This Saturday will likely start off with the coldest overnight low temperatures so far this season for NYC. Strong high pressure will be moving in from the west overnight and during the day, the center of this high pressure will be passing almost directly overhead. As a result, we should see a sunny day with diminishing winds, though high temperatures will be much below normal (69°F) with the day starting off pretty chilly. Quite a contrast between this past Wednesday when we set a new daily record high across the city with temperatures peaking at 92-93°F! Autumn is certainly in the air.

My Forecast
High: 61°F | Low: 48°F | Max sustained winds: 14 mph | Total precipitation: 0.00″ – verification will come from METAR data for the period between 2AM Saturday and 2AM Sunday (06Z Saturday to 06Z Sunday) at LGA (LaGuardia Airport), and the KLGA Daily Climate Report.

Verification
High: 60°F | Low: 46°F | Max sustained winds: 20 mph | Total precipitation: 0.00″ –
Temperature forecast turned out to be pretty good, especially high temperatures. Off by 2°F on low temperatures, which ended up 1°F colder than statistical guidance. I believe this was due to slightly stronger cold air advection than expected leading into the forecast period. Off by 6 mph for max sustained winds, which qualifies as a minor bust. The highest wind speed direction was northeast. Judging by METAR data, winds were coming form this direction sometime between 7-8AM Saturday. I think it’s possible that the pressure gradient was tighter than forecast the day before, which would have resulted in the potential for faster winds like these.

Synoptic Set Up
A strong area of high pressure is forecast to be the dominant feature at the surface for much of the Northeast, Mid-Atlantic, and Ohio Valley during the forecast period. The center of this high pressure will be moving east during this time and will be nearly overhead for the hours of peak solar heating tomorrow. This high pressure will be accompanied by a dry airmass throughout nearly the entire atmospheric column. Reference the images below, which show NAM model output of relative humidity and winds at 300 mb, 500 mb, and 850 mb respectively. The brown hues shown indicate very dry conditions.

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High Temperatures
GFS, NAM, and NBM are in good agreement that tomorrow will be a much below average day in terms of high temperatures. These forecast sources depict high temperatures ranging from 58-62°F, which is 7-11ºF below normal for this time of year. They also show winds that are veering from northerly to east-southeast, which makes sense given the forecast position and track of the high pressure center. Although northeasterly and easterly are onshore wind directions, the near surface layer of the atmosphere is forecast to start off so dry that it would take quite a while for moisture off the ocean to make an impact and produce clouds that could eat into high temperatures. Furthermore, sea surface temperatures at this time are actually ranging in the upper-60s, so onshore flow should actually serve to bring warmer air in. EKDMOS 50th percentile for high temperatures is about 60°F. NAM is a touch cooler at 58°F while GFS and NBM call for 62°F. NAM seems to suggest slightly more cloudiness than GFS. I’m going with a high of 61°F, which I think is a reasonable balance that incorporates the data points above.

Low Temperatures
Just as is the case with high temperatures, statistical guidance similarly shows that low temperatures could be as much as 8°F below normal for this time of year. These temperatures are more typical of the end of October. This is a result of decent cold air advection leading into the beginning of the forecast period. Clear and dry conditions should carry though the overnight hours. However, winds should be strong enough so as to limit radiational cooling. EKDMOS has 47°F as the 10th percentile, and 50°F as 50th percentile so going with 48°F seems plausible when considering that statistical guidance is nearly unanimous calling for 47°F as the low.

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Max Sustained Winds
Climatologically, winds during October can be quite strong from the northeast, but easterly to southeasterly winds are typically the weakest. Since the center of the high pressure should be moving overhead during the forecast period, the pressure gradient should be decreasing, leading to diminishing winds. Statistical guidance looks on point calling for peak winds at the beginning of forecast period. Even with a well-mixed layer forecast to form during the afternoon tomorrow, winds aloft will be weak, so wind speeds should be subdued. NBM came in with max wind speeds well below the other statistical guidance at only 6 knots. EKDMOS 50th percentile is about 10 knots, thus a forecast of 12 knots here should be OK.

Total Precipitation
This is the easiest part of the forecast as strong high pressure and very dry air throughout most of the atmospheric column makes it nearly impossible for precipitation to materialize.