Robert Dewey - Imminent snow?

Todd Rittinger

Todd Rittinger

EF4
Joined
Jun 10, 2004
Messages
390
Location
Kitchener, ON, Canada
Robert, Nick, and any other Michiganites: it would appear that a snow event is inbound for you and us (S.O.), but on another mailing list, a student whose forecasting ability has proven quite accurate in the past, is thinking a dud. Since my skills are still non-existent, what do you think?

Be well, TR
 
Well, it certainly is interesting... The one thing I notice the most is the very dry air in the low levels, with dewpoints down to nearly -20F in southern Ontario, to around -6F or so in lower MI. Usually, this tends to stop significant snows in it's tracks. On the other hand, the latest 18Z ETA and GFS have bumped up QPF across this region to nearly 0.50 inches, and with a nice ratio of 1 to 20 or 1 to 25 would mean nearly a foot of snow for quite a few places. Supporting that would be the weak stability in the lower mid levels, as well as a very strong TROWAL that lingers around between 700-500mb in the >80% humidity layer and good vertical velocity through tomorrow afternoon which to me would point to mesoscale banding of snows. Again though, it's highly dependant on whether or not the dry low level air can be over come, so certainly not an easy forecast. I would tend to go middle of the road and call for around 6 inches, but if it appears that the low level dry air won't be too much of a problem, I would say 8-12 inches.
 
It's the dry air that I keep missing for some reason...ok, so am I correct in this reasoning:

Dry air is dry for a reason, that is, it is colder air, therefore more dense (like my brain lol) and lower ability to retain water. The only way for this air to be able to retain moisture (significant amounts) is to increase in temperature. This warmer air then has to pass over moisture (large water body for example) or be mingled with air already containing moisture. A large and dense enough cold dry air mass has enough clout to divert an "inbound" moist air mass or stop it.

Am I on track with this concept?

Thanx everyone, I'll have this down yet...

Be well, TR
 
It's the dry air that I keep missing for some reason...ok, so am I correct in this reasoning:

Dry air is dry for a reason, that is, it is colder air, therefore more dense (like my brain lol) and lower ability to retain water. The only way for this air to be able to retain moisture (significant amounts) is to increase in temperature. This warmer air then has to pass over moisture (large water body for example) or be mingled with air already containing moisture. A large and dense enough cold dry air mass has enough clout to divert an "inbound" moist air mass or stop it.

Am I on track with this concept?

Thanx everyone, I'll have this down yet...

Be well, TR
Click to expand...

Yes, you pretty much got it. I have been watching the METAR data over the past few hours, and the temps/dwpts are pretty erratic. For example, my site is reporting 6F with a dewpoint of -7F right now, while the "thumb" of MI is reporting a temp of -6F. The station just to the west of me has a temp of 9F, while the dewpoint has dropped to -15F. Another way the atmosphere can moisten up is when the precip does move overhead, there will be extensive virga (precip not reaching the ground)... That will tend to add moisture to the air through evaporation processes, and slowly get things going. You can usually identify the dry air by "donut holes" around radar sites (although, interestingly with the dry air, there doesn't appear to be any such signatures... yet).

A quick look at the latest 00Z ETA actually bumps up QPF even further, which is amazing given the bone dry and cold air in place (mixing ratios barely make 2.5g/kg at 750mb). It still maintains a very strong TROWAL between 700-500mb, very strong frontogenesis along the axis of the TROWAL, and >80% humidity through the column (tomorrow morning/afternoon), and again, weak stability. This would all point towards a nice convective snow band setting up shop somewhere across the lower lakes, but where exactly would be the question (as well as dry air problems at the beginning of the event)...
 
RE: moisture and precipitation

A quick answer: There is low moisture content in the low levels of the atmosphere with this system (I haven't looked at the data - just read the previous posts in the thread). Therefore, precipitation is generated aloft and falls though the dry layer before it reaches the ground. Much of the precipitation will evaporate before it reaches the ground (also lowering the temperature - evaporative cooling). By contrast, if the air in the lower levels is moist, less of the precipitation evaporates as it falls. BTW, dry air is air whos dew point is significantly lower then the temperature (dew point spread). Relative humidity is also a way to express moisture.

To illustrate my point, here's two soundings to compare (00Z, 01/22/05).

http://weather.cod.edu/analysis/analysis.sound.html

First, click on "MPX" = Minneapolis. Note that from the surface though the upper levels (300 mb and above) the dew point is very close to the temperature. This is a "saturated sounding" - and a winter storm is in progress there. All levels of the atmosphere - including the lower levels - are moist (relative to the air temperature).

Now, click on "KAPX" = Gaylord MI. Here, the upper levels are nearly saturated - between about 300 mb and 600 mb - this is from upper clouds and "blowoff" out of ahead of the ongoing winter storm in the upper-Midwest. There is a very dry layer between an inversion at 880 mb and 650 mb. Most precipitation falling from the saturated upper levels will evaporate in this dry layer until the layer is moistened. Hope that makes sense - bill
 
Robert and Bill, thanx muchly for those clear explanations!

Robert, my skewt reading ability is still on par with a one year old trying to walk :wink: , however, the text information shows me this:

6 850 1432 -13.3 -42.3
7 799 1906 -9.5 -47.5
8 776 2131 -9.3 -43.3

A large differential between temp and dew point (as referred to by Bill) in this spread . This is then the reflection of the layer of very dry air?

Bill, a summary then on heating and cooling with regards to precip:

Condesation releases latent heat resulting in a heating effect, and evaporation traps the latent heat energy resulting in a cooling effect?

I'm going to nail this all down if it kills me (writing his will now.. 8) )

Be well, TR
 
Robert and Bill, thanx muchly for those clear explanations!

Robert, my skewt reading ability is still on par with a one year old trying to walk :wink: , however, the text information shows me this:

6 850 1432 -13.3 -42.3
7 799 1906 -9.5 -47.5
8 776 2131 -9.3 -43.3

A large differential between temp and dew point (as referred to by Bill) in this spread . This is then the reflection of the layer of very dry air?
Click to expand...

Yes, notice how the RH through a layer from about 850-575mb is below 35% (or close to it), that's some pretty dry air.

Finally, we are getting a radar site that shows the dry air... Check out how the precipitation seems to be blocked by this radar site: http://weather.cod.edu/analysis/paulradar.pl?GRR (click loop on the left to see it in action).

The reason this occurs, is because the snow evaporates before reaching the ground... The further from the radar the beam get's, the higher above the ground level it gets. So, chances are, you are looking at virga away from the radar site, whereas you don't see anything near the radar because the beam isn't high enough (i.e. snow isn't reaching that low to the ground yet)... I hope I am making sense
:lol:
 
Most of the 00Z data is in, and all of the models are heavier with the QPF, and a bit further north with the storm track. ETA/RUC/GFS all agree that a general 0.50 inches of QPF should fall across my location (ETA/RUC slightly heavier, around 0.55 inches, 03Z RUC is looking very strong with low level forcing). This would indicate about a foot of snow for the I-69/I-96 corridor southward. Given the low and mid level setup of the rapidly deepening low, orientation of the TROWAL, weak stability, good FGEN forcing/vertical motion, and deep moisture, it appears quite possible that a mesoscale band of snow may setup, hammering areas located within, while leaving areas just north of the band in the subsidence (i.e. alot less snow).

Further east, for Todds area... QPF drops off slightly (most likely due to the dry air and very cold temps across your region). It's still quite generous though, given the cold air, at 0.40 to 0.45 inches. That would yeild a general 6-8 inch (15cm to 20cm) snowfall, barring any changes, minor or not.

Haven't really checked any wind fields yet, but I am guessing they remained the same as the last few runs, with gusts in the 25-35MPH range, most of which will be after the snow is done falling...

EDIT: Just wanted to update the wind speeds... Decided to take an in depth analysis of the BUFKIT ETA, and it's showing nearly 55knts mixing down to the SFC around the DTW (Detroit) area. Winds really pick up around 16Z, which coincides with the snowfall ending between 18-21Z (with light snow thereafter)... If these winds materialize, it could be very nasty from WI/IL to MI/IN and sliding into Ontario, Canada. After reading NWS discussions, looks like a nice 10-14 inch event will unfold across IN/MI (BUFKIT shows 13.8 inches), again, with the 50-55knts mixing down...
 
Bill, a summary then on heating and cooling with regards to precip:

Condesation releases latent heat resulting in a heating effect, and evaporation traps the latent heat energy resulting in a cooling effect?
Click to expand...

You got it! Energy input/output is required for a "phase change". Evaporation requires or *removes* heat from the surrounding area – the atmosphere - hence, "evaporative cooling" when, for example, virga falls (rain or snow that evaporates before it hits the ground). The opposite process is true for condensation. BTW, a similar situation exists for the phase change from solid (ice) to liquid - or the other way around. For example, melting ice takes requires *much* more extra energy beyond what is required to warm ice from 31.9F to water at 32.1F, then is required to raise the same water at 32.1F to 32.3F - the same temperature differential.

I will take a second and relate this discussion to "breaking the cap" in a convective WX situation. There are often times when the cap (a mid-level inversion inhibiting convection) is very strong. Occasionally, an upper-level disturbance will spread mid-level clouds and often some virga into the initiation target area. This virga can cool the layer above the inversion several degrees (often it doesn't require much to "break the cap" – sometimes even a degree or two will suffice), and convection is initiated. - bill
 
Pretty interesting... After watching the radar here for about an hour, I watched what appeared to be clutter (sometimes it's an airplane), approaching from the southeast... Then, the "clutter" started growing in size and density (thus ruling out clutter)... It's now over my area, so I decided to take a look outside... Since that little "blip" moved over, we have had about half inch of snow, with big flakes coming down... Just got me by surprise how this developed and 'retrograded' into the main precip shield...

[Broken External Image]:http://69.14.190.10/DTX.BREF1.gif
 
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