Heavy rain to impact our area today

Last night and early this morning, many areas, especially north and west of NYC had quite the icing event. Saturday’s Arctic air was a bit more stubborn to leave than forecast, as only a light southerly flow was not enough to escort it out. However, temperatures aloft were able to warm more easily, since wind flow above the ground is stronger. This means that precipitation would melt when it hit the warm air aloft (into rain) but then freeze when it hit the colder ground, giving a lot of the area freezing rain. We are starting to see a transition to plain rain in Westchester county and other immediate NYC suburbs, but most other further north and west suburbs could continue to have ice until around noon or slightly longer. Even as we write this, it is in the 40s in most of Long Island, but only 30 degrees in Danbury airport.

In its wake will be heavy rain for this afternoon. The rain is mostly light across the area, but should become moderate around 11:00am, and will get heavier after 1:00pm. The heaviest rains for our area will be between 2:00pm and 6:00pm, particularly between 3:00pm and 5:00pm, before ending between 8:00pm and 9:00pm. For further east suburbs like Suffolk county, you can push these timeframes back an hour or two.

Today's GFS model gives most of the area a solid stripe of 1.5 to 1.75" of rain today.

Today’s GFS model gives most of the area a solid stripe of 1.5 to 1.75″ of rain today.

Rainfall amounts generally between 1-2″ are expected, with the heaviest generally along the I-95 corridor and just east of the I-95 corridor. It is possible that localized heavier banding of rain could give some areas over 2″ of rain, and/or some flash-flooding, but the fast movement of the system should prevent widespread flooding.

Further support for heavy downpours comes from a map we don’t often use during the winter: a map of precipitable water. We usually use this feature during the warm season, since warm airmasses can hold more moisture. This airmass isn’t even that warm for winter standards, but it is somewhat warm and very moist.

Today's NAM model shows precipitable water values approaching 1", which means that heavy downpours are possible this afternoon.

Today’s NAM model shows precipitable water values approaching 1″, which means that heavy downpours are possible this afternoon.

Precipitable water is essentially the amount of water that would result at the ground if the atmosphere in its current state were “squeezed” like a towel. Amounts are approaching 1″, which is high for this time of year, and even some surface-based CAPE (instability) is present to our south over the Atlantic Ocean. This means that even some convective heavy downpours and thunderstorms could develop offshore. They would weaken before they got here and probably not result in any thunderstorms given our stable atmosphere, but they could still get here as heavy downpours which could create flash-flooding issues this afternoon.

After the rain clears this evening, winds will shift to westerly and bring in a chilly, but not that cold of an airmass. During the day, temperatures will rise to around 40 in most spots, and 40s from NYC and east (warmer the further east you go), but should generally fall back to the low 30s in most locales tonight. We do think urban locations will remain above freezing, but most suburbs — particularly north and west of NYC — do have the risk of standing water freezing, which could make Monday morning’s commute a bit difficult.

High temperatures on Monday will be in the upper 30s to low 40s with breezy conditions: around average for this time of year.