Warm, Rainy End to the Week; Arctic Blast to Follow Next Week.

While models have generally been consistent with a massive warm-up across the country for the past few weeks, what they have not done is consistently handled a potential Arctic wave towards the middle of next week, which could in fact signal the long-awaited "pattern change". The first part of this blog post will focus on this week's forecast, while the second part will focus on the potential wave of Arctic air next week, what models are showing, and what will likely happen.

Section One: Warm, Rainy End to the Week.

One to two weeks ago, models were showing a very prolonged, and very strong warm-up across two-thirds of the nation. Record temperatures were being "progged" in the Northeast and surrounding states. Now, it is likely that this does NOT happen. Models have trended towards a several day period of temperatures in the 40's and 50's, with Friday and Saturday reaching the maximum of the warm trend.

These are the MOS forecasts for high temperatures over coming days. As you can see, there are still clear differences that need to be ironed out in the short and medium range forecast.

This is the NAM sounding for 12z Friday morning (7 a.m.). Using the initial temperature profile analysis, and "mixing" the shallow boundary layer, I would expect temperatures to reach the middle and upper 60's. BUT, at the same time, showers and heavy thunderstorms will be moving through the area as well Friday morning, which will temper high temperatures. Even after heavy rain moves out of the area, cloud cover will keep high temperatures in the upper 50's. If we see cloud cover "thin" out by the afternoon hours, we may actually see temperatures rise into the lower 60's, but at this time it is unclear whether this will happen. In fact, current trends in the NAM suggest that precipitation will likely exit the region more slowly than previously suggested, giving even more credence to high temperatures in the middle to upper 50's.

The above image is that of the 18z NAM's radar reflectivity forecast for 7 a.m. Friday morning (basically showing where and how heavy precipitation is). As I stated earlier, the NAM has been trending slower and stronger with the wave of precipitation (with the possibility of thunderstorms at times Friday morning). The only difference then between the 12z NAM and the 18z NAM is that the 18z NAM is only bringing 0.4 to 0.6 inches of rain to the region, and the 12z NAM is forecasting 0.9 to 1.2 inches of rain for the Friday morning period. At this point, I'm not sure exactly what rainfall totals will be, but given the general trend towards a stronger upper level system, and better return flow from the Gulf of Mexico, a consensus between the 12z and 18z NAM may be warranted, giving the area in between 0.6 and 0.9 inches of rainfall. Another bias to take into consideration is the fact that the NAM tends to overdevelop thunderstorms in situations such as these, thus indicating a lower precipitation amount in reality, then what the model is forecasting. I'll keep you updated on this.

These two charts are comparing the 500 millibar vorticity forecast from 00z last night (NAM's run) to the 18z run from the NAM this afternoon. As is indicated, the NAM is trending slower, farther east, and stronger than was forecasted yesterday. This will focus stronger forcing on the area, which may in fact account for higher rainfall totals. At this point, again, we shall see what happens, but current indications suggest that we could actually get some decent heavy rainfall this Friday. 

As for Saturday night into Sunday, an approaching cold front, in tandem with a strong upper level trough could produce a few more showers, but nearly as extensive as Friday morning's rainfall.

Second section: Arctic wave and Winter Weather

In this section I will focus on the potential for an Arctic "blast" into the region by early next week (in the Monday/Tuesday timeframe). 

As you can see, the 12z GFS forecast charts show a slowly moving "strong" cold front moving past the region. Notice also how slow this cold front will be moving. Normally, slow-moving cold fronts are associated with the type of front known as an "anafront". An anafront produces widespread precipitation behind the frontal zone. If this were to occur, we should see periods of light snow as a possibility next Tuesday. But given the model inconsistency as of late, it would be really hard to make a forecast just based on this. After this timeframe, the GFS brings a very cold air mass into the region, and is very "stubborn" with keeping the Arctic air in the area. 

The above 12z ECMWF forecasts indicates a much quicker, and drier frontal passage with this cold front. Not only this, but the ECMWF is slightly colder, as well as more "progressive". As of right now, I'm not leaning towards either solution, as they both have the possibility of panning out. My main concern in this section is to elaborate on a potential wave of very cold air in the timeframe of January 15-17. If the ECMWF were to verify, there would be no back-end snowstorm, and possibly very little precipitation. 

What do the teleconnection forecasts have to say for the rest of the month? They continue to indicate a much more substantial wave of Arctic air to return to the Midwest and possibly the Eastern states after the 20-23rd. It seems as if the MJO may keep "in-check" the entire Pacific longwave pattern (which would limit North Atlantic blocking), which will delay the arrival of a truly sustained Arctic air mass.

The MJO forecasted phases are expected to move into Phases 6 and 7 during the same timeframe, which seems to be in conflict with the AO/NAO going negative during this same timeframe. Further analysis of tropical convection would also indicate that this is likely to hold, and the MJO may not head into Phase 8 as quickly as originally thought. This would validate the "pattern change" dates that I originally forecasted of the 20th-27th. I do expect that the MJO will eventually either become favorable or move into the infamous "Circle of Death". When the MJO becomes inactive, storm systems moving onto the Pacific Coast generally become more infrequent, and thus usually drier conditions out west.

As for the expected magnitude of this cold air? It is possible that it could be well below normal, breaching records in some areas of the Northern Plains, especially by the end of the month. I expect this to occur, mainly because of the well above normal snow cover across North America, as well as the well above average snow depth across Southern Canada, which will modify pre-existing air masses to become colder (regardless of strong downslope flow).

As for the longer range forecast, the latest analysis of the tropical Pacific continues to indicate more favorable conditions to the development and strengthening of a weak La Nina in coming weeks. I will continue to monitor this, as it could have a large impact on our spring/summer weather this year. Have a great afternoon and evening!