Transition day today leads to a nice end to the week

The cold front that had helped to cause the thunderstorms and heavy downpours over the past couple of days is finally working its way through the area, which will gradually clear our skies as the day goes on. However, the front will be slow to move through the area, given the strong western Atlantic ridging. Combining this with an extremely powerful mid and upper level disturbance, placing us in the right-entrance region of the jet stream, there will still be plenty of clouds around and some showers, with locally heavy downpours. That being said, considering the cold front has shifted to the east, the biggest threat for heavy rain should stay to the east of NYC.

4-panel plot of this morning's NAM run, valid for 10:00am. Precipitation still hangs around the coast, with a cooler airmass behind it (top-left), a closed 850mb low in SE Canada leads to very chilly readings in the upper Midwest (top-right), a powerful 500mb low with strong vorticity that helped provide our messy weather still lingers (bottom-left), and a powerful jet streak at 300mb leads to upward vertical motion for clouds and rain (bottom-right). (New York Metro Weather Models)

4-panel plot of this morning’s NAM run, valid for 11:00am. Precipitation still hangs around the coast, with a cooler airmass behind it (top-left), a closed 850mb low in SE Canada leads to very chilly readings in the upper Midwest (top-right), a powerful 500mb low with strong vorticity that helped provide our messy weather still lingers (bottom-left), and a powerful jet streak at 300mb leads to upward vertical motion for clouds and rain (bottom-right). (New York Metro Weather Models)

The powerful 500mb low is actually a piece of the Polar Vortex, which explains why it is so strong and is bringing chilly temperatures to the Midwest, where temperatures even fell into the 30s this morning! Out ahead of it, strong southwest flow persisted for days, providing a conveyor belt of moisture and instability for thunderstorms. Now that the cold front is finally moving eastward, the conveyor belt also shifts eastward, which puts a limit to the coverage of rain. Most of New Jersey should stay rain-free, but clouds will remain for most of the day, thanks to the 100 + knot jet streak shown in the bottom-right panel. Long Island may have to deal with some rain and locally heavy downpours as we head through the morning.

The cold front is evidenced well by the precipitable water values —  a measure of the moisture in the entire atmosphere. Once the cold front moves through, the moisture in the atmosphere will decrease rapidly.

Valid 11:00am: The NAM's precipitable water values are still very high along the immediate coast, which yields the potential for heavy rain. Note the very dry air just to our west -- that will quickly arrive this evening. (New York Metro Weather Models)

Valid 11:00am: The NAM’s precipitable water values are still very high along the immediate coast, which yields the potential for heavy rain. Note the very dry air just to our west — that will quickly arrive this evening. (New York Metro Weather Models)

Precipitable water values near 2″, as well as 30-50 knots of wind shear and a little bit of surface-based CAPE will further trigger heavy downpours this morning along the coast. Considering all of the rain we’ve had over the past couple of days, the ground is saturated and could easily become flooded with any downpour. Fortunately, much drier air resides just to our west — as the strong moisture gradient beautifully illustrates where the cold front is.

Peaks of sun are possible for western portions of the area during the early-afternoon hours, and during the late-afternoon and evening for NYC and east. Tonight will feel quite pleasant compared to the past few nights, as temperatures fall to seasonable to slightly below seasonable levels, with much less humidity.

This morning's NAM valid for 5:00am shows temperatures dropping into the upper 50s in interior locations -- otherwise primarily 60s (mid 60s in the city) for the rest of the area. (New York Metro Weather Models)

This morning’s NAM valid for 5:00am shows temperatures dropping into the upper 50s in interior locations — otherwise primarily 60s (mid 60s in the city) for the rest of the area. (New York Metro Weather Models)

Moving forward to tomorrow, high pressure should dominate the region, bringing dry, northerly winds and very pleasant conditions. The very chilly weather the Midwest is getting will not make it into our area, as the airmass will become quite modified with the piece of the Polar Vortex lifting back into Canada. Temperatures should generally be in the upper 70s to lower 80s tomorrow with sunny skies and low humidity.

Tomorrow night may see more widespread 50s, as the high pressure more firmly asserts itself across our region. The city and the immediate coast should stay in the low 60s, however. But with continued low humidity, it should be a great night to keep the windows open and turn off those ACs.

This morning's NAM model shows low temperatures in the upper 50s for much of the area tomorrow night. (New York Metro Weather Models)

This morning’s NAM model shows low temperatures in the upper 50s for much of the area tomorrow night. (New York Metro Weather Models)

Friday looks to be another gorgeous day as well, with high pressure remaining under control. There is a bit of a model discrepancy right now between the NAM and the GFS for the weekend — the NAM is slower to move high pressure out to sea, whereas the GFS moves it out quicker. The faster the high pressure moves out, the faster we get a bit of return southerly flow, which could lead to some scattered showers and thunderstorms on Saturday and Sunday. However, the weather pattern as a whole will lack moisture, which tends to prevent thunderstorm activity, and there will not be a strong storm system too close-by, which leads us to believe the weekend will have more sun than clouds. Temperatures should rise into the low-to-mid 80s, with around a 20-30% chance of an afternoon shower or thunderstorm — particularly in western zones.

Major Heatwave Potential in the Plains early next week: Something else interesting we’ve stumbled upon while looking at the data is the increasing chance for a major heatwave in the Great Plains — with temperatures exceeding 100 degrees. A large heat ridge is forecast to build on Monday, leading to 850mb temperatures approaching 30C! The large, expansive, bubbling 500mb and 300mb ridge is a classic indicator of a building heat wave.

This morning's GFS shows a classic pattern for extreme heat to develop in the Plains early next week. (New York Metro Weather Models)

This morning’s GFS shows a classic pattern for extreme heat to develop in the Plains early next week. (New York Metro Weather Models)

What does this mean for us? Sometimes when a large heat ridge builds into the Plains, we get a bit of a weak trough just to the east of it — or in our area. This type of trough leads to NW flow at 500mb, which is dry. Additionally, the very warm airmass is very close-by, and can often advect into our area in the mid and upper levels. This mid-level heat can lead to a large stable layer in the middle of the atmosphere, but with much stronger instability just above that stable layer. For the most part, this means sunny skies, but late in the afternoon with enough heating at the ground, that stable layer can break, and the atmosphere taps into the unstable layer above the now-broken stable layer. Combine this with weak trough we might have, and you have the potential for severe weather — dry mid-levels with an unstable middle and upper atmosphere are often our best setups for severe weather. That being said, this is highly speculative, but the chances are there later next week for a more unique severe weather setup.

Last night's GFS showed temperatures of 105 degrees in South Dakota on Monday, with much more pleasant conditions in our area. (New York Metro Weather Models)

Last night’s GFS showed temperatures of 105 degrees in South Dakota on Monday, with much more pleasant conditions in our area. (New York Metro Weather Models)

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