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nj2va

850mb Low Track

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Why does the track of 850 low matter so much for snowfall?

There are old rules of thumb about the thermal structure around an H85 low, such as snowfall is not possible until H85 temps are at or below -3 C to the nw of the low center, etc. This is a poor man's way of determining if a warm nose around or above H85 may persist or perhaps if the sfc-based layer is cold enough to support snow. H85 flow is also a good proxy in determining the degree of llvl moisture flux into the storm system. The best llvl forcing in terms of frontogenesis shows up more clearly in geostrophic flow (regions off the ground mostly unaffected by friction)...generally in the H85-H7 layer. The configuration and modification of frontogenetical areas through time can give a good indication of where the heavier snowfall rates may occur.

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There are old rules of thumb about the thermal structure around an H85 low, such as snowfall is not possible until H85 temps are at or below -3 C to the nw of the low center, etc. This is a poor man's way of determining if a warm nose around or above H85 may persist or perhaps if the sfc-based layer is cold enough to support snow. H85 flow is also a good proxy in determining the degree of llvl moisture flux into the storm system. The best llvl forcing in terms of frontogenesis shows up more clearly in geostrophic flow (regions off the ground mostly unaffected by friction)...generally in the H85-H7 layer. The configuration and modification of frontogenetical areas through time can give a good indication of where the heavier snowfall rates may occur.

Totally agree, of course, but I was thinking much more basic...

If the 850 low goes to your west, you're in the warm sector and getting rain most likely (unless there is a very cold antecedent air mass). If it goes east, then you have a shot at seeing snow.

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Totally agree, of course, but I was thinking much more basic...

If the 850 low goes to your west, you're in the warm sector and getting rain most likely (unless there is a very cold antecedent air mass). If it goes east, then you have a shot at seeing snow.

That's it, it's really hard to get snow with an 850 track to the northwest of DC. once the cold advection kicks in you get too much drying from downsloping. I actually, think the old rules of thumb concerning the 850 are pretty useful and the models often keep too much precipitation to the west of a storm when the 850 low has a bad track anywhere east of the appalachians. For the past storm, you had used the raw nam, DCA should have had .35" of liquid falling as snow. I doubt it got a fourth of that.

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Thanks for the replies. I guess my next question is, what is the 850 low?

Simply put, it's where the cyclone is located at the H85 level. The flow is counterclockwise and there is a definite center of circulation or a closed height field. In this case, the H85 low center is located over sw/rn Ontario within the closed 126 DM contour.

post-866-0-48324500-1323553080.jpg

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