12/09 - SNOW STORM: GREAT LAKES

[Robert Dewey]

I decided to make this into a seperate thread... The other thread was more focused on the Southern Plains, and there is a great disconnect between the two locations...

With that said, the latest 00Z models are looking interesting for the Great Lakes region. NAM and GFS have both shifted further northwest with the lead shortwave, and pull in a disturbance from the Gulf region. That brings a pretty good surge of moisture northward into the colder air...

A decent TROWAL sets up in the 600-500MB level with a decent upglide flow... 700MB mixing ratios aren't very impressive though - but would still yeild a 4 inch snowfall over a 6 hour period (2G/KG).

Of REAL interest is what the cross sections are showing. First, between 00Z FRI and 09Z FRI very strong CSI is present... In fact, the environment is conditionally unstable in the 700-500MB layer for a short period (3 hours). That's within the saturated layer, which extends well into the -15C to -20C isotherm... Second - to top it off, strong forcing is present in the form of very intense omega and frontogenesis values. The setup looks nearly perfect for an intense band of potentially convective snowfall, oriented in a northeast to southwesterly fashion across lower MI.

Using just the model QPF of 0.35-0.50 inches, that would yeild a 4-6 inch snowfall on a 1 to 12 ratio. I expect a "fluffy" type of snow given the relatively cold profile and good snow growth potential, but I have found that 12 to 1 usually works the best over lower MI. Throwing in the conditional instability and the potential for convective banding would really throw QPF off as well.

It's still a solid 48 hours away, and forecasting conditional instability that far out is already a problem - not to mention this is the first NAM run to show the northward trend. In addition, there also appears to be a wedge of dry air in the 925-850MB layer on the cross sections - which would likely be slow to erode. I have seen plenty of events killed by a wedge of dry air that just won't erode.

 

[rdale]

It does look very interesting, but I think most of the action should wind down before 12Z on the 9th so I'd modify the thread to say 12/8 instead if we want to match our convective dimension ;>

I'm not terribly excited about the convective / banding content of this storm - but I do think ratios could be higher than 12 to 1 (UWM output says pretty good chance of higher than 15:1.) I'll go with a good 3-6" for the southeast corner of MI / with slightly more for northern IN/OH.

 

[Joel Wright]

The models have been pretty consistant the last few days in bringing the northern part of the storm further north into the very cold airmass in place over the midwest/great lakes. Southern IA and northern MO into IL look to get a nice snow tomorrow. The new 12z NAM paints over .30" liquid in these areas. The local WFO is estimating around a 20:1 ratio snow event. This would indicate a potential 4-6"+ snow along this track. I'm concerned about the dry air in place that will eat the front edge of the precip as it approaches.

We already have 4" of the white stuff covering the landscape already, so with this new snow we'll really be building an early season snowpack.. 8)

 

[Robert Dewey]

Well, I'm not too excited about the 12Z run for lower MI (though areas further south/west in IL would benefit from the higher QPF). With this run, 0.25 inches of QPF BARELY makes it into far southern lower MI, with most locations seeing 0.15 to 0.25. With a 15 to 1 snow ratio, that would be roughly 3-5 inches (solid advisory criteria stuff).

The TROWAL actually appears to be quite a bit stronger than the 00Z NAM, thought it's slightly further southeast. Decent CSI is still present, mainly over the southeastern portion of the state (closer to the TROWAL axis). In fact - between 700-500MB - saturated theta-e surfaces actually decrease with height. There is still a hint of conditional instability as well on the theta-e surfaces - mainly from the SFC to about 900MB, and then another small layer up near 600-500MB. That could possibly generate a narrow band of heavier and potentially convective snowfall anywhere between LAN / PHN southeastward to CLE, depending on where the TROWAL and instability setup.

As for forcing, omega values are still strong - but not like the 00Z NAM. Frontogenesis is considerably weaker at 700MB, only showing 16-18 units versus 30-36 units that the 00Z NAM was showing. The best omega, moisture, and CSI/CI are still located within the best snowgrowth layer.

Like Joel, I still have concerns about low level dry air. The cross sections still indicate a dry wedge between the SFC and 850MB. The NAM erodes this layer pretty quickly... But if that process slows down any, it would significantly cut down on accumulations.

I'm going to say 3 to 6 inches at this point, and see what future models show... The prediction of any banding is usually a real-time thing, so it's useless to try and fit that into a snowfall forecast (especially when we're talking about a slight potential for a 25-50 mile wide band of heavier snow with a high bust potential). But with that said, IF a band does develop then a few isolated reports of 6-9 inches wouldn't seem all that unreasonable. Right now, the best chance appears to be in northwestern OH and far southeastern lower MI.

EDIT: 12Z FSL RUC and 09Z SREF data is in... FSL RUC has storm total QPF of around 0.50 to 0.60 inches over MI, while the SREF shows 0.25 inches (with a 0.25 inch spread). GFS is very similar to it's 00Z run, which brings the 0.25 inch region further northward into southern lower MI. I really like the resolution on the FSL RUC and it usually captures mesoscale features pretty well... But almost all of the models are showing roughly 0.25 inches... NCEP stopped running it's WRF model, what's going on with that? I'd be interested in that output as well.

 

[Robert Dewey]

Anyone want to know why I do my own forecasts? Here's a perfect example... Our local news station (NBC affiliate WDIV) just said

"a monster snowstorm is on the way, we can expect anywhere from 2 to 4 inches!"

Come on now, that gets a laugh from the entire viewing audience - not just the met-oriented people.

Anyway, 18Z NAM is in and it's now back around 0.40 inches on the QPF - with a small bullseye of 0.63 inches over southwestern Ontario. The thermal profile is very cold, and would offer high snow to liquid ratios, not to mention the origins of the storm is favorable for a high ratio. Basically, the 18Z NAM is similar to what the 00Z or 06Z NAM had - it's a bit further northwest.

 

[mikegeukes]

Some links for folks, who are not familar with the term TROWAL

TROWAL stands for TROugh of Warm air ALoft

ANIMATION OF THE TROWAL AIRSTREAM
http://marrella.meteor.wisc.edu/trowal.html

Creating The Perfect Snowstorm
http://www.eas.slu.edu/CIPS/Presentations/...003/tsld001.htm
http://www.crh.noaa.gov/eax/science/Winter...w-Processes.ppt

Heavy Banded Snow
http://meted.ucar.edu/norlat/bandedsnow/

Physical Processes Associated with Heavy Snowfall
http://www.crh.noaa.gov/eax/science/Winter...w-Processes.ppt

Processes Important for Heavy Snowfall
http://www.eas.slu.edu/CIPS/Presentations/...WX-Workshop.PPT

STRUCTURE OF THE TROWAL AND THE TROWAL AIRSTREAM
http://marrella.meteor.wisc.edu/cyclwkshp.html

TROWAL
http://rammb.cira.colostate.edu/visit/trow.../definition.asp

TROWAL Identification
http://rammb.cira.colostate.edu/visit/trowal.html

What is a TROWAL?
http://www.eas.slu.edu/CIPS/Presentations/..._02/tsld003.htm

A few links on TROWAL and Heavy Snowbands.

Mike
http://mgweather.com

 

[Robert Dewey]

This is where the TROWAL sets up (forecasted to set up, rather): http://69.14.190.10/images/CSI_3.GIF

The 12Z NAM had that TROWAL further south over CLE - where the cold "conveyor belt" is now located.

Also, some of those papers state that you need an occluded system to get a TROWAL... I don't think that's really accurate as proven with this current system. We have a rather strong TROWAL, yet the SFC system is very weak - hardly occluded...

Also, here is a cross section oriented perpendicular to the 850-500MB thermal wind / thickness: http://69.14.190.10/images/CSI_2.GIF

As you can see by the negative MPVg* (Saturated Geo. Potential Vorticity), there is a decent layer of CSI. It's also located within the best thermal profile, with decent omega values. It's hard to see, but there are RH contours too - much of the CSI region is >80% RH. Those conditions exist between MBS and CLE from roughly 04Z (exrapolating) to 09Z.

All in all, things look pretty favorable for a heavy snow band, possibly convective. The 00Z models will be the real "meat" - the 18Z doesn't have the benefit of any upper air data and may not have captured the mid and upper level dynamics correctly (i.e. too strong or too weak). We shall see...

 

[Robert Dewey]

Well, I shall continue to fill this thread up with my nonsense...

The 00Z models are looking pretty good for a heavy snowfall from northern IL to the northeast (another thread). In fact, the 00Z models are stronger than the 18Z, 12Z, and previous 00Z runs. Lower MI looks to be in the best position to see the heaviest snowfall, as areas further south into central IN/OH seem to get dryslotted (though the NWS disagrees). In addition, the further north one goes the colder the profile gets, and a higher snow to liquid ratio.

Right now, the NAM / GEM / FSL RUC / and NGM are all on board with +0.50 inches of QPF over a good chunk of southern/southeastern lower MI. The track of the upper level system is actually from the central plains, which typically yeilds a 15 to 1 snow ratio (versus a storm from the Gulf) - and the cold profile certainly supports a fluffy snow. That would yield a rather widespread area of 5-8 inches of snow with local areas of up to 10 inches (local 10 inch amounts based only on the NAM fields).

NAM still holds on to a rather strong (but progressive) TROWAL up at the 600MB region over MI accompanied by near-saturation, very strong frontogenesis, and intense vertical velocities. NAM continues with decent instability in the 700-500MB layer in the form of CSI, and the CI (conditional instability) layer is STILL present across extreme eastern MI / southern Ontario. The NAM has been very consistant with these features, regardless of the QPF output.

The conditions which are favorable for convective banding (TROWAL, CSI, CI and the associated forcing) persist for at least 6 hours. In fact, forcing and instability are evident as soon as 21-00Z (southwest to northeast across lower MI), at which point the low level dry air wedge is completely erroded according to the NAM.

If the 00Z models hold their own and are factual, then I would say a good 6-8 inch snow is in store for the southern third of lower MI.

The big IF is convective banding, which could add quite a bit more snowfall to localized areas. Convective bands are typically very narrow in comparison to the larger snow band, giving them a high bust potential. I am quite confident that banding will occur, but I'm not so confident that any band will 1) remain stationary or 2) persist for several hours - let alone the exact location of any such bands.

EDIT: Just going through some forecast soundings on the NAM for MTC, and found this quite interesting. At 06Z/09, a parcel lifted from 675MB yields LI values of almost 0C... 700-500MB lapse rates also rise to 7.1C/km. Those are some pretty impressive numbers, possibly indicating true convective instability.

 

[Robert Dewey]

The 06Z models still on track with over 0.50 inches of QPF across southeast lower MI. NCEP has us in a low risk for >4 inches - in fact, they have a higher risk further south which doesn't make sense. All of the models dry slot that area, which would cut down on totals.

Further north, I am concerned about a wedge of dry air in the 950-850MB layer around 21Z-00Z. RH values within that layer drop to 25-30%. Meanwhile, RH values of >85% stream in between that layer and 500MB with decent vertical velocity developing. But, most of that precipitation should be in the form of virga if the low level dry air hangs on. In fact, the National 1KM NEXRAD is showing quite a few echoes across IL and IN, while SFC obs aren't showing any precipitation...

What's interesting is the snowfall totals of 8-10 inches in MO. Observations indicated that 0.30 to 0.45 inches of QPF fell with a snow ratio of just over 20 to 1. The GFS seemed to verify the best - the NAM had 0.63 inches over that region, which is too high. As I said earlier, QPF on all models (GFS included) across parts of southern lower MI are upwards of 0.50 inches, with the NAM showing local maximums of a hair over 0.60 inches.

 

[Joel Wright]

Well, so far we've only had a dusting. I'm starting to wonder if model QPF will verify. It's still showing over .25" liquid, but the snow is very unimpressive to say the least. It looks like eastern IL on eastward will get the best snow.



There must be some sort of dome around this area too, deflecting any kind of storm system away.

 

[Robert Dewey]

Well, so far we've only had a dusting. I'm starting to wonder if model QPF will verify. It's still showing over .25" liquid, but the snow is very unimpressive to say the least. It looks like eastern IL on eastward will get the best snow.



There must be some sort of dome around this area too, deflecting any kind of storm system away.

Yeah, pretty much anywhere south of DVN to FDY line will be dry slotted. The comma head is where it will be at as the system wraps around into the TROWAL. I am very surprised that HPC STILL has the best chances for heavy snow south of that line - are they sleeping?

 

[Dan Cook]

Snowing really good here now. Sticking too.

 

[Gary Stofer]

Silly question, off topic for a moment. Why is it we see so much virga during the winter months? South of my location in Greenwood, the IND radar is indicating 35-40 dBZ reflectivity, yet ground observations indicate nothing is happening.

 

[Robert Dewey]

Silly question, off topic for a moment. Why is it we see so much virga during the winter months? South of my location in Greenwood, the IND radar is indicating 35-40 dBZ reflectivity, yet ground observations indicate nothing is happening.

That echo appears at roughly at 1850Z. If you are looking at SFC obs to verify the NEXRAD data, there is a 50 minute gap (assuming the stations are hourly METARS). Taking the NEXRAD back to 1800Z shows a classic donut hole around KIND NEXRAD site, with heavy reflectivies just to the south.

Looking at RUC2 soundings, that entire region shows a saturated profile, so I bet snow should start flying at your location pretty soon...

 

[Gary Stofer]

Thanks Robert. I didn't know there was such a delay with the observations. Well, within 5 minutes of that intense band passing over us, it finally started to come down. A few minutes later it was quite heavy, we've got well over an inch on the ground now. In fact as of this moment the grass here is not visible, so its probably over an inch. This pic was taken 10 minutes after the first precip started to fall.