The National Weather Service forecast office in Wilmington OH (Twitter link) has noted via tweets during the day on 18 December 2023 (link, link, link) that road visibility conditions are variable in the extreme as snow showers move through Indiana and Ohio. The Day Cloud Phase distinction imagery below at 1701, 1801 and 1901 UTC on 18 December suggests that the color of the RGB might be related to snowfall. This can occur because the Day Cloud Phase Distinction RGB color signature changes as a cloud glaciates. Consider, for example, the reddish cloud that extends at 1701 UTC from Benton Harbor MI (KBEH) to the east of South Bend (KSBN) and through Goshen (KGSN) and Fort Wayne (KFWA); note that snow is associated with that cloud. What might you expect to be occurring in Paulding County (the east of Fort Wayne) or in Jay County (to the south of Fort Wayne) where a similar-colored signal exists?

Cloud Top Phase for the same 3 hours as above, shown below, indicate (in dark green) mixed-phase clouds (that is, liquid and ice co-existing) and ice clouds (in red) over the region where snow is occurring. This should not surprise: appreciable snow requires the presence of ice within the cloud for ice crystals to grow at the expense of small water droplets. Of greater interest is the back edge of the mixed phase/ice region; by 1901 UTC most of the cloud tops over eastern Indiana are shown (light green) to be entirely supercooled liquid water. Will snow continue to occur in such conditions? There have been cases where supercooled cloud-top droplets nevertheless supported snowfall (blog post; FDTD Satellite Applications webinar). At 1901 UTC, however, there were not many observations of snow underneath clouds that were supercooled.

The Slider Juxtaposes below compare Day Cloud Phase Distinction and Level Cloud Top Phase a 1701 UTC (below) and 1901 UTC (bottom).

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The GOES-16 GLM instrument has occasionally observed lightning over southern Ohio and West Virginia during the afternoon. How did LightningCast Probabiity do in predicting the flashes? Imagery from 1901 to 1951 UTC on the 18th, below, from this website, doesn’t show large values of LightningCast probability in regions where GLM observations occurred. However, small LightningCast probabilities are noted occasionally within the environment that is spawning occasional GLM observations.

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These snow showers and lightning events occurred on the back side of a very large extratropical cyclones that moved up the east coast of the United States, bringing heavy rains from Georgia to Maine. The 2-day animation of GOES-16 hourly airmass RGB images (created using geo2grid), below, shows the evolution of the system. On 18 December, a strong potential vorticity anomaly was present over the Ohio River Valley as suggested by the orange color in the RGB. (Click here to see a model cross section at 1800 UTC — the corresponding airmass RGB is here) in the midwest, created from the awesome TropicalTidbits website that confirms that suspicion).

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10-minute JMA Himawari-9 AHI Shortwave Infrared (3.9 µm) and Clean Infrared Window (10.4 µm) images (above) showed he development of a pyrocumulonimbus (pyroCb) cloud northwest of Tamworth (YSTW) in New South Wales, Australia on 18 December 2023. The coldest cloud-top Infrared Window brightness temperature was -53.4ºC at 1230 UTC.

The southern flank of the large wildfire complex flared up after a convective outflow boundary moved southwestward across the area between 1000-1100 UTC — the pyroCb cloud then developed by 1200 UTC.

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As discussed in this blog post, heavy rainfall had been occurring across the Samoan Islands on 14 December 2023. The South Pacific Convergence Zone (SPCZ) remained in the vicinity of the Samoan Islands on 16 December (surface analyses) — and 1-minute Mesoscale Domain Sector GOES-18 (GOES-West) Infrared images (above) showed another round of deep convection that produced heavy rainfall over the island of Tutuila, which resulted in flash flooding, rockslides/landslides and wind damage (Local Storm Reports). In addition, the 1326 UTC Pago Pago (NSTU) METAR indicated that a brief thunderstorm (TS) developed (below), which ended 18 minutes later at 1344 UTC.

The GOES-18 Infrared image at 1250 UTC (below) included a cursor sample of the 10.3 µm brightness temperature (white) and Level 2 Rain Rate derived product (green) just west of Tutuila — the Rain Rate was 3.85 in/hr, highlighting the heavy rain potential of these convective showers and thunderstorms. As of 1800 UTC (7 AM local time) on 16 December, Pago Pago had received 3.93 inches of rain in a 6-hour period (text | graph); a total of 4.04 inches of rain was recorded for the calendar day on 16 December (adding to the 3.46 inches on 13 December and 3.59 inches on 14 December). In addition, note that the southeast wind at Pago Pago was gusting to 40 knots (46 mph) at that time — and a heavy rain shower was reducing the surface visibility to 1/4 mile.

AWIPS Users: a variety of GOES RGB types combined with Level 2 Derived Product Readouts is available under the “Satellite -> Local Menu Items” menu (below). In the above example, the VIS/IR Sandwich Combo RGB was used to display the Rain Rate — with the RGB components (and the other L2 derived products) subsequently unloaded, to just display the 10.3 µm Infrared imagery along with the Rain Rate.

It bears mention that the Total Precipitable Water value derived from NSTU rawinsonde data at 1200 UTC on 16 December was 2.87 inches (above) — which was a record maximum value for that date/time, according to this SPC site (below).

MIMIC TPW images covering the 4-day period from 13-16 December (below) showed the SPCZ stream of moisture as it edged southwestward to spend more time streaming across American Samoa.

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Airmass RGB imagery every hour from 1000 UTC on 14 December through 1900 UTC on 15 December, below, overlain on top of GOES-18 Level 2 Total Preciptable Water, show strong convection in between Tutuila and the Manu’a islands of American Samoa mostly ending after 0000 UTC on 15 December, but the airmass given the green signal in the RGB (click here for help in interpreting the airmass RGB) remains tropical.

GsMap estimates of 24-h rainfall, below, show that the heaviest rains — a maximum of 150+ mm — occurred in between Tutuila and the Manu’a Islands, but >2″ rains occurred over Tutuila, Ofu and Olosega. Rock slides were reported over western Tutuila (click here for the Local Storm Report).

MIMIC Total Precipitable Water fields, below, (from here) for the 24 hours ending 1900 UTC on 15 December, show abundant moisture over all of Samoa with more arriving from the east. Accordingly, Flood Watches (and a small craft advisory) continue through Saturday.

The MetopB Advanced Scatterometer (ASCAT) winds, below, show the strong winds associated with convection south of the Manu’a Islands at 2100 UTC on 14 December. Note also the cyclonic circulation to the north of Samoa.

GOES-18 Visible Imagery, below, from this site, show strong convection to the north of Samoa at 2000 UTC. Given the moisture shown in both the MIMIC TPW and GOES-18 TPW fields that surround the Samoan islands, however, isolated heavy showers may occur at any time throughout Samoa, and showers were occurring over Tutuila at 2100 UTC on 15th. The small line of clouds responsible is apparent in the imagery below towards the end of the animation.

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More heavy rain occurred across parts of American Samoa on 16 December, as shown in this blog post.

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