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Chances for wintry precipitation continue in the Tennessee Valley.  This morning and afternoon, another batch of precipitation was moving across areas of the lower to mid Mississippi Valley.  Ground reports indicate that much of the precipitation reaching the surface is a mix of snow and sleet in eastern Arkansas and northern Mississippi.  Drier air in the lower levels may prevent some of the precipitation from reaching the surface on the northern extent of the precipitation shield.  A perusal of SPoRT LIS soil temperatures in AWIPS shows that skin surface temperatures are a little below freezing in much of northern and western Mississippi and in parts of northwestern Alabama (Figure 1).  In the image below, white colors indicate temps around 32°F, while deeper blues (lower OH Valley and mid-Mississippi Valley) are below 28°F.    

Figure 1.  3-km Average Skin Surface Temperature (F), 15 UTC 23 Feb 2015

Figure 1. 3-km Average Skin Surface Temperature (F), 15 UTC 23 Feb 2015

Of course, it is important to point out that these data were valid at 15 UTC (from the 15 UTC analysis).  Skin surface temperatures, especially, can exhibit relatively large changes in values during periods of radiational heating/cooling.  Thus, today’s breaks in the clouds and resulting insolation will cause further warning in skin surface temperatures particularly in parts of northern Alabama.  Next, let’s take a look at the 0-10 cm soil temperatures (Figure 2).

Figure 2.  3-km 0-10 cm SPoRT LIS Soil Temperatures (F), 15 UTC 23 Feb 2015

Figure 2. 3-km 0-10 cm SPoRT LIS Soil Temperatures (F), 15 UTC 23 Feb 2015

Notice that temperatures in the deeper soil layer are generally above freezing (per the SPoRT LIS) despite air temperatures well below freezing.  Air temperatures were warmer across the region during much of the weekend, so the sub-surface is still warmer, but slowly responding to the recent cold air advection over the last 24 hours.

So, the take away here is that any accumulations will probably be immediate (due to the below freezing skin temperatures and snowfall rates), but limited, as warmth from the deeper sub-surface tends to melt precipitation slowly from below.

One of the great features of AWIPS II, is that it allows for the visualization of multiple layers.  Following is a time lapse of regional NEXRAD radars (composite) over the SPoRT LIS skin surface temperatures (Figure 3) during the late morning into the early afternoon.  The 15 UTC SPoRT LIS soil temperature image is static due to its more limited 6-hourly resolution.  These soil temperature data/products are currently being transitioned in a testbed phase to NWS offices in Houston, Huntsville and Raleigh.

Figure 3.  Loop of SPoRT LIS surface skin temperatures (background image, 15 UTC) overlaid with NEXRAD regional radar composite (0.5 degree), 1718-1912 UTC 23 Feb 2015

Figure 3. Loop of SPoRT LIS surface skin temperatures (background image, 15 UTC) overlaid with NEXRAD regional radar composite (0.5 degree), 1718-1912 UTC 23 Feb 2015

Recently, three NWS offices participated in an assessment of several SPoRT Land Information System (LIS) soil moisture products for applicability to drought monitoring and flood threat applications.  Currently, two soil temperature products are being sent to the same WFOs (Houston, Huntsville, and Raleigh) in an initial testbed phase during this winter.  These soil temperature variables are 3-km average surface skin temperature and 0-10 cm soil temperature.  Due to the recent very cold air temperatures, both skin and 0-10 cm soil temperatures were quite cold across the Tennessee Valley.  However, temperatures were a little warmer further to the south over portions of the lower Mississippi Valley and the Gulf Coastal Plan region.  Nevertheless, the SPoRT LIS still indicated that soil skin temperatures were cold enough for the potential for freezing precipitation to accumulate on surfaces.  Take a look at this image of combined SPoRT LIS average skin surface temperature and regional radar data from earlier this morning (Figure 1).

SPoRT LIS 3-km Average Skin Surface Temperature (0900 UTC 20 Feb 2015) overlaid with regional composite 0.5 WSR-88D Reflectivity (~1454 UTC)

Figure 1.  SPoRT LIS 3-km Average Skin Surface Temperature (0900 UTC 20 Feb 2015) overlaid with regional composite 0.5 WSR-88D Reflectivity (~1454 UTC)

Since the radar data are displayed on top of the SPoRT LIS soil temperatures, the soil temperature scale does not show in AWIPS II…unfortunately.  However, the white colors indicate temps ranging from -1C to +1C.  The cyan colors found generally along coastal Louisiana and Texas and portions of Florida, are above freezing, while the deeper blues represent values well below freezing.  Notice that average skin surface temperatures according to the SPoRT LIS across much of Mississippi were below freezing, despite temps at or above freezing in southern Mississippi (although the data latency should also be noted…the LIS data were valid at 09 UTC).  Icy conditions with freezing rain were reported around portions of central and southern Mississippi with the precipitation that moved across the area.   The Jackson NWS office had issued a Freezing Rain Advisory to address this winter weather threat.

Soil temperatures can be very difficult to come by, and forecasters often have to make assumptions about these values.  Here, the value of the LIS data can be seen to address the threat for freezing precipitation accumulation in a winter weather event.

A quick-moving upper-level trough and associated cold front moved across the TN and lower OH valleys this morning, producing snow showers around the region.  The Nighttime Microphysics RGB image below from the VIIRS instrument (Image 1), from around 0805 UTC, shows low/mid clouds associated with the trough. Notice most of the clouds appear light green/yellow and or green/red, indicating predominantly low/mid cloud types.  A synthesis of radar and sounding data indicates these clouds were dominated by snow/ice crystals and perhaps some super-cooled liquid droplets.  Reports of only light snow have been received at surface stations in the region this morning.

Suomi NPP VIIRS Nighttime Microphysics RGB ~0805 UTC 18 Feb 2015

Image 1.  Suomi NPP VIIRS Nighttime Microphysics RGB ~0805 UTC 18 Feb 2015

Notice the narrow swath of darker green colors stretching from NE Mississippi northeastward into northern middle Tennessee.  Although it may not be apparent, the green color contribution (green represents the 10.8-3.9 µm channel difference in the RGB recipe) in this swath was actually lower than in adjacent clouds. In addition to lower contributions of green, red and blue color contributions were also lower in this narrow swath.  The resulting interpretation is that this area was composed of lower, warmer clouds.  Due to the variations in green color contribution across the cloud deck, it was immediately unclear whether there was a mix of super-cooled liquid water and ice/snow crystals.  Radar imagery from the Nashville, TN and Columbus, MS radars from about the same time indicate mostly uniformity in falling hydrometeors.  The image below (Image 2) shows Correlation Coefficient values from the KOHX and KGWX radars at ~0806 UTC.

Image 2.  VIIRS Nighttime Microphysics RGB overlaid with Correlation Coefficient (CC) values from KOHX and KGWX radars

Image 2. VIIRS Nighttime Microphysics RGB overlaid with Correlation Coefficient (CC) values from KOHX and KGWX radars

A small area of slightly lower CC values can be seen to the northwest of Nashville and an inspection of ZDR values (not shown) indicated slight/moderate wet snowflakes.  Although, standard reflectivity imagery from the same time shows little in the way of precipitation in that area (Image 3).  It should be noted that precipitation echoes were somewhat absent from the darker green swath.  However, since these clouds and any resulting precipitation (if present) was relatively low, echoes were below the lowest scans of the regional radars.

Image 3.  VIIRS Nighttime Microphysics RGB overlaid with 0.5 Reflectivity from KOHX and KGWX radars

Image 3. VIIRS Nighttime Microphysics RGB overlaid with 0.5 Reflectivity from KOHX and KGWX radars

Incidentally, the KHTX radar was down for a needed repair and thus not available during this time.  One advantage of this type of satellite imagery is that it is safer to make inferences about the presence of precipitation, and in some cases, perhaps even precipitation intensity and type, given similar RGB cloud characteristics.  During an event such as this where standard radar data may not be available at a given location, the value of the imagery becomes even more apparent.

The prospect for additional snow and mixed precipitation events over the upcoming week or so will offer more interesting observations of the Nighttime Microphysics RGB.  More posts to follow soon!

It is the time of the year when deep plumes of tropical moisture known as atmospheric rivers tend to bring heavy precipitation to the west coast.  A particularly strong atmospheric river impacted the west coast this weekend (6-8 February) bringing over 10 inches of rainfall to parts of northern California and nearly 20 inches of snowfall across portions of the Sierra Nevada.  The CIRA total layered precipitable water product clearly shows the large amount of moisture streaming into California from the tropics.

Image 1.  SPoRT-CIRA total layered precipitable water valid on 7 February 2015 at 12 UTC.

Image 1. CIRA total layered precipitable water valid on 7 February 2015 at 12 UTC.

However, there is another component to this atmospheric river event that does not get much attention but they can significantly impact precipitation from these storms.  They are dust aerosols originating from the deserts in Asia, the Middle East, and Africa, and pollution aerosols from the megacities of eastern Asia.  These aerosols are transported across the Pacific most frequently during the late winter and early spring when the wind pattern is ideal.  In fact, a currently ongoing NOAA-led field investigation (i.e. CalWater 2) is focused on investigating the interaction between aerosols and atmospheric rivers to better understand how they modify precipitation.  SPoRT is supporting this effort by disseminating a near-real time product that combines aerosol optical depth (AOD) retrievals from geostationary and polar-orbiting satellites to monitor and track the long-range transport of aerosols.  Extensive cloud cover over the Pacific can often make it difficult to track the aerosols via satellites, therefore, the SPoRT product takes advantage of MODIS, VIIRS, MTSAT, and GOES AOD retrievals in order to provide a more comprehensive look at aerosol activity across the Pacific.  SPoRT is disseminating 6-hourly and daily AOD composites throughout the extent of CalWater 2 field campaign set to end mid-March with the overall goal of supporting their aerosol forecasting and flight planning activities.  Shortly before the atmospheric river made landfall on 7 February, the SPoRT daily AOD composite shows unusually high AOD of 0.3 to 0.5 (Image 2) in the vicinity of the atmospheric river in Image 1.  Higher values of AOD indicate an increasing amount or loading of aerosols in the atmosphere.  AOD is typically minimal (< 0.2) over the eastern Pacific when long-range transported aerosols are not present.

Image 1. NASA SPoRT daily AOD composite valid on 7 February 2015 at 00 UTC.

Image 2. NASA SPoRT daily AOD composite valid on 7 February 2015 at 00 UTC.

The CalWater 2 field investigators flew an aircraft equipped with advanced instruments directly through this atmospheric river on 7 February, which sampled an abundance of long-range transported dust according to CalWater investigator Dr. Jessie Creamean.  Further investigation is warranted to determine the source region of the dust.  With weeks to go still in the CalWater 2 campaign, field investigators are hoping for more opportunities to fly through aerosol laden atmospheric rivers.  If the trend from the past week continues, the CalWater 2 team should be able to gather a wealth of unique measurements that will ultimately lead to improved forecasting of these atmospheric river events.

Aerosol activity over the Pacific was relatively calm during January, but February kicked off with a bang.  The SPoRT daily AOD composite on 3 February shows a very thick, extensive aerosol plume propagating from eastern Asia to the western Pacific (Image 3).  Closer analysis suggested this plume was a mixture of dust and pollution aerosols.

Image 3.  NASA SPoRT daily AOD composite valid on 3 February 2015 at 00 UTC.

Image 3. NASA SPoRT daily AOD composite valid on 3 February 2015 at 00 UTC.

SPoRT AOD composites can be found at http://weather.msfc.nasa.gov/sport/aod/aodCalendarView.html

We’re near full-moon phase now so I thought I’d take a moment to show some recent VIIRS Day-Night Band (DNB) observations from the Suomi-NPP satellite. This first DNB image from the early morning of Feb 2nd, shows a broad swath of snow on the ground across portions of the Midwest, in addition to a variety of clouds across the Tennessee and Ohio Valleys and further east.  The snow field in the DNB image (Image 1) is fairly easily distinguishable from the clouds due to presence of the river valleys cutting across the snow field in the image.  Notice the edge of the snowpack extends about as far south as Kansas City, but is mainly on the northern half of the metro area.  In this case, having the city lights as a reference can be operationally beneficial…for help in determining surface snow impacts.

VIIRS Day-Night Band Radiance RGB

Image 1.  VIIRS Day-Night Band Radiance RGB ~0806 UTC 2 Feb 2015.  The

This VIIRS Nighttime Microphysics RGB can further help to distinguish snow from clouds (Image 2).  Notice that the snow doesn’t really show up well in this RGB (although it encompasses the slightly more orange hues against the surface pink colors).  Toggling these two images in AWIPS II would allow a forecast to more readily distinguish snow from clouds .

Image 2.  Suomi-NPP VIIRS Nighttime Microphysics RGB 0806 UTC 2 Feb 2015

Image 2. Suomi-NPP VIIRS Nighttime Microphysics RGB ~0806 UTC 2 Feb 2015

Following are a couple of DNB images from the past couple of mornings.  Notice that the snowpack remains but has been shrinking slightly at its edges over the past couple of days.

Image 3.  Suomi-NPP VIIRS DNB Radiance RGB 0728 UTC 4 Feb 2015

Image 3. Suomi-NPP VIIRS DNB Radiance RGB ~0747 UTC 3 Feb 2015

Image 4.  Suomi-NPP VIIRS DNB Radiance RGB ~0728 UTC 4 Feb 2015

Image 4. Suomi-NPP VIIRS DNB Radiance RGB ~0728 UTC 4 Feb 2015

SPoRT and NESDIS are currently collaborating on an assessment of a snowfall rate (SFR) product that includes data from the Suomi-NPP Advanced Technology Microwave Sounder (ATMS) instrument.  ATMS provides more channels, better resolution, and a wider swath than previous operational microwave sounders, like the Advanced Microwave Sounding Unit (AMSU) and Microwave Humidity Sounder (MHS).  The SFR product uses information in microwave channels to estimate liquid-equivalent snowfall rates that forecasters can use for pinpointing the locations of the heaviest snowfall during winter weather events.  These observations are being provided in near-real-time (less than 30 minutes latency) through access to data from direct broadcast provided by the University of Wisconsin/CIMSS.

The northeast is currently experiencing a historic blizzard with areas of New England recording more than a foot of snow with more snow to come later today.  Below is an example of the output from the ATMS SFR product depicting heavy snow over most of New England.  This image indicates that the heaviest snowfall at this time was centered over southeastern Connecticut with rates anywhere from 1-1.5 inches of solid snow per hour.

ATMS SFR depicting the 2015 Blizzard at 0650 UTC (around 2:00 A.M. local time) on 27 January 2015.

ATMS SFR depicting the 2015 Blizzard at 0650 UTC (around 2:00 A.M. local time) on 27 January 2015.

Coupled with other microwave sensors on board other NOAA and European satellites, up to 10 swaths of observations are available to provide observations of where the heaviest snowfall is falling and allows forecasters to track these features when used in conjunction with GOES imagery and radar.

David Barjenbruch, a forecaster at the Denver/Boulder WFO working with SPoRT and NESDIS to evaluate the NESDIS Snowfall Rate product this winter, provided a nice case study for consideration.  He writes:

We received more snow across the Front Range of Colorado on Wednesday, January 21, and just wanted to attach a few comparisons of SFR, radar, obs, and METARS.  At 17Z, the SFR unfortunately missed the moderate snow across the Denver metro area (KDEN…1mm water in last hour…and KBJC 1/2 mile visibility in moderate snow, and 1/4 mile at KAPA…0.8mm water in last hour). Area of main concern is highlighted in pink, while at the same time it also overestimated snow toward the east toward ITR in eastern Colorado.

Comparison between radar and SFR product at 17Z on 21 Jan 2015

Comparison between radar (left) and SFR Product (right) at 17Z on 21 Jan 2015.  Circled area denotes region where SFR missed accumulating precipitation in the Front Range.  SFR overestimates snow in eastern CO.

 

Comparison between radar and SFR at 20Z on 21 Jan 2015.

Comparison between radar (left) and SFR Product (right) at 20Z on 21 Jan 2015.  SFR captures some of the snow in southern CO but continues to overestimate snowfall in parts of eastern CO.

After a fairly widespread precipitation event Wednesday morning, the snow turned more convective in the afternoon.  Checking the 2320Z radar, we had an intense convective cell which moved southwest across the western sections of the Denver metro area (highlighted in pink again).  This particular cluster of convection produced anywhere from 1.5 to 3″ of snow (2-5.5mm water) in an hour or less.

Comparison between radar and SFR Product at 2320Z on 21 Jan 2015.

Comparison between radar and SFR Product at 2320Z on 21 Jan 2015.  Circled area denotes convective snowfall that was not captured by the SFR.

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