO/PNA/AO/NAO - - - - 16 + + - - 13 - + - - 11 + - - - 10 - - + + 9 + + + + 6 - + + + 3 Some take aways from the chart... the most common snowfall combo is a pattern driven by a -EPO ridge in conjunction with atlantic blocking. The next most common is a +PNA ridge with atlantic blocking The 3rd most common is a combo EPO/PNA ridge with atlantic blocking. So....atlantic blocking with ANY pacific help from either the PNA, EPO, or both...accounts for 40 of the 88 storms. After that we are left with storms when only ONE factor was working in favor of it. For those.... 10 were purely a function of High Lat blocking overcoming a crap pacific 9 were purely from an EPO ridge with no PNA or High Latitude Help (All of them were an east based EPO ridge) 6 were purely from a favorable PNA ridge and no other help Only 3 came from a combo EPO/PNA ridge with no blocking. So for everyone that throws around 1993....that was one of only 3 storms that hit with that look. A huge EPO/PNA ridge with no blocking is just cold/dry and progressive. Usually a quick cold shot then a warm up. One last note... by far the single biggest factor if we had to work with only one thing to get snow...is the Hudson Bay Ridge. We had 16 storms where the only really positive feature working in our favor was a Hudson Bay Ridge. Most of those storms don't show up on the chart because that feature often falls in between the typical index domains and in a lot of those storms all the other factors looked like crap. Apparently a Hudson Block is a local enough phenomenon to overcome pattern issues in a lot of other areas. It was by far the biggest way we got a snowstorm in an otherwise crap pattern. Also there were no cases with a -AO as the ONLY factor working in favor. That is not because the AO isnt important its just really hard to have a -AO without there being an EPO or NAO ridge because there is some overlap of those domains. It's also hard not to have EPO or NAO help...since the TPV being located near one usually opens the door to ridging near the other. And if we did have lower heights all across both domains the AO would likely be very positive. I will show some of the common composites to these patterns in following posts

I have been seeing some of the same questions posted wrt patterns lately and been getting some PMs about it so I thought I would post some stuff I have been working on for a while wrt snow climo and patterns. Others can add to this also. I did a study of all warning level snowfall events at BWI going back to 1948 and I will post the results below but first I thought I would give some info. One disclaimer: most of this research and information is focused on warning criteria snowfalls. We can fluke our way to a minor advisory level event in almost any pattern during mid winter. It really muddies things if we include a lot of 1-3" type events. But there are clear patterns if we exclude minor events and only look at what patterns are right for 5"+ events. I am not dismissing minor snowfalls but its really hard to identify patterns for a 1-2" snowfall. Some common pattern driver indexes we reference The colder phase of these indexes are -EPO (ridge), +PNA (Ridge), -AO (weaker TPV over the pole), -NAO (ridging). But colder doesn't always equal snow. There is no one pattern for snow. The trick is getting the jet right so that we get a storm into our "box". The most common tracks to get a significant storm into that box are shown. But there are a variety of pattern combinations to get the jet wavelengths right. Changing one factor can change what we need from another. But the key is getting the jet aligned for the track of storms to end up just to out south or southeast. For example...most of us know about the idea -NAO pattern... But...a -EPO pattern can work too or a Hudson Bay Ridge I will go into specific patterns and what percentage of our warning criteria snowfalls came from those patterns in the next few posts

You didn’t ruffle my feathers, relax. I do think you misinterpreted “hypothetical” and/or/if type analysis as forecast. I did say that if the pac ridge is correct we need NAO help. And I do tend to buy the pac ridge as likely. But I’m still unsold on the NAO. For the record I predicted about avg snowfall this year but I never expected a putely pac driven pattern to work. Issue is I interpreted the analogs to indicate we would get some NAO help this year. But what’s happening now isn’t a good omen. When the AO and NAO go this positive it can be a persistent issue. Add in that we are in a long term +NAO cycle and I am not as optimistic as I was 2 weeks ago. The pac isn’t all that different from what I expected. The issue is I’m seeing some early signs we might not get the high latitude help we need. I’m not punting a damn thing yet though. And even if it becomes apparent we’re very unlikely to have a good winter I will still track in hopes of one fluke storm. Some of those dreg analog years had at least one decent storm.

What happened? I miss something?

Thanks. Honestly others seem to have been more offended by him than me. Anyone is free to question and challenge anything I say. I’m a big boy. I wish I could analyze a bad pattern hypothetically without it meaning I punted winter though. But I don’t take anything personally. I’m also used to dealing with teenagers everyday so....

My problem was obviously making my analysis before the eggnog not after.

On the other hand...if we assume we will not get any help from the NAO...this is the composite of all snowstorms since 1948 that featured an EPO ridge without NAO/AO help. Notice the location of the EPO ridge. It is VERY possible to get a snowstorm from the EPO alone...its actually only slightly below the NAO in terms of if we had to work with only one factor...HOWEVER that EPO ridge must be east based in NW Canada not out over the Pacific. So if we assume the AO/NAO do not improve we would need that ridge to end up significantly further east than projected currently.

If we get a dominant central Pac ridge pattern...there have been 10 January/February warning level snowfall events in our area with that regime, but every one had a pretty significant NAO block. This is the loading pattern a couple days before those 10 events. The point is for that ridge alignment to work would require an equally anomalous NAO block.

It’s not personal...lol. I understand people don’t want to hear it doesn’t look good.

Most of us were on Eastern then. And I remember discussing the chances for snow late/Jan into Feb from well in advance. Yes there was a mild period but even then there were markers. The high latitudes remained conducive. And extreme blocking tends to recycle. I haven’t punted anything yet but I stand by what I said. If....We get to mid January and that pac pattern locks in with a raging positive AO were in big trouble. That look has NEVER ended well and it can be very persistent and hard to break out of. That look has accounted for some of our very worst winters like 2002 and 2012. Im not rooting for that. I want snow. But it’s hard to ignore what’s showing up in all guidance. Me saying that look sucks won’t change anything. Hopefully it never comes to that. There are ways out. Maybe the guidance is misplacing the ridge and it ends up creeping into NW Canada over the top. That’s an ok look. Or the NAO could flip suddenly. But we are flirting with what is the absolute worst look we could get for winter prospects on guidance right now imo. That’s based on all past seasons with a similar pattern. If me pointing that out bothers you...well sorry but not sorry .

I’m still not invested. My opinion isn’t changed by a couple day 9 op runs. There are very few decent snowstorms from the current pattern shown that period. There are a lot of very minor 1-2” type snows in that look. But it’s mostly a cold dry look that usually leads to suppressed sheared our waves followed by a warmup. Once in a while we get very lucky and I’ll root for that but at long leads climo beats op runs 90% of the time. As for your second point. I spent most of Nov and Dec very positive. I thought there were enough positive signs. In the past I have been very bullish. I have often pointed out looks I felt good about from really long range. Some hit. Some didn’t. But right now I see things I really don’t like. You can question my accuracy but at least my thought are authentic. If you want smoke blown up your arse I’m not the right one to look too. “if the guidance is right about the pac pattern pray the NAO flips or were toast.” I assume that is what you’re referencing. Sorry but that’s a factual statement. If that anomalous pac ridge is real and we don’t get some high latitude help history says we have almost no chance of snow of any consequence.

Yea me seeing where the pattern might be headed made it happen. Lol

Unless the Atlantic improves I fear that’s very likely what’s coming. That anomalous central pac ridge is on all guidance and it’s supported by the mjo progression and recent history (last year). I fear it’s likeky to be a reality. If so without NAO help that will lead to a huge ridge in the east. Last year we actually had just enough Atlantic help at times to prevent an all our SE ridge fest. If the AO/NAO stay positive and the pac ridge is real we will torch and it will likely last a long time. Im still working on the climo post. Family stuff came up yesterday. But I did finish the analysts of all warning level events going back to 1948. There are some Jan/Feb storms with a strong central pac ridge but they all had extreme NAO help. Not a single one had a ridge there with no NAO block. Reality is if the guidance is right about the pac pattern pray the NAO flips or were toast. Ironically I know people are rooting for that pac ridge to shift east but our better chance according to the pattern analogs is for Atlantic blocking to help retrograde that ridge a bit. It would put a trough into AK and cut off any chance of really cold air but also split the jet out west and let the stj cut under which would give us a fighting chance at snow. There were a few March events with a pac ridge absent NAO help once wavelengths shorten and funky stuff can happen. But it’s a shutout look in Jan Feb. Possibly the worst one there is.

I have to agree with that assessment. That’s why I’m stuck on the NAO. Every analog to what we are seeing so far this winter failed to develop into a favorable PNA pattern that winter. Those that did not get NAO help were utter disaster fail years. Those that got “some” NAO help salvaged respectable but still blah totals. A few got excellent blocking periods and had a very snowy period later in winter. Our fate rests in the Atlantic being able to offset the central pac ridge pna trough imo.

Leesburgs sass aside (he doesn’t get that I LIKE doing this win or lose) I’m not too invested in the Jan 6-12 period. It could work. But we’ve seen a LOT of these epo driven gradient looks the last few years look great at day 8-15 only to become mostly rain (maybe some slop to start) waves that track to our north. That’s what climo says that type pattern favors. But it does work out every so often. Usually if we get lucky with a transient 50/50 or tpv lobe in the right spot. It could happen. Just not the most likely outcome.

Looking at all the New Years period comps to this pattern that’s the only realistic save. It wasn’t all bad. Some (about 1/3) of the analogs to this current mess were in went on to a snowy period sometime later in winter. But they all did it the way you described. With an Atlantic ridge that retrogrades forcing the pac jet to split and cut under. I know some are rooting for that pac ridge to shift east (and that would work) but that’s not the most likely way we evolve to a better look according to the analogs.

It looks good through day 12 then reverts to this bwfore that there was a significant improvement with several big hits and numerous small ones in the day 10-13 range. I am hesitant though because it’s got the kind of gradient look that teases us day 10 only to become congrats upper Midwest and New England come game time. But the look at the end has a silver lining. The tpv is vacating Greenland and it’s easy to envision ridging building into the NAO space after that given the evolution day 13-15. Honestly imo that’s our best chance at a sustained favorable pattern. Imo the pac has shown it’s cards and frankly it’s a similar pattern to last year. The wpo/epo ridge is likely to remain a bit west of where we need it absent any blocking. But the qbo is more favorable this year so if we can a period of NAO help that pac becomes workable. I’m hopeful we see signs of the next -NAM period showing soon.

12z GEFS really degraded our snow chances day 10-15. Lost the eastern Canada ridging idea. It’s building a better Atlantic pattern way out at day 16 but it pushed the process back 5 days, back to fantasy range again. Perhaps it’s the right idea but was rushing it but until I see the eps move that way I am skeptical. EPS has been crushing the GEFS lately.

You are trying to learn. Don’t apologize that’s awesome! It’s much preferred to those that know nothing, make no effort to learn anything, and fill up the threads with nonsense year after year. Ive been getting the same kind of questions both in the threads and in PMs lately so later today (probably this evening) I’m going to revive the old winter weather classroom thread we had with explanations of various pattern drivers and analogs to past storms from various patterns with explanations of what made them work. I’ve been working on a project for a while looking at past mid atl snows and categorizing them by pattern type. I intent to finish today and post those results also. Something to do during the down time.

Have fun.

EPS is less enthusiastic but it did take a baby step away from the ledge imo. Slightly better ridging into AK. Some hints at improvement on the Atlantic side.

First let me clarify the point I was making based on history 1. If we get a strong central pac ridge the first half of Jan, that is highly likely to persist most of winter. 2. That pattern can only work if we get blocking 3. If signs of blocking don’t show up by mid January it’s highly unlikely things improve. History suggests years where blocking helped it showed up by mid January. So while im not punting anything now, if we get to say January 10th and we’ve been in a persistent central pac ridge +AO/NAO regime and guidance shows no light at the end of the tunnel then at that point yes I would say shut out the lights wrt realistic hope this season ends well. Still could get a fluke storm but chances of hitting climo would be extremely slim. What is my confidence that is what happens... 2/5 maybe. My gut still says this shouldn’t be a dumpster fire year. Historically enso neutral years with our qbo and north pac sst profile have been decent to good. There were some hints of a way out of that fate on guidance today. On the other hand climate changes may have rendered the analogs useless. Enso neutral years have been skewing less snowy lately. And I can’t ignore the evidence right in front of me wrt the pac pattern and the long term lack of blocking. I won’t pretend I’m not a lot less optimistic now than 2 weeks ago. So I guess right now I’m just waiting to see what the next 2 weeks shows this winter to be with an open mind. But unlike years when I argued we could easily pull a late save, history suggests the pattern guidance has been showing lately is not the kind of thing we recover from if that sets in for any appreciable amount of time.

That’s literally our only realistic way to salvage the pac look this year.

Why can’t there be analysis without some people exaggerating it to extremes. Good or bad. When someone says the pattern looks good suddenly they promised a blizzard. When someone says it looks bad suddenly they cancelled winter. Nobody punted January today. I’m glad HM is optimistic. But he also thought it would snow around the holidays. He did nail the -AO period but he missed the pac muting it’s impact. He also missed the AO then going positive again before the pac relaxed. No one is perfect. I say that because the only thing people said, including myself, was that if (somehow everyone misses the if) we get a prolonged central pac ridge with +AO/NAO pattern that’s a really bad sign for winter. And guess what it is. Doesn’t mean we will. Long range guidance is wrong a lot. But if 15 days from now we’re dealing with that look, HM is probably wrong and this winter is toast. But im not against him. I want it to snow. I hope he is right! But seeing a huge central pac ridge and +AO develop as we head into January isnt a good thing. Anyone who thinks that’s no big deal is kidding themselves. But we still have a long way to go before it’s time to give up.