By Kate Bilo

By Kate Bilo

PHILADELPHIA (CBS) — We continue to monitor a developing system for Sunday night and Monday that looks like one of our bigger winter storms of this stormy season. This storm is tricky to forecast for a number of reasons. First, this being the start of March, we’ll be dealing with temperatures hovering above and around freezing at the start of the event, and we’re monitoring the timing of a few pushes of cold air at varying levels of the atmosphere as the storm gets underway. The timing of the cold and the level at which it arrives will be key in determining precipitation type with this storm, and a surface temperature of 33 vs. 30 makes a huge difference, as does the temperature at 850 millibars which will start out above freezing (rain) and finish up well below freezing (snow or sleet).

Ignore this paragraph if you don’t like technical weather speak!
(A simplistic way of looking at winter precipitation type is to look at the surface temperature and that 850 temperature. If both are above freezing, any precipitation falls as rain. If both are below freezing, it’s most likely snow. If the surface temp is above freezing but the 850 is below freezing, precipitation will fall as sleet or ice pellets, and if the surface is sub-freezing and the 850 is above freezing, precipitation will fall as rain and freeze on contact (i.e. freezing rain). So when we devise a forecast, we look at the temperatures for each time period to determine which type is falling and how much of that type. It gets even trickier when, for example, you get a cold injection at around 925 millibars, or just above the surface, meaning the precipitation falls through a layer of freezing air.)

Okay, so meteorology 101 aside, my point is that we’re looking at a mixed bag of winter weather and, much like real estate, it’s all about LOCATION LOCATION LOCATION. The model runs this afternoon have showed some trends – they are keeping the storm a bit further south (i.e. colder) and also a bit less robust, keeping the precipitation slightly lighter, especially through Sunday night. We do want a colder solution, as this limits the threat for ice, which none of us want.

As of this writing, our thoughts are as follows:
-precipitation begins late Sunday afternoon, mainly as rain (though sleet/snow for N Lehigh Valley and Poconos)
-overnight Sunday night as colder air moves in, rain will mix with sleet and snow, and eventually transition to all snow by early Monday morning
-height of the storm is Monday morning through mid-afternoon, with potentially heavy snow rates and 6-10″ of snow falling with locally higher amounts possible. The highest amounts are expected in the upper left quadrant formed by the intersection of the Turnpike and I-95 (i.e. Philly to the north and west).
-areas south and east will have a longer period of mixing but will eventually end as a thump of snow/sleet, around 3-6″ for areas just south and east of the city, lower amounts for areas along immediate coast.
-precip exits later Monday evening.

Now, the axis of the heaviest snow can shift. As of yesterday, it looked to be a jackpot zone along I-80. Now, it looks further South, perhaps along the I-78 corridor or even along I-76 with the southward trend in the models today. That remains to be seen, as does the extent of the mixing. However, the main point is that EVERYONE should prepare for a major winter mess on Monday. Plan for school closures and plan to stay off the roads as much as possible. Also, we only need 6″ of snow to put as at #2 for snowiest winters ever, and we’ve got a good shot with this system.

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