climate, geography, weather

Southwest Florida Temperatures Dip Below 70 Degrees (H. Michael Mogil, CCM, CBM, NWA-DS*)

Temperatures and dew points across southwest Florida finally dipped below 70 degrees Fahrenheit (F), yesterday morning (Oct. 25, 2017); this is a sure sign that the area has finally entered the autumn season. The last 70-degree temperature reading at Naples Municipal Airport (call sign APF), that did not involve cooler thunderstorm outflow winds, was back on May 11 of this year. Similar comparisons can be found across southwest Florida.

The dew point drop was almost more spectacular than that of the temperature (Fig. 1). The dew point stood at 74 degrees around 9:00 p.m. last evening (Oct. 24, 2017); by 11:00 a.m. E.D.T. (Oct. 25, 2017), a scant 14-hour period, the dew point had dropped to 45 degrees (a drop of almost 30 degrees)! Dew point temperatures that low are much more typical during later in autumn and during winter.

 

Southwest Florida’s annual seasonal march of temperature and dew point is easily shown within this Köppen climate classification system map (Fig. 2). Southwest Florida is firmly in the Aw category, a tropical wet-dry climate region characterized by high-sun angle rains and low-sun angle dryness. The framework was devised by Wladimir Köppen, a German botanist-climatologist in the late 1800’s and was continually re-examined and revised until his death, around the start of World War II. The basis of his classification system involved climatic boundaries that matched those of various vegetation zones (biomes). This is, of course, tied directly to temperature and precipitation transitions, the two main natural drivers of plant survival.

According to Dr. Troy Kimmel, a professor at the University of Texas – Austin, an Aw climate defines a savanna, a location that “has an average monthly temperature of 64.4 degrees Fahrenheit or greater and more than two dry months (monthly precipitation below 2.40 inches).”

This doesn’t mean that warm, humid and/or rainy weather won’t return to southwest Florida during the next few months. However, it does signify that time of year when cold fronts reach and pass by southwest Florida. Both of these (warmer, more humid, rainy weather and the passage of yet another cold front) are on tap for the upcoming Saturday-Tuesday period.

In honor of this transition, I opened my house yesterday morning and gave my air-conditioner a much-needed, even if temporary, respite. My checkbook is already heaving a sign of relief.

© 2017 H. Michael Mogil

Originally posted 10/26/17

* 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, geography, weather

A Tropical Storm Season Update – Aug. 1, 2017 (H. Michael Mogil, CCM, CBM, NWA-DS*)

So far, the Eastern Pacific (EPAC) Basin (the North Pacific Ocean east of 140 degrees West longitude) 2017 hurricane season is “on fire.” For July alone, Accumulated Cyclone Energy (ACE), which measures the combined strength and duration of tropical storms and hurricanes, ran at 2.5 times the recent monthly average. This made July 2017 the fifth most active July on record. On the other hand, the Atlantic Basin remained “on vacation.”

This isn’t unusual for the two basins, based on a long-term (50-year) seasonal study by Matt Bolton (student at St. Leo University, St. Leo, FL and intern at How The Weatherworks) and myself. The authors also found that inter-connectivities among the ocean basins worldwide tended to keep global tropical cyclone numbers fairly constant from year to year.

According to NOAA’s National Hurricane Center (NHC), the July 2017 EPAC basin saw five named storms, with two reaching major hurricane status and three having sustained winds reaching triple digits. Fernanda’s sustained winds peaked at 145 miles per hour.

Based on a 30-year (1981-2010) NHC climatology, three to four named storms typically form in the basin in July, with two becoming hurricanes and one of those reaching major hurricane intensity. This offers further testimony that July was an active time in the Eastern Pacific.

For the North Atlantic, Caribbean Sea, and the Gulf of Mexico (considered the North Atlantic Basin), two named storms and one unnamed tropical depression formed in July 2017. Based on a 30-year climatology (1981-2010), one named storm typically forms in the basin in July, with a hurricane forming once every other year. ACE has been low, noted NHC, because all storms have been, “…relatively weak and short lived.” In fact, none of the five named Atlantic storms have had sustained winds above 60 miles per hour.

Fig. 1 summarizes activity in the two basins for the current hurricane season through July 31, 2017.

An NHC-compiled climatology for the period 1966 to 2009 indicated that the mid-point of tropical season in both the Atlantic and Eastern Pacific basins occurs during the late August to early September time period. Recent activity does not a hurricane season make. Hence, the current activity in either basin may or may not be representative of what the entire season will bring.

© 2017 H. Michael Mogil

Originally posted 8/6/17

* 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.

geography, weather

A Look at GOES Water Vapor Imagery (H. Michael Mogil, CCM, CBM, NWA-DS*)

This feature is about GOES (Geostationary Operational Environmental Satellite) Water Vapor (WV) imagery. This type of satellite imagery (just one of many) is based on a specific band of data within the infrared radiation (IR) spectrum. It is collected from a geostationary satellite positioned roughly 22,000 miles above the Equator near the longitude of Florida and a longitude just west of the U.S. West Coast. With the satellites racing around their larger orbit path at an angular speed that matches that of the Earth’s surface, the satellite never sees the earth moving (hence, geo- or Earth- stationary). As a result, animations show cloud and/or moisture movement only. These two U.S.-operated satellites are part of a global network of satellites supported by several Nations and/or organizations.

I’m a big fan of WV imagery because it allows meteorologists and others to see the motions (called animations) of clouds and locations of moist/dry areas. Even when the image is not color-enhanced, the various white to black grey shades provide important discrimination. Thunderstorms, with cold, high, cloud tops appear as the brightest white; cirrus (ice crystal) clouds are white, but typically not as bright. Moist areas are generally grey; dry areas are black (see scaling, warm to cold, at lower left of Fig. 1). It would be easy to see this representation as something similar to a topographic or relief map, with thunderstorms representing the mountains and dry zones representing dried lake beds.

In this image (Fig. 1) and associated animation, there is a lot to see. For example, there is a bright area that develops across eastern Oklahoma and moves into southeast Missouri. To its north, it is easy to see a small-scale swirl in the clouds and moisture areas. Further northwest, a larger swirl (the center of the main storm system) was moving into southwestern Minnesota.

Since the WV imagery is based on IR (temperature) data, it is easy to ascertain ground-based features when very dry air covers a region (such as west Texas, New Mexico, Colorado, and western Kansas). Notice the roughly north-line that separates a light grey region to the west and a dark area to the east. This marks the front-range of the Rocky Mountains, with snow-covered, colder mountains to the west and low-altitude, warmer, plains to the east (Fig. 2).Near the end of the animation, a cloud rapidly develops over far west Texas, just to the northeast of El Paso. The western edge of the cloud stays locked in place, as nearby clouds continue to move by from the west. This cloud feature formed as moist air bumped into the Guadalupe Mountains (and Guadalupe Peak, 8751 feet elevation) and was forced to rise (Fig. 3). Rising air cools by expansion (lower pressure as one goes up in the atmosphere) and this often leads to condensation and cloud formation. Since the feature causing the cloud to form doesn’t move, the cloud feature continually reforms on its western edge. From the ground, this would likely appear as a standing wave cloud,

Finally, across New Mexico, notice that some grey clouds move over the snow-covered, colder ground areas. These clouds are warmer than the underlying ground and provide some information about the vertical atmospheric temperature profile in the area.

The bottom line to all of this is that GOES weather satellite imagery allows meteorologists and others to view larger geographic regions and both large- and small-scale weather features.

And this limited perspective of satellite capabilities is just with current GOES capabilities and just one IR channel. Wait until GOES R, launched last November, gets fully checked out and starts to deliver operational data and images. The WOW factor will likely be incredible!

© 2017 H. Michael Mogil

Originally posted 3/1/17

* 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, geography, weather

A real cold front is heading to Florida (H. Michael Mogil, CCM, CBM, DMS)

South Florida experiences “cool front” passages fairly often during the chillier months of the year. These bring wind shifts (winds northerly and northwesterly, quickly turning to northeasterly), and a modest drop in temperatures and a more significant drop in dew point temperatures. In summer months, very weak “cool fronts” occasionally reach as far south as places like Tampa and Orlando, and even less frequently to Naples and Fort Lauderdale.

fig001-sfc-fcst-161209-12zNow, a bona fide arctic cold front is enroute to the sunshine state and should pass by the Naples area on Thursday evening and be fully in control by Saturday (Fig. 1). No, arctic temperatures won’t reach this far south, but the temperature drop will be substantial. From daytime highs in the low to mid 80’s, temperatures in the Naples-Fort Myers area will tumble into the upper 60’s to near 70 by Friday. Overnight lows, which have remained in the uber-humid low to mid 70’s, will dip into the upper 40’s inland and lower 50’s in coastal population centers. This means that the Naples area will transition from temperatures that have been running about 6 degrees warmer than average so far this month to some 10 degrees colder than average by Friday and Friday night. That’s a 16-degree swing! For this time of year, the average high and low temperatures in Naples are 78 and 58, respectively.

Gusty winds will make the chill feel worse than it really is.

Although rain chances for the next several days are only 20 to 30 percent, once the initial non-arctic front passes through the Naples area by tonight, cloudy skies will rule until the arctic front sweeps things clear by Saturday.

Then, as quickly as it came, the cold air will be replaced by moderating temperatures and humidity as winds swing to the northeast by Saturday.

Keep looking to the northwest, though. Given the evolving upper level flow pattern, a string of Pacific weather systems, and periodic southward pushes of the polar vortex, additional cold air outbursts may be heading our way soon. Regardless of how the temperature evolves, dry weather should dominate for the next several weeks.

© 2016 H. Michael Mogil

Originally posted 12/6/16

biology, geography, learning, oceanography, weather

Red tide still affecting many southwest Florida beaches (H. Michael Mogil, CCM, CBM, DMS)

For weeks, a “red tide” (such tides are also referred to as a “harmful algal bloom” or HAB) has affected many beaches from Lee County, FL northward. With the departure of Hurricane Matthew a little over a week ago, the HAB was pushed southward and eastward, finally reaching beaches in Collier County. A day earlier (Fri., Oct. 6, 2016), news reports from Sarasota, FL indicated that with increasing winds and winds blowing from land to water (the approach of Matthew), the tide in its area would likely lessen. Unfortunately, the tide had to go somewhere. On Sat., Oct. 7, 2016, the HAB’s smelly signature arrived in Naples, FL. The tide has since returned to Sarasota area beaches, as well.

According to Florida’s Fish and Wildlife Conservation Commission, a “red tide is a naturally-occurring microscopic alga that has been documented along Florida’s Gulf Coast since the 1840’s and occurs nearly every year. Blooms, or higher-than-normal concentrations, of the Florida red tide alga, Karenia brevis, frequently occur in the Gulf of Mexico at this time of year (late summer or early fall). Red tide begins in the Gulf of Mexico 10 to 40 miles offshore and can be transported in (to) shore by winds and currents.”

fig001-san-diego-area-algal-bloom-imageAs the name “red tide” suggests, this bloom of algae often turns the water red (Fig. 1).

A HAB has two potential components. First, according to NOAA’s National Ocean Service (NOS), are the harmful toxins that some (not all) algal blooms create. Florida’s “red tide” bloom is of the type that can kill fish and make shellfish dangerous to eat. These same toxins may also make the surrounding air difficult to breathe. Both of these can have local economic impacts (fishing and tourism).

Another way the algal bloom (even without the presence of harmful toxins) can harm marine life involves the natural end-of-life of the bloom. According to NOS, “…when masses of algae die and decompose, the decaying process can deplete oxygen in the water, causing the water to become so low in oxygen that animals either leave the area or die.”

fig002-dead-fish-at-vanderbilt-beach-naples-161007Either way, with onshore winds and/or an ocean current with a coastward component, there is the potential for dead fish to wash up on the beach (as I personally witnessed 10 days ago on a north Naples beach). The presence of hundreds to tens of thousands of dead fish makes a beach a very unattractive and very odiferous place to visit (Fig. 2). This setting can easily ruin vacations and impact a local area’s tourism industry.

Local agencies and volunteers often work quickly to clean up the dead fish. However, as long as the tide sits nearby, fish deposition on the shore often continues.

fig003-sfl-watch-warning-map-161017-0630amedtCurrently, the National Weather Service (NWS) offices in Miami and Tampa Bay have posted “beach hazards statements” advising of the HAB’s potential human impacts through this Monday evening (Oct. 17, 2016) along many southwest Florida beaches (including those in Collier, Pineallis, Sarasota and Manatee counties). See Fig. 3 for affected areas in southwest Florida.  These statements note that, the red tide affecting the area may cause, “…possible respiratory irritation in some coastal areas. The irritation can include coughing, sneezing, and/or tearing eyes.” The statements further note that, “…people with respiratory conditions such as asthma, emphysema and other pulmonary diseases may be more sensitive (to these conditions).” The statements also urge people to check out current “red tide” conditions at the following web sites:

http://tidesandcurrents,noaa.gov/hab

http://mote.org/beaches

http://myfwc.com/redtidestatus and consider visiting nearby unaffected beaches.

For health information, people should check with their physician, as appropriate. People can also get medical information at the floridahealth.gov web site or by calling the Poison Control Center at 1-800-222-1222

For links to “red tide” conditions across other coastal states, visit NOAA’s “red tide” web site.

© 2016 H. Michael Mogil

Originally posted 10/17/16

climate, geography, weather

THEWEATHERMOGIL:: Colorado’s summer monsoon begins

Dateline: Vail, CO

During the months of May and June, in many parts of the Nation, you might start hearing talk about the soon-to-arrive, “monsoon.” But, do you really know what the monsoon is?

The North American Monsoon provides summer thunderstorms from California to Colorado and from Mexico to Montana. The area of coverage is basically the southwestern U.S. and Northwest Mexico.

It is sometimes referred to as the Southwest monsoon or the Mexican Monsoon.

Not to be excluded, Florida experiences a similar monsoon.

When it comes to describing a monsoon, a lot of people tend to think about the torrential flooding rains that typically occur in India and Southeast Asia each summer. The U.S.’ summer monsoon is not quite as dramatic, but it does provide lots of precipitation. Flooding rains, due to slow moving thunderstorms and/or repeated thunderstorm transits, are often part of the “wet” monsoon season.

Yet, the word monsoon (Arabic origin) simply means a seasonal wind reversal. Rainfall is not the driver for the monsoon; rather, it is the result!

Hence, what starts as a “wet” event (linked to a certain wind flow pattern), ends with the onset of a “dry” event (with a different wind flow pattern). Both are monsoon events, even though most folks key on the “wet” portion only.

Fig001-seasonal-SLP-wind-patternsFigure 1 can be used to describe all three of the above-mentioned different geographically-based seasonal monsoon patterns. Note how the locations of pressure systems flip-flop between land and water locations based on solar heating variations and land-water differences. This, in turn, results in significant wind direction shifts seasonally (even if not day-to-day). Wind shifts allow different types of air masses to move into or away from certain areas.

Fig002-US-radar-map-160630-early amLinked to all these ground level (or “surface”) wind patterns, there are accompanying upper level pressure and wind patterns. For example, consider the upper level high-pressure system (5 to 10 miles above the Earth’s surface) that had parked itself across Arizona, Nevada and California for much of June. That high has now shifted eastward, allowing the upper level monsoonal wind circulation to transition. In the case of Colorado, my home base this week, the high’s movement to a location south of Colorado has allowed thunderstorms to finally develop across western parts of the state as well as much of the intermountain western states (Fig. 2).

Fig003-upper-winds-160630-21ZAccording to Julie Malingowski of the National Weather Service’s Grand Junction Forecast Office, “Synoptically, the classic monsoon pattern in Colorado is defined with high-pressure over the Southern Great Plains. This allows a clockwise/southeasterly tap from the Gulf of Mexico.” Then, moisture-laden easterly waves on the south side of the high ride the high into the Four Corners. This week, there is no closed high-pressure system, but the upper level ridge has moved to the south of Colorado. This is allowing Pacific moisture to move into the southwest U.S. and Colorado (Fig. 3).

Fig004-7-day-qpf-thru-160706-12ZThunderstorms are now mentioned in statewide forecasts for the southwest U.S. for most of the next four days. Then, the monsoon is expected to relax somewhat next week. However, the 7-day quantitative precipitation forecast map clearly shows that the upper high will be relocating even further to the east (Fig. 4), suggesting a quick return to monsoonal conditions.

Since the “wet” monsoon is summer-season based, it is not unusual for record-breaking heat to herald its arrival. Such was the case across much of the southwest U.S. earlier in June. Places like Denver and Grand Junction, CO, Flagstaff and Phoenix, AZ, Albuquerque, NM and Los Angeles, CA all reported very hot days just before the onset of the monsoon.

However, even with media hype about the excessive, sometimes record-breaking, warmth, temperature averages for the first 28 days of June, at all of the above inland locations, were only in the 3.7 to 4.7 above average range. Both Los Angeles airport and downtown locations reported readings only a couple of degrees above average.

Now that the monsoon has begun in many locations, temperatures have dropped to near to just slightly above seasonal averages.

© 2016 H. Michael Mogil

Originally posted 6/30/16

geography, weather

THEWEATHERMOGIL:: Latitudes and altitudes, Colorado style

Dateline: Vail, CO

This article, which addresses the effects of altitude and latitude on temperature, could be for most anyplace(s) in the 50 states. But the western U.S. (including Alaska and Hawaii), with incredibly wild swings of altitude across relatively small distances, offers some of the best settings. Since I am stationed in Vail, CO this week, the “Centennial State” will be my focus.

According to netstate.com, Colorado’s elevation ranges from a low of 3,315 feet above sea level on the Arkansas River at the Kansas-Colorado border to a peak altitude of 14,440 feet atop Mt. Elbert. Pike’s Peak, the state’s most famous mountain, tops out at a scant 330 feet below Elbert’s. The state’s mean elevation of 6,800 feet makes it the highest state in the Nation.

Early morning temperatures today (Sun., Jun. 26, 2016), provide some insights into the effect of altitude (i.e. terrain) and latitude. For latitude, one only has to look across Kansas and Nebraska (Fig. 1). As is typically the case, from about 25 degrees latitude poleward, temperatures are lower the further one is from the Equator.Fig001-colorado-temps-160626-12Z

Across Colorado, latitude is partially masked by terrain effects on dew point and temperature. Dew points early on Jun. 26 were highest (mid-50’s) across the southeastern Plains (east of the Continental Divide – represented by the ochre colored line extending mostly north-south across the eastern part of the state). Dew points dropped into the 40’s across the northeast Plains. Across mountainous areas from the Continental Divide westward, dew points were mostly in the 30s.

When talking about low temperatures, dew point is extremely important. The dew point near the ground acts as a brake on nighttime cooling. It is hard for temperatures to fall below the air’s dew point temperature.

Since air temperatures normally drop off with increasing altitude, it is not a surprise that sunrise temperature readings were lower at higher mountain altitudes. The relationship isn’t one-to-one due to several reasons, including local terrain variations, geographic features (e.g., nearby river, nearby lake, nearby urban center) and the presence or absence of wind. Wind can mix the air near the ground and keep surface temperatures warmer.

Most notable is that the temperature at Craig (elevation 6,918 feet), in the northwest part of the state, dipped to 35 degrees. Eagle (elevation 6,547 feet), to the southeast of Craig only dipped to 46 degrees. But, Carol B., who works at a Vail timeshare resort, reported that at her home in Edwards, a community about 15 miles due east of Eagle, frost graced her windshield this morning.

While it may be summer, early on Jun. 26, there was a hint of cooler times across parts of Colorado.

© 2016 H. Michael Mogil

Originally posted 6/26/16

astronomy, geography, geology, weather

THEWEATHERMOGIL:: When geology and meteorology meet

In the Earth-Space Science arena, we often discuss meteorology, hydrology, oceanography, astronomy and geology. In recent years, a curious blend of meteorology and astronomy, known as “Space Weather,” has emerged. The National Weather Service, now routinely issues “Space Weather” forecasts that address parameters such as the solar wind and the transport of electromagnetic surges toward Earth and our orbiting spacecraft. While not weather, as we know it Earth, its impacts are more weather-focused (electricity) than astronomy-based (planetary motions).

Another, not-so-unusual, relationship involves geology and meteorology. At the Earth-atmosphere interface, mountains and coastlines affect wind flow; mountains can lead to rain-shadow effects (wet on one side of the mountain and dry on the other); changes in elevation and/or the mountains themselves can lead to intense thunderstorm rainfall and flash flooding; and terrain, even without a mountain presence, can help to channel precipitation runoff into narrow canyons or waterways and lead to significant localized and/or deadly flash flooding.

Add volcanoes to the mix and now the geologic events themselves can cause meteorological impacts. The recent eruption of the Pavlof volcano in the Aleutian Island chain is a prime example (Fig. 1).

NATL001-USCG-Pavlof-ash-plume-tstorm-shape-160328-130pmAST?

First, any heating of the exterior or near exterior of a volcanic mountain (which in northern climes is often ice and/or snow covered), can lead to rapid snowmelt and cause avalanches, mudslides and/or flooding. It is easy to see the effect that melting has in this image of the Pavlof volcano (Fig. 2).

NATL002-USCG-Pavlof-ash-plume-flow-160328-319pmAST?

The main risk from volcanoes happens during and following an eruption. Based on the force of the ejected air and gas cloud, volcanic dust and particles can be lifted to altitudes high above the Earth’s surface. The Pavlof eruption of last weekend was relatively “tame,” sending ash only four to five miles aloft. The eruption of Mount Pinatubo (June 1991) sent ash so high that it reached well into the stratosphere, where ash was still detected years later.

The eruption process, once started, resembles the convective process of regular thunderstorm formation. In fact, volcanic clouds, as they grow from the eruption, often look like “dirty” thunderstorm clouds (towers, anvils, precipitation, virga and/or “fall” streaks, lightning and sometimes waterspouts). Fig. 1 and Fig. 3 typify what volcanic thunderstorms look like.

NATL003-USCG-Pavlof-ash-plume-160328-1117amAST?

In the short-term, volcanic ash and dust can lead to significant aviation impacts. In 1982, British Airlines Flight 9, flying at 37,000 feet, entered an ash cloud near Jakarta, Indonesia. The flight crew was not aware that such an ash cloud was in their flight path. As a result, the plane’s four engines “flamed out” and the plane started a growingly steep descent. The pilot was able to restart the engines, but not until the plane had descended to 12,000 feet. Following another “flame out” of one engine, the pilot was able to safely land the plane. A review following the incident demonstrated that the ash cloud was associated with an eruption from Mount Galunggung, some 110 miles southeast of Jakarta.

A similar experience occurred in late 1989 following an eruption of Mount Redoubt, AK. Here, a KLM flight, carrying 231 passengers to Anchorage, suffered a drop of several thousand feet following a “flame out” of its engines.

Since the ash cloud plume from the recently-erupted Pavlof volcano extended several hundred miles into Alaska (Fig. 4), flight operations at Alaska Airlines were put on hold (41 cancellations) on Monday. With the volcano eruption lessening, and the threat level lowered accordingly, flight operations at Alaska Airlines are expected to resume yesterday.

NATL004-GOES-Pavlof-ash-plume-160328-1900Z

As noted by Alaska Airlines spokeswoman Bobbie Egan, and most likely echoed by every airline executive worldwide, “We just simply will not fly when ash is present.”

Now, the United States Geologic Survey (USGS) routinely monitors volcanoes for all types of hazards and issues advisories or alerts for a wide range of target audiences. The USGS and other agencies use a sophisticated array of sensors to track earth movements, changes in lava distribution and aircraft to monitor actual or imminent eruptions. In the U.S., NOAA routinely provides satellite support, helping to track any ash clouds and monitor their density. Worldwide, a total of 9 Volcano Ash Advisory Centers (VAACs) provide such support for international aviation. The bottom line is that flight cancellations rather than emergency landings or actions are now the rule.

Although the Pavlof volcano has calmed down a bit and alert levels have been lowered, Pavlof and several other volcanoes worldwide remain on active USGS monitoring lists.

© 2016 H. Michael Mogil

Originally posted 3/30/16; updated 3/31/16

geography, weather

THEWEATHERMOGIL:: It’s all about source regions

March has been a mostly mild month across much of the U.S. That’s because upper-level and lower-level winds have been blowing primarily from the west, southwest or south across the Nation. Yes, there have been occasional bursts of chillier air, and these come on the heels of winds from northwest, north or northeast.

This is an important factor in the weather forecasting world because winds from certain directions carry air from certain “source regions.” “Source regions” are large geographical areas that help certain temperature and moisture characteristics develop in the lower atmosphere.

Consider the Gulf of Mexico. It’s a large, relatively homogenous, region filled with “bathtub-like water.” Hence, heat and moisture from it’s waters can create a warm and humid air mass above it. Similarly, air that sits over an icy landscape becomes cold and dry (cold air cannot hold much moisture).

Once formed, these air masses (large geographic regions with similar temperature and moisture characteristics) can move. When they do, significant changes in local weather conditions, possibly far-removed from source region origins, can take place.

Today, a storm system is exiting the Great Lakes, allowing a little chilly air to move southward into the U.S. (Fig. 1a). But, west and southwesterly winds are already coming into play across the western Plains states. These will initially spread warm and dry air northward through the Nation’s mid-section. As the winds across the rest of the Plains states and Mississippi River Valley become southerly, warm and moist air from the Gulf of Mexico will be drawn northward. This will set the stage for yet another bout of severe weather as the week unfolds. In fact, NOAA’s Storm Prediction Center is indicating a large region of severe storm potential on Mar. 30 from Iowa southward to Louisiana (Fig. 2).

NATL001-sfc-fcst-160328-160402   NATL002-spc-fcst-160330

However, by the start of April, a very chilly air mass will charge southward from Canada, thanks to a significant shift in upper level winds (Fig. 3). By Apr. 2, cold air will cover the region from the northern and central Plains states to the Northeast and Middle-Atlantic (Fig. 1b).

NATL003-ua-fcst-160328-160402

Then, winds return to westerly across the U.S., until around Apr. 8, when another significant system is expected to race into the Northeast, bringing another brief period of cold temperatures into the Great Lakes and Northeast. Leading up to this Canadian air mass intrusion, another Gulf of Mexico air mass will help set the stage for severe storms across parts of the eastern half of the Nation.

© 2016 H. Michael Mogil

Originally posted 3/28/16