Wednesday, April 4, 2007

Significant winter storm underway

Snow and sleet greeted Keene, NH this morning, however temperatures above freezing allowed road conditions to be okay. Snow and sleet, mixing with rain, has continued throughout the day. Sleet accumulation of .5" has occurred. Roads are starting to become slick even in salted areas. Unsalted roads are in very poor condition, and require slow travel. These conditions will only worsen throughout the evening. The first batch of precipitation is starting to exit the region, however the real storm is still ahead. Secondary development occuring off the Virginia coast will move northward and pull colder air into New England. Precipitation will change to all snow by 8pm, and will fall heavily through 3am. A chance of snow will continue through 8am Thursday.

The period between 6pm and 3am will be very hazardous. Snow will fall at rates of 1 to 2 inches per hour, on top of the ice accumulation earlier today. If you must travel tonight, remember that there is a sheet of ice underneath the accumulating snow. Drive will extreme caution.

The possibility exists for scattered power outages tonight. Accumulations over 4 inches of wet, sticky snow (about 8:1) will result in some downed trees and powerlines.

Snow accumulation forecast:
The temperature profiles are not ideal for maximum snow growth, however since it will fall on the diurnal maximum, the snow could come down very heavily for a 6 hour period. The 12z forecast sounding for 8pm this evening from the GFS keeps us just below freezing and is steady up to around 800mb (1800m). The ETA forecast sounding is about the same at the surface, but about 1C colder by 650m and cools to around -10C around 700mb (2600m). The RUC forecast sounding is close to the GFS but with a 500mb thickness of 5594m, which would not support maximum snow growth. The NGM forecast sounding is the warmest, with a surface temperate around 3C and 0C around 650m. So we have a general consensus for a below freezing surface temperature by 8pm, but 500mb thickness is shaky since both the GFS and RUC are above 5400m. Im not totally convinced we will be switched to all snow by then. However, definitely by 9pm. The GFS drops below by 2am (the next forecast time).

The MOS forecasts are fairly low on snow accumulations with a range of 1 to 4. The op GFS and NAM are consistent with over an inch for QPF after 8am this morning.
The NWS zone forecast is calling for 4 to 8.
The point forecast is going with 4 to 6.

To throw some other forecasts out there:
WMUR is calling for 6 to 10.
Intellicast is calling for 3 to 5.
Weather.com is calling for 6 to 10.

So there is a considerable range out there, mainly due to the temperature complications. I'll forecast in line with the point forecast, under the zone and warning text.

Snow and sleet accumulation forecast for 00z 4/4 through 12z 4/5: 4" - 6"

As for snowday potential, I'm starting to think precipitation will end too early for a snowday. At least a delay is all but certain at this point, because we will be talking about heavy accumulations of snow and ice.

-----

Cold weather will stick around through next week, and we will have a few more shots at snow accumulation. Winter is far from over.

No comments: