Cold Front & Snow Expected Over the Next 10 Days, Travel Impacts Expected

A cold front arrives today in Oregon and it's going to make the weather in Southern Oregon quite interesting over the next 10 days. Here's a tl;dr -Diamond Lake and Crater Lake could get a few feet of snow between Wednesday morning and Friday morning. Then lots more snow from the next system arriving this weekend. -Travel is expected to begin being disrupted on I-5 starting as early as Friday/Saturday over the passes and higher elevations. By Sunday, a significant amount of snow is expected in the passes and travelers need to be prepared and check conditions before heading out. -Snow will likely be seen across Douglas County between Friday and Sunday, mostly in the higher elevations but the lower elevations could see some as well. Here is the full forecast discussion from this morning from the NWS in Medford: A prolonged period of cold and wet weather is expected across SW Oregon and Northern California over the next 10 days. Significant winter travel impacts are expected, first at mid to higher elevations today through Friday morning and then, likely, at low to middle elevations from Friday/Christmas Eve through approximately New Year`s Day. If you will be traveling during this time period, be sure to check the latest road conditions and forecasts before you decide to hit the road. For everyone, including those who live locally, now is a great time to prepare for the upcoming wintry weather by packing a winter travel/survival kit and preparing for colder, potentially snowy weather. It is likely that elevations down to between 500 and 1500 feet will be impacted by snow at some point during the next 10 days. This morning an impressively deep and high amplitude trough of low pressure exists across the eastern Pacific Ocean with an impressively large and high amplitude high pressure ridge west of the trough over the central North Pacific extending well into the Bering Sea to near the Arctic Circle. The trough, which was mostly closed off from the main polar jet stream flow yesterday, is now starting to merge back into the main flow. This is resulting in increasing southwest flow over NorCal and SW Oregon this morning that`s likely to max out at about 150 knots at the 300mb level this evening into Thursday morning. Scattered light showers this morning with snow levels mostly in the 3kft to 4kft range are likely to increase across Northern California and west of the Cascades today, especially along and near the coast. Model guidance has trended wetter for this afternoon through Thursday morning as the above mentioned jet stream energy pumps subtropical moisture northeastward across the area. This has resulted in the need to upgrade the Winter Weather Advisory in effect for elevations above 4kft in the Mount Shasta area to a Winter Storm Warning valid late this afternoon through early Thursday morning. Numerical models suggest around 1.2 inches of water is likely to fall in Mount Shasta City and the surrounding area during a 12 hour time period this evening through Thursday morning. This is, in part, due to a stronger, longer duration, and more southerly flow at the 850mb level in that area. Elsewhere, headlines have remained about the same for this frontal system. Snow levels are likely to be in the 4-5kft range. The Winter Weather Advisory for areas east of the Cascades has been expanded some, mostly in Klamath County, due to more precipitation expected there tonight into early Thursday. Late Thursday morning through Friday morning a cold front will move southward across the area with rain and snow showers as the snow levels lower from ~4kft to ~2kft. Winter Weather Advisories remain in effect for some areas for this front. We`ll need to be on the lookout for the potential need to expand the advisories to other areas west of the Cascades if current wetter trends noted in the 06Z NAM12 continue and show up in the 12Z SPC HREF guidance. Christmas Eve Day/Friday begins the time period of most concern for travel impacts for areas west of the Cascades. A Winter Storm Watch has been issued for much of the area west of the Cascades for elevations above 1500 feet from midday Friday through Monday morning. The cold upper level trough swings in from the Pacific Friday bringing temperatures of -30 to -35C at 500mb. This cold air, along with plenty of vorticity and instability, is likely to bring convective precipitation across the west side. This can result in snow levels dipping lower in moderate to heavy showers, also leading to potential accumulations. In fact, some rumbles of thunder are not outside of the realm of possibility for even inland areas on Christmas Eve into Christmas Day. Snow levels near 2500 feet on Friday are likely to fall to around 1500 feet Saturday/Christmas morning. From Christmas Day through New Year`s Day, approximately 5 shortwave troughs move through the area with 850mb temperatures between -3C and -12C. A rough rule of thumb for snow here at 1370ft MSL at the Rogue Valley International Airport is 850mb temperatures of about -6C. Said another way, National Blend of Model probabilities indicate a 75% chance of 6 inches or more of snowfall at the Medford Airport in a 72 hour period from the 26th through the 28th with about a 60% probability of 8 inches or more. While the amount of snowfall in this blend of models may be a bit overdone and the amount of snowfall compared to that on the ground at any given time is likely to be less than that, even just an inch of snow on the ground at the airport can yield significant travel impacts. For mountainous areas such as Sexton and Canyon Mountain Pass on upward, snow`s likely to be measured in feet of low density snow during this same time period due to the cold temperatures and precipitation we`re expecting. Again, if you`ll be needing to travel at all in the next 10 days, now is a good time to get yourself and your means of travel ready for wintry travel if you have not already done so. Also take note that this period of wintry weather is likely to bring with it and behind the coldest temperatures we`ve recorded, thus far, this fall-winter. ~BTL