High pressure dominates the east coast today. Despite the very cold temperatures yesterday and today, a moderating trend will develop by the end of the week. This high pressure system over us will move off the coast, but it will be blocking any storms from affecting us through Friday.
This Saturday, a coastal low pressure system is expected to develop around Delaware Bay Friday night and move slowly northeastward through the day Saturday as an upper air low catches up with the surface low. The models are in remarkable agreement about this system right now.
Ordinarily, this would be a snowstorm for us but not this time. What’s lacking, unsurprisingly for this winter, is a cold high pressure system to the immediate northwest.
GFS ensemble Precip rate forecast for 7 PM Saturday. White line is 32º surface temp line.
So Saturday will be rainy for most of the day, with QPF values near 1.00 inches water.
As the upper low moves into position, some cold air may be enough to provide some snow flurries late Saturday night, but temperatures are expected to remain above freezing.
Update Sat 03:35 PM — The changeover to sleet has occurred in the time frame predicted by the NAM model.
The 1PM run of the NAM, which just became available, predicts some freezing rain here between 5-6 PM.
NAM 18Z model forecast for 6 PM- Freezing rain in red, sleet -pink, green rain
Update Sat 12:14 PM — Latest Canadian Global, just available, is very similar to the lower accumulations of the NAM.
CMC GDPS Model Snow accumulation totals 7 PM
Update Sat 11:55 AM — Earlier, I said I was going with the NAM model’s lower accumulations, but the latest GFS just available still insists on more. It has been very consistent and it’s something that needs to be considered as a possibility.
Latest GFS model continues with higher snow totals at 7 PM. Click for larger image.
Update Sat 09:04 AM — NAM data just became available. I think we’re going to go with these much lower snow/sleet totals—
Last night’s models have continued the dichotomy between the global models and the short range high resolution models.
The short range (NAM, NAM NEST) have further reduced accumulations. Here’s the latest NAM snow/sleet depth for 7 PM tonight—
NAM Snow/sleet depth at 7 PM Saturday. Click for larger image
These models continue with a warming above 6000 feet, changing the precip to sleet by 3-4 PM.
The global models (GFS, CMC, ECMWF) continue with lower level cold and dynamic cooling from heavier precip. They maintain snow levels almost 1 inch over the above totals.
Just as last night, I’m leaning towards the NAM and the lower snow/sleet totals shown above. I’m assuming the GFS is over-doing the cold air and precip.
Tonight’s models have converged on a forecast similar to that predicted by the new NBM—
Snow moves in between 10 AM and 12 noon, although intermittent flurries possible earlier.
A changeover to a mix of snow and sleet occurs about 3-4 pm and a the mix changes to mostly rain about 6 pm.
Most likely accumulations of snow and sleet from the latest NAM before the changeover to all rain—
NAM snow depth forecast 6PM Saturday
Just as I was going to post this, the latest GFS model became available. The GFS has a burst of snow late in the afternoon. It has been consistently in the high range. Here’s the latest GFS snow totals by 7 pm —
GFS snow depth
I don’t know what to make of this. It’s something to consider as a possibility.
Update Fri 7:30 PM — Latest high resolution Canadian and GEFS ensemble are similar to the NBM accumulation forecast posted below.
For the start time: Latest High Resolution Window Ensemble Forecast Model (HREF) has very some very light flurries moving in from the west, starting as early as 9-10 AM:
HREF Ensemble Precipitation Forecast.. Light flurries moves in as early as 9:30 AM Saturday.
Update Fri 04:30 PM — NBM data back online. Here’s the latest median snow/sleet accumulation for Saturday 7PM:
National Blend of Models (NBM) median snow/sleet 7PM Saturday. Click for larger image.
… from earlier this afternoon—
An interesting winter mixed precipitation type scenario setting up for Saturday. There are differences between the short range regional high resolution models and global models.
Most models have snow starting here about 10AM-12 noon. with some intermittent light snow flurries possible earlier. (The Canadian models have a later start) Precipitation ends about 7-9 PM Saturday.
As for snow/sleet accumulation totals:
Today’s global models (GFS CMC-GDPS amd ECMWF) have all increased their snow/sleet totals somewhat from previous days. Here’s the latest GFS snow/sleet totals:
GFS Snow-sleet depth totals for 6 PM Saturday. Click for larger image
Meanwhile, the NAM and higher resolution models have very different predictions due to a warmer profile at about 6000 feet.
NAM Snow totals Saturday 6 PM
Both sets of models have similar QPF values- total precip falling as water. (0.45-0.55 inches water.)
However, the NAM and other high resolution models have sleet as the predominant PTYPE (pink) by 3 PM:
NAM 3 PM PTYPE Sleet in pink, freezing rain red
The GFS doesn’t have the influx of warm air at and above 6000 feet, maintaining snow as the principal precipitation type (Purple) at 3PM:
GFS PTYPE 3 PM (shading) and critical 3 D thermal profiles (lines)
It’s funny, in past years, I put heavy emphasis on the NAM model thermal profiles.
However, the new GFS (new last year) has outperformed the NAM for storm tracks this year. We’re going to find out whether it’s better with thermal profiles.
I think the reality will be somewhere in the middle- an average of the GFS and NAM above. I’ll be checking again later this evening.
