Heavy rain, strong winds, and potentially thunder highlight Saturday

The snow and freezing rain this morning has left the immediate area, and gave most areas a coating to 1″ of snow, and also very treacherous roads. Now that the warm front has passed the area, temperatures have risen above freezing, moisture has increased, and dense fog is beginning to develop. An extremely saturated low-level profile will continue to promote an increase in fog as the night goes on, so all motorists should be extremely careful when traveling tonight, as visibilities will be below 1/4 of a mile at times.

However, a freezing rain advisory still exists for interior Central and Northern NJ, interior southern NY, and interior southern CT, as cold air has still remained at the surface. Areas close to NYC should change over to plain rain by midnight with less than a tenth of an inch of ice, and areas a bit further inland may remain below freezing until 4:00am, as a tenth of an inch of freezing rain could fall. Travel especially carefully in these areas. For more information on these advisories, see the embedded links. 

Moving forward to tomorrow, the weather will continue to get worse. A storm system will be deepening and cutting into the Great Lakes, leaving a cold front in its wake — all the way down to the Gulf of Mexico.

Today’s 18z NAM valid for tomorrow at 1:00pm shows a 988mb low in SE Canada with a cold front draped across the Appalachians.

The NAM model shows the cold front quite well. East and southeast of the 988mb low in SE Canada, winds are generally southerly and southeasterly; whereas to the south, west, and southwest of that low, the winds are generally westerly and southwesterly. Additionally, a classic indicator of a cold front is a pressure trough. The isobars are the solid black lines. In the area where the different wind directions are converging, there is a very well-defined pressure trough in Ohio and Kentucky, further proving the location of the cold front. It perfectly matches the region where the wind is changing in direction.

Today’s 18z NAM valid for 4:00pm tomorrow shows an area of very high precipitable water values streaming into the northern Mid Atlantic, an indicator of a significantly moist atmospheric column. This supports the potential for heavy downpours.

Although there is not a whole lot of cold air behind the front, the front is still powerful due to significant moisture differences, as well as an abundance of warm air and moisture out ahead of the front. The high preciptable water values shown above, as well as surface temperatures rising into the upper 50 and low 60s leads to low-density air that can easily be lifted. This warm and moist air being lifted by the cold front will help to trigger an area of heavy rain, and potentially enough convection to lead to a squall line with embedded thunderstorms.

Generally, the more organized, long duration rainfall is located closer to the storm system. However, with the weak temperature gradient to the north with the lack of any true cold air, and with the temperature differences being much more significant on the warm side of the system, most of the precipitation will occur in advance of the cold front. This means that initially, with the cold front being far away, rain may be relatively scattered and disorganized, with only a few occasional period of moderate rain throughout the morning and early afternoon. This is because out ahead of the cold front in this type of warm and moist airmass, one of the major precipitation triggers is convection, which often dumps most of its heavy rain in a short period of time, rather than a long-duration moderate rain/precipitation event as you would see in a typical nor’easter.

Today’s 18z NAM shows the potential for a squall-line to form in eastern PA, and head towards our area during the late afternoon and early-evening timeframe.

As one would expect, the NAM model eventually shows an area of very heavy rain developing in Eastern PA and heading towards NYC in the 4:00 – 7:00pm timeframe. It shows rainfall amounts between 0.50″ and 1.00″ in some spots in a 3-hour period. Most of our rainfall will fall in the 4:00 – 10:00pm time period, and anything before and after that will just be scattered areas of light rain. Regardless, it will be foggy, so travel conditions are still going to be poor — but do not expect heavy downpours until 4:00 and afterward.

Generally speaking, 1-2″ of rain can ultimately be expected with this storm system — the specific amount depending on the exact strength, location, and placement of the heaviest rain within the squall-line, as a few localized areas could receive 2.5″ of rain. Considering a lot of this rain could fall at once, the risk of flash flooding is quite high during tomorrow evening’s commute, so traveling is not advised unless it is absolutely necessary. Additionally, anyone near a local river or stream could have enhanced flooding due to the previous ice jams.

Another reason why traveling could be quite hazardous — especially crossing bridges — is the strong wind threat, which will be analyzed below, via a forecast model sounding for NYC, valid for 4:00pm tomorrow.

18z NAM forecast model sounding valid for 4:00pm shows favorable conditions for a squall line with very strong winds above the surface. The question becomes how much of those strong winds can be mixed down to the surface.

The first thing that comes to mind is the extremely strong wind profile throughout the atmosphere. There are winds of 50-60 knots located just above the surface, and they increase in strength as one travels higher in the atmosphere.

The next thing that stands out is how moist the atmosphere is, as the temperature line (red) and dewpoint line (dotted black) are located very close to each other throughout the whole atmosphere, and are in exact tandem through around 950mb. The complete saturation up to 950mb explains the dense fog, and the near-saturated entire column explains the high precipitable water values, and the potential for extremely heavy rain.

The fact that the wind profile is strong, yet unidirectional favors the formation of a linear squall line, given enough lift and instability. There is a pretty strong inversion at the surface up through around 900mb, which will be a hindrance to mixing strong winds down, since the relatively dense and cooler air at the surface relative to warmer and less dense area associated with the low-level jet will prevent those strong winds from “sinking” and they may stay “floating” above that inversion.

That being said, one feature that could support enough downward momentum to get those winds down to the surface is a squall-line. Although surface instability is lacking, the instability becomes pretty impressive as one heads above 900mb throughout the entire atmosphere, as the temperature cools pretty readily with height. Particularly impressive is the instability between 850mb and 700mb — this leads to MUCAPE values near 500 J/KG in some spots. This may help to promote enough lifting at that layer of the atmosphere for not only heavy rain, but even thunder, associated with the squall-line. With all the moisture that’s in the air, the support is there for torrential downpours and embedded thunder, which could support enough downward momentum to mix those 50-60 knot winds down to the surface.

Accordingly, the National Weather Service has issued a wind advisory for New York City and Long Island. Although the 50-60 knot winds may only be mixed down during the squall-line in gusts, the potential still exists for winds to be sustained between 20 and 30 miles per hour, with wind gusts between 45 and even 60 miles per hour in some located spots, throughout the entire area. Although winds will be gusty throughout the entire afternoon, the timeframe for the 45+mph wind gusts will generally be between 4:00pm and 8:00pm — when the squall-line is hitting the area.

All in all, traveling will be extremely difficult tomorrow, especially during the late afternoon and early-evening, due to dense fog, heavy downpours, and very strong winds. Rumbles of thunder are possible as well. The rain and wind should not be too significant in nature until around 4:00pm and afterward, so if you must travel, we’d advise you to get it done before that time. The heaviest rain should be over by 10:00pm, and only lingering showers and drizzle will be around through midnight — with a drying and calming trend thereafter due to the cold front passing. Fog will also dissipate after midnight. Sunday looks to be a partly sunny and breezy day with temperatures around 50.

As always, stay tuned to our website and our social media accounts for more updates. Enjoy your weekend, everyone.