January 2022 EZNews

Updated: May 26

Hello and welcome to the 9th edition of EZNews!


Happy New Year! At EZWxBrief, we really appreciate all of those subscribers that have recently joined, renewed their membership or signed up for auto-renewal. Due to the holidays, there were no new releases during this last month. For those new to EZWxBrief, please remember that for the best user experience, EZWxBrief is designed to run as a progressive web app (PWA) and must be installed on your device. You won't find EZWxBrief in the App Store or Google Play store. See the Pilots Guide for more information on how to install EZWxBrief as a PWA.


A change of heart


We get some really good feedback from our customers and certainly appreciate hearing from you. In some cases, customers are just not sure how to get started with this new progressive web app which is totally understandable. With any new application there will be a learning curve. For example, we received the following feedback this past month after a customer signed up for a 14-day trial...


"The app is a bit clunky and less intuitive than I had hoped for. I know with time and practice, it will become second nature. However, it was a hard first start as I muddled through your app trying to make a weather decision on a flight that took me from Texas to Tennessee over a 10 hour period."


We suggested to this customer to invest a some time and view the EZWxBrief YouTube channel videos and take a look at the 140+ page Pilots Guide where you can find a quick start guide near the beginning. Here's the response we received after that suggestion...


"I wanted to let you know I started my subscription. This tool is the best thing since the invention of GPS and the moving map! Thanks for sharing your talents and passion with the world.


Recently EZWXBrief helped me secure the confidence to fly VFR to a $100 hamburger after a frontal passage left Texas in an oddball weather situation which was rapidly changing. It was a matter of timing and information and your product cleared the fog in my mind. I should tell you I fly a 1946 Globe Swift with an original panel and use ForeFlight with a Stratus ADS-B to find my way because paper maps are soooo early 2000’s. I’m never one to push the weather, and admittedly I’m probably overly conservative in my decision making because as an ex-USAF and current airline Captain, I don’t want to add my name to the list of experienced pilots who inadvertently flew VMC into IMC. EZWxBrief fills the void. I thoroughly enjoy the Departure Advisor tool.


I have watched a fair number of your videos and learned much. Very helpful. Thanks and keep them coming."


As always, if you have any questions about using EZWxBrief or run into an issue or bug, the best way to reach us is to send an email through our Contact Us page on the EZWxBrief website. This is so we can consolidate all of your feedback, questions and comments into our support tracking software. Please understand that we do not offer telephone support. Our support team usually responds to email support requests within 24 hours, but often much sooner. We appreciate your help and feedback and look forward to hearing from you!


A frontal passage as seen on the Skew-T


A strong front passed through the Southeast U.S. recently and can be easily seen on the forecast soundings and analyses. Below is what the surface analysis chart looked like at 0900Z on January 3, 2022. There's a strong cold front moving through the midlands of South Carolina. A Skew-T log (p) diagram is a great drill down tool to examine the weather at a specific location. For this case, let's look at the Columbia Metro Airport (KCAE) located in central South Carolina.

Before we get into the details, let's review the typical changes expected at the surface during the passage of a front? First, a cold frontal system shown on the surface analysis chart shown above is a trough of low pressure. That is, the front marks the point of the lowest pressure in the region. Think about digging a V-shaped trough in the ground to bury a cable or a pipe. The bottom of the trough represents the location of the lowest point. Similarly, the front is the point of the lowest pressure in that area. Therefore, as a front approaches your location (in this analysis, the Columbia Metro Airport), the surface pressure will continue to fall and then the pressure will rise once the front passes. Frontal systems are also areas of surface convergence. If you were to pour water on each side of that trough you dug, the water would converge and meet at the lowest point.


Often the surface observations (METARs) will show a PRESFR or PRESRR in the remarks which indicates that the pressure is falling rapidly as the front approaches or rising rapidly after the front has passed, respectively. This can be seen for the two surface observations below from KCAE, the first being prior to the passage of the cold front and the second after the passage of the cold front.


Prior:

KCAE 030856Z 19011G18KT 10SM BKN020 BKN028 OVC033 19/17 A2946 RMK AO2 RAE48 PRESFR


After:

KCAE 031537Z 31014G24KT 2SM RA BR BKN009 OVC016 08/07 A3001 RMK AO2 PRESRR


Next, it is extremely common for there to be a shift in the wind direction as the front passes. For a cold front moving from west to east as is the case here, the surface winds are typically out of the south, southwest or southeast prior to the frontal passage and they shift around to the west, northwest or north and persist in this direction after the passage of the cold front. You can see in the METARs above from Columbia Metro Airport shows the classic wind direction shift. The wind direction at 0856Z is 180° (southerly) prior to the frontal passage and at 1537Z the wind direction shifted to 310° (northwesterly) after the cold front moved through.

Surface analysis chart valid at 0900Z prior to the passage of the cold front at the Columbia Metro Airport.

What about wind speed? Turns out the wind speed isn't a great indicator of the passage of a front. The winds can be strong and gusty before or after the passage. In some cases with weaker cold fronts or with the passage of a warm front, the winds can be very light. This is a reflection of the pressure gradient associated with the front. Though there are many factors, a stronger the pressure gradient across the front, the more significant the wind speed will be.


Many fronts will also see a rapid change in temperature. A cold front, for example, will see a slight temperature increase right before a cold front passes (called a thermal ridge) and then as the front passes, the temperature will drop. In the two METARs above, the temperature fell from 19°C to 8°C in just 6 hours. Keep in mind that other factors such as downsloping winds on the lee of a mountain range can cause a northwest wind to go through adiabatic compression causing the temperature change across the front to be negligible. This happens quite frequently on the lee side of the Appalachian Mountains.


Similar to the temperature, the dewpoint temperature will also drop with the passage of a front. For much of the U.S. a northwest wind is from a continental or dry source, so the air behind the cold front is usually rather dry, thus, lowering the dewpoint temperature when it passes. You can see in the METARs above, the dewpoint fell from 17°C to 7°C in that same six hour period.

Surface analysis chart valid at 1200Z after the passage of the cold front at the Columbia Metro Airport.

What about clouds and precipitation? Cold fronts typically usher in cooler, usually drier weather. But the weather that occurs when a cold front passes by can be wet and wild or sometimes dry and mild. Anafronts are characterized by postfrontal cloudiness and precipitation, while katafronts typically have precipitation in a band along or ahead of the cold front.


So what does a frontal passage look on a forecast sounding or analysis? Below is the sounding analysis valid at 0900Z for KCAE just prior to the frontal passage. Surface winds are out of the south and those shift around to the southwest aloft. For the Southeast region, this is a warm, very moist pattern given this wind direction. The surface temperature is on the warmer side and the air is generally unstable with a most unstable CAPE value of 940 J/kg.

After the cold front passes through KCAE, the surface winds shift around to the north and the temperature at the surface drops significantly as shown below using the forecast sounding valid at 2000Z. The air is fairly stable behind the cold front with a lifted index of +21 and very dry conditions aloft. Also notice the deep isothermal layer (temperature remaining the same with height) between 3,000 and 10,000 feet MSL. This is created as the cold, dense and dry air aloft flows in behind the cold front. This air subsides which causes the air above the boundary layer to go through adiabatic compression and heat up quite a bit. This process also helps it dry out even more creating a very large dewpoint depression in the free atmosphere aloft. This sets up the classic stratocumulus cloud signature below with a well mixed boundary layer and an unstable lapse rate near the surface and very dry conditions aloft.

You can access Skew-T diagrams in EZWxBrief by visiting the EZAirport page. Put in the airport identifier of interest. Then find the menu symbol at the top and select Skew-T Diagram from the menu. It will launch the soundings tool from NOAA in a new browser window.

Using the NOAA tool available from the EZAirport page in EZWxBrief, you can easily overlay these two soundings by holding down the Shift key and selecting both diagrams from the choices at the bottom. From this you can quickly compare the two diagrams including the shift in the wind direction and wind speed as well as the change in the overall temperature and dewpoint profiles. It's a cool way to see how the weather is expected to change over a range in time at a particular location or you can even use multiple locations.


What's new with EZWxBrief?


There have been no major changes for this release worth discussing. We expect Version 1.0.8 to be released later this month.


Most pilots are weatherwise, but some are otherwise™


Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief

CFI & former NWS meteorologist






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