Christmas time-frame discussion and what the guidance is showing:
A big pattern shift by the end of next week into Christmas weekend will occur for the Southern Plains potentially bringing a favorable setup for wintry precipitation and the coldest air of the season. Guidance has indicated anomalously cool temperatures during the Christmas time-frame for the Southern Plains due to a long-wave trough digging into the region (see Fig. 1).
Fig. 1: GEFS 500mb Indicating Long-Wave Trough Across the Central-U.S.
The reason the guidance is showing this pattern is because the eastern Pacific Oscillation (EPO) will tank after the 18th of December (see Fig. 2), which favors a ridging over northwestern parts of North America. The ridging in this region is favorable for arctic air to invade parts of the lower-48. To evaluate which parts of the lower-48 may see the most anomalously cold temperatures, it is important to evaluate the Pacific/North American (PNA) phase. The PNA will become negative over the weekend (see Fig. 2). A negative PNA is most favorable for the coldest temperatures to occur in the western and central parts of the lower-48. The Southeast is generally ‘protected’ from the cold air is this scenario due to amplification of the SE ridge when the PNA is negative.
Fig. 2: Forecasts of Teleconnection Indices
Forecast breakdown for this time-frame:
It appears the advertised arctic airmass will move into Oklahoma and Texas on Thursday of next week. The exact timing is uncertain, but numerical guidance usually struggles with arctic airmasses and is too slow with the movement of such a dense airmass (however, guidance is in concurrence of a Thursday arrival). Out west, an upper-level low will develop at the base of the aforementioned long-wave trough, possibly cutting off, setting the stage for overrunning precipitation. The timing of the potential precipitation event(s) is unknown at this time, but it does appear the first round of precipitation may occur next Friday night into Saturday. Potentially followed by another round or two of precipitation depending on the evolution of the upper-level low and track between the 22nd and 26th of December.
While too early to establish an exact area, amounts, timing, and precipitation type; confidence, is increasing that somewhere across Texas/Oklahoma will see wintry precipitation periodically between the dates listed above. With the shallow nature of an arctic airmass, it is possible a freezing rain scenario may develop across central and eastern Texas. This is because surface temperatures appear to be below freezing at this time, but just above our heads, temperatures may be above freezing. Further west, the airmass is deeper, thus, more of a snow setup comes to fruition if precipitation is present.
The GFS is showing such a scenario by next Saturday in north Texas. Forecast soundings show the surface well below freezing with a very deep warm layer (above freezing) just a couple thousand feet above our heads (see Fig. 3). This means snowflakes would develop, fall, melt just above our heads, then those melted snowflakes would fall as rain at the surface but immediately freeze upon contact. This is not what we want because freezing rain can be devastating. Further west in Texas, near the upper-level low, temperatures aloft are much colder so snow is more likely in this region (see Fig. 4).
Fig. 3: GFS Forecast Sounding Next Saturday (north Texas)
Fig. 4: GFS Forecast Sounding Next Saturday (west Texas)
Again, it is too early for exact predictions, but this is something I am monitoring. The finer details will be worked out over the next several days, but I am becoming more confident in the large scale setup across North America. Models do not handle arctic airmasses well, so if the airmass is colder and deeper than forecast, if precipitation occurs, it is possible more of a sleet/snow scenario may evolve. If temperatures are warmer, then potentially a cold rain–if precipitation develops. I will have updates as needed.