Late Feb/Early March Potential Winter Weather

OHX spoke of the Thursday night-Friday system having a lot of deep moisture and basically said that QPF might currently be underdone by the models so it would possibly bear watching for being a decent snow event.

OHX has been right quite a few times this year in its forcast this winter.

John1122 wrote:OHX spoke of the Thursday night-Friday system having a lot of deep moisture and basically said that QPF might currently be underdone by the models so it would possibly bear watching for being a decent snow event.

GFS tonight was slightly better with precip for that time period so things are headed in the right direction. Will be interesting to see if GFS is right on day time temps, if so it will be rain during the day and probably snow during the night.

Dyersburg Weather wrote:Most of the time in an argument on here both sides have somewhat of a point but not this time. THIS IS NOT A VIRGA SITUATION !!!

Geez. What is it when the upper portion of the column is saturated and the lower portion of the column is not?

Virga is precip that never reaches the ground. Here it's almost always due to low dew points when very very cold air is in place and overrunning precip is moving in from the S and SW.

In the situation we'll be facing this coming week, the atmosphere will be wringing out any and all available moisture, which will be carried along with these spokes of energy pinwheeling around the trough. Usually they pick up moisture across the Great Lakes and lift it with the elevation rise in Eastern Kentucky and Eastern Middle and Eastern Tennessee too.

This is the quote from OHX I mentioned earlier about the system Thursday and Friday.

MODELS ALL CONVERGE TO A DESCENT AMOUNT OF DEEP
MOISTURE ENHANCEMENT WITH THIS SYSTEM.

That moisture will be wrung from the clouds and it will reach the surface.

The whole "Virga being precip" thing has went right over Snowdogs head. This has never been a Virga situation..this is also another reason you cant forecast using only soundings..lol

snowdog wrote:Geez. What is it when the upper portion of the column is saturated and the lower portion of the column is not?

Hmm..the base of a cloud?

It's not just forecasting from soundings, the soundings were showing moisture in the column from around 700-850 MB. That is the normal snow growth zone as long as you can get some -10c temps somewhere in that area. If the moisture exists in 100mb of the column it's generally considered deep moisture.

This is a dry column sounding.

On the otherhand, this is for Saturday evening.

It's a bit saturated.

OHX likes Friday-Friday nite timeframe.

ECMWF AND GFS BOTH DEVELOP A SECONDARY 50H LOW PRESSURE AREA OVER
MISSOURI ON FRIDAY AND BRING THE LOW PRESSURE CENTER SOUTHEASTWARD
ACROSS MIDDLE TENNESSEE FRIDAY NIGHT AND SATURDAY MORNING. BELIEVE
THIS WILL BE OUR BEST CHANCE FOR AN ACCUMULATING SNOW, AND SINCE
TEMPS IN THE MID STATE WILL LIKELY COOL TO THE MIDDLE AND UPPER
20S BY SATURDAY MORNING, WE COULD BE LOOKING AT SOME SLIPPERY
ROADS DEVELOPING.

MRX still playing the rain game, has 30% rain/snow with a low of 26 for me Friday night. OHX is going all snow for their Plateau areas with 60% pops Friday night and a low of 26.

Snowdog - you're wrong on this one. I hate to agree with Toot, especially since he went Godwin's Law on you, but still.

You can be completely dry at H7, and if you've got plenty of moisture at H850, and really cold air aloft/northwest flow, you will get snow showers.

Its generally going to be smaller flakes because of where the snow growth region is. But many of our snow events are from the nw flow wringing out all the moisture left over at h850

jmundie wrote:I hate to agree with Toot, especially since he went Godwin's Law on you, but still.

LMAO

I dont mind being wrong, if so I will gladly admit it. I just want to learn from it. What is this sounding showing below? From todays 06z gfs for Nashville at hour 90. Is the majority of that precip making it to the ground or not? To me it looks like the surface portion up to maybe 900mb isn't yet saturated. Thus you would probably be seeing some very very light drizzle if anything at all. Where am I going wrong here?

That looks like snow flurries to me. Im not trying to be a smartass and im certainly no expert on soundings. That said a sounding does not show precip...it shows winds and various temperatures that helps one figure out the vertical profile of the atmosphere at a given time and location. There is a major difference between moisture and precipitation.

That's showing a dewpoint of probably 32 with a temp of 35 at the surface with the column saturated from above 700 down to about 925. You don't have to have 100% humidity to get precip to the surface.

In actual practice, surface relative humidity is almost meaningless in determining if precipitation will occur or not. The lift and instability release that produces precipitation occurs aloft and not specifically at the surface. Often precipitation will reach the surface with an initial surface RH well less than 50%. To determine if precipitation is likely, synoptic vertical velocity and instability release potential is studied along with assessing column RH and soundings.

Column RH is the average RH within a deep vertical column of air. Column RH can be used to assess how close a deep layer of vertical air is to saturation. Column RH is given in FOUS data for three layers and on the graphical synoptic models. A popular column RH is the 850 mb to 500 mb column average RH. If the column RH is near saturation and ample lift occurs, precipitation is likely.

Here is what OHX sees for that timeframe.

Friday Night: Cloudy. A chance of rain and snow in the evening... Then snow likely after midnight. Light snow accumulation possible. Lows in the lower 30s. Chance of precipitation 60 percent.

John1122 wrote:That's showing a dewpoint of probably 32 with a temp of 35 at the surface with the column saturated from above 700 down to about 925. You don't have to have 100% humidity to get precip to the surface.

John thanks for the quoted part of your reply. So looking at the column in question from my last post, for the Nashville area it shows 925mb RH 90-95% and Surface RH around 80% with very little vertical velocity at 850mb. Dewpoints and temps are as you mentioned. How would you use that data to come to a conclusion on how much precip. is making it to the ground?

snowdog wrote:

John1122 wrote:That's showing a dewpoint of probably 32 with a temp of 35 at the surface with the column saturated from above 700 down to about 925. You don't have to have 100% humidity to get precip to the surface.

John thanks for the quoted part of your reply. So looking at the column in question from my last post, for the Nashville area it shows 925mb RH 90-95% and Surface RH around 80% with very little vertical velocity at 850mb. Dewpoints and temps are as you mentioned. How would you use that data to come to a conclusion on how much precip. is making it to the ground?

That's where you get into the higher math in meteorology, you have to calculate precipatable water, lifted condensation level and some other factors. In a situation like this there isn't a ton of precipatable water in the column, but these shortwaves do a good job of wringing out what's there.

snowdog wrote:

John1122 wrote:That's showing a dewpoint of probably 32 with a temp of 35 at the surface with the column saturated from above 700 down to about 925. You don't have to have 100% humidity to get precip to the surface.

John thanks for the quoted part of your reply. So looking at the column in question from my last post, for the Nashville area it shows 925mb RH 90-95% and Surface RH around 80% with very little vertical velocity at 850mb. Dewpoints and temps are as you mentioned. How would you use that data to come to a conclusion on how much precip. is making it to the ground?

Snowdog - your virga sounding generally from top to bottom has a saturated look, then a really serious dry layer below that (where the dewpoint line goes way to the left)

That often happens in situations where a cold airmass is in place and warmer moist air is overrunning the cold. The cold, dry air hangs tough at the lower levels.

BTW - its been raining off and on here all day, and our surface humidity is only 59%

, Feb 26
Drizzle. Overcast. Cool. 51°F 41°F 16 mph / SW 70% 43°F Minimal 74% 0.06"
Wed, Feb 27
Flurries late. Cloudy. Chilly. 42°F 37°F 19 mph / WSW 67% 32°F Minimal 35% <0.01"
Thu, Feb 28
Cloudy. Chilly. 41°F 34°F 11 mph / WNW 67% 33°F Minimal 37%
Fri, Mar 1
Light mixture of precip. Cloudy. Chilly. 43°F 33°F 9 mph / NW 63% 35°F Minimal 42% <0.01"
Sat, Mar 2
An icy mix changing to snow. Overcast. Chilly. 36°F 27°F 11 mph / NNW 81% 27°F Minimal 54% 0.06"
Sun, Mar 3
Decreasing cloudiness. Chilly. 34°F 20°F 9 mph / NNW 61% 24°F Low 30%
Mon, Mar 4
Scattered clouds. Chilly. 43°F 20°F 8 mph / SE 45% 36°F Low 19%
Tue, Mar 5
Partly cloudy. Cool. 46°F 32°F 15 mph / NNW 68% 39°F Low 10%
Wed, Mar 6
Sunny. Cool. 51°F 29°F 9 mph / N 47% 48°F Low 0%
Thu, Mar 7
High level clouds. Cool. 56°F 35°F 8 mph / SE 49% 54°F Low 0%
Fri, Mar 8
More sun than clouds. Cool. 59°F 39°F 4 mph / E 55% 59°F Low 10%
Sat, Mar 9
Rain late. Mostly cloudy. Cool. 59°F 39°F 14 mph / ENE 79% 57°F Minimal 100% 0.73"
Sun, Mar 10
Heavy rain early. Mostly cloudy. Cool. 49°F 27°F 19 mph / NNW 63% 43°F Low 100% 12.08"
Mon, Mar 11
Partly cloudy. Cool. 46°F 31°F 13 mph / W 63% 41°F Low 10%
Tue, Mar 12
More sun than clouds. Cool. 55°F 32°F 4 mph / S 69% 55°F Low 0%

Charts

Look at March 10 it shows 12 inches of rain forcast for WhiteHouse. Wonder what model they use.

jmundie wrote:Snowdog - your virga sounding generally from top to bottom has a saturated look, then a really serious dry layer below that (where the dewpoint line goes way to the left)

That often happens in situations where a cold airmass is in place and warmer moist air is overrunning the cold. The cold, dry air hangs tough at the lower levels.

BTW - its been raining off and on here all day, and our surface humidity is only 59%

Yeah I thought I had a pretty good understanding of a virga profile. I guess what tripped me up was seeing the 925mb to surface layer not quite saturated (but not as extreme as a true virga profile) and assumed it to be mostly virga with possibly very little moisture making it to the ground.

Also, in my defense some what, the whole virga thing started this weekend when the modeling was very poor for this time frame and the moisture that was being modeled wasn't as deep as it is now being depicted by models. Evidently it still isn't close to being right precip. wise as OHX seems to think this one may over perform.

Looking at the GFS for Fri into Sunday looks like and ULL swings down with light snow or flurries.

The NAM giving The Great East TN valley the big middle finger over the next few days