16 January 2007 Weather Discussion

This is a sequential collection of SLP maps for every 3 hours between 2100Z Sunday 14 January 2007 and 1800Z Tuesday 16 January 2007. Note the stationary front that extends across the eastern U.S. during the first 6 hours of the period. At 0300Z on 15 January, a distinct low center develops along with a warm front across the Ohio Valley and a cold front slightly west of the Mississippi River. The low center and warm front then move northeastward, and the cold front moves southeastward. After 1200Z on 15 January, the warm front develops a bend as the warm air mass moves faster towards the north on either side of the Appalachian Range than it does in the mountainous area. The Appalachians also retard the eastward movement of the cold front after it reaches them at 2100Z on 15 January. The final map at 1800Z on 16 January shows that the low center has moved northeastward to the tip of Newfoundland with a surface trough and cold front extending southwestward over the Atlantic, Florida, and the Gulf of Mexico. High pressure now prevails over the central U.S. and Great Basin.

This is a sequential collection of radar maps for the same times as the SLP maps. Blue indicates light precipitation and green and yellow indicate heavier precipitation. During the beginning of the period the precipitation has little spatial organization but then becomes arranged in bands along the cold front and ahead of the warm front. The precipitation pattern appears to break up around 2100Z on 15 January when the cold front reaches the Appalachian Range. Although the mountains may have disrupted the front, it is also possible that the high topography blocks the radar signal such that precipitation can no longer be easily seen. Precipitation is also not seen over the Atlantic Ocean due to the limited range of the radar. On 16 January additional areas of precipitation can be seen over the U.S. The first set occurs southeast of the Great Lakes and is probably "lake-effect" snow that is generated when cold air behind the cold front flows over the relatively warm water. The cause of the large area of precipitation in Texas that is unconnected to the main cold front will be discussed later. There is also a band of precipitation associated with a cold front coming into the Pacific Northwest from the Pacific Ocean.

This is a sequential collection of visible satellite images for the same times (with a 15 minute offset) as the SLP maps. The first image of the set occurs during local afternoon (2115Z or 1515 CST) of January 14 with several dark nighttime images following. Note that the shadow of the large elevated cloud band produced by the warm conveyor belt of the cyclone can be seen over the central U.S. in the first morning image of January 15 (1515Z or 0915 CST). The shadow of a low-level cloud layer behind the main band can also been seen over Iowa. The large cloud shield associated with the warm conveyor belt continues to move eastward while leaving a widespread area of low-level cloudiness or snow behind it. Similar shadows occur on the morning of January 16. An area of widespread cloudiness behind a rope cloud marking the location of the surface cold front can be seen over the Gulf of Mexico on 1815Z and 2115 Z of 15 January and 1515Z and 1815Z of 16 January. These clouds are generated by convection as cold air passes over the warm water.

This is a sequential collection of window-channel infrared satellite images for the same times (with a 15 minute offset) as the SLP maps. Dark areas are warm and emit much radiation while bright areas are cold and emit less radiation. Since water vapor has little absorption at the measured wavelength, clouds but not clear sky block radiation emitted by the surface. Cloudless areas of Canada and mountainous areas of the western U.S. are not as dark as cloudless areas of Mexico and the Atlantic because the former regions are colder. Land areas are also darker during day than during night due to the diurnal cycle of temperature. This phenomenon does not occur over the ocean due to its large thermal inertia. The first couple of satellite images from 14 January show a somewhat incoherent cloud mass over the U.S. At the same times as the cyclone and surface fronts develop in the SLP maps, the clouds become organized into a band stretching northeastward over the U.S. and then turning directly eastward near Canada. This cloud structure has a high top since it is bright and it is generated by the warm conveyor belt. It is difficult to see the areas of low-level cloudiness trailing the main cloud band and over the Gulf of Mexico because they have warm tops and emit almost as much radiation as the cloudless surface. On 16 January, the main cloud structure associated with the cyclone moves over the Atlantic but remains connected to a cloud band stretching down to Mexico.

This is a sequential collection of water vapor infrared satellite images for the same times (with a 15 minute offset) as the SLP maps. Since water vapor absorbs at the measured wavelengths clouds and water vapor in the upper troposphere block radiation emitted by the surface. The cloud features in these images are similar to those in the window-channel infrared images, but several new features can be seen. Immediately west of the cloud band produced by the warm conveyor belt is a dark band corresponding to the dry tongue that allows radiation emitted by the relatively warm surface escape to space. It is also interesting to note that the clouds over Mexico and Texas on 16 January occur in a large band of slightly darker gray, indicating substantial water vapor around the clouds. This is a stream of moisture drawn up from the tropics that is producing the precipitation seen over Texas in the radar maps. The darkness of the subtropical Atlantic shows that little water vapor is present in the upper toposphere since this is the downwelling region of the Hadley circulation.

This map shows wind at the 200 hPa level, which is near the tropopause, on 00Z 16 January 2007. Darker purple corresponds to faster wind speed and the location of the jet stream. The reason why moisture is being drawn up from Mexico is that the jet stream has split over North America with a weak branch over Canada and a strong branch dipping down to the subtropical eastern North Pacific. Note how the jet stream passes parallel to the surface cold front and then turns east.

This is a skew-T chart that shows temperature (right) and dew point (left) measured by a balloon launched from Albany, NY on 12Z 15 January 2007. The closeness of the temperature and dew point lines indicate that the entire troposphere is saturated. The straight blue lines slanting towards the upper right are lines of constant temperature (hence "skew-T"). Note how temperature near the surface is less than 0 C whereas temperature near the 800 hPa level is more than 0 C. This is opposite the usual condition where temperature decreases with height. The SLP map for 12Z 15 January 2007 indicates that Albany is north of an approaching surface warm front. Recall that warm fronts tilt forward with height. Even though the surface part of the warm front has not yet reached Albany, the elevated portion of the warm front has already passed through. This is also consistent with southeasterly wind at the surface (typical for ahead of a warm front) and the southwesterly wind aloft (typical for behind a warm front). The atmosphere is saturated because the warm conveyor belt is passing over Albany and producing precipitation (as seen in the infrared image and radar map for 12Z 15 January 2007). The big change in temperature near the 200 hPa level marks the tropopause, and the nearly isothermal layer above (with a temperature around -60 C) is the stratosphere.

This is a skew-T chart for San Diego on 12Z (0400 PST) of 16 January 2007. The temperature and dew point lines are very far apart, indicating that the troposphere is very dry. The relatively cold layer extremely close to the surface is generated by nighttime radiative cooling, and the big change in temperature near the 300 hPa level marks the location of the tropopause. The SLP map for 12Z 16 January 2007 shows high pressure over the Great Basin and a weak trough off the coast of California. This pattern is typical for "Santa Ana" conditions, where inland high pressure drives downslope flow over Southern California. Because the air over the western U.S. is unusually cold, the temperature of San Diego is not as warm as it typically is during a Santa Ana.