Dense Fog Advisories over the Plains

Dense Fog Advisories were issued over parts of the central and northern Plains states on Friday January 5. For example, from the North Platte Office (similar warnings were issued by Billings, Rapid City and Bismark offices):

URGENT – WEATHER MESSAGE
National Weather Service North Platte NE
634 AM CST Fri Jan 5 2018

…Areas of dense fog likely this morning…

.Areas of fog reducing visibilities below one quarter mile at
times will be likely from parts of southwest into the central
Nebraska Sandhills this morning. With the fog occurring where
temperatures are below freezing, some slick spots may develop on
area roads and sidewalks as well.

NEZ025-026-037-038-059-071-051800-
/O.NEW.KLBF.FG.Y.0001.180105T1234Z-180105T1800Z/
Thomas-Blaine-Logan-Custer-Lincoln-Frontier-
Including the cities of Thedford, Halsey, Dunning, Purdum,
Brewster, Stapleton, Broken Bow, North Platte, Curtis, Eustis,
and Maywood
634 AM CST Fri Jan 5 2018

…DENSE FOG ADVISORY IN EFFECT UNTIL NOON CST TODAY…

The National Weather Service in North Platte has issued a Dense
Fog Advisory, which is in effect until noon CST today.

* Visibilities…as low as one quarter mile or less at times.

* Timing…Through the morning hours with visibilities improving
after noon CST.

* Impacts…Hazardous driving conditions due to low visibility.
Fog may freeze on area roads and walkways as well.

PRECAUTIONARY/PREPAREDNESS ACTIONS…

A Dense Fog Advisory means visibilities will frequently be
reduced to less than one quarter mile. If driving, slow down, use
your headlights, and leave plenty of distance ahead of you.

&&

$$

JWS

GOES-16 IFR Probability fields captured the development of these regions of dense fog. The animation from 0400-1200 UTC on 5 January is below. Highest values of IFR Probability are consistent in the areas where IFR Conditions are developing and where Dense Fog Advisories were issued.

GOES-16 IFR Probability, 0402 – 1207 UTC on 5 January 2018 (Click to animate)

Note that IFR Probability fields are fairly high over Iowa and the eastern Dakotas, regions where mid-level stratus was widespread but where IFR observations did not occur. On this day, Low IFR Probability fields better screened out this region of mid-level stratus. The toggle below compares IFR Probability and Low IFR Probability on 0957 UTC. The region where dense fog advisories were issued shows high values in both fields. The stratus deck over Iowa and the eastern Dakotas shows much smaller values of Low IFR Probability.

GOES-16 also has a ‘Fog Product’ brightness temperature difference (10.3 – 3.9) that has historically been used to detect low clouds. However, when cirrus clouds are present, as on 5 January, the efficacy of this product in fog detection is affected. Although fog and stratus detection is identifiable underneath the moving cirrus (the same is true in the Advanced NightTime Microphysics RGB product below), identifying the low cloud as stratus or fog from satellite data is a challenge because a consistent color married to IFR Probability does not exist.

GOES-16 ‘Fog Product’ Brightness Temperature Difference (10.3 µm – 3.9 µm), 0402 – 1207 UTC, 5 January 2017 (Click to animate)

GOES-16 Advanced Nighttime Microphysics RGB, 0402-1207 UTC on 5 January 2018 (Click to animate)

GOES-16 IFR Probability fields maintain a consistent look from night to day. Both the (10.3 µm – 3.9 µm) Brightness Temperature Difference field and the Advanced Nighttime Microphysics RGB (that uses the ‘Fog Product’ BTD) will change because the increase in reflected solar radiation at 3.9 µm will change the sign of the Brightness Temperature Difference field. There is a Daytime Day/Snow/Fog RGB Product in AWIPS, and the toggle below from 1612 UTC on 5 January compares IFR Probability and the Day/Snow/Fog RGB. As with the nighttime products, the presence of high (or mid-level) clouds makes it difficult to use the RGB alone to identify regions of fog/low stratus. In contrast, the IFR Probability field continues to correctly identify where the obstructions to visibility exist.

Dense Fog over the Deep South

GOES-16 IFR Probability fields, 1312 UTC on 18 December 2017 (Click to enlarge). Ceilings and visibilities are also plotted.

Dense Fog was widespread across the south on Monday morning, 18 December 2017 (See the screen capture below from this site at 1319 UTC).  The GOES-16 IFR Probability field, above, highlights the regions of IFR and near-IFR conditions very well. It has a flat character over much of central Mississippi and Alabama.  These are regions where multiple cloud decks are preventing the satellite from viewing the near-surface clouds, and where Rapid Refresh data are being used as the sole predictor for the probability of IFR conditions.  In contrast, the IFR Probability field over much of Tennessee and Arkansas (for example) has a pixelated look to it — there are small variations over very small distances:  over these two states, higher clouds are not preventing the satellite from viewing near-surface clouds, and satellite data can also be used as a predictor in the IFR Probability fields (See the 10.3 µm – 3.9 µm Brightness Temperature difference field below).

Screen Capture from http://www.weather.gov at 1319 UTC on 18 December 2017 (Click to enlarge). Grey regions are under Dense Fog Advisories.

When high clouds are present, fog detection techniques that rely solely on satellite data struggle to detect low clouds.  Compare the above field, for example, to the 10.3 µm – 3.9 µm Brightness Temperature Difference field (sometimes called the ‘Fog Product’).  In the enhancement used (the default enhancement in AWIPS), fog is depicted as blue (a positive value in the brightness temperature difference) and cirrus/high clouds in black.  There is little signal of Fog over a region where Dense Fog advisories have been issued. Similarly, the Advanced Nighttime Microphysics RGB, below, that relies on the 10.3 µm – 3.9 µm to highlight low clouds that might be fog, also is not indicating fog (a light cream/cyan color, typically) over much of the Deep South.  When cirrus clouds are present, its use, like that of the Fog Brightness Temperature Difference, is of dubious value.

The last figure, at the bottom, toggles between all three fields at 1312 UTC.

GOES-16 Fog Brightness Temperature Difference (10.3 µm – 3.9 µm) field, 1312 UTC on 18 December 2017  (Click to enlarge)

Advanced Nighttime Microphysics RGB, 1312 UTC on 18 December 2017  (Click to enlarge)

GOES-16 IFR Probability, GOES-16 ‘Fog’ Brightness Temperature Difference (10.3 µm – 3.9 µm), and Advanced Nighttime Microphysics RGB, all at 1312 UTC on 18 December 2017 (Click to enlarge)

Dense Fog over Idaho

GOES-16 IFR Probability fields, 0502-1302 UTC on 15 December 2017 (Click to enlarge)

GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing

GOES-16 is now in the operational GOES-East position (but not, yet, technically operational) and GOES-16 data started flowing shortly after 1500 UTC on Thursday 14 December. GOES-16 produces excellent imagery over the western United States despite the satellite’s station at 75.2 West Longitude. The animation above shows GOES-16 IFR Probability fields over Idaho, with large values over the Snake River; High Pressure over the region has capped moisture (and pollutants) in the valley, and reduced visibilities are a result. (Click here for the Boise Sounding from 0000 UTC on 15 December from this site) The Pocatello Idaho Forecast Office of the NWS issued (at bottom) Dense Fog Advisories that were valid in the morning of 15 December 2017.

The excellent temporal resolution allows for close monitoring of the eastern edge of the region of fog, expanding eastward from the Snake River Valley into Wyoming and Montana.

The animation above shows consistent GOES-16 IFR Probabilities over the Snake River, and observations of low ceilings and reduced visibilities.  Note that over the eastern part of the Valley, from Pocatello to Idaho Falls and Rexburg, the character of the IFR Probability field at times loses all pixelation.  During this time (around 1000 UTC), model data (in the form of low-level saturation in the Rapid Refresh Model) are contributing to the IFR Probability Field, but satellite data are not because of high-level cirrus.  The animation, below, of the Nighttime Fog Brightness Temperature Difference (10.3 µm – 3.9 µm), confirms the presence of cirrus (they appear grey/black in the color enhancement).  It also suggests why that field alone rather than a fused field such as GOES-R IFR Probability can struggle to detect fog in regions of cirrus.

GOES-16 Brightness Temperature Difference Field (10.3 µm – 3.9 µm), 0502-1302 UTC on 15 December 2017 (Click to animate)

Products that use only satellite data, such as the Brightness Temperature Difference field, above, or the Advanced Nighttime Microphysics RGB Product, below, that uses the (10.3 µm – 3.9 µm) Brightness Temperature Difference field as the ‘Green’ component, will always struggle to detect fog in regions of cirrus. Of course, the superb temporal resolution of GOES-16 mitigates that effect, as in this case; it’s obvious in this animation what is going on: a band of cirrus is moving over the fog, but it not likely affecting it.  A single snapshot of the scene, however, might not impart the true character of surface conditions.

Advanced NIghttime Microphysics RGB Composite, 0502-1302 UTC on 15 December 2017 (Click to enlarge)

Screencapture of WFO PIH (Pocatello Idaho) Website from 1320 UTC on 15 December 2017 (Click to enlarge)

GOES-16 IFR Probabilities in AWIPS

Visible Imagery and GOES-16 IFR Probability Fields, 2202 UTC on 29 November 2017 (Click to enlarge)

GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing

GOES-16 IFR Probabilities can now be displayed in AWIPS.  This site has included the product for several months now.  The imagery above, however, is from AWIPS, showing a toggle between Visible GOES-16 Imagery (with and without observations of ceiling heights and visibilities), MVFR (Marginal Visible Flight Rules) Probabilities, IFR (Instrument Flight Rules) Probabilities and LIFR (Low Instrument Flight Rules) Probabilities.  GOES-16 IFR Probability fields are Bayesian and have been trained using about 2 months’ worth of GOES-16 Data.  The LDM data feed will be providing data once GOES-16 Data are flowing again, sometime between 14 December and 20 December, when GOES-16 is on station at 75.2 º W Longitude.

GOES-16 IFR Probability, 0642-1137 UTC on 30 November 2017 (Click to enlarge)

Dense Fog covered parts of Florida early on 30 November, and the animated GOES-16 IFR Probability field, above, shows the benefit of GOES-16’s routine 5-minute temporal cadence:  the motion of the fog field is well-captured, and it’s straightforward to use the field to estimate the onset of IFR conditions.  The Advanced Nigthtime Microphysics RGB for the same time period is shown below, and that product does not well indicate the widespread nature of the reduced ceilings and visibilities over northern Florida.

GOES-16 Night-time Microphysics RGB, 0642-1137 UTC on 30 November 2017 (Click to enlarge)

Screen Capture for http://www.weather.gov at 1148 UTC on 30 November 2017 (Click to enlarge)

Dense Fog Advisories were widespread over the southeastern US on the morning of 30 November, as shown by the screen capture above, from 1148 UTC 30 November. The toggle below shows IFR Probabilities and the Advanced Microphysics RGB for 1147 UTC on 30 November.  The 10.3 µm – 3.9 µm Brightness Temperature Difference (BTD) for the same time shown at the bottom. Evidence of multiple cloud decks is apparent in the image. Such mid- and high-level clouds result in an ambiguous signal as far as fog detection goes in both the BTD and in the RGB. IFR Probabilities give a consistent signal in those regions that relies on Rapid Refresh Model output suggesting low-level saturation is present.

GOES-16 IFR Probabilities and the GOES-16 Advanced Microphysics RGB at 1147 UTC on 30 November 2017 (Click to enlarge)

GOES-16 Fog Brightness Temperature Difference (10.3 µm – 3.9 µm), 1147 UTC on 30 November 2017 (Click to enlarge)

Dense Fog Advisories over Memphis

Dense Fog Advisories were issued over Memphis and adjacent portions of the mid-south on Tuesday morning, 7 November. (Click here for a 1230 UTC screen capture from the Memphis National Weather Service webpage). The Advisory text is at the bottom of the post.

GOES-16 Brightness Temmperature Difference field (10.3 µm – 3.9 µm) from 0502 through 1252 UTC on 7 November 2017 (Click to enlarge)

GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing

GOES-R IFR Probabilities are computed using Legacy (Operational) GOES (GOES-13 and GOES-15) and Rapid Refresh model information; Preliminary IFR Probability fields computed with GOES-16 data are available here. IFR Probability fields based on GOES-16 will be available via LDM Request when GOES-16 becomes operational as GOES-East (currently scheduled for some time between 11 and 20 December 2017).

The animation above shows the GOES-16 Brightness Temperature Difference Product (at every 10 minutes, rather than the typical 5-minute temporal cadence of GOES-16 over the continental US) during the night on 7 November.  In the default color enhancement shown, clouds made up of water droplets are show up as cyan and blue whereas higher clouds are black.  Note also the effects of increasing solar reflectance at the end of the animation:  the brightness temperature difference is switching sign as increasing amounts of 3.9 µm radiation are reflected off the clouds.

It could be difficult to use the animation above, alone, to heighten situational awareness of a developing region of fog because of the confounding effects of higher clouds.  Additionally, infrared satellite imagery is challenged to detect cloud thickness:  are the stratus clouds detected (cyan/blue in the enhancement) mid-level, low-level or both?

IFR Probability fields can screen out regions of mid-level stratus, regions that are not so important from the point of view of transportation.  This is because Rapid Refresh Model output is used as a predictor in the statistical model underlying IFR Probability fields.  If the Rapid Refresh Fields do not show low-level near-saturation, the IFR Conditions are less likely.

Consider, for example, the animation below of IFR Probability fields computed for GOES-13 data.  At the beginning of the animation, the fields clearly distinguish between regions where dense fog is occurring near Memphis, and where mid-level stratus is more common (over northern Mississippi).  As dawn approaches, reports of fog become more widespread over Mississippi — but the product has given a timely alert to how conditions might differ over a short region that was not possible with the single brightness temperature difference product alone.

GOES-13 IFR Probability fields, hourly from 0315-1215 UTC on 7 November 2017 (Click to enlarge)

GOES-R IFR Probabilities are available via an LDM feed to National Weather Service Offices. At present, IFR/Low IFR and Marginal IFR Probabilities (and Cloud Thickness) fields that are sent are those created by the operational GOES-East Satellite, GOES-13.  IFR Probability Products based on GOES-16 are being produced now, however, and are available here. The short animation below shows a behavior similar to the product based on GOES-13, but GOES-16 has far better temporal and spatial resolution!  Click here for a toggle between GOES-13 IFR Probabilities, GOES-16 Brightness Temperature Difference Fields, and GOES-16 IFR Probabilities at 0715 UTC.

When GOES-16 is operational as GOES-East, currently scheduled to occur between 11 and 20 December 2017, the LDM feed will supply GOES-East IFR Probabilities computed with GOES-16 data.

GOES-16 IFR Probabilities, 0402-0557 UTC on 7 November 2017 (Click to enlarge)

Suomi NPP flew over the region at 0740 UTC on 7 November, and there was ample illumination to see the clouds. Multiple cloud decks and levels are apparent below.

Suomi-NPP Day Night Visible Imagery (0.70 µm) Near-Constant Contrast Product, 0741 UTC on 7 November 2017 (Click to enlarge)

URGENT – WEATHER MESSAGE
National Weather Service Memphis TN
1228 AM CST Tue Nov 7 2017

…A Dense Fog Advisory is in Effect for Portions of the Midsouth
including the Memphis Metro Area…

ARZ036-048-049-MSZ001>004-007-008-TNZ088>090-071500-
/O.EXT.KMEG.FG.Y.0025.171107T0700Z-171107T1500Z/
Crittenden-St. Francis-Lee AR-DeSoto-Marshall-Benton MS-Tippah-
Tunica-Tate-Shelby-Fayette-Hardeman-
Including the cities of West Memphis, Forrest City, Marianna,
Southaven, Olive Branch, Holly Springs, Ashland, Ripley MS,
Tunica, Senatobia, Bartlett, Germantown, Collierville, Memphis,
Millington, Somerville, Oakland, and Bolivar
1228 AM CST Tue Nov 7 2017

…DENSE FOG ADVISORY NOW IN EFFECT UNTIL 9 AM CST THIS MORNING…

* VISIBILITY…Less than one-half mile.

* TIMING…Through 9 AM CST Tuesday.

* IMPACTS…Dense fog will most commonplace outside of the
Memphis urban center and near bodies of water. Travel may
become difficult due to limited visibilities.

PRECAUTIONARY/PREPAREDNESS ACTIONS…

A Dense Fog Advisory means visibilities will frequently be
reduced to less than one quarter mile. If driving…slow down…
use your low beam headlights…and leave plenty of distance ahead
of you.

&&

$$

JAB

GOES-16 IFR Probability with Dense Fog in the Upper Midwest

GOES-16 IFR Probability fields, 0932-1157 UTC on 23 October 2017 (Click to animate)

GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing.

Dense Fog developed over the upper Midwest on Monday morning, 23 October 2017, and Advisories were issued as shown below.  GOES-R IFR Probabilities are now being created using GOES-16 data, those data are now available at this link.  The uniformity of the IFR Probability fields shown above over WI suggest that high-level clouds are present, and the GOES-16 satellite could not therefore view the fog/stratus near the ground: only Rapid Refresh data were used to create GOES-R IFR Probability values.

GOES-R IFR Probability fields available to NWS Field Offices via LDM are still being computed with GOES-13 and GOES-15 data.  When GOES-16 becomes operational as GOES-East at 75.2º W Longitude, planned for December, IFR Probabilities available through the LDM will be created with GOES-16 and GOES-15 data. The switchover will happen when GOES-16 becomes operational.

Screenshot of NWS webpage from Sullivan, WI at 1200 UTC on 23 October 2017. Dense Fog advisories are in place from SW Wisconsin to NE Wisconsin. Note also the Radar imagery showing departing showers. (Click to enlarge)

Resolution and Fog Detection in the mountains of the Appalachia

GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing.

Clear skies and light winds allowed for the development of Radiation Fog over parts of West Virginia and surrounding states early on Tuesday 17 October 2017, and IFR conditions were widespread (Source).

GOES-16 and GOES-13 both monitored this event, and toggles between the two brightness temperature difference fields, from 0315, 0415 and 0615 on 17 October, are shown below.  The better spatial resolution (and better precision) of the GOES-16 ABI is apparent in the imagery:  At 0415 UTC, GOES-16 is detecting river fog in many valleys in West Virginia and Kentucky, and is better resolving the detected fog over the Allegheny River in northwest Pennsylvania.  By 0615 UTC, the fog is also detected by GOES-13 — meaning it has increased in size so that the GOES-13 pixel size can detect it.  GOES-16 had a lead time of about 1-2 hours on GOES-13 in this case!

GOES-13 and GOES-16 Brightness Temperature Difference Fields, 0315 UTC on 17 October (Click to enlarge)

GOES-13 and GOES-16 Brightness Temperature Difference Fields, 0415 UTC on 17 October (Click to enlarge)

GOES-13 and GOES-16 Brightness Temperature Difference Fields, 0615 UTC on 17 October (Click to enlarge)

At present, GOES-R IFR Probabilities are created using GOES-13 data (or GOES-15 data for the western United States).  As might be expected, the GOES-R IFR Probabilities also lagged the GOES-16 Brightness Temperature Difference field in alerting to fog.  The short animation below shows values hourly from 0315-0615.  Very small IFR Probabilities are present at 0315 and 0415 — because, in part, the GOES-13 Imager cannot resolve the developing Fog/Stratus (and the Rapid Refresh model lacks the horizontal resolution to reproduce what is happening in narrow river valleys).  By 0515, and 0615 especially, GOES-R IFR probabilities are suggesting that a region of fog is likely.

One other thing to note:  There is a small signal in the brightness temperature difference field at 0315 UTC, above, in northwest Ohio in both GOES-13 and GOES-16 imagery.  There is no corresponding signal in the IFR Probability field.  The conclusion is that this is mid-level stratus that is not affecting surface visibility or ceilings.

GOES-R IFR Probabilities, hourly from 0315-0615 UTC on 17 October 2017 (Click to enlarge)

GOES-R IFR Probabilities, supplied via an LDM feed to National Weather Service Offices, will use GOES-13 (or GOES-15) imagery until GOES-16 is operational at 75.2 degrees West Longitude.  When GOES-16 is operational, GOES-R IFR Probabilities will use GOES-16 data, and that data will flow through the LDM.  In the meantime, GOES-16 IFR Probabilities can be viewed at this site.

Fog returned the following morning.  Link.

The GOES-16 Version of GOES-R IFR Probability

GOES-R IFR Probability fields computed using GOES-16 Data and Rapid Refresh Model Output, 1042 UTC on 3 October 2017 (Click to enlarge)

GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing

The suite of GOES-R IFR Probability products are now being computed using GOES-16 data, with a 5-minute temporal cadence over CONUS.  Products include IFR Probability (above), Low IFR Probability, MVFR Probability and Cloud Height.  These products are only available for now at http://cimss.ssec.wisc.edu/geocat ;  however, work is progressing to have the fields available through an LDM feed.  The GOES-R IFR Probability fields computed using Legacy GOES (GOES-13 and GOES-15) continue to flow to the National Weather Service via the LDM.

IFR Conditions in Pennsylvania and Oregon

GOES-16 Brightness Temperature Difference (10.3 µm – 3.9 µm) at 0912 UTC on 2 October 2017 over the Mid-Atlantic States (click to enlarge)

GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing.

The images above show the GOES-16 Brightness Temperature Difference at the same time at two places over the United States: The mid-Atlantic States (above) and Oregon and surrounding States (below).  The ‘Fog’ Product, as this Brightness Temperature Difference is commonly called, in reality identifies only clouds that are made up of water droplets — that is, stratus.  A cloud made up of water droplets emits 10.3 µm radiation nearly as a blackbody does. Thus, the computation of Brightness Temperature — which computation assumes a blackbody emission — results is a temperature close to that which might be observed.  In contrast, those water droplets do not emit 3.9 µm radiation as a blackbody would.  Thus, the amount of radiation detected by the satellite is smaller than would be detected if blackbody emissions were occurring, and the computation of blackbody temperature therefore yields a colder temperature, and the brightness temperature difference field, above, will show clouds made up of water droplets as positive, or cyan in the enhancement above.

The River Valleys of the northeast show a very strong signal that suggests Radiation Fog is developing over the relatively warm waters in the Valleys.  The Delaware, Hudson, Mohawk, Connecticut, Susquehanna, Allegheny, Monongahela, and others — all show a signature that one would associate with fog.  A signal is also apparent from southern New Jersey southwestward through the Piedmont of North Carolina.  Would you expect there to be fog there as well, given the signal?

The State of Oregon at the same time shows a very strong signal in the ‘Fog’ Product.  A clue that this might be only stratus, and not visibility-restricting fog, lies in the structure of the clouds — they do not seem to be constrained by topographic features as is common with fog.

GOES-16 Brightness Temperature Difference (10.3 µm – 3.9 µm) at 0912 UTC on 2 October 2017 over Oregon and adjacent States (click to enlarge)

GOES-R IFR Probabilities are computed using Legacy GOES (GOES-13 and GOES-15) and Rapid Refresh model information; Preliminary IFR Probability fields computed with GOES-16 data are available here.  These GOES-16 fields should be available via LDM Request when GOES-16 becomes operational as GOES-East.

GOES-R IFR Probability Fields use both the Brightness Temperature Difference field (10.7 µm – 3.9 µm) from heritage GOES instruments and information about low-level saturation from Rapid Refresh Model output.  The horizontal resolution on GOES-13 and GOES-15 is coarser than on GOES-16 (4 kilometers at the sub-satellite point vs. 2 kilometers), so small river valleys will not be resolved.  (It is also difficult for the Rapid Refresh model to resolve small valleys).

GOES-R IFR Probability fields at 0915 UTC, along with 0900 UTC surface observations of ceilings and visibility (Click to enlarge)

The IFR Probability Fields, above, show some signal over the river valleys of the northeast; that signal is mostly satellite-based, but the poor resolution of GOES-13 means that fog/stratus in the river valleys is not well-resolved. Still, a seasoned forecaster could likely interpret the small signals that are developing to mean fog is in the Valleys.  (And restrictions to ceilings and visibilities are certainly reported in the river valleys of the Mid-Atlantic and Northeast)   IFR Probabilities are also noticeable over southeast Virginia, although widespread surface observations showing IFR Conditions are not present.  (Such observations are somewhat more common near sunrise, at 1130 UTC).

IFR Probabilities are much less widespread over Oregon, with most of the signal over western Oregon related to the topography.  In this example, IFR Probabilities are ably screening out regions where elevated stratus is creating a strong signal for the satellite in the Brightness Temperature Difference field.

Dense Fog over Missouri and over Alabama

GOES-R IFR Probability Fields, Hourly from 0215-1315 UTC on 19 September 2017 (Click to enlarge)

GOES-R IFR Probabilities are computed using Legacy GOES (GOES-13 and GOES-15) and Rapid Refresh model information; GOES-16 data will be incorporated into the IFR Probability algorithm in late 2017

Dense fog developed over Missouri on Tuesday 19 September and Dense Fog Advisories were issued. The animation above shows the hourly development of GOES-R IFR Probability fields; values increased from northern Missouri to southern Missouri as dense fog developed, first north of I-70, then south into the rest of the state. The morning of 19 September was mostly devoid of mid-level and high-level clouds over Missouri (exception: west-central Missouri starting after 0900 UTC), and that kind of night means that traditional methods of fog detection work well. The brightness temperature difference field between the shortwave Infrared and the Longwave Infrared (3.9 µm and 10.3 µm on GOES-16, 3.9 µm and 10.7 µm on GOES-13) shows the fog development.

Note that the IFR Probability field, above, does not show fog dissipating around sunrise. That’s in contrast to the Brightness Temperature Difference field below. As the sun rises, the amount of solar radiation at 3.9 µm that is reflected off the clouds increases; this changes the brightness temperature difference from positive (cyan in the color enhancement shown) to negative (grey or black in the enhancement shown).

GOES-16 has better spatial resolution than GOES-13; thus, the small valley fogs that can develop in the rugged (ish) terrain of southern Missouri are resolved in GOES-16, but not in GOES-13. When GOES-R IFR Probability is created using GOES-16 data (slated to begin in late 2017), the resolution improvements in GOES-16 will migrate to IFR Probability fields.

GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing.

GOES-16 Brightness Temperature Difference (10.3 µm – 3.9 µm), hourly from 0412 to 1112 UTC on 19 September (Click to enlarge)


==========================================================
On 20 September, Dense Fog developed over Tennessee and Alabama, leading to the issuance of Dense Fog Advisories. The GOES-R IFR Probability field, below, shows good agreement between high probabilities and reduced ceilings/visibilities.

GOES-R IFR Probability Fields, Hourly from 0415-1315 UTC on 20 September 2017 (Click to enlarge)

As on the 19th over Missouri, top, this was a night with relatively few middle- and upper-level cloud decks. On such nights, the GOES-16 Brightness Temperature Difference field can capably identify regions of stratus (it’s up to a human to decide if the stratus deck extends to the surface; on this night, much of the stratus did). The 2-hour animation of Brightness Temperature Difference, below, highlights two particular strengths of GOES-16: Better spatial resolution that allows small valleys to be sampled correctly, and good temporal resolution (every 5 minutes vs. every 15 minutes for GOES-13) that allows superior monitoring of the cloud evolution with time. Note that the rising sun is eroding the GOES-16 Brightness Temperature Difference signal by the end of the animation below.

GOES-16 Brightness Temperature Difference (10.3 µm – 3.9 µm), hourly from 1002 to 1202 UTC on 20 September (Click to enlarge)

The Brightness Temperature Difference field (10.3 µm – 3.9 µm) is a key component to the Nighttime Microphysics Red/Green/Blue Composite. As the toggle below shows, the Brightness Temperature Difference field overwhelmingly controls the region identified by the RGB as one with a potential for fog.