Feeds:
Posts
Comments

An upper level closed low near Baja and a backdoor front combined to bring considerable precipitation to New Mexico during the day and overnight period of March 19, 2015.  Ample moisture surged northward over New Mexico ahead of the closed low, and precipitable water as measured by the soundings at Albuquerque, NM and El Paso, TX tied for the fourth highest value for March since 1950 (at both locations). Additional support for this event came in the form of a back door cold front which raced through the eastern plains of New Mexico during the day on March 19.  Because of warm temperatures, Winter Storm Warnings (blue) and Advisories (yellow)  were limited to the New Mexico northern high terrain including the Jemez Mountains, San Juan Mountains and Sangre de Cristo mountains, as shown in the figure below.

WSW

Two NESDIS Snowfall Rate (SFR) products were available for review the morning following the event.  The first is from 0353Z on 20 March 2015, or about 10pm MDT on the evening of March 19, and is shown in the figure below with the 04Z surface observations.  The east to northeast flow in the eastern half of the state indicates the progress of the back door front.  Most locations in central and eastern New Mexico are reporting rain, including Raton (KRTN) just to the east of the active snow area in the SFR product. Angel Fire (KAXX) just to the south of the area is reporting snow.  At this time, the SFR product appears to do a good job in distinguishing between rain and snow despite the fact the Angel Fire is just outside the SFR active area.

NESDIS_SFR_20150320_0353Z_obs

The 0.5 reflectivity mosaic at the same time illustrates beam blockage that impacts the area east of Albuquerque, but also the limited radar coverage in northern New Mexico, though there are weak echos associated with the snow report at KAXX as well as the rain at KRTN.  Also note convection in western Texas – earlier in the evening one-inch hail was reported in eastern New Mexico.  This is an example of the interesting regimes that can impact our CWA in that we can have winter weather warnings and severe weather at concurrent times.

0_5reflectivity_20150320_0400Z_obs

Similar graphics are shown for 0855Z, or 3am MDT, on the morning March 20th. Activity has weakened considerably and the WSW is about to be cancelled.  Still, light snowfall rates are depicted by the SFR over the northern high terrain.  The metar observation at Angel Fire, KAXX, is still reporting snow.

NESDIS_SFR_20150320_0855Z_obs

The 0.5 reflectivity mosaic illustrates that the only isolated precipitation continues over western and central New Mexico, with no returns over the northern high terrain.

0_5reflectivity_20150320_0854Z_obs

In the image below, the 0855Z SFR product is combined with the awips hi-res topography map to illustrate the agreement with the SFR and the highest terrain of the southern San Juan and northern Sangre de Cristo Mountains in northern New Mexico.

SFR_0855Z_withTerrain

One of the frustrations with evaluating the NESDIS SFR product is that consecutive products can be separated by long periods of time, in this case by 5 hours. However, substantial snow accumulations were reported in the Sangre de Cristo mountains – from 6 to 19 inches. Thus the area depicted by the SFR product seems to be fairly accurate, but the evaluation is rates is more difficult.

In addition to snow, widespread rainfall reports ranged from one quarter of an inch to one inch. Early this morning, the following DOT report was posted – the combination of rain and snow resulted in rock slides on at least two roads in northern New Mexico.

DOTinfo

NWS Raleigh along with WFOs Houston and Huntsville have been participating in an assessment of several NASA SPoRT Land Information System (LIS) soil moisture products during the past year. During the winter, the same WFOs have been receiving two LIS soil temperature products as a part of an initial testbed. These soil temperature products are the 3-km average surface skin temperature and 0-10 cm soil temperature.

Past experience has shown that the temperature of the soil and ground surface can have an impact on snow and ice accumulation. It was hoped that the availability of the SPoRT 0-10 cm soil temperature and the skin temperature products would provide useful information to forecasters. In addition, the SPoRT analysis products would complement a network of mesonet point observations managed by the NC State Climate Office that provide 10cm soil temperatures.

The numerous winter weather precipitation events in central NC this year provided an opportunity to view and evaluate the SPoRT products. At WFO Raleigh, the SPoRT soil temperature data was informally viewed and consulted to evaluate its utility.

Fig 1. AWIPS 2 display of surface METAR data and composite regional radar reflectivity from 1000 UTC on 09 January, 2015.

Fig 1. AWIPS 2 display of surface METAR data and composite regional radar reflectivity from 1000 UTC on 09 January, 2015.

On Friday, January 9th, 2015, areas of freezing rain fell across the southern and central Coastal Plain of North Carolina during the pre-dawn hours. Surface air temperatures ranged in the mid to upper 20s at 10 UTC or 5 AM EST (Fig. 1). The precipitation was driven by convergent low level flow that resulted in a small region of ascent and saturation across portions of southeastern North Carolina. The precipitation followed a period of 24 to 36 consecutive hours of sub-freezing air temperatures.

Fig 2. AWIPS 2 display of NASA SPoRT 3-km average surface skin temperature (top) and the NASA SPoRT 0-10 cm soil temperature (bottom) valid at 0900 UTC on 09 January, 2015.

Fig 2. AWIPS 2 display of NASA SPoRT 3-km average surface skin temperature (top) and the NASA SPoRT 0-10 cm soil temperature (bottom) valid at 0900 UTC on 09 January, 2015.

Given the cold antecedent conditions, the ground across the area was very cold. The SPoRT 3-km average surface skin temperature as displayed in AWIPS 2 is shown in the top of Fig. 2. Skin temperatures in the region of freezing rain ranged between 20 and 25 degrees. In addition, the SPoRT 0-10 cm soil average temperature (bottom of Fig. 2) indicated that the average soil temperature in the layer from the surface to 10cm or 4 inches below the ground averaged between 25 and 30 degrees. This indicated that not only was the top of the soil column well below freezing but the soil immediately below the ground was below freezing and wouldn’t be providing significant heat to warm the ground near the surface.

Fig 3. Snapshot of a WECT-TV news story on the car accidents resulting from the freezing rain on 09 January, 2015.

Fig 3. Snapshot of a WECT-TV news story on the car accidents resulting from the freezing rain on 09 January, 2015.

Given this environment, the precipitation fell as an area of light freezing rain across the Coastal Plain which resulted in icy conditions on many roadways with numerous car accidents across Sampson, Bladen, Duplin and nearby counties during the morning (Fig. 3). While the amount and extent of precipitation was in question just hours prior to the event, the soil temperature data provided increased confidence that any precipitation that fell would have a significant impact.

Fig 4. NC State Climate Office CRONOS display of surface temperatures and composite regional radar reflectivity from 1000 UTC on 27 January, 2015.

Fig 4. NC State Climate Office CRONOS display of surface temperatures and composite regional radar reflectivity from 1000 UTC on 27 January, 2015.

In another event, during the early morning hours on Tuesday, January 27th, 2015, a short wave trough and the associated surface trough produced an area of precipitation that moved across central NC. The precipitation was in the form of snow and snow showers near the Virginia border across the northern Piedmont and northern Coastal Plain of North Carolina with air temperatures hovering near freezing (Fig. 4). Temperatures were warmer to the south where the precipitation fell as rain or mixed rain and snow.

Fig 5. Web page display of the NASA SPoRT 3-km average surface skin temperature (top) and the NASA SPoRT 0-10 cm soil temperature (bottom) valid at 1000 UTC on 27 January, 2015.

Fig 5. Web page display of the NASA SPoRT 3-km average surface skin temperature (top) and the NASA SPoRT 0-10 cm soil temperature (bottom) valid at 1000 UTC on 27 January, 2015.

The ground across northern NC was cool that morning but the soil was generally above freezing. The SPoRT 3-km average surface skin temperature as displayed via the web is shown in the top of Fig 5 with the skin temperatures in the region where precipitation was falling ranging around or a few degrees above freezing. Skin temperatures cooled to the west behind the surface trough and the precipitation, as cooler and drier air moved into the area. In addition, the SPoRT 0-10 cm average soil temperature (bottom of Fig.5) indicated that the average soil temperature in the layer from the surface down to 10cm below the ground averaged in the upper 30s to lower 40s. These values were consistent with the soil temperature observations from the NC State Climate Office’s ECNONET network (not shown) which reported 10 cm soil temperatures in the upper 30s to lower 40s.

Fig 6. Snapshot of a WRAL-TV photo taken near Henderson, NC at around 0630 UTC on 27 January, 2015.

Fig 6. Snapshot of a WRAL-TV photo taken near Henderson, NC at around 0630 UTC on 27 January, 2015.

In locations where snow was falling, air temperatures fell to around 32 degrees, the surface skin temperatures were near or just above freezing, and soil temperatures deeper in the ground at 10 cm were in the upper 30s to lower 40s. Given these conditions, it’s possible to conclude that the snow would not accumulate efficiently or much at all. However, the snow fell steadily for at least short periods of time with radar reflectivities in the 25-35 dBZ range. The snow rates were significant enough to produce accumulations of a quarter to a half inch with a few locations reporting an inch or more of snow with some accumulations noted on roadways (Fig. 6).

Snow accumulation forecasting, especially in the South, can be very problematic. In our region, snow accumulation events require so many variables to come together just right and even the slightest accumulation can have a significant impact. Forecasters have many variables to consider including the amount of precipitation expected to fall, the rate, the air temperature, the moisture profile, the soil and skin temperatures, solar impact, and more. In this case, the precipitation rate overcame the other marginal factors for accumulating snow, and more snow accumulated than was anticipated (Fig. 7).

Fig 7. Subjective analysis of accumulated snow in inches from the 26-27 January, 2015 winter storm.

Fig 7. Subjective analysis of accumulated snow in inches from the 26-27 January, 2015 winter storm.

The NASA SPoRT 0-10 cm soil temperature and skin temperature products were examined this winter at WFO Raleigh. The initial results were generally favorable and the additional information was fairly well received. It is important however, that the soil temperature information is used in good context of other observational or forecast information and is used as part of the process. In short, the soil temperature data is one piece of the puzzle, its importance should not be overvalued. There is also some feedback on some technical and delivery issues we noted at Raleigh that we have shared with the developers.

Yet another bout of snow, sleet and ice recently affected much of the Tennessee and Ohio Valley regions.  Although clouds were clearing in western portions of this region, allowing for a broad scale satellite view of the newly laid snow/ice field, eastern portions remained cloud-covered until sunset.  While ground reports contain valuable information about the depth of snow and/or ice, they’re only point measurements, so assumptions often have to be made about the spatial extent of the snow, until satellite observations are available (unless clouds obscure).  So, those observations would have to wait until the next day, during visible sunlight hours…or would they?  Well, not exactly…which is the point of this blog post.

The image below (Image 1) is a Snow/Cloud RGB produced by SPoRT and disseminated to collaborative NWS field offices.  The green colors represent the background surface (grass, trees, cities, etc.), while the deeper reds represent snow/ice cover.  White colors depict clouds, while reddish-white represents very cold clouds containing ice crystal clouds.  Notice the swath of snow that is visible from NE Texas into the Midwest.  Meanwhile, clouds obscure any snow/ice in eastern areas.

Image 1.  MODIS  Snow/Cloud RGB 1631 UTC 5 March 2015

Image 1. MODIS Snow/Cloud RGB 1631 UTC 5 March 2015

Clouds had pushed eastward by sunset, but did still not move far enough to provide a clear indication of the eastward extent of the snow/ice field that had just fallen.  However, once the VIIRS Day-Night Band imagery became available later that night, the spatial extent of the snow and ice could be fairly easily observed.  Notice in the next image (Image 2) the snow and ice cover that was apparent over portions of the Tennessee and Ohio Valley region.

Image 2.  Suomi-NPP VIIRS Day-Night Band Radiance RGB, 0805 UTC 6 March 2015.  The solid white line indicates the extent of the snow/ice cover.  Clouds are also in the image over portions of the southern Appalachians and the Gulf into the Atlantic Coastal Plain.

Image 2. Suomi-NPP VIIRS Day-Night Band Radiance RGB, 0805 UTC 6 March 2015. The solid white line indicates the extent of the snow/ice cover. Clouds are also in the image over portions of the southern Appalachians and the Gulf into the Atlantic Coastal Plain.

This type of imagery can be helpful for operational forecasters when trying to assess the potential societal impacts of lingering snow and ice, and also the impacts on sensible parameters such as temperatures and relative humidity, which can help improve weather forecasts.

NASA SPoRT has developed a real-time application of the NASA Land Information System (LIS) that runs over much of the central and eastern United States. The LIS produces several products, including a suite of soil moisture products that can be used to help assess drought and flooding potential. There are four LIS soil moisture products that are being assessed by WFO Raleigh forecasters in AWIPS-2. The products are also available online at http://weather.msfc.nasa.gov/sport/case_studies/lis_SEUS.html for the Southeast and http://weather.msfc.nasa.gov/sport/case_studies/lis_NC.html for North Carolina.

After a dry start to the month, multiple rainfall events occurred across central NC during the middle and latter portion of December 2014. During the afternoon of 22 December, forecasters were analyzing rainfall from the previous 24 hours which ranged from a tenth of an inch in the Northwest Piedmont to an inch or more across the eastern Piedmont and Coastal Plain regions. This event was the 3rd fairly significant rainfall event (>0.5 inches) over the central NC since 10 December. Forecasters became concerned as another storm system would impact the Carolinas during the next few days and was expected to produce between 1.25 to 2.0 inches of rain from 22 to 25 December (see Fig. 1). This additional rain had the potential to produce some flooding on main stem rivers, especially across the Coastal Plain.

Fig. 1. WPC 72-hour QPF forecast valid 12 UTC 22 December through 12 UTC 25 December, 2014.

Fig. 1. WPC 72-hour QPF forecast valid 12 UTC 22 December through 12 UTC 25 December, 2014.

A SPoRT-LIS field that forecasters have found useful is the one-week change in total column relative soil moisture (RSM, 0-2 m). The RSM is the ratio of the current volumetric soil moisture between the wilting and saturation points for a given soil type, with values scaling between 0% (wilting) and 100% (saturation). The one-week change product valid at 15 UTC on 22 December, just prior to the rain event, is shown in Fig. 2 with the NWS CWAs outlined in yellow. Note that multiple significant rainfall events occurred across central NC during the previous two weeks. Not surprisingly, this product indicated that much of central NC had experienced a relative soil moisture increase from the previous week.

Fig. 2. The SPoRT LIS one-week change in total column relative soil moisture valid at 15 UTC on 22 December 2014 with the WFO CWAs outlined in yellow.

Fig. 2. The SPoRT LIS one-week change in total column relative soil moisture valid at 15 UTC on 22 December 2014 with the WFO CWAs outlined in yellow.

Another SPoRT-LIS field that forecasters found useful is the SPoRT LIS 0-200 cm Relative Soil Moisture (%) analysis product. The LIS 0-200 cm Relative Soil Moisture (RSOIM) analysis from 15 UTC on 22 December is shown in Fig. 3. The RSOM values in the area outlined by the red box across the northern and central portions of the Coastal Plain are highlighted in the deeper and darker green shading and generally exceed 55% and in many locations exceed 60%. Subjective analysis of the RSOIM product by previously by WFO Huntsville AL during several synoptic rainfall events suggests that when the 0-200 cm RSOIM values exceed 55%, the risk of flooding on larger rivers increases substantially.

Fig. 3. The SPoRT LIS 0-200 cm relative soil moisture (%) analysis valid at 15 UTC on 22 December 2014 with the WFO CWAs outlined in yellow.

Fig. 3. The SPoRT LIS 0-200 cm relative soil moisture (%) analysis valid at 15 UTC on 22 December 2014 with the WFO CWAs outlined in yellow.

Significant rain did fall across central NC during the days leading up to Christmas. An analysis of precipitation across central NC from 23 to 25 December shown in Fig. 4 indicates a large area of 2.0 to 2.5 inches of rain fell across the Coastal Plain of NC with an average of around 2.0 to 2.25 inches across the Tar and Neuse River basins. These same locations were noted in Fig. 2 with RSOIM values that exceeded 55%.

Fig. 4. An analysis of precipitation across central NC from 23 to 25 December indicates a large area of 2.0 to 2.5 inches of rain across the Coastal Plain of NC with lesser amounts in the 1.0 to 2.0 range across the western and northern Piedmont of NC.

Fig. 4. An analysis of precipitation across central NC from 23 to 25 December indicates a large area of 2.0 to 2.5 inches of rain across the Coastal Plain of NC with lesser amounts in the 1.0 to 2.0 range across the western and northern Piedmont of NC.

The significant rain combined with the wet antecedent conditions did result in flooding at several forecast points across central NC with a few locations in the Coastal Plain reaching moderate flooding. The observed hydrograph and multiple forecast traces for Smithfield NC (SMFN7) on the Neuse River is shown in Fig. 5. The observed stage is noted by the nearly continuous red dots surrounded by blue circles that exceed the orange horizontal line (flood stage) and the red horizontal line (moderate flood stage). The river exceeded flood stage at Smithfield during the afternoon of 24 December and reached moderate flooding less than 24 hours later.

Fig. 5. The observed hydrograph and multiple forecast traces for Smithfield NC (SMFN7) on the Neuse River from 12 UTC on 23 December through 12 UTC on 27 December.  The observed stage is noted by the nearly continuous red dots surrounded by blue circles that exceed the orange horizontal line (flood stage) and the red horizontal line (moderate flood stage) while the forecast traces are noted by the narrower lines with dots every 6 hours.

Fig. 5. The observed hydrograph and multiple forecast traces for Smithfield NC (SMFN7) on the Neuse River from 12 UTC on 23 December through 12 UTC on 27 December. The observed stage is noted by the nearly continuous red dots surrounded by blue circles that exceed the orange horizontal line (flood stage) and the red horizontal line (moderate flood stage) while the forecast traces are noted by the narrower lines with dots every 6 hours.

Several days prior to flooding, the hydrologic situation was discussed in the NWS Raleigh Area Forecast Discussion (AFD) issued at 230 PM EST on Monday, 22 December (text shown below). In the AFD, the antecedent conditions were discussed with multiple SPoRT LIS products referenced. While the rainfall during the following few days exceeded the initial forecast and the anticipated impacts, the availability of the SPoRT LIS products lead to increased awareness of the flooding potential which proved especially helpful as the observed rainfall amounts increased and river levels rose.

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC

.HYDROLOGY…
AS OF 230 PM MONDAY…

ANTECEDENT RAINFALL OVER THE PAST 24 HOURS RANGED FROM A MINIMA OF ABOUT A TENTH OF AN INCH IN THE NORTHWEST PIEDMONT (THE UPPER YADKIN/PEE DEE AND UPPER HAW RIVER BASINS) TO A STRIPE OF 1 INCH PLUS ACROSS THE SOUTHEAST (THE CENTRAL NEUSE AND CENTRAL CAPE FEAR BASINS). WILL SEE ONLY MINOR RISES ON THE MAINSTEM RIVERS IN RESPONSE…BUT THIS IS THE 3RD FAIRLY SIGNIFICANT RAINFALL EVENT (>0.5 INCHES) OVER THE AREA SINCE 12/10. RELATIVE SOIL MOISTURE PERCENTAGES IN THE 0-200 CM COLUMN HAVE BEEN INCREASING…WITH HIGHER PERCENTILES NEARER THE SURFACE…SO QUICKER RUNOFF IS EXPECTED FROM OUR UPCOMING RAIN EVENT.

CURRENT QUANTITATIVE PRECIP FORECASTS FROM THE GEFS AND NAEFS ENSEMBLES ARE IN LOCKSTEP AGREEMENT AT PRESENT…WITH HEAVIEST RAIN (~1.3-1.5 INCHES) FROM TOMORROW NIGHT THROUGH CHRISTMAS MORNING. RAINFALL AMOUNTS IN THIS RANGE COULD POTENTIALLY CAUSE SOME MINOR FLOODING ON THE NEUSE RIVER AND TAR RIVER LATE ON CHRISTMAS DAY…BUT IT WOULD BE LOW IMPACT WITH THOSE RIVERS BARELY REACHING MINOR FLOOD STAGE. THE UPSHOT…RIVER FLOODING WILL ONLY BE AN ISSUE IF RAINFALL FORECASTS BEGIN TRENDING HIGHER…INTO THE 2 INCH PLUS RANGE.
&&

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!

Follow

Get every new post delivered to your Inbox.

Join 659 other followers