A large intrusion of cold air into the central and eastern portion of the United States this weekend ushered in the coldest air of the season across the Northern Plains and Great Lakes region. It was slightly unusual in that there was no significant surface low/intense shortwave trough and associated sharp cold front. What starts as a mid-upper level weakly baroclinic stationary trough centered over the Hudson Bay eventually develops into a case of lower to mid-level frontogenesis, incredible cold air advection, and synoptic subsidence incited by a low amplitude upper level wave disturbance.
The analysis fields of the GFS on the 30th September, 12Z depict the trough over the Hudson Bay:
The upper tropospheric wave disturbance (12z) is seen here over northern Alberta:
By 0Z the 31st, the analysis fields of the NAM/GFS nicely capture the intensifying wave disturbance as it begins to interact with the larger trough over the Hudson Bay:
The disturbance is quite evident in the WV loop:
This is also captured nicely in the RUC Analysis field at 0Z. Note the lack of a low-level wave as no low level mass response has developed at this point in time.
Since there has yet to be any low level mass response, it makes sense there is a total lack of cold air advection in the low levels (850 hpa) along the southern Canada border into northern MN:
By 18Z the 1st October, the disturbance had progressed over the Great Lakes with the weak phasing essentially complete. Note the significant subsidence behind the wave disturbance.
Here is how the 12Z GFS has it analyzed (at 500 hpa) 6 hours later (same time as the WV image):
MPX (Minneapolis) shows the weak surface windshift and ensuing cold air advection at noon. Note the extreme pressure rises on the right as the high not only propagates southeast but builds with increasing large scale subsidence. Update: Is it a “cold front”? It is worth noting there is a 4-5 hour period of pressure falls (approximately one mb) before the rapid pressure rise begins. It certainly has “frontal” characteristics.
By 0Z the 2nd, the effect of the strong synoptic subsidence is clear. The low level wind fields have shifted to nearly 360 along the Canadian border with strong low level cold air advection over the Northern Plains:
At the surface, by 6z, note how far the surface high has shifted and built, now to 1032 mb over northern MN. Also worth noting is the co-location of the surface high with the region of strongest low level (850 hpa) cold air advection. Remember, from the Quasigeostrophic Omega equation, cold air advection results in synoptic descent.
Our good friend the Omega Equation! Thermal advection noted (also note it is not differential thermal advection, we will get to that later):
By 12Z the 2nd of October, the upper trough deepened slightly and has carved itself out over the Northern Plains.
This time, our friend the QG Height Tendency Equation can shed some light.
In the height tendency equation, thermal advection is now differential. In this case, low level thermal advection decreasing with height leads to height falls.
Thermal advection at 700 hpa:
And at 850 hpa:
QG Chi works! Significant low level cold air advection (shown at 850 hpa) with weaker cold air advection aloft (shown at 700 hpa) leads to upper level height falls (resulting in the upper level trough).
By the morning of the 3rd, the coldest temps of Autumn developed resulting in the first large scale frost across portions of the Northern Plains and Great Lakes:
Temps dropped down to 25 in Ely, a notoriously cold spot in northern MN. The 12Z sounding from International Falls, MN:
This particular event goes to show not all “cold air outbreaks” are driven by strong baroclinic waves with associated powerful surface lows/cold fronts.
Also worth noting here is the obvious (and very important!) fact that meteorology does not happen at static pressure levels only (850 hpa, 500 hpa, 250 hpa, etc.). Pressure analysis is obviously very convenient, and in most cases, a good analysis can be made with that alone. It is certainly more realistic to depict the atmosphere as 3-Dimensional, but the lack of good internet weather charts inhibits the use of isentropic analysis, model cross sections, etc. A good meteorologist will fill in the details and make certain to remember the atmosphere does not happen at four levels only. The well known Dr. Doswell talks about other techniques which are also useful in synoptic analysis: http://www.cimms.ou.edu/~doswell/ipvdisc/IPV.html .