weather, weather safety/preparedness

Long duration lake-effect snow event underway (H. Michael Mogil, CCM, CBM, NWA-DS*)

The National Weather Service has issued “lake effect” snow warnings and advisories for areas to the east of Lakes Erie and Ontario through Sunday evening (Fig. 1). These forecasts call for locally heavy snow and wind-blown snow with associated hazardous driving conditions. Forecasts for areas in New York State downwind of these lakes indicate that snowfall amounts will be within a 15 and 40 inch range (locally higher amounts expected) along with wind gusts to 45 miles per hour. The heaviest snowfalls are expected in the Tug Hill Plateau area (downwind from Lake Ontario, near Watertown and Montague, NY), with 12-hour snowfalls, throughout the period, frequently in the six to 12 inch range. Already, early this Friday morning, Watertown reported wind gusts in excess of 40 miles per hour.

While not yet issued, look for similar forecasts to be posted for other downwind areas of the entire Great Lakes region during the next several days.

This entire weather scenario is linked to a quasi-stationary surface and upper air low-pressure system across eastern Canada (Fig. 2). The system’s counter-

clockwise circulation will send winds across the Great Lakes, mostly from a west to northwest direction.

With the Great Lakes mostly ice free (Fig. 3), colder air, moving over relatively warmer water, can gain heat and moisture. This allows clouds (often low-topped) to develop in bands across the lakes (Fig. 4). As these bands make landfall, the air they are riding on experiences increased frictional drag

(friction is higher as winds blow over land than water). The result is a low-level convergence effect, adding uplift to the clouds. In areas where terrain increases inland, there is an added orography-lift effect (e.g., the Tug Hill Plateau area). In a snowfall event earlier this winter season, parts of the Tug Hill Plateau were targeted with over four feet of snow in a three-day period.

Due to the nature of “lake effect” snow events, the National Weather Service in Buffalo, NY noted early this Friday morning that, “…in lake effect snow, the weather can vary from locally heavy snow in narrow bands to clear skies just a few miles away. If you will be traveling across the region, be prepared for rapid changes in road and visibility conditions.”

© 2017 H. Michael Mogil

Originally posted 1/27/17

* 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, weather safety/preparedness

Enhanced severe weather threat for southeast U.S. (H. Michael Mogil, CCM, CBM, NWA-DS*)

Batten down the hatches (and be prepared to take shelter today and tonight) if you live or are traveling across the southeast U.S. The Storm Prediction has issued a highly unusual severe weather outlook for the region – calling for a moderate to high severe weather risk for South Carolina, Georgia, a small part of southeast Alabama and much of northern and

central Florida (Fig. 1). According to news reports, the storm system behind today’s severe weather threat has already claimed the lives of at least six people across Mississippi and Georgia.

Fig. 2 and Fig. 3 show the radar and satellite images, respectively, late this Sunday morning. What is most telling is that the current strong thunderstorm activity is occurring in a region of dry air aloft (the oranges and blacks on the water vapor satellite image – Fig. 3). This means that rising air

inside the thunderstorms must be moving at very high velocity to counter this dryness. Hence, many of the stronger thunderstorms are already likely acting as “supercells (isolated storms ahead of a line of storms).” Two apparent supercells can be seen in the northern Gulf of Mexico (Fig. 2).

As a new low-pressure system develops across the southeast today, and moves to northern Georgia and eastern Tennessee by tonight,

(Fig. 4), a very strong jet stream aloft (winds greater than 130 miles per hour) and a very warm and unstable air mass (dew points above 70 degrees F) will all be factoring into the storm evolution as the day and night progress.

The local storm potential graphic (Fig. 5) indicates just how strong the storms may be, even removed from the main threat region.

As the newly forming low develops, it will move to the northeast and spread a large shield of rain along the entire east coast during the next several days. Except for parts of New England, the atmosphere is just too warm for snow.

Although winds will be strong across the southeast today, coastal wave action, higher than average tides,

rip current danger and other coastal and boating dangers (including offshore gale warnings in the Gulf of Mexico) will be the watchwords across the west coast of the Florida peninsula into the Gulf beginning tonight and continuing until Tuesday (Fig. 6). That is because strong west and northwest winds will arrive once the cold front clears the area (Fig. 4).

Although skies may be sunny to partly cloudy across much of the Florida peninsula now, conditions will rapidly deteriorate during the latter part of the calendar day and early Monday morning.

Stay tuned to the National Weather Service, local broadcast media outlets and any tailored private weather subscriptions you may have for watches, warnings and advisories.

If you have loose objects outdoors, consider bringing these inside before the storms arrive.

© 2017 H. Michael Mogil

Originally posted 1/22/17

* 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.

climate, weather

Sierra Nevada snowpack increases dramatically; drought conditions wane (H. Michael Mogil, CCM, CBM, NWA-DS*)

When the computer models started to forecast incredible precipitation amounts for the Sierra Nevada Mountains of California and Nevada earlier this month, I thought the numbers were somewhat exuberant. After all, California has been in the throes of an extensive and hard-hitting drought for several years. However, this storm event (and the one on the western horizon) are welcome news for a state that lives in drought.

According to the National Weather Service (NWS) Forecast Office in Reno, the early January stormy period (Jan. 2 – 13, 2017) transformed the Sierra snowpack from a sub-average value to one that is pushing 200 percent of seasonal average (Fig. 1). This snowpack provides California’s dry season river runoff, water for agricultural and human uses and other aspects of California’s existence. The snowfall is also helping to boost ski resort business (at least, once folks can get to the ski areas).

It’s easy to see the impact of this precipitation on the California drought (Fig. 2). Note that the data cutoff for Drought Monitor maps is each Tuesday at 7:00 a.m. E.S.T., even though the maps are published each Thursday at 8:30 a.m. E.S.T. Hence, next week’s maps will likely show a further reduction in drought coverage across California.

Observations indicate that the high Sierra received between 9 and 15 feet of snow since the start of 2017. In the Tahoe Basin, within the “rain shadow”** of the Sierras, 2 to 5 feet of snow fell, except for the west shore of Lake Tahoe which received

between 6 and 8 feet of snow. For the Virginia Range (located just east of Lake Tahoe), reports indicated over 2 feet of snow had fallen, while along Highway 395 between Bridgeport and Lee Vining (on the east side of Yosemite National Park) between 1 and 4 feet of snow was reported.

Forecasters see a brief break in the stormy weather pattern through early next week. Then, more valley rains and mountain snows are on the menu (Fig. 3).

–  –  –  –  –  –  –  –

** The “rain shadow” lies downwind from a mountain range. It typically receives lesser precipitation amounts because upslope winds on one side of the mountain receives the heaviest precipitation, while downslope winds on the rain shadow side receive less precipitation. For the Sierra’s, west winds provide the upslope across much of California; as the winds cross over the mountains, lighter precipitation occurs across western Nevada and the Lake Tahoe Basin.

© 2017 H. Michael Mogil

Originally posted 1/14/17

* 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.

climate, weather

A Significant Dixie chill (H. Michael Mogil, CCM, CBM, NWA-DS*)

 

Across the South, this Sunday morning, Jan. 8, 2017, folks were pulling out their winter clothing. Temperatures that plunged to below freezing levels across Texas and much of the Gulf Coast east to the western Florida Panhandle on Saturday morning did so again this morning. However, today, the sub-freezing chill also made it nearly as far south across Florida as a Tampa-Orlando line. Even in parts of far south Florida temperatures in the upper 30’s and lower 40’s led to highly unusual wind chill readings that dipped to near 32 degrees. It’s easy to see the expanse of the cold weather by viewing this National Weather Service (NWS) “watch-warning” map (Fig. 1). Wind chill advisories and freeze and hard freeze warnings covered the entire Gulf Coast from the Rio Grande Valley in south Texas to Naples and nearby locales in southwest Florida. In south Florida, palm trees were shivering (swaying) in the cold winds!

                             T

There are several culprits at work, here. First, a significant upper level trough (Fig. 2) has allowed northwesterly winds to drive a surface level high-pressure system southward. This high-pressure system, an unusually strong one, had a central pressure of 1043 millibars (30.80 inches of mercury) near Dallas, TX early this morning (Fig. 3). The record high-pressure reading at Dallas (records dating back to 1898) was 31.06 inches during an extreme arctic outbreak on Dec. 24, 1982. The record high pressure in Dallas during January was 31.05 inches in 1962.

As northerly winds blew south from the Plains into Texas yesterday morning, and over the entire Gulf Coast today, the trajectory involved passage over an extensive snow cover. Hence, air that would have normally passed over warmer ground did not. This allowed cold air to penetrate much further southward than expected. Early on Jan. 7, snow was observed on the ground in every one of the 48 contiguous states except Florida (Fig. 4).

In short, this has been a highly unusual, but not unprecedented, arctic outbreak.

The good news is that the high is moving to the east and warmer air is slated to return to many south and southeastern locales fairly quickly.

© 2017 H. Michael Mogil

Originally posted 1/8/17

* 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, weather safety/preparedness

Ground fog! (H. Michael Mogil, CCM, CBM, NWA-DS*)

Dense fog advisories (Fig. 1) as well as the fog they describe (Fig. 2), covered southwest Florida early this Friday morning (Jan. 6, 2017). Some places had 0-0 visibility (that is, zero feet both horizontally and vertically); other places had visibilities of several miles horizontally and several hundreds of feet vertically (Fig. 3). Visibility is defined as how far one can see cloud bases (vertically) or known ground-based objects (horizontally).

   

It is the variation in fog thickness horizontally (and associated visibility) that came to the forefront this morning as I walked my dog, Pepper, around our southwest Florida neighborhood.

  

First, looking up my street, visibility was about a half mile, the measured length of my street (Fig. 4). As soon as Pepper and I walked onto the golf course, fog density increased and visibility decreased (Fig. 5). As we approached the location of my COCORAHS (Community Collaborative Rain Hail and Snow Network) rain gage, which is adjacent to a large pond, the fog density increased further and visibility tumbled down to a few hundred feet (Fig. 6). All images were taken within 10 minutes of one another.

These three scenarios are described in the table below:

Location Fog density Visibility
Street light ½ mile
Golf course dense ~ ¼ mile
Near pond very dense few hundred feet

For the street, warmer air temperatures resulted in less dense fog. Over the golf course, temperatures were slightly cooler, allowing for somewhat dense fog to develop. In the pond area, cooler air temperatures and higher atmospheric moisture contributed to the densest fog.

Note that these conditions contribute to the formation of fog. But, local winds, drainage winds, and other factors can allow fog areas to move. Not surprisingly, this can result in highly variable fog density along roadways. Common sense dictates that motorists of all types of vehicles (cars, buses, trucks) drive more slowly and pay even greater than average attention to potentially rapidly changing visibility.

While the dense fog has lifted already, it will be back again, especially during the upcoming winter months.

– – – – –

© 2017 H. Michael Mogil

Originally posted 1/6/17

* 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.

 

astronomy, weather

The Moon and Venus are getting closer (H. Michael Mogil, CCM, CBM, DMS)

If you happened to look to the west-southwest tonight just after sunset, and the clouds didn’t get in the way, you got to see the Moon catching up to Venus. Venus is the brightest planet in the sky right now and continues to impress in the evening sky.

The other night, a faint waxing crescent moon appeared on the western horizon just after the sun set. During the past two nights, as the sky darkened, the moon has become more easily visible in a position higher in the sky. Tonight, it lies just beneath Venus.

Mars, a very faint, reddish, planet by Venus’ standards, sits well above and slightly to the left of Venus.

All of these celestial bodies are annotated in Fig. 1 (image taken in Naples, FL).

Tomorrow evening, according to Earth & Sky, the Moon will sit above Venus, but below Mars.

Hope the clouds don’t ruin an impressive New Year’s sky show for my readers.

Enjoy!

© 2017 H. Michael Mogil

Originally posted 1/1/17

weather

What a difference a wind direction makes (H. Michael Mogil, CCM, CBM, DMS)

The other day, I wrote about the arrival of a strong cold front in south Florida. That front arrived with a significant chill down. Low temperature readings, this Saturday morning (Dec. 31, 2016), were in the 40’s across many areas of southwest Florida. Even with relatively light north winds, wind chill readings dipped into the 30’s to the north of a Fort Myers-Lake Okeechobee line. Dew point readings tumbled into the teens and twenties in many areas (Fig. 1).

But, with brilliant sunshine and a wind shift to the southeast, temperatures soared into the 70’s across most of south Florida by mid-day Saturday (Fig. 2) and into the mid- to upper- 70’s across southwest Florida by Saturday afternoon. Of even more significance was that dew point readings jumped from the 20’s into the low and mid 50’s in most places (Naples shown in Fig. 3).

This transition was linked to a change in wind direction. During the day on Friday and into Saturday morning, winds arrived from the north and northeast (Fig. 1); this kept the trajectory of the arriving continental Polar air mass passing over land. On Saturday, the trajectory of air was from water first and over the Florida peninsula next (Fig. 2). The result was that a continental Polar air mass had been replaced quickly by a maritime Tropical one.

And the trend will continue. By today, New Year’s Day, highs across southwest Florida reached the low- to mid- 80’s. Morning lows only dipped into the mid 60’s to lower 70’s along the southeast coast and low 60’s in most other coastal locations. These warmer day- and night- time readings should be the rule (plus or minus a few degrees) for most of the upcoming week.

I hope to be writing, soon, about examples in which wind direction and/or variations in wind direction play a significant role in an evolving weather event. Other GWCC writers have already posted regarding lake effect snow showers and squalls. Stay tuned!

© 2017 H. Michael Mogil

Originally posted 1/1/17