LOADED FOR BEAR...
If you like your weather active and a challenge to forecast, the pattern is loaded for bear. The next two weeks leading up to Christmas look to be energetic and progressively colder. If things go the way models are trending today, we might even get that white Christmas that's eluded us for quite a few years.
To that point, some parts of my northern counties near the HWY 20 corridor have seen mixed precipitation turn to snow overnight and into Friday morning. Slushy wet accumulations of up to an inch (maybe two) are on the table here. Some freezing rain preceding the snow has likely contributed to some slick travel conditions, especially on untreated and elevated surfaces. The influx of warm air ahead of the system has kept the heavier snows just north of my area, something I was concerned about in my previous post Thursday.
Further south, thermal parameters in the rest of my area are expected to be warm enough for a rain event, a welcome one at that.. At this time, winter weather advisories are out for these counties near HWY 20 until noon Friday. Problems should be minimal from HWY 20 south. In fact, it's possible the advisory could be canceled earlier if trends warrant.
Whether you're getting rain or snow, Friday is going to be a poor day, particularly in the morning when precipitation is still ongoing. It will taper off early from SW to NE as the center passes off to the east. Clouds will likely hold into Saturday and temperatures will move very little so I'm expecting near steady temperatures (even Friday night). Look for readings in the north of 31-37 while the south is more in the 37-43 degree category during the period.
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Monday a strong closed low develops over the Plains that has all the attributes of a lollygagger. Cut-off from the westerlies, it will take the system the entire week to cross the Plains and exit the Midwest. Initially, the storm spawns brisk south winds that draw mild air and moisture into the deepening center. My region being in the warm sector sees above normal temperatures and eventually periods of rain (maybe even some thunderstorms). The rain holds off until late Tuesday but could around on again, off again, into Wednesday night. The potential is there for at least an inch or rain, perhaps even more. The GFS shows these 48 hours totals Tuesday morning through Thursday morning.
While it will be quite mild aloft, we may never get the true warm front through the area which generally keeps surface temperatures in the upper 30s north and the 40s elsewhere Monday and Tuesday. Wednesday the warm front may get close enough for the far south to reach the 50s far south but the rest of the region remains closer to a 40-45 degree range.
Wednesday night the upper air low is nearly overhead which puts us in the eye of the storm. Winds will ease and rain comes to an end. Eventually later Thursday and Friday the core creeps far enough east to bring a renewed round of wind and colder readings. Snow showers wrap into the area on the back-side of the low. Some minor accumulations are possible with highs in the low to mid 30s.
Temperature will turn significantly colder by next weekend and should remain below normal up to Christmas. My rational for this is best explained by examining the 500mb jet stream pattern. December 16th, notice all the red up in Canada. That indicates upper air heights are high all the way from Alaska to Greenland. That's known as a block and the high pressure exerted by it forces the could air relative to average into the U.S. That allows temperatures to go down a step below average.
Now I take you to December 23rd notice the much of the red has faded as the higher heights retrograde westward. That big red paint bomb over Alaska depicts a ridge injecting cold into the pattern.
This is a classic negative EPO (Eastern Pacific Oscillation). The negatives are 5-7 standard deviations below the neutral state.
A strong negative EPO such as that teleconnects to plenty of cold in the central U.S. It may not be that we get a massive dump of Arctic air (although that is on the table). Rather, it may just be a steady supply of cold that's not barbaric.
Once again. look at the EURO 500mb flow December 23rd.