Following the passage of a strong cold front on 08 February, the northwesterly flow of air with surface temperatures in the 30s F on 09 February allowed for a narrow “lake effect” (or in this case, river effect) snow band to form over Wheeler Lake, which created accumulating snowfall to the southeast (downwind) of the lake. This lake effect snow band could be seen in a RealEarth composite of Suomi NPP VIIRS / Aqua MODIS true-color Red/Green/Blue (RGB) images and radar reflectivity (below). The lake effect plume began to shift northward during the afternoon hours, as surface winds briefly backed to a more westerly direction.On 10 February, the northwesterly flow of cold air was less pronounced, but was still enough to allow for a narrow lake effect plume to be seen early in the day on 1-minute interval GOES-14 Super Rapid Scan Operations for GOES-R (SRSO-R) images (below; also available as a large 89 Mbyte animated GIF). As the clouds cleared during the afternoon hours, small patches of white snow cover could be seen just southeast of Wheeler Lake. In a comparison of Terra MODIS true-color and false-color RGB images (below), the presence of snow cover (cyan in the false-color image) could be seen between the lines of cumulus clouds. Data from NOHRSC (below) showed that as much as 3.0 inches of total snowfall was measured downwind of Wheeler Lake (in the higher elevation of the Union Hill area) during the 09-11 February period, and the snow depth on the morning of 10 February was 2.5 inches at that location (enough to be seen on the GOES-14 visible images above). Additional information and images of this event can be found here. ]]>
A comparison of GOES-14 Visible (0.63 µm, 1-km resolution) and Shortwave Infrared (3.9 µm, 4-km resolution) images (below; also available as a large 71 Mbyte animated GIF) offered evidence that the cloud material within each “hole punch” was composed of ice crystals, which exhibited colder (lighter gray) IR brightness temperatures than the surrounding supercooled water droplet clouds. It is likely that many of the hole punch features were caused by aircraft ascending from or descending to the Charlotte Douglas International Airport in North Carolina (KCLT).In a comparison 1-km resolution POES AVHRR Visible (0.86 µm) and Infrared (12.0 µm) images (below), the cloud-top IR brightness temperatures in the vicinity of the hole punch features were only as cold as -20 to -24º C (cyan to blue color enhancement), which again is supportive of the cloud layer being composed of supercooled water droplets. ]]>
A closer view of the GOES-14 Visible (0.63 µm) images (below; also available as a large 85 Mbyte animated GIF) revealed the rapid motion of low-altitude clouds when gaps in the high-altitude clouds were present. Very strong winds were caused by the strong pressure gradient, with gusts as high as 72 mph, and a large Royal Caribbean cruise ship experienced some damage due to the winds (media report 1 | media report 2). The corresponding GOES-14 Water Vapor (6.5 µm) images, which also extend further in time after dark, are available here.A comparison of 1-km resolution POES AVHRR Visible (0.86 µm) and Infrared (12.0 µm) images at 2202 UTC (below) displayed greater detail of the classic “cusp” signature of high clouds, indicative of an intensifying surface cyclone (VISIT lesson). At the time, wind gusts to 60 knots were seen at one the buoys off the coast of North Carolina.
At 0137 UTC, a closed-off low level circulation center could be seen on a POES AVHRR Infrared (12.0 µm) image (below).]]>
There are two animations a the top of this blog post, one with a 1-minute timestep, above, and one with a 15-minute timestep, below. The strong winter storm that hit Colorado on Monday 1 February (Blog Post) was accompanied by multiple cloud layers and snow during the day on Monday was not steady. Was it related to the holes that are present in the clouds? How easy is it to track the different clouds to predict the arrival, overhead, of a gap in the high clouds? Especially for the low clouds in eastern Colorado in this example, cloud hole tracking can be done with more confidence with 1-minute imagery. Decision Support related to short time-scale variability in snow accumulations can be done with more confidence with the 1-minute imagery.
On 7 February 2016, GOES-14 in SRSO-R monitored the development of a very strong storm over the Atlantic Ocean (blog post) east of the United States. Consider the animations below, starting with the standard GOES-East time steps (nominally every 15 minutes with some gaps). If you are monitoring the storm development, or the motion of the individual convective clouds, the 15-minute temporal gaps are insufficient for confident detection of cloud motions. When, for example, does the surface circulation first appear? Do the cloud towers that appear in the 15-minute animation persist over the course of 15 minutes, or do they decay and reappear? In the succeeding animations below, at 5- and 1-minute intervals, increasing amounts of detail are present because the better temporal resolution is convincingly following features. Additionally, the precise timing of events is better captured.
The differences between 1-, 5- and 15-minute time steps are visualized in the rocking animation below. The right-most panel has a 15-minute timestep always, the middle panel starts with a 15-minute time step before switching to 5-minute, and the left-most panel shows 15-minute, 5-minute and 1-minute time steps. Note how the convective towers appear and disappear on timescales that make resolution in the 5-minute time step difficult and in the 15-minute timestep impossible. The region below is excised from the animations above, and is over the ocean south of the developing low pressure system.]]>
Farther to the south, as moisture from the Gulf of Mexico was drawn northward (GOES-14 sounder Total Precipitable Water derived product images) in advance of the eastward-moving cold frontal boundary (surface analyses) associated with the aforementioned Upper Midwest storm, areas of strong to severe thunderstorms developed across the Mississippi River and Tennessee River Valley regions during the afternoon and evening hours. GOES-14 Infrared Window (10.7 µm) images (below; also available as a large 208-Mbyte animated GIF) showed the cold cloud-top IR brightness temperatures (orange to red color enhancement) exhibited by the widespread convective activity.Taking a closer look at the severe thunderstorms which produced multiple tornadoes from eastern Mississippi into far western Alabama (SPC storm reports), GOES-14 Visible (0.63 µm) images (above; also available as a large 66-Mbyte animated GIF) revealed numerous overshooting tops; the counties where tornadoes were reported are indicated by their dashed red outlines. Another visible image animation from RAMMB/CIRA is available here. NWS storm damage surveys (Jackson MS | Birmingham AL) found EF-1 to EF-2 damage in both Mississippi and Alabama.
The corresponding GOES-14 Infrared Window (10.7 µm) images (below; also available as a large 37-Mbyte animated GIF) indicated that the coldest cloud-top IR brightness temperatures were in the -50º to -60º range (darker orange to red color enhancement), which was at or above the tropopause level according the Jackson MS and Birmingham AL rawinsonde data.]]>
During the afternoon hours, GOES-14 Visible (0.63 µm) images (below; also available as a large 91 Mbyte animated GIF) revealed the hazy signature of areas of blowing dust across southwest Texas, both ahead of and also in the wake of a cold frontal passage (surface analyses). Much of the blowing dust ahead of the cold front originated from dry lake beds in northern Mexico, which was then transported northeastward across Texas by strong southwesterly winds (an enhanced visible MP4 animation which shows the blowing dust better is available here). Blowing dust along and behind the cold front restricted the surface visibility to 1.0 miles at Big Spring (KBPG) and 2.5 miles at Midland (KMAF). Also note that early in the animation — beginning at 1800 UTC — there were small convective bands moving northeastward over the El Paso area, which produced light to moderate accumulating snow that reduced surface visibility to 1.0 miles at El Paso and Biggs Army Air Field (KBIF), and 2.0 miles at Ciudad Juarez, Mexico (MMCS).GOES-14 Shortwave Infrared (3.9 µm) images (below; also available as a large 52 Mbyte animated GIF) showed the “hot spot” signature (darker black to red pixels) associated with a large grass fire which developed in the Big Bend National Park area, beginning around 2300 UTC. The hot spot was seen to diminish not long after the arrival of cooler air (lighter shades of gray) behind the cold front. Surface air temperatures were quite warm in Texas ahead of the cold front, with daytime highs of 91º F at Del Rio (KDRT) and 95º F — the highest temperature recorded for the day in the lower 48 states — farther to the southeast at Cotulla. GOES-14 Water Vapor (6.5 µm) images (below; also available as a large 57 Mbyte animated GIF) showed a broad ascending belt of moisture curving cyclonically over central and eastern Colorado, where moderate snow and significant accumulations were occurring at a number of locations. A blog post discussing this ascending belt of moisture in more detail can be found here; a YouTube animation of GOES-14 Infrared Window (10.7 µm) images is available here.
===== 02 February Update =====During the subsequent overnight hours, an undular bore developed along and just ahead of the advancing cold front, as seen in GOES-14 Shortwave Infrared (3.9 µm) images (below; also available as a large 107 Mbyte animated GIF). A detailed view of the undular bore was also captured at 0859 UTC (3:59 AM local time) on Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images (below). ]]>
Nighttime images of Suomi NPP VIIRS Shortwave Infrared (3.9 µm), Longwave Infrared Window (11.45 µm), and Day/Night Band (0.7 µm) images at 0817 UTC on 25 January (below, courtesy of William Straka, SSEC) revealed the hot spot of the erupting volcano summit (orange pixels), and ample illumination from the Moon allowed the plume to be seen on the Day/Night Band image. The large areas of bright city lights from Mexico City and Pueblo (located northwest and southeast of Popocatépetl, respectively) are also very apparent on the Day/Night Band image.A legacy product for use in the detection of volcanic ash plumes is the Infrared “Split-Window” (11-12 µm) brightness temperature difference product (below), which showed the plume streaming eastward to northeastward during the 24-25 January period. Taking advantage of the multi-spectral imagery available from the MODIS and VIIRS instruments on the Terra/Aqua ans Suomi NPP satellites, quantitative products can be derived such as Ash Height, Ash Loading, Ash Effective Radius, and Ash Probability from the NOAA/CIMSS Volcanic Coud Monitoring (below). ]]>
*As the GOES-13 Sounder continues to be off-line due to an anomaly (Link), the principle driver of this product over the eastern US is now GPS data.Cold air is also present. The MODIS Land Surface Temperature product from 0731 UTC on 22 January shows temperatures (in cloud-free regions) colder than -5º C southward into Virginia. Dewpoints in this region are colder than -10º C. High Pressure over the East Coast is promoting cold air damming along the Appalachians as well. Suomi-NPP carries on board two instruments that provide vertical profiles of moisture and temperature in the atmosphere, the CrIS and the ATMS. NUCAPS Soundings combine information from those two soundings. NUCAPS Soundings sites from the early morning Suomi NPP Pass on 22 January are shown superimposed on the MODIS Land Surface Image below; five sounding sites (highlighted in red) were selected: northwest of Boston, over western Connecticut, New York City, Washington DC and central Virginia. These soundings all have common attributes: They are dry (although the vertical profiles from DC and Virginia show the most moisture: ~0.3″ of total precipitable water), they are too warm near the surface (detection of low-level inversions from satellite data is difficult) and they show lapse rates at mid-levels that suggest vigorous ascent may be possible. The 0600, 1200 and 1800 UTC Soundings from KIAD (below) also show dry air (at least initially: total precipitable water doubled from 0.24″ at 1200 UTC to 0.49″ at 1800 UTC) and steep mid-level lapse rates. The Aqua Satellite, bearing the MODIS instrument, overflew the eastern United States shortly before 1900 UTC on 22 January 2016. MODIS samples the atmosphere at 36 different wavelengths, and selected images are shown below.
The toggle between the Visible (0.65 µm) and the ‘Snow Ice’ Channel in MODIS (1.63 µm), below, highlights regions of ice clouds. Ice particles absorb radiation with wavelength of 1.63 µm but water droplets scatter such radiation. Thus, regions in visible imagery that are white that include mostly ice crystals (or snow on the ground), for example the cirrus shield on the East Coast, will appear dark in the 1.63 µm imagery but bright in visible because clouds are highly reflective to visible light. Water-based clouds (over Mississippi, for example, or southeastern West Virginia; in fact, low clouds are apparent just to the west of the cirrus shield associated with the developing baroclinic leaf, from West Virginia southward to Savannah Georgia!) will appear bright in both channels.MODIS also includes a channel (1.38 µm) in a region in the electromagnetic spectrum where strong water vapor absorption occurs; this channel is ideal for high cloud detection. (GOES-R will also detect radiation at this wavelength) The toggle below shows the Visible (0.65 µm), Cirrus channel (1.38 µm) and Infrared window channel (11.02 µm) from MODIS. The storm at mid-day on 22 January was producing an extensive cirrus shield that had the classic baroclinic leaf structure (a structure that was also evident in the infrared window channel). Careful inspection of the visible and near-infrared channels from MODIS reveals transverse banding (features commonly associated with turbulence) along the western edge of the Cirrus Shield along the East Coast. The toggle below of the MODIS Water Vapor imagery (6.8 µm) shows distinct transverse banding. Pilot reports of turbulence with this system are widespread. To better monitor the long-duration storm, the GOES-13 (GOES-East) satellite was placed into Rapid Scan Operations (RSO) mode for a 2-day period beginning at 1215 UTC on 22 January. During RSO, images are available as frequently as every 5-7 minutes, an improvement over the routine 15-minute image interval (note: the ABI instrument on GOES-R will be able to provide images as often as every minute, or even every 30 seconds). Animations of RSO Visible (0.63 µm), Water Vapor (6.5 µm), and Infrared window (10.7 µm) imagery during the daylight portion of Day 1 of the storm are shown below. Though they lack the temporal resolution provided by geostationary satellites such as GOES, polar-orbiting satellites such as the NOAA series (with their AVHRR instrument), Terra and Aqua (with their MODIS instrument), and Suomi NPP (with the VIIRS instrument) offer imagery with significantly improved spatial resolution. Shown below is a series of AVHRR, MODIS, and VIIRS Infrared window channel images (10.8 µm, 11.0 µm, and 11.45 µm, respectively) on 22 January, with the data projected into a 1-km AWIPS-I grid. Areas with cloud-top IR brightness temperatures in the -50º to -60º C range (orange to red color enhancement) can be seen as the storm moved across the eastern US. Excellent detail can also be seen in a series of 1-km resolution MODIS Water Vapor (6.7 µm) images spanning the 21-22 January period, shown below. One interesting aspect to note was that the cold front associated with the intensifying storm had moved southward across the Gulf of Mexico (surface analyses), crossed the mountainous terrain of Mexico, and emerged as an area of strong gap winds over the Pacific Ocean south of Mexico (in the Gulf of Tehuantepec). The leading edge of the gap wind flow, known as a Tehuano wind (or a “Tehuantepecer”), was marked by a thin arc cloud fanning out away from the southerrn coast of Mexico, with hazy plumes of blowing dust seen streaming southward off the coast as the strong northerly winds persisted during the day.
===== 23 January Update =====
As the surface low deepened to a minimum central pressure of 983 hPa and moved northeastward just off the US East Coast (surface analyses), GOES-13 Visible (0.63 µm) images, below, showed the moisture — with some embedded convective elements, judging from the texture and shadowing of the cloud tops — moving inland from the Atlantic Ocean north of the storm. Thundersnow was in fact reported at a number of locations. A similar animation of GOES-13 Visible images covering the daylight portions of the 22-23 January period is available here, with the entire 48-hour Infrared window channel (10.7 µm) animation here.Consecutive Suomi NPP VIIRS true-color RGB images at 1652 and 1828 UTC, below, provided a more detailed view of the convective elements that were moving inland north of the storm center.
===== 24 January Update =====
Shown above is a 72-hour animation of the MIMIC TPW product (from 00 UTC on 21 January to 00 UTC on 24 January), which — as mentioned at the beginning of this blog post — revealed the large amount of moisture-rich air that was drawn northward and subsequently wrapped into the storm. South of Mexico, a narrow tongue of dry air (a signature of the aforementioned Tehuano wind event) was also clearly seen, moving southwestward over the Pacific Ocean.The entire 48-hour period of Rapid Scan Operations GOES-13 Water Vapor (6.5 µm) imagery with plots of surface weather symbols (above; also available as a large 66 Mbyte animated GIF) depicted the evolution of the storm as it moved across the Eastern US from 1215 UTC on 22 January to 1215 UTC on 24 January. The storm produced widespread heavy snowfall, areas of freezing rain and sleet, hurricane-force winds (peak gusts), and coastal flooding (WPC storm summary | NWS impacts statement | Capital Weather Gang blog) — it was ranked a Category 4 on the NESIS scale, and the 4th most intense since 1950 (NCEI overview). Features seen on the water vapor imagery included the development of a well-defined dry slot, cold conveyor belt, and elongated comma head / deformation zone that helped to produce the prolonged period of heavy snow. Interesting gravity waves were also seen within the offshore dry slot on 23 January, which appeared to be propagating westward back toward the coast. Larger-scale GOES-13 animations covering the entire 48-hour RSO period are also available [Water Vapor (6.5 µm): MP4 | animated GIF ; Infrared window channel (10.7 µm): MP4 | animated GIF].
The illumination of a Full Moon helped to provide a vivid “visible image at night” using the Suomi NPP VIIRS 0.7 µm Day/Night Band (below), highlighting the clouds associated with the departing storm along and just off the US East Coast, as well as the vast areas inland that were snow-covered. In the toggle between the corresponding Infrared window (11.45 µm) image, cloud streets due to cold air streaming southward and southeastward across the Gulf of Mexico toward Cuba were also seen.With the arrival of daylight on the morning of 24 January, the expansive area of snow cover was very apparent on GOES-13 Visible (0.63 µm) images, shown below. Snow depth values (inches) are plotted in cyan — 12 UTC depths for the earlier images, and 18 UTC depths for the later images. The 18 UTC snow depth values were a bit less at many locations (due to compaction and/or melting), and parts of the extreme southern and southeastern edges of the snow cover were seen to melt away during the late morning hours. False-color Red/Green/Blue (RGB) images made using Terra MODIS Visible (0.65 µm) and Snow/Ice (1.61 µm) images, below, showed how such RGB images can be useful for the discrimination of snow/ice (shades of red) vs. supercooled clouds (shades of white). Bare ground appears as shades of cyan. The full-resolution Terra MODIS true-color RGB images viewed using the SSEC RealEarth web map server, below, showed even better detail, including the very sharp northern edge of the snow cover from New York and Connecticut to Massachusetts. The wider swath of the VIIRS instrument on the Suomi NPP satellite provided a good true-color vs false-color image comparison, shown below. In this particular RGB image, show/ice (and glaciated ice crystal clouds) appear as shades of cyan, while supercooled water droplet clouds appear as shades of white. Even greater detail could be seen in a 30-meter resolution Landsat-8 false-color RGB image, centered over the Washington DC area (below). Snow and ice also appear as shades of cyan in this image — ice can be seen in parts of the Potomac and Anacostia Rivers. The partially-plowed runway network of Reagan National Airport appears at the bottom center of the image. A close-up Landsat-8 view of the Baltimore, Maryland area (below) also showed some ice forming in a few of the rivers. Widespread strong gusts occurred with this storm, as shown in the Table above (from the National Weather Service). The hourly animation, below, of GOES-13 Water Vapor (6.5 µm) with Wind Gusts superimposed, shows that the strongest gusts occurred as the storm’s dry slot, depicted as darker shades in the water vapor imagery and a region which is often associated with strong subsidence, was nearby. Suomi NPP VIIRS Visible and Near-Infrared imagery, below, shows the extent of the snowcover on 25 January. The benefit of multi-spectral imagery (as is available today from Suomi NPP, and will also be available from GOES-R) appears by comparing the 0.64 µm, 0.86 µm and 1.61 µm channels. For example, regions of snow vs. no snow are less distinct in the 0.86 µm (over northwest Connecticut, for example), but land/water differences are accentuated. Comparing the visible and the 1.61 µm brings out snow/ice features. The band of snow over southern New England is dark in the 1.61 µm because snow/ice absorbs radiation at that wavelength. Snow is highly reflective in the visible, however, and it appears bright white on that image. This comparison of visible and 1.61 µm can also be used to highlight ice clouds (as noted higher up in this blog post). RGB Imagery allows a one-image perspective (vs. an image toggle) to highlight features. The RGB image from Suomi NPP VIIRS imagery (0.64 µm and 1.61 µm) below shows snow cover (shades of red) over the Northeast. In the mid-Atlantic states, thin patchy clouds are present. The brightness of these clouds in the 1.61 µm channel suggests they are composed of supercooled water droplets.
When strong storms appear in Forecast Models, it is tempting to trace back in the atmosphere where the system originates. The water vapor animation above, from GOES-13, ends with the future East Coast Storm developing over the lower Mississippi River Valley (where it produced severe weather). The rocking animation, below, shows that the storm moved in from the Pacific Ocean on January 19th.
How did the impulse traverse the Pacific Ocean? The animation above, from GOES-15, shows the system approaching the West Coast at the end of the day on Monday 18 January. It is by no means the strongest-looking system in this animation, and it has the characteristics, at 2100 UTC on 18 January, of a developing cyclone west of San Francisco. The rocking animation, below, shows that the system crossed the Pacific Basin, moving well north of Hawaii, in about two days. What did the storm do before GOES-15 could view it?
The animation from Himawari-8, above (note that Himawari-8 produces Full Disk imagery every 10 minutes; GOES-13 and GOES-15 time resolution for full disk imagery is every 3 hours, and the time resolution of the Himawari-8 animation here is degraded significantly to match that of GOES-13 and GOES-15; GOES-R will have temporal resolution equivalent to Himawari), shows a series of impulses moving off the coast of Asia, with the final impulse maintaining integrity into the central Pacific (shown here at 2100 UTC on 16 January 2016; here is the view from GOES-15 at the same time). The rocking animation, below, allows for easier tracing of these impulses. The impulse that would track across the Pacific, enter North America and become a strong storm appears to be a small enhancement in the Water Vapor imagery at 0600 UTC on Friday 15 January southwest of Japan, north or Taiwan, just emerging off the coast of Asia. It can be traced back to a large low pressure system over Siberia at the beginning of the Himawari-8 animation (on 13 January).
This animation strings together Himawari-8, GOES-15 and GOES-13 Water Vapor imagery from 14 January through 21 January, showing the evolution, in 3-hour steps (the native GOES-13/GOES-15 temporal resolution for Full Disk Imagery), of the system as it moves from Asia across the Pacific into North America. The images below, are annotated water vapor imagery stepping back in time showing the impulse.]]>