3/17 All Zones Early Afternoon Update on Potential Snowfall This Weekend

Today will be a beautiful day, with lots of sunshine for much of the day. The large closed upper-level low influence our weather for last few days, will finally be lifting further north over the Canadian Maritimes. This will lead to rising mid-level heights the over region , allowing for temperatures to warm up into upper 30s to lower 40s for highs this afternoon. While these temperatures may feel a little more comfortable, compared the last few days, they are still well below normal for this time of year.

However, more inclement wintry weather is store, beginning later tonight and Saturday morning. More clouds will arrive and thicken late this afternoon and evening. Then a band of precipitation will start moving late tonight over Western NJ and Southeast PA ahead of warm/occluded front approaching the region. Some mid-level frontogenesis suggest this precipitation may be a little steadier side just northwest of NYC.  But confluence will slow the down this front to southwest and delay precipitation until perhaps after daybreak on Saturday for areas further east.

Thermal profiles on model soundings support this precipitation may start some snowflakes, then will quickly mix with more rain over Central New Jersey, New York City and Long Island and areas further south. Due to lighter intensify and more mixing, little or no accumulations are expected for these areas. Over Interior, colder temperatures just above surface will support more snow  to continue and could accumulate a couple inches by noon tomorrow, especially on colder surfaces. But this first round of precipitation isn’t expected to cause too many significant hazards, around the region.

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But then low pressure will weaken over the Great Lakes region tomorrow as it transfers another area low pressure off the Virginia coast by tomorrow night. An inverted trough will be being developing over region with precipitation becoming steadier for parts of the region, during Saturday afternoon.  Then the coastal low will cause winds to turn more northerly by Saturday night. Delivering a little more colder air in the lower levels. Thus changing any rain or mix over to all snow again throughout the region, during the evening and overnight hours.

Also increasing lift from 500mb PVA and jet streak streaming northward along Mid-Atlantic coast will lead to enhanced mid-level lift and instability with omega and steep mid-level lapse rates in the snow growth region. This will lead in heavier banding with some dynamic cooling tomorrow night into Sunday morning. Surface temperatures will fall to near or few degrees below freezing through the area with snow becoming moderate to possibly heavy at times in some portion of the region. This will support snow to start accumulating on more surfaces. But there still some high uncertainty exactly where this will occur with some disagreement between the models.

12z NAM model showing steep 500-700mb level lapse rates over the region

12z/17 NAM model showing some mid-level instability with  steep 500-700mb level lapse rates that could enhance more moderate to heavy banding of snow Saturday evening.

Snowfall totals at the end, will depend exactly where this heavy banding setup Saturday night into early Sunday morning. The 0z/17 ECMWF model overnight still suggest this will be further south over Central and Southern NJ where boundary temperatures may support more mixing or snow to melt when it reaches the surface. Another thing to watch is that models are slowing down the 500mb closed low when it reaching the Mid-Atlantic coast. This could allow for the coastal storm to organize by reforming and vertically stacking it’s mid-level centers further north. As such the 700mb low reforms just offshore on some the models allow more frontogentic banding wrapping around into the late morning and afternoon hours over eastern parts of Long Island and Coastal CT. If this heavy enough with temperatures near freezing, more additional accumulations are possible.

The latest 12z NAM today suggest this potential the most and support a moderate to significant borderline snowfall for Long Island and Coastal CT, even considering it’s biases with overdoing amount of precipitation.However the 12z RGEM model run today shows this vorticity around the 500mb low bit more messy. This mean best forcing with PVA could be a little further east or northeast with heavier banding over Southeast New England later Saturday night into Sunday. A coupled upper-level jet also is shifting bit too far east too allow for northwest expansion of banding into the local region.

12z RGEM 500mb voriticity 2am Sunday

12z RGEM late Saturday night and early Sunday morning  showing 500mb closed low back over WV/MD/VA  with some phased shortwave energy underneath it. But also strong vorticity and lift moving just east of Long Island, resulting more subsidence possibly for Long Island and heavier banding shifting toward Southeast New England

At this time, with the high uncertainty, we are still maintaining our snowfall total forecast for light accumulations. But will be likely be reevaluating our forecast later this afternoon and evening, as new model data comes in. Nevertheless, there will still be huge bust potential on either the high or low side, with snowfall totals, if model guidance is poor in handling banding and boundary-layer cooling. The storm should move far enough offshore to for more clearing by Sunday evening.

Next week overall is looking more progressive with a few frontal systems moving through with some light precipitation. Temperatures will moderate to near or above normal, deeper troughiness out West causes more ridging to build over the East by late week. Stay tuned for more zone forecast updates with more analysis on rest of model suite coming later this afternoon and evening, on this weekend’s storm.