Upon waking up to another chilly morning, most of us are probably wondering when Spring is coming. For those west of NYC, it probably arrives on Monday and Tuesday, but for those east of New York City, we might have to wait until April 15th or so. Forecast models are struggling immensely with the positioning of a warm front for next week, which throws lots of wrenches into the forecast.
A storm system will head up the coast on Friday and shoot into the Canadian Maritimes this weekend, and strengthen as it does so. That will help to keep things relatively chilly for the weekend, with the cooler northerly flow behind it. Once that storm system gets far enough to our north, heights will begin to rise, and this is when the potential warm-up begins. This is also when the European model and the GFS model really begin to diverge. The biggest difference is the way they handle a huge upper level low in Canada. The GFS splits the ULL into two and elongates it, while the Euro keeps it one large entity that dominates the pattern. The differences begin as soon as 84 hours (Sunday morning), as the GFS is already showing signs of splitting the feature and elongating it, while the Euro is much more powerful. Implications of this will be explained shortly.
One thing we do know is that it is clear that there are still remnants of the past blocking pattern in place. There are cutoff portions of very high heights in northern Canada; thus favoring closed off ULLs to form downstream. These patterns do not tend to run very warm, but the good news is that the core of the blocking for the most part is oriented a bit to the west of the NAO regions. Both models agree on a vigorous shortwave entering the Pacific NW, but it will not be allowed to race eastward, due to the ULL and the blocking. Thus, it is instead forced southward, underneath the block.
Let’s fast forward a little bit to Monday afternoon. That storm system is forced to the south on both models, so it is digging into the west coast states. This helps to raise the heights towards the east, as opposed to a zonal flow, which should help raise temperatures into the low to mid 60s for areas west of New York City. However, we can clearly see the differences magnify with time, which has major implications down the road. Since the GFS splits and elongates the ULL, part of it gets trapped underneath the block, so the block is not exerting as much influence in forcing the storm to the south, since a piece of the ULL is underneath it. The European, however, keeps the ULL much more powerful and as one feature to the east of where the GFS has it. So instead of a piece of the ULL getting caught underneath the block, the storm itself is caught underneath the block, which forces it much further south.
For one thing, this has major implications on the potential severe weather outbreak in the southern Plains early next week. The European model’s setup would be much more conducive for stronger storms because the more amplified storm system allows for moisture and instability to surge into the southern Plains states. As far as we’re concerned, the impacts in the modeling differences do not mean too terribly much, as both show height rises entering our area and a moderate SW flow, which is an offshore flow for areas west of NYC. This yields a smaller marine influence for these areas; however, east of NYC, a SW flow does have some marine fetch, so the warm-up could be muted somewhat, given the very cold water temperatures in the beginning of Spring. Remember, the temperature change of the ocean greatly lags that of the land, given water’s greater specific heat capacity than air, which is why water temperatures are at their coldest in March, and still plenty cold in early April. More details regarding the specific nature of the airmass for next week will be illustrated shortly.
As we move forward in time, we can see the differences magnify even further. On Tuesday afternoon, the GFS has that western split of the ULL that was underneath the block retrograde even further to the southwest, into the longwave trough associated with our storm system. This helps the buckle the heights and yields further height rises into Canada, which is a warmer pattern for us. However, since the Euro does not split the ULL, nor elongate it, there is no ULL piece between the storm and the block, thus the Euro is interpreting the storm system itself as a new ULL that forms underneath the block, which helps for it to stall and nearly cut off. This helps it to disconnect it from the main, large ULL piece to the northeast. Since they are disconnected, the ULL is more powerful and compresses the height field greatly in Canada. This prevents height rises from fully making it northward, which brings a return to colder conditions come Wednesday and beyond.
By the time we get to 162 hours, which is Wednesday afternoon, the GFS has turned that ULL into a split, elongated mess, and it almost appears to be “sliding” into our large, relatively progressive longwave trough, which helps to further amplify the trough; pumping the heights out ahead of it. If the GFS were to be right, temperatures would easily reach the 70s for much of the region on Wednesday, as a warm front on the east side of the storm is allowed to shoot northward. However, the Euro keeps our main storm system relatively cut off and slow to move, while the ULL is powerful and sitting just to our north. This yields a confluent flow and a strong high pressure system at the surface to form in SE Canada, bringing cold, northerly winds, and compressing the height rises. The warm front to the east of the low is thus positioned much further south, yielding the warmth to only make it to the Mid Atlantic. The temperature gradient with this would be quite impressive. Either way, there is going to be quite the warm sector in the southeast states, with a cold front draped in the southern Plains through the Tennessee and Ohio Valley regions, along with a warm front somewhere in the Mid Atlantic or Northeast (the exact position, as mentioned before, is still uncertain). Thus, severe weather activity will really ramp up in the Plains and southeast states come next week.
To be quite honest, I have a feeling the European model has the correct idea here. There still appears to be enough blocking in Canada to yield a more powerful ULL. Additionally, that block should help to greatly force the storm system to dive to the south and nearly cut off, further removing it from the flow and allowing the ULL to become more dominant, as opposed to the height rises being dominant. Models are often too aggressive in eroding a blocking pattern’s effects, and considering how high the height anomalies still are in Canada, I feel that the pattern should be acting more “blocky” than not. And strong blocks this time of year usually lead to cut-off lows. Also, if the storm is more cut off from the flow, then it’s harder to imagine the ULL “sliding” into our longwave trough and further yielding height rises, since the trough will be closed and cut off as opposed to more open for phasing like the GFS shows. The Euro would yield temperatures struggling to get into the 50s, with plenty of clouds and rain for Wednesday through next weekend.
That being said, this does not mean that Sunday, Monday, and Tuesday won’t be nice days weather-wise. It’s Wednesday and a few days after that where I am concerned might not be nearly as Spring-like as some hope. However, if the GFS has the right idea instead of the Euro, Wednesday and the days after will yield a May-like pattern.
Let’s take a look at the local pattern for early next week. As alluded to before with the previous 500mb images, the pattern starts to become a bit more amplified, but is still somewhat zonal for Monday afternoon. The digging trough yields a weak southwest flow, which will initially help to warm things up pretty nicely. A weak wave of low pressure will move along the ridge axis, as the main storm system is still to the west, explaining the existent, but weaker SW flow. But as temperatures peak in the early afternoon, the difference between the land and the ocean will increase, helping to trigger a seabreeze developing in the afternoon (remember how cold the ocean temperatures are this time of year, as I alluded to earlier!).
Considering that the southwest flow is pretty weak, and that a large dome of high pressure is located offshore, the onshore flow–via a clockwise flow around a high pressure–and seabreeze might be able to quickly and efficiently penetrate into the region, since the weak SW flow will lose the “tug of war” with the onshore flow. This would yield a quick temperature drop during the mid-afternoon on Monday, after perhaps initially getting to the low 60s. NYC and east will probably be stuck in the upper 50s, due to their proximity to the ocean.
As we head into Tuesday, the trough to our west amplifies, which helps to increase our SW flow. Additionally, the weak wave to our west on Monday will have moved more northeastward, which puts a dent on the dome of high pressure offshore, especially the northern part of it. This helps to reduce the cold, onshore flow, and increase the SW flow instead. Thus, although a seabreeze will still try to form, the onshore flow will be weaker and lose the “tug of war” with the stronger SW flow, which could help bump up the temperatures by a few degrees; making widespread 60s to maybe even near 70 in some spots, especially western locations, much more likely. East of NYC will also warm a little due to the weaker offshore flow, but they will still suffer the effects of the SW flow “fetching” the colder ocean water, and the seabreeze. Coastal areas really prefer a more downsloping, offshore westerly flow (or even NW flow assuming cold air is not being advected) for warmth this time of year, which may be something that could bump up tomorrow’s temperatures more than forecast, if the storm misses. Either way, Tuesday of next week should be a very nice day even for the coastal areas, especially considering how cool it’s been recently. But the coastal regions may have to wait a bit longer to actually join the true “warm party”, because of the onshore flow.
So, when will a sustained period of Spring really start? If the GFS is right, it’s Wednesday, but if the European is right, we may have to wait a little longer. Come April 15 or so, once the blocking finally gets disintegrated, height rises will have a much easier time getting into Canada, and the strongest height anomalies will be to our west, helping to co-locate a stronger high pressure system onshore in the SE states, which yields offshore winds. This helps to bring a warm, westerly, downsloping flow, with minimal onshore flow. Thus, I will make a prediction that our area will see widespread 70s (yes, even to the coast!) and even some 80s during the April 15-20 period. The GFS ensembles show this quite well.
Stay tuned to all of our latest updates, as we will be fine-tuning next week’s forecast as the days go on! Have a great day, everyone.