I’ve been looking at the latest GFS and NAM models from this morning. Sunday night into Tuesday promises some interesting (at least for this season) winter weather.
Two systems will affect us, one Sunday evening and the other starts Monday evening. The first approaches Sunday with light snow developing sometime during the evening and ends early morning Monday.
There are differences in the models with the QPF. The NAM is showing 0.23 inches water, the GFS less. With the prior two winter weather events, the NAM out-performed the GFS. So it appears that 2, possibly 3 inches of snow will have accumulated when it ends Monday morning.
Monday will be mostly cloudy as another more intense storm moves to our west. Cold air in place at the surface with warm, moist air over-running the the cold air will result in snow initially.
The GFS has the snow starting early on Monday evening. Critical temperatures in the upper atmosphere warm by 3-5 AM Tuesday morning, with the snow changing to sleet and freezing rain at that time. It’s possible that 3-4 inches of snow will have accumulated before the changeover.
Surface temperatures may remain at or below freezing until 7 AM Tuesday, so things will be icy and messy Tuesday morning.
North and west of the city, the freezing rain and sleet may be prolonged into late morning.
By noon on Tuesday everything should have transitioned to all rain. This looks messy and the temperature profiles and QPF predictions will likely change in the next few days. Stay tuned.
A front moved through today, associated with a dip in the jetstream that will allow cold air to move in for the next few days.
The frontal boundary stalled to our south and multiple models are showing the development of low pressure that will move along the front bringing significant precipitation. Both the NAM and GFS show QPF values, as high as 0.98 inches of water.
There are currently differences between the timing and thermal profiles of the models and those differences result in significant differences in the precipitation type (rain /sleet/snow).
The NAM is warmer and shows little snow until the afternoon. The GFS is colder and faster, and shows as much as 4-6 inches of snow!
We also need to include consideration of sun angle in April and resultant solar insolation through clouds along with daytime vs nighttime Spring snows. Too early to make a call, but interesting weather for April. Stay tuned.
Wednesday 10:45 pm -Tonight’s NAM continues with a thermal profile that is too warm for snow during much of the storm around Philadelphia.
Upper Bucks and Montgomery County as well as western Chester county may have over 4 inches, as measured on grassy surfaces, but the immediate PHL area is mostly rain and some sleet, changing to snow late in the game. A light coating possible. Expect changes
The latest NAM data is coming in as I type this. Looking at the trends-
QPF values have stabilized at about 1.00 inches water.
Temperatures at critical levels levels of the atmosphere decrease enough to support a changeover to snow about 9-11am in Philadelphia, earlier north and west of the city.
Prior to the changeover to snow, expect continued sleet and freezing rain. So snow changeover may be a bit later than previously thought .
Based on the actual QPF expected to fall as snow and factoring in March sun angle through clouds, I’m thinking that about 6-9 inches fall in Philadelphia and immediate suburbs, 10-12 in far northwest suburbs.
Winds will be high at 30 -35 mph with higher gusts.
So, the snow transition starts a bit later here and QPF values have decreased somewhat and winds remain a big problem.
important update 11:30pm. Correct winds to 20-25 mph with higher gusts. Reduce snowfall to 4-7 inches in Philadelphia based on GFS NAM blend
The models have been advertising another coastal snow storm development for Tuesday night into Wednesday. Not all of the major models are totally on-board with this scenario, but it still appears more than likely. The wind intensity of this storm will NOT be anything like this past Friday, although snowfall rates may look impressive during the day.
Incredibly, the current range is anywhere from 6-10 inches of snow for Philadelphia , based on the GFS (6″) and the NAM (10″).
Current timing- light snow develops during the evening Tuesday and increases in intensity after daybreak Wednesday, as the secondary coastal low develops and intensifies.
There’s still some thought that warm air might mix in, but right now the NAM and GFS critical temperatures all support snow.
I’ve given much thought about this past storm I’m not going to get into the March sun angle, solar insolation and ground temperature effects, since it made giving an accurate accumulation forecast impossible. I guess in March, it’s really about snowfall rate, not accumulation since accumulations mean much less in March— the snow can start melting as soon as it falls.
I’ll keep an eye on this one and will update tomorrow!
Sunday 10pm: The latest NAM data shows a significant snowstorm for Philadelphia on Wednesday. QPF values are almost two inches water. Critical temperatures show it falling as heavy wet snow. Still too early to be sure, but this could be a major snowfall.
11pm: Tonight’s GFS confirms a major snowstorm likely for PHL. We’ll be waking up to heavy snow falling on Wednesday, snowfall increases for the rest of the day. Current models suggest as much as 15 inches! Too soon to hang our hat on this.