The First Snow Of The Season Arrives Late Tonight After A Cold Rain

Hello everyone!

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4:15PM Radar Shows Rain Slowly Expanding NW. Image Credit: Accuweather, Annotations By Me

Rain is already spreading across the area this afternoon as a plume of moisture moves north. The immediate coastline and the inland portion of the midcoast is currently seeing light to moderate rain and this will continue through the evening with rain slowly expanding NW.  Light to moderate rain should continue throughout most of the early part of the night mainly east of the Turnpike.

12Z GFS Giving Us An Overview Of The Storm, Map Credit: Weatherbell, Annotations By Me.
12Z GFS Giving Us An Overview Of The Storm, Map Credit: Weatherbell, Annotations By Me.

The main feature of this storm will be a sharp gradient on the back side of the system with 5 or 10 miles making up the difference between a few sprinkles and a half inch of rain ending as an inch of snow. As a result, the forecast is especially touchy for areas west of the Midcoast where 1-4″ is likely depending on elevation. Once you get to the I-295/I-95 corridor, a very sharp dropoff in snow totals is likely with some areas seeing up to 2″ and others but a few midnight flakes.

20Z HRRR Model IDEA On How The Storm Winds Down Tomorrow Morning. Map Credit: Weatherbell, Annotations By Me
20Z HRRR Model IDEA On How The Storm Winds Down Tomorrow Morning. Map Credit: Weatherbell, Annotations By Me

 

All precip, rain or snow, comes to an end early tomorrow morning with everyone west of I-95 dry by sunup and everyone else soon thereafter. The rest of tomorrow looks cool but nice with clearing skies and NW winds bringing in cooler and drier air.

18Z NAM Showing The Negatively Tilted Trough And The Winds Out Ahead Of It Directing The Storm Westward Up The Coast. Map Credit: Weatherbell, Annotations By Me
18Z NAM Showing The Negatively Tilted Trough And The Winds Out Ahead Of It Directing The Storm Westward Up The Coast. Map Credit: Weatherbell, Annotations By Me

Every storm has a surprise and by virtue of surprises, they largely remain unknown but if there was one place I had to guess there would be a surprise, it’s where there’s been one already. There were indications that the trough at the 500mb level would be negatively tilted with this storm for days. Finally, the surface forecasts caught up to that and in the past 24 hours the forecast has gone from ‘chance of a flurry midcoast’ to ‘many people get their first accumulating snow’. This trend has smoothed out with the afternoon guidance today but don’t be surprised if this thing comes a tad farther west and instead of a dusting, you end up with 2″ of snow. After all, it would really only take 25 miles to make that big of a difference for some.

12Z GFS Showing NW Winds Bringing In Colder (And Drier) Air Which Could Shut Off Precip Early
12Z GFS Showing NW Winds Bringing In Colder (And Drier) Air Which Could Shut Off Precip Early

The other, equally possible surprise is that the same cold NW winds bringing in the cold air to change things over to snow also come with enough dry air to shut off the precip early, especially because NW winds downslope off the mountains which dries the air even more. For a full explanation of downsloping in relation to temperature, check out my neat weather tidbit from a few weeks ago. While that post talks specifically about temperature, the same principle applies to moisture in that when air rises, moisture condenses and forms clouds/precip etc. just like it cools and when it sinks, the opposite happens. The post also includes a general overview on downsloping which applies to this forecast.

While the last two paragraphs could reasonably be perceived as me covering myself in case the forecast goes wrong, my hope is that in case it does, you can at least know why. As I’ve said several times, part of my goal here is to educate you about why weather happens as well as to tell you what’s likely to happen.

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Speaking of what’s likely to happen, here’s a look at how much snow to expect when all is said and done. All in all, not a big deal but it still will be nice to see the flakes flying in November.

I’ll have another update on tomorrow’s weather in the morning.

-Jack

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