Feb 11, 2014 Evening Update

Thursday’s Storm Update: Models Trending West

snowprelim

The outlook for Thursday remains complex with more uncertainty than usual, and some of the 12z model guidance has further complicated the outlook for the area. Through this morning, the ECMWF was the westernmost model, while the CMC and UKMET were fairly consistent with the heavy snow axis not far west of the I-95 corridor with the changeover to rain only extending up to Long Island and/or NYC before retreating eastward, and the GFS gradually continued to trend towards a stronger low pressure closer to the coast as expected. With at least somewhat more data sampling of the lead northern stream shortwave which has entered the northwest US, and continuing minor adjustments of the southern stream shortwave over the southern US, the 12z and 18z runs have adjusted towards a slightly stronger and deeper trough phasing slightly further west, with the result a stronger closed mid level low tracking closer to the coast than previous runs have indicated. Along with the orientation of the negatively tilted trough, this allows more mid level warmth to surge into the area before the mid level low passes through and winds turn northerly, supporting a further inland extent of rain and sleet.

The ECMWF is fairly close at this time to the CMC and UKMET, which have also shifted towards a stronger mid level low and a stronger surface low pressure closer to the coast, siding closer to a consensus between the three models. Until this point, it has not been easy to rely solely on the ECMWF especially given its recurring tendency to overamplify storms in the medium range and especially its less impressive than usual verification this winter, although based on the latest trends with the handling of the shortwaves over the western US, this would support a scenario closer to the ECMWF as a more realistic outcome than the earlier GFS and NAM runs, which continue to trend west but remain east of the remainder of the model guidance. Additionally, the ECMWF continues to have support from its ensemble mean, while the SREF and GFS ensemble mean continue to hint at additional possible westward corrections. Given the changes above, for tonight’s preliminary snow map I am siding closer to the ECM/CMC/UKMET than the GFS and NAM, which places the storm track and heavy snow axis west of this morning’s outlook.

 

Forecast Update: The next uncertainty to determine is the inland extent of mixing with rain. As the trough becomes negatively tilted with the 500mb low ultimately becoming closed off southeast of the area, the 850mb and 925mb closed lows will gain more of a SSW-NNE orientation, with mid level warm air advection throughout the late morning and afternoon hours with a SE flow turning easterly prior to the arrival of the mid level low late in the evening hours, which is expected to track east of NYC with winds turning northerly. With the warm air advection initially expected, along with strong mid level lifting and frontogenesis tracking northeast from the Mid Atlantic region, a band of heavy snow is expected to move through in the area in the morning hours regardless of the afternoon changeover, with snow rates near 1-2 inches per hour possible. Afterwards, mid level warming is likely to lead to a changeover to rain over Long Island, NYC and coastal NJ, with the current forecast timing for the changeover after at least 1-4pm. The colder guidance keeps the mixing line just over NYC, while the warmer models bring mixing with sleet and freezing rain as far inland as NW NJ and the Hudson Valley; at this time, I introduced mixing with rain into the outlook for NYC, NE NJ and southern CT, although the mid to late afternoon hours are also expected to feature lighter precipitation rates with the best dynamics lifting northeast of the area and the deformation banding setting up west of NYC. Back-end precipitation is likely to move through in the evening before precipitation ends by at least 12-2am, with some locations changing back to snow; this part of the storm is tough to predict given occasional tendency for models to struggle handling wrap-around snow totals.

 

With the scenario above, the preliminary snow outlook has been slightly revised to 3-7″ of snow in Long Island and SE CT, 4-9″ of snow in NYC and the north/west suburbs of NYC, and 6-12″ over the rest of northern NJ into SE NY and interior CT. This range will be narrowed down with Wednesday morning’s forecast discussion.

A preliminary snow map has been posted in the top of this post. The updated outlook has also been reflected in the 8-day forecast, updated as of 9pm.

 

 

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