© Fernando Caracena, 2018
This post continues the discussion of 16 January 2018 entitled, "The Current Arctic Outbreak 17 January 2018". In the typographical error of the date was contained the prediction for the next day.
An analysis of the 500 mb height field Fig.1 for the late afternoon of 16 January, which is 0000 GMT 17 January, shows that the trough of the morning had propagated south-eastward very slowly.
By early morning (1200 GMT) on 17 January the deep trough over the Arctic airmass was moving toward the Eastern Seaboard, leaving rare snow falls in its wake in the deep South. A wind-chill map depicts the deep discomfort with the cold felt across the United States.
The colored areas in the above depictions (Fig. 1 and 2) are contoured in terms of an approximately conserved quantity called quasi-geostrophic (QG) potential vorticity (PV). In the above depictions of the 500 mb surface, it can serve as a visual check on the continuity of the analyses from one time frame to the other, each based on a separate set of independent observations. Basically, the continuity of these blobs of colored-in QGPV reassures us that there is dynamical continuity here.
It should be mentioned here that although the objectively analysed 500 mb height field was depicted here, this field is just one part of sets of analyses of 40 atmospheric levels every 25 mb (in this case) throughout the full depth covered by the sounding data. The three dimensional analyses consist here of a grid of points having a horizontal spacing of about 80 km and a vertical spacing of 25 mb. The horizontal and vertical grid scales are both adjustable, but ultimately the real real resolution of the analysis is determined by the time and space scales of the observations. These can be sharpened by however, by a technique called four dimensional data assimilation.