You could always ignore Thursday and focus on the lovely early autumn weather forecast for Friday and Saturday.
But that could, of course, be a dangerous miscalculation.
A flash flood watch will begin at 8 a.m. and last until 2 a.m. Friday as a cold front works from west to east and significant south to north precipitation is pushed over the Lehigh Valley and northwest New Jersey, the National Weather Service said.
“One concern today with this front continues to be the flash flooding threat with the area of greatest concern being the Lehigh Valley and Poconos where (mountain) enhancement is likely to dump several inches of rain with localized amounts possibly exceeding three inches,” the Mount Holly, New Jersey, office’s forecast discussion says. “A swath of (precipitation) values exceeding two inches just to our west continues to move towards our region this morning.
“A big factor at play will be exactly how quickly the front moves east with a rather abrupt end to the rainfall as the dry slot moves into the region from west to east. Latest guidance has suggested this may push eastward a little more quickly, which has resulted in a slight decrease in rainfall totals, though the risk for localized amounts exceeding three to four inches still remaining across eastern Pennsylvania. A flash flood watch remains in effect for much of the region along and west of the I-95 corridor.”
The weather service, in its flash flood watch, sees the southern Poconos possibly getting up to five inches of rain.
In his morning forecast video, EPAWA Weather Consulting meteorologist Bobby Martrich said the region could see scattered showers early on, but the real event should be closer to 3 p.m. and it should be done between 6 p.m. and 8 p.m.
“It’s a three- to five-hour-wide window of opportunity for moderate to heavy rainfall,” he said.
The National Weather Service lays out the flooding threat of Sept. 23, 2021.National Weather Service
Streams, creeks, urban areas and places of poor drainage could suddenly flood, the weather service said.
Timing has been an issue for days, with models being 36 hours apart the better part of a week ago and still 24 hours apart two days ago, Martrich said. But they converged on Wednesday afternoon and the forecast became more clear, he said.
“As soon as the rain ends, within an hour or two, you’re going to have clear skies,” he predicted.
Any wind will be out ahead of the front, Martrich said, and gusts could hit 30 mph, the weather service said. Once the front arrives, the wind will become calm and eventually, as the front moves away, dead calm, Martrich said.
There is a “non-zero” chance of a “brief” tornado, Martrich said. Some of the conditions are ripe for severe weather with the strong thunderstorms moving through, Martrich and the weather service said. But there isn’t great instability in the upper cloud level, Martrich added.
“It’s probably about a 1% chance,” he said. “It’s not impossible”, but this will be “primarily a rain event,” he said.
The weather service offered, “With the more southeasterly flow at the surface and southerly to south-southwesterly winds in this jet, the threat for tornadoes continues to remain. Any updrafts that can tap into this pre-existing low-level wind shear will easily be able to rotate. The more surface heating and sunshine we see early today, the greater the threat is likely to be.”
The Storm Prediction Center sees a marginal chance of severe weather, the lowest point on its five-point scale.
Once the passing front’s work is done, cooler, clear weather is expected, with highs closer to 70, which would be below the normal of 75 and well below recent highs in the 80s.
There will be cooler nights and crisp mornings, Martrich said.
“It will actually play the part of autumn by that point,” he said.
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Tony Rhodin can be reached at arhodin@lehighvalleylive.com.
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