Tuesday Update: Predictability Increases for Sunday to Tuesday Time Frame, Powerful Coastal Storm Still Quite Possible

In this post...

An update on the honest assessment of predictability
A description of the forecast weather pattern
Impacts deemed as likely
Potential impacts
Perhaps most important: A direct warning to mariners and those on the water

After a post that focused on an honest assessment of probability and predictability yesterday, I had mixed emotions that the brief "worst case scenario" assessment was the snippet that circulated far and wide. On one hand, I wondered if it's wiser to not put such snippets out well in advance, for fear a few sentences of an honest discussion on predictability will become all you're remembered for - and this is likely why so many are focusing on the chances a major event WON'T happen - there is fear of being branded someone of hype if you step too far and the wrong quote is circulated. On the other hand - and I think this is the more important one - if all of your statements are honest and not hype, even just snippets of the assessment get many folks talking and hopefully a large number are driven to read these posts, where you can garner a full explanation of the factors at play. Because the fact is, probability and predictability is increasing quickly, and the potential impact is so significant that we have to start getting the honest assessment out about what is at stake, and what the chances are of verification. With today's available technology to guide us, it would be somewhat neglectful not to provide an overview of the honest potential, whether guaranteed to unfold or not, and I hope to tackle that here.

So...let's begin the same way we did on Monday - with a quick assessment of predictability. The most important factor at a forecast time range of several days is to assess the forecast jet stream pattern that dominates storm-steering. On the big picture the keys are these - we have a large and deep storm south of Labrador this weekend, a building high pressure ridge over Atlantic Canada, and a strengthening jet stream trough (dip) over the Central United States, with Tropical Cyclone Sandy not too far east of the trough. The map included here is valid Saturday evening and depicts these features. It's important to note the overall predictability of the placement of each of these features now stands at 90%+ on the position of Sandy, 90%+ on the position of the ridge, and either side of 70% on the position of the Central US trough. This means that while details still will vary, the jet stream pattern is becoming quite predictable - that is, while you may still hear of uncertainty regarding specific details, predictability has increased dramatically on overall evolution.

For those familiar with the Greek alphabet, I encourage you to look at the map above (you can click on it to enlarge) and note the start of a process that leads to the development of the Greek letter "omega" off the U.S. East Coast. In the world of weather, this is called an "omega block" and it results in a substantial slowing or stalling of the atmosphere. This is key to our forecast for Sunday to Tuesday, because it implies that Sandy will have a very difficult time penetrating the developing block - in other words, she will be hard-pressed to escape east over the Atlantic Ocean. In fact, when looking at the average solutions of all major guidance, they all show a similar end-game - no matter how far east Sandy tries to run, she gets caught and eventually backs northwest to a position near the New England coast by early next week. In fact, there is a 50-80% predictability of this solution verifying. In other words, though we can wonder about the wind speed, wave height and precipitation amounts, and there is still a corresponding 30-50% chance she escapes (which is certainly not a negligible chance) there is quickly diminishing doubt that New England, especially, will be impacted in some way by this storm. Even the most east-leaning guidance averages eventually land near New England. In fact, in conversation with colleague Tim Kelley today, he made the excellent point that - with this kind of blocking developing - it may be more likely that if the storm were to miss New England that would be because it went into the Mid-Atlantic, instead, not out to sea! Though that is not the most likely scenario, it certainly cannot be ruled out.

The end-game on this storm is that, while possible it rides east or heads into the Mid-Atlantic, the highest probability of all solutions is for a New England impact. The stakes are high - if this storm does indeed accelerate northward, as the weather pattern described above certainly would favor, and maintains strength while doing so, the impact will be high and may be severe - including damaging onshore coastal wind, locally flooding rain, battering waves producing substantial beach erosion, and significant coastal flooding. There's your snippet. But for those of you who do read these posts in their entirety, please understand that there is plenty of room for deviation on impact strength, though I think the window for storm track deviation is closing rapidly. Note in the image at left, that the wind field associated with this storm, regardless of its exact track, is HUGE! A large area of wind 2-3 standard deviations above normal.

What I feel confident with:

A slug of rain and wind is likely Sunday night through Monday - this is likely given the combination of an approaching jet stream trough, cold front and any interaction that may occur with tropical moisture. High probability 1"+, moderate probability 2"+ for some, TBD higher.
Even in a most meager solution, the probability of at least minor coastal flooding and beach erosion is rather high for Sunday night and Monday high tide cycles, given astronomical high tide combined with prolonged onshore wind.

What I lack confidence on, but believe it entirely possible and early preps should be made for:

Wind: Tropical cyclones undergoing non-tropical transition always include a huge, expanding wind field. This storm will be no exception. In fact, while placement of the storm may differ, there is tremendous agreement on several hundred miles of strong wind around this storm, meaning we should be ready for especially coastal areas to see damaging onshore wind if worst-case scenario verifies.
Coastal: No place will the impacts be felt more than the New England coast, and this is where we can start thinking of preps. If you have winter storm preparations that take a few days to complete to protect from battering waves and coastal flooding, start them now. If the storm eases, you'll have them done in advance - if the storm barrels in, you'll be glad you did them in advance. A worst-case scenario would be days of coastal flooding at high tide - Sunday night through Tuesday - and days of battering waves for substantial beach erosion for all east and southeast facing beaches.
Mariners: The Navy NOGAPS WaveWatch 3 model puts waves of 36 to 42 feet into the Cape and Islands Sunday night (see map below, click to enlarge), and at one point the storm produces a remarkably large area of 20+ foot seas from 600 miles east of Daytona Beach, Florida, to Provincetown, Massachusetts. Though this forecast may NOT be correct on specific timing, this should put into perspective why I'm concerned about the storm - even a fraction of this would be a significant event. Though you have some time, we may need to pull some boats from exposed locations, and you absolutely must monitor for updates before making plans for late-season fishing trips, as even a miss of several hundred miles would still bring life-threatening seas to New England waters.
Rain: Communities should plan to have leaves cleared out of catch basins and storm drains by Sunday afternoon, to avoid ponding of water and localized flooding. It's too early to determine if rainfall amounts will end up being high enough to result in any freshwater river flooding - we'll see how things shape up as we get closer.

In summary, it's quite likely New England will feel impact from this storm - the questions focus on just how severe those impacts will be, but given that the potential for significant and severe damage, especially to coastal communities, does exist, it would be irresponsible for me not to give full disclosure for those who need a few days of preparation time. Yours Truly is hoping we can back down and call of the dogs on this one, but at this point, I simply can't say that's the case.