30 January 2007 Weather Discussion

This is a SLP/1000-500mb thickness map for 1200Z Tuesday 30 January 2007. There is a low center in the Gulf of Alaska with warm advection occuring to the northeast and cold advection occuring to the southeast. The regions of advection can easily be identified by where colored SLP contours cross the black thickness contours. Warm advection occurs where the geostrophic wind (parallel to the SLP contours with low pressure to the left) goes from large thickness (warm) to small thickness (cold), and cold advection occurs where the geostrophic wind goes from small thickness to large thickness. The warm front is located at the warm side of the region of warm advection, and the cold front is located at the warm side of the region of cold advection. Another cyclone is located over the central U.S., and a third cyclone is over the northwestern Atlantic. These latter two cyclones have occluded fronts where the SLP contours and thickness contours are rather parallel (no substantial temperature advection).

This is a SLP map with fronts for 1200Z Tuesday 30 January 2007. Note how the fronts in this map line up with the fronts identified in the SLP/thickness map.

This is a window channel IR image for 1200Z Tuesday 30 January 2007 from GOES-west. Note the high-top cloud region over the Alaskan Peninsula that occurs north of the surface warm front and the cloud band extending southward along the surface cold front. There are also a few mid-top clouds over southern California associated with the precipitation that occurred on this day.

This is a window channel IR image for 1200Z Tuesday 30 January 2007 from GOES-east. Note the high-top cloud band stretching across the southeastern U.S. that is ahead of the surface cold front. There is also another high-top cloud band over the western Atlantic that is associated with the cyclone there. A large area of low-level convective clouds occurs in the region of cold advection over the relatively warm Atlantic. A close look indicates that these clouds are arranged in lines that happen to be parallel to the near-surface wind (compare to the SLP/thickness map).

This is a skew-T chart for San Diego on 12Z 30 January 2007. There are several nearly saturated layers, which is consistent with the precipitation that occurred overnight. The dew point is not so close to temperature above the 500 hPa level, indicating that the upper troposphere is not saturated. This is consistent with the IR image that showed mid-top clouds instead of high-top clouds or low-top clouds. Winds in the lower troposphere are very weak, which is consistent with the very large spacing between SLP contours over southern California (weak pressure gradient corresponds to weak geostrophic wind).

This is a skew-T chart for a station on the Alaskan Penisula on 12Z 30 January 2007. Recall that this location is in a region of warm advection ahead of the surface warm front. Note that the winds turn anticyclonically (clockwise in the NH) with height from southeasterly near the surface to southerly in the upper troposphere. The atmosphere is nearly saturated all the way up to the tropopause, which is consistent with the high-top clouds seen in the IR image and the location of the station under the warm conveyor belt.

This is a skew-T chart for a station in Illinois on 12Z 30 January 2007. Recall that this location is in a region of cold advection behind the surface cold front. Note that the winds turn cyclonically (counterclockwise in the NH) from northwesterly near the surface to westerly in the upper troposphere. The atmosphere is not close to saturated except near the surface, but it is difficult to detect any low-level clouds in the IR image because they have nearly the same temperature as the cold surface.

This is a 500 mb height/vorticity map for 1200Z Tuesday 30 January 2007. Cyclonic vorticity (positive red color in the NH) occurs in the cutoff low offshore from Southern California and in the large trough over the north central U.S. Anticyclonic vorticity (negative blue color in the NH) occurs in the large ridge over western Canada. Note the strong dipole in positive and negative vorticity over the southwestern area of the Gulf of Alaska that is not associated with any turning of atmospheric winds. The geostrophic wind at 500 mb instead goes straight towards the north, and the contour spacing is closest in between the positive and negative vorticity centers. This means the strongest wind is located between the vorticity centers with weaker wind on either side. The horizontal shear in wind speed creates anticyclonic (clockwise in the NH) turning on the left side of the jet and cyclonic (counterclockwise in the NH) turning on the right side of the jet.

This is map that shows pressure vertical velocity at 700 mb. Because pressure decreases with height, negative values (blue) correspond to upward motion and positive values (orange) correspond to downward motion. The upward motion over southern California appears to be associated with cyclonic vorticity advection. Other areas of upward motion in the map are associated with cyclonic vorticity advection or warm advection. Areas of downward motion are generally associated with anticyclonic vorticity advection or cold advection. Sometimes the effects of vorticity advection oppose the effects of temperature advection so that no vertical motion occurs.