I’ve been reviewing the latest NAM and GFS data.  Here’s the latest on the expected storm for Sunday afternoon into Sunday night.  Total QPF values 0.50 GFS and 0.74 NAM

Basic scenario: snow to sleet and rain in the city, then back to snow. Details below:

Precipitation starts as wet snow in the afternoon, about 2-4 PM.  Initially, surface temperatures are above freezing in areas near the city, preventing accumulation.  Temperatures  then drop due to dynamic cooling and there’s some snow accumulation in the city.  However…

During the early evening, the snow in the city will mix with and change to sleet and rain.  By 9 PM, it will be mostly rain and some sleet in the city.  The northern extent of this changeover is depicted in the following map at 10 PM:

North of this white line, snow continues throughout the storm.

South of this white line (10PM), the sleet and rain will gradually change back to snow about 12-1 AM  and will continue as snow until morning.

North, the QPF values are less, but the temperatures are colder; calculations for the snow totals become a bit complicated.

North of this white line, final accumulations will be 4-8 inches.

South of this white line, the final accumulations will be 2-5 from the city northward.  These are preliminary estimates.

This is a complex storm. We won’t have a final handle on the thermal profiles  and final QPF until tomorrow morning’s model runs.

[su_box title=”Winter Weather Update: Tuesday 8 PM” box_color=”#defcdc” title_color=”#000000″]A quick review of the today’s models. We’re back to 4-5 inches of snow, changing to sleet about 5 pm. Tonight’s NAM data starts becoming available after 9 pm and I’ll update sometime after 9:40. [/su_box]

[su_box title=”Winter Weather Update: Tues 9:30 AM” box_color=”#defcdc” title_color=”#000000″]A change in the forecast, based on the latest NAM. Precipitation starts about 10 AM, earlier to the west and southwest. Snow until about 4-5 PM, then a changeover to sleet and then some freezing rain.

The revision in the forecast- most of the QPF will be arriving after the changeover to sleet.. about 3 inches of snow expected, more north and especially west, before a changeover to an extended period of heavy sleet and sleet mixed with rain, then rain.

I’ll be updating this evening. Likely update time is around 9:30 PM.[/su_box]

I’m waiting for the latest NAM data.  Last night’s NAM and GFS moved the precipitation start time later to 10-11 AM (with the exception of the FV3-GFS which still maintains an earlier start.)

NAM QPF values for the morning and afternoon increased to 0.80 inches water, but some of that will be falling as sleet.   The NAM maintains freezing temperatures into the evening.  At the current time, I’m still with 4-6 inches, leaning towards the higher amount, before a changeover to an extended period of sleet.   Snowfall will be heavy in the early afternoon.

I’ll  be updating with the NAM data this morning, before 10 AM.

A very busy schedule at work today, so I probably won’t get a chance to further update until this evening.  (It will be this evening’s data that will be most important.)