Soil Moisture Conditions over Southeast Texas Prior to Hurricane Harvey

As much-anticipated Hurricane Harvey approaches the southern and eastern coast of Texas today, it is worth examining the pre-existing soil moisture over the region to understand the capacity of the land surface to absorb the upcoming rainfall.  Granted, the amount of rainfall simulated by numerical guidance is off-the-charts high (e.g., today’s 0600 UTC initialized NAM model [Fig. 1] shows 84-hour maximum accumulated rainfall of over 60″ between Corpus Christie and Houston!!).  Thus, extreme flooding is anticipated, regardless of the amount that can be absorbed by the soils.

Figure 1.  The NCEP/NAM model 84-hour forecast of total accumulated precipitation (inches) over Southeastern Texas, from the simulation initialized at 0600 UTC 25 August 2017 [image courtesy of College of DuPage forecast page].

SPoRT manages a real-time simulation of the NASA Land Information System (hereafter, “SPoRT-LIS“), running over the Continental U.S. at ~3-km grid resolution.  The SPoRT-LIS product is a Noah land surface model climatological and real-time simulation over 4 model soil layers (0-10, 10-40, 40-100, and 100-200 cm).  The climatological simulation spans 1981-2013 and forms the basis for daily-updated total-column soil moisture percentiles (forthcoming in Fig. 3), in order to place current soil moisture values into historical context.  For real-time output, the Noah simulation is regularly updated four times per day as an extension of the long-term climatology simulation.  It includes NOAA/NESDIS daily global VIIRS Green Vegetation Fraction data, and the real-time SPoRT-LIS component also incorporates quantitative precipitation estimates (QPE) from the Multi-Radar Multi-Sensor (MRMS) gauge-corrected radar product.  The climatological SPoRT-LIS is based exclusively on atmospheric analysis input from the NOAA/NASA North American Land Data Assimilation System – version 2.

Relative Soil Moisture output from the SPoRT-LIS over the 0-100 cm layer is shown in Fig. 2 over Southeastern Texas and Louisiana at 1200 UTC this morning.  A marked gradient between very dry soils to the west and moist soils to the east occurs in the vicinity of the greater Houston metropolitan area.  The soils in the region bounded by Corpus Christi, San Antonio, Austin, and Houston (areas forecast to have the greatest rainfall from Hurricane Harvey) are extremely dry prior to Harvey’s landfall.  This dryness will help to some extent in absorbing the initial rainfall from Hurricane Harvey.  But with such excessive rainfall being forecast over a prolonged time period (3-5+ days), it won’t be long before the upper portions of the soil column saturates and widespread areal flooding occurs.  In addition, the high forecast rainfall rates could easily result in flash flooding (despite prevailing soil dryness), especially further inland where terrain plays a more important role in runoff and flash flooding.

The total column relative soil moisture percentile from 24 August shows that historically-speaking, the soil moisture is slightly drier than normal, particularly along the coastal plain between Corpus Christi and Houston (Fig. 3).  In this corridor, the soil moisture is generally between the 10th and 30th percentile compared to the 1981-2013 climatological distribution for 24 August.

Figure 2.  SPoRT-LIS relative soil moisture (RSM) distribution in the 0-1 meter layer across Southeastern Texas and Louisiana, valid 1200 UTC 25 August 2017.  RSM values of 0% represent wilting (vegetation cannot extract moisture from soil) and 100% represents saturation (subsequent rainfall becomes runoff).

Figure 3.  Total column (0-2 m) relative soil moisture percentile valid 24 Aug 2017, as compared to all 24 August soil moisture values from a 33-year climatological simulation of the SPoRT-LIS.

Finally, an hourly animation of the 1-day changes in 0-10 cm (top model layer) relative soil moisture show that the near-surface soils are quickly moistening between Corpus Christi and Houston, as the initial rainbands of Hurricane Harvey began impacting the coastal plain this morning.  As the soils continue to moisten rapidly from the top-down, subsequent rainfall will quickly lead to runoff and flooding.

Figure 4.  Hourly animation of 1-day change in top-layer (0-10 cm) relative soil moisture, for the time period spanning 0000-1400 UTC 25 August 2017.  Each hourly image is a simple difference in 0-10 cm relative soil moisture between the current and previous day at the same valid hour.  Line contours depict one-hour QPE from the MRMS product, as input to the real-time SPoRT-LIS.

Nighttime Microphysics for GOES-16

The Nighttime Microphysics RGB Imagery, provided by S-NPP VIIRS in above image, efficiently highlights the low cloud and fog areas in aqua to dull gray, to allow forecasters to better see where hazards exist to transportation (aviation, public, or marine).  This VIIRS image also provides forecasters with a look at the new geostationary capabilities that will be available soon with GOES-16 ABI.  This Nighttime Microphysics RGB Imagery was originally created by EUMETSAT around 2006, transitioned by NASA/SPoRT to forecasters within the NOAA Satellite Proving Ground over the last 5 years, and recently adopted by GOES-16 as one of the many RGB products that will be available to better utilize the ABI three fold increase in the number of bands over the current GOES imager. Currently, the Nighttime Microphysics RGB Imagery from VIIRS as well as several AVHRR and MODIS instruments is regularly used by forecasters in operations, which has allowed them to gain experience in preparation for this new capability from GOES-16.

On February 8, 2017 Dense Fog Advisories were in place across the Gulf Coast and parts of the Southeast (see image below) and there have been many similar events in the region for this winter.

Near 1000 UTC (~4:00am CST) large areas of low ceilings and visibility were occurring in the advisory regions, as seen in the first image of the post.  In the images below, take a look at how the Nighttime Microphysics RGB Imagery (this time from NOAA-19/AVHRR) compares to using a single longwave infrared channel in the split scene of the Gulf Coast region and then compare this with the same scene where only the Nighttime Microphysics  RGB Imagery is shown.  Note that the fog and clear areas can look similar in the infrared image and that the fog itself is a bit warmer than the ground areas in Texas. For help interpreting these types of images, NASA/SPoRT has RGB Quick Guides available at  https://weather.msfc.nasa.gov/sport/training/ .

VIIRS Day-Night Band Imagery and Fog Detection

Working midnight shifts this past weekend, I had the opportunity to take a look at the VIIRS Day-Night Band Imagery for the detection and analysis of fog.  Early Monday morning, the observation at Ft. Payne was indicating fog with 1/2 statute mile visibility.  However, the presence of thin cirrus over parts of the area did not allow for the observation of ground phenomena, including fog, in the region via traditional Shortwave IR imagery (Image 1).  However, low clouds and fog were observed in the VIIRS Day-Night Band imagery since the cirrus were sufficiently translucent in the visible portion of the spectrum (Image 2).

Image 1. VIIRS 3.9 µm IR image provided by NASA SPoRT, valid 0728 UTC 22 Aug 2016. Fog cannot be observed in the 3.9 um imagery since the cirrus are sufficiently opaque at this wavelength.

Image 2. VIIRS Day-Night Band Reflectance provided by NASA SPoRT, valid 0728 UTC 22 August 2016. Fog can be seen in the narrow Paint Rock Valley of western Jackson County (in northeastern Alabama). Despite the observation of fog at Ft. Payne (DeKalb County AL, –located to the SE of Jackson County), fog cannot be readily observed in the imagery, suggesting that the fog was very localized and perhaps shallow.

I could show the standard fog product imagery (11-3.9 µm), but the story is essentially the same as that of the 3.9 µm imagery of course.  The ability to see through thin cirrus is one of the primary advantages offered by the VIIRS Day-Night Band imagery and thus is among its most useful applications, operationally speaking.  These imagery are a part of the JPSS Proving Ground and have been available in AWIPS here at the HUN office for several years now, including other SPoRT collaborative partners.

In this particular case, it was operationally advantageous to see that the extent of the fog was not widespread and was just confined to some of the more fog-prone valley locations, especially the Paint Rock Valley, and may have only been highly localized to Ft. Payne, or even just the Ft Payne airport observation location.  Had the fog been observed through a larger area in Jackson and especially in DeKalb Counties, then a dense fog advisory might have been necessary.

 

Snow Cover Blankets Northeastern New Mexico

A potent winter storm system impacted portions of New Mexico on March 26, 2016, ending an extended stretch of very dry weather. Snowfall amounts of 3 to 9 inches were reported from the Sangre de Cristo Mountains eastward across the northeast plains. The MODIS and VIIRS satellite products proved useful for illustrating the extent of snow cover in both the daytime and nighttime scenes. The images below are graphical briefings posted to the NWS Albuquerque web page and shared via Twitter after this much needed snowfall event.

Graphical briefing (part one) showing the extent of snow cover during the nighttime and daytime periods on March 27, 2016.

Graphical briefing (part two) showing the extent of snow cover through RGBs on March 27, 2016.

LEO Perspective of River-Effect Snow in North Alabama

The cold air outbreak over the eastern United States had impacts far and wide, including the development of snow showers all the way into northern Alabama.  However, between unseasonably low 850 mb temperatures and northwesterly flow, the outbreak also caused a semi-persistent band of snow to develop along the Tennessee River (downwind of a reservoir known as “Lake Wheeler”).

While most of the river-effect monitoring occurred with radar, the late-morning MODIS overpass captured one of the narrow river-effect bands (and did so more effectively than the lower-resolution GOES-East Imagery).

Figure 1. MODIS visible image, valid 1644 UTC 9 February 2016.  Larger lakes are outlined in blue, and the river-effect band is circled in yellow.

Snowfall reports from underneath the band have indicated 2 to 3 inches of snow, compared to the 1-2 inches reported with heavy or persistent snow showers elsewhere.  Unfortunately, orbit timing and cloud cover have not allowed us to view the snow swath using the Snow-Cloud RGB.  However, the Snow-Cloud RGB from the edge of this morning’s MODIS pass still illustrated the river-effect band persistence.

Figure 2. MODIS Snow-Cloud RGB image, valid 1549 UTC 10 February 2016.  The Tennessee River is the dark blue feature in the center of the image; the river effect band is circled in red.

Channel Differencing and RGB Issues Over Desert Locations…

Recently, I had the opportunity to travel to the Tucson NWS office and work with forecasters there concerning a number of experimental data sets transitioned by the SPoRT group.  Primarily, this involved the SPoRT LIS, GPM Constellation and IMERG, and NESDIS QPE data sets.  However, I also had the opportunity to see how other products were being utilized by forecasters.  While taking a look at the Nighttime Microphysics RGB image, I was initially perplexed by the apparent presence of fog and low clouds in parts of the desert southwest.  The first image below is a 4-panel image from AWIPS, showing the Longwave (LW) and Shortwave (SW) IR, the LW-SW IR channel difference, and the Nighttime Microphysics RGB from the VIIRS instrument on the morning of Sept 23rd.

Image 1. Suomi-NPP VIIRS imagery valid 0915 UTC 23 Sep 2015, Longwave IR (upper left), Shortwave IR (upper right), LW-SW IR channel difference (“fog product”, lower left), and the Nighttime Microphysics RGB (lower right).

The difference in brightness temperatures between the LW and SW IR channels in parts of SW Arizona, SE California and areas of NW Mexico around the Gulf of California, results in relatively large positive values.  Notice the yellow colors that appear in these areas in the channel difference imagery (image 1, lower right), and the corresponding appearance of white-aqua colors in the Nighttime Microphysics RGB (the 10.8-3.9 channel difference represents the green color component of the RGB recipe).  For a forecaster accustomed to looking at these imagery in other parts of the country (and those will less sandy surfaces), these channel difference values and colors in the RGB would suggest the presence of low stratus and/or fog.  However, no clouds or fog were present in those locations during the morning.  You can, however, see some low clouds in portions of central and eastern New Mexico, as indicated by the brighter white-aqua colors.

So, what is going on here?  Well, as eluded to above, it’s the presence of dry sand.  The image below (courtesy of COMET) shows the IR emissivity over several different surface features: tree leaves, red clay, dry sand, and water.

Image 2. IR emissivity vs. wavelength of several surface features, including tree leaves, red clay, dry sand, and water.  (image courtesy of COMET)

Notice that the emissivity over dry sand changes fairly substantially through portions of the SW and LW portion of the spectrum, and is lower at 3.9 µm than at 10.8 µm.  The channel difference between 10.8 and 3.9 µm will result in positive values (given clear sky conditions of course) over dry sandy areas, thus mimicking the presence of low clouds and/or fog, as would be the interpretation in other areas.  The next image below demonstrates the LW and SW IR brightness temperatures and differences, along with the Nighttime Microphysics RGB, as sampled over a clear, dry sandy area.

Image 3. Suomi-NPP VIIRS image from 0902 UTC 25 Sep 2015, LW IR (upper left), SW IR (upper right), LW-SW IR channel difference (lower left), and the Nighttime Microphysics RGB (lower right).

Notice the substantial resulting green color contribution in the Nighttime Microphysics RGB (lower right in above image).  These colors are very similar to colors that would be indicative of fog and other low cloud features as they traditionally appear under similar temperature conditions in other areas outside of dry, sandy areas (image 4 below).

Image 4. Nighttime Microphysics image depicting fog and low clouds (white-aqua colors) in portions of the southern and central Appalachian region.

Soil Moisture Analysis of Fort Craig Wildfire

NWS Albuquerque recently began ingesting the updated SPoRT CONUS LIS products in our new AWIPS II system as part of our continued collaboration with SPoRT. These products have already peaked the interest of several local, state, and federal partners. Short-term drought conditions have improved steadily since late winter as more frequent and widespread precipitation events impacted the state. Overall, deep-layer soil moisture conditions have improved substantially compared to this time last year (Fig. 1).

Figure 1. Deep soil moisture (0-200cm) 1-year change valid 12Z 27 July 2015.

The SPoRT LIS products have become a valuable tool for drought monitoring during our monthly drought workshops. Several state and federal partners noted on our most recent call in late July that these new products provided an additional layer of situational awareness and infuse more science into the drought monitoring process. These products have also peaked the interest of our fire weather community, in particular Incident Meteorologist Brent Wachter. New Mexico during late July is generally under the influence of higher humidity with periodic wetting rainfall events. The convective nature of the precipitation however tends to bring about a patchwork of “have’s and have-nots”. The Fort Craig wildfire broke out in a dry pocket of south central Socorro County within the middle Rio Grande Valley during the afternoon of 26 July 2015. The New Mexico State Climatologist, Dave DuBois, captured the wildfire on camera and posted the image to Twitter shortly thereafter (Fig. 2).

Figure 2. A distant view of the Fort Craig wildfire captured by the New Mexico State Climatologist, Dave DuBois, around 830am, July 27, 2015.

The SPoRT LIS 0-10cm volumetric soil moisture at 12Z 28 July 2015 showed the corresponding dry area where the wildfire developed (Fig. 3). Les Owen from the New Mexico Department of Agriculture also noted this area of drying within Socorro County in what he called his “windshield survey” in mid to late July. The Fort Craig fire grew to nearly 700 acres over the course of two days. The NASA SPoRT soil moisture imagery showed the dry area quite well and the fire was located smack dab in the middle of it.

FIgure 3. NASA SPoRT 0-10cm volumetric soil moisture within Socorro County valid 12Z 28 July 2015. Note the large dry area in near surface soil moisture in response to the recent dry stretch. The location of the Fort Craig wildfire is indicated by the home identifier.

Several storms then impacted the area late on the 28th and the 29th leading to some natural fire suppression and reduction in active fire behavior. The follow-up SPoRT imagery at 12Z 30 July 2015 showed the increase in 0-10cm relative soil moisture over the same area (Fig. 4). The high resolution imagery could be useful in determining fuel dryness for potential fire starts from human activities, cloud to ground lightning ignitions, as well as highlight potential active fire behavior areas. We will continue to assess the possible applications of the SPoRT LIS products as we move through the remainder of the 2015 monsoon season.

Figure 4. NASA SPoRT 0-10cm relative soil moisture within Socorro County valid 12Z 30 July 2015. Note the dramatic increase in near surface soil moisture values in response to the active storm pattern. The location of the Fort Craig wildfire is indicated by the home identifier.