Evening PM Zones Update: A Cooler Pattern

Good evening, everyone! Another pleasant and warm day was had today with high temperatures generally in the 70s, with a good amount of mixing behind a cold front. However, surface pressures have begun to rise, which is evidence that Canadian high pressure is building in. This will allow for a cooler airmass to move in, winds to calm, and for skies to generally remain clear. There is a weak shortwave trough that will be passing over the region, which may briefly yield an increase in mid and high clouds in the early and mid-evening hours, but this shortwave trough is extremely positively tilted, so shortwave ridging is right on its heels — combined with the Canadian high pressure, subsidence should outweigh the forcing for lift, so the increase in clouds will be minimal and then should decrease again later in the overnight. Forecast soundings do show some mixing remaining in the low-levels tonight, so while winds will decrease, they may not go completely calm. This could prevent ideal radiational cooling conditions, but we will still have favorable enough radiational cooling for most areas to fall into the low to mid 40s tonight, with some mid to upper 30s in the interior valleys and Pine Barrens.

Tuesday will be a very pleasant day with the high pressure still in control, but its change in positioning will lead to some changes in our sensible weather going forward.

The high pressure will strengthen to near 1040mb and initially be situated in New England. This supports weak wind flow and a decent amount of mixing from sunshine as the surface flow from the high still has mainly a land component with a weak gradient, so we should still be able to warm into the low to mid 60s on Tuesday with mostly sunny skies. However, as this strong high pressure moves further offshore, it will also slow down its movement. This will allow a weak low pressure just north of the Great Lakes to catch up with it and increase the pressure gradient. This, combined with the high being offshore and now having more of an ocean/onshore component will allow somewhat deeper southeast flow to come in off the ocean.

Notice the streamlines on the bottom left panel as well as the shading. This is showing 10-20 knot southeasterly winds just off the coast, so winds may become somewhat gusty along the coast — but should drop off inland. Notice the tight pressure gradient as well, but also notice how it’s tightest on the southeast side of the bottom right panel, coinciding with the strongest winds on the bottom left panel. It has already dropped into the 40s for most of the area at this time, and temperatures will continue to drop the rest of the night with widespread lows in the upper 30s to low 40s. Clouds will also increase with this marine layer.

As a result, Wednesday will remain quite chilly with the onshore flow and marine layer. Notice how at 2:00pm, temperatures are still holding in the upper 40s for the most part, though areas a bit inland in SE NY have risen to the low to mid 50s. Also notice how on parts of Long Island, it is still in the low 40s — and if we look at the steamlines, we can see why. High temperatures may end up a few degrees warmer than this, but it goes to show the influence of this marine layer.

This strong marine layer interacting with the relatively warmer land will result in some low-level convergence and frontal boundaries, which may start to wreak a bit more havoc. Remember that strong winds decelerating in the same direction can act as convergence — particularly when they go from ocean to land and you get frictional convergence. This may eventually lead to some scattered showers on Wednesday night into Thursday, but nothing hazardous.

On Thursday, a stronger storm will be traversing the Great Lakes, and its associated cold front will have strong southwesterly winds out ahead of it. These winds will converge with the onshore/south/southeast winds, which will help provide additional synoptic lift.

Notice in the above image we have a strong closed 500mb low in the Great Lakes. This will help provide plenty of synoptic lift — something we did not have on Sunday. Also notice this leads to the main jet streak being suppressed to its south in the Ohio Valley, but it’s pretty strong. Notice how the height gradient goes from sharp our west to wide in our area and how the wind barbs start somewhat diverging — this is upper-level diffluence. This is not as good as upper-level divergence is for upward vertical motion, but it can still help to provide some.

Precipitable water values will also be trending above normal with a little more subtropical moisture connection. The low’s position to our northwest may also place us in the warm sector, which supports the potential for convection with heavy rainfall as well. There is still a chance that this system may pass by a tad too far north for us to have the strongest lift and we are also not in the true upper-level divergence sector, so uncertainty is still high with this system.

As we head later in the weekend, we may have to watch for a potential larger storm system that could give widespread heavier rainfall. We will have more details on this in tomorrow’s premium article, but we will very briefly discuss it here.

A strong shortwave will be able to tap into Gulf moisture and also slow down, allowing ample time to gather further moisture. It may then try to head northeastward — almost like a gradual Nor’Easter evolution. The ECMWF model shows a large cutoff low, which leads to a long-duration heavy rain event from Sunday through Tuesday, with 2-5″ of rain falling area-wide. The GFS (above) is a bit more progressive — notice how it’s not cut off at 500mb — but it’s still an intriguing setup for a quick hit of heavy rain on Sunday. It is still too early to say which model has the correct idea, but we will have to watch this closely.