PM Zones: Warmth on Hiatus

Good afternoon! We continue to have chilly weather with a mix of sun and clouds, thanks to a large blocking pattern forcing an upper-level low to linger over the area. The upper-level low has a very chilly airmass underneath it, particularly from 850mb and above. In fact, this chilly airmass led to snow in Norfolk, CT this morning! Generally, highs only held in the upper 50s across the area, and it actually could’ve been worse had it not been for downsloping NW flow and deep mixing in the atmosphere. But the NW flow and mixing makes the surface warmer, which increases the difference between the surface temperatures and the very chilly airmass aloft, which increases atmospheric instability, and leads to an increase in clouds — thus the frequent periods of cloud-cover today. These clouds also contained a few isolated showers, but the downsloping component kept these a bit more infrequent than days past.

Today's 12z GFS valid for Tuesday morning shows overnight lows dipping into the 30s for areas west of I-95 in NJ.

Today’s 12z GFS valid for Tuesday morning shows overnight lows dipping into the 30s for areas west of I-95 in NJ.

Forecasting low temperatures will be tricky tonight. The airmass overhead is quite chilly and winds will be calming, which leads to temperatures dropping pretty easily. However, forecast soundings still show the atmosphere not decoupling because of all of the mixing still in the atmosphere, and the clouds may also still linger. If these forecast soundings are correct, then the above GFS forecast low temperatures are probably too cold, and NJ and other NYC suburbs would have lows in the low to mid 40s. But if the atmosphere can become just stable enough via the sun going down and some initial afternoon clouds would thus go away, then temperatures can drop quickly and the above GFS would be right with lows widespread in the upper 30s in NYC suburbs, and even colder in the Pine Barrens and more elevated interior valleys. However, there will be shortwave energy rotating on the backside of the upper-level low and heading towards our area, so clouds aloft may increase again later at night. We thus think the above GFS image is a bit too cold and that most areas will remain above 40 degrees. NYC itself, regardless, will hold in the 45-49F range. Areas well to the north and west in the Adirondacks and W PA will be very cold as evidenced above, since they are on the other side of this shortwave energy and thus will see subsidence and more clearing.

This afternoon's NAM model valid for 2:00pm on Tuesday still shows the upper-level low stalling just north of our area, leading to continued below average temperatures.

This afternoon’s NAM model valid for 2:00pm on Tuesday still shows the upper-level low stalling just north of our area, leading to continued below average temperatures.

As we head into tomorrow afternoon, it is more of the same story. The upper-level low will continue to sit near the area, allowing for instability to remain present as well as areas of vorticity rotating cyclonically into the area. This will lead to afternoon cumulus clouds, which combined with the chilly airmass will mean temperatures will hold to similar levels to today. Fortunately, there should still be enough downsloping NW flow for breaks of sunshine. Notice how 850mb temperatures are solidly below 0C across the region, but also notice how the wind barbs at that level are very light. This means that even with deep afternoon mixing, winds will remain very light tomorrow. The light wind flow means that sea-breezes can thus form along the coast, making it even chillier there than inland areas. With the core of the instability remaining to our northeast, any afternoon instability showers will generally remain in CT. 

Tuesday night may actually be chillier than Monday night as those faster NW wind barbs aloft shown to our southwest in the above image move into the area, which is a byproduct of upper-level confluence. Upper-level confluence usually leads to more subsidence beneath, so clouds may actually clear a bit. Frost is possible in interior valleys.

Today's NAM model valid for Wednesday afternoon shows light winds with temperatures rising into the mid 60s.

Today’s NAM model valid for Wednesday afternoon shows light winds with temperatures rising into the mid 60s.

For the same reasons that Tuesday night may be chillier than Monday night may actually allow Wednesday to be a tad warmer than Tuesday. This is because that confluent zone has a tight height gradient, so when that moves over the region, our heights aloft quickly get a tad higher, so the airmass is a bit warmer. When combined with a bit more sunshine, this should lead to temperatures reaching the mid 60s. Afternoon cumulus clouds are still possible with low-level instability still being present.

We are still tracking a potential coastal storm for this weekend. The large-scale features are very favorable for a large storm with heavy rain with a massive west-based blocking pattern forcing energy underneath to amplify and phase, and then the blocking gradually weakens enough to allow the storm to come up the coast. However, the nuances make the forecast a bit trickier, as now the blocking pattern may become so strong, that energy beneath it near the Great Lakes may become even more cutoff and larger and slower to move, rather than a smaller and more mobile feature. This makes phasing between the streams a bit more difficult, as the large bowling ball orientation may instead end up dominating the flow and shearing out the system. We still think the blocking pattern will be large enough to buckle the flow enough for amplification and thus a hit from this system, but confidence is not as high in a hit with this as it was for Friday’s deluge. Even with a lack of phasing, the bowling ball low on its own may trigger some showers and thunderstorms out ahead of it on Saturday night and Sunday, but this scenario wouldn’t be nearly as impactful as a phased Nor’Easter.