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Question: Is it possible for a high pressure system to bring rain? I know that most storms are brought by a low pressure system, but is it possible? — Luke
Answer: This is a good example of how, in order to make weather understandable and more easily communicated, we often greatly simplify what are in fact very complex patterns and processes in the atmosphere. While we commonly associate low pressure with precipitation and high pressure with fair skies (and rightly so in many cases), there are exceptions and there are situations in which interactions of relatively low pressure at one altitude correspond to relatively high pressure at another, sometimes resulting in precipitation. That's a long way of saying that it IS possible for precipitation to occur when there is a high pressure system in place. One common example for us is the "cold air damming" pattern, in which high pressure extends into the state from the north or northeast, creating a north or northeasterly wind flow and bringing a shallow layer of cold, dense air into the region. Sometimes, that air undercuts and lifts warmer, moister air that condenses into clouds and precipitation. In many cases, this only yields sprinkles, drizzle or very light rain, but in some instances the precipitation can be more substantial. Another example would be high pressure building in from the west in the winter, producing strong northwesterly winds for our state. For a lot of us, that is generally a dry pattern. However, it isn't uncommon in the mountains for air flowing up the western slopes within the eastern or northeastern portions of the surface high to be moist enough to produce flurries or snow showers there. One other note is that tropical cyclones often are best organized and set up to intensify if the low pressure center near the surface is lined up nicely with a high pressure center in the upper atmosphere more or less directly above it.
Feb. 10, 2017 | Tags: fronts & airmasses, general meteorology, rain
Question: Why is it that the air sometimes smells noticeably fresher? This evening is one of those times. — K. Schaffer
Answer: There can be a number of reasons, and it varies depending on the kind of "un-fresh" smell that existed before. Sometimes it is a simple matter of the air being stagnant or blowing from the direction of a source of pollutants, and then having the wind direction shift or the speeds pick up such that turbulent mixing brings cleaner air down from a few hundred or few thousand feet above the ground. There can also be a fresh smell in the wake of a rainfall in which the rain captures some of the particles in the air and scours them out, depositing them on the ground. In the case of the evening you wrote in, we had a warm afternoon followed by a cold frontal passage that occurred around mid-afternoon. You may have been noticing the effects of cooler, less humid air flowing in from a different direction and perhaps a mixing effect as well. In the case of this front, we went from a high temperature of 76 that day to a high of 55 the next, indicating a notable airmass change.
Feb. 7, 2017 | Tags: air quality, fronts & airmasses
Question: I got a weather surprise today (January 26) around noon as I was bike riding West on Hillsborough Street near NCSU. I was going to try to experience the passage of the strong cold front that I had been tracking over time on the personal weather stations to the West of us. As the front got to somewhere between Cary and West Raleigh a narrow line of heavy rain and wind similar to a squall line suddenly blew up within roughly a 5-minute period. From looking at myfox8 interactive radar website when I got home it seemed to have blown up around I-540. A lot of times this sudden development happens with summer thunderstorms between Durham and Raleigh or around I-95 well to the East. Can you explain the dynamics of this type of pop-up squall line? — Dave Crotts
Answer: The frontal structure that day appears to have been of the "split front" variety in which a cold front aloft decouples from and surges well ahead of a lagging surface cold front. In these situations, there is usually a band of fairly widespread precipitation that is located along a "warm conveyor belt" of moist air that is acted on in part by upward motions associated with a broad upper-level trough following along behind both frontal boundaries. That band of precipitation is what occurred through the late night/early morning hours on Thursday. The cold front aloft brought a deep layer of cooler, drier air into place that caused some clearing in the wake of the morning rain, but left a fairly moist, shallow layer of warmer air near the surface along and ahead of the surface cold front. By the time that front moved in to the Raleigh area, the break in cloud cover had allowed significant warming at the surface, leading to substantial instability due to the warm surface and cooling airmass aloft. At about the same time, the surface front provided some measure of lift to the low-level air along it (due to drier, cooler, and thus more dense air undercutting the warmer, more humid air) and at about the same time a concentration of higher winds aloft (a subtle jet streak) moved into position to also provide a measure of enhanced upward motion. Where the two features overlapped, we saw a northeast to southwest oriented line of showers develop, and they briefly become rather intense over a small area. In short order, the surface front moved on to the east, the jet streak pulled away northeast, and skies cleared again. You can read briefly about split fronts and see a couple of helpful diagrams at www4.ncsu.edu/~nwsfo/storage/training/jets/split.html.
Feb. 3, 2017 | Tags: cool sites, fronts & airmasses, past weather
Question: If warm air comes from the southwest and the northwest, then where does cold air come from? — Aliyah
Answer: The development of colder or warmer air than is currently in place over a given area is actually much more complex than that, and includes factors such as the presence or absence of clouds and precipitation, whether on the large scale air over the region is rising or sinking, and the extent to which warmer or cooler air than average has been carried especially far north or south by other weather systems upwind of our location. Because of that, it is possible under the right circumstances for warmer air or colder air to flow in from any direction. Even with all that being the case, it is true that in a broad, general sense, warmer air tends to originate south of us and vice versa, so colder air most often moves in with winds having a northerly component (northwest, north, northeast, etc) while warmer air is more likely to move in with winds having a southerly component.
Dec. 22, 2016 | Tags: cold, fronts & airmasses, general meteorology, heat
Question: Will we have another unseasonably warm end of November and December like last year? — Mark Womack
Answer: There isn't a very strong influencing climate signal to base a confident forecast about that on, though a weak La Nina pattern in the Pacific does suggest a slight tilt of odds toward warmer and drier than normal weather overall for December into January. In the shorter term, most models lean toward a range of temperatures not too far off normal for the most part as we head through the first half or so of November (after a rather warm start in the first few days of the month), while the Climate Prediction Center suggests the latter half of November has about equal chances of turning out above, near, or below normal for temperatures.
Nov. 7, 2016 | Tags: fronts & airmasses
Question: Where is Fall? Why is it 80 degrees in October? — Kendall Terashima
Answer: In the first three weeks of October, we had several days with highs in the upper 70s to mid 80s, on the warm side of normal though mostly below record levels. A couple of times during the month, we've had upper-level high pressure ridges build over the eastern U.S., which has stalled frontal boundaries and cooler/drier airmasses off to our northwest, leaving us with stretches of fairly warm weather. We've also had a couple of shorter intervals with cooler highs in the upper 60s and low 70s. By the time this answer appears, we're likely to have had another front push through with a notably cooler and less humid airmass that feels plenty autumn-like, with highs in the 60s to around 70 and dew points in the 30s and 40s.
Oct. 23, 2016 | Tags: fronts & airmasses, heat, humidity/dew point, normals
Question: What are the chances that Disturbance 1 will catch up to and merge with Matthew? — Bob
Answer: You're referring to a disturbance well northeast of Matthew early in the first week of October that was officially designated as "Invest AL98" that later became Tropical Storm Nicole. At the time we're writing this, the chance that it will catch up to and be absorbed directly by the circulation of Matthew appears very slim, as most computer projections maintain the two entities as separate areas of rotation. It does appear there's some potential for the Nicole circulation to be absorbed into a cold frontal boundary that will trail southward from a non-tropical system far to its north by Monday or Tuesday.
Oct. 7, 2016 | Tags: fronts & airmasses, hurricanes
Question: This morning when checking the weather maps I noticed an odd-colored purple frontal boundary stretching from Winnepeg south through much of western Minnesota. Wikipedia tells me it is an occluded front, and being someone who watches the local news the majority of evenings, it struck me as odd that I'd never heard the term before. Is it that I've not been a good weather citizen all these years, or are they uncommon to see? How often do we encounter them in the Piedmont? — Matt in Durham
Answer: Occlusions are quite common overall as a part of the general organization of mid-latitude synoptic storm systems. In somewhat oversimplified terms, they arise where cold fronts catch up to warm fronts, such that the warmer airmass between those two boundaries is lifted off the ground in and along the vicinity of the occluded front. For much of the year, they tend to occur most frequently north or northwest of us, while more typical warm fronts, cold fronts and stationary fronts extending south of the primary low pressure areas affect us. However, during the winter months, we tend to see a few occluded fronts track across our state when surface low pressure systems follow a notably more southerly path than at other times of year. The character of weather associated with them falls somewhere between classic warm front and cold fronts, and can lean closer to either, depending on the distribution of temperature and moisture around a particular storm system. Now that you're aware of them, you'll likely notice them on weather maps rather often, in many cases running just east and/or southeast from slow-moving, deep low pressure centers.
Sep. 30, 2016 | Tags: fronts & airmasses, general meteorology
Question: Why do the prevailing winds change in September and October? — Brian
Answer: In September and October, as we transition out of summer and into the Fall, the jest stream begins to move on average to a more southerly position and as a result we begin to have more frequent passages of the polar front south of our position. However, during this early part of the transition to a colder regime, it is common from fronts to move a short distance south of us and either stall or wash out, while a ridge of surface high pressure centered near New England or off the northeastern coast of the U.S. either stalls for a few days or moves slowly, with the high pressure ridge extending for several days down the eastern seaboard, focused on the east side of the Appalachian mountains. This high pressure position results in winds anywhere from northerly to easterly for several days at a time on a number of occasions during those months, leading to the prevailing direction being northeasterly. That said, as with the remainder of the year, it is possible to have winds from any direction during September and October, but they are somewhere between north and east often enough to average out to northeasterly. For the rest of the year, winds can also be variable, but they are frequently enough between south and west for that to be the predominant direction for all other months. During the summer, for example, winds are often fairly light but strongly favor a southwesterly direction due to the near-stagnant position of the Bermuda High. During the core of winter there is a noticeable tendency for winds to shift to northwesterly behind cold fronts, northeasterly for a short time with high pressure areas northwest of us, and southwesterly as high pressure centers move to our east or southeast and new cold fronts approach from the west or northwest.
Sep. 20, 2016 | Tags: fronts & airmasses, general meteorology, winds
Question: What is the difference between a 'front' and a high/low pressure system? — John Sides
Answer: Pressure systems are characterized by how the atmospheric pressure varies horizontally, such that a high pressure center, for example, is a location where the pressure become lower in all directions away from the center. Low pressure centers are the opposite, such that the pressure is higher in all directions away from the center. It is also possible to have "systems" that aren't centers, such as elongated ridges of high pressure or troughs of low pressure.
A front, on the other hand, is a zone of contrast (a "boundary," of sorts) between airmasses having significantly different temperature and/or moisture characteristics. Strong fronts have sharp differences of temperature, moisture (and usually wind direction) over a short distance, but fronts can also be weak and rather diffuse, with more gradual changes in the character of the airmass. The name and symbol for a front depends largely on its movement. Where colder air replaces warmer air, you have a cold front, and vice versa for a warm front. A frontal boundary can also be stalled in place, or nearly so, in which case it is called a stationary front.
Generally, frontal boundaries exist on the periphery of large high pressure systems, while many (but not all) low pressure systems form along or very close to frontal zones.
May. 25, 2016 | Tags: fronts & airmasses, general meteorology
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Published: 2007-10-09 14:40:00
Updated: 2014-06-24 16:06:51