6:30 PM Update on Christmas Eve Storm

A premium update is available on both the unsettled weather this week and a storm threat around December 28th that could bring accumulating snow.  It also goes more in depth on the long-range forecast.  This brief free update will focus primarily on the Christmas Eve storm, detailing the expected conditions through the next couple of days.

I just want to lay out my expectations now that the storm will start having impacts tomorrow.  Certain parts of this forecast, especially those that are in the next couple of days, are higher confidence than the parts of the forecast for Wednesday and Thursday, where more details still need to get figured out.  First, tomorrow there will be some light snow breaking out across the region.  Snow may stick inland, whereas at the coast it may be a rain/snow mix that does not stick even if some snow does mix in.  By tomorrow evening light precipitation will be all rain at the coast and a rain/snow mix inland, and overnight all precipitation will turn to light rain.  The entire day on Tuesday will just be cloudy with drizzle and/or light scattered showers but very minimal rain accumulation.  Temperatures will be steady in the mid 40s as warmer air floods the region.  It is on Wednesday that heavier rain begins.  Heavy rain will be off and on, with models disagreeing on the exact timing of this rain.  However, one heavy burst of rain looks to move through Wednesday morning, with the steadiest heavier rain falling Wednesday evening into early Thursday morning.  Some models do have heavy rain falling until 9 or 10 AM on Christmas Day, while others have precipitation ending overnight Wednesday night with primarily clearing for early Christmas morning.  I continue to lean towards the latter, thinking that a storm like this will dry out quickly on the backside, and it is unlikely past 6 or 7 AM the region sees much precipitation, if any.  In terms of winds, the strongest winds look to come Wednesday evening into Thursday morning.  Overnight there will be a lull as the front moves through, as winds in front of the front will be warm coming from the south, and winds behind will be colder coming from the northeast.  My current thoughts for winds are that coastal areas see sustained winds around 25 mph with gusts to 45 mph, maybe with an isolated 50 mph gust.  Inland will see sustained winds around 20 mph with gusts to maybe 40.  This means that coastal areas may need a Wind Advisory, but inland areas won’t need anything most likely, and even the coast should not see too many impacts from the winds. Flight delays may be possible Christmas Eve due to the winds and the rain, however.  In terms of storm total rain from this storm, my current estimate is that there are widespread amounts of 1-2+ inches, which continues to be a very wide range due to the long duration of the event and the number of variables.  I had previously expected it to be drier, but some of our shorter range weather models have trended wetter, and indicated this will be a mesoscale event where total rainfall amounts could vary drastically over smaller areas due to smaller cells of heavy rain forming.  The result is that there is a l0w-end threat for some isolated flash flooding, though if the drier scenario verifies then there would be no flooding threats at all.  Any flooding would come during the day on Wednesday, and despite it not being a huge threat, I’ll continue to watch that closely.  So that’s everything I’m looking at in this storm for the coming week.  Gusty winds and heavy rain are likely, but neither are expected to be bad enough to cause significant impacts.

After this, there continues to be a threat for wintry precipitation on December 28/29, but models wildly disagree.  Now we have the ECMWF showing a strong low pressure center forming at the base and potentially riding up along the front.  Similarly, the CMC weather model shows a similar setup to the ECMWF, though not as strong and warmer.  However, the GFS, DGEX, NAVGEM, and their associated ensembles show basically nothing for this storm.  This is a classic setup of our model guidance, with the American models primarily too progressive shunting the storm out to sea, and the international weather models likely too aggressive and amplified, resulting in some heavy precipitation along this front.  The result is that it is too early to be very confident on this storm with all the moving parts, though on Premium I offer up additional analysis of where I think the models may trend from here.

I’ll have more on this storm through the week, and will continue to have some free long-range updates as well, so be sure to stay tuned!

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