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January 15th-17th 2024 Arctic Blast/Snow Event


John1122
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5 minutes ago, fountainguy97 said:

Agreed. It's like having two discussions at once. Nowcasting reports and then discussion on last second models for the areas it hasn't begun yet.

 

 

It is driving me nuts.  I LOVE the observations.  It is the best part of the storm.  The problem is that in NE TN, the storm is 12 hours behind initial reports.  BTW, big slug of precip headed TRI's way...I would think rates will flip rain to snow when it arrives.  We have hit the leading edge of the precip here.

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10 minutes ago, Carvers Gap said:

For several days, the NAM busted badly in the eastern valley as it didn't have any snow east of the Plateau.  Globals did much better over here.  The final run of the GFS before the event is probably going to verify for the most part with the exception of the foothills which mostmodels basically failed w/ until the last minute..  The Canadian sniffed out the problems in the eastern foothills first.  Once the NAM came on board yesterday, it also had it.  The HRRR also had it to some extent when it came into range.  It is also worth pointing out that the NAM doing well in middle and west TN while not doing well here is easily explainable.  It was closer in time to the event and those areas got cold quickly(better ratios).  On this end, we were still several hours behind from a modeling standpoint...and it overdid accums on the Plateau and Cumberlands which was feedback.  I do think the NC lee side low caused it problems.  Once it lost that feedback(and we got closer in time), it did much better. It didn't dial-in here until about 12-16 hours to go.   The other problem is that as the system backed and slowed, the NAM tended to feedback.  The ICON is probably gonna be the big winner in all of this statewide, and that is probably a shock to most - me for sure.   The 3k NAM was particularly good.  The 12k NAM was terrible.  The RAP and HRRR have been really good as well.  I don't like the RAP, but it has done decently this go around.  

As MRX noted, no model had the warm air pushing to I-81 in NE TN.....personally, I think the front stalled at dawn at the Plateau and then daytime temps caused issues.  The front stalling on the Plateau is not unusual.  As Tellico noted the other day, Knoxville has a nice pathway for cold air from the Plateau.  Chattanooga and TRI do not.   The big red flag was when temps in NE TN yesterday beat forecast highs by 8 degrees.  That produced a lot of warm air to scour as we got cloudy at dark, and warm air couldn't scour.  Then it go banked against the Apps this AM.

The NAM itself is I think a false positive.  It got some things right, but for the wrong reason IMO.  It got incorrectly amped and missed the track, but its snow totals worked out.  Why?  The cold air moved aggressively into middle and west TN and allowed rates to be crazy good.  I have roughy 4" of snow right now.  My ratio might be 8:1.  If it was colder, I would likely have double that.  So, whichever model moved the cold in most aggressively is correct...and I don't which one that is....probably the RGEM? The other thing is that the over-amping of the NAM gave it some semblance of expanding the precip shield.  Its precip shield was juiced due to feedback and had a bad track in the East - plenty of snow fell east of the Apps which it didn't have.  In reality, that precip shield was going to fill in as the rush of cold air squeezed every drop of moisture out of the air - it was impressive.  The entry point of heavy precip was decently modeled and didn't change much.  

Still looking through some of the reports from middle and west TN which are incredibly helpful.  I think the main bust(and let's be honest, nobody cares if a model busts and they get more snow than shown), is the foothills...and that may take some time to figure out.  

Great analysis...I think in the end the wave that generates the Lee Low wasn't well handled. Once in range the models start being able to hone in so to speak. Combine it with the plateau slowing things down naturally...it stalled and pushed back the Arctic air, especially at the downslope prone areas. With the pressure stabilizing now, more areas are going to be on the correct side of the Lee. Allows the artic to resume its push.

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11 minutes ago, Carvers Gap said:

For several days, the NAM busted badly in the eastern valley as it didn't have any snow east of the Plateau.  Globals did much better over here.  The final run of the GFS before the event is probably going to verify for the most part with the exception of the foothills which mostmodels basically failed w/ until the last minute..  The Canadian sniffed out the problems in the eastern foothills first.  Once the NAM came on board yesterday, it also had it.  The HRRR also had it to some extent when it came into range.  It is also worth pointing out that the NAM doing well in middle and west TN while not doing well here is easily explainable.  It was closer in time to the event and those areas got cold quickly(better ratios).  On this end, we were still several hours behind from a modeling standpoint...and it overdid accums on the Plateau and Cumberlands which was feedback.  I do think the NC lee side low caused it problems.  Once it lost that feedback(and we got closer in time), it did much better. It didn't dial-in here until about 12-16 hours to go.   The other problem is that as the system backed and slowed, the NAM tended to feedback.  The ICON is probably gonna be the big winner in all of this statewide, and that is probably a shock to most - me for sure.   The 3k NAM was particularly good.  The 12k NAM was terrible.  The RAP and HRRR have been really good as well.  I don't like the RAP, but it has done decently this go around.  

As MRX noted, no model had the warm air pushing to I-81 in NE TN.....personally, I think the front stalled at dawn at the Plateau and then daytime temps caused issues.  The front stalling on the Plateau is not unusual.  As Tellico noted the other day, Knoxville has a nice pathway for cold air from the Plateau.  Chattanooga and TRI do not.   The big red flag was when temps in NE TN yesterday beat forecast highs by 8 degrees.  That produced a lot of warm air to scour as we got cloudy at dark, and warm air couldn't scour.  Then it go banked against the Apps this AM.

The NAM itself is I think a false positive.  It got some things right, but for the wrong reason IMO.  It got incorrectly amped and missed the track, but its snow totals worked out.  Why?  The cold air moved aggressively into middle and west TN and allowed rates to be crazy good.  I have roughy 4" of snow right now.  My ratio might be 8:1.  If it was colder, I would likely have double that.  So, whichever model moved the cold in most aggressively is correct...and I don't which one that is....probably the RGEM? The other thing is that the over-amping of the NAM gave it some semblance of expanding the precip shield.  Its precip shield was juiced due to feedback and had a bad track in the East - plenty of snow fell east of the Apps which it didn't have.  In reality, that precip shield was going to fill in as the rush of cold air squeezed every drop of moisture out of the air - it was impressive.  The entry point of heavy precip was decently modeled and didn't change much.  

Still looking through some of the reports from middle and west TN which are incredibly helpful.  I think the main bust(and let's be honest, nobody cares if a model busts and they get more snow than shown), is the foothills...and that may take some time to figure out.  

Excellent post man ! I agree on your reasoning on all points. We were fortunate here in that we were colder than TRI and Ratios better but, rates have been light for the most part as flakes have been small. Portions of the County have been experiencing larger flakes, generally near mountains. I currently have about 4.5 inches. 

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7 minutes ago, TellicoWx said:

Great analysis...I think in the end the wave that generates the Lee Low wasn't well handled. Once in range the models start being able to hone in so to speak. Combine it with the plateau slowing things down naturally...it stalled and pushed back the Arctic air, especially at the downslope prone areas. With the pressure stabilizing now, more areas are going to be on the correct side of the Lee. Allows the artic to resume its push.

Forgot to mention the lee side slp.   Normally that is a huge plus, but when the cold front stalled....it didn't help areas with marginal temps.  Never did I imagine that warm air would seep all the way to I-81(further in some cases).  Crazy dynamics.  Hopefully, as it gets dark and rates increase...maybe some areas will switch back to snow and get thumped.

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10 minutes ago, Daniel Boone said:

Excellent post man ! I agree on your reasoning on all points. We were fortunate here in that we were colder than TRI and Ratios better but, rates have been light for the most part as flakes have been small. Portions of the County have been experiencing larger flakes, generally near mountains. I currently have about 4.5 inches. 

We have kicked and clawed our way to 4" of snow.  Rates right now are decent.  Within an hour we should be dealing with cooling due to darkness and higher rates.  Hopefully, that will force the rain/snow line to recede.

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Just now, Carvers Gap said:

We have kicked and clawed our way to 4" of snow.  Rates right now are decent.  Within an hour we should be dealing with cooling due to darkness and higher rates.  Hopefully, that will force the rain/snow line to recede.

Shouldn't heavier rates also help bring cooler air down as well?

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13 minutes ago, John1122 said:

I've probably not had a flake bigger than a nickel all day today. This reminds me of upslope. We had an event around Christmas in the 2010s where it was about 48 hours of it and I got 7 or 8 inches. Never snows hard, it just keeps snowing and snowing.

Same here.  It’s my first time in 24 years living here that I’ve witnessed it snow that long (24hrs).  Slow and steady but it never let up.  I did have some waves on the backend that produced some decent sized flakes though.  Currently sitting at a balmy 10 degrees.

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5 minutes ago, Dsty2001 said:

Shouldn't heavier rates also help bring cooler air down as well?

It is the evaporation due to heavy precip that cools the air column.  Kind of like if you get wet on a cold day, and you feel colder.   Once that column cools, snow can travel the entire length of its trip without melting.  I suppose it could pull some cold air down, but I would guess it is more due to evaporational cooling.  If somebody has a better answer, have at it. I am a bit over my skis so to speak.  When we aren't fighting daylight....that is also another factor in cooling. 

 

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TRI has been above freezing all day.  I think warming yesterday (clouds moved in later and at perfectly the wrong time) seeded the problems for today.  We got too warm yesterday afternoon due to sun.  Then clouds moved in and capped that warmth so it couldn't radiate out.   Time of day IMO was the single, biggest determining factor of who got what.  

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Just now, Carvers Gap said:

TRI has been above freezing all day.  I think warming yesterday (clouds moved in later and at perfectly the wrong time) seeded the problems for today.  We got too warm yesterday afternoon due to sun.  Then clouds moved in and capped that warmth so it couldn't radiate out.   Time of day IMO was the single, biggest determining factor of who got what.  

That definitely didn't help matters..more heating makes it harder to scour out, especially if you lose radiational cooling. Bath tub theory..clouds act as a partial clog to drain out the heat.

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1 minute ago, Carvers Gap said:

TRI has been above freezing all day.  I think warming yesterday (clouds moved in later and at perfectly the wrong time) seeded the problems for today.  We got too warm yesterday afternoon due to sun.  Then clouds moved in and capped that warmth so it couldn't radiate out.   Time of day IMO was the single, biggest determining factor of who got what.  

Not sure that was all it was. That didn’t help.  Timing for us has been terrible. We did make it to the upper 20s around our region by daybreak.  Just couldn’t stay there with the warming of the day and a slight downslope off the mountains.  I didn’t think it would happen, but remember leaving a post saying I never totally discount it around here.

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