Category Archives: Deep South

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.

Fog over the lower Mississippi River Valley

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GOES-East IFR Probabilities (upper left), GOES-East Traditional Brightness Temperature Difference (10.7 µm – 3.9 µm) (upper left), VIIRS Day/Night Band from Suomi/NPP (lower left), VIIRS Brightness Temperature Difference (10.80 µm – 3.74 µm) from Suomi/NPP (lower right) at 0315 UTC on 6 November 2012

IFR Conditions developed over the deep south overnight on the 5th/6th of November.  How did the GOES-R IFR Probabilities capture this event, and how do the fields compare to the traditional brightness temperature difference fields?  At 0315 UTC, near-IFR conditions have developed over central Mississippi and over southwest Missouri, the two regions where IFR probabilities are diagnosed to be highest.  Rapid Refresh Model data are appropriately de-emphasizing the satellite signal in regions where IFR conditions are not reported (northeast Arkansas, for example).

Four hours later (below), at 0715 UTC, the area of IFR conditions over Mississippi has expanded somewhat, and the IFR Probability field continues to suggest — strongly — that IFR conditions are present.  Both the traditional brightness temperature difference field and the IFR probability field suggest a sharp western edge to the fog/low stratus over north-central Louisiana, and that sharp edge is confirmed in the Day/Night band from VIIRS on Suomi/NPP.  The Brightness Temperature Difference fields from both GOES and from VIIRS suggest one large field of fog, but the IFR probability field has two separate fields:  one over Mississippi/extreme southern Arkansas and Louisiana, and one over southern Missouri.

GOES-East IFR Probabilities (upper left), GOES-East Traditional Brightness Temperature Difference (10.7 µm – 3.9 µm) (upper left), VIIRS Day/Night Band from Suomi/NPP (lower left), VIIRS Brightness Temperature Difference (10.80 µm – 3.74 µm) from Suomi/NPP (lower right) at approximately 0715 UTC on 6 November 2012

Fog on Texas and Louisiana Gulf Coasts

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Fog was anticipated to develop over the coastal sections of Texas and Louisiana starting late on Oct 31 2012.  From the 0149 UTC 1 November Houston Forecast Discussion:    The 0921 UTC Forecast Discussion from Lake Charles (above) describes increasing fog possibilities — and the 0453 UTC AFD (below) mentions patchy fog.

For both WFOs, the GOES-R IFR Probability field shows a good picture of the evolving fog/low stratus as it develops.

000
FXUS64 KHGX 010149
AFDHGX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HOUSTON/GALVESTON TX
849 PM CDT WED OCT 31 2012

.DISCUSSION...
CURRENT FCST IS ON TRACK. ONLY TWEAKS TO GRIDS WERE TO MOVE UP
TIMING OF FOG FORMATION. WOULDN`T DOUBT IF A DENSE FOG ADVSY
MIGHT BE REQUIRED FOR SOME LOCATIONS...ESP SW. WILL KEEP AN EYE ON
TRENDS. DIFFUSE WIND SHIFT AND SLIGHTLY LOWER DEWPOINTS WILL
PROBABLY MOVE INTO NE ZONES LATER TONIGHT THEN STALL/WASHOUT.
 

 
000
FXUS64 KLCH 010921
AFDLCH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAKE CHARLES LA
421 AM CDT THU NOV 1 2012

.DISCUSSION...TRAPPED LOW LEVEL MOISTURE AND CLEAR SKIES HAVE
ALLOWED AREAS OF FOG TO DEVELOP THIS MORNING ACROSS THE CWA. AT THIS
TIME PATCHY DENSE FOG HAS ALSO DEVELOPED IN CALCASIEU PARISH AND
JEFFERSON COUNTY. VISIBILITIES ARE SLOWLY DROPPING ELSEWHERE AND
IF CONDITIONS CONTINUE TO DETERIORATE A DENSE FOG ADV MAY BE NEEDED
THIS MORNING. 



The 0921 UTC Forecast Discussion from Lake Charles (above) describes increasing fog possibilities — and the 0453 UTC AFD (below) mentions patchy fog.

  

 
 
000
FXUS64 KLCH 010453
AFDLCH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAKE CHARLES LA
1153 PM CDT WED OCT 31 2012

.DISCUSSION...
01/06Z TAF ISSUANCE.

&&

.AVIATION...
FEW CHANGES TO THE TAFS THIS EVENING WITH WINDS NEARLY CALM ACRS
THE AREA. T/TD SPREAD NARROWING AT BPT AND LCH AND COULD SEE SOME
PATCHY FOG DEVELOP AT THESE SITES WITHIN THE NEXT HOUR OR TWO.
CANNOT RULE OUT FOG AT OTHER TAF SITES...BUT EXPECT ONSET A LITTLE
LATER AS DEWPOINT DEPRESSIONS ARE SLIGHTLY LARGER. VFR CONDITIONS
EXPECTED TO PREVAIL WITH THE EXCEPTION OF PERIODIC MVFR OR BRIEF
IFR VISBYS BETWEEN NOW AND 14Z. LT WINDS WILL GRADUALLY BECOME
SWLY THURS AFTN. 24 


For both WFOs, the GOES-R IFR Probability field shows a good picture of the evolving fog/low stratus as it develops.  The every-hour loop below, starting at 0315 UTC, shows the steady increase in probabilities along the Louisiana and Texas Gulf Coasts.  Note the relatively low probabilities in and around Houston — an apparent break between IFR conditions to the north and east and those to the south.  The Houston airport observations did not fall to IFR criteria although those criteria were common to the north and south.  Also, the IFR probabilities downplay the brightness temperature difference signal over central Texas where IFR conditions do not occur.











GOES-R IFR Probabilities computed from GOES-East (upper left), Traditional Brightness temperature Difference product (10.7 µm – 3.9  µm) (upper right), GOES-R IFR Probabilities computed from MODIS (lower left), Suomi/NPP Day/Night Band (lower left)






The imagery below shows GOES-R and MODIS imagery at the same time (immediately below) and GOES-R and Suomi/NPP imagery at the same time (bottom).  Note that the fog that develops is not of sufficient thickness to block views of the city lights.











GOES-R IFR Probabilities computed from GOES-East (upper left),  Traditional Brightness temperature Difference product (10.7 µm – 3.9  µm)  (upper right), GOES-R IFR Probabilities computed from MODIS (lower  left), Suomi/NPP Day/Night Band (lower left)
















GOES-R IFR Probabilities computed from GOES-East (upper left),  Traditional Brightness temperature Difference product (10.7  µm- 3.9  µm)  (upper right), GOES-R IFR Probabilities computed from MODIS (lower  left), Suomi/NPP Day/Night Band (lower left)








 

					

		
		
	


	

	

				

			
						

				Day/Night band and fog detection
			

										

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Day/Night band from VIIRS on Suomi/NPP from 0714 UTC on 4 October 2012






The day/night band imagery, above, shows fog/low clouds in the Hill Country west of San Antonio and Austin in south-central Texas.  Additionally, there are low clouds over the Gulf of Mexico, with cloud street structures that suggest a south-southeasterly wind bringing moisture inland from the Gulf.  The 0855 UTC day/night band image over the same domain, below, shows an expansion of the fog/low cloud signal.











Day/Night band from VIIRS on Suomi/NPP from 0855 UTC on 4 October 2012















Day/Night band from Suomi/NPP with observations overlain.






Observations suggest that the northern edge of the cloud streets over the extreme western Gulf of Mexico is the edge of a moisture gradient, and that that gradient extends inland to where the fog and low stratus are occurring.  How did the GOES-R IFR Probability field perform on this day?  The 0702 UTC (below) and 0915 UTC IFR probability fields show an increas in the areal extent of higher probabilities over the course of the night, consistent with the overnight cooling and the continued feed of moisture from the Gulf.  By 0900 UTC, IFR observations are common in and near the region where IFR probabilities are high.  This is a good example of how the Day/Night band and IFR Probabilities can be used in concert to understand the evolution of the fog/low stratus field over south Texas.









					

		
		
	


	

				

			
						

				IFR Conditions in Isaac
			

										

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GOES-R IFR Probability Field associated with Tropical Storm Isaac






The multiple cloud layers associated with a tropical system, and the extensive cirrus shield that is spawned by rainbands, makes it very difficult for the traditional brightness temperature difference product to be used to highlight regions of low stratus and fog.  This is when model data are vital in a fused product to highlight regions of aviation hazards, as shown in this loop.  There is a change in probability as the Sun rises that is associated with changes in the weights given to various predictors.  The color table suggests a very abrupt change.  In reality, values increase from 10 to 15 percentage points.


Note how well the IFR probability field matches the regions of IFR and near IFR conditions in the loop above.  This is testament to the accuracy of the model data, and to the relationships developed between model fields and IFR observations.

					

		
		
	


	

				

			
						

				Fog over Louisiana and Mississippi
			

										

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GOES-R IFR Probabilities computed from GOES-East Imager data (upper left), GOES-East brightness temperature difference (upper right), GOES-R Cloud Thickness of the highest liquid water cloud layer (bottom left), Suomi/NPP Day/Night band (bottom right), all from 0830-0900 UTC on 23 July 2012.















Enhanced 11-micrometer imagery, 0831 UTC 23 July 2012






Fog formed over the southern Mississippi Valley in the early morning of July 23 in a region where high clouds associated with a westward-tracking wave made detection difficult via the traditional brightness temperature difference method.  The imagery above shows relatively high IFR probabilities over southwestern Louisiana where IFR conditions are occurring.  Two items should jump out.  The IFR probabilities are highest where both satellite and model predictors are high, and that occurs in west-central Louisiana.  In regions to the south and west, where higher clouds exist (and satellite predictors are therefore low), probabilities are a bit lower in a region where only model predictors are being used.  However, IFR conditions are present.  Note how the character of the IFR probability field changes from the region where satellite data are used (much more spatially variable) to the region where mostly model data are used (more spatially uniform).  It is very important when interpreting the probability fields to be aware of the presence of high clouds that limit the inclusion of satellite data in the predictors.













GOES-R IFR Probabilities computed from GOES-East Imager data (upper  left), GOES-East brightness temperature difference (upper right), GOES-R  Cloud Thickness of the highest liquid water cloud layer (bottom left),  Suomi/NPP Day/Night band (bottom right), all from 1145-1200 UTC on 23  July 2012.















GOES-R IFR Probabilities computed from GOES-East Imager data (upper  left), GOES-East brightness temperature difference (upper right), GOES-R  Cloud Thickness of the highest liquid water cloud layer (bottom left),  Suomi/NPP Day/Night band (bottom right), all from 1215 UTC on 23  July 2012.






The two images above show how the probabilities change as the predictors used change from nighttime values (at 1145 UTC) to daytime values (1215 UTC).  At 1145 UTC, probabilities over Louisiana are near 40%, and these probabilities are driven largely by model data, because of high clouds.  There are several airports reporting IFR conditions at 1200 UTC.  Probabilities jump to around 55% at 1215 UTC.













GOES-R IFR Probabilities computed from GOES-East Imager data (upper  left), GOES-East brightness temperature difference (upper right), GOES-R  Cloud Thickness of the highest liquid water cloud layer (bottom left), GOES-East Visible imagery (bottom right), all from around 1300 UTC on 23  July 2012.






At 1300/1400 UTC, the GOES-R IFR probabilities and cloudt thickness fields neatly overlap the visible imagery observations of cloudiness over Mississippi and over western Louisiana, with a pronounced break in central Louisiana.