weather

Darby departs Oahu and then the heavy rains arrive (H. Michael Mogil, CCM, CBM, DMS)

Fig001-Darby-advisory-11pmHST-160724Just after 7:00 p.m. H.S.T. on Sunday (3:00 a.m. E.D.T. Monday), departing Tropical Storm Darby was starting to pass by Kauai, the northernmost of the populated Hawaiian Island chain. At that time, Darby was about 100 miles to the northwest of Honolulu (island of Oahu) (Fig. 1).

As part of a luckily-timed vacation to the 50th state, I was thrilled to have sort of experienced a weak tropical storm pass by some five thousand miles from home. So much for “being lucky” and so much for a meteorologist on vacation!

Fig002-TCU-Koolau-Mountains-400pmHST-160724Three hours earlier (around 4:00 p.m. H.S.T.), clearing skies in the wake of Darby allowed the convective process to begin in earnest on Oahu. At this time, surface sunlight heating and upslope mountain wind flow allowed towering cumulus clouds to build along the southern slopes of the Koolau Mountains to the east and northeast of Waikiki Beach (Fig. 2). To the east of this convective development, a lingering north-south feeder band, associated with Darby, was slowly moving to the west.

Around 7:00 p.m. H.S.T., these two convective forces converged and heavy thunderstorms (yes, thunderstorms) began in earnest. Within 30 minutes, streets in Waikiki looked like lakes. Water began rising to curb level and, in some cases, rose high enough (and ran off roofs fast enough), that at least one restaurant had to close due to flooding.

Fig003-waikiki-flooding-around700pmHST-160724We left that restaurant (without even being seated), and raced to a restaurant just a street away. Umbrellas were hard pressed to offer much protection, as water splashed up on us from the sidewalk and water-covered streets below (Fig. 3).

Upon arriving at the restaurant and trying to dry off a little, my cell phone alerted me to the flash flood warning for Oahu. The warning talked about rainfall rates of one to two inches per hour. A storm report noted that water was one foot deep on Keeaumoko Street in front of a Walmart store (located about a mile and a half west-northwest of my then location).

As we ate a rushed dinner, sitting partially water-logged in the restaurant, we listened to the sound of torrential rainfall outside. Interspersed, came rumbles of thunder and flashes of lightning. That’s when a report from the local emergency office indicated that water had ponded so deeply on Interstate Highway H-1 (a major east-west route along the southern part of Oahu) that the six lane highway had been decreased to two usable travel lanes. The report noted that water was pouring in from nearby School Street. Minutes later, a report was issued about H-1 being closed due to water inundation and/or stalled cars in the water.

Fig004-waikiki-flooding-around930pmHST-160724Dinner completed, we headed back to our hotel and had to cross streets still filled with runoff and ponded water (Fig. 4). Lightning continued to flash overhead.

Returning to our room, we watched and listened as the rain continued unabated until almost 10 p.m. H.S.T. Then, rain continued, although at a lesser rate, until after 1.00 a.m. H.S.T.

It was clear from observing the movement of the lightning sources (and confirming this with radar) that Waikiki and the surrounding area experienced “training” thunderstorms. One storm moved through and was followed immediately another and then another. This is much like railroad cars moving across a fixed point on railroad tracks. I estimate that at least 5 or 6 thunderstorms passed over Waikiki between 7:00 p.m. H.S.T and 10:00 p.m. H.S.T.

In my more than 60 years involved in meteorology, and having lived in many locations across the U.S., including south Florida, I don’t recall having ever personally experiencing such a long duration, heavy rainfall, training event.

Fig005-rainfall-estimates-Oahu-160724-16-0725Fig. 5 shows estimated rainfall across Oahu for three three-hour periods spanning the time period from 5:00 p.m. H.S.T. on Sunday to 2:00 a.m. H.S.T.

Rainfall reports were hard to come by overnight. However, recognizing the steeply sloped terrain to the north of Waikiki, it seemed that significant flooding had to have occurred in normal valley/canyon flood-prone zones. The morning 24-hour rainfall collective showed at least four sites across Oahu that reported more than 10 inches of rain (Luluku – 10.56”; Waihee Pump – 10.38”; Maunaloa – 10.31”; and Nuuanu Upper – 10.17”). It would not surprise me to find other reports with even higher numbers.

Stay tuned and we’ll find out tomorrow. Meanwhile, heavy rainfall, associated with a line of training thunderstorms was falling across northwest parts of Oahu this morning.

So much for an attempt at a meteorologically-correct vacation!

© 2016 H. Michael Mogil

Originally posted 7/25/16

weather, weather safety/preparedness

Darby’s rainfall, wind and surf affecting the Hawaiian Islands (H. Michael Mogil, CCM, CBM, DMS)

Fig001-darby-160724-west-of-Big-IslandTropical Storm Darby continues to plod along to the west, refusing to make that anticipated turn to the northwest. As a result, instead of skirting the Hawaiian Islands on their northeast flanks, Darby has been trekking just to the southwest of the island chain (Fig. 1). This has allowed him to weaken a bit more quickly because the stronger northern sections of the storm passed over the mountainous Big Island of Hawaii. With mountain peaks topping 10,000 feet there, Darby’s low-level circulation has been significantly disrupted. Still, wind gusts in mountain passes and at higher elevations have been significantly stronger than winds at airports nearer the coast. Fig002-darby-hawaii-geographyFor example, in the Koolan Range in the east-central part of Oahu, winds have gusted to 60 miles per hour. In the Kaupo Gap on Maui (south facing slopes of Mount Haleakala), winds have gusted to 51 miles per hour (Fig. 2). Other reports from the Hawaii mesonet showed sustained winds of 41 miles per hour, gusting to 55 miles per hour at just northeast of Kawaihae and 41 mile per hour sustained winds with gusts to 53 miles per hour at Kamuela (both on the Big island).Fig003-darby-hawaii-arrival-in-newspaper

However, winds are not really the big story about Darcy, even though forecasts and advisories (including media stories such as the headline in the Honolulu Advertiser on Jul. 23, 2016 (Fig. 3) have been headlining the storm and its winds. Rather, rainfall and surf continue to offer the greatest threats.

According to several National Weather Service (NWS) issuances early on Sun., Jul. 24 (Hawaiian Standard Time):

  • A flash flood watch remains in effect for today on the Big Island of Hawaii; through tonight for Maui, Molokai, Lani and Kahoolawe; through Monday morning for Oahu; and through Monday afternoon for Kauai and Niihau.
  • A flood advisory has been issued for parts of Maui. During overnight hours, radar showed heavy rainfall (rates of two inches per hour) on southeast-facing slopes of Mount Haleakala and the west Maui mountains.
  • A flash flood watch replaced the flash flood warning for parts of the Big Island of Hawaii early this morning. Still, heavy rainfall had fallen on southeast parts of the Big Island (with some locations topping six inches). Hawaii County Civil Defense has reported that Highway 11 near mile marker 59 was closed due to flooding.
  • The NWS provided additional very specific details about the potential for heavy rainfall across the island chain – “… rain bands around Tropical Storm Darby will bring frequent showers and isolated thunderstorms … with periods of heavy rainfall and flash flooding possible.” For Oahu, storm total rainfall amounts of three to five inches are possible) with isolated amounts up to 10 inches in higher terrain. For other islands still in Darby’s path (Kahoolawe, Lanai, Maui and Molokai), 5 to 10 inches of rainfall is expected with local amounts of up to 15 inches. The Big Island, starting to move into a lesser heavier rainfall potential, can still expect 5 to 8 inches of rainfall.
  • “In addition to flood prone areas…heavy rain events of this size may cause flooding in areas outside designated flood zones. Low spots in roads can become dangerous and impassible due to severe runoff. High amounts of debris in streams and gulches may clog bridges and culverts resulting in dangerous flooding (even outside of normal channels).”

The heaviest official rainfall report through early this morning was 2.11 inches at Hilo on the east side of the Big Island. However, there were two reports of more than six inches on the northwest part of Maui.Fig004-hawaii-rainfall-estimates-3-hr-ending-160724-2amHST

NWS radar estimates also show much higher amounts on the south-facing sides of Mauna Loa (Big Island) and along some of northeast-facing slopes of the northwest-southeast mountain ranges on some of the islands (Fig. 4).

Finally, and not to be forgotten, a high surf warning is posted for east-facing shores of Maui and the Big Island and a high surf advisory covers east-facing shores of Kauai, Oahu, and Molokai, both until 6:00 p.m. H.S.T. today. Waves of 6 to 10 feet are expected.

© 2016 H. Michael Mogil

Originally posted 7/24/16

weather, weather safety/preparedness

Darby to bring wind and rain to parts of the Hawaiian Islands (H. Michael Mogil, CCM, CBM, DMS)

Tropical Storm Darby, now in his 11th day, is on a westward course at 14 miles per hour toward the Aloha State. At 11:00 a.m. H.S.T. (5:00 p.m. E.D.T.) Darby, with sustained winds of 65 miles per hour, was located some 755 miles southeast of Honolulu, Oahu (and about 560 miles east of Hilo, Big Island) (Fig. 1). Forecasts keep Darby as a strong tropical storm until he starts to interact with the Hawaiian Island mountains on Sunday and Monday. Even with this interaction, weakening is expected to be slow.

Fig001-Darby-fcst-map-160721-1100HST

This suggests a long period of fairly strong winds over eastern parts of the island chain; in addition, flash flood watches have been posted for Maui and the Big Island of Hawaii due enhanced tropical storm rainfall linked to mountain-generated upslope winds. With an island landscape punctuated by many narrow, steep canyons, channeling of rainfall and the flow of runoff can be quick. Coastal areas throughout Hawaii, especially on north- and east-facing beaches, can expect rough, high surf. Offshore, tropical storm warnings have been posted around much of the island chain (Fig. 2).

Fig002-watch-warning-map-160721-1100HST

National Weather Service forecasters continue to emphasize that residents and visitors in the Hawaiian Islands should carefully monitor the progress of Darby, as he could eventually have impacts on all islands. They also note that, “it is important not to focus too closely on the exact track and intensity forecasts because the average track error 72 hours out (into the future) is near 100 miles, while the average intensity error is about 15 knots (20 miles per hour). In addition, the hazards of a tropical cyclone can extend over a broad area well away from the center.”

For what it’s worth, I start my trip to Hawaii this Saturday and will be arriving on Oahu just before the storm’s effects arrive. So, look for my on-the-scene reports as the weekend unfolds.

© 2016 H. Michael Mogil

Originally posted 7/21/16

weather

THEWEATHERMOGIL:: Just what is an MCS, anyway?

Take a look at the enhanced (false-colored) infrared satellite image in Fig. 1. See that huge red blob across Kansas? That feature is what meteorologists call an MCS or “Meso-scale Convective System.”

Fig001-GOES-EIR-160702.0715ZBy official National Weather Service definition, “An MCS is, “a complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC.”

This now begs the question, “What is an MCC?”

According to the American Meteorological Society’s glossary, a “Mesoscale Convective Complex (MCC) is a large Mesoscale Convective System (MCS), generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., “roundness”), based on the cloud shield as seen on infrared satellite photographs: *Size: Area of cloud top -32 degrees C or less: 100,000 square kilometers or more (slightly smaller than the state of Ohio), and area of cloud top -52 degrees C or less: 50,000 square kilometers or more. *Duration: Size criteria must be met for at least 6 hours. *Eccentricity: Minor/major axis at least 0.7. MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest. During peak intensity, the primary threat shifts toward heavy rain and flooding.”

Fig002-radar-160702.0848ZFor practical purposes, let’s not utilize such exacting scientific distinctions. Instead, let’s just realize than an MCS is a large, organized, cluster of thunderstorms in which severe weather, but most likely excessive rainfall, is likely.

While the satellite imagery clearly shows the character of an MCS, radar data (Fig. 2) also shows its structure. The leading edge is marked by the most intense convection or thunderstorms. That is followed by a region of lighter rainfall, which is followed by another zone of heavier rainfall.

Fig003-sfc-map-160702.09Z

Fig004-sfc-map-160702.10Z

Surface weather maps (Fig. 3 and Fig. 4) clearly show the outflow boundary on the east and south sides of the MCS and also the diverging surface wind flow associated with a well-formed MCS.

Fig005-flash-flood-watches-160702The outflow boundary is expected to become stationary across Kansas and Missouri as the day unfolds. That boundary will become the focus of new thunderstorm development and allow for multiple thunderstorm transits across the region. Hence, flash flood watches have been posted (Fig. 5) for rainfall of 4 to 6 inches (with locally heavier amounts) during the next 24 to 48 hours.

© 2016 H. Michael Mogil

Originally posted 7/2/16