Cloudy Dreary Work Week Ahead

"FWIW, the 12Z Euro does give a significant to major winter storm to parts of the southeast US 12/12-13 thanks to a very weak Miller A combined with significant CAD. The very weak low is just ESE of Brownsville early on 12/6 and it moves across central FL on 12/13. This is quite far south and is the kind of track that can result in unusually far south significant SE wintry precip.. when the cold air to the north is ample. The QPF is generous (1-1.5" from SW GA to SE NC) which just goes to show that very a lot of precipitation can be associated with very weak Miller A's in the SE US. Many in history were that way.

This Miller A generates sig. to major snow from N GA across C & N SC to S & C NC. Also, due to strong wedging, there appears to be quite a bit of sleet in C and N GA, much of SC,and far southern NC. To the south of this, there appears to be sig. ZR over C and even northern portions of S GA as well as S and C SC. Keep in mind that the Euro has a warm bias at 2 meters when there is sig. evap. cooling."

For those of you that follow me on twitter @RaleighWx you know I have been concerned about ice potential this Sunday for a while now, despite each model cycle oscillation on the exact thermal structure in place.

The reason why was that the models have been unanimous in moving a strong 1035-1040mb high into the northeast US Saturday night and creating a strong classical CAD (cold air damming setup) event. This high pressure is associated with a very cold air mass for this time of year so there was no question that the cold/dry air associated with it could be sufficient for an ice event.

The big question was always the timing of the arrival of the cold air and the moisture. Earlier in the week it looked like moisture was going to arrive Saturday night with the cold air associated with the high pressure not filtering in till Saturday night and Sunday not early enough to set up a cold (below freezing) wedge east of the mountains. This scenario would lead to only minor ice or no ice for those south of Virginia as the transport of cold air would not be sufficient and enough.

 However, recent model trends have shown this cold surface air arriving earlier. And in fact today’s 12z NAM shows it arriving Saturday night with dewpoints in the teens even reaching down into northern NC late Saturday night.

This results in strong evaporational cooling Sunday morning and an entrenched strong surface cold air mass east of the mountains. Moisture overruns this early Sunday morning and we get an icy day Sunday with anywhere from 0.25 to 0.75 inches of QPF falling in the form of mostly freezing rain from prone regions in the mountains to around or just north of Charlotte to Sanford to Rocky Mount and north and west of there including the Triangle. In fact the 12z NAM keeps temps near freezing through at least 7pm Sunday.

The 12z GFS is very similar in overall features but a bit lighter with precip (0.25 to 0.5 inches) and not quite as cold Sunday although close. It keeps the surface freezing line around or just north of the Triangle along the I-40 corridor. However, it could be that the coarse nature of the GFS model compared to the higher resolution of the NAM means the GFS model isn’t resolving the complicated thermal structure in the lower levels correctly. The 12z GGEM is also colder than previous runs and very similar to the NAM in showing a significant ice event Sunday.

The ECMWF trended cooler too but taken literally it does keep the surface freezing line further west and confines ice to the northern foothills.

 So what is the bottom line? I think that we will receive a moderate to in some places significant ice event on Sunday during the day. Temperatures will fall Sunday night and increasing drier air at the surface will advect into the area from the NE as the CAD wedge establishes itself. Precipitation will likely break out in the wee hours Sunday morning and begin as rain but quickly change over to freezing rain for areas as far south as the NC/SC border in the foothills/southern piedmont. This will continue into much of NC west of I-95Sunday morning. As winds aloft increase from the SW we will see the warm nose strengthen and slowly the CADS wedge will weaken along the southern/eastern edges with temperatures warming above freezing Sunday morning to lunch time from the southern piedmont (CLT to the NC coastal plain) with perhaps a 0.1 inch of glaze possible. Further north and west including the Triangle, Triad and into the northern foothills the freezing rain will last longer with more significant accumulation. For now I would go with a general 0.25 inch freezing rain glaze for those areas (See map) with likely the usual areas of the Triad to Hickory to getting more than the Triangle. The foothills near the escarpment and the far NW piedmont near the Va border could see damaging ice accrual up to 0.5 inches.

Again see the map for my ideas. The general trend of cooler air arriving earlier and ice being a bigger problem is undeniable the last 24 hours. If the 12z Nam scenario is correct these totals could need to be increased. I will update as needed.

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