Category Archives: Cloud Thickness

Predicting the dissipation time of Radiation Fog

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GOES-R IFR Probabilities (Upper Left), GOES Brightness Temperature Difference (10.7 µm – 3.9 µm) (Upper Right), GOES-R Cloud Thickness (Lower Left), GOES Visible (0.62 µm) (Lower Right) at 1400 UTC on 27 December 2012.

GOES-R Cloud thickness can be used to predict how long it will take radiation fog and low stratus to burn off after developing overnight.  This case from the high plains of Colorado, on December 27th, is typical.  At 1402 UTC, La Junta Colorado is in a region of enhanced IFR Probability, with 2-mile visibilty and 400-foot ceilings.  The Cloud Thickness at this time, the last image available before twilight conditions, was as much as 1200 feet.  This scatterplot suggests that the fog will be gone in 4-5 hours.  The 1732 UTC image, below, shows the final remnant of low cloud persisting (it was not present at 1815 UTC).  Although difficult to see in the visible imagery, perhaps because of snow-covered ground, it shows up well in both the IFR probability field, the Brightness Temperature Difference field, and the Cloud Thickness field.

As above, but at 1732 UTC.

IFR conditions under high clouds in the Northeast

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GOES-R IFR Probabilities computed using GOES-East (Upper Left), GOES-East Brightness Temperature Difference (10.7 µm- 3.9 µm, The ‘traditional’ fog detection product) (Upper Right), GOES-R Cloud Thickness computed using GOES-East (Lower Left), GOES-East Window Channel (10.7 µm) Brightness Temperature (Lower Right)

When high clouds overspread an area, the traditional brightness temperature difference product cannot be used to highlight areas of fog and low stratus because radiation emissions are originating from high clouds, not from the water-based low clouds.  In this example from Monday morning, 17 December 2012, IFR conditions, causing airport flight delays, are commons from Washington DC to New York, and the GOES-R IFR probability product highlights the area where IFR (and near-IFR) conditions prevail.  Modest values (around 50%) occur where the satellite predictors do not provide a fog/low stratus signal;  however, the Rapid Refresh model data does show high probability of fog and low stratus.  Where the Satellite does contribute to the product (that is, in Pennsylvania north and east of the high cloud deck), IFR probabilities are very high.

Note also that the GOES-R Cloud Thickness product (bottom left), is computed only for the highest water-based cloud (in non-twilight conditions).  It is therefore not shown under the cirrus canopy, over southern New Jersey, Delaware, and Chesapeake Bay.

Florida Radiation Fog

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GOES-R IFR Probabilities (Upper Left) computed from GOES-East, GOES-East Brightness Temperature Differences (10.7 µm – 3.9 µm) (Upper right), GOES-R Cloud Thickness product (Lower Left), GOES-East Visible Imagery (0.63 µm) (Lower Right), hourly from 0615 UTC through 1215 UTC on 14 December 2012

Light north winds over Florida over night were accompanied by the development of low stratus and fog that were captured well by the traditional fog detection product, the brightness temperature difference between 10.7 µm and 3.9 µm that occurs because of emissivity differences in water clouds at the two wavelengths.  Because of the strong satellite signal (upper right), the GOES-R IFR Probabilities over Florida were quite high.  When mid-level and high clouds are absent, IFR probabilities in regions of fog development can easily exceed 90%, as shown above.

Note, however, what happens at sunrise.  The increase in reflected 3.9 µm solar radiation causes the brightness temperature signal to vanish.  (At later times, the sign of the brightness temperature difference will flip).  The GOES-R IFR probability product maintains a coherent signal through sunrise, although values shift somewhat, as can be seen by the southwest to northeast boundary in the final image of the loop over Florida and extreme southeast Georgia.

Cloud thickness, bottom left, can be used to estimate when radiation-induced stratus or fog will ‘burn off’ — thicker clouds will take longer to dissipate.

Note in this loop that the eastern coastline of Mobile Bay shows a signal in the Brightness Temperature Difference.  This signal is likely an artifact of poor co-registration between Bands 2 and 4 (the 3.9 µm and 10.7 µm channels, respectively) on GOES-13.  NESDIS scientists and engineers are working to mitigate this time-dependent error.

The Fog/Low Stratus products can be compared to data from Suomi/NPP.  The VIIRS instrument includes a day/night band that uses reflected moonlight as a light source (below).  Unfortunately, because the moon was new on December 13th, 2012, very little reflected light is available.  Nevertheless, smearing of city lights over Florida does suggest the presence of fog.

As above, but with Day/Night band from VIIRS on Suomi/NPP instead of GOES-13 0.63 µm visible imagery.  Images from ~0630 UTC on 14 December.

Gulf of Mexico Fog

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GOES-R IFR Probabilities computed from GOES-East (Upper Left), GOES-East Brightness Temperature Difference (10.7 µm – 3.9µm ) (Upper Right), GOES-East Visible imagery (0.63 µm) (Lower Left), GOES-R Cloud Thickness (Lower Right), all from ~2030 UTC on 9 December 2012

Fog was present in the northeast Gulf of Mexico at sunset on December 9th, and the GOES-R Products was able to track its evolution as it slowly moved onshore.  Late-in-the-day visible imagery from 2030 UTC 9 December (above) shows the fog bank tucked into the northeast corner of the Gulf of Mexico.  By 0100 UTC on 10 December (below), some of the fog as moved inland, and visibilities/ceilings at Crystal River have decreased to IFR conditions.  Note in the 0100 UTC image the presence of mid-level clouds is creating a hole in the Brightness Temperature Difference Signal over the eastern Gulf of Mexico just northwest of Tampa, and in the Cloud Thickness product.  IFR Probabilities in this region are considerably reduced, as well.

As at top, but for 0100 UTC on 10 December
As above but for 0402 UTC on 10 December

By 0400 UTC on 10 December, the fog/low stratus continue to press inland, as ceilings and visibilities decrease.  Note also the presence of higher clouds in the brightness temperature difference product.  In these regions, satellite predictors are not use to compute the IFR probability product;  consequently, IFR Probabilities over and around Apalachee Bay in the extreme northeast Gulf of Mexico are somewhat reduced; in addition, the field is considerably flatter (compared to the more pixelated character close to Tampa).  At 0802 UTC (below), the presence of mid-level clouds peaks.  Note that the brightness temperature difference product shows very little strong signal over the northern part of the Florida peninsula, despite significant visibility obscurations there (consider Gainesville, for example, with a 1/4-mile visibility).

As above but for 0802 UTC on 10 December
As above, but for 1102 UTC on 10 December

By 1102 UTC, the mid-level cloudiness has dissipated, allowing the brightness temperature difference to have a distinct signal.  Consequently, the IFR probability field increases.  The combination of both satellite predictors and model (Rapid Refresh) predictors being used allows for a consistent signal throughout the night over this evolving fog/stratus deck.  In addition, the signal over the ocean, at the end of the day on 9 December, in a region where surface observations are not routine, could serve as an alert to the early development of nocturnal fog.

Note that the Tampa Bay office of the National Weather Service noted the IFR probability field in its forecast discussion:

 
000
FXUS62 KTBW 100034
AFDTBW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TAMPA BAY RUSKIN FL
734 PM EST SUN DEC 9 2012

.FOR THE EVENING UPDATE...
SURFACE HIGH PRESSURE WILL REMAIN OVER THE REGION WITH A STRONG
MID LEVEL SUBSIDENCE INVERSION AND STABLE AIR MASS OVER THE
FORECAST AREA. AREA OF FOG...LOCALLY DENSE...HAS PERSISTED THROUGH
THE DAY OVER THE COASTAL WATERS NORTH OF VENICE AND LOCALLY
ONSHORE FROM ABOUT ANNA MARIA ISLAND NORTH TO CEDAR KEY. AREA OF
LOCALLY DENSE FOG WILL GRADUALLY PUSH ONSHORE THE COASTAL
COUNTIES THIS EVENING...WITH FOG DEVELOPING OVER INTERIOR BECOMING
LOCALLY DENSE AFTER MIDNIGHT. CURRENT ZONES ARE ON TRACK WITH NO
UPDATE PLANNED.

&&

.AVIATION...
GOES-R IFR AND LIFR PROBABILITY PRODUCTS NICELY OUTLINE AN AREA OF
SEA FOG THAT HAS LINGERED ACROSS NEAR SHORE WATERS FROM BIG BEND
SOUTH TO AROUND VENICE. THIS SEA FOG IS RIGHT ALONG THE COASTLINE
AND IS VISIBLE ON SOME BEACH WEB CAMS IN THE AREA AND THEREFORE IS
POISED TO MOVE INLAND ONCE TEMPS FALL SOME. THE BIG QUESTION IN ALL
THIS IS WHETHER THE WEAK WRLY WINDS ASSOC W/ THE SEA BREEZE
CIRCULATION WILL CONTINUE LATE ENOUGH TO PUSH THE SEA FOG BANK
INLAND IMPACTING TAMPA BAY AREA SITES. PREVIOUS TAFS WERE COUNTING
ON THIS OCCURRING AND WILL CONTINUE CLOSE TO THE FORECAST ALTHOUGH
HAVE NOTED 18Z MAV AND LATEST LAMP HAVE STRONGLY BACKED OFF ON THE
FOG FOG FORECAST AND NOW CONFINED FOG TO MUCH LATER IN THE NIGHT
CLOSER TO SUNRISE. WILL HAVE TO WATCH CLOSE AND AMEND AS NEEDED IN
NEXT FEW HOURS AS ONSET OF LIFR CONDITIONS IS BIGGEST QUESTION MARK
TONIGHT. LIFR CONDITIONS ARE PRETTY CERTAIN AROUND 12Z AND WILL SEE
SLOW IMPROVEMENT TO MVFR LEVELS THROUGH 18Z. MVFR LEVELS MAY LINGER
AND CONTINUE BEYOND 18Z AS MOISTURE SURGES INTO THE AREA LEADING TO
SCT SHRA ACROSS THE ENTIRE AREA AND POSSIBLY A FEW TSTMS MAINLY
AROUND OR EAST OF SRN SITES (FMY, RSW, PGD). MODELS SUGGEST
POTENTIAL DEVELOPMENT OF MORE WIDESPREAD SEAFOG EVENT LATE TOMORROW
AS MOISTURE CONTINUES TO INCREASE AND A MORE PERSISTENT SW FLOW
DEVELOPS. 
 
 
MODIS-based GOES-R IFR Probabilities, 0334 UTC 10 December (above) and 0745 UTC 10 December (below)

MODIS-based GOES-R IFR Probabilities, above, show a similar evolution of the field, from fairly constrained over northern Florida at 0334 UTC, and determined by both satellite and model fields, to more widespread but deteremined more by model fields only at 0745 UTC.

Fog in California’s San Joaquin Valley

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GOES-R IFR Probabilities, computed from GOES-West (Upper Left), GOES-West Visible Imagery (0.62 µm) (Upper Right), GOES-R Cloud Thickness (Lower Left), GOES-West Brightness Temperature Difference (10.7 µm – 3.9 µm) (Lower Right)

Fog is persisting in California’s San Joaquin Valley today, and the GOES-R IFR Probability and Cloud Thickness products both are describing its spatial extent.  The lowest clouds are banked against the western side of the valley, with visibilities lowest, and IFR probabilities highest, from Bakersfield to Fresno to points north.  The Sacramento Valley (not shown) is not showing visibilities near IFR conditions, and the IFR probabilities in that part of California are lower.

The impact of higher clouds

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GOES-R IFR Probabilities computed from GOES-East (Upper Left), GOES-East Brightness Temperature Difference (10.7 µm – 3.9 µm) (Upper Right), GOES-R Cloud Thickness (Lower Left), GOES-East 10.7 µm Imagery (Lower Right) at 0700 UTC on 6 December 2012.

Upper-level clouds, such as those apparent in both the 10.7 µm imagery and the brightness temperature difference imagery, above, lower right and upper right, respectively, have an impact on the GOES-R IFR Probability and GOES-R Cloud Thickness products.  The most obvious impact is in the Cloud Thickness product (lower left), which product is not computed in regions where high clouds are present.  The GOES-R IFR Probabilities are computed underneath high clouds, using mostly Rapid Refresh model data to determine the probability of fog and low stratus.  However, because cloud predictors are not used, IFR probabilities are somewhat lower.  In addition, the character of the field is flatter, reflecting the smoother fields that are present in the model.  This is especially obvious over northeast Louisiana and extreme east Texas in the IFR Probability image above.  Thus, the heritage product, the brightness temperature difference, gives no information from southwest Louisiana northward into southern Arkansas, but the fused GOES-R IFR Probabilities do suggest enhanced possibilities of IFR conditions in regions where reduced visibilities are reported:  central and northeast Louisiana and east Texas.  IFR Probabilities are much lower over southwest Louisiana where IFR conditions are not reported.

As above, but for 1245 UTC on 6 December 2012.

By 1245 UTC, the high clouds have lifted northeast and dissipated somewhat, so the heritage brightness temperature difference product gives information over the entire lower Mississippi River valley and over east Texas, where IFR conditions were widespread underneath very high GOES-R IFR Probabilities.  The GOES-R Cloud Thickness product indicates cloud thicknesses up to near 1200 feet, suggesting a burn-off time for radiation fog of around 5 hours.

Day/Night Band (from Suomi/NPP) imagery over Louisiana and East Texas, 0733 UTC on 6 December 2012. 

 The Day/Night band image derived from data from Suomi/NPP from 0733 UTC on 6 December, above, shows the higher and lower clouds over Texas and Louisiana.  The clouds between Houston and Dallas/Ft. Worth are low clouds, but the high clouds over Louisiana inhibit the detection of low clouds.  In addition, although the Day/Night band can give a good outline of where the clouds are, it does not show where the visibility restrictions consistent with IFR conditions are.

Visible Imagery from 2000 UTC

Visible imagery from 2000 UTC, above, shows that, although the fog has lifted, it has not burned off in 5 hours as predicted by the thickness/burn-off time relationship (here).  This may be related to the very low sun angle in December.

(Added:  This image loop shows how thin cirrus can show up in the brightness temperature difference product at night (this example is from Suomi/NPP even when fog-bound valleys are plainly evident in the Day/Night band at the same time!)

Stratus vs Fog

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GOES-R IFR Probabilties over Wisconsin, computed from GOES-East (Upper left), GOES-East brightness temperature difference (10.7 µm – 3.9 µm) (Upper right), GOES-East Visible imagery (0.62 µm) (Lower left), GOES-R Cloud thickness (Lower right)

Stratus and fog look very similar from the satellite’s perspective, both during the day and at night.  That is why it is important to include surface information in a product that detects fog and low stratus.  The IFR Probability product over stratus-bound Wisconsin at mid-day on November 30 2012 shows a diagonal stripe of higher probabilities from southwestern Wisconsin to north-central Wisconsin.  Airports that are reporting IFR or near-IFR conditions are located within this stripe.  Over the rest of the state, where IFR probabilities are lower, the large majority of airports are reporting visibilities and ceilings exceeding IFR limitations.  The character of the visibile satellite data, and of the brightness temperature difference product, gives very little indication that surface visibilities are reduced primarily from northeast Iowa/southwest Wisconsin to north-central Wisconsin.

Continuity through sunrise

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GOES-R IFR Probabilities (Upper Left), computed from GOES-East, Brightness Temperature Difference (10.7 µm – 3.9 µm) computed from GOES-East, GOES-R Cloud Thickness (Lower left), Ceiling and Visibility observations (Lower right)

The animation of the GOES-R IFR Probability product, above, shows one of its strengths:  it has a similar look during night and day.  The traditional fog product created by the brightness temperature difference between 10.7 µm and 3.9 µm data from GOES-East, switches sign as the sun rises and the amount of reflected 3.9 µm radiation increases.  The IFR Probability maintains a steady signal that matches observed IFR conditions.

Note that the Brightness temperature difference product shows a signal along the Louisiana/Texas border (over the Toledo Bend Reservoir on the Sabine River), and also over Lake Sam Rayburn, Lake Livingston and Lake Conroe in east Texas.  It is possible that there is shallow fog over these bodies of water (post-sunrise imagery shows no signal);  the signal might also arise from the approximately 1-pixel co-registration error between the 3.9 µm and 10.7 µm channels on GOES-13. This possibly erroneous signal in the brightness temperature difference does propagate into the GOES-R IFR probability field.

Dense Fog at O’Hare

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GOES-R IFR Probabilities, computed from GOES-East, and surface observations of ceilings and visibilities, hourly from 00:15 UTC to 13:15 UTC on 21 November 2012
Heritage Fog Detection product, that is, brightness temperature difference 10.7 – 3.9, hourly from 00:15 UTC to 13:15 UTC on 21 November 2012

Dense fog has developed over the midwest on one of the busiest travel days of the year.  The GOES-R IFR Probabilities, top loop, above, show the fog initially over the Mississippi River Valley (this fog had actually formed the night before and persisted through the day — link)  and then spreading eastward towards Chicago’s O’Hare Airport.  The animation of GOES-R IFR probabilities and surface observations depicts the widespread nature of the fog.  Compare the fields above to the fields of the brightness temperature difference, the Heritage Fog Detection product, just above.  Several notable differences are obvious.  Note, for example, that at the start of the loop, much of Michigan is covered by a strong brightness temperature difference signal, but IFR conditions are not common.  The GOES-R IFR Probabilities correctly shows low probabilities of reduced visibilities under the elevated stratus deck over Michigan.  There are several upper cloud features that propagate across Illinois during the course of the night.  These features prevent the Heritage Cloud Product from detecting low clouds.  For example — the region over southern Wisconsin at 0515 UTC.  The IFR probabilities decrease in this region (and acquire the characteristically smooth appearance that arises when model predictors predominate in the computation of probabilities), and rebound once the higher clouds move off to the south and east.  The 0515 UTC images for both products are below. 

GOES-R IFR Probabilities from 0515 UTC on 21 November
Heritage Fog Detection from 0515 UTC on 21 November

The Wednesday before Thanksgiving is a very busy travel day through O’Hare, the timing of the fog dissipation is therefore of utmost importance.  The depth of the fog/low stratus at that time just before twilight conditions (when the product is not computed) can be related to dissipation time, as shown in this graph.  The last cloud thickness estimate just before sunrise, below, was around 1000 feet over O’Hare, suggesting that the Fog will burn off about 3 hours after sunrise (13:00Z).

GOES-R Cloud Thickness from 1245 UTC on 21 November

The images below show the fog clearing across the area. Note how the areas where the last cloud thickness estimate before sunrise was thicker (Central Illinois and SW Wisconsin where thicknesses were close to 1200 ft) takes longer to clear than areas where the clouds were thinner (E Wisconsin and NE Illinois around O’Hare where thicknesses were around 1000 ft).

GOES-R IFR probabilities (top left), GOES-R cloud thickness (top right), heritage 3.9-11 micron BTD product (bottom left) and visible satellite image (bottom right) for Nov 21, 2012 at 16:15 UTC. Surface observations of ceiling and visibility are in blue.

In the above image at 16:15 UTC (about 3hrs after sunrise) the fog has cleared near O’Hare, in SE Wisconsin and other areas where pre-sunrise cloud thicknesses were ~1000 ft. In the image below at 18:15 UTC (about 5 hours after sunrise) the fog has cleared everywhere except central Illinois where the largest pre-sunrise thicknesses were estimated.

GOES-R IFR probabilities (top left), GOES-R cloud thickness (top right), heritage 3.9-11 micron BTD product (bottom left) and visible satellite image (bottom right) for Nov 21, 2012 at 18:15 UTC. Surface observations of ceiling and visibility are in blue.
As seen below, by 19:32 UTC (about 6.5 hrs after sunrise) the last small pockets of fog finish burning off. It was noted by a NWS forecaster in Sullivan, WI that a strong low-level inversion was present that took longer to erode than usual. This likely resulted in some of these lingering areas of fog to persist a little longer than was estimated using the last pre-sunrise GOES-R cloud thickness relationship. The complete animation is available here.
GOES-R IFR probabilities (top left), GOES-R cloud thickness (top right), heritage 3.9-11 micron BTD product (bottom left) and visible satellite image (bottom right) for Nov 21, 2012 at 19:32 UTC. Surface observations of ceiling and visibility are in blue.

Fog was considerably less dense at the Lakefront in Chicago.  Various web-cam captures, below, demonstrate the variable denseness of the fog in Chicago, and also in Madison, WI.

North-Facing Webcam, Madison WI, time as indicated.  Source.

North-Facing Webcam from Field Museum in Chicago, ca. 1445 UTC 21 November.  Source.

Kennedy Expressway at Cumberland Ave, ca. 1445 UTC 21 November 2012.  Source.

Dense Fog over the Upper Midwest

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GOES-R IFR Probabilities computed from GOES-East (upper left), GOES-East Traditional Brightness temperature difference, 10.7 µm  – 3.9 µm (upper right), GOES-R Cloud Thickness (lower left), GOES-East Visible (0.63 µm ) imagery (lower right), from approximately 0230 UTC 20 Nov 2012.

Dense fog developed over portions of the upper Mississippi Valley early in the morning on 20 November 2012.  This event forced delayed openings for many eastern Iowa schools.  How well did the GOES-R Fused Data products do for this event?  The animation above shows the quick development of high IFR probabilities over Iowa;  development over northwest Wisconsin, where IFR conditions were observed, was delayed.  Why?  The imagery above, from 0230 UTC, shows IFR probabilities increasing over eastern Iowa where near IFR conditions are already developing.  At 0330 UTC, below, ceilings and visibilities continue to lower over eastern Iowa where IFR Probabilities increase.  Note that the traditional brightness temperature difference field shows a strong signal over Wisconsin and Illinois — but IFR conditions there are not widespread, and IFR probabilities are except over southwest Wisconsin.  The 0330 imagery also hints at higher clouds moving in from the west over southwest Minnesota and western Iowa/eastern Nebraska, where the darker regions in the brightness temperature difference product exists.

GOES-R IFR Probabilities computed from GOES-East (upper left), GOES-East Traditional Brightness temperature difference, 10.7 µm  – 3.9 µm (upper right), GOES-R Cloud Thickness (lower left), GOES-East Visible (0.63 µm ) imagery (lower right), from approximately 0330 UTC 20 Nov 2012.
GOES-R IFR Probabilities computed from GOES-East (upper left), GOES-East Traditional Brightness temperature difference, 10.7 µm  – 3.9 µm (upper right), GOES-R Cloud Thickness (lower left), GOES-East Visible (0.63 µm ) imagery (lower right), from approximately 0530 UTC 20 Nov 2012.

Two hours later, at 0530 UTC, IFR conditions are widespread over eastern Iowa where the traditional brightness temperature difference product has a signal, and where GOES-East-based GOES-R IFR Probabilities are very high.  The Traditional brightness temperature difference signal from GOES-East continues to have a strong signal over central Wisconsin southward into Illinois where, except for southwest Wisconsin, IFR conditions are not common.  Note the development of IFR conditions in northwest Wisconsin, in a region where IFR probabilities are low.  In this region, satellite detection of low clouds is complicated by higher clouds (indicated by the dark region in the brightness temperature difference product).  In addition, low-level relative humidity fields at this time in the Rapid Refresh model are lower than they are over Iowa (see the animation of fields used to compute IFR probabilities at the bottom of this post).

GOES-R IFR Probabilities computed from GOES-East (upper left), GOES-East Traditional Brightness temperature difference, 10.7 µm  – 3.9 µm (upper right), GOES-R Cloud Thickness (lower left), GOES-East Visible (0.63 µm ) imagery (lower right), from approximately 0730 UTC 20 Nov 2012.

At 0730 UTC, above, mid-level clouds over eastern Iowa are having an effect on IFR probabilities there.  Because satellite predictors cannot give a strong indication of fog/low stratus when mid-level (or higher) clouds are present, IFR probabilities will decrease.  This is happening over portions of eastern Iowa where IFR conditions persist.  Probabilities fall, and the field acquires a much smoother look;  in addition, Cloud thickness is not computed.  These occurrences all are a consequence of the presence of higher clouds, as depicted by the darker grey enhancement in the traditional brightness temperature difference field.  Note that IFR probabilities continue to be fairly low over northwest Wisconsin where IFR or near-IFR conditions are present.

GOES-R IFR Probabilities computed from GOES-East (upper left), GOES-East Traditional Brightness temperature difference, 10.7 µm  – 3.9 µm (upper right), GOES-R Cloud Thickness (lower left), GOES-East Visible (0.63 µm ) imagery (lower right), from approximately 0930 UTC 20 Nov 2012.

By 0930 UTC, IFR probabilities finally do increase over northwest Wisconsin where IFR or near-IFR conditions are present (similarly, they increase over eastern Wisconsin near Lake Michigan).  Both the satellite signal and the Rapid Refresh signal have started to suggest low clouds near the surface, as observed.  An animation of fields important to the computation of IFR probabilities is below.  Note how the near-surface relative humidity saturates first over eastern Iowa;  that saturation is slow to spread northward into northwest Wisconsin.

Heritage Fog Algorithm (10.7 µm – 3.9 µm ) from GOES-East (upper left, white = fog/low stratus), GOES-R Cloud Type (upper right), GOES-East 10.7 µm imagery (lower left), Peak Rapid Refresh Model Relative Humidity below 500 m (lower right), hourly from 0432 UTC through 0932 UTC, 20 November 2012.  Data from this site.

The last pre-twilight Cloud Thickness product can be used to guess the dissipation of radiation fog.  Those data are shown below.  Cloud thickness over eastern Iowa and southwest Wisconsin peaks around 1200 feet.  This graph suggests, then, a dissipation time about 5 hours after sunrise.

GOES-R IFR Probabilities computed from GOES-East (upper left), GOES-East Traditional Brightness temperature difference, 10.7 µm  – 3.9 µm (upper right), GOES-R Cloud Thickness (lower left), GOES-East Visible (0.63 µm ) imagery (lower right), from approximately 1300 UTC 20 Nov 2012.
GOES-East Visible Imagery, 1745 UTC

Note how well the low clouds over eastern Iowa at 1745 UTC align with the cloud thickness field at 1300 UTC!

GOES-East visible imagery and station ceilings/visibilities, 2002 UTC

(Update:  The low sun angle of mid- to late-November is making it difficult for the fog to burn off.  As of 2000 UTC, stratus persists)