Whenever discussing a forecast in the medium range, I'm always very quick to point out the inherent uncertainty associated with extended forecasts. You may remember in the past I've referenced "windows of uncertainty" and "windows of certainty" - and the numbers support these: that is, there are some atmospheric patterns that are quite predictable, even several days out, and others that are challenging right up to hours before an event. The first step in ascertaining predictability of an event is to determine how much agreement or disagreement exists on the jet stream pattern in the given region, at the forecast time period. I should preface this discussion by disclosing without hesitation that the predictability for the weather pattern in the Northeast US on Monday 10/29, the center of the timeframe in question, is between 10 and 18 percent. That is, of all the possible solutions, you'll find no more than 18% agreement on even the most likely scenario. This qualifies as a "very low" predictability event.
That said, the stakes in the forecast are high. A combination of satellite observations and surface reports indicate that Tropical Depression #18 has formed south of Jamaica this Monday morning, and is forecast by the National Hurricane Center to track north, strengthening to become Tropical Storm Sandy, perhaps by day's end. This storm is expected to continue tracking north-northeast through the week, crossing the Bahamas and entering the Western Atlantic by the weekend. There is always inherent uncertainty in a tropical forecast, and Sandy will be no exception, with
intensity guidance by Saturday morning ranging from a 32 mph depression, to a nearly 80 mph hurricane.
Meanwhile, the overall jet stream pattern that steers storms - both tropical and non-tropical - will be evolving to favor an amplifying (strenghthening) trough - or dip in the jet stream - over the Central US, a building ridge (bump in the jet stream) over extreme eastern New England and Atlantic Canada, and a position somewhere off the U.S. East Coast of the tropical system, however strong it may be. Interestingly, there is extremely high agreement on the position of each of these features, which is the cause for great interest in the meteorological community.
Remembering that the jet stream flows from west to east, looking at the forecast setup on the map included here for Saturday night paints a picture of a south to north jet stream wind developing along the Appalachian Mountains, and a large ridge, or "block" developing just east of New England. These are two very important components, as the developing southerly flow will be eager to accelerate anything off the East Coast northward, and the developing block to our east will slow the atmosphere, meaning the southerly jet stream flow that develops Saturday night into Sunday may stay in place into Monday.
This creates a dynamic atmospheric setup in which the potential exists for the tropical entity - however strong Sandy will be at that point - off the East Coast to interact with the digging trough and associated energy and cold air over the Central US, nudging toward the East Coast. You don't have to be a meteorologist to put the ingredients of this recipe together! A tropical system, loaded with warmth, moisture and attendant tropical energy, moving north ahead of a strong jet stream pointed up the coast, slowed by a high pressure ridge to the east, and clashing with cold air surging in from the west. If all comes together, the potential exists for a very strong coastal storm with widespread damaging wind, destructive coastal flooding, battering surf for substantial beach erosion and flooding rain for some of the Mid-Atlantic and Northeast corridor.
So how likely is this, and where do I think we stand with this potential storm? Here's a quick breakdown of what I see:
- Storm development along the East Coast, along with a slug of rain and wind from the Mid-Atlantic to New England, is likely in the Sunday to Tuesday timeframe
- It is not too early to conjecture there will be heavy rain and gusty wind with whatever system forms, and at least some large surf and some degree of beach erosion
- Is IS too early to speculate on the strength of the storm, and duration of impact for some very specific reasons...
- Strength of Tropical Storm Sandy at time of interaction with jet stream still unknown - huge factor.
- Strength of blocking ridge over Atlantic Canada still unknown - likely to be strong enough to slow the flow of the atmosphere, but by how much?
- As a result of uncertainty with the blocking ridge, resultant uncertainty with position and strength of jet stream wind on east side of developing trough moving east from Central U.S.
- Uncertainty with how much of Sandy's energy moves north, and how much of it is allowed to shear northeast, over the ocean (dependent upon the above factors)
So...where this leaves us is a forecast of a likely coastal creature with impact of heavy rain and gusty wind sometime Sunday-Tuesday timeframe, but just how much and just how strong remains to be seen. As mentioned above, part of the reason for focus at this point is that, if everything comes together in a worst-case scenario, this would be an extremely damaging storm for much of the coastal Eastern United States, and particularly the Mid-Atlantic and Northeast corridor.