Author Archives: Scott Lindstrom

Fog along the East Coast

GOES-R IFR Probability Fields computed from GOES-East and Rapid Refresh Data, hourly from 2300 7 May through 1200 8 May, and surface observations of ceilings/visibility (Click to enlarge)

GOES-R IFR Probability Fields showed large values at sunset over the Atlantic Ocean east of New Jersey and the Delmarva Peninsula. As night progressed, that fog penetrated inland. The IFR Probability field accurately depicts the region where visibilities due to fog were reduced. The 0400 UTC image in the animation above (reproduced below), has qualities that highlight the benefit of a fused product. The Ocean to the east of the southern Delmarva peninsula is overlain with multiple cloud layers that make satellite detection of low clouds/fog problematic. In this region, satellite data cannot be used as a predictor and the GOES-R IFR Probability field is a flat field. Because the GOES-R IFR Probability product includes information from the Rapid Refresh model (2-3 hour model forecasts, typically) and because saturation is indicated in the lowest 1000 feet of the model, IFR Probabilities over the ocean are high in a region where satellite data cannot be used as a predictor.

As above, but for 0400 UTC 8 May 2015 only (Click to enlarge)

GOES-R IFR Probabilities can also be computed using MODIS data, which data has better spatial resolution than GOES (1-km vs. nominal 4-km). The toggle below of the MODIS brightness temperature difference and the GOES-R IFR Probability shows a very sharp edge to the expanding fog field over New Jersey.

MODIS Brightness Temperature Difference (11µm – 3.9µm) and MODIS-based GOES-R IFR Probabilities, ~0250 UTC on 8 May, 2015 (Click to enlarge)

The Gulf Stream is apparent in the Brightness Temperature Difference field above and IFR Probabilities are high over the ocean between the coast and the Gulf Stream. In the absence of observations, how much should those high IFR Probabilities be believed. There is high dewpoint air (mid-50s Fahrenheit) along the East Coast at this time, and advection fog could be occurring, for example. Suomi NPP also overflew the region shortly after midnight. The toggle below, of brightness temperature difference and the Day Night Band confirms the presence of (presumably) low clouds over the cold Shelf Water of the mid-Atlantic bight.

Suomi NPP Brightness Temperature Difference (11.35 µm – 3.74 µm) and Suomi NPP Day Night Band Visible Imagery (0.70 µm) at night, 0643 UTC on 8 May, 2015

Dense Fog over the Midwest US

Fog and low clouds developed north of a stationary front draped across the midwest early in the morning on May 6th. Dense fog advisories were hoisted from Iowa to northwestern Ohio.

The animation above shows the increase in IFR Probabilities overnight as the dense fog developed. Note the difference in IFR Probability that arises when satellite data can be used as a predictor (that is, when the developing fog/low stratus is not overlain by higher clouds). Northwest Ohio until about 1200 UTC is a region where low clouds are viewed. There, satellite predictors can be used in the computation of IFR Probability fields. Accordingly, values are larger and there is more small-scale variability (the field looks more pixelated). In contrast, the field over Iowa for much of the animation is relatively flat. Here, even through values are comparatively low, interpret them knowing that satellite predictors are not being used because of the presence of middle/higher clouds that preclude the ability of satellite detection of low clouds.

An animation of the traditional brightness temperature difference field, here, from 0500-1000 UTC (after 1100 UTC, increasing reflected solar radiation makes the brightness temperature difference field less useful as a fog/stratus detection device). Compare the regions where IFR Probabilities are largest with the regions of strong Brightness Temperature Difference Signals.

IFR Probability fields above define the region of reduced visibilities very well. This suggests the Rapid Refresh model and the satellite (where its use was possible) were both in accord with the development of fog/low stratus in this region of the country.

The 1115 UTC image in the animation above, shown below, includes the day/night boundary artifact. This is the straight line, roughly parallel to the terminator (it will stretch directly north-south on the Equinoxes), that parallels the Lake Michigan shoreline at Chicago. To the right, daytime predictors are used and IFR Probabilities are somewhat larger (67% vs. 51%) than they are where nighttime predictors are used to the west.

Fog over Nebraska under high clouds

GOES-R IFR Probabilities, 0315-1215 UTC, 5 May 2015 (Click to enlarge)

Dense Fog developed over the Hastings, Nebraska WFO overnight, leading to the issuance of Dense Fog Advisories. The GOES-R IFR Probabilities, above, show a steady increase in probabilities over the night as the fog develops. The relatively flat nature of the IFR Probability field is characteristic of GOES-R IFR Probabilities that do not include information from satellite (that is, only model fields are being used here to educe IFR probabilities). IFR Probability fields are a fused product, typically blending information from model fields and from satellite data. However, this was a case of fog developing under an extensive cirrus shield so that satellite data were not used as a predictors. The 10.7µm – 3.9µm Brightness Temperature Difference field, shown below, gives no information about surface conditions. That the IFR Probability fields neatly overlap the region of developing IFR conditions is testimony to the accuracy of the model field in simulating the lower part of the troposphere.

When only model data are used, as above, features in the field that are parallel to surface topography contours can become evident in the GOES-R IFR Probability fields. This is related to interpolation of the lowest 1000 feet of model relative humidity fields (moisture information that is used as a predictor in the computation of the IFR Probability) in regions of sloping topography.

In the animation above, note that the IFR Probabilities increase in the final frame. Over most of the scene, at 1215 UTC, the sun has risen and Daytime Predictors are being used to compute IFR Probabilities. (The dividing line between Daytime — to the east — and nighttime — to the west — is visible stretching north-northwest to south-southeast from the extreme northeast corner of Colorado). IFR Probabilities are somewhat higher during the day (compared to night) because visible imagery is incorporated into the satellite predictors; more accurate cloud clearing means that IFR Probabilities increase just a bit.

GOES-13 Brightness temperature difference fields (10.7 µm – 3.9 µm) over the Great Plains, 0630-1300 UTC, 5 May 2015 (Click to enlarge)

Suomi NPP overflew Nebraska, giving a view of the extensive cirrus shield. The Day Night Band gave crisp imagery as the Moon was very nearly full.

Suomi NPP Day Night Band Visible Imagery (0.70 µm) at 0740 UTC on 5 May 2015 (Click to enlarge)

Short Training Video

The video below is a short (11-minute) description of IFR Probability.

Alternatively, click the Link.

Dense Fog over the Red River of the North

Dense Fog advisories were issued by the National Weather Service in Grand Forks as visibilities in the WFO dropped to near zero. How did the IFR Probability Fields and traditional Brightness Temperature Difference Fields capture this event? The animation below shows the brightness temperature difference field (10.7 µm – 3.9 µm) from GOES-13. Initially, a swath of mid-level and upper-level clouds covered the Red River Valley (this system had produced very light rains on Monday the 27th), but the clouds moved east and dense fog quickly developed (Cavalier, ND, for example, showed reduced visibility already at 0400 UTC).

GOES-13 Brightness Temperature Difference (10.7 µm – 3.9 µm) hourly from 0315 to 1115 UTC on 28 April 2015, along with surface plots of ceilings and visibility (Click to enlarge)

The IFR Probability fields for the same time, below, better capture the horizontal extent of the fog. For example, the strong signal in the Brightness Temperature Difference field over South Dakota at the end of the animation, above, is not present in the IFR Probability fields. IFR Conditions are not occurring over South Dakota. The good match between the developing IFR Probability fields and the developing fog testifies to the satellite view of the fog and the accurate simulation of this event by the Rapid Refresh model.

GOES-R IFR Probability Fields hourly from 0315 to 1115 UTC on 28 April 2015, along with surface plots of ceilings and visibility (Click to enlarge)

Geostationary GOES fields give good temporal resolution to the evolving field. Polar orbiting satellites, such as Suomi NPP (carrying the VIIRS instrument) and Terra/Aqua (each carrying MODIS) each gave snapshot views of the developing fog. At 0355, IFR Probabilities are low, and the Red River valley is mostly obscured by higher clouds. Four hours later, at 0805 UTC, dense fog has developed and IFR probabilities are large.

Terra MODIS Brightness Temperature Difference (11µm – 3.9µm) and IFR Probability fields, ~0355 UTC on 28 April 2015 (Click to enlarge)

Aqua MODIS Brightness Temperature Difference (11µm – 3.9µm) and IFR Probability fields, ~0805 UTC on 28 April 2015 (Click to enlarge)

Suomi NPP also viewed the fog field. The toggle between the Day Night Band and the Brightness Temperature Difference field (11.45µm – 3.74µm), below, shows evidence of fog in the visible Day Night band imagery. The lights of western North Dakota’s oil shale fields are also evident.

Polar-orbiting satellites give excellent high-resolution imagery of fog fields. When used in concert with the excellent time resolution of GOES imagery, a complete picture of the evolving fog field can be drawn.

Toggle between Day Night Band (0.70 µm) and Brightness Temperature Difference (11.45µm – 3.74µm) field from VIIRS on Suomi NPP at 0815 UTC (Click to enlarge)

Fog along Lake Erie

GOES-R IFR Probabilities, 0800 through 1400 UTC on 27 April 2015 (Click to enlarge)

Fog developed along Lake Erie after sunrise on Monday 27 April 2015. The fog and low ceilings were associated with a line of light showers, so multiple cloud layers were present. These layers inhibit satellite detection of fog/low stratus. GOES-R IFR Probabilities, above, computed using GOES-13 Satellite data and Rapid Refresh Model Output show very low probabilities at 0800 and 1015 UTC (stratus clouds are observed); at 1215 UTC, IFR Probabilities increase in the counties adjacent to Lake Erie in Pennsylvania and New York; at 1315 and 1400 UTC, IFR Probabilities are high, and IFR conditions are observed in both Erie PA and Dunkirk NY.

GOES-13 Brightness Temperature Difference Fields, 0400 through 1000 UTC on 27 April 2015 (Click to enlarge)

Brightness Temperature Difference fields overnight, above, showed evidence of water-based clouds over much of the area. The fields are moving south, however, leaving the lakeshore behind. The toggle below is of Brightness Temperature Difference and IFR Probability from 1215 UTC. It is far more difficult to relate features in the brightness temperature difference field with reductions in observations at the surface than it is to relate IFR Probability fields with surface observations. Note also that the character of the brightness temperature difference field below has changed because reflected solar radiation at 3.9 µm has become important.

Toggle between GOES-R IFR Probability fields and GOES East Brightness Temperature Difference Fields at 1215 UTC on 27 April 2015 (Click to enlarge)

Fog over Indiana

GOES-R IFR Probabilities over Indiana and surrounding states, 0200-1215 UTC on 17 April (Click to enlarge)

For developed over Indiana and surrounding states during the morning of April 17th. An hourly animation of GOES-R IFR Probabilities, from 0200 through 1215 UTC, computed from GOES-East and Rapid Refresh Data is shown above. Fog is developing at 0200 UTC, already over portions of western Indiana, and IFR Probabilities increase quickly. By 0700 UTC, large regions show reduced visibilities and IFR Probabilities exceeding 85%.

High clouds moving in from the west and southwest starting at about 0800 UTC have an impact on the IFR Probability fields as well. Only Rapid Refresh Data are used to compute IFR Probabilities where mid-level and high clouds prevent satellite detection of low clouds. As a result, the character of the field changes: it becomes flatter (less pixelated) and values decrease (because probability is not so certain when satellite data cannot be used to validate Model predictions).

The final image in the animation above, at 1215 UTC, was computed just after sunrise. Note that IFR Probability values generally increase. This is especially notable in regions where mid-level and high-level clouds are present (over southern Illinois and southern Indiana). Probabilities are higher because the satellite can detect clouds are present. There are also regions at 1215 UTC where IFR Probabilities rapidly drops to zero. This is likely a difficulty in the Cloud Typing algorithm that occurs with very low sun angle (as discussed here). Holes at 1215 UTC have filled in by 1300 UTC.

Cloud thickness can give a first estimate of cloud dissipation time. This link shows a scatterplot with a best-fit line that relates dissipation time to Cloud Thickness from a past study. The Cloud Thickness used is the final one computed before twilight conditions, and that is shown below. (Note that cloud thickness is not computed in regions where mid-level and high-level clouds exist) Much of the fog over Indiana is relatively thin — less than 800 feet thick — with a few regions that exceed 1000 feet. Burn-off of this fog should be relatively quick, and most of the Dense Fog Advisories expired at 1400 UTC.

GOES-R Cloud Thickness just before sunrise, 17 April 2015 (Click to enlarge)

URGENT - WEATHER MESSAGE
NATIONAL WEATHER SERVICE NORTHERN INDIANA
600 AM EDT FRI APR 17 2015

INZ020-022-023-032>034-171400-
/O.NEW.KIWX.FG.Y.0005.150417T1000Z-150417T1400Z/
WHITE-CASS IN-MIAMI-GRANT-BLACKFORD-JAY-
INCLUDING THE CITIES OF...MONTICELLO...BROOKSTON...MONON...
LOGANSPORT...ROYAL CENTER...PERU...GRISSOM AFB...MEXICO...
MARION...GAS CITY...UPLAND...HARTFORD CITY...MONTPELIER...
PORTLAND...DUNKIRK
600 AM EDT FRI APR 17 2015

...DENSE FOG ADVISORY IN EFFECT UNTIL 10 AM EDT THIS MORNING...

THE NATIONAL WEATHER SERVICE IN NORTHERN INDIANA HAS ISSUED A
DENSE FOG ADVISORY...WHICH IS IN EFFECT UNTIL 10 AM EDT THIS
MORNING.

* VISIBILITY: A QUARTER MILE OR LESS.

* IMPACTS: VERY HAZARDOUS DRIVING CONDITIONS...WHICH WILL LEAD TO
  TRAVEL DELAYS. PLEASE LEAVE EARLY IF TRAVELING THIS MORNING AND
  ALLOW EXTRA TIME TO REACH YOUR DESTINATION.

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.

GOES and MODIS IFR Probabilities over Alaska

GOES-R IFR Probabilities computed from MODIS and from GOES-15, both at ~0700 UTC on 14 April 2015 (Click to enlarge)

At very high latitudes, limb effects can alter the brightness temperature difference between 10.7 µm and 3.9 µm. GOES also has very large pixel sizes at high latitudes. The image above toggles between the GOES-R IFR Probability computed using MODIS and GOES-15. Observations — scant over Alaska — of ceilings and observations are superimposed on the imagery. GOES-based GOES-R IFR Probabilities are elevated over much of Alaska; in contrast, MODIS-based IFR Probabilities show larger values in only a few regions.

MODIS-based values at high latitudes are available frequently compared to lower latitudes. At 0830/0845 UTC, below, MODIS data show a slow expansion in values (note that eastern Alaska was not viewed by MODIS at this time). At about 1100 UTC, the slow areal increase in IFR Probabilities continues.

GOES-R IFR Probabilities computed from MODIS and from GOES-15, both at ~0830 UTC on 14 April 2015 (Click to enlarge)

GOES-R IFR Probabilities computed from MODIS and from GOES-15, both at ~1100 UTC on 14 April 2015 (Click to enlarge)

The 1100 UTC MODIS pass was over only eastern Alaska, and it shows relatively large values in some spots of northeastern Alaska. The high values from the GOES-based GOES-R IFR Probabilities over central Alaska can probably be discounted. Note, however, that the highest GOES-based GOES-R IFR Probabilities do have a counterpart in the MODIS-based field.

At 1400 UTC, no MODIS pass was available. The GOES-based image, below, again has large values over northern Alaska (with corroborating surface observations at Point Lay (where snow is falling) and at Atqasuk (where freezing fog is reported). MODIS-based data from earlier in the day adds confidence to the discounting of widespread modest (40-50%) IFR Probability values over central Alaska.

GOES-R IFR Probabilities computed from GOES-15 at 1400 UTC on 14 April 2015 (Click to enlarge)

Use moving IFR Probability Fields as a forecast aid

GOES-R IFR Probabilities, hourly from 0200 through 1300 UTC on 7 April 2015 (Click to enlarge)

Denver International Airport had a period of restricted visibility during the morning of 7 April, starting around 0830 UTC, when northeast winds ushered in low ceilings and reduced visibilities. High Probabilities in the IFR Probability fields shift west and south with time, demonstrating how the fields can be used to anticipate the development of IFR conditions.

Low Clouds and Fog over Oklahoma and Arkansas

Brightness temperature Difference field (10.7µm – 3.9µm) from GOES East over the southern Plains, 1000 UTC on 6 April 2015 (Click to enlarge)

The heritage, traditional method for detecting fog and low stratus is the brightness temperature difference product, seen above (with an enhancement) at 1000 UTC on 6 April. It is difficult to discern a difference in the field over regions where IFR conditions are reported versus regions where IFR conditions are absent. In contrast, the IFR Probability Field from the same time, below, neatly outlines the regions of IFR conditions, and, importantly, does not highlight regions — such as Fort Smith AR and Poteau, OK — where IFR conditions are not present.

GOES-R IFR Probability Fields, 1000 UTC on 6 April 2015 UTC (Click to enlarge)

GOES-R Fog Product Examples

Fog detection fusing GOES, Terra/Aqua or Suomi/NPP Satellite data with Model output