Slick spots possible Sunday night into Monday, close shave for Monday night, Christmas update

12/20/2020

Catch the rays if you can Sunday

A bit warmer for the day. Wind is generally light away from the coast. A southerly breeze ahead of the weak frontal boundary approaching the region may raise speed in the 10-15 mph along the beaches in the afternoon.

NAM model idea of 1-hour liquid equivalent precipitation from 1 PM Sunday to 7 AM Monday

Southwestern areas may catch a brief rain / mix or snow shower later in the afternoon. Front passes through the region overnight and ends over northern and eastern areas around daylight Monday.

Some areas may not get any accumulation. I do have a bit of concern for some ocean enhancement for Portland areas east, which accounts for a slight raise in snow totals. Expect some slick travel in spots Monday morning.

Close call for snow for eastern areas Monday Night

GEM model idea of surface pressure and precipitation from 10 AM Monday to 7 PM Tuesday

I've mentioned this offshore frontal boundary in my previous updates in regards to Monday night. Quite frankly, I am still a bit concerned about it. Most guidance is a bit more progressive with the position of the front and storm initiation to affect the Canadian Maritimes. This GEM model idea is the outlier, showing a slight delay in the forward progress of the front. I am not one that likes surprises very much when it comes to snowfall especially, so I am throwing this idea out there as a possibility that eastern areas could pick up an inch or two of snow Monday night into early Tuesday.

If the rest of the model ideas are correct, the eastern shorelines may get a slight brush of snow showers or flurries, and that would be it.

A sloppy wet kiss likely for Christmas,
but there is a slight chance it could turn colder

European model idea of precipitable water content from 1 AM Thursday to 7 PM Saturday

I want to define this loop here by saying this is an idea of the amount of precipitable water in the air column, NOT rainfall amounts. I present this idea to show the amount of atmospheric moisture associated with the storm that is on the way for Christmas.

In a nutshell, this is typical of what we see in July.

For this time of the year, it's seriously abnormal. Anomalous values could exceed higher than 400% of normal. This storm is very rich in moisture and has tropical characteristics. As a result, heavy rain, thunderstorms, potential for localized flash flooding, urban street flooding, ponding on roadways, fog, and gusty south/southeast wind which could bring power outages are all on the table for this event.

Ensemble ideas are looking at 1-1½" of rain likely at this point. That is a good starting point, and if ideas hold together, amounts could double that in places, especially the mountains.

GEM model idea of surface pressure and precipitation from 7 PM Wednesday to 7 AM Saturday

Timing as of Sunday morning appears that some outflow showers are possible in the afternoon for southern areas Christmas Eve Thursday, but the main slug of moisture works through Christmas Day Friday, ending overnight into Saturday morning.

BE ADVISED that the temperatures fall below freezing behind the system and everything will FREEZE UP overnight Christmas night into Saturday morning. There is flash-freeze potential on the backside of this, but it's too early to nail that down just yet. Between the rain and run off from melting, Saturday morning could be skating rink across the entire state.

For those in ski country looking for a glimmer of hope that the precipitation type changes to a more frozen variety, I will present this...

If low pressure forms over the Gulf of Maine along this front heading into Christmas Day, it could effectively shut down the warm air aloft and turn in colder, which would mean snow. I will say this idea is remote, and the chances are very slim, but it is not a slam dunk that this storm stays wet through duration.

Special thanks to William Watson at NWS Gray for pointing this idea out.

Temperature outlook through Saturday

A roller coaster ride on temperatures through the week. Typical high and low for northern Maine is 25° and 9°. For southern areas, 36° and 19°. Temperatures will run generally above normal everywhere through the week ahead.

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- Mike

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