technology, weather

Rain temporarily leaves Southwest Florida (H. Michael Mogil, CCM, CBM, NWA-DS*)

The focus during tropical cyclone events is typically on high winds, coastal storm surge, heavy coastal and inland rainfall, and possible flooding. However, if one is located far enough away from the storm’s circulation, atmospheric processes may lead to less rainfall. Such is the case for southwest Florida during the past few days. And while rainfall didn’t vanish entirely from the southwest Florida area, it certainly dropped off dramatically.

Fig. 1 shows the upper level wind flow as determined by weather satellite and radiosonde (weather balloon) data. Computer programs can track cloud elements and compute their motion (and, hence, winds). Combining radiosonde wind and temperature data and infrared satellite cloud temperature measurements, computer programs can assign winds into specific altitude bands.

Late on Jun. 21, 2017, it is easy to see this altitudinal variation near Tropical Storm Cindy (located just offshore from the Texas-Louisiana coast). Winds shown in green show a counterclockwise (low-pressure) circulation. Mid-altitude winds are tagged to be between 20,000 feet to 25,000 feet above ground level or at pressure altitudes of 350mb to 500mb. Pressure is measured in millibars (mb) and decreases as one goes higher in the atmosphere (due to less air above).

To the north and east of Cindy’s mid-altitude circulation, higher altitude winds (shown in blue) actually spin slightly clockwise (indicating a high-pressure system). This differential circulation pattern is often found in well-developed tropical storms and hurricanes.

This upper-level circulation leads to a ridge or high-pressure system to the east of the storm’s circulation. In this high-pressure system, sinking air dominates. This is in contrast to the rising air motion (and associated clouds and precipitation) within Cindy’s circulation. Note that precipitation can still extend quite a distance from the storm’s center, but is mostly confined within the low- and mid-level counter-clockwise circulation pattern (Fig. 2). As a result of these upper and lower level circulation considerations, southwest Florida experienced less cloudiness and less shower and thunderstorm activity during the past two days.

As Cindy landfalls, weakens and moves to the northeast (see Fig. 2 – the elongated shape of the storm’s circulation is oriented in the direction of future movement), the upper ridge over Florida will slowly weaken and a more usual daytime shower and thunderstorm pattern will return to southwest Florida.

© 2017 H. Michael Mogil

Originally posted 6/22/17

* The National Weather Association Digital Seal (NWA-DS) is awarded to individuals who pass stringent meteorological testing and evaluation of written weather content. H. Michael Mogil was awarded the second such seal and is a strong advocate for its use by weather bloggers.

weather

Two Atlantic Tropical Weather Systems (H. Michael Mogil, CCM, CBM, NWA-DS*)

During the past two days, the tropical Atlantic has come to life. Two weather systems are in the news this Monday morning (Fig. 1).

The first is Potential Tropical Storm TWO, that is moving rapidly westward toward the southern Windward Islands. This very low-latitude tropical weather system is expected to reach tropical storm intensity before it reaches the island chain on later today, tonight or early Tuesday (Fig. 2). As of 8:00 a.m. E.D.T., the system was not yet a named tropical storm even with sustained winds of 40 miles per hour.

According to David Zelinsky, a meteorologist at the National Hurricane Center (NHC), “Although the winds meet the threshold for a tropical storm, the system does not yet possess a closed well-defined surface circulation (emphasis mine), so it isn’t classified as one. Since the system still has the potential to bring tropical storm impacts, the NHC now has the option to issue advisories for ‘Potential Tropical Cyclones.’ This way, we can still issue all of the appropriate warning products on time, rather than waiting for the system to finish developing.”

The second system is of more immediate interest to the U.S. This involves a, “broad area of low-pressure located near the east coast of the Yucatan Peninsula.” The system is lopsided with most of the strong winds (as high as gale force) and heavy rainfall located on the east and northeast sides of the system.

NHC, in its 8:00 a.m. E.D.T intermediate advisory, noted that, “the low-pressure area still lacks a well-defined center of circulation.”

Lacking a center, and being so close to land, this system will likely await its transit into the Gulf of Mexico before intensifying. However, during the next few days, there is a very high likelihood of the system warranting a tropical storm name.

© 2017 H. Michael Mogil

Originally posted 6/19/17

* The National Weather Association Digital Seal (NWA-DS) is awarded to individuals who pass stringent meteorological testing and evaluation of written weather content. H. Michael Mogil was awarded the second such seal and is a strong advocate for its use by weather bloggers.

hydrology, weather

South Florida Rainfall Records Going Under Water (H. Michael Mogil, CCM, CBM, NWA-DS*)

A few days ago, I noted that southwest Florida “rainfall for June 2017 (now almost only half over) was at rarified levels.” Since I’ve had a chance to compare observed rainfall to longer-term records (using the NOAA Regional Climate Center data base), it is clear that the deluge so far this month has started to submerge many existing records.

Recognize, however, that many locations do not have official long-term records; hence, the numbers shown for record comparisons are limited to only a few sites. And, even for some of these, records are rather short-term. Regardless, the numbers are more than impressive across the entire region. They are even more impressive when one realizes that less than a month ago, everyone was worried about drought and forest fires!

Looking at a radar – rain gauge data mix (Fig. 1), it is easy to see where the heaviest rainfall has occurred. Collier and Broward Counties have some locations with an excess of 20 inches so far this month.

Thus far this month (through early morning on Jun.18, 2017), Naples Airport, FL (APF) has received 11.40 inches of precipitation. The average for the entire month of June is about 8.80 inches. Based on the Naples area period of record (1942-2017), the 11.40 inches ranks 17th on the list of highest monthly amounts. The record June rainfall occurred in 2005 (20.51 inches); the second highest amount was recorded in 1947 (17.97 inches).

Regional Southwest Airport (RSW) in Fort Myers has received 12.96 inches so far this month, making this the 5th wettest June at RSW (records have been kept there only since 1998). The record occurred in 2005 (20.85 inches).

With an even shorter period of record, Marco Island has already measured 20.90 inches of rain this June. The record (25.21 inches) also occurred in 2005.

Rainfall, year-to-date, at Marco Island is now at roughly 60% of the total average yearly rainfall of about 50 inches.

Given the weather pattern that we are in and the abundant tropical moisture supply, it is likely that some of these long-term records could be challenged and broken as the month continues. And, to think, there’s still 13 days left in the month!

© 2017 H. Michael Mogil

Originally posted 6/18/17

* The National Weather Association Digital Seal (NWA-DS) is awarded to individuals who pass stringent meteorological testing and evaluation of written weather content. H. Michael Mogil was awarded the second such seal and is a strong advocate for its use by weather bloggers.

oceanography, weather

A major source of a hurricane’s energy

Everyone knows something about tropical cyclones, the broad class of tropical low-pressure systems that includes hurricanes and typhoons (Fig. 1). Many of us know when and where they are more likely to form and how they are named. If you were asked about the most favorable conditions under which these tropical systems develop, among the first things you would probably say would be sea surface temperature. Some of you may also know that a sea surface water temperature above about 80oF is a very favorable factor. No wonder that hurricanes form over low-latitude ocean areas.

A hurricane or its antecedent weather system could be thought as an engine, drawing heat energy from the water body underneath it. The system does this in two ways. First, air touching warm water is warmed by contact (conduction), and air heated from below wants to rise (convection)*. Second, water evaporates from the warm ocean surface (with latent heat trapped in the water vapor) and is then carried upward by rising air currents (updrafts). The latent heat is released during the phase change from water vapor to water droplets (condensation and cloud formation) and that heat is then distributed vertically within the updraft (Fig. 2).

Now, think about a weather situation modeled in your kitchen. Imagine that you have a bowl with hot, steaming soup. Would you rather cool it down by blowing on the whole bowl or a small portion of the soup in a spoon? Almost everybody would choose the second option. Well, now think about the force of the winds in a hurricane. Don’t you think they are strong enough to lower the water temperature? And if you take also into account the large amounts of chilled precipitation that falls on the ocean surface, then you would realize that we need something else to keep providing the necessary energy for hurricanes.

The depth of the isothermal layer (a layer in the ocean that has an almost constant temperature) plays an important role in sustaining and intensifying hurricanes and tropical cyclones. Considering its depth and its temperature, we could then estimate the available potential energy for the hurricane’s heat engine. It is important to note that the hurricane circulation is strong enough and its pressure low enough to actually lift the surface of the ocean, causing deeper layers with a lower temperature to rise, thereby squeezing this isothermal layer somewhat. Depending on the strength of the storm’s circulation, how low the atmospheric pressure is in the center of the storm, and the storm’s speed of motion, the effect can be quite strong. It could possibly even generate a cool surface water “footprint.”  Obviously, such cool “footprints” can weaken a hurricane.

Conversely, we can say that there must be certain areas where a tropical cyclone could intensify. Tropical meteorologists call these “warm pools.” Forecasters can monitor these and ascertain how they affect a hurricane’s growth and/or intensification.

*Sensible heat is almost negligible, since it depends of the temperature difference between the water surface and the air, which in the tropics is usually less than 1oC ( Emanuel, K: The theory of hurricanes, 1991).

© 2017 Mayguen Ojeda

Originally posted 6/17/17

climate, weather

More heavy rainfall for southwest Florida (H. Michael Mogil, CCM, CBM, NWA-DS*)

Rainfall for June (now almost only half over) is already at rarified levels. Many locations across southwest Florida have logged 15 to 20 inch values. And more rainfall is anticipated.

In fact, for the Naples area (my home base), the official National Weather Service (NWS) forecast is calling for 50 to 70 percent daytime rainfall probabilities each day for the next week.

And, no wonder. The atmosphere above the now, so-called “Sunshine State,” is overburdened with moisture (Fig. 1). According to NOAA’s (National Oceanic and Atmospheric Administration)’s Storm Prediction Center (SPC), precipitable water (PW) was at 1.92 inches at Miami, FL early this Thursday morning. PW is the measure of all water vapor in a column above a radiosonde (weather balloon) sounding location. For Jun. 15, the climatological average PW at Miami, FL is 1.66 inches and a 90 percentile PW is 2.00 inches. In short, if all the water vapor above a point could be condensed into liquid water and then squeezed out (like a sponge), and no water vapor moved in or out of the atmospheric column, 1.92 inches of rain would result.

Further, winds throughout the atmospheric column from the ground to some 50,000 feet above ground level are less than 15 miles per hour. That means that any storms that develop will not move very fast.

While outflow boundaries (areas marking where low-level winds blowing out of thunderstorm areas meet the environmental air mass) may interact with other outflow boundaries and sea breezes to enhance thunderstorm formation or localized movement, most storms will be slow-moving, prolific, rainfall producers. Many locations could see short-period downpours of one to two inches or more during the upcoming week or until the weather pattern changes.

© 2017 H. Michael Mogil

Originally posted 6/15/17

* The National Weather Association Digital Seal (NWA-DS) is awarded to individuals who pass stringent meteorological testing and evaluation of written weather content. H. Michael Mogil was awarded the second such seal and is a strong advocate for its use by weather bloggers.

technology, weather

Tornado watchers from the sky

Last November, a new weather satellite (GOES-R*), was launched from the Kennedy Space Center in Florida (Fig. 1). This new space dweller is equipped with modern and powerful instruments, opening numerous possibilities for improved weather analysis and forecasting.

Today, I would like to focus on a brand-new instrument – The Geostationary Lightning Mapper (GLM). Scientists believe that this could be the key to improved tornado forecasting.  Studies have shown that there is a positive relationship between changes in the lightning pattern inside a storm and the formation of a tornado. But, before we get into that, let’s talk about thunderstorm formation, particularly that of tornadic thunderstorms.

In areas with vertical differences in wind direction and speed (what meteorologists call wind shear), there is a potential for the formation of horizontal rolls (Fig. 2).

When a powerful thunderstorm develops, the lifting air associated with it tilts these rolls, turning them into more vertical positions (one with rising air and one with sinking air) as shown in Fig. 3.

Under some circumstances, the upward rotation can extend to the ground generating a violent column of rotating air, known as a tornado. Unfortunately, there are still limitations to pin-pointing the onset or potential onset of tornado formation (i.e., the warning process). Severe weather forecasters typically recognize the larger-scale conditions and general geographic areas in which tornadoes can possibly form (watch process).

Once a watch is issued (or localized conditions suggest possible tornado or severe weather development), storm spotters and Doppler radar become the key tools available to local National Weather Service forecasters. Trained spotters can recognize cloud features that suggest possible severe weather. Radar, however, can probe the interior of the thunderstorms. Determining the presence of certain echoes (radar patterns based on the size and distribution of water droplets) can indicate the presence of a rotating core that is often linked to the occurrence of tornadoes.  Doppler radars are also capable of detecting areas of rotation (based on the wind movement toward/away from the radar).  Still, radars have limitations regarding vertical and horizontal coverage and resolution. At large distances, the radar may even “overshoot” the tornadic circulation, passing high above it. So, depending on the distance from the storm to the radar and other factors, forecasters may not be able to detect a tornadic signature.

Recent research indicates there is a close relationship between lightning and tornado formation. A considerable and sudden increase in lightning flash rates (lightning frequency), known as a “jump,” has been observed to take place just before tornado formation. These jumps have been registered 20-25 minutes before the tornado occurs (i.e., about 10 minutes more than the current average tornado warning lead-time). Until now, the only possible way to measure the lightning activity inside the storms, was to use the National Lightning Detection Network, which was the most accurate way to record time, strength and number of strokes of cloud to ground lightning flashes. But, the more intense and severe the storm is, the more important the intra-cloud lightning (cloud to cloud strokes) may be.

That is why the meteorological world is celebrating the launch of GOES 16. This satellite is opening new ways to make huge advancements in weather observing, analysis and forecasting. With the arrival of the GLM, the satellite is capable of detecting intra-cloud, cloud-to-ground and cloud-to-air lightning.

With the 2017 tornado season well underway, scientists have already been looking at GLM’s capabilities and reliability in detecting tornadoes.

*The name of the satellite was changed from GOES R to GOES 16, once the spacecraft reached its stationary orbit on November 29th.

© 2017 Mayguen Ojeda

Originally posted 6/14/17

weather

Funnel cloud in Naples (H. Michael Mogil, CCM, CBM, NWA-DS*)

At around 7:30 p.m. E.D.T. this Friday evening, my wife and I spied a well-formed funnel cloud west of our location in North Naples, FL (Fig. 1). The funnel was likely near or just offshore from Vanderbilt Beach. Radar images showed a developing shower or thunderstorm near the funnel’s location (Fig. 2).

About 15 minutes later, the lowered cloud base associated with the funnel had moved to our northwest and the funnel was difficult to see behind a grove of palm trees. At this time, the funnel was becoming wrapped in rain.

© 2017 H. Michael Mogil