textproduct: Gray - Portland

WHAT HAS CHANGED

Wildfire smoke from western Ontario has prevented temperatures from climbing as high as previous forecast. Therefore the Heat Advisory has been canceled.

Main focus continues to the threat for severe storms this evening through tonight. The threat for severe storms has extended longer into tonight and perhaps into early Wednesday morning.

KEY MESSAGES

1. Severe storms, locally significant, remain likely into tonight. The greatest threat window for significant severe storms will be 6 pm to midnight across the western Maine mountains and Coos County. Severe storms remain possible south of the mountains with a time of arrival around 8 pm and may persist well into tonight.

2. Additional cold fronts will approach Wednesday and Thursday bringing chances for showers and storms and a slight cooling trend into the weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Latest radar composite and satellite imagery show clusters of storms tracking ESE across Quebec into NW Maine. These storms are associated with a potent jet streak across Ontario and Quebec that will slowly sag into northern New England this evening. This jet streak combined with strong height falls and a southward moving cold front will spread strong forcing for ascent over the area this evening with this support lasting well into tonight. Smoke from wildfires over western Ontario and convective cloud debris have hindered surface warming today. Despite weaker surface heating, steep mid level lapse rates around 7.5C/km will aid in building instability through this evening. Latest SPC mesoanalysis suggest ML CAPE will build in excess of 2000 J/kg ahead of a broken line of storms that approaches northern NH and NW Maine this evening. This strong instability combined with effective shear 50-60 kts will support supercells, some potentially intense. Any semi-discrete cells this evening will be capable of all severe hazards including hail size to 2 inches, wind gusts to 75 mph, and perhaps a strong (EF2) tornado. SPC maintains an Enhanced Risk (level 3/5) across the north with a rare 10 percent tornado probability with significant hatching to account for a tornado to EF2 strength. CAMs remain mixed with respect to upscale growth, while there is a general consensus that storms will grow upscale into linear segments leading to damaging winds becoming the primary threat as they move south of the mountains.

Storms will track southward through the first half of tonight with recent runs of the HRRR suggesting that storms may continue into early Thursday morning across coastal Maine and central NH. This set up is particularly rare as latest guidance suggest these storms will still potentially be severe well past sunset thanks to the strong forcing and steep mid level lapse rates. Therefore it is encouraged that people have multiple ways to receive a warning and one that could wake them up in the middle of the night. It is also important people are aware of their surroundings with respect to trees and have access to sturdy shelters as popular areas for camping are in the areas of highest risk. SPC has a Slight Risk (level 2/5) to the Maine coast and into central NH. Model soundings suggest DCAPE will remain elevated to 700-1000 J/kg into tonight which will promote damaging wind gusts. Large hail will also remain possible overnight.

The cold front will eventually push south through the area Wednesday morning with thunderstorms ending around sunrise per recent runs of the HRRR.

KEY MESSAGE 2 DESCRIPTION...

Troughing will deepen over eastern North America Wednesday through the end of the week. Humidity will largely be gone Wednesday with highs in the 80s to low 90s. Recent HRRR runs suggest wildfire smoke may reside over the area which could inhibit warming. Despite lower boundary level moisture, CAMs suggest enough instability will build Wednesday afternoon for scattered showers and isolated thunderstorms. Temperatures tick down a coupe of degrees Thursday while another cold front approaches. CAMs suggest MU CAPE will build to around 1000 J/kg with ample shear for organized updrafts. Therefore another round of storms are possible Thursday, some could be strong to severe, while not as intense as storms tonight.

High pressure builds in Friday for fair weather. A short wave moving within the long wave trough over eastern North America will approach over the weekend. This will bring increasing chances for showers late Saturday into Sunday morning.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Through 18z Wednesday: Scattered TS move south across ME and NH between 00Z and 12Z. These storms may be severe, with large hail, damaging winds, and possible tornado. Mainly VFR outside of TS with the best chance for storms near the US/CAN border through HIE/AUG/RKD, but will continue to monitor chances further south and west. Mainly VFR Wednesday with NW wind gusts to 25 kts.

Outlook:

Wednesday Night: Mainly VFR.

Thursday-Saturday: Mainly VFR conditions with WNW flow prevailing. SHRA/TSRA Thursday afternoon/evening may bring TEMPO restrictions.

Saturday night-Sunday: Chances for rain may bring a period of at least MVFR.

MARINE

SCA wind gusts and wave heights remain in the region through this evening. Winds shift NW behind a cold front Wednesday morning with gusts 20 kts or less. Another cold front crosses Thursday with SW winds increasing ahead of the front and then shifting offshore Friday morning. Winds and seas likely remain below SCA thresholds into the weekend.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None NH...None MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ150>152.