Have to confess … I am not sure what some forecasters are looking at this morning for the Sun-Sun night weather system. Based on 850, 925, and surface temps, at least 90% of the precipitation will be frozen, meaning snow or sleet. And the average from 6 models is 4/10ths of an inch which gives 4” of snow/sleet.
So why the heavy hit on ice? I am a little puzzled with “nuisance wintery mix”? 1 of snow and a trace of ice are a nuisance. But 4” of snow or ¼” of ice is potentially dangerous, enough to shut down schools! I digress …
The number one thing for a forecaster is to see what is going on out there:
1. Temps near zero this morning. Kind of like last weekend before that storm … hmmmm.
2. Last weekend it was supposed to ice … how did that work out? We had no ice to speak of.
3. Last weekend, a low level jet pointed right at the Washington DC, Baltimore, Philly, Central NJ areas as the bulls-eye for heavy precipitation. Guess who got the heavy snow? Bingo. Same areas.
4. This weekend. Guess where the low level jet is pointing? Bingo. Over our area.
5. Last weekend … big high over us then moving away to our east with cold air in place … same this weekend.
6. Last weekend … cold front with weak storm to our north … same this weekend.
7. Last weekend … strong push of warm air coming north … same this weekend.
These are just observations. They are not model projections. The set up this weekend is very similar to last weekend’s system. Both are weak and not well organized. Both have a decent to good supply of moisture, and are driven by warm air transport over cold air. Both are attacking the high pressure as it leaves, and doing so quickly. This means the cold has little time to get out of the way. Again both are similar.
What is the biggest difference? The amount of moisture for Sun-Sunday night is maybe about 2/3 of that which fell last weekend. So look at the average snowfall for that storm (5-12”) and take 2/3rds of that (3-8”) as your starting point. Now for location: we ought to be on the southern half of the heavier snow, meaning those north of us will get more snow and us less. Again this is a starting point. Now you look at models for precipitation amounts. And they vary: .20, .70, .40, .25, .35, .45 … and the average of the six: 0.39 of an inch.
So here is my forecast for Sun-Mon storm and early look at Tue-Wed system featuring a brief warm-up:
Saturday night: Clear with clouds arriving overnight. Cold. Low 10 but rising toward morning.
Sunday: Cloudy, some very light snow in morning, but steady snow arrives early afternoon. Becoming heavy by mid-afternoon into the evening. High 28.
Sunday night: Steady at times to heavy snow, possibly mixing with/changing to some sleet/freezing rain before ending by midnight. Total snow: 3-5” (locally more, esp. to the north). Total ice: trace.
Monday: Clearing skies. Colder toward evening. High: 38, low 10.
1. On paper this looks like a little ice, but big rain system. However, not so fast. Fresh cold air will be in place, and with a fast flow of moisture from the positive tilted trough, will mean the warm air will be attaching the cold high quickly. If the precipitation is fast and steady, it will thump some snow. So expect some snow Tue, but then changing to ice then rain Tue night into early Wed.
2. Temps spike into the mid-40s Wed morning.
3. More rain or wet snow possible Thu.