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Archive for the ‘RGB’ Category

This was one of those storms that people will talk about for years, especially those that were directly affected by it.  It all started with three separate shortwaves that all phased together once off the Mid-Atlantic coast, far enough offshore to limit any direct effects save for some unusual late season snow and gusty winds the next day.  The highest impact area included Cape Cod, Nantucket, Nova Scotia, and New Foundland.  I’m sure any ships that were in the vicinity were not happy with this situation!

GOES-Sounder RGB Air Mass animation valid 03/24/14-03/26/14.

GOES-Sounder RGB Air Mass animation valid 03/24/14-03/26/14.

The evolution of the nor’easter can be seen in the GOES Sounder RGB Air Mass animation above.  A southern stream system originating in the Gulf of Mexico moved east of Florida while two other shortwaves dropped southeast out of Canada.  All of the pieces combined near the North Carolina coastline, but the explosive deepening took place as the combined system moved northeast away from the Mid-Atlantic.  There appears to be a few stratospheric intrusions, but the most impressive intrusion occurs with the final shortwave as noted by the dark oranges and reds that appear at the end of the day on 03/25.  When models are forecasting a phasing situation, this product can be quite useful in identifying the features and observing the stratospheric drying seemingly “bleed” from one shortwave to the other.

MODIS RGB Air Mass product valid at 1540 UTC on 03/26/14.

MODIS RGB Air Mass product valid at 1540 UTC on 03/26/14.

MODIS RGB Air Mass product with ASCAT winds overlaid valid at 1540 UTC on 03/26/14.

MODIS RGB Air Mass product with ASCAT winds overlaid valid at 1540 UTC on 03/26/14.

The two MODIS RGB Air Mass products above show the nor’easter near peak intensity.  Notice how distinct the gradient between oranges and greens is in this image, almost as though you can see the upper portion of the frontogenesis, well behind the actual front.  The intensity of the stratospheric intrusion is quite evident by the dark pinks near the center of the cyclone.  The second image shows the wind field overlaid from ASCATB.  Notice the large area of hurricane force winds (red wind barbs) near the bent-back front, in the comma-head of the cyclone.  This area of wind affected parts of Southeast Massachusetts, including Nantucket where winds gusted from 60-85 mph.  Nantucket recorded a wind gust of 82 mph and about 10″ of snow.  Meanwhile, Nova Scotia bore the brunt of this beast with wind gusts of 129 mph at the Bay of Fundy and 115 mph in Wreckhouse.  Waves were equally impressive with altimeter readings between 40-50 ft and a buoy report of 52.5 ft.

GOES-13 Infrared imagery with the GLD-360 30-minute lightning density product overlaid.

GOES-13 Infrared imagery with the GLD-360 30-minute lightning density product overlaid.

Another interesting aspect of this storm was the two distinct areas of thunderstorms that erupted.  I overlaid the OPC and TAFB offshore zones for reference.  Notice well east of the Bahamas there are possible supercell thunderstorms associated with the southern shortwave energy.  Meanwhile, as the strong northern stream shortwaves exit the NC coastline, two areas of thunderstorms developed with the easternmost storm exhibiting supercell characteristics.  Although the lightning was not as intense with this northern area, I would speculate that the storms were associated with very strong wind gusts due to the dry air associated with the stratospheric intrusion.

VIIRS Visible image valid at 1719 UTC on 03/26/14.

VIIRS Visible image valid at 1719 UTC on 03/26/14.

VIIRS Visible image with the 18 UTC OPC surface analysis overlaid.

VIIRS Visible image with the 18 UTC OPC surface analysis overlaid.

I’ll finish this entry with two VIIRS Visible images above showing the majestic beauty of this nor’easter.  The 18 UTC OPC surface analysis depicts the storm at a maximum intensity of 955 mb, after a 45 mb drop in 24 hours!  This qualifies as one of the most explosive cyclones on record.  Another tidbit. . .this was the strongest storm in this part of the Atlantic since Hurricane Sandy (2012).

Thanks for reading!

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VIIRS True Color RGB imagery produced by NASA/SPoRT.  Southwest region domain at 1836UTC, 11 March 2014.

VIIRS True Color RGB imagery produced by NASA/SPoRT. Southwest region domain at 1836UTC, 11 March 2014.

In the southwest CONUS region, severe to extreme drought conditions exist in many areas.  In particular southwest Colorado, northeast New Mexico and the Texas and Oklahoma panhandle areas are very dry according to the U.S. Drought Monitor.  A building high pressure area developed a strong pressure gradient across these areas during the afternoon of 11 March 2014, resulting in 20-30 kt sustained northerly winds with gusts over 40 kt. Combined with the dry conditions, WFOs in the southwest have been anticipating blow dust events to be large and more frequent with strong Spring cyclones. VIIRS True Color RGB imagery (above) shows the blowing dust in Colorado and Texas, but the clouds in Colorado and Kansas have a similar color and the dry ground characteristics in Texas also look similar in color to the dust.  To provide a more efficient analysis of the blowing dust, VIIRS and MODIS can be used to create an RGB imagery product that shows blowing dust in shades of magenta to differentiate it from clouds and ground features.  This is done using the EUMETSAT recipe for the “Dust RGB” per their “Best Practices” after years of experience with the MeteoSat Second Generation SEVIRI instrument.  This geostationary instrument has similar capabilities to that of the future GOES-R ABI instrument.  Hence VIIRS and MODIS provide operational utility now and demonstrate future capabilities that all U.S. forecasters can use to be ready for the next generation of satellite products.  The VIIRS and MODIS passes show three times from this afternoon to aid forecasters with tracking the dust event.

20140311_1836_sport_viirs_swregion_dust_annotated

MODIS Dust RGB Imagery for 1941UTC 11 March 2014

MODIS Dust RGB Imagery for 1941UTC 11 March 2014

VIIRS Dust RGB Imagery for 2019UTC 11 March 2014

VIIRS Dust RGB Imagery for 2019UTC 11 March 2014

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Well, it’s February and it’s the East Pacific off of California, so the short answer is no.  But. . .what an amazing structure, right?  We haven’t seen anything this good looking in the tropical Atlantic in years!  But I digress. . .

MODIS RGB Air Mass product valid at 0621 UTC on 02/28/14.  The blue lines are the boundaries of OPC (north), TAFB (south), and Hawaii (west)

MODIS RGB Air Mass product valid at 0621 UTC on 02/28/14. The blue lines are the boundaries of OPC (north), TAFB (south), and Hawaii (west)

MODIS RGB Air Mass product valid at 1032 UTC on 02/28/14.

MODIS RGB Air Mass product valid at 1032 UTC on 02/28/14.

The first image was collected four hours before the second image and you can see how the center of the intense storm developed an “eye-like” feature (images courtesy of NASA SPoRT).  Notice the distribution of the pinks and reds in both images as well.  That is dry, stratospheric air filling the center of the strong upper-level low (~300-500 mb).  The second area shows an additional area of pink approaching the southern California coast.  This area is associated with strong instability that has led to rare California thunderstorms.

So, how do we know if there is stratospheric air?

AIRS Total Column Ozone product valid at 2200 UTC on 02/27/14.

AIRS Total Column Ozone product valid at 2200 UTC on 02/27/14.

AIRS Ozone Anomaly Product valid at 2200 UTC on 02/27/14.

AIRS Ozone Anomaly Product valid at 2200 UTC on 02/27/14.

The first image above is the AIRS Total Column Ozone product developed at NASA SPoRT.  The color bar on the left is not correct.  The main idea is that the warmer (cooler) the colors, the more (less) ozone is in the atmospheric column.  The green colors indicate ozone levels above 200 Dobson Units (ozone unit of measurement) with the magenta areas indicating ~500 Dobson Units.  The second image shows the AIRS Ozone Anomaly product with the first level of blue indicating 125% of normal, while the yellow region indicates >200% of normal ozone at that latitude and geographic location.  Stratospheric air is associated with high levels of ozone and potential vorticity which can help identify the strength of the upper-level low.  These images show the connection of this ozone pocket with the “reservoir” of ozone located in the northern latitudes at this time of year.

AIRS Total Column Ozone Product valid at 1000 UTC on 02/28/14.

AIRS Total Column Ozone Product valid at 1000 UTC on 02/28/14.

AIRS Ozone Anomaly valid at 1000 UTC on 02/28/14.

AIRS Ozone Anomaly valid at 1000 UTC on 02/28/14.

As the upper-low cut off and became stacked over the surface low (~971 mb), you can see how the high concentration of ozone becomes more focused over the storm.  Once again, the magenta coloring indicates ozone levels >500 Dobson Units.  The anomalies are more incredible with a large area of >200% of normal directly west of southern California.

I will continue to work with forecasters at OPC, TAFB, SAB, and WPC on discovering ways to use these products in conjunction with the RGB Air Mass products to gauge storm strength and look for signals upstream of developing tropopause folds and stratospheric intrusions.

GOES-15 Visible imagery with the GLD-360 30-minute lightning density product overlaid.

GOES-15 Visible imagery with the GLD-360 30-minute lightning density product overlaid.

The ozone isn’t the only impressive part of this storm.  Notice the occasional bursts of lightning within the spiral bands of the parent storm.  Although not completely unusual, this is a great indicator of how much energy is available to this storm.

GOES-Sounder RGB Air Mass product with GLD-360 lightning strikes overlaid.

GOES-Sounder RGB Air Mass product with GLD-360 lightning strikes overlaid.

I put together a longer animation of the GOES-Sounder RGB Air Mass product with the GLD-360 lightning strikes overlaid.  Note the first system that came ashore in California earlier this week, then moved over the four-corners regions with plenty of lightning, especially for this time of year.  The current storm is seen lurking offshore with more lightning developing in a band of thunderstorms that moved from Los Angeles to just north of San Diego.  This system will be responsible for the next bought of winter weather for the Midwest to the Mid-Atlantic next week.

Thanks for reading and as always, feel free to contact me with questions and feedback!

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In the early morning hours of Wednesday, January 29th a deck of low stratus clouds developed over the Copper River Basin in Alaska.  The RGB Night-Time Microphysics product derived from SNPP VIIRS instrument at 1321UTC (4:21am local Alaska time) is shown in the following screen capture from the National Weather Service’s AWIPS workstation at WFO Fairbanks, Alaska.   This view is zoomed into the southern portion of mainland Alaska; the Copper River Basin is northeast of Anchorage and includes the community of Gulkana.  The 1253UTC METAR observation from Gulkana indicated an overcast ceiling of 500ft above ground, with seven miles of horizontal visibility.  The RGB NT Micro depicts the stratus deck with a gray-yellow color, and one can see the low clouds confined by the higher terrain and covering the broad Cooper River Basin as well as following the more narrow Copper River itself as it flows southeast of Gulkana and eventually into the Gulf of Alaska.

Copper Basin annotated

A comparison of the RGB NT Micro product with different VIIRS products from the same SNPP pass is presented in the following 4-panel screen capture.  The RGB NT Micro is in the upper-left, the Day-Night Band is in the upper-right, the 11.45 micron IR is in the lower-right, and the traditional channel differencing fog product is in the lower-left.  The deck of stratus clouds over the Copper River Basin is also evident in the longwave IR imagery and the fog product.  The clouds are thin enough that the city lights of are evident through the cloud layer in the Day-Night Band.  In this example, it appears that the stratus deck is most evident in the RGB NT Micro and the fog product, and least evident in the Day-Night Band.

Copper Basin 4-Panel

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During a brief period in mid-Janaury, when the “Polar Vortex” was safely docked in its home port of Alaska, temperatures over Alaska’s Interior plunged into the 40s and 50s below zero.  Under clear skies and light winds, surface-based radiation inversions decouple a thin bottom layer of air from the free troposphere and produce extreme stratification of temperatures in the vertical over Interior Alaska’s complex terrain.  Long-wave Infrared imagery from polar-orbiting satellites allows forecasters to “see” the topography during these events, since the coldest air settles into the valley bottoms and ground temperatures monotonically moderate with increasing height.

The 2013-2014 winter is the inaugural season for SPoRT’s RGB Night-time Microphysics product.  Given that longwave IR contributes the “B” channel of the RGB NT Micro, one might wonder if topographic features might also be evident in the RGB NT Micro during a cold clear Alaska night.

This figure is a four-panel screen capture from an AWIPS workstation at National Weather Service WFO Fairbanks, Alaska.  All four panels show imagery from the 1321Z (5:21am Alaska Standard Time) pass of the SNPP VIIRS instrument on Monday, January 13th, 2014.  The area shown is zoomed into the “Upper Yukon Valley” of Alaska’s Interior.  The village of Fort Yukon, on the Yukon River, is marked with the star in each panel.  Conditions at this time in the Upper Yukon Valley were clear, calm, and cold, with surface temperatures around Fort Yukon in the 40s below zero.

Upper Yukon 4 Panel annotated

The upper-left panel is the RGB NT Micro using the default color enhancement, the upper-right panel is the Day-Night Band, the lower-right panel is the 11 micron longwave IR, and the lower-left panel is the traditional channel differencing “fog product.”  Topographic features are clearly evident in the DNB image in the upper-right, as well as in the longwave IR image in the lower-right.  But interestingly, the topography is almost entirely non-evident in the RGB NT Micro in the upper-left, despite longwave IR being one of the three components of the RGB image.

Learning to use a new product includes discovering what that product is capable of depicting and *not* depicting under a variety of weather conditions.  The Alaskan winter is an extreme and unusual environment, and this one example indicates that one should not expect to see terrain features in the RGB NT Micro under conditions of clear skies and extreme temperature stratification.  Correspondingly, a “smooth” look to RGB NT Micro imagery in this case does *not* indicate a broad cloud deck covering the terrain.

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SPoRT is conducting an assessment of RGB imagery for Aviation and Cloud Analysis with Alaska WFO partners.  A 17 minute training module for high-latitude application of nighttime RGBs with an Alaska example was created by SPoRT to support this assessment (see SPoRT training page to download or launch module). The Juneau WFO provided feedback for 1/24/14.  Here is a part of their feedback regarding the value of the Nighttime Microphysics RGB imagery from MODIS and VIIRS and an example image from AWIPS/D2d.

WFO Juneau feedback:
“… the microphysics image was very helpful in picking out where the fog and low clouds were in the complex terrain that is the SE panhandle. most of the fog this morning was confined to the narrower valleys and channels while the wider channels were mostly clear. This is possibly due to higher winds still present in the wider channels limiting fog formation there. It also showed little or no fog and low clouds out in the gulf. The microphysics image was very helpful with figuring out fog for zone and marine forecasts. It also helped out with the TAFs with seeing if there were any higher clouds layers above the fog layer.”

AJK_Ntmicro_assess_feedback_for_blog_20140124_0736_annotated

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The following is an evaluation of a low cloud and fog (IFR/MVFR ceilings and MVFR visibility) case in South Texas using the 0900 UTC 2 January 2014 Nighttime (NT) Microphysics and MODIS-VIIRS 11-3.9 micron 1-km products. (The 0900 UTC 2 January 2014 MODIS-VIIRS 11 micron 1-km product and corresponding METAR observations were also utilized in the analysis.)

During the early morning hours of 2 January 2014, a drier air mass moved southward across South Texas with mid and upper level drying advancing faster than drying at the surface (note relative humidity/equivalent potential temperature cross-section in Figure 1, and surface equivalent potential temperature in Figure 2.) Thus the moisture profile ahead of the surface boundary became consistent with the development of fog or a mixture of low clouds and fog.

Cross Section: Relative Humidity and Equivalent Potential Temperature  0900 UTC 2 January 2014

Figure 1: Cross Section: Relative Humidity and Equivalent Potential Temperature 0900 UTC 2 January 2014 (WFO CRP local WRF-ARW 3-hour forecast)

Surface Equivalent Potential Temperature 0900 UTC 2 January 2014 (WFO CRP local WRF-ARW 3-hour forecast)

Figure 2: Surface Equivalent Potential Temperature 0900 UTC 2 January 2014 (WFO CRP local WRF-ARW 3-hour forecast)

At 0900 UTC, NT Microphysics imagery was available (Figure 3.) At this time, the fog was located primarily over Deep South Texas and over the Coastal Waters. This evaluation is focused on the region within the circle depicted in Figure 3.

0900 UTC 2 January 2014 MODIS-VIIRS Nighttime Microphysics. Yellow circle depicts the region relevant to this analysis

Figure 3: 0900 UTC 2 January 2014 MODIS-VIIRS Nighttime Microphysics. Yellow circle depicts the region relevant to this analysis

The MODIS-VIIRS 11-3.9 1-km micron imagery (Figure 4) revealed mainly yellow color, which suggests low clouds or fog. Note that the corresponding METAR observations LRD (Laredo International Airport, Laredo Texas) and HBV (Jim Hogg County Airport, Hebbronville Texas) depict low clouds (1100 feet and 1600 feet ceilings, respectively), and no fog. The southern observation APY (Zapata County Airport, Zapata Texas) depicts a combination of four (4) statute miles in fog (mist) and 800 feet AGL ceiling, yet is surrounded by limited blue color in the spectral difference imagery suggestive of higher clouds. In fact, SPoRT MODIS-VIIRS 11 micron 1-km imagery (Figure 5) depicts high clouds that correspond to the blue color in the spectral difference imagery. Thus, the 11-3.9 micron imagery does not identify the low cloud/fog mixture at APY.

0900 UTC 2 January 2014 MODIS-VIIRS 11-3.9 micron Imagery. Red Circle depicts the region relevant to this analysis.

Figure 4: 0900 UTC 2 January 2014 MODIS-VIIRS 11-3.9 micron Imagery. Red Circle depicts the region relevant to this analysis.

0900 UTC 2 January 2014 MODIS-VIIRS 11 micron Imagery. Circle depicts region relevant to this analysis.

Figure 5: 0900 UTC 2 January 2014 MODIS-VIIRS 11 micron Imagery. Circle depicts region relevant to this analysis.

However, the SPoRT NT Microphysics product (Figure 3) does provide the information necessary to distinguish between the APY and the LRD/HBV regions, yet additional analysis is needed. Note that the northern observations are located in predominately dull white-colored regions, yet the cloud/fog location is surrounded by dark blue-colored regions. A technique was utilized to determine the actual RGB values before interpretation of the imagery. A local application (D2D Image Maker) was utilized to generate PNG-formatted files of the NT Microphysics Imagery. These files were then imported into Adobe Photoshop computer software in order to determine the specific Red, Green, and Blue contributions at each point in the imagery.

The following are the approximate RGB values representative of the region surrounding each of the three (3) METAR sites, and the corresponding visibility restriction and cloud portion of the METAR observation at 0900 UTC 2 January 2014. (RGB values were determined in the following manner: For each color, determine the value at four points along the ~0.5 mile radius surrounding the location of the METAR, then compute the arithmetic average.)

KLRD: R123, G148, B144 10SM OVC011
KHBV: R132, G132, B156 10SM OVC016
KAPY: R100, G61, B130 4SM BR OVC008

Note that the blue and red contributions were significant, suggestive of a lower level/near surface phenomenon with significant optical depth (low clouds and/or fog). Note that although the resultant color of the NT Microphysics imagery suggests that the region surrounding APY has a higher concentration of blue, the actual blue contribution for APY is less relative to the other sites. (It’s important to note that the blue is not bright, which gives a clue that the blue contribution, though visible, is not necessarily high.)

When examining the RGB value differences between the locations, there is a significant difference in green contribution between the APY region and that of the other two regions; the green contribution of APY is significantly lower. A lower concentration of green for the combination MVFR visibility and MVFR/IFR ceiling (fog and cloud mixture) region of APY is consistent with a greater likelihood for fog.

This case illustrates the utility of using NT Microphysics imagery to separate the combination MVFR visibility and MVFR/IFR ceiling regions from the MVFR/IFR ceiling-only regions, if knowledge of the actual RGB values is available.

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I’ve posted about this a few times, but this morning offered another good example of the ability of the Suomi NPP VIIRS Day-Night Band RGB imagery to show low clouds beneath thin cirrus.  First, take a look at the 11-3.9 µm imagery (with the standard color curve) from the VIIRS instrument, valid this morning at approx 0749 UTC.  Focus your attention mainly over southern Texas.

Suomi NPP VIIRS 11-3.9 µm image valid 19 Dec 2013 0749 UTC

Image 1.  Suomi NPP VIIRS 11-3.9 µm image valid 19 Dec 2013 0749 UTC

In the image above, high, cirrus clouds will appear as blue colors, while lower clouds appear yellow.  Notice the bank of low clouds in the northern Gulf extending into portions of extreme SE Texas and southern Louisiana.  Notice that some low clouds can also be observed in and around the thin cirrus to the southwest in southern Texas.  These cloud features show up arguably better in the VIIRS Day-Night Band image from the same time (image 2).

Image 2.  Soumi NPP VIIRS Nighttime Microphysics RGB 19 Dec 0749 UTC

Image 2. Soumi NPP VIIRS Nighttime Microphysics RGB 19 Dec 0749 UTC

With the Nighttime Microphysics RGB, the low clouds appear as an off-white, while the high thin cirrus are the deep reds and blues spreading across much of the CONUS in the image.  Notice the low clouds in southern Texas still cannot be observed very well beneath the cirrus canopy, as the cirrus are mostly opaque to the longer IR wavelengths.  Now, let’s take a look at the VIIRS Day-Night Band imagery.  I like the Day-Night Band Radiance RGB best for this particular situation (image 3).

Image 3.  Suomi NPP VIIRS Day-Night Band Radiance RGB 19 Dec 2013 0749 UTC

Image 3. Suomi NPP VIIRS Day-Night Band Radiance RGB 19 Dec 2013 0749 UTC

Since the cirrus are mostly translucent in the visible portion of the spectrum, the low clouds can be seen beneath.  Also, given that the Day-Night Band Radiance RGB has an IR component, this results in better delineation between high and low clouds.  In this situation, forecasters would have a much better idea of the extent of the low cloud deck.  Observations from 0800Z indicated the cloud bases were around 1-2 kft in that area of southern Texas, which could have significant impacts on sensible weather and forecasts, especially for aviation.

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Strong North Atlantic Storm

I had this posted on the OPC Facebook page yesterday afternoon and thought it would make an interesting, short blog post:

There is a strong, hurricane-force storm affecting Ireland and Great Britain today. As part of our GOES-R and JPSS Proving Ground activities, some new experimental products are being introduced to forecasters.

This image shows the SEVIRI (Met-10) RGB Air Mass product overlaid with a new AIRS Ozone product and ASCAT winds. The green numbers represent concentration of ozone, which correlates well with downward momentum of stratospheric air (high in ozone and potential vorticity). This storm has greater than 400 Dobson units (black circle), which means lots of descending air near the comma-head.

This ASCAT image was chosen as it shows storm-force winds in two locations (> 42 kts), but some stronger (> 56 kts) near the comma head co-located with the high ozone readings. This storm is officially designated a hurricane-force storm based on a later ASCAT-B pass (not shown) which showed winds greater than 64 kts. This is one way forecasters can combine data sets to fully assess the situation and even provide more confidence in a forecast.

For more information on this storm and the official high seas forecast, please visit OPC’s webpage at: www.opc.ncep.noaa.gov

SEVIRI RGB Air Mass product overlaid with the AIRS Total Column Ozone and ASCAT winds valid at 1400 UTC on 12/18/13. The black circle highlights the descending stratospheric intrusion near the comma-head/bent back front.

SEVIRI RGB Air Mass product overlaid with the AIRS Total Column Ozone and ASCAT winds valid at 1400 UTC on 12/18/13. The black circle highlights the descending stratospheric intrusion near the comma-head/bent back front.

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Late yesterday evening (Dec 17th) fog began forming along coastal areas of Lousiana and Texas.  By 10 pm CST, visibilities at some locations along the coast had already dropped to less than 1 SM.  The fog continued to intensify, with visibilities falling to around 1/4 SM or less at many locations during the early morning hours this morning (Dec 18th).   By 2 am CST (0800 UTC), the visibility had fallen to near 0 SM in portions of SW Louisiana as noted by the observation at Jennings (3R7, near Fenton in the image, image 1).

GOES 11-3.9 Spectral Difference Image, with Ceiling (AGL) and Visibility (SM) observations 18 Dec 2013 0800/0815 UTC

Image 1.  GOES 11-3.9 Spectral Difference Image, with Ceiling (AGL) and Visibility (SM) observations 18 Dec 2013 0800/0815 UTC

In this standard GOES spectral difference imagery however, the fog is very diffcult to visually discern, likely due to it’s very shallow depth.   There is some indication of the fog, with slightly brighter pixels in areas of southern LA and SE Texas.  However, notice the better (albeit slightly) detection of the fog in the Nighttime Microphysics RGB products from the VIIRS and MODIS instruments below (images 2 and 3, respectively).  The fog in these images appears as a pinkish-gray color.

Image 2.  Suomi NPP VIIRS Nighttime Microphysics RGB 18 Dec 2013 0808 UTC

Image 2. Suomi NPP VIIRS Nighttime Microphysics RGB 18 Dec 2013 0808 UTC

Image 3.  Aqua MODIS Nighttime Microphysics RGB 18 Dec 2013 0804 UTC

Image 3. Aqua MODIS Nighttime Microphysics RGB 18 Dec 2013 0804 UTC

I think the fog is a little easier to see in the MODIS image, which may be due to higher resolution and small differences in channel wavelengths between the VIIRS and MODIS instruments.  Nevertheless, the fog in all of the imagery is rather subtle and will require development of pattern recognition by forecasters.  Sampling the color contributions, I found that the primary changes between areas of fog and areas without occurred in the green color (assigned the ~10.8-3.9/3.7 channel difference), which would be expected.  For example, when taking a color sample from the pinkish-gray band of fog in SW Louisiana (near Jennings) from the MODIS image, I came up with: Red-180, Green-137, Blue-165.  Meanwhile, sampling of a pixel in central Louisiana without fog: Red-167, Green-99, Blue-150.

So, how did the fog appear in the Day-Night Band RGBs?  Not very well at all, as you can see in the next couple of images…

Image 4.  Suomi NPP VIIRS Day-Night Band Radiance RGB 18 Dec 2013 0808 UTC

Image 4. Suomi NPP VIIRS Day-Night Band Radiance RGB 18 Dec 2013 0808 UTC

Image 5.  Suomi NPP Day-Night Band Reflectance RGB 18 Dec 2013 0808 UTC

Image 5. Suomi NPP Day-Night Band Reflectance RGB 18 Dec 2013 0808 UTC

I wasn’t able to detect any fog at all in the Day-Night Band RGB imagery.  However, there is still potentially important information to glean from all of this.  If the fog is evident (even slightly) in the NT Microphysics RGB imagery mainly due to the 10.8-3.7/3.9 channel difference, but is essentially translucent in the visible spectrum (Day-Night Band), then it is likely very shallow.  This could be helpful for determining the duration of the fog during a change of conditions, such as the development of mixing after sunrise (i.e. shallow fog dissipation will be quicker than thick fog dissipation).

Notice in the GOES visible loop below (images at 0445 UTC and 0601 UTC) that the fog dissipated very quickly after sunrise (click the expanded image to obtain the loop).

Loop of GOES visible images at 1445 and 1601 UTC with observations of

Loop of GOES visible images at 1445 and 1601 UTC with ceiling (AGL) and visibility (SM) observations  oat 1500 and 1600 UTC

The Corpus Christi, Houston and Slidell offices all issued Dense Fog Advisories or Special Weather Statements concerning the fog in their respective County Warning/Forecast Areas during the early morning hours.

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