SIGNIFICANT pattern change for NA over the next few weeks...

[Wnghs2007]

Ok....you just warn me when you see the first hints of anything winter coming for me. Ok wxguy25? Thanks bud.

[yoda]

EXCELLENT discussion as usual Wxguy. I understood everything. Interesting to see this pattern change, along with the -AO and +PNA.

I am busy now, so I wil post more later on my thoughts about this phasing event/possible winter event in the OV/NE/GL/MA... but we will see...

KC, I don't see any real cold for you for about the next two weeks...

[Wnghs2007]

EXCELLENT discussion as usual Wxguy. I understood everything. Interesting to see this pattern change, along with the -AO and +PNA.

I am busy now, so I wil post more later on my thoughts about this phasing event/possible winter event in the OV/NE/GL/MA... but we will see...

KC, I don't see any real cold for you for about the next two weeks...

Are you sure you and Dt are not related. Your writting styles are very similar.

[yoda]

EXCELLENT discussion as usual Wxguy. I understood everything. Interesting to see this pattern change, along with the -AO and +PNA.

I am busy now, so I wil post more later on my thoughts about this phasing event/possible winter event in the OV/NE/GL/MA... but we will see...

KC, I don't see any real cold for you for about the next two weeks...

Are you sure you and Dt are not related. Your writting styles are very similar.

No, but we do live in the same state... why do you think that?

[wxguy25]

EXCELLENT discussion as usual Wxguy. I understood everything. Interesting to see this pattern change, along with the -AO and +PNA.

I am busy now, so I wil post more later on my thoughts about this phasing event/possible winter event in the OV/NE/GL/MA... but we will see...

KC, I don't see any real cold for you for about the next two weeks...

I don’t see this as being a phasing situation. instead the front that comes into the OV and lakes on D6 or 7 stalls b/c of the resistance from the ridge and a new waves develops on the front and rides it northward.

The SFC set-up is a CLASSIC "COL" pattern w/ two high and low pressure centers adjacent to one another. You can get some really heavy frontogenetically forced precipitation events w/ a COL in place since the circulations around the Low in the SW quad and the high in the SE develop strong WAA which collides w/ the area of CAA produced by northerly flow from the Northwestern high and Northeastern low pressure systems. This results in the intensification of the thermal gradient and frontogenesis.

Ok...to put to rest any misunderstandings, Ill explain it like this:

Picture a compass w/ the North, South east and West Cardinal directions, Place an area of low pressure in the SW and NE quadrants, and a High in the NW and SE quads, then envision the flow around those features. You will see that when you have the surface low and high pressure systems arranged in such a manner, the WAA (from the south) and CAA (from the North) contribute to LLVL convergence of the isotherms and, thereby, an intensification of thermal gradient over a short distance (frontogenesis).

D7 h850 temps/MSLP

D7 500mb height/absolute vorticity

[Chris the Weather Man]

wxguy25... On WWBB, People have been saying (Weenies) That it will be 01-02... I been trying to tell them..... The Pattern does even look like 01-02......

[wxguy25]

wxguy25... On WWBB, People have been saying (Weenies) That it will be 01-02... I been trying to tell them..... The Pattern does even look like 01-02......

Thats why THEY are weenies.

[Chris the Weather Man]

wxguy25... On WWBB, People have been saying (Weenies) That it will be 01-02... I been trying to tell them..... The Pattern does even look like 01-02......

Thats why THEY are weenies.

Indeed!

[Brett Adair]

That massive trough near the 22nd does show a bit of a pattern change for the eastern 2/3 of the CONUS. Hmmm....looks to be a nice beginning to winter especially across the mid section with some of the critical height lines coming together to make a frozen event, but also looks to be the makings of some severe wx for eastern and southern portions ahead of the frontal boundary.

[CaptinCrunch]

The Latest GFS models have been posted.

[gboudx]

It's been a while. What number represents the freeze line?

[yoda]

It's been a while. What number represents the freeze line?

It changes really, but its the 540 DM line.

[wxguy25]

It changes really, but its the 540 DM line.

To Build on that...

It's been a while. What number represents the freeze line?

What you see there is the 1000-500mb thickness field (plotted in geopotential meters). Since Thicknesses are a function of both moisture content and temperature, we use the average VIRTUAL TEMPERATURE (hereafter, Tv) between 1000 and 500 mb to compute the thickness of that layer. You can increase (decrease) thickness by either Increasing (decreasing) the Moisture content, increasing (decreasing) temperature, or both at the same time.

The resultant effect will modify the Tv to conform to the changes above, and thusly adjust the thickness of the layer. 1000-500mb thicknesses are a function of the average virtual temperature between the 1000 and 500mb.

The value that relatively signifies freezing is the 540 thickness line (5400 geopotential meters) however—I use the term relatively since a 1000-500mb thickness value ≤ 5400 gpm does NOT ALWAYS mean that the Surface temperature will be below 32F or that P-type will be SN or ZR or IP and ties into the concept of Warm and Cold thickness biasing, where, the 540 thickness value will not be a viable predictor of the actual precipitation type.

The first example is a Cold Thickness Bias. In this situation you may have a 1000-500mb thickness of 5340-5400 but b/c temperatures in the lowest levels may be close to/above freezing the precip type will fall in the form of Rain even in spite of favorable thicknesses. The reason why this happens is b/c the colder layer aloft is probably deeper than the shallow warm layer, so since the thickness is affected more heavily by the deeper cold layer, you get a thickness which would inaccurately represent snow. One may also observe an increase CAA w/ height which can intensify the effect.

Differential advection/WAA can cause layers of the atmosphere to remain nearly consistent (isothermal) or actually rise w/ height (i.e. temperature inversion). This is Warm thickness biasing.

When it comes to the issue of Precip type, DONT UNDER ANY CIRCUMSTANCES use 1000-500mb thicknesses when making a precipitation type forecast, since it alone, is very rudimentary and prone to gross errors which can cause you to BUST as a result of some of the factors just looked at.
A more practical way would be to break up the 1000-500mb layer, and assess two key aspects of it separately, using 1000-850mb thicknesses, and 850-700mb thicknesses, since these will be more sensitive to smaller changes in the vertical temperature profile over a fixed location. But even still, this is not perfect. For example sake, you may have Low level thicknesses favorable for SN, but the 850-700mb layer may not be b/c WAA forced a level to rise above freezing. In these cases, consider ZRA/IP.

The MOST effective method for assessing precipitation type is examining soundings.

The 12z GFS at 168hrs (which Chris linked) has an example of cold thickness biasing over the Mid Atlantic region.



Notice the 0C isotherm ( freezing line/32F) is running across PA, MD, VA, but 1000-500mb thicknesses are running well BELOW 5400 gpm.

[yoda]

Excellent discussion Wxguy. I now understand how to use those darn things correctly...

The GFS always has a cold bias in the winter, so when I see it showing a storm w/in 72 hrs, and it has model support, I raise the temps by a degree or two to offset it.... if the other models show it a tad warmer.

Anyway, any new news about next week?

[wxguy25]

Excellent discussion Wxguy. I now understand how to use those darn things correctly...

The GFS always has a cold bias in the winter, so when I see it showing a storm w/in 72 hrs, and it has model support, I raise the temps by a degree or two to offset it.... if the other models show it a tad warmer.

Anyway, any new news about next week?

Yes, typical GFS drivel. Lets wait to see what the EURO has to offer.

[yoda]

The Euro, eh? Is that a good model to use in long-term forecasting during the winter?

[wxguy25]

The Euro, eh? Is that a good model to use in long-term forecasting during the winter?

C'mon YOU KNOW the answer to that..

And its Medium range actually.

[yoda]

The Euro, eh? Is that a good model to use in long-term forecasting during the winter?

C'mon YOU KNOW the answer to that..

And its Medium range actually.

I know...

But I meant does it forecast winter storms? Or do we use it mainly for the PV? Or something else?

[wxguy25]

The Euro, eh? Is that a good model to use in long-term forecasting during the winter?

C'mon YOU KNOW the answer to that..

And its Medium range actually.

I know...

But I meant does it forecast winter storms? Or do we use it mainly for the PV? Or something else?

I use it for everything. The only issue the model has is holding back too much energy in the Southern branch. Otherwise its superior to the GFS and most others. Especially in the extended.

[yoda]

Ah ok thanks. Yeah, always have to watch that Southern and Northern Branch of the Jet streams...