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Posted at 11:15 AM ET, 02/19/2013

Rain favored over snow in storms for next week; a more snow-friendly pattern around March 1?


Spring is rapidly approaching. Average temperatures are starting to rise in response to the sun which at this time of year is about the same in strength as in early to mid-October. By early next week the average maximum temperature crests 50.

Through Friday, temperatures will probably run near or slightly below normal with highs being in the low-to-mid 40s. Maximum temperatures should climb into the upper 40s by Saturday and then into the low 50s by Sunday, Monday, and Tuesday.

The pattern should bring a few chances of precipitation in the area over the next two weeks. Precipitation is possible Friday, over the weekend, and again towards Tuesday. The exact timing of any precipitation and amounts are much less certain.

Most of the precipitation through next Tuesday should be in the form of rain though the precipitation Friday could start as a wintry mix especially in the colder areas north and west of the city.

Once a front passes by next Tuesday, things get more complicated. Some models suggest that the storm track could shift south putting us on the cold side of a storm. That *might* give us better snow chances towards the beginning of March. Should this storm occur, it would usher in a period of cooler than normal temperatures in its wake.

Technical discussion

Friday through Tuesday


The pressure pattern from the GFS ensembles (to the right) gives a feel for why precipitation at least through the weekend is expected to be mostly rain.

The spaghetti diagram of pressures from the ensemble members (top panel) and ensemble mean (bottom panel) are pretty consistent at showing a low tracking towards the Great Lakes by Friday evening while also showing an area of higher pressure just north of Maine.

Having the pressure higher in Maine than over the Great Lakes almost always provides us with winds originating from the Atlantic where temperatures are above freezing. Therefore, even if we start off with cold air damming (with a north wind nosing down the spine of Appalachians), unless the air is truly frigid to begin with, we end up warming enough to change any precipitation to rain as flow becomes more easterly. Soundings from the 06Z GFS suggest that precipitation could start as a little sleet or freezing rain Friday but would warm enough for rain on Saturday.

A number of the GFS ensemble members then take a weak low from the Gulf of Mexico and track it to our south. That would give us a chance of rain this weekend. However, some members forecast little additional rainfall until Monday or Tuesday - see the plume diagram below.


When the lines from the various members are rising, it signifies precipitation falling. The rate of the rise equates to the precipitation intensity. Note on the plume diagram how most members predict mostly rain but differ on the timing. Also note, some members do not have rising lines; in another words, precipitation is not a given and it may end up mostly dry.

After the weekend, the next chance of precipitation is Tuesday. The European ensemble mean forecast for Tuesday evening shows a low over the Great Lakes region which again suggests there will be a strong enough easterly wind component (off the ocean) at low levels to keep the surface temperatures warm enough to rain. It also forecasts the temperatures at around 5000 ft to be above freezing.


The big question mark is whether the Great Lakes low shown Tuesday will be followed by another passing to our south which could offer us snow chances towards the end of the month.

The Feb 28 and beyond pattern

The ensemble mean forecast below shows some of the features that make the period interesting if you like snow.


Note the red area of above normal heights extends from northern Europe across Greenland and most of Canada. A strongly negative North Atlantic Oscillation (NAO) is in place at the same time that ridging is starting to show over west. Having these above normal heights or blocking across Canada usually helps displace cold air southward.

The blocking to our north also suppresses the jet stream forcing it to our south. Note how closely packed the lines are over the Southeast (representing the position of the jet stream, on the left panel). West of the dip in the jet, the pattern is favorable for a high pressure system to develop across southern Canada to deliver some cold air.

However, the spaghetti diagram (right panel above) suggests that the look of the ensemble mean trough over the East on the left hand image may be misleading. On the right hand panel, look at the blue lines, and the number of them that form circles; also, note how they differ in location. When you average the upper lows or troughs that are defined by all of the circles, it results in the rather broad trough on the ensemble mean which may not be representative of the actual flow.

The blue circles suggest the troughs on the various individual ensemble members are much better defined (rather than the broad trough shown in the mean) but there is a lot of uncertainty about their location. The blue circles located farthest to the southwest would support a storm or low pressure center forming off the coast far enough south to give us a chance at snow. The more northerly trough positions probably reflect primary low tracking to our north and with the low redeveloping too far north to give us snow. Only time will tell which is right.

Behind the possible February 28 or March 1 storm, the upper flow is forecast to become northwesterly bringing us cooler weather.

Conclusion

The next 10 days are expected to be wetter than normal but the chances of significant snow before February 28 are low. The pattern looks more interesting in terms of snow chances around Feb 28 or March 1. But, even then, there is a lot of uncertainty about the evolution of any storm and what it might mean for us. Once that last potential storm exits the area, another shot of cooler than normal weather is expected.

By  |  11:15 AM ET, 02/19/2013

Categories:  Latest, Winter Storms

 
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