'The situation for our area has escalated quickly': Winter storm warning issued for the Lehigh Valley for 7 to 10 inches of snow

BETHLEHEM, Pa. — The storm system set to move into our region is an interesting one, with significant model adjustments through the weekend that leave us with one big question.

How much snow is coming?

Through Saturday, noticeable trends appeared to show diminishing returns for the Lehigh Valley in terms of accumulation.

The National Weather Service issued a winter storm watch for 4 to 8 inches of snow, then dropped that to 3 to 5 inches Sunday morning, citing low confidence that warning criteria snowfall could be met.

But throughout the day, there were fast-changing and notable trends in the modeling that put the Lehigh Valley teetering on the edge of a significant snowfall event.

Early Monday, the weather service issued a winter storm warning in effect for the area for 7 to 10 inches of snow, with locally higher amounts over the higher elevations.

The warning is in effect from midnight to 3 p.m. Tuesday.

"The situation for our area has escalated quickly, not only with respect to the high-resolution models but the larger scale global models as well," the latest forecast discussion said.

EPAWA meteorologist Bobby Martrich also said there would be big adjustments between his “first call” and “final call” maps because of a notable southeastern shift in the modeling.

Just after 11 a.m. Monday, Martrich said snowfall totals in the Lehigh Valley were likely to exceed six inches, with the northern half of the area doing better than the southern half, though not by a whole lot.

A final call map will be issued at 4 p.m., he said.

Ahead of the storm, the Pennsylvania Department of Transportation and Pennsylvania Turnpike Commission are advising motorists to avoid unnecessary travel on Tuesday. Additionally, the agencies will implement various speed and vehicle restrictions throughout the storm.

The breakdown

The setup involves a low-pressure system that should be along and just west of the Appalachians this evening followed by a transfer to a rapidly intensifying coastal low by Tuesday morning.

This new low is expected to track through Delmarva and toward Cape May, New Jersey, and Atlantic City before moving up the coast.

In this setup, the storm would begin as rain for almost all areas before the coastal low takes over and begins to draw in colder air.

It’s at this point meteorologists will tell you that these systems like to do what they want at the moment, with a battle between warm and cold air — or rain and snow — playing out.

Dynamic cooling

The main process at play that will determine how much rain and snow end up falling is known as dynamic cooling.

The process occurs when a strengthening low-pressure system creates enough vertical motion, or lift, to produce heavy precipitation. As the air rises, it cools within the atmosphere.

But with enough lift occurring, it can cause rain to change over to wet snow even with warm air in place aloft.

Forecasters say this looks to be crucial to the forecast on Tuesday morning with a change from rain to snow set to take place.

If the rain is falling hard enough where strong cooling occurs, this could bring a burst of accumulating snow.

Questions — and caution — remain

For meteorologists, properly communicating this storm with high levels of uncertainty and sensitivity in the storm track will be a challenge, with notable shifts in the models carrying the potential for big changes in snowfall totals.

As the cold air aloft is relatively weak, it will depend on how much lift and dynamic cooling we get that will change rain to snow.

Meteorologists caution that a farther-south storm doesn’t mean a much colder storm, and that given the lack of an antecedent cold air mass, temperatures will be near or above freezing throughout the event.

That translates to a need for caution with 10:1 (10 inches of snow for every inch of rain) snow maps, though Martrich said places such as the Lehigh Valley and points south — considered a marginal area for snowfall — likely will see a flip from rain to snow faster Tuesday morning.

“Along that gradient, there is some pretty heavy snow,” he said.

Main Points

• Precipitation will arrive Monday night, most likely between 9 and 10 p.m. 
• Warm air ahead of the system should make the initial round of precipitation fall as rain for most of the area
• Rain is expected to flip to snow by 4 a.m. Tuesday
• Snowfall rates of more than 1 inch per hour are expected at times Tuesday morning, making for a treacherous morning commute.