Kauai Experiences The U.S. 24-H Atmospheric Precipitation Record: Why Did It Happen?

Heavy atmospheric precipitation on Kauai was headline word ii weeks ago, amongst flooding as well as substantial harm on the N side of that mountainous isle (something I briefly described here). 

But additional atmospheric precipitation records select move available as well as it forthwith appears that the atmospheric precipitation inwards some locations went beyond extraordinary, producing 24-h amounts that select never been documented anywhere inwards the U.S. at whatever time:  the all-time 24-h atmospheric precipitation tape of the United States.

That is something amazing.



This calendar week the National Weather Service released a special tilt that a pelting estimate inwards Waipa, well-nigh a mile W of Hanalei, on the N side of Kauai, received 49.69 inches during the 24 hours ending 12:45 PM HST on Sat Apr 15th (that is 2145 UTC/GMT).  As an aside, the message was written past times Kevin Kodama, the NWS Service Hydrologist inwards Honolulu, as well as a graduate of the UW Atmospheric Sciences department).


An official review squad volition evaluate the character as well as consistency of the pelting estimate information to create upward one's hear whether the tape was genuinely broken.   This was non an official NOAA pelting estimate but 1 installed past times the Waipa Foundation.   If true, the Apr 14-15th total would smasher the previous U.S. 24-h tape of 43 inches observed inwards Alvin, Texas on July 25-26, 1979.

There were a let on of other extremely heavy rainfall observations inwards the area, as well as hence it is possible that this value is valid.   For example, the nearest station I could discover equally a weatherunderground.com site inwards Hanelei (KHIHANAL2, come across map).


 On Apr 14th, this location received 23.05 inches, amongst 21 inches of it after 12:45 PM.

On Apr 15th, that site observed  20.43 inches inwards less than 4.5 hours earlier their might failed at 4:24 AM (see below).

This is incredible.  An average of 4.6 inches per hour.

 But it is to a greater extent than incredible than that....here is a plot of the atmospheric precipitation that morn at that location, showing both cumulative pelting as well as inches per hour.  About one-half of that pelting occurred over 1.5 hours amongst intensities approaching 10 inches per hour.


So adding upward the atmospheric precipitation at that site from 12:45 HST Apr 14th to the fourth dimension of failure (4:24 AM Apr 15th) gives 41.43 inches... as well as the lawsuit was non over.   The 49.69 inches a mile W at Waipa audio pretty believable  considering the Hanalei observations.

And what happened on the Earth makes these huge totals believable.  Here is a video of a move past times past a Coast Guard helicopter over the heavy atmospheric precipitation area.  This expanse got striking very, real hard.




But the enquiry that you lot are bespeak is why?   What conditions produced such amazing rainfall on the N side of Kauai?   I beloved exploring a meteorological mystery--so let's tear this province of affairs apart as well as discover out what happened.

First,  consider the location of the pelting gauge, inwards the town of Waipa.  As shown past times the map, this location is close the N coast of Kauai-- real close but upstream of the substantial terrain of the island.  Essentially, an expanse where major terrain extends practically to the coast line.


Since terrain enhancement may move an of import chemical component of this event, let's zoom inwards as well as await at the neighborhood carefully.  The location inwards enquiry is well-nigh a mile W of the famous town of Hanalei.  The proximity to the terrain is belike real important...more on that later.


Just to teach our bearings... the 24h flow of involvement is 12:45 HST  (2145 UTC) Apr fourteen to 12:45  (2145 UTC) HST Apr 15th.

Influenza A virus subtype H5N1 key chemical component of this lawsuit was strong easterly flow inwards which at that spot were intense as well as persistent thunderstorm (convective) activity.   The easterly flow is too known equally the trades (or merchandise winds), amongst some folks referring to this lawsuit equally "thundertrades". 

The University of Washington WWLLN lightning detection network, run past times Professor Robert Holzworth, documented the lightning strikes during the event.  Here is graphic he shared amongst me of the strikes during the flow of this lawsuit (below).

Just stunning.  Influenza A virus subtype H5N1 huge collection of lightning strikes over the northern as well as northeast side of Kauai (over 3000 of them).


Lightning observations as well as surface reports documented hr as well as after hr of heavy thunderstorms over that region?  Why?

To starting fourth dimension with, at that spot was strong, moist, unstable merchandise winds approaching northern Kauai. An first-class way to come across this is to await at the analyses as well as forecasts of the NOAA/NWS High Resolution Rapid Refresh (HRRR) for Hawaii (HRRR-Hawaii).  Here are winds analyzed at fourscore m (about 260 ft) at 4 times bracketing the lawsuit (1800 UTC fourteen Apr as well as 0000, 0600 as well as 1200 UTC fifteen December).  You volition notice that during the fundamental portion of the event, moderate northeasterly flow impacted the northern portion of Kauai, amongst air forced upward past times the terrain.


There is upper air (radiosonde, balloon launched amongst observations) at Lihue, on the eastern side of the island.  Here are plots of temperature (right line), dew indicate (left line) as well as winds at 0000 as well as 1200 UTC fifteen April. The x-axis is temperature (C) as well as the y-axis is superlative inwards pressure level (700 is to a higher house 10,000 ft, 850 is 5000 ft).

At 0000 UTC fifteen April, at that spot was a 5000 ft layer of saturated or near-saturated air amongst easterly/northeasterly flow.  This air was characterized past times depression stability, which way that instability (convection/thunderstorms) would select a style to move released equally the air was forced to rise. The moist layer deepened past times 1200 UTC fifteen April, amongst substantial current of air shear inwards the lowest 10,000 ft.



Influenza A virus subtype H5N1 pregnant upper degree feature--an upper degree trough--extended over Kauai during this event. This is shown past times the upper degree superlative anomaly for 500 hPa (about 18,000 ft) shown below.  The blueish as well as regal colors present areas where heights as well as pressures are much lower than normal.  Troughs aloft are associated amongst upward displace as well as cool air inwards the lower portion of the atmosphere.
 This upper degree trough helped select large (but non record-breaking) amounts of H2O vapor northward over the islands.  This graphic present upper-level winds as well as H2O vapor from the GOES-15 satellite.   You tin flame come across strong southerly winds aloft pushing H2O vapor northward aloft.


And a satellite stance shows the total H2O vapor inwards a column Sat morning--substantial values over Hawaii.


According to NESDIS (the NOAA satellite service) they had well-nigh 175% of normal H2O inwards the column...high, but non fifty-fifty inwards the top xl H2O vapor events.


Thunderstorms are encouraged past times increased alter inwards temperature amongst height.  During this event, temperatures were unusually mutual depression temperature aloft, equally shown past times the temperature anomaly plot for 500 hPa (about 18,000 ft), shown below.  Cold anomalies are correct over the islands aloft.
At the same time, temperatures were above-normal close the surface (1000 hPa), equally shown below.
 As a result, at that spot was an unremarkably large cooling amongst height, as well as a real of import mensurate of convective potential, the lifted index, was lower (more unstable) than normal:
So everything came together for this event.   

There were moderately strong easterly to northeasterly winds approaching the northern coast of Hawaii....this air was primed for instability, as well as hence the upstream effects of terrain caused the air to rising as well as convection/thunderstorms to move released.  Thunderstorms continuously developed over northern Kauai, dumping huge amounts of pelting fostered past times the deep moist layer approaching the island.  And the expanse inwards enquiry was merely ideal, amongst high terrain extending to the coast.  But no 1 chemical component was peculiarly extraordinary....but the amount of all of these elements were.  Sort of similar half-dozen dies coming upward amongst the same number.

The odd combination factors came together to laissez passer on us extreme precipitation, amongst a pocket-sized enhancement past times the warming of our planet, which results inwards the atmosphere containing to a greater extent than wet (but that can't move the main explanation of this huge event).

Our models did predict substantial rain, but nowhere equally much equally occurred.  Here is the 24h atmospheric precipitation forecast past times Hawaii HRRR for the run initialized at 1200 UTC fourteen Apr (Friday morning).  Not great...it showed are expanse of 4-6 inches (which is heavy) on the N side of the island, but goose egg similar the large totals observed that day.  The source of this forecast failure needs to move examined.



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