textproduct: Gray - Portland

WHAT HAS CHANGED

MCS over Quebec has shown signs of strengthening with latest radar scans showing +60 dBZ to 37,500 feet. Surface obs in the vicinity show peak winds around 35 mph and are likely muted by low level inversions. Latest time of arrival has the MCS reaching the International Border of Maine and NH between 930 and 10 AM. Have updated PoPs as latest runs of the HRRR bring the MCS through western Maine through late morning and added damaging winds wording based on latest radar trends. Will be watching this closely for the potential need to issue Severe Thunderstorm warnings. After this MCS passes the focus will be later this evening as the cold front approaches bringing another round of severe storms.

KEY MESSAGES

1. Hot today with greater humidity creating widespread heat indices between 95 and 100. If spending time outdoors today, take breaks in the shade and remain well hydrated. Humidity decreases for Wednesday and Thursday, remaining warm.

2. Severe weather, potentially locally significant, is likely from late afternoon into the early overnight hours today. The highest threat for severe weather is expected to be from 6 PM to 2 AM. Damaging winds, large hail, tornadoes, and frequent lightning are all possible.

3. A second cold front will approach the region Wednesday night, bringing the chance for additional showers or storms, but also a cooling trend into late week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Temperatures rapidly climb this morning after a mild night. Aloft, 850mb temperatures will increase towards 20C through the morning and afternoon. Resultant surface temperatures swell into the 90s for most locations outside of the mountains. Given enough sun, portions of northern NH may also push into the lower 90s.

The widespread heat will be met with increased humidity as a moist airmass surges in from the west. Mean dew point temperatures push several degrees higher than Monday, with values topping in the upper 60s to around 70. This addition will be responsible for heat index values pushing into 95 to 100 range, warranting the continuation of the Heat Advisory for a bulk of the forecast area (sans the Maine Midcoast and western mountains, and far northern Coos County, NH) today.

Thankfully, the warmest plume of temps aloft move across the area later this evening and overnight, not aligning with daytime heating and mixing. This creates a very warm setup for overnight, but drier air will be arriving through Wed morning. The plume is progressive, and should be south of the area by the time Wednesday morning arrives. However, it's pace slows as it drapes across far southern New England and the Delmarva region. Thus warm temperatures remain in the forecast for the southern half of the forecast area Wed and Thurs, albeit amid a drier (lower humidity) airmass.

NE winds increase Wed with drier air arriving. While forecast temperatures again rise around 90 for areas outside of the foothills and mountains, the breeze and lower humidity should keep heat indices below 90. Overnight lows will offer some recovery hours, as they dip into the upper 50s to mid 60s Wed night, and cooler Thurs night.

KEY MESSAGE 2 DESCRIPTION...

Scattered severe thunderstorms are expected this evening through the first half of tonight. The Storm Prediction Center continues to have northern NH and the western ME mountains into an Enhanced Risk (Level 3/5), a Slight Risk (Level 2/5) for central NH and interior western ME, and a Marginal Risk (Level 1/5) for coastal ME and portions of southern NH.

Strong southwesterly flow aloft combined with a strong H5 s/wv trof for this time of the year and trailing cold front will set the stage for a rather favorable environment for severe thunderstorms to develop, especially given the hot temperatures and modestly high dew points that will be in place at the surface. This will be further enhanced through steep height falls and an elevated mixed layer, which will help to support a severe threat well after sunset.

Latest CAMs and RRFS continue to show significant MUCAPE in the 2000-3000 J/KG range, MLCAPE of around 2000 J/KG, and 0-6km effective bulk shear of 50-60 kts. Forecast soundings also show some drier air within the low-lvls, which is helping to boost DCAPE into the 800-1000 J/KG range. In addition, mid-lvl lapse rates of up to 7.5C/KM are possible, which given the expected shear will allow for robust updrafts to develop and sustain themselves. Given this environment, supercell structures are expected with bowing segments also possible as storm outflow boundaries potentially collide. Damaging winds along with large hail remain the primary hazards but low level hodographs and 0-1 KM SRH also support a tornado threat.

Storms will develop late this afternoon into early this evening across southern Quebec before approaching northern NH and northwestern ME between roughly 6PM-8PM. Storms are then expected to move south through the rest of the evening and first half of tonight. There continues to be some guidance that shows isolated discrete cells to develop within the 3PM-6PM timeframe though. The severe threat should diminish within the 1AM-3AM timeframe.

This is not a yearly, typical threat of severe storms, and it becomes even more dangerous as the threat continues after dark. It is highly advised to have multiple ways to receive warnings, especially ones that will wake you up with the threat continuing into the late evening and early overnight hours. The highest risk areas also include many popular camping and recreation areas, which make those who are outdoors extremely vulnerable to falling trees.

KEY MESSAGE 3 DESCRIPTION...

As the mid-CONUS 500mb ridge begins to weaken, a NW flow regime continues across northern New England. Embedded will be another cold front that looks to cross New England Thursday. While the airmass is drier than the early week episode, there remains the chance for daytime showers or thunderstorms with its passage.

Due to less moisture, instability looks to be the more uncertain parameter, with deep layer shear a-plenty through the day.

A cooler air mass moves in behind the front that will help keep high temperatures in the 70s and 80s Friday into the weekend. Additional disturbances rotating through the trough will bring chances for showers over the weekend while chances are low in the 20 to 40 percent range.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Through 06z Wednesday: VFR through this morning, but can't rule out some valley fog in vicinity of AUG or RKD towards daybreak. A more unstable airmass will lead to a isolated TS today, with more widespread TS likely this evening through midnight local. These storms may be severe, with large hail, damaging winds, and possible tornado. The best chance for these storms will be towards the US/CAN border through AUG/IZG/LEB, but will continue to monitor chances further south and east.

Outlook:

Wednesday/Wednesday Night: VFR conditions with daytime NW wind gusts up to 25 kts.

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

MARINE

SCA wind gusts and wave heights remain in the region through today, but may subside briefly this morning before again increasing this afternoon and tonight. These then shift NW behind a cold front Wednesday morning.

Winds and seas generally stay below SCA thresholds Wednesday into Saturday. Offshore winds prevail Wednesday and turn SW ahead of a front Thursday. Winds turn back offshore Friday.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Heat Advisory until 8 PM EDT this evening for MEZ012>014- 018>021-023-024-033. NH...Heat Advisory until 8 PM EDT this evening for NHZ002>015. MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ150>152.