ABOUT RAIN TO SNOW FOR VA MD DEL WAV western NC eastern TN se KY on 3 JAN

12/30/21 1900 est — NEXT UPDATE 7PM FRI DEC 31

As I discussed in the video the various weather models have been showing the possibility of a second area of LOW pressure developing on the strong cold front that arrives on January 2nd. Initially, the models were quite strong with this possibility last weekend but then begin to weaken this possibility back on Monday and Tuesday. However, starting on Wednesday the models began increasing this threat once again. This kind of model uncertainty can be a sign of a risky forecast but in this case the models appear to be increasing the chance of the rain changing to snow for several hours in the Middle Atlantic region, as we get closer to the event. Usually, model agreement that is increasing as you get closer to the event is a good sign that the threat is real and may be increasing. Parenthetically, the opposite is also true — if the model uncertainty is increasing as you get closer to the event it is usually a warning sign that this is a risky situation with high bust potential.

This image shows the current upper are patterns valid for Friday night (left side) and again on Sunday evening January 2nd (right side). It is important to understand what these upper air maps are showing in order to understand and accurately assess the risk of the rain changing to snow with some accumulation in the Middle Atlantic region on the morning of January 3rd. The map on the right hand side shows the upper air pattern for North America valid on Friday evening. There was a massive Trough in the jet stream which has been situated across western Canada and the West Coast for the past few weeks. This huge Tough has caused an equally strong Ridge to dominate the weather pattern east of the Mississippi River which is why December has been so mild and snowless, even in places such as the Great Lakes and New England.

However, this huge Trough is finally on the move and it is sweeping out of the Rockies into the Plains and the Midwest, eventually reaching the East Coast by January 3rd. As the front moves east, it will bring significant snow to the central Rockies the central Plains and the Upper Midwest on December 31 into January 1. To the south and east of the front where the temperatures will stay quite mild, there will be widespread heavy rain and thunderstorms some of which will bring severe thunderstorms and possible tornadoes to the Ohio and Tennessee Valley regions on January 1st.

The KEY feature here is a piece of energy at the bottom of this Trough in AZ and NM which I have marked with it black X. By Sunday night, that feature has become much stronger on the GFS model and we can see this on the map on the right. Here the black X is centered in ARK and MS and represents a very strong piece of energy at the southern end of the cold front.

It is possible that the GFS may be over-developing this piece of energy in the southern jet stream as it moves from AZ and NM to ARK and MS on Sunday night. The other weather models are not quite as strong with this piece of energy which would mean a weaker than the surface LOW early on January 3rd and therefore less snow in the Middle Atlantic region on Monday morning.

This image below shows the sequence of events on January 1st. The initial cold front is pushing through the northern TX, ARK, western TN, central KY, southern OH, into western PA, and western NY. Further to the east, temperatures will be quite warm across the southeastern states into VA, MD, eastern PA and NJ.. Widespread heavy rains and severe thunderstorms are likely along the front in ARK, TN, KY, southern IND, and OH. Areas north of the front will see heavy snow in most of KS, northern MO, southern IA, and Northern ILL.

By early in the morning of January 2nd the cold front will have pushed into the Mid-Atlantic region running on the line from Boston to NYC to Charlottesville, into eastern TN. At this point, the models are showing some kind of LOW pressure trying to develop in northern MS or AL. Conditions east of the front are still quite mild but generally dry from GA to the Delmarva. Heavy rain continues to fall in southern New England, much of NY State, PA, WV, eastern OH, TN, KY, MS, and Al. Heavy snow continues across northern ILL, MI, northern Indiana and the eastern Great Lakes.

This next set of maps is from the 6z operational or regular run of the GFS model early on Thursday morning. Here the model that is most aggressive with the development of the second LOW in GA and SC while the cold HIGH pressure area in MO brings down strong north winds which allows the temperatures to crash. The result is a changeover to heavy snow in eastern TN. western NC, southeast KY, much of WV and most of western and central VA, MD, DEL, and southern NJ.

A case can be made that the GFS model is overdoing the development of the LOW on the cold front early on Monday morning and it is advancing the cold air from north to south too quickly. Taken verbatim or at face value, the GFS model produces this kind of snowfall pattern by midday on Monday with the heaviest snows in central and eastern VA — including the Richmond Metro area, Fredericksburg , the Northern Neck and across the Delmarva. Notice that the snow, according to this map does NOT reach Charlottesville Washington D.C Baltimore Philadelphia or New York Metro areas.

The 6z Thursday European ensemble model is also fairly aggressive or bullish on this possibility of the rain changing to snow for several hours on the morning of January 3rd. Here the European model generally agrees with the GFS and shows the rain changing to snow in western NC, southwest VA, to central and southern VA including Richmond and the Hampton Roads area and across the southern Delmarva.

Indeed this map shows the comparison of possible snowfall between the 0Z Thursday European ensemble and the 6z Euro ensemble. As you can see the snow amounts have increased to some degree although it’s still very light but the 6z EURO shows snow across most of VA, MD, DEL, and NJ.

This next image shows the comparison between the European ensemble and GFS ensemble with the depiction of the second LOW pressure area on the front early on Monday morning. We have highlighted the key area in black and you can see a lot of LOW pressure areas (represented by the Red L or the red Numbers) The model agreement here is striking. This leans support to the idea that the LOW pressure area that develops on the cold front will be fairly significant and the risk of the rain changing to snow is fairly strong.

Finally here is the midday Thursday GFS model and it continues to show the same kind of scenario the rain changes to snow early Monday morning that lasts until about noon with temperatures around 32 degrees in central Virginia and near 30° in the Shenandoah Valley, northern Virginia, as well as in MD and northern DEL.

18z THURS EUROPPEAN

Taking verbatim this model also produces some snow in these areas with minor accumulations of an inch or two.

All that being said, it must be kept in mind that ground temperatures are warm for early January and the ground will be quite wet. WxRisk is fairly confident that the rain is going to change to snow in some portion of WV, the mountains of western NC, most of VA, MD DEL, southeast PA and NJ possibly into New York City and southern New England Sunday night into Monday morning. But the issue is accumulation. The GFS solution going to have to really work out perfectly in order to get the kind of snow accumulation that model is showing. Most of that will be on the grassy surfaces, picnic tables, the back porch, trees and shrubs , not on the major or secondary roads.

Skies will clear out rapidly Monday afternoon and evening and the Gusty North winds will bring in cold dry air which should dry up most of the ground during the evening hours. Temperatures will be cold enough to support some slush refreezing but the dry north winds will also help dry out many areas and keep any refreezing problems and black ice to a minimum early Tuesday morning.

NEXT UPDATE 7PM FRI DEC 31