09 January 2007 Weather Discussion

This is the sea level pressure (SLP) map with fronts for 1200Z Tuesday 09 January 2007. Recall that 1200Z (noon at the Prime Meridian) corresponds to 4:00 AM Pacific Standard Time. A cold front has just passed from North America and gone over the Atlantic Ocean. Note how the cold front lies along a trough in SLP. The cold front connects to a warm front east of Newfoundland, and an occluded front extends east to a low center the the Gulf of St. Lawrence.

This SLP map is from twelve hours earlier (0000Z 09 January 2007). We will look at other charts for this time because the extratropical cyclone is not as close to the edge of the map. Twelve hours earlier the low center was northwest of Maine and the cold front was closer to the East Coast. The cyclone is traveling towards the northeast, which is a typical direction for cyclone propagation. Note how the cyclone was less occluded (shorter occluded front) at 0000Z than at 1200Z. Cyclones typically occlude as they age.

This is an 850 mb map for 0000Z 09 January 2007. It shows temperature (colored lines), relative humidity (shading), and winds (flags) at the level in the atmosphere where the pressure is 850 mb (hPa). Typical values for SLP are around 1000 mb (hPa), and the 850 mb level is about 1500 m above sea level. Note how the warm edge of the temperature gradient (closely spaced isotherms) off the East Coast corresponds to the general position of the surface cold front on the SLP map. Also note how the warm edge of the temperature gradient east of New England corresponds to the general position of the surface warm front on the SLP map. If you look carefully, you will see a tongue of warmer air extending from the "elbow" of the isotherms westward into the interior of Canada that corresponds to the general position of the occluded front on the SLP map. I am looking at temperature gradients at 850 mb level rather than at the surface because heat transfer from the ocean will smooth out strong gradients in the surface layer of the atmosphere (this effect does not occur over land).

This is a detailed SLP map for 0000Z 09 January 2007 with many more station symbols from land stations, buoys, and ships than the previous SLP map. Note how the wind direction shifts from one side of the front to the other. Winds are typically southerly (from the south) in the warm sector (ahead of the cold front and behind the warm front). For this cyclone, the warm sector is out over the Atlantic. Winds are typically westerly or northwesterly in the cold sector (behind the cold front). For this cyclone, the cold sector is over the East Coast of the U.S. Winds are typically easterly or southeasterly ahead of the warm front, the area near the Gulf of St. Lawrence for this cyclone.

This map shows surface winds retreived by a satellite scatterometer. Note how the location of the surface cold front is distinctly marked by the abrupt wind shift from southwesterly ahead to northwesterly behind. The exact time of the retrieval is not clear to me, but it appears to occur sometime later in the day on 09 January 2007.

This is the 1445Z (0945 EST) 09 January 2007 visible satellite image from GOES-East, one of the U.S. geostationary satellites. The boomerang-shaped cloud structure off the East Coast is associated with the cold and warm fronts of the extratropical cyclone seen in the previous maps. The dark line extending eastward from Newfoundland is the shadow of the higher cloud deck to the south on the lower cloud deck to the north.

This is a subsection of the 1545Z (1045 EST) 09 January 2007 visible satellite image. If you look carefully at the cloud band off the East Coast you will see a narrow bright line near the eastern edge of the main cloud band. This is called a "rope cloud" and marks the location of the surface cold front.

This is a small subsection of the 1615Z (1115 EST) 09 January 2007 visible satellite image. Note the small patchy clouds west of the main cold front cloud band. These are shallow convective clouds generated by the flow of cold air over the warm Gulf Stream. Some these clouds are arranged in lines corresponding to secondary "roll" circulations in the atmospheric boundary layer. This weather phenomenon is called a "cold air outbreak", but this particular one is rather weak because the air behind the cold front is not especially cold.