Stakes Are High

9:35 Update: In my opinion, kudos to the NAM and NMM models on the precipitation intensity with this storm. The 18z NAM called for 9/10ths of an inch of QPF at Chatham, with the majority of that coming AFTER 00z. Snow totals across the Cape are now exceeding 8 inches in some areas and the National Weather Service in Taunton, recognizing this, re-issued the Winter Storm Warning after dropping it earlier in the evening - a move that was a bit unusual in the first place. Near blizzard conditions are occurring across the Cape, and with temperatures falling into the 20s now (something that I was not expecting by any means) blowing and drifting snow has increased dramatically. Totals will approach a foot and drifts of 2 to 3 feet will be common.

Sunday Evening Update: A deep ocean storm is spinning southeast of Nantucket this evening and sending waves of snow westward and onto the Cape this evening. Snow has been falling most of the day, but really picked up in intensity late this afternoon and this evening. Heavy snow has been reported across the Cape and visibilities have occasionally lowered to nearly zero as near whiteout conditions have developed. A particularly intense band of snow crossed the region between 6 and 8PM, with snow fall rates of 1 to 2 inches per hour. Here at CapeCodWeather.Net in Yarmouth Port between 5 and 6 inches of snow has fallen as of 8:30PM. A report from Marstons Mills indicates 6 to 7 inches.

The temperature has dropped from the mid 30s to the upper 20s to near 30F which has allowed for a more powdery snow over the last several hours. Not only will this lead to greater blowing and drifting of the snow but this may allow for a bit more accumulation than expected. North winds are beginning to pick up across the region, with sustained speeds of 30 mph and gusts to 45 mph common. Nantucket's 8PM observation reports a sustained wind of 46 mph with gusts to 54 mph. The strongest winds are expected from midnight until 5AM.

The National Weather Service downgraded the Winter Storm Warning to a Snow Advisory. However, based on observations around the region and radar trends, I suspect we will still end up with snow totals close to what was predicted, with 6 to 10 inch amounts common across the Cape. Drifts to 2 feet are possible in spots.

Friday's Post: The ocean effect snow just didn't get firing the way I thought it would - there were certainly plenty of clouds and flurries, but no organized bands of precipitation. Parts of the Cape saw a coating of snow during the evening, but that was more related to the passage of the arctic front than true ocean effect snow. There was a lot of dry air pouring southward across the region (as evidence by this morning's upper air sounding in Chatham which reveals a nearly -30C dewpoint!) which could have hampered things a bit. Either way, the lingering clouds and flurries this morning will shift offshore and sunshine will be the theme for most of Friday and Saturday.

I'll have to put this poor performance behind me though as he next big forecast challenge is quickly approaching, and this one is far more difficult and far more important.

f you follow these posts regularly you know that the jet stream is expected to undergo a transition over the next few days as a quasi-blocking pattern develops for a few days. You also know that this upcoming pattern has the potential to impact on our upcoming weather in a big way.

Just as a blocking upper level pattern yields a slow progression of ridges and troughs in the jet stream, it causes slower moving air masses and can make storm systems move at a snail's pace. Upper level blocks can leave locations in the same general weather conditions for several days on end (can be nice weather, can be horrendous weather). They are common in the spring and are typically the reason behind our pro-longed stretches of nasty cold spring weather. There's been very little upper level blocking at any latitude this winter, which has accounted for the rapidly moving weather systems, changeable conditions and lack of sustained cold air. This is at least temporarily going to change, and depending on just how this evolving jet stream pattern unfolds, we could see some VERY interesting weather Sunday and Monday.

The key to the forecast hinges on where the upper level block develops and where the upper level winds close off - as this will directly impact the placement of its reflective ocean storm in the western Atlantic. We will be following a piece of energy embedded in the jet stream that will eject eastward from the major west coast storm system (the one bringing all the snow and rain to central and southern CA) and traverse the country over the next 48 hours. This energy will slide across the central US, through the Ohio Valley and will eventually form a closed upper level low pressure system somewhere east of the Mid-Atlantic on Sunday. As this upper low takes shape, a correspoding surface low pressure center will form and rapidly deepen into a powerful storm center.

The placement of this surface low will be critical to our Sunday and Monday weather. I mentioned earlier in the week that one of the best medium range computer charts, the ECMWF (European Model), was placing this storm awfully close to the region - perhaps close enough to graze the Cape with snow and wind. The GFS, on the otherhand, was much less threatening, as was the Canadian model. But when the Euro is on to something, its hard to ignore it, and low and behold, computer guidance is slowly starting to converge on the more western theme lead by the ECMWF several days ago. The Canadian model stalls out a 986 mb storm southeast of Nantucket and leaves it there spinning in place for 36 hours - pounding the Cape. Not unlike the NOGAPS yesterday. Now that the storm has moved into the sites of the NAM model, it too has a powerhouse storm's western periphery scraping the Cape Sunday night and Monday. The GFS is further south, but finally has the system.

So why, or what are, the concerns?

1. Snow or snow eventually transitioning to rain Sunday afternoon into Monday night. Precipitation will pinwheel north and westward around the developing ocean storm and head towards the Cape later Sunday. The atmosphere should be cold enough at that time to support snow. But, a fascinating aspect of this set up will be as the storm is intensifying it will also be exhausting the supply of cold air over New England. So if precipitation can rotate westward and onto the Cape - which as of right now is still in question but becoming more likely - it may start as snow and stay snow for quite some time before warmer air eventually allows rain and sleet to get involved. Way too early to really think amounts or intensity or precip-type however. Sometimes these cold pools of air aloft can keep things all snow.

2. Wind - the trick to this whole deal is not necessarily the strength of the ocean storm alone but the presence of a surface high pressure nosing in to our north. The difference in pressure between these two features is likely to produce a strong north to northeast wind, regardless of whether the storm is ever close enough to produce any precipitation. So minimally a 20 to 30 mph sustained northeast wind is likely. BUT, that said, if the NAM model were on track, a major windstorm would be in the cards. In fact, the 06z and 12z runs of the NAM today are jaw-dropping in terms of wind. It's a bit early to buy such a dramatic forecast, but that's one scenario out there.

Suffice it to say, there's plenty to watch over the next 48 hours and don't be fooled by forecasts of a quiet weekend with no big storms in site...they are in site...it just depends on how close they come...