Mar 1, 2014 Evening Update

[notice]Sunday 11am Update: Today’s forecast is in progress and will be posted at 12pm.[/notice]

7:30 PM Update: Snow Map Posted

snowmapIn continuation with the forecast update earlier today, which reviewed the differences in the model guidance leading to recent changes, the main theme today has been a continued south trend from yesterday’s model outputs, continuously shifting the axis of heavy snow well to the south of the area. The remainder of today’s model guidance, including the 12z ECMWF and the 18z NAM and GFS, have continued this southern trend, with liquid-equivalent precipitation totals for NYC as low as 0.27″ on the ECMWF and 0.30″ on the GFS. Focusing on the upper levels, the trend continued towards a stronger polar vortex and a slightly slower southern shortwave, allowing for more suppression of heights in the northeast US ahead of the low pressure, pushing the frontal boundary further south and leading to a slightly weaker and further south system.

As noted earlier today, this southern trend is valid given the differences between current initialized conditions and the modeled conditions from as recently as 2 days ago, which decreases the likelihood of over 10 inches of snow across the area as was depicted as recently as yesterday’s 6z GFS run. The main question at this time, however, remains if the models have overdone the south trend, or will remain the same or perhaps shift even further south. Comparing the latest observations with the model guidance, there do not appear to be any obvious initialization errors, but with additional slight north or south shifts likely as the model guidance gradually narrows down the range of the positioning and strength of the key features involved. With the strength of the polar vortex, however, and recent trends with the handling of this feature, there does not appear to be a high probability of a significant shift north to produce widespread 6-8+ inch accumulations across the area.

At this time, I opted to narrow down the forecast range in the first snow map, which in the process slightly lowered forecast totals for parts of the area. Again, it should be noted that this is a lower than usual confidence forecast for a day 2 outlook considering the handling of the key features, as well as the sharp precipitation gradient involved with the precipitation shield, but confidence is increasing that most of the snow for the area will fall on Sunday night through early Monday morning as the frontal boundary shifts south, with the main snowfall from the wave of low pressure on Monday remaining mostly, if not entirely south of the area. I currently did not side with snow totals as low as the 12z ECM and 18z GFS, but will continue to monitor the situation with additional slight up/down revision possible to the forecast. The main area of uncertainty at this time is the northward extent of the 4″ and 6″ zones, which may be shifted slightly north or south depending on future trends. The next forecast update will be posted on Sunday morning.

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