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The US Northeast was hit surprisingly hard by Hurricane Ida, dumping a phenomenal amount of rain on the region, causing significant flooding and tornadic activity. I'm having difficulty understanding why the hurricane was able to travel so far inland with as much strength, particularly with regards to precipitation, given several mitigating factors:

This article from LiveScience suggests that a brown ocean effect occurred, which allowed Ida to retain strength further inland than normal due to the marshy areas north of Louisiana, but I would think that impact would've dissipated upon hitting the mountains.

Additionally, per several news articles the rainfall amounts in the Northeast were peaking between 5-10 inches (130-260 mm). But somehow, this is more than totals seen for Tennessee, which saw around 5 inches (130 mm).

Were there other sources that could've caused Ida's precipitation capacity to increase as it traveled more than 1,200 miles (1,900 km) overland?

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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

Storm When Pressure Sustained Winds
Typical daily pressures ~ 1005-1020 mb
Tropical Storm Fay July 2020 1000 mb 45 mph
Remnants of Ida Sept 2021 998-1000 mb 30-40 mph
Tropical Storm Henri Aug 2021 989 mb 60 mph
2017 Nor'easter Feb 2017 969 mb 43 mph
Hurricane Irene Aug 2011 965 mb 65 mph
2021 Nor'easter Feb 2021 960 mb 80 mph
Hurricane Sandy Oct 2012 946 mb 80 mph
Ida at Louisiana landfall Aug 2021 930 mb 150 mph

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:

enter image description here

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?

enter image description here (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.

Enter image description here (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!!!

enter image description here Remind me to update with Henri and Ida images for direct comparison

A swath of like 5-8 inch rainfall in all of them, a few higher 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):

enter image description here

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.

Enter image description here

(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.

Enter image description here

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:

Enter image description here

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:

Enter image description here

Enter image description here

both rare levels of risk, especially for the northeast (ditto 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.

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    $\begingroup$ Given your commentary on my answer I hoped you'd put something together, and you haven't disappointed. $\endgroup$
    – Ash
    Sep 12 at 1:20
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    $\begingroup$ To all: for a broad idea of what can bring heavy rainfall, it may also be good to look at a somewhat related question on Hurricane Harvey from 2017... I never focused that much on tropical cyclone flooding, but seems to be a topic that I always end up in around here! 8-) $\endgroup$ Sep 12 at 1:24
  • $\begingroup$ @JeopardyTempest And why shouldn't you focus on tropical cyclone flooding if you are the resident expert here ? $\endgroup$
    – gansub
    Sep 12 at 4:46
  • $\begingroup$ @JeopardyTempest If you go here - worldview.earthdata.nasa.gov and you choose soil moisture 36 km resolution (the 9km resolution data is not available yet) and you choose 1 Sep data soil moisture in the NE area is relatively saturated. So brown ocean effect still in play in the bottom of the 9th ? $\endgroup$
    – gansub
    Sep 12 at 5:02
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    $\begingroup$ @JeopardyTempest GCMS do not have resolution to resolve contributions from soil moisture or from lakes or swamps. Only a mesoscale model can do that. $\endgroup$
    – gansub
    Sep 12 at 8:05
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You're asking about several different aspects so let me try and get them in order.

The slow drop off in rainfall is in fact because of the Brown Ocean Effect just a bigger one than we've seen before. Once Ida made landfall it was crossed one hot, saturated watershed after another and fed on their heat and moisture to extend its range. Because of how unseasonably wet and hot the land was Ida was, in effect, over open, tropical, water until it hit the Appalachian watershed divide.

The Appalachians average only 910m (3000ft) so they don't block as much weather as the name "mountains" suggests they should, there is very little rain shadow effect across the range and then only around the higher peaks. They made very little difference to Ida just as they have to many other weather systems.

The higher precipitation at the northeastern end of Ida's track was largely down to the fact that it finally lost it's brown ocean and started to lose strength and dump rain in a way that is more typical of a land bound tropical storm. It also ran into colder air that force it to lose yet more moisture.

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    $\begingroup$ Very very poor/misleading graphic and flawed conclusions, as mobile.twitter.com/nwswpc/status/1433448454898999299 and origin.wpc.ncep.noaa.gov/discussions/nfdscc4.html indicate clearly, other than right near landfall, Ida produced it's heaviest rainfall over the northeast. The movement speed seemed fairly common by that point (4 days to transit from the Gulf to Canada). $\endgroup$ Sep 11 at 9:07
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    $\begingroup$ The brown ocean effect as noted in the Wiki article and papers like Shen et al and Evans et al is about [re]strengthening the storm itself... and is firstmost about excess surface water which even with local heavy rains in TN a few days earlier, RFC data suggest was mainly only a factor in S LA [much of the midAtlantic has been below normal in recent months) $\endgroup$ Sep 11 at 9:33
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    $\begingroup$ ... indeed the cyclone weakened rapidly further inland, downgraded to a TD over central MS, much further south than many storms make it. If it was "in effect over tropical water" until the Appalachians, it wouldn't have died like that. So in the traditional meaning of the term, the "brown ocean effect" wasn't a real player further north, as it didn't restrengthen. Certainly some of the moisture over the south may've made it further north, and what you say about the Appalachians and airmass meetup have some truth, but overall this answer is fairly poor and misleading :-( $\endgroup$ Sep 11 at 9:51
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    $\begingroup$ @JeopardyTempest's data syncs with what I was seeing when I was researching this a bit, can you expand on your answer to discuss this impact in greater detail? Based on the graphic you're depicting, it seems that maybe when considering overall volume the storm's precipitation reduced by virtue of the width of the impact band narrowing but that doesn't account for the startling high rainfall peaks observed in PA, NJ, NY, etc. The observed precipitation depths were akin to at least a 100-year storm, 24-hour storm if not greater. I would expect that from a hurricane off the ocean, not overland. $\endgroup$ Sep 11 at 17:33
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    $\begingroup$ That graphic really looks like a forecast to me, not a map of actual precipitation totals measured after the fact. Can you cite where you got it from? $\endgroup$ Sep 11 at 19:31
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A hurricane is a system dynamic of moisture and warm air. As they go north, they get colder, and when they run out of energy, the moisture condenses in cooler climate. Precipitation is inevitable.

An average thunderstorm cloud contains enough water drops to fill up approximately 275 million gallons, and a hurricane can store several billion.

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  • $\begingroup$ The missing thing with what you say is that air cooling doesn't really directly mean precipitation, because without rising motion to cause more widespread mass-condensation, you've just got fog (you also have the issue of how it cools... self-cooling is slow in humid air... and if the air is cooling by mixing with a separate source of cooler air, that also mixes the dryness of the new air in too). The big thing is that warm air rises over cool air because it's less dense... so encountering cool air provides the lift needed. Not by directly adding cool to the air, but lift\adiabatic cooling. $\endgroup$ Sep 13 at 2:17

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