Warm Air Causes Snow?

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By Quincy Vagell on November 29, 2012, 3:40pm Last modified: November 30, 2012, 4:48pm

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Warm air running over colder air at the surface (over-running) is expected to bring some light precipitation to Connecticut. The time-frame looks like late Friday night into the day on Saturday. Right now, there is the potential for a good amount of the precipitation to fall in the form of snow.

Snow flurries could creep into northwestern Connecticut by Friday evening (4 to 8 p.m.). Any snow at that time would be very light and may have trouble reaching the ground.
Overnight, an area of light snow is expected to break out across much of the state after midnight. The bulk of the moisture is expected to impact the northeastern half of the state, but most places should see snow through daybreak Saturday.
By Saturday morning, as temperatures warm, the light snow may change over to a mix of drizzle, sleet and light rain for portions of Fairfield, New Haven and Middlesex counties before ending by late morning. Some light snow or snow showers may persist through a good portion of the day in northern and northeastern Connecticut.

How much snow?
Precipitation totals look to be on the low side.
Some areas outlined in blue could see between one and two inches of snowfall:

This is a tricky little system to forecast. Over-running situations prove to be problematic as sometimes they "over-perform." This means that the forecast may call for little or no precipitation and then the system may dump a few inches of snow. Also, if the atmosphere is moist enough early on, precipitation can move in ahead of schedule.
This setup looks relatively minor at this point, but stay tuned to future forecasts.

Below is a graphic that helps explain the situation. Warm, moist air is running over colder, drier air near the surface. Precipitation forms and falls in the form of snow, if the ground is cold enough. If the cold layer is not thick enough, snow can melt into a wintry mix or even plain rain.

A strong area of high pressure will lock in cold air near the surface for Friday night into Saturday morning. The combination of cold air near the surface and moisture riding ahead of a warm boundary will lead to over-running precipitation. The predominate precipitation type looks looks to be snow as the column is pretty cold from 850mb down to the surface. However, after sunrise, warming takes place rather quickly across southwestern Connecticut. Combine this with very light precipitation and it's likely that portions of Fairfield and New Haven counties switch over to some sleet or possibly even light rain/drizzle.
The bulk of the precipitation is expected to impact northern and eastern sections of the state. The model consensus is for between 0.1 and 0.2 inches of precipitation there. HPC was predicting a maximum of 0.17 inches. This could result in at least one to two inches of snowfall. Later computer model runs will be monitored closely to see if this changes. Right now, the trend has been for a slightly further northeast positioning of this band. If this happens, portions of lower Fairfield County may see little if any precipitation at all.
During the day, warm air bleeds slowly across the state. The northwest and northeast valleys may have trouble getting out of the low to mid-30's Saturday afternoon, while coastal sections could easily bump into the lower 40's. 

Stay tuned to WXedge.com for the latest forecasts and discussions.

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Quincy Vagell

Town: Naugatuck, CT  

Reporting for WXedge since January 2012.

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