First, it's good to note that it really wasn't much of Ida that went that far... in terms of the strength of the low pressure (and the resultant wind), while the swampy waters in southern Louisiana may've helped it maintain category 4 strength for an extra few hours inland, it decayed rapidly soon after, becoming a depression in Mississippi, and extratropical near the Kentucky/West Virginia border. By the time it passed through the northeast, it was just a 998-1000 mb low pressure with sustained winds of only 30-40 mph. As a low pressure system it was quite pedestrian in even fairly recent history:
(Lower on the list = more intense low pressure), values are mostly peaks, at landfall if applicable
|Typical daily pressures
|~ 1005-1020 mb
|Tropical Storm Fay
|Remnants of Ida
|Tropical Storm Henri
|Ida at Louisiana landfall
Most tornadoes and many heavy rainfall events come outside of hurricanes, and a large number come from fairly weak lows. They're both so much about how specific localized conditions lineup, not so much the strength of the low pressure itself.
Now, first to get a couple of the secondary questions out of the way:
In regard to rain shadows, they're just as much about wind direction and surrounding factors.
In the fairly arid intermountain western US for example, storms come in with fairly limited total moisture as is (due to the cold east Pacific temperatures), then wring out quite a bit of what they have as widespread precipitation forced by its air rising over the Cascades and Sierras... but as important is that pre-storm wind directions from the northeast/east/southeast/south have very little access to water due to the basin nature of the whole region and large distances to water. In comparison, most precipitation comes ahead of low pressures in the northeast because this is the wind direction:
As transitioning tropical cyclones work north from the Gulf (or regular lows work east from the Plains), they drop plenty of precipitation, and sure you can pick out some extra moisture is deposited from rising air on the windward side of the Appalachians... but the pre-storm fetch for the east coast is over the warm Gulf-Stream of the western Atlantic, so moisture is well preserved... as the northeast coastal areas often finds out too well in the winter months when snow piles in from nor'easters. Moisture profile isn't about where the center of the low pressure goes, but the flow around it. In nor'easters, a just-offshore track tends to favor more snow not because a track over the Appalachians would steal the moisture, but because of how it maximizes the onshore flow most perpendicularly to the strongest temperature gradient to create the most lift (and because a track offshore means you're in the cold sector, a path over land means you spend more time in the warm sector (= perhaps not snow). The low profile of the Appalachians and ability of air to flow around it from ample water areas of the Gulf, Atlantic, and even Great Lakes means there really isn't any great rainshadow in the eastern US.
In terms of tornadoes, the overall count was fairly regular for tropical cyclones.
Now for rainfall... how much was there?
(remind me in a few months to update with a better graphic that the WPC produces for tropical cyclones)
Is that extraordinary? Yes... and no... and yes... and no... and then very much yes!!!
Those are some rainfall records listed up above, so it's certainly a notable amount in spots.
However, certainly compared to rainfall when tropical cyclones stall further south, it's a drop in the bucket. In this very event, Ida, a few spots in Louisiana and Mississippi had nearly double the rain that even some of the harder hit spots in the northeast got. Spots along the northeast Texas Gulf Coast got nearly 60 inches (1500 mm) of rain from Hurricane Harvey in 2017, and then another 40 inches (1000 mm) from weak Tropical Storm Imelda in 2019 (and looks like they could see another potential event next week).
Still, one person's mountain is another person's molehill. The northeast doesn't have particularly great permeable soil.
(Source NASA's Earth Observatory)
And that's where there's soil. The northeast is... kind of known as a heavily urbanized area.
So a comparable amount of rain in the New York metro versus most of Louisiana means more.
But... what's most surprising to me is that, when you compare the rainfall maps to recent events overall... it really doesn't stand out much!!!
A swath of like 5-8 inch rainfall in all three of the (at some point) hurricanes, each with a few higher inches spots. Not off the charts.
Yet, at least from what we saw on television from afar around the country, it seemed like this storm's (freshwater) flooding was comparably quite extreme. (Though honestly the picture from the media often makes it seem like total widespread devastation when it's really not. Many areas in southeastern Louisiana are quite reasonable, as were decent amounts of Houston in Harvey and south/central Florida in Hurricane Irma. It does seem harder to get a real feel of the true scope of an event from the media anymore ☹️) But based upon the proliferation of awful videos of raging waters and collapsing basements around NYC and satellite images like these, and the list of different areas involved, it seems to have been fairly notable compared to most systems.
So where does that leave us. Get to the point, was it really that bad?!?
The answers looks be yes. For two reasons I can see/guess at.
#1 is that the rainfall came two weeks after similar rainfall from Henri. The soil was saturated, so less rainfall went into the soil. The great Coast NOAA hurricane finder suggests only three times in past history has New York City had two tropical cyclones (or remnants) pass within 60 miles of NYC within a month... 1888, 1893, and 1985. And the rainfall analyses from 1985-Henri and Gloria produced less rain and most of it further from the I-95 corridor. So having two storms in quick succession (and additionally, suggestions the NYC area was also about 4" above average in July), both happening to line up the heavy rain, made things tough.
And #2 is the rainfall rate. Here are the greatest 1 hour rainfalls in modern history at Times Square (from Iowa State's dataset):
Making other plots there's a range of results... New Brunswick/Somerville, NJ, another badly hit area, similarly blew out their record... other spots weren't quite as bad but some still up flirting with records (as some did with Henri the week before), like Laguardia (2 of top 3 were in Ida) and Trenton (slight record)... whereas Philadelphia and Providence were nowhere on the list, showing it was a fairly narrow swath.
Why such heavy rainfall rates?
Because it's getting well beyond a week since the event, data is a bit hard to come by on some key things (many of the best sites keep their data only a few hours to days because of the size... it's something I wish we'd focus on improving, but there's enough to piece together some things these days at least). But I think it actually kind of relates back full circle in connection to the other topic touched on... tornadoes. Intense tornadoes are quite rare in the northeast. The central reason that both intense rain (like >2-3") and tornadoes are scarcer up that way is that very intense deep convection is quite infrequent in the region because it's pretty rare to get instability/strong lift (hot moist air at the surface, cold air aloft, things like strong fronts [all giving the energy for rapidly rising air, allowing moisture to build faster/larger raindrop size]) and helicity (rotation of the winds with height which allowing the storms to maintain intensity) to line up. SPC's sounding analysis shows that summer has some decent instability (CAPE) in the northeast, but lower helicity/deep layer shear, as the area is usually far removed from the jet stream and it's fairly rare to gets the enhancement a tropical cyclone can offer. Whereas the ingredients were all reasonably in place for strong persisting updrafts = tornado potential and very heavy rainfall rates.
(Here's a fantastic radar track video of Ida's US life... you can see the deep convection flare up brighter in the brighter yellow and reddish colors in a fairly tight band over NJ, also seen in the white deeper clouds that show up in the same area on satellite.)
Why'd it come together like this? Going to be hard to answer too confidently with a quick analysis with limited data.
Was there extra moisture, perhaps from global warming? Sea surface temperatures certainly are above normal in the region, as they are this year in much of the Atlantic basin.
But a rough try at lining up NYC sounding data shows that while precipitable water level was very high at NYC, above the daily record, it wasn't miles off the charts for this time of year:
And plotting record dewpoints shows the surface moisture was also not extraordinary whatsoever (for even just fall months, which have lower absolute humidity than summer) at Central Park (obs vs records) or Somerville (obs vs records).
Did the extra intensity from the existing rainfall / "brown ocean effect" over the deep south sustaining the storm a bit longer and giving it perhaps a bit larger of moisture envelope, shift the common ET-transition tornado zone caused by dry slot interaction further up north than the more common mid-Atlantic/Tennessee Valley areas? Maybe? But it would take a lot more to prove.
Or was it just a small band happened to come together just right to produce a few supercells, as does happen on slight occasion (tornadoes are rare in the northeast, but not entirely unknown, usually having a modest regional outbreak every few years in areas like MD/PA/NJ/NY)? Though it certainly was fairly above the typical.
Regardless, the models did absolutely remarkable with it, leading to very strong forecasts for both tornadoes and rainfall by the morning of:
both rare levels of risk, especially for the northeast (ditto success on NHC forecast skill).
The extreme flood damage was quite clearly due in very large part to the excessive rain rates and wet soils from recent storms. What were the exact causes for those factors? It's a complex task to unravel, and pinning it to one specific thing is often dubious meteorologically... just as there are a lot of factors that lead to any particular tornado outbreak or heavy rain event -- that's why they have historically been very hard to forecast until the advent of improved computer models.