Forecast Discussion October 10, 2021 2:53 PM EDT
Our attention will turn to the increasingly dangerous Hurricane Wanda. As Wanda has just made landfall as a powerful category 5 hurricane near the Big Bend region of Florida, it can only weaken so much in the 18-24 hours it will take for the circulation to plow into our region. NHC forecasts Wanda to become an extratropical cyclone as it comes through our region. However, with 850mb winds as high as 120kts and 925mb winds as high as 100kts sustained tropical storm-force winds and hurricane force wind gusts (75-90mph) can be expected across the region, particularly along and east of Blue Ridge where the right front quadrant of the storm will pass. As a result, a Hurricane Warning is in effect for all areas except eastern WV and the far western panhandle of Maryland where a Tropical Storm Warning is in effect. Areas back towards I-81 will have the highest chance of significant to extreme flash flooding where tropical moisture will interact with the strong cold front approaching from the west allowing heavy rainbands to set up along and west of the low-pressure track. Rainfall amounts of 5-10” with localized amounts of up to 15” can be expected in this region. To the east of the Blue Ridge rainfall amounts of 2-5” w/ localized amounts of up to 7” will be more commonplace.
Showers and heavy downpours will increase in coverage as the night wears on. With some limited instability (SBCAPE: 400-800 J/kg) and decent low-level shear (Effective SRH: 150-300 m2/s2) a few supercells may also form with the threat of a brief tornado or two. Temperatures will only fall into the mid-70s with the tropical airmass in place with increasing SE winds.
By dawn bands of heavy rain w/ embedded supercells will start moving through the region in earnest w/ SE winds gusting to around 40mph or so. By the late morning hours when the strongest low-level winds move overhead (100+kts around 1km above the surface) winds could gust as high as 90mph potentially causing extensive structural damage. Simultaneously, bands of heavy rain will be setting up over the mountains and I-81 corridor with 1-3”+/hr rainfall rates commonplace. By this time, the low-pressure center will be moving very quickly owing to the strong s/w trough moving in from the west so the strongest winds and heaviest rains will only last during a brief 3-6 hour period. While low-level shear will be extremely strong limited instability owing to the saturated atmosphere will make the tornado threat marginal at best. Torrential rain and flash flooding will be the main threat with any convection given the 2.5"+ Pwats. By the mid-late afternoon hours the center of Wanda will be well to the north/northeast in NE PA with the strong front blasting through. This will bring a quick end to any remaining showers. Winds will still be gusting in the 40 to 50mph range out of the northwest behind the front as temperatures fall from the upper 70s and into the upper 50s by dusk.
From Tuesday onwards through the foreseeable future (at least the next week) a significantly chillier airmass will be in place with dailt highs struggling to get out of the 50s and morning lows in the 30s being commonplace in the greater metropolitan area. The NW suburbs and the mountains may get down into the upper 20s. Frost and freeze headlines will be needed for much of the region for Wednesday morning.