A storm system is going to push through the region tomorrow. This is the storm we talked about some last week. But models have consistently shown this as being primarily a rain system, although it could start out as snow, with some accumulations for our area before the changeover occurs.

The situation is this: A low pressure system moves over our area late tomorrow and early Tuesday, pulling in moist air from the Gulf of Mexico. Temperatures at the mid-level of the atmosphere may be cold enough for snow to start, even though surface temperatures are projected to never drop below freezing tomorrow night. As the storm moves in, a warm front will be pushing northward as southerly winds deliver that warm Gulf air. That’s projected to raise the mid-level temps to above freezing, resulting in only rain for most areas.

The sweet spot for snow-lovers in this area may be northwest Tennessee. There, the NWS in Memphis has issued a winter storm watch, and calls for up to 4-8″ of snow, as heavy precip starts as snow and a transition to rain may never take place.

Precipitation fields will be weakening as the system moves further east, meaning we may not even see a half-inch of liquid precip from this system. The NWS in Morristown is calling for us to start as rain tomorrow night, transitioning to snow at the end as the low departs and colder temperatures settle in. However, both the GFS and NAM models are depicting a scenario that would bring snow to start before a changeover to rain. The GFS has very light precip ahead of the warmer air, but shortly after midnight, the warmer air above the surface arrives slightly ahead of the best moisture. The NAM model, on the other hand, is colder than the GFS and certainly needs to be watched. It doesn’t have warmer air pushing in aloft until Tuesday morning, with some decent precipitation pushing through Monday night. A literal interpretation of the NAM would mean travel problems Tuesday morning. But the NAM hasn’t been especially reliable as of late. With the big snow last weekend, the NAM consistently showed much more precipitation than any other global model, but it didn’t pan out.

If the models were over-estimating the warm air advection from the Gulf, this system could really surprise some people. Usually, the opposite happens: Models tend to under-estimate the warm air advection…which could mean the warmer air arrives quicker than either the GFS or the NAM are showing, and the NWS’s forecast of only rain for our area is correct. One thing’s for sure: With those south winds roaring both at the surface and above it on Tuesday, it’s hard to imagine that our temps won’t be pushed above 40 degrees.

So, all in all, it doesn’t look like a major system. But the front end of it could be interesting.