Advected Layer Precipitable Water (ALPW) product for 5 February 2020 Severe and Flood event
February 6th, 2020 by Dan Bikos
Posted in: Heavy Rain and Flooding Issues, POES, Severe Weather, | Comments closed
Thin fog over snow in southwest Kansas – 29 January 2020
January 29th, 2020 by Dan BikosA snow event in southwest Kansas on 28/29 January 2020 led to widespread snow amounts of 4 to 8 inches, with locally higher amounts (above 12″). On the 29th, a thin layer of fog developed over the snow covered land. Inspect the GOES-16 imagery:
Upper left: Visible (0.64 micron) band
Upper right: CIRA Snow Cloud Layers RGB
Lower left: Day Snow Fog RGB
Lower right: Day Cloud Phase Distinction RGB
In the visible imagery, the fog is undetectable due to the lack of contrast (the snow cover and fog are the same color).
In the CIRA Snow Cloud Layers RGB, snow cover is white while low-cloud/fog is yellowish-green. Note how we can see through the fog since it is thin, the snow cover on the ground can be seen.
In the Day Snow Fog RGB, snow cover is red, low clouds / fog are light purple. The transparency in this product is less than the previous RGB so that the snow cover under the fog is somewhat more subtle.
In the Day Cloud Phase Distinction RGB, snow cover is green, low-cloud/fog is cyan and there is sufficient transparency to view the snow cover under the thin fog.
The key takeaway point is to make use of RGB products to discern fog from snow cover, the visible imagery alone makes it much more challenging to make this discrimination.
As an example of data fusion, note this tweet from the NWS WFO in Dodge City which confirms the fog via web-cams:
GOES-16 composite satellite imagery really distinguishes snow cover areas from the low clouds and fog across southwest KS today #kswx pic.twitter.com/I7PR80SDoB
— NWS Dodge City (@NWSDodgeCity) January 29, 2020
Posted in: Fog, | Comments closed
10 January 2020 – Southern Plains Severe Weather event with Elevated Mixed Layer (EML)
January 10th, 2020 by Dan Bikos
Alternately, also available here:
ALPW Loop:
Posted in: Severe Weather, | Comments closed
Snow Squalls in Pennsylvania on 8 January 2020
January 8th, 2020 by Dan BikosBy Dan Bikos and Bill Line
On 8 January 2020 numerous snow squalls moved across Pennsylvania leading to the issuance of multiple snow squall warnings by the NWS. One of the challenges with this event is that it spanned the time period around sunrise. Obviously radar and IR satellite imagery is not affected by this, but the Day Cloud Phase Distinction RGB, which has been shown to be quite useful for snow squall events, can only be used during the daytime. One alternative to continue monitoring during this time period is to make use of the nighttime microphysics RGB at night. This RGB product makes use of the 10.3 minus 3.9 micron band difference for the green component which discriminates between water (positive) and ice (negative) clouds at night.
The following 4 panel GOES-16 loop:
Upper-left: Nighttime Microphysics RGB
Upper-right: Daytime Cloud Phase Distinction RGB
Lower-left: MRMS 1-km composite reflectivity
Lower right: IR (10.3 micron) band with METARs
First look at the Daytime Cloud Phase Distinction RGB in the upper-right, later in the loop when there is daylight this product does a good job in delineating which clouds are glaciated (green and also orange/red shades) versus water clouds (cyan/lavendar). Compare the cloud tops that are glaciated with MRMS reflectivity and we see that there is good agreement in delineating the more intense snow squalls from the less intense regions of snow.
Now trace backwards in time these glaciated clouds through the sunrise period into the overnight hours and notice how they appear in the nighttime microphyics RGB in the upper-left. The glaciated clouds appear tan to darker shades of red and also correspond pretty well with regions of higher reflectivity observed from MRMS.
We include IR imagery (with METARs) to illustrate only a loose correspondence between colder cloud tops and higher regions of reflectivity, the RGB products definitely do a better job since they can delineate ice versus water cloud tops.
Next, we’ll focus in on a modified Day Cloud Phase Distinction RGB product that allows you to see further towards the nighttime hours compared to the default product:
Upper-left: Nighttime Microphysics RGB
Upper-right: Modified Daytime Cloud Phase Distinction RGB
Lower-left: MRMS 1-km composite reflectivity
Lower right: IR (10.3 micron) band with METARs and GLM Flash Extent Density
The modifications to the Daytime Cloud Phase Distinction RGB are the same as shown in this blog entry.
Compare the modified Daytime Cloud Phase Distinction RGB to the default in the previous loop and notice that you get more images towards the nighttime hours around sunrise.
We included the GLM Flash Extent Density data as an overlay in the lower-right panel, note that there was some lightning activity associated with some of the snow squalls which further increases confidence that these were high-impact snow squalls.
A social media post illustrates the significant impacts of these snow squalls.
Later the same day during the early afternoon, a snow squall quickly moved through Philadelphia, prompting the issuance of a snow squall warning by the local NWS office. The Day Cloud Phase Distinction RGB at 1-min temporal resolution provides an alternative/supplement to radar imagery for tracking the precise evolution of the snow squall (see animation immediately below). Considering the environment in which we are viewing this RGB, the combination of convective cloud elements, cloud top glaciation, and rapid motion all apparent in the 1-min RGB imagery indicate snow squall potential with this feature.
Nearby surface observations measured wind gusts to 46 mph in associated with the snow squall, and photos on social media show the approach of the snow squall into Philadelphia.
Before and after a snow squall moved through Philadelphia this afternoon – as seen from the Comcast Technology Center. This line had a snow squall warning associated with it. Photo by @psumeteo alum Eric Mailloux. #snowsquall pic.twitter.com/BQ4uwZ0LfJ
— Weather World (@WeatherWorldPSU) January 8, 2020
Posted in: Winter Weather, | Comments closed
Elevated Mixed Layer event on 16 December 2019
December 16th, 2019 by Dan BikosOn 16 December 2019, SPC issued an enhanced risk of severe thunderstorms for portions of Louisiana and Mississippi:
One of the favorable ingredients for this severe weather setup was the presence of an Elevated Mixed Layer (EML) which is depicted in the 12Z Jackson, MS sounding:
The EML is bounded between the capping inversion around 750 mb and the higher RH around 500 mb. Within this layer, the relative humidity is quite low with a source region off the elevated plateau of northern Mexico. Lapse-rates within this layer are relatively high, 700-500 mb lapse rate is 7.6 degrees C / km and the maximum is shown in the magenta layer on the sounding of 8.1 degrees C / km.
At times, the EML may show itself as a region of warmer brightness temperatures in the GOES 7.3 micron imagery. In this particular case, the region of warmer brightness temperatures may be traced back to diurnal heating from the previous day over the elevated Mexican plateau and advecting northeastward across Texas, Louisiana and Mississippi:
Clouds develop during the early morning hours which obscure the EML signature, however it may still be tracked via the Advected Layer Precipitable Water (ALPW) product:
ALPW is a product based on microwave instruments aboard polar satellites which can see through most clouds. The 700-500 mb layer is useful in that it depicts the source region of the dry air associated with the EML and it can be tracked towards Mississippi. This dry air exists underneath a moist air mass with origins from the Gulf of Mexico as seen in ALPW in the lower layers. ALPW allows a four dimensional perspective of assessing moisture in the vertical in time.
More information on EML tracking via satellite imagery and products:
Posted in: Convection, Severe Weather, | Comments closed