New static Imagery from the HREF model

With WeatherSpork version 4.3.0, we’ve extended the static Imagery to include aviation forecasts from the High Resolution Ensemble Forecast (HREF) model.  The HREF is updated four times a day and provides a forecast lead time of 36 hours at one hour time increments.  In WeatherSpork you can find these in the Imagery view right below Graphical MOS under HREF Model as shown below.

These six choices provide the probability of a flight category of Marginal VFR (MVFR), IFR and Low IFR (LIFR) as well as the probability of a ceiling below 3,000 ft, 2,000 ft and 1,000 ft AGL.  In the sample below, the colors represent probabilities shown on the scale at the bottom of the chart.  Colors such as red and purple are indicative of ceilings that are forecast to be less than 3,000 feet as they are in northern Ohio; of course, they could be much less than 3,000 feet in this area.  Keep in mind that non-filled areas do not mean clear skies per se.  This simply means that there is a very low chance the ceiling will be less than 3,000 feet in these areas. This 21 hr forecast is valid at 1500Z on April 28, 2018.

The probabilistic ceiling forecasts above are additive in nature.  In other words if there’s a high probability of a ceiling less than 1,000 feet, there’s also a high probability of a ceiling less than 2,000 feet and 1,000 feet respectively.

Flight category combines both the ceiling and visibility as shown in the table below. These flight categories are sometimes improperly referred to as flight rules in many flight planning apps.

In addition to the probabilistic forecasts for ceiling, the HREF also provides a probabilistic forecast for specific flight categories that include LIFR, IFR and MVFR.  The example below has a forecast lead time of 22 hours and is valid at 1600Z on April 28, 2018.  Similar to the probabilistic ceiling forecasts above, higher probabilities of a MVFR flight category are shown in red and purple colors like you see in Indiana and Ohio in the example below.  Non-filled areas simply mean that the chances of a MVFR flight category are very low.  Unlike the probabilistic ceiling forecasts above, these forecasts are not additive.  That is, in an area where there’s a high probability of a MVFR ceiling, there may be a very low probability of a LIFR ceiling.

“Most pilots are weatherwise, but some are otherwise.”

Scott Dennstaedt
Weather Systems Engineer
Co-founder, WeatherSpork

An introduction to Model Output Statistics or MOS

A majority of certificated pilots have never heard of Model Output Statistics or what is referred to as MOS (pronounced moss).  MOS isn’t new and has been around since the 1960s, but is just starting to be leveraged by many pilots.  Back in 2016 the FAA added a brief description of MOS to the Aviation Weather Services advisory circular AC 00-45H Change 1 so it is now officially recognized by the FAA as supplemental guidance for preflight planning. WeatherSpork makes heavy use of this automated guidance which drives the signature feature in WeatherSpork called the Wheels Up Departure Advisor (WUDA) found in the Map, Grid and Route Profile views.  MOS is also used by meteorologists as one of several tools to issue terminal aerodrome forecasts (TAFs).

This video below was an audio and screen recording of a presentation given by Scott Dennstaedt to a local flight school in Charlotte.  Watch this video to learn a bit more about MOS and its advantages as flight planning guidance.

“Most pilots are weatherwise, but some are otherwise.”

Scott Dennstaedt
Weather Systems Engineer
Co-founder, WeatherSpork

Dangers of airframe icing in stratocumulus clouds

Normally pilots look forward to improving weather conditions after the passage of a cold front. The cold, dense air behind the front becomes negatively buoyant and sinks which tends to dry out the air. Moderate northwesterly winds will often prevail on the cold side of the front making for moderate mechanical turbulence sometimes extending up to 8,000 feet AGL. Other than some turbulent air, we don’t typically encounter much in the way of adverse weather behind such a cold front with few clouds, no precipitation and unlimited visibility.

During the spring, how many pilots think about the airframe icing threat that can occur in an overcast stratocumulus deck after the passage of cold front? Even a thin stratocumulus cloud deck like the one shown above can contain a liquid water contents approaching 0.5 g/m3 – especially near the tops. When the temperature is just right, these harmless-looking clouds can surprise a pilot with some moderate or even severe icing while climbing or descending through them.  This is especially concerning to those pilots flying aircraft without certified ice protection systems (IPSs).

Stratocumulus decks have very distinct characteristics from other clouds. Although not completely smooth on top like a stratus deck, they have rather even tops with a quilted-like or lumpy appearance when viewed from above. While a stratocumulus deck can be broken or even scattered it is quite common for these cloud decks to be overcast when they occur after the passage of a cold front. They are rooted in the boundary layer near the surface similar to other cumuliform clouds, but an overcast stratocumulus deck can extend for hundreds of miles making them difficult to avoid.

Let’s examine a case in the middle of April near Atlanta, Georgia where many pilots were reporting moderate icing in one of these stratocumulus decks.

This story starts out in the prior day.  A strong cold front moved through the Atlanta area during the late morning, bringing with it severe thunderstorms with tornadoes and microbursts in the afternoon from northern Virginia down to the peninsula of Florida as shown above. Temperatures ahead of the front climbed into the low 80s.  As the cold front (shown below) moved off the southeastern Atlantic coast the following day, this set the stage for a cold northwesterly wind to push a cold air mass over a fairly warm and wet ground in the Atlanta metro area.  A warm and moist surface with cold air moving in aloft is the perfect recipe for the genesis of a juicy stratocumlus cloud deck.

Skies were generally overcast all morning throughout the Atlanta area as can be seen by the surface observations below.  Ceilings were marginal VFR ranging from 2,100 feet to 2,600 feet.  In an overcast cloud deck, there’s no real way to know its depth from looking at the clouds from below.  Depending on the sun angle, a darker cloud base is indicative of more condensate, but this was early in the morning where the bases will generally be darker.  But in the Atlanta region, this was definitely an overcast stratocumulus deck like the one shown in the picture below.

KPDK 161453Z 30013G21KT 10SM OVC021 04/M01 A3000 RMK AO2 SLP167 T00391011 53014
KPDK 161353Z 27011G18KT 10SM OVC021 03/M01 A2998 RMK AO2 SLP160 T00331011
KPDK 161253Z 26013KT 10SM OVC022 03/M01 A2996 RMK AO2 SLP156 T00331011
KPDK 161153Z 28011KT 10SM BKN021 OVC028 04/M01 A2995 RMK AO2 SLP151 60000 70060 T00391006
KPDK 161139Z 26010KT 10SM BKN026 OVC033 04/M01 A2996 RMK AO2 T00391006

Temperatures at the surface during the morning were a chilly +3°C to +4°C.  But one characteristic of stratocumulus clouds is that there is very unstable air below.  This means the temperature decreases at the dry adiabatic lapse rate of 3°C for every 1,000 feet gain in altitude.  Except for right at the surface, this lapse rate is the most unstable that unsaturated air can be.  This is best seen on a thermodynamic chart called a Skew-T log (p) diagram.  The Skew-T analysis below for Dekalb-Peachtree Airport (KPDK) captured through the WeatherSpork Airports view demonstrates a textbook stratocumulus signature.

As can be seen above, the stratocumulus clouds extend from the bases at 2,500 feet MSL where the temperature and dewpoint converge with saturated conditions to the tops at roughly 5,500 feet MSL or where the temperature and dewpoint diverge.  That’s a depth of roughly 3,000 feet at 14Z (they likely varied in depth over the region).  Notice the large lapse rate below the cloud bases with nearly moist absolute instability within the actual cloud deck itself.  This is the most unstable lapse rate that can occur in saturated air. Remember that this area had received a fair amount of rain, so there was plenty of fuel being pulled into these clouds from below.

The tops are capped by a very strong temperature inversion which limits vertical cloud growth which is the classic signature to stratocumulus clouds.  So you can think of this “system” as a lid on a pot of boiling water.  The unstable air ascends, expands and cools to produce these clouds and given that the clouds are also unstable inside, the momentum in the capped updrafts gives those clouds the lumpy appearance when viewed from above.  In fact, taking a closer look at this sounding analysis shown below with a parcel lapse rate added, there is a fair amount of convective available potential energy (CAPE) that allows for efficient transfer of water vapor into condensate (liquid drops).  It’s like squeezing the water out of a sponge.

While not specifically shown on the Skew-T diagram, the graph below is a trace of the liquid water content of an instrumented research aircraft that climbed through one of these cloud decks.  The liquid water content is shown increasing to the right on the X-axis.  Height is shown on the Y-axis.  Because these clouds are rooted in the boundary layer, the have a median volumetric diameter drop size of less than 50 microns (small-drop icing scenario).  A liquid water content value at the top of this cloud deck shown in the graph below of 0.5 g/m3 is huge.  The liquid water content can be significant enough to overwhelm many aircraft when flying in the tops of these clouds.

Temperature is also important.  The cloud top will typically exhibit the coldest temperatures in these clouds.  In this case for Atlanta, the cloud top temperature is in the perfect range at -7°C if you want to see icing conditions.  That guarantees an all-liquid process making for a juicy cloud containing no ice crystals.  You can also see the cloud top temperature in the color-enhanced infrared satellite image below.  Colors in the image are based on temperature in degrees Celsius.  When clouds are present, this is the temperature of the cloud top, otherwise it’s the temperature of the surface of the earth. The stratocumulus deck shows up nicely over northern Georgia as a yellow color implying a cloud top temperatures of -8°C which matches what is shown in the Skew-T diagram.

Some of this cloud cover in extreme northwest Georgia was covered under an icing G-AIRMET valid at 15Z for widespread moderate icing from the surface to 10,000 feet MSL (shown below).

Lastly, there were a number of pilot weather reports for icing in and around the Atlanta region that included an urgent pilot report from a Boeing 737 at 5,000 feet MSL south of Atlanta.  The moderate ice reports during the morning tended to be around the tops of this cloud deck.

ATL UUA /OV ATL085020/TM 1245/FL050/TP B737/TA -3/IC MODERATE RIME

If you are not a member of WeatherSpork, your best online source for aviation weather and education, please consider becoming part of the WeatherSpork family.

“Most pilots are weatherwise, but some are otherwise.”

Scott Dennstaedt
Weather Systems Engineer
Co-founder, WeatherSpork

Thunderstorm planning with WeatherSpork, Part 3

Part 1 and Part 2 of this three video series showed a window of opportunity to depart Savannah, Georgia during the morning hours between 14Z and 15Z arriving in Charlotte, North Carolina before 17Z.  Was this guidance on the money?  Watch Part 3 of this video series to see how it all played out.  And stop back to this post and read some of the final thoughts below…

As shown below this was a very dangerous weather system that included many tornado and severe thunderstorm warnings up and down the Piedmont regions of North and South Carolina.  The line of relatively low-topped thunderstorms began to impact the Charlotte terminal area around 19Z and that matched the guidance from earlier in the day.

As I mentioned in the video, very strong winds at the surface were possible as heavy rain showers transferred the momentum of that strong low level jet down to the surface.  In fact, the wind gusted to 59 knots (68 mph) at the Charlotte Douglas Airport at 1913Z based on the METAR remarks below.

KCLT 151916Z 21035G59KT 1/4SM +TSRA BR BKN037CB BKN050 OVC065 21/17 A2980 RMK AO2 PK WND 20059/1913 LTG DSNT ALQDS RAB12 TSE01B02 PRESRR FRQ LTGICCCCG SW-W TS SW-W MOV NE P0000 T02110172 RVRNO

You can see in this color-enhanced IR satellite image below two patches of dark blue.  These were coincident with the two tornado warnings (red polygons) shown in the image above.  This satellite depictions shows cloud top temperatures in degrees Celsius.  The colder the temperatures, the higher the cloud tops.  In many cases, these overshooting tops are cells that will exhibit severe characteristics. These two cells within this line of thunderstorms likely spawned a tornado.

These two cells show up as well on the visible satellite (below), although not as good as the IR satellite image given the time of day.  If this occurred during sunset, you would have seen the higher tops cast a shadow on the clouds below. Nevertheless, the red arrows point to those two cells and the orange arrows show their approximate track.  The red X is there I live.   As this southernmost cell moved north-northeast, there was a heavy rain-wrapped circulation that passed through my neighborhood.  The driving heavy rain ended very abruptly as the backside of that cell passed…very typical of these kinds of mesoscale events.

Well I hope this video series was useful.  Feel free to send a link of these videos to all of your pilot friends and post them through your social media accounts or to your favorite aviation forums.

“Most pilots are weatherwise, but some are otherwise.”

Scott Dennstaedt
Weather Systems Engineer
Co-founder, WeatherSpork

 

Thunderstorm planning with WeatherSpork, Part 2

Part 1 of this series of three videos focused on how to use WeatherSpork to find the best time of departure for a trip the following day given a threat of thunderstorms along the proposed route.  Part 2 of this series takes place on the morning of this flight.  Watch this 20 minute video below to see if yesterday’s guidance was still on track.

“Most pilots are weatherwise, but some are otherwise.”

Scott Dennstaedt
Weather Systems Engineer
Co-founder, WeatherSpork

Thunderstorms a threat? Use WeatherSpork to plan your departure, Part 1

It’s that time of the year again.  Snow is finally beginning to melt in the northern tier of states and temps in the deep south are topping 90 degrees on a regular basis.  Moreover, G-AIRMETs for for airframe ice begin to morph into convective SIGMETs.  WeatherSpork is your best tool to find that perfect time to depart…or that perfect time to head to your comfy chair, put your feet up, relax and get caught up in a good novel.  Here’s a 30 minute video that will show you some of the advantages of using WeatherSpork to plan your departure when thunderstorms are in the forecast.  Enjoy!

“Most pilots are weatherwise, but some are otherwise.”

Scott Dennstaedt
Weather Systems Engineer
Co-founder, WeatherSpork

A shortcut from the Route Profile to Airports view

In WeatherSpork we will be adding a number of shortcuts to cut down on the number of clicks or taps needed to navigate to other important guidance within the app.  Here’s a video that demonstrates one such shortcut to get to the Airports view for a specific station identified as a fly-by airport in the Route Profile view.

“Most pilots are weatherwise, but some are otherwise.”

Scott Dennstaedt
Weather Systems Engineer
Co-founder, WeatherSpork

Don’t forget the big weather picture!

One of the slickest features of WeatherSpork that many of our members are raving about is the vertical depiction within the route profile and meteogram views.  The route profile like the one shown below is an incredibly useful tool to visualize how the weather will impact your route in four dimensions (fourth dimension being time).  However, one of those dimensions is pretty narrow.  The corridor used in WeatherSpork only considers a narrow slice of the atmosphere.  It’s important to acknowledge that this tiny corridor is like driving down the road looking through a straw.

It’s very possible that your route could be bumping up against some serious weather.  Just by shifting your route by 75 to 100 nm may put you in that weather.  Or perhaps you end up running late;  a change of just two or three hours might also cause you to be in weather you hadn’t planned to encounter based on the original narrow corridor shown in the vertical profile.  These vertical views have such a huge glance value, that they can easily lure you into the trap that everything looks great until you stray from the original plan.

My advice is simple; don’t forget the big weather picture.  Looking at the surface analysis, prog charts. constant pressure charts and convective outlooks are still a must.  You can find all of this other great guidance in the WeatherSpork Imagery view such as this 500 mb chart below.  Don’t know how to interpret some of these charts?  No problem.  WeatherSpork has dozens of online workshop available that explain how to use much of the guidance found in the Imagery view.  Or for a more personal touch, consider one-on-one online training or attending one of several live workshops through our partner AvWxWorkshops.com.  This unique training will challenge your most basic understanding of the weather.

SLD added to CIP/FIP severity Imagery

The Imagery view in WeatherSpork includes the latest automated analyses and forecasts for airframe ice.  These are referred to as the Current Icing Product (CIP) and Forecast Icing Product (FIP), respectively. Both the analysis and forecast have three basic components to include probability, severity and supercooled large drop (SLD) icing.  Based on WeatherSpork customer feedback, we have recently added the SLD threat to the icing analysis and forecast severity charts like the one shown below.

What is SLD?

SLD stands for supercooled LARGE drop icing.  Many pilots like to say it stands for supercooled “liquid” drops. Yes, they are liquid, but they are missing the point. For icing certification standards a large drop environment is one that has a median volumetric diameter (MVD) of greater than 50 microns.  This means that if you could line up all of the drops in a cloud by its size and found the median size, that would be the MVD for that environment.  Just for reference, the average human hair is 100 microns in diameter.  So, just as a drop becomes visible to the naked eye, it’s considered beyond the certification standards of all aircraft.

SLD is shown as a red-hatched overlay on the ADDS icing severity chart like the one shown below. Keep in mind that the SLD analysis or forecast isn’t a calibrated probability.  In other words, SLD will be included even when there’s just a 5 percent chance of it occurring.  So it’s properly referred to as an SLD “potential” field.

Icing analyses

CIP (pronounced “sip”) is an analysis of the current icing situation.  This means that it is always valid in the recent past.  It is updated hourly and depicts the icing scenario at the top of the most recent hour.  It becomes available in WeatherSpork around 25 minutes past each hour.  For example, the analysis valid at 1600Z will be available at 1625Z.  You will find this SLD analysis in the Imagery view under Icing Analyses::Current Icing Product::ADDS Severity.

If you want to assess the actual likelihood of SLD valid at the top of the most recent hour, WeatherSpork provides a CIP SLD analysis as a separate field.  You will find this in the Imagery view under Icing Analyses::Current Icing Product::SLD Potential as shown below.  Warm colors such as red and orange represent a greater likelihood of SLD where as cooler colors such as green and blue are indicative of a low likelihood.

FIP, on the other hand, is the forecast counterpart and you will find this in the Imagery view under Icing Forecasts by Altitude::ADDS Severity. You can choose from a lead time of 2, 3, 6, 9, 12, 15 and 18 hours as shown below and it will also have an SLD overlay hatched in red.

 

“Most pilots are weatherwise, but some are otherwise.”

Scott Dennstaedt
Weather Systems Engineer
Co-founder, WeatherSpork

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