Category Archives: Plains

Persistent fog and freezing fog over New Mexico and Texas

GOES-R IFR Probability fields, 0415-1745 UTC on 30 December 2015 (Click to enlarge)

Fog with sub-freezing temperatures developed over New Mexico and west Texas early on the 30th of December, and persisted into mid-day. How well did conventional (and newer) algorithms designed to detect fog perform? The GOES-R IFR Probability fields, above, hourly from 0415 UTC through 1745 UTC on 30 December, show highest IFR Probabilities initially along the Pecos River in New Mexico. Airports at both Artesia and Roswell reported IFR conditions continuously during the period shown. IFR Conditions developed over Texas west of a line from about Breckenridge (in Stephens County) to Vernon Texas (in Wilbarger County). The southern extent of the ice fog was near a Midland (in Midland County) to Coleman (in Coleman County) line. High IFR Probabilities were common over Texas where the Fog/Freezing Fog was occurring.

The Brightness Temperature Difference field for the same times are shown below. The Brightness Temperature Difference field captures the presence of water-based clouds along the Pecos River in New Mexico — both at night (orange enhancement) and during the day (black enhancement). The Brightness Temperature Difference field tells you something about the top of the cloud only, however; it cannot give information about the cloud base. (In contrast, the GOES-R IFR Probability product, because it fuses satellite data with surface information derived from Rapid Refresh Model output, a distinction between mid-level stratus and low fog is possible). In addition, there are regions in the brightness temperature difference field where no strong signal occurs even though fog is present (Hobbs, NM in Lea County and Seminole TX in Gaines County, for example).

GOES-13 Brightness Temperature Difference Fields (10.7 µm – 3.9 µm), 0415-1745 UTC on 30 December 2015 (Click to enlarge)

High Cirrus over west Texas and Fog on the Ground

GOES-based GOES-R IFR Probabilities and GOES-13 Brightness Temperature Differences (10.7 µm – 3.9 µm), 1100 UTC on 30 November 2015 (Click to enlarge)

Extratropical cyclones are accompanied most time with multiple clouds levels. In the example above, a cirrus shield accompanies a subtropical jet over the southern boundary of the United States. That cirrus prevents the satellite from seeing any low clouds that may be present. To detect/diagnose low clouds and fog in such regions, other data sets must be used. For GOES-R IFR Probability fields (shown in the toggle above), that other data set is low-level saturation in the Rapid Refresh model. If the model suggests saturation is present, IFR Probability fields will show a strong signal even where cirrus shields prevent the satellite from viewing water-based clouds near the surface. That is the case above. IFR conditions are widespread in the scene above are are diagnosed quite well by the IFR Probability fields.

Fog over Kansas

GOES-R IFR Probabilities (Upper left, computed from GOES-13 and lower left, computed from GOES-15), GOES-13 Brightness Temperature Difference fields (10.7 µm – 3.9 µm, Upper right) and GOES-R Cloud Thickness (Lower Right), hourly from 0500 through 1300 UTC 10 November 2015 (Click to enlarge)

Dense Fog Advisories were hoisted over western Kansas in response to the development of dense fog in the early morning on 10 November 2015. The hourly evolution of GOES-R IFR Probabilities (top left, bottom left), GOES-13 Brightness Temperature Difference fields (10.7 µm – 3.9 µm, upper right) and GOES-R Cloud Thickness fields (Bottom right) are shown above.

Both IFR Probability fields show highest probabilities over western Kansas where observations report IFR conditions. Portions of northern, central and eastern Kansas have strong returns in the GOES-13 Brightness Temperature Difference field, but IFR Probabilities are small and IFR conditions are not reported. In these regions, Rapid Refresh Model model output does not show low-level saturation and the IFR Probability algorithm correctly recognizes that IFR conditions are not likely despite the presence of stratus. It is the fusing of data in this way that gives the GOES-R IFR Probability field superior statistics (compared to the Brightness Temperature Difference field) in fog/low cloud detection.

Maximum cloud depth observed is more then 1200 feet. This suggests a long time before fog dissipation unless winds increase over Kansas.

GOES-R IFR Probabilities based on GOES-West show somewhat higher values than those based on GOES-East data; this difference arises because of the very oblique view of Kansas from GOES-15.

Widespread Fog over the central United States

GOES-R IFR Probability Field and GOES-13 Brightness Temperature Difference (10.7µm – 3.9µm) at 1145 UTC on 3 November 2015. Surface-based observations of ceilings and visibilities are plotted (Click to enlarge)

Fog was widespread over the central United States on the morning of 3 November, and Dense Fog Advisories were commonplace. The toggle above compares the Brightness Temperature Difference field (10.7 µm – 3.9 µm) with the GOES-R IFR Probability field at 1145 UTC on 3 November. (This toggle is much faster) IFR Probabilities use near-surface information in the Rapid Refresh Model to screen out regions where Brightness Temperature Difference signals are showing elevated stratus rather than fog (For example, the region around Columbus MS). IFR Probabilities are also low in regions with a modest brightness temperature difference return (most of eastern Nebraska, west Texas, southern Indiana along the Ohio River); these regions do not include stations observing IFR Conditions.

Benefit of fused data over Oklahoma

GOES-R IFR Probabilities (Upper Left), GOES-R Cloud Thickness (Lower Left), GOES-13 Brightness Temperature Difference (10.7 µm – 3.9 µm) (Upper Right), GOES-13 Water Vapor Infrared Imagery (6.5 µm) (Lower Right), all at 1100 UTC on 28 October 2015 (Click to enlarge)

Fused Data is beneficial in the detection of low ceilings and reduced visibilities. Consider the region in northeastern Oklahoma where cirrus has overspread low clouds and dense fog is reported on the ground. The satellite view of the low clouds is obscured by the cirrus; the brightness temperature difference signal is not one associated with fog/low stratus. However, GOES-R IFR Probability fields maintain a signal because Rapid Refresh model output is used; saturation is occurring in the lowest 1000 feet of the model over eastern OK, and the GOES-R IFR Probability is larger there as a result. (Dense Fog Advisories were issued for this event).

IFR Probability vs. Brightness Temperature Difference

GOES-R IFR Probability and GOES-13 Brightness Temperature Difference (10.7 µm – 3.9 µm) at 1000 UTC on 21 September 2015 (Click to enlarge)

The toggle above compares GOES-R IFR Probability fields and GOES-13 Brightness Temperature Difference fields at 1000 UTC on 21 September 2015. A shortcoming of Brightness Temperature Difference fields, or indeed of any low cloud detection algorithm that relies solely on cloud-top measurements, is that low stratus that does not reduce visibility and fog that does reduce visibility can look very similar from the cloud top. By incorporating surface and near-surface moisture information from the Rapid Refresh Model, the GOES-R IFR Probability algorithm can correctly screen out regions of stratus and highlight only those regions where fog and low stratus might affect transporation. In the example above, Guymon OK and Lamar CO are at the outer edge of the highest IFR Probability, and both report IFR conditions. South and west of those stations, IFR Probabilities drop quickly, but brightness temperature difference signals remain strong.

Dense Fog over Kansas

Screen Captures of NWS Pages at Dodge City (left) and Wichita (Right), 0700 CST 31 August 2015 (click to enlarge)

Dense Fog Advisories were issued by both Dodge City and Wichita WFOs on the morning of 31 August 2015. GOES-R IFR Probabilities in this case did a better job of outlining where IFR conditions were occurring, distinguishing those regions from regions of low stratus. Each of the paired images below shows Brightness Temperature Difference fields on top and IFR Probability fields on bottom, for four different times during the night (0530, 0715, 0915, 1115 UTC). IFR Probability fields show a signal that is confined mostly to regions where IFR conditions are developing (for the earlier times) or observed (at 0915 and 1115 UTC). Thus, IFR Probability in this case refines the brightness temperature difference signal, highlighting regions only where low clouds/fog are present (eastern Kansas), rather than regions where stratus clouds are present (western Kansas). The incorporation of near-surface saturation as predicted in the Rapid Refresh model is key to screening out regions of mid-level stratus when low stratus and fog are the more important field.

GOES-13 Brightness Temperature Difference (10.7µm – 3.9µm) (top) and GOES-R IFR Probabilities (bottom), 0530 UTC 31 August 2015 (Click to enlarge)

GOES-13 Brightness Temperature Difference (10.7µm – 3.9µm) (top) and GOES-R IFR Probabilities (bottom), 0715 UTC 31 August 2015 (Click to enlarge)

GOES-13 Brightness Temperature Difference (10.7µm – 3.9µm) (top) and GOES-R IFR Probabilities (bottom), 0915 UTC 31 August 2015 (Click to enlarge)

GOES-13 Brightness Temperature Difference (10.7µm – 3.9µm) (top) and GOES-R IFR Probabilities (bottom), 1115 UTC 31 August 2015 (Click to enlarge)

When the sun rises, solar radiation with a wavelength of 3.9 µm will alter the brightness temperature difference field. At night, a water-based cloud will not emit 3.9 µm radiation as a blackbody and be perceived as colder (compared to 10.7 µm). During the day, the relatively large amount of 3.9 µm radiation from the sun reflected off the cloud will make the cloud appear to be warmer (compared to 10.7 µm). Thus the signal in the brightness temperature difference field flips. In contrast, the IFR Probability field signal is not significantly perturbed by sunrise. The 1315 UTC image, below, is an example of this. The GOES-R IFR probability field also benefits from better cloud clearing during the day.

GOES-13 Brightness Temperature Difference (10.7µm – 3.9µm) (top) and GOES-R IFR Probabilities (bottom), 1315 UTC 31 August 2015 (Click to enlarge)

Dense Fog over central Nebraska

GOES-R IFR Probability (Upper Left), GOES-R Low IFR Probability (Lower Left), GOES-13 Brightness Temperature Difference Field (10.7 µm – 3.9 µm) (Upper Right), GOES-R Cloud Thickness (Lower Right), Times as Indicated (Click to enlarge)

GOES-R IFR Probability fields, above, suggest IFR conditions are developing over central Nebraska during the early morning of 18 August 2015. Several aspects of the field require comment. At the beginning of the animation, there are noticeable north-south oriented lines in the IFR and Low-IFR Probability fields. These are artifacts of determining the saturation in the lowest 1000 feet of the model as discussed here. As the model saturation deepened over the course of the night, those parallel lines disappeared.

The flat nature of the field announces that satellite data are not being used in the computation of IFR Probability because multiple cloud layers are present. Model fields lack the fine spatial resolution of GOES Satellite data. Where breaks in the clouds do occur, GOES-R IFR Probability fields increase in value (because cloud predictors can be used, and their use enhances the ability of the algorithm to predict whether fog/low clouds are present): Temper your interpretation of the magnitude of the IFR Probability with knowledge of presence of clouds.

Note that the Brightness Temperature Difference field at 0500 UTC has much larger values. Stray Light is beginning to impinge on satellite fields as the calendar gets closer to the Equinox. This issue will remain through mid-October. Multiple cloud decks over Nebraska on 18 August greatly hampered the ability of the brightness temperature difference field to identify regions of low clouds.

The GOES-R Cloud Thickness fields are displayed; these are derived from an empirical relationship between 3.9 µm emmissivity and cloud thickness derived from SODAR data off the west coast of the United States. When multiple cloud layers are present, this field is not computed (nor is it displayed around sunrise/sunset). Thus, you should not see Cloud Thickness fields in regions where GOES-R IFR Probability fields are very smooth (suggesting that only model data are being used). The regions where Cloud Thickness is displayed, above, correspond to regions in the IFR Probability field that are pixelated (that is, where satellite data are being used).

Fog formation is aided by increasingly long nights. Nights now are above 90 minutes longer in Hastings than they were at the Summer Solstice.

The text for the advisory is below:

URGENT - WEATHER MESSAGE
NATIONAL WEATHER SERVICE HASTINGS NE
608 AM CDT TUE AUG 18 2015

...DENSE FOG PERSISTING FOR A FEW HOURS THIS MORNING...

NEZ048-049-061>064-073>077-181400-
/O.NEW.KGID.FG.Y.0006.150818T1108Z-150818T1400Z/
MERRICK-POLK-BUFFALO-HALL-HAMILTON-YORK-PHELPS-KEARNEY-ADAMS-CLAY-
FILLMORE-
INCLUDING THE CITIES OF...CENTRAL CITY...STROMSBURG...OSCEOLA...
SHELBY...POLK...KEARNEY...GRAND ISLAND...AURORA...YORK...
HOLDREGE...MINDEN...HASTINGS...SUTTON...HARVARD...CLAY CENTER...
EDGAR...FAIRFIELD...GENEVA...EXETER...FAIRMONT
608 AM CDT TUE AUG 18 2015

...DENSE FOG ADVISORY IN EFFECT UNTIL 9 AM CDT THIS MORNING...

THE NATIONAL WEATHER SERVICE IN HASTINGS HAS ISSUED A DENSE FOG
ADVISORY...WHICH IS IN EFFECT UNTIL 9 AM CDT THIS MORNING.

* VISIBILITY...REDUCED TO 1/4 MILE OR LESS ACROSS MUCH OF THE
  ADVISORY AREA...INCLUDING THE INTERSTATE 80 AND HIGHWAY 6
  CORRIDORS. THE COMBINATION OF RAIN MOVING INTO PARTS OF THE
  AREA...ALONG WITH A COLD FRONT MOVING IN FROM THE WEST...SHOULD
  HELP TO GRADUALLY IMPROVE DENSE FOG ISSUES.

* IMPACTS...DENSE FOG WILL GREATLY AFFECT THE MORNING COMMUTE IN
  SOME AREAS...AND MOTORISTS ARE URGED TO DRIVE WITH CAUTION AND
  PREPARE FOR RAPID CHANGES IN VISIBILITY OVER SHORT DISTANCES.

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.

&&

$$

PFANNKUCH

Dense Fog over Kansas and Nebraska

GOESR_IFRPROB_05August2015_0400_1300anim

GOES-R IFR Probability fields computed from GOES-13 and Rapid Refresh data, hourly from 0400 through 1315 UTC, 5 August 2015 (Click to enlarge)

Dense fog developed over the High Plains of Kansas and Nebraska overnight on 4-5 August 2015, and Advisories were issued by the North Platte, NE, Goodland KS and Dodge City KS WFOs. The animation above, of GOES-R IFR Probability fields computed from GOES-13 data, shows a slow expansion in the area of highest probabilities. The IFR Probability field has less spatial variability over eastern Kansas and eastern Nebraska where strong convection prevented the satellite from detecting low clouds; there, only model data predictors (from model fields that vary smoothly) could be used in the computation of the GOES-R IFR Probability fields and the IFR Probability field therefore has a smoother look.

GOES-13 Brightness Temperature Difference Fields (10.7 µm – 3.9 µm) at 0700, 0915 and 1100 UTC on 5 August 2015 (click to enlarge)

The GOES-13 Brightness Temperature Difference field (10.7 µm – 3.9 µm), above, at 0700, 0915 and 1100 UTC, similarly shows an expansion in the detection of water-based clouds over the High Plains. However, the field overpredicts the region of IFR conditions. The toggle between 1100 UTC IFR Probability and Brightness Temperature Difference, below, highlights how the IFR Probability can screen out regions (Southwestern Kansas, eastern Colorado) where low clouds are present, but IFR Conditions may not be. The toggle also shows how GOES-R IFR Probability can give information in regions where the Brightness Temperature Difference field has a signal for high clouds only (that is, under the convection in eastern Kansas)

Toggle between GOES-R IFR Probability and GOES-13 Brightness Temperature Difference (10.7 µm – 3.9 µm), 1100 UTC, 5 August 2015 (Click to enlarge)

Suomi NPP overflew Kansas around 0800 UTC on 5 August, and the Day Night Band imagery, below, showed both the strong convection (complete with streaks associated with lightning) and the developing low clouds. Brightness Temperature Difference (11.45 µm – 3.74 µm) fields from Suomi NPP (Link) confirm the presence of water-based clouds (yellow and orange in the enhancement used). The strong convection over eastern Kansas has multiple overshooting tops still at 0854 UTC (Link).

Suomi NPP Day Night Visible (0.70 µm) band, 0854 UTC 5 August 2015 (Click to enlarge)

Lead Time with GOES-R IFR Probabilities and Brightness Temperature Difference

A small region of dense fog developed over northeast Colorado and western Nebraska during the early morning on June 1st 2015. How did the GOES-R and traditional products handle this event? The animation below shows IFR Probabilities from 0730-0800 UTC on 1 June. Probabilities jump from <10% to about 20% at 0800 UTC in a region centered on Holyoke, CO, just south of I-76 in northeast Colorado. The Brightness Temperature Difference Field for the same 3 times, below the IFR Probabilities, shows a signal moving over the region but not substantially changing. (From this, one could conclude that the Rapid Refresh model data might be driving increase in the IFR Probability field)

GOES-R IFR Probabilities, 0730-0800 UTC on 1 June (Click to enlarge)

GOES-East Brightness Temperature Difference fields (10.7 µm – 3.9 µm) at 0730, 0745 and 0800 UTC, 1 June 2015 (Click to enlarge)

Toggle between GOES-R IFR Probabilities and GOES-East Brightness Temperature Difference Fields, 0915 UTC on 1 June (Click to enlarge)

By 0915 UTC (above), IFR Probabilities and GOES-13 Brightness Temperature Difference fields show a strong signal over NE Colorado where IFR Conditions occur/are developing. IFR Probability fields have provided more lead-time in the development of this region of low ceilings and visibilities. By 1100 UTC (below), a stronger, more widespread signal is apparent in both fields. At 1230 UTC (bottom), the rising sun has altered the brightness temperature field so it gives no useful information on low clouds; this highlights an advantage of GOES-R IFR Probability fields: A consistent signal through sunrise.

Toggle between GOES-R IFR Probabilities and GOES-East Brightness Temperature Difference Fields, 1100 UTC on 1 June (Click to enlarge)

Toggle between GOES-R IFR Probabilities and GOES-East Brightness Temperature Difference Fields, 1230 UTC on 1 June (Click to enlarge)

GOES-R Fog Product Examples

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