Let's examine some predictors:
- Surface temp: If <= 0c either freezing rain, sleet or snow is likely, though if the sub-freezing layer at the surface is very shallow and/or WAA (warm air advection) is occurring either freezing rain or even plain rain are likely. The surface temp alone isn't a great predictor of snow vs rain, but is a good predictor of wintry precip vs rain.
- 925 mb temp: A better measure of snow vs rain or freezing rain because the warm layer in an overrunning/warm front event is often noticeable at 925 mb (2500'). If this layer is above 0c snow can generally be ruled out and freezing rain as well unless the surface temp is at or below freezing. In a moist adiabatic or isothermal profile a 925 mb temp of 0 to -1c can be enough for snow down to the hills (500-1000 feet) and even flakes or a mix below that.
- 850 mb temp: Similar to 925 mb temp, generally if both 925 mb temp and 850 mb temp are above freezing any surface cold air is very shallow and the only potential is for freezing rain. In an isothermal or inverted layer, the 850 mb temp can be 0c and snow can still occur assuming lower levels are at or below freezing. This isn't a common scenario in western WA/OR except during periods of easterly/offshore flow near the Cascade gaps and the Gorge. In a moist adiabatic profile (usually present with some component of onshore flow) generally a 850 mb temp of at least -8c is necessary for snow down to the surface (below 500 feet).
- 850 mb thickness: Less commonly utilized (but arguably superior) metric in determining precip type at low elevations. 1290-1292 meters indicates the entire 1000-850 mb layer is at or below freezing and suggests a 90% chance of frozen precip type (snow generally). 1300 is a typical rain/snow guess east of the Rockies.
- 500 mb thickness: Probably the worst metric for determining precip type, but the "first guess" for meteorologists east of the Rockies. It's fairly useless over and west of the Rockies. Studies on precip type vs 500 mb thickness in the UK have established a rough 50/50 rain/snow value at around 522 at coastal locations near sea level in onshore flow scenarios. A rough 50/50 value of 526 based on observations at Portland, OR.
- Other considerations: Modeled wet bulb zero elevation (either in terms of pressure level or in meters/feet). Not always available or easily determined from model output. If available a very useful value along with 850 mb thickness.
1. 850 mb thickness: look for values at or below 1300 meters south and west of your location, or at a lower elevation than your location.
2. 925 mb temp: look for values at or below 0c, preferably -1 or -2c for better odds of accumulation. Beware of 925 mb temps even a degree above freezing.
3. 850 mb temp: look for values at or below -8c unless offshore flow is present and surface dewpoint depressions will allow for evaporative cooling.
4. 500 mb thickness: if this is all you have look for values at or below 525 or 526 (usually the maps are in increments of 3 dam or 30 meters so ideally you would look for 525 and 522 lines). This isn't a guarantee of snow but values above 526 are fairly unlikely to yield snow in the lowlands without offshore flow.
Rough guide to snow level based on 500 mb thickness and 850 mb temps, assuming moist adiabatic thermal profile between the surface and 700 mb
Sea level (coastal): 524 and -9c
Sea level (inland): 526 and -8c
500': 528 and -7c
1000': 530 and -6c
2000': 534 and -4c
3000': 538 and -2c
4000': 542 and 0c
5000': 546 and +2c
6000': 550 and +4c
Note: NWS Salt Lake gives a critical 500 mb thickness of 543 at Salt Lake City (elev 4,200') which suggests the above list is fairly accurate at mid-slope level in the Cascades. Marine influence is most pronounced in the lower levels.