The 12/13-14/2007 Snowfall: Initial Thoughts

[donsutherland1]

Since yesterday, some of the models have been suggesting that a system that will track rapidly east-northeastward had the potential to produce accumulating snows in some of the big cities from the northern Mid-Atlantic region into southern New England. Although the 12/11 12z run of the NAM pushed the system much farther to the south, the GFS has maintained a reasonably consistent track. Considering that the 12/11 12z run of the GFS enjoys ensemble support, my thinking leans in the direction of the GFS. In some areas, the precipitation could initially begin as rain or sleet before changing to snow. The precipitation could end as rain in coastal Connecticut and in New York City.

At this time, I believe the system has the potential to lay down a stripe of 4"-8" of snow across a portion of southern New England. Along the coastline and in New York City, there will be precipitation-type issues that could hold down any accumulations. Farther north, lower qpf should translate into lower accumulations. My initial thoughts are as follows:

High Risk of 2" or more; Moderate risk of 4" or more:
Albany, Concord, Portland, Providence

High Risk of 4" or more; Moderate risk of 6" or more:
Boston, Hartford, Worcester

Lesser amounts are possible across northwestern New Jersey, New York City and its immediate northern and western suburbs, and along Long Island and the Connecticut coastline.

[Category 5]

What are you thinking for the Philadelphia area?

[donsutherland1]

Some large differences between the NAM and GFS have persisted overnight. For example, on the 6z run, Bufkit shows 13.2" for LGA, while the GFS brings almost all sleet there. At this time, I continue to lean toward the GFS, though much uncertainty remains.

My initial snowfall estimates are:

Albany: 3"-6"
Allentown: 1"-3"
Boston: 4"-8"
Concord: 3"-6"
Hartford: 4"-8"
New York City: 2"-4"
Newark: 1"-3"
Portland: 2"-4"
Providence: 4"-8"
Philadelphia: 1" or less
Worcester: 4"-8"

[donsutherland1]

Category 5,

Unfortunately, I believe PHL will likely see 1" or less.

[donsutherland1]

My snowfall estimates are updated to include Toronto. After reviewing the 18z guidance, the only other change that I have is to increase the snowfall at Worcester to 5"-10".

Albany: 3"-6"
Allentown: 1"-3"
Boston: 4"-8"
Concord: 3"-6"
Hartford: 4"-8"
New York City: 2"-4"
Newark: 1"-3"
Portland: 2"-4"
Providence: 4"-8"
Philadelphia: 1" or less
Toronto: 1"-3" (2.5 cm-7.5 cm)
Worcester: 5"-10"

[JBG]

Is that for NYC itself?

[Stephanie]

This has been a rather active December for Winter Weather. Very interesting.

Great analysis as usual Don!

[btangy]

One thing to factor in are snow ratios inland may be quite a bit higher than what we normally witness. Snowflake growth will be ideal with maximum rising motion coinciding with the -15C region, and the rest of the column is fairly cold. Will get low density, fluffy snow as a result where the boundary layer stays well below freezing. May be looking at 12:1 to 15:1 ratios along and near the MA pike instead of the usual <10:1.

Also, NAM is coming in to much better agreement with the GFS in its 0Z run shifting the axis of heaviest snowfall further N. Could be quite a nasty evening commute through most of MA tomorrow.

[donsutherland1]

After reviewing the 0z and 6z guidance along with current and forecast soundings, it does appear that NYC south and west will have more sleet than snow. M final estimates are as follows:

Albany: 4"-8"
Allentown: 1"-3"
Boston: 6"-12"
Concord: 3"-6"
Hartford: 4"-8"
New York City: 1"-3"
Newark: 1"-3"
Portland: 2"-4"
Providence: 4"-8"
Philadelphia: 1" or less
Toronto: 1"-3" (2.5 cm-7.5 cm)
Worcester: 6"-12"

[cpdaman]

Don i must say i appreciate your forecasts every week.

IMO systems with change over, and snow/sleet lines make forecasting accumulations for certain city's almost impossible.

do you have any thoughts (mid afternoon) not necessarily update on your forecast amounts , but of how far that sleet line will get into eastern and southern newengand. (nevermind it never reached the prov-taunton area)

http://cam.warmwinds.com/cam_java.htm Naraganset RI (S. RI coast) is still snow via webcam at 345

as of 530pm i have confirmation from a spotter/friend in providence of over 6.5 inches and heavy snow

thank you for another good forecast, look forward to your sat nite/sunday forecast

[JonathanBelles]

So Far, from family in upstate NY, i'm hearing about 6-8" of snow.

[Category 5]

We had nothing but rain.

[donsutherland1]

Thanks Cpdaman.

The forecasts can be challenging for some locations. I had been worried that some sleet would reach Providence, hence I stayed with 4"-8" figures. A complete verification will be posted tomorrow or Saturday.

Best wishes.

[Stephanie]

Yep, rain, rain and more rain. Not a surprise.

[donsutherland1]

Verification

The idea that southern New England faced a significant snowfall worked out reasonably well. However, just 7 of the 12 (58%) sites saw snowfall that fell within my expected range. The largest error occurred at Hartford where 2.6" more snow fell than I had anticipated. The overall error of the sites falling outside my forecast range was 0.8". 4/5 (80%) of those sites had an error of less than 1" outside my forecast range.

From 12/12/2007 8:00 am:
Albany: 3"-6"; Actual: 8.2"; Error: 2.2"
Allentown: 1"-3"; Actual: 0.9"; Error: 0.1"
Boston: 4"-8"; Actual: 10.1"; Error: 2.1"
Concord: 3"-6"; Actual: 5.4"; Within range
Hartford: 4"-8"; Actual: 10.6"; Error: 2.6"
New York City: 2"-4"; Actual: 0.2"; Error: 1.8"
Newark: 1"-3"; Actual: 1.1"; Within range
Portland: 2"-4"; Actual: 3.5"; Within range
Providence: 4"-8"; Actual: 8.2"; Error: 0.2"
Philadelphia: 1" or less; Actual: None; Within range
Worcester: 4"-8"; Actual: 9.3"; Error: 1.3"

From 12/12/2007 6:30 pm:
Albany: 3"-6"; Actual: 8.2"; Error: 2.2"
Allentown: 1"-3"; Actual: 0.9"; Error: 0.1"
Boston: 4"-8"; Actual: 10.1"; Error: 2.1"
Concord: 3"-6"; Actual: 5.4"; Within range
Hartford: 4"-8"; Actual: 10.6"; Error: 2.6"
New York City: 2"-4"; Actual: 0.2"; Error: 1.8"
Newark: 1"-3"; Actual: 1.1"; Within range
Portland: 2"-4"; Actual: 3.5"; Within range
Providence: 4"-8"; Actual: 8.2"; Error: 0.2"
Philadelphia: 1" or less; Actual: None; Within range
Toronto: 1"-3" (2.5 cm-7.5 cm); Actual: 1.2" (3.0 cm); Within range
Worcester: 5"-10"; Actual: 9.3"; Within range

From 12/13/2007 7:30 am:
Albany: 4"-8"; Actual: 8.2"; Error: 0.2"
Allentown: 1"-3"; Actual: 0.9"; Error: 0.1"
Boston: 6"-12"; Actual: 10.1"; Within range
Concord: 3"-6"; Actual: 5.4"; Within range
Hartford: 4"-8"; Actual: 10.6"; Error: 2.6"
New York City: 1"-3"; Actual: 0.2"; Error: 0.8"
Newark: 1"-3"; Actual: 1.1"; Within range
Portland: 2"-4"; Actual: 3.5"; Within range
Providence: 4"-8"; Actual: 8.2"; Error: 0.2"
Philadelphia: 1" or less; Actual: None; Within range
Toronto: 1"-3" (2.5 cm-7.5 cm); Actual: 1.2" (3.0 cm); Within range
Worcester: 6"-12"; Actual: 9.3"; Within range