MIMIC Total Precipitable Water (TPW) fields, below, show a band a rich moisture over the northern Indian Ocean between the Equator and 15^oN Latitude. By 17 October in the animation, cyclonic motion is diagnosed in the TPW fields between 60^o and 70^o E Longitude. By 18 October, an invest was declared. The toggle below (imagery from the CIMSS/SSEC Tropical Weather Website) shows shear values, Sea-Surface Temperatures, and low-level convergence and upper-level divergence. Shear values are favorable, and a good divergence/convergence couplet over warm waters is apparent.

Metop-B overflew the system shortly after 0500 UTC on 19 October (link) and a closed circulation was apparent near 11^oN, 63^oE. Imagery and winds derived from FY-4A, below, (from this NSMC website) show strong diffluence above the developing system at 1200 UTC on 19 October. The upper-level wind barbs (in red) and mid-tropospheric wind barbs (in green) are nearly parallel, suggesting low shear values.

By 1200 UTC on 20 October, below, a storm has formed in the region of low shear, and landfall on the southern shore of the Arabian Peninsula is forecast.

By 2100 UTC on 21 October 2023, below, a well-developed system was present. Satellite-derived winds suggest an outflow jet to the east along the northern perimeter of the storm.

RCM-2 overflew Tej shortly after 1400 UTC on 22 October 2023 (link). The SAR wind analysis, below, shows a well-defined eye structure at that time to the north of Socotra Island. The wind analysis (here) shows peak winds over 100 knots.

View only this post Read Less

Strong gap winds accelerating out of the Copper River Valley — located along the coast of southcentral Alaska — were lofting fine particles of glacial silt/sand, and transporting those aerosols southward across the Gulf of Alaska during 20-22 October 2023. 10-minute GOES-18 (GOES-West) Dust Red-Green-Blue (RGB) images created using Geo2Grid (above) displayed the plume of airborne silt/dust (shades of pink) during that 48-hour period.

During the daylight hours on 21 October, GOES-18 True Color RGB images (below) showed the long plume as it curled southeastward across the Gulf of Alaska. The hazy plume of blowing silt/dust could be seen moving down the narrow Copper River Valley before emerging from the coast.

The strong gap wind flow resulted from a tight coastal pressure gradient as a ridge of high pressure began building across southern Interior Alaska (above), which forced surface winds to accelerate down the Copper River Valley (located northeast and east of Cordova, as seen in the topography image below) toward the Gulf of Alaska.

Glacial silt from the riverbed of the Copper River has made it over the Cordova area as seen from the photos below! Credit to the Cordova Telecom Cooperative (left pic looking East from the West side of the Delta) and the FAA (right pic looking Southwest from the airport).#AKwx pic.twitter.com/Au114HULMv

— NWS Anchorage (@NWSAnchorage) October 21, 2023

Prior to sunrise on 22 October, the western edge of the plume briefly drifted across Middleton Island (PAMD), where it reduced the surface visibility to 6 miles from 1300-1500 UTC (below).

View only this post Read Less

GOES-18 (GOES-West) Air Mass RGB images (above) showed the progression of a strong cold front — associated with a deepening Alberta Clipper type of midlatitude cyclone — which produced strong winds across parts of central/eastern Montana and western North Dakota / South Dakota during the afternoon and evening hours on 17 October 2023. Notable METAR site peak wind gusts included 61 knots (70 mph) at Jordan MT (KJDN) and Buffalo SD (K2WX), and 60 knots (69 mph) at Dickinson, ND (KDIK); in addition, a mesonet station at Beach ND recorded a peak wind gust of 66 knots (76 mph).

It's getting windy out west. Showers are bringing down gusty winds from above the surface. Froid, Beach, Sentinal Butte and Beach have all had gusts above 60 mph in last hour. Beach "winning" with 76 mph gust. @NWSBismarck has just issued a High Wind Warning for the area. #ndwx pic.twitter.com/NMhZ511hMh

— NDAWN (@NDAWNmesonet) October 17, 2023

The brighter shades of orange-red in the Air Mass RGB imagery highlighted dry, ozone-rich air indicative of the lower tropopause associated with this disturbance. Contours of the “dynamic tropopause” — taken to be the pressure of the RAP40 model PV1.5 surface — portrayed the tropopause extending down to the 650 hPa pressure level over far northeastern Montana (above). A SW-to-NE oriented cross section from western Montana to the US/Canada border (below) also showed the pronounced lowering of stratospheric values of Potential Vorticity (greater than 1.5 PVU) over northeastern Montana.

2 examples of GOES-18 Mid-level Water Vapor (6.9 µm) imagery with plots of associated Derived Motion Winds (above) showed speeds greater than 50 knots. A plot of rawinsonde data from Glasgow, Montana at 0000 UTC on 18 October (below) depicted steep mid-level (7.3 C/km) and boundary layer (near dry adiabatic) temperature lapse rates, which were aiding the downward transport of momentum (strong winds) from the middle troposphere to the surface. The aforementioned low tropopause was also evident in the temperature profile, near 370 hPa (which had descended dramatically since 1200 UTC) — with very dry air above 500 hPa.

View only this post Read Less

GOES-R Satellites (GOES-16 as GOES-East and GOES-18 as GOES-West) provide single-channel observations, such as the clean window infrared image above that can be used to identify features. In the example above, a mid-level cloud deck stretches southeastward from the Samoan islands that surround 170^oW Longitude. Higher clouds with with embedded convection are apparent in the northeastern quadrant of the animation, with a noticeable convective feature forming from 1000 to 1100 UTC. A useful Level 2 product to include in a Clean Window infrared image is the Total Precipitable Water: this is a clear-sky only product and thus gives information where the Band 13 image is returning surface information. The animation below has TPW underlain under the Band 13 image and the default AWIPS enhancement for TPW has been altered: the driest value is 0.5 (inches) rather than 0.0. The TPW distribution allows a viewer to note that the cloud band to the southeast of the Samoan Islands is aligned with a moisture gradient. That deep convective complex is developing near the southern gradient of that deepest moisture. The driest atmosphere is south of 20^oS in the image below.

Derived Motion Wind vectors are another Level 2 product, and they can be plotted as a function of pressure, as shown below. This added information provides useful context to the infrared imagery. At lower levels (surface-900 mb), cloud motion is brisk and from the east and southeast at 20-25 knots. From 775-900 mb, motion is more easterly; derived vectors have appeared on top of the cloud band that stretches southeastward from the Samoan islands. The convection that develops between 1000 and 1100 UTC is within a region of cyclonic winds. Between 600-775 mb, winds are more westerly, and the convection that is developing between 1000 and 1100 UTC is within a region of diffluence associated with anticyclonic motion. Winds from 450-600 mb also show diffluence in the region where convection developed.

The animation below shows the derived winds all plotted on top of each other. AWIPS will also plot vectors such that the wind barb color is a function of wind speed. Low-level winds at this time are mostly derived from the 3.9 µm imagery. High-level winds are derived from Water Vapor channels and from 11.2 µm imagery.

The pronounced shift in wind direction with height is also apparent from the 1200 UTC 17 October 2023 Rawinsonde from Pago Pago, below (from this link).

The persistent winds have raised seas around American Samoa. The buoy at Aunu’u below shows seas climbing to above 10 feet before sunrise American Samoa Time on the 17th. One other recent change that has happened is the cessation of the very long-period southerly swell that had persisted for several days through yesterday.

---

In this environment with persistent winds, Tutuila experienced rain activity that was unforecast by numerical models. Night Microphysics RGB imagery, below, from the CSPP Geosphere site, show the evolution of cloud lines from light violet to violet with a reddish tint as clouds grow in height and start to precipitate.

_______________

Two other GOES-18 Level 2 derived products that helped to further characterize the rain-producing cloud lines were Cloud Top Height (above) and Cloud Top Phase (below). As cloud-top heights increased and reached colder temperatures, their cloud-top phase changed from water (cyan) to supercooled (light green) then to mixed supercooled/ice (darker green). Note that not all of the hourly METAR surface reports were plotted by AWIPS: at 0820 UTC and 0850 UTC, heavy rain reduced the surface visibility to 1.0 and 0.5 mile, respectively, at Pago Pago International Airport on Tutuila Island, American Samoa (plot | text).

Cursor sampling of GOES-18 Cloud Top Phase, Cloud Top Height and Cloud Top Temperature at 1130 UTC — when the maximum height of the cloud line in the vicinity of Tutuila was 22036 ft — is shown below.

View only this post Read Less