textproduct: Gray - Portland

WHAT HAS CHANGED

A Severe Thunderstorm Watch has been issued for western portions of Maine and north and central New Hampshire through 4am. Current radar shows one severe thunderstorm producing prolific lightning in northern New Hampshire, crossing through the White Mountains. Should this storm maintain strength, communities through the Maine Lakes Region and then towards southern coastal Maine could be next through midnight.

KEY MESSAGES

1. Severe storms, locally significant, remain likely through tonight. The greatest threat window for significant severe storms will be through 4am across the western Maine mountains and north/central New Hampshire. Severe storms remain possible south of the mountains as well.

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...

A potent jet streak across Ontario and Quebec 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 through tonight. Despite weaker daytime 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 70 mph, and perhaps a tornado or two. SPC maintains an Enhanced Risk (level 3/5) across the north with significant hatching. 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 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 /01Z WEDNESDAY THROUGH SUNDAY/

Through 00z Thursday: Scattered TS move south across ME and NH between 00Z and 12Z. These storms may be severe, with large hail, damaging winds, and a possible tornado. Mainly VFR outside of TS, but will continue to monitor chances further south and west. Wildfire smoke should remain aloft, but could mix down in showers or storms overnight. Mainly VFR Wednesday with NW wind gusts to 25 to 30 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.