Sun. Eve: Warmer, then Cooler with Mid-Week Storms

Synopsis

High pressure just north of the US-Canadian Border and an upper-level trough have helped usher in some cooler air to start the work week. Pretty soon the high pressure will head eastward and set-up a southerly flow which will allow warmer air to move in on Tuesday and Wednesday. A cold front over the northern Mississippi & Lower Missouri River Valleys will slide on through Wednesday, which will touch off showers and thunderstorms throughout the day. The front is offshore by Thursday morning, so clearing and cooler conditions will return to the state. A weak upper-level disturbance could touch off a rain or snow shower overnight while high pressure passes to our south. Then, another system from the Southern Plains heads in for the weekend.

Models Used: 00z NAM & Hi-Res NAM, 18z GFS, 12z ECMWF; glanced at 12z UKMET & CMC and 18z NAMs for Wednesday

Short-Term – Monday and Tuesday

The high pressure system is actually allowing cold-air damming to take place, albeit with temperatures well above freezing. This will help keep temperatures in the 50s for Monday across the northern highlands. The high pressure system will exit eastward and the southerly flow will be able to overcome the cold-air damming that set up over the weekend. Temperatures on Tuesday will be near 70 in parts of the State – possibly near Philadelphia and Pittsburgh with 60s for almost ALL of PA.

Be aware that MOS will have trouble with the temperatures once the southerly flow starts-up. Use the 850-mb/ mixing method plus de-bias the cooler MOS temperatures.

Medium-Range – Wednesday

This was the toughest part of the forecast because while most of the guidance put the would-be squall-line in Ohio on Wednesday AM, the 00z Hi-Res NAM had it in while the European & Canadian model had it in western/ northwestern Ohio around the same time. We put it in West-Central Ohio at 12z Wednesday… pretty similar to the WPC discussion. The set-up and timeline of the front will be almost identical to Friday, with possibly less cloud-cover during the day. There's still uncertainty with 60 hours to go, so we stayed with the possibility of a strong storm. The SPC's Day 3 Outlook should be consulted before proceeding with the forecast for strong storms.

Longer Range – Thursday through early Saturday

The upper-level trough will be slower, which actually could hamper severe weather on Wednesday (less shear than on the 19^th). The 850-mb temps will decrease dramatically, so cooler air at the surface on Thursday is very likely. The upper-level trough will move through overnight which could touch off a rain or graupel shower. Friday will be sunny as high pressure passes to our south. A cut-off low will approach on Friday night, which could touch off a shower in Pittsburgh as the sun rises on Saturday.

-Jaron Breen