Gotta love when a plan comes together...meteorologically speaking of course.  It's TOP news across most of the country right now and the incoming storm is pretty much developing as models predicted so really all there is to add at this point are the numbers and then strap yourself in for the ride.  It'll be worth it!  You can tell I don't have to travel anywhere this weekend...

 

The Big Picture

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To keep things short (no, not really), we've got all the fixins' to make one mammoth storm over the next 48 hours.  High up overhead, a potent jet streak (basically a jet stream on steroids) with loads of Arctic air is beginning to drop into the Midwest.  Along the leading edge of this jet's "sweet spot" at ground level, a surface low has spun up as expected and will deepen rapidly as the upper atmosphere links with the lower like a massive chimney and causes the storm to explode.  At the time of this advisory, the center of circulation was over the panhandle of Oklahoma but the track should shortly turn northeast up through the Mississippi River Valley towards the Great Lakes and reach Ohio and Lake Erie by around daybreak Thursday.  As it reaches this region on a continuing northeasterly track into southern Canada at peak intensity, we have a great change of witnessing some holiday "bombogenesis" with this one.  One of my favorite meteorological phenomena to watch by the numbers, a storm qualifies for this elite status when the barometric pressure drops more than 24 millibars in 24 hours or less (top diagram).  Looking at this evening's model runs, we should well exceed that threshold...truly groovy.   You guys know what I usually say right about now but I'll keep it festive.  By late evening on Christmas Eve, the center of the storm will be well into Quebec but the residual effects of the storm's passage will still be very evident in our area.  Here's what I am expecting...

 

Temperatures - The Big Story

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With intense storms like this, it all boils down to what side of it you're on...much like a hurricane.  Since the path of this storm has not deviated much even from a few days ago (the models nailed it pretty well), we will be experiencing intense warm air advection as tropical air is drawn up from the south along the eastern side of the storm center.  From about midnight tonight, temperatures will steadily rise regardless of night or day to near 55F in the pre dawn hours Friday morning.  However, as the storm passes by to our west, a powerful (like...uh...nuclear) blast of Arctic air on the back side of the storm will crash into the region like a runaway train (middle diagram).  This blast is significant enough that freeze warnings are already going up all the way to the Gulf Coast and states of emergency have been declared in several states.  Between 3 AM Friday and 3 AM Saturday, we'll go from 55F to a comfy 12F and will likely remain below 20F until Sunday when we might hit 25F (glass half full).  At the earliest, I do not see temperatures even rebounding above freezing until Tuesday at the earliest.

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With the low temps. and accompanying winds will also come significant wind chills.  At this point, I do not think we will reach Wind Chill Advisory thresholds near the Philly region, NJ and DE (-10F) but it's going to be close overnight Christmas Eve (bottom diagram).  I do, however, think we could hit Advisory thresholds in some of the northern counties and Poconos (-15F).  Right now, models are suggesting wind chill values around -20F overnight Christmas Eve.  We'll have to see if this bears out. 

 

Wind and Rain  - The OTHER Big Story

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As the storm bombs out Friday (the wingnuts at the Weather Channel are calling it Elliott), it will create a very impressive pressure gradient and wind field that, if the timing is right, could cause more damage than anything else.  For most of the metro region, the wind will noticeably pick up around lunch time Thursday but not peak until Friday afternoon.  Steady winds of 20-30 mph are expected with gusts in the 40-50 mph range.  In areas of low relief (NJ/DE shore points) and in higher elevations, the impact could be even higher.  I mentioned the other day that I expected Wind Advisories to be posted at some point and the NWS is suggesting the same thing although they have not been issued yet - more likely tomorrow as everything takes shape.  Although our sustained winds might be just below the threshold for issuance, our potential wind gusts will definitely qualify so expect this some time tomorrow.  These winds will stick with us through Saturday.

 

The additional wind impacts will center on timing and actual rainfall.  Most of the region is expecting between 1"-2" of rainfall which is about the maximum rainfall you can have with air of this temperature so, if the ground is saturated, we could see widespread infrastructure impact (especially power outages). 

 

Rainfall should begin in the metro area by around lunchtime at the latest and then intensify rapidly with peak amounts falling Thursday evening into Friday morning.  It's right around the tail end of this rainfall Friday that the winds will also reach near peak intensity...oh goodie.  Just before the rainfall ends by around lunchtime Friday, we might get a quick burst of snow which will be the visual cue that the temperatures are getting ready to crash. 

 

Ice, Ice...Baby?

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Our main risk for ice Christmas Eve and Christmas Day will not come in the form of precipitation; it will be from standing water after the storm as the temperatures plummet.  If we're lucky, the incoming dry air in the wake of the storm will evaporate light coatings of leftover water before the cold air freezes it but that's a gamble.  Near Philly, our last gasp of above freezing temperatures will occur around lunchtime Friday...any time after that will have a serious ice risk for standing water still on the ground.  We've also got the risk for a flash freeze.  As the temperatures continue to drop, salt will also lose its effectiveness to melt the ice and by Saturday, it will basically be useless.

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So, in summary:

• Rubber boots and umbrella for Thursday

• New umbrella purchased with a gift card to replace the one you just lost in the wind on Friday

• Purina Yeti Chow for Saturday 

 

Hey y'all!  It's officially winter and most of us are on a break!  Enjoy the holiday, have some fun, make some French toast pops and enjoy the ride.  Happy holidays everyone!

 

2021

rs