Notices by Daniel Swain (weatherwest@mastodon.social)

Unfortunately, #Melissa has high potential to have devastating effects in Jamaica (even greater than Gilbert in 1988) despite the island's extensive hurricane experience. This one's looking like a worst-case scenario in terms of strength, duration, and "angle of approach."

This will not only be hurricane with top-of-scale intensity, but will also be slow-moving so wind/rain will linger for *days* in affected areas. Its slow movement will further amplify already extreme flood threat, particularly in mountainous parts of Jamaica & Haiti. #Melissa

Jamaica, in particular, will likely suffer catastrophic impacts from now rapidly-intensifying (but slow-moving) Hurricane #Melissa. A *Category 5* landfall now appears to be the most likely outcome, & 30-40+ inches of *additional* rain in mountains will lead to extreme flooding.

This evening's update from the U.S. National Hurricane Center on #Melissa is ominous, and explicitly mentions: "catastrophic and life-threatening flash flooding...extensive infrastructural damage...and potentially prolonged isolation of communities" in both Jamaica & Haiti.

While only Jamaica appears to be at risk of a direct strike from a Cat 4/5 storm, Haiti and Dominican Rep. have already been receiving torrential rainfall from #Melissa & this will continue for days to come. Haiti in particular is highly vulnerable to flash flood/landslides.

I'll host a live "virtual office hour" session on Tue, Aug 26 @ 4pm PT to discuss the recent SW heat (and ongoing PacNW heat) as well as the widespread monsoonal thunderstorm and wildfire outbreak in recent days. Expect a live radar/satellite tour! #CAwx #CAfire https://youtube.com/live/y03Qsjsujqw?feature=share

Hot weather will continue across PacNW, with gradual cooling across CA. Most mtn thunderstorms will be "wet," including some local heavy downpour/flash flood risk, but dry lightning/gusty outflows still likely on storm peripheries & at lower elevs (with elevated wildfire risk).

A relatively late-season monsoonal surge combined with continued ridging over PacNW will bring an active weather pattern to CA, OR, & WA next 48-72 hours. Widespread mountain thunderstorm activity will continue, with some isolated activity possible elsewhere... #CAwx #ORwx #WAwx

Due to mass confusion, clarifying again:

Tsunami Watch: Tsunami of indeterminate magnitude is possible; stand by.
Tsunami Advisory: Tsunami is likely *but will probably be small.* Evacuations unlikely.
Tsunami Warning: Tsunami is likely *and may be large.* Evacuations needed.

As of ~6pm Pacific Time on Jul 29, 2025, Tsunami *Warnings* are in effect for portions of West Pacific close to epicenter as well as Alaska and Hawaii. A #Tsunami Watch is also in effect for entire U.S./Canada West Coast, including all of coastal California.

Update for folks: Following a very large (M 8.7) and relatively shallow undersea subduction zone quake in the West Pacific east of Kamchatka, Russia, there is a considerable threat of a damaging basin-wide #tsunami. For late-breaking details, visit: https://tsunami.gov

Why is #GlobalWarming fundamentally a "rate of change" problem? It's happening very fast relative to most natural episodes of climate warming and cooling on Earth, and too quickly for ecosystems & societies to easily adapt. Therein lies the problem. https://www.youtube.com/watch?v=rMm8C0aEC9c

This is the basis for "Expanding Atmospheric Sponge" effect that we coined to help visualize practical implications stemming from more heavier downpours (i.e., flash floods) & also more extreme evaporative demand (e.g., faster-developing droughts, more intense wildfires).

Increased temperatures rapidly raises "ceiling" on both precipitation & evaporation intensity--but same is *not* true for typical/average values of either! Regional mean precip can increase or decrease with warming, and actual evaporation is constrained by local H20 availability.

The discourse surrounding precipitation changes in a warming climate (both public discussion and even scientific one at times) is complicated by widespread conflation of changes in averages vs extremes (and also actual vs *potential* evaporation/evaporative demand). [Thread]

Further, the resulting (long-predicted) relative humidity decrease over land now appears to be stronger than earlier expected for reasons that remain TBD. Possible culprits include underestimated land-atmosphere feedbacks or possibly ENSO variability changes contrary to models.

Continental interior and/or arid land areas, which have "functionally finite" water in soils and plants potentially available to contribute to evapotranspiration versus "functionally infinite" water over oceans, typically constrain actual evaporation relative to its potential.

The "expanding atmospheric sponge" and its role in driving more extreme rainfall and faster/deeper droughts is enough to explain a large majority of why climate change is so clearly linked to these types of extreme events. But there is also more to the story, as discussed below.

The analogy works well even w/caveats: A larger sponge only has *potential* to absorb more water--but will not necessarily do so unless water is present! Likewise, even a huge saturated sponge will not actually relinquish water unless actively wrung out. https://www.nature.com/articles/s43017-024-00624-z

However, the fact that most of the *very most extreme* precip events in any given location are usually associated with strong/ long-range anomalous moisture transport from elsewhere (usually warmer ocean regions) means they will still increase even across continental interiors!