VISIT: Meteorological Interpretation Blog

2-3 April 2019 East Coast Low – ALPW analysis

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Nebraska flooding

The past two weeks Nebraska has been inundated with heavy precipitation, in the forms of rain and snow. Nebraska was significantly affected by the ‘record-breaking’ mid-latitude cyclone that past through the area from 13-15 March 2019.  Refer to the GOES-16 10.3um infrared satellite imagery below, seen from 5Z, 13 March 2019 to 22Z, 14 March 2019. Throughout the animation, notice the large areal extent of the cyclone and the cold convective cloud tops (i.e. cold brightness temperatures) indicating varying levels of precipitation in Eastern Nebraska.

From this storm, plus subsequent storms thereafter, extreme flooding has transpired throughout eastern Nebraska. The image below displays the observed precipitation values over Nebraska the last 14 days, valid at 12Z, 22 March 2019. Product is provided from the National Weather Service (NWS) – Advanced Hydrologic Prediction Services (AHPS). Precipitation values ranged from 1-5 inches throughout Nebraska.

Coinciding with the precipitation values is polar-orbiting satellite data that observes the magnitude of flooding via Suomi-National Polar-orbiting Partnership (S-NPP) Visible Infrared Imaging Radiometer Suite (VIIRS) Flood Areal Extent product. The product (seen in animation below) shows the areas of flooding (i.e. yellow, orange, red colors) in Eastern Nebraska and Iowa between 15-21 March 2019. 17 and 19 March were omitted due to cloud obscuration observed over the area. VIIRS Flood Areal Extent calculates the floodwater fraction percentage of a pixel (i.e. from 0-100%, green-to-red colors), and discriminates between different scene types. Note in the legend: LD = land (brown), SI = supra/snow ice (mixed water and ice, or water over ice, seen in purple), IC = ice (river or lake ice, seen in aqua), CL = clouds (grey), CS = cloud shadows (dark grey), and WA = open water (blue). Spatial resolution of the product is at 375-m resolution. Within the animation, see the evolution of the floodwaters as they increase in width or move along the rivers.

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Polar orbiting and geostationary lake ice monitoring

Monitoring lake ice coverage over the Great Lakes via satellite is vital and affects shipping industries, tourism and recreation, especially over the winter months when ice develops, grows and expands over the lakes. According to the Great Lakes Surface Environmental Analysis (GLSEA) and NOAA CoastWatch, the total ice coverage between all 5 lakes is 80% as of 8 March 2019. GLSEA and NOAA CoastWatch’s diagram below highlights lake ice coverage (i.e. ice depicted as grey, dark grey and black colors) and the areas of open water seen via different shades of blue (i.e. represented via water temperatures, ~0°C-5°C).

Satellite observations, combined with other data sets, are vital in producing ice coverage percentages over the Great Lakes. On 8 March 2019, under moderate clear-sky conditions, polar-orbiting and geostationary satellites observe the Great Lakes at high spatial resolution. Note, geostationary observations express high temporal resolution as well, however polar-orbiting observations exhibit coarser temporal resolution. Satellite imagery and products are shown below and are provided from RAMSDIS Online, CIRA SLIDER and RealEarth.

S-NPP Day/Night Band (DNB) – Solar Reflectance (0.7um) at 1850Z, 8 March 2019

DNB solar reflectance acts like ‘daytime visible imagery’ (i.e. 0.64um) where DNB’s satellite sensor observes the solar reflectance from atmospheric or surface features that exhibit high albedos. DNB provides imagery (below) at 750-m resolution and shows open water, land, ice and clouds, above and around the Great Lakes. However, how can users differentiate between the aforementioned scene types?

S-NPP VIIRS Snow/Cloud Layers at 1850Z, 8 March 2019

Look no further than to the polar-orbiting VIIRS Snow/Cloud Layers product which is at 750-m resolution. Observing the same domain as DNB, the VIIRS Snow/Cloud Layers differentiates between land (green), snow and ice (white), low (yellow) and high (pink) clouds and bodies of water (dark blue/black).

GOES-16 Snow/Cloud Layers from 1832-1927Z, 8 March 2019

Now incorporate a similar product, except derived from the geostationary satellite, GOES-16, users can observe the Great Lakes at high temporal resolution. Temporal resolution is from the CONUS sector, that is, 5-minute geostationary data. Notice the lake ice movement (i.e. moving white features over the Great Lakes) along with the low and high clouds moving to the east. Lake ice motion can be seen more conspicuously over Lake Huron.

S-NPP VIIRS Flood Detection Product at 1900Z, 8 March 2019

Additionally, another polar-orbiting product that users can observe the Great Lakes and differentiate between surface and atmospheric features, is the VIIRS Flood Detection product. Product is at 375-m resolution, discriminates between different scene types: ice = aqua, supra-snow ice (water on top of ice, or melting ice) = purple, open water = blue, clouds = grey, snow = white, and land = brown. The product also calculates the floodwater fraction percentage from 0-100% (green-red colors) as well. VIIRS Flood product can be accessed in AWIPS-II via LDM.

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3 March 2019 – Severe thunderstorm and heavy rainfall event

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Radar and satellite ‘snowfall rate’ observations

Forecasting snowfall and snowfall rates can be quite challenging, especially in radar-limited and or radar-deprived regions. A polar-orbiting satellite ‘Snowfall Rate’ product can be used together with radar observations to help anticipate snowfall rates, identify snowfall areal extent and snowfall maximas. To highlight the product’s capabilities, refer to the following snowfall case event over Northern Colorado, between 3-15Z, 2 March 2019 comprising of surface, radar and satellite observations.

Surface Observations over Northern Colorado from 3-15Z, 2 March 2019.

The 13-hr loop shows a decrease in air temperatures across Northern Colorado, exhibiting below-freezing temperatures. Over time, notice a surface low develop, just north of Denver, CO as southeasterly, upslope flow moved into the area. Additionally, the surface low in complement with an upper-level jet maxima (not pictured) and an increase in low-to-mid level moisture, produced enhanced snowfall totals over Northern Colorado.

Radar observations over Northern Colorado from 3-15Z, 2 March 2019.

Base Reflectivity radar observations (via Denver radar from the COD website) during the same time period, shows the evolution of higher reflectivity values (between 15-35 dBZ) observed over Northern Colorado. Notably from 11-13Z, a bright ‘snow band’ (an elongated reflectivity maxima) was observed, indicating heavy precipitation, or in this case, heavy snowfall over Larimer and Weld counties. But what snowfall rates are being observed? This is where the Snowfall Rate (SFR) product can be utilized.

Collocated Snowfall Rate (SFR) product and Radar Observations at ~11Z, 2 March 2019.

The SFR product is derived from passive microwave observations via polar-orbiting satellites, where SFR observations are displayed in units of liquid equivalent ‘inches per hour’. The image below is a direct comparison of SFR in relation to the radar (albeit, offset by two minutes) at ~11Z, 2 March 2019. Notice the line of higher liquid equivalent snowfall rates (0.04-0.1 inches per hour) correspond well with the bright ‘snow band’ seen in the radar. Additionally, the SFR product has the ability to observe the snowfall rate areal extent, seen throughout Wyoming, Nebraska and South Dakota via 1 satellite swath (i.e. DMSP). In contrast, the Denver radar exhibits a limited range, where an adjacent radar needs to be utilized to see snowfall occurring in nearby domains (i.e. users would need to refer to the Cheyenne, WY or North Platte, NE radars).

Snowfall Analysis and snow totals (ending ~9AM MST, 2 March 2019).

Via NWS/NOAA snowfall analyses, note the snow totals from this event, ranging from 0-6 inches at low elevations, and 6-plus inches at higher elevations.

For interested readers, NASA-SPoRT has an additional product, denoted as the Merged SFR product (i.e. incorporates radar and SFR together) that can be accessed online via the following link.

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