For the first half of this week, New England remains embedded within a trough. An upper-level low pressure system has parked itself just to the north of New England and a cold pool will center itself over New England through midweek. This will keep chilly temperatures around for the region with highs topping out in the 40s across northern areas and 50s south. On top of that, widespread morning lows in the 20s and 30s will bring frost and freeze chances across the region through Friday morning.
The upper-level low will also cycle scattered rain and snow showers into northern New England through midweek. These will mostly be confined to the higher elevations. The most widespread of these showers will likely come Tuesday evening into the overnight hours. By Tuesday night, most of the rain showers will have transitioned to light snow showers. Southern and central New England will more likely stay completely dry during this time.
Below: GFS showing potential weather this evening:
For nearly all of October thus far, the same general pattern has been more or less stuck in place across the United States. This pattern has been a general ridge-in-the-west-trough-in-the-east setup. This has bottled up warm temperatures in the west while seasonable to below average temperatures have dominated for the east. This has also kept much of the west dry and has prevented larger storms from forming and traveling across the country (not including tropical weather, which is independent from this setup).
This has been a rather stubborn pattern, but all weather patterns must change eventually, and this one will do just that heading into the latter part of this week. Thursday will act as a transition day from the cooler weather to the more mild and sunny weather. This will come as an expansive area of high pressure nudges eastward while the upper-level low pulls away from the region.
Below: 500mb height anomaly for Thursday showing a trough pulling away from the east coast with a ridge across the center of the country and a trough digging into the west:
The building high pressure will aid in keeping nights chilly through Friday morning. After the morning, however, temperatures will really begin to nudge upward for New England. By Friday, a more trough-in-the-west-ridge-in-the-east pattern will set up for the United States, bringing more mild and sunny weather to New England while portions of the west cool down. The eastward shift of the high pressure will allow for a more southerly influence for the east coast.
The true mild air will be felt by Saturday afternoon with highs well into the 60s for most of New England and possibly push into the low 70s for the typical warm spots of southern New England. With high pressure overhead, the overnights will still see a decent drop with cool morning lows continuing. Overnight lows should remain above frost levels by late in the weekend.
The ridge does look to begin to flatten by late this weekend, but the expansive high pressure looks to hold firm through the weekend and into at least early next week. This flattening of the ridge may allow the more mild temperatures to spread back westward. This will likely keep the mild air mass in place for New England through midweek next week and may last for much of next week. This would support mid 60s to low 70s for highs through midweek with milder overnight lows (40s and low 50s) as well.
Below: Climate Prediction Center 6-10 day outlook. This currently covers October 20-24. You can see above average temperatures favored for much of the United States, but more so for the east:
This change in pace can also be seen by looking at the NAO and PNA index values. For the first half of this month, these values were negative for the NAO (east) and positive for the PNA (west). These values will be swapping with positive values for the NAO and negative values for the PNA. This is currently looking to hold for much of the remainder of October. How this translates in the long term will remain to be seen, but in the shorter term, it means mild temperatures returning for the east.
This strong and expansive ridge of high pressure also means there will be very limited shower chances after the upper-low departs around the middle of this week. No large scale low pressure systems will enter the picture and it may be difficult for a front to push through the region. A cold front may approach northernmost New England late this weekend, but likely won't be able to push into the region very far before lifting back north. This will keep dry, mainly sunny and overall calm weather around for the extended.
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