textproduct: Gray - Portland

WHAT HAS CHANGED

A Heat Advisory has been issued for portions of far southern New Hampshire Tuesday. Temperatures will climb into the lower 90s into the early afternoon, with heat index values up to 95.

A Marginal Risk for severe thunderstorms has been expanded to include all of New Hampshire and much of southwestern Maine Tuesday.

KEY MESSAGES

1. Hot, well above normal temperatures move into the region for Tuesday and Wednesday. Widespread highs in the upper 80s to lower 90s are expected Tuesday, with warm overnight lows in the upper 60s to around 70. Another round of hot temperatures is possible Wednesday, mainly focused from the interior to the coast.

2. An approaching cold front will bring chances for thunderstorms Tuesday and chances for showers and storms Wednesday when the front crosses. Any thunderstorm that develops Tuesday will have the potential to become severe.

3. High pressure builds in Thursday and Friday for mostly dry and cooler weather. Uncertainty exists going into the weekend as moisture approaches from the southwest while high pressure may keep things dry through Saturday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Broad warm sector across the eastern CONUS is evident on 850mb analysis. Upper jet remains well north of the Great Lakes, and low level SW winds will begin to advect warmer low level temperatures tonight. Brief high pres moves overhead tonight bringing mostly clear skies and calm winds. The incoming low level warm airmass makes its presence known, developing a very steep low level inversion through the evening hours. This is especially pronounced along the ME coast where surface winds become onshore with some fog potentially moving inland late tonight. By Tuesday morning, clear skies should allow full sun for rapid surface heating through the morning hours.

The incoming airmass will near record max climo for temps at 850mb over much of New England, sitting in the 99.5 percentile for southern NH and through coastal/interior ME. Daytime mixing will easily tap into this air pushing surface temperatures into the upper 80s, with low 90s for the southern half of NH. This won't be as humid as our summer heat events as the low level moisture is thin. However being the first hot, summer-like heat of the season, went ahead with a Heat Advisory for Hillsborough and interior Rockingham counties in New Hampshire. Heat index values will largely remain in the lower 90s here, but will approach 95 through the afternoon.

Southwest breeze continues into the Tues overnight hours, and this will keep temps warm. Lows only dip into the upper 60s to around 70 for much of the interior and coast Tuesday night, mitigating temp recovery. This sets up another warm day for Wednesday, but current forecast highs fall short of those on Tuesday. The hot temps will be more focused on coastal/interior ME and SE NH as a cold front approaches from the west. This should bring more seasonable temps for the mountains and foothills depending on convective activity. As the front pushes off the coast Wed evening, a more refreshing night is expected with lows in the upper 30s to mid 40s.

KEY MESSAGE 2 DESCRIPTION...

Low pressure and attendant warm front will be lifting northeastward through Quebec Tuesday with the trailing cold front draped southwestward through the Great Lakes region. This will leave northern New England within the warm sector Tuesday into Tuesday night as the cold front is progged to cross Wednesday. Temperatures climbing into the upper 80s to low 90s with dew points around 60 will yield SB CAPE upwards of 1500 J/kg Tuesday afternoon. Deep layer shear of 25 to 35 kts will support some organized convection while a limiting factor will be that the main source of lift, the cold front, will remain well west of the area through Tuesday night. The 12Z HREF suggests that widely scattered storms will remain possible in the absence of lift with few containing robust rotating updrafts. The best storm chances will be during the afternoon across the interior while a few cam solutions suggest storms could track all the way to the coast. The primary hazard will be damaging winds as model soundings show inverted V profiles with dew point depressions greater than 20 degrees. Mid level lapse rates are not impressive while hail will still be possible with any strong rotating updraft. CAMs suggest the chances for storms will diminish Tuesday night, however, as the forecast area remains in the warm sector some overnight convection cannot be ruled out.

The cold front will finally arrive into western New England Wednesday morning and will push offshore Wednesday afternoon. Available CAM guidance suggests that a corridor of SB CAPE around 1000 J/kg will develop ahead of the front with deep layer shear around 35 kts. Most solutions push the front through the area prior to peak heating limiting thunderstorm potential to areas south of the mountains. Dry air arriving aloft may also limit any sustained updrafts with the latest outlook from SPC keeping the Marginal Risk south of the area for Wednesday.

KEY MESSAGE 3 DESCRIPTION...

High pressure builds in from the WNW Wednesday night into Thursday bringing a period of breezy northwest winds. Skies look to be mostly sunny Thursday withs highs ranging from the 50s north to mid 60s along the coast. High pressure will center over southern New England Friday allowing winds to relax across the south while a short wave moving through eastern Canada will maintain breezy conditions across the mountains and central Maine. Highs on Friday will range from the upper 50s north to upper 60s south of the mountains under mostly fair skies. Short waves ejecting out of a trough out of the central US will send moisture towards New England. Global models suggest that high pressure over the Northeast will help suppress this moisture into Saturday into Sunday. The NBM generally keeps PoPs less than 50 percent through the weekend which seems reasonable at this time range.

AVIATION /19Z MONDAY THROUGH SATURDAY/

Through 18z Tuesday...VFR through this evening as winds become calm. Onshore flow along the ME coast could develop low stratus and/or fog to RKD/PWM at or after midnight local. This may only be in vicinity of PSM. Fog should then lift Tues morning after sunrise and a offshore wind shift for PWM south. 35kt LLWS possible for a duration tonight as a steep surface inversion develops.

SW winds increase after sunrise Tuesday, with gusts 15 to 20 kt. Isolated to scattered showers develop into early afternoon, with TS possible through to the evening. TS may be focused along a line, passing west to east. MVFR vis possible in SHRA.

Outlook:

Tuesday night: VFR as SHRA chances exit the coast in the evening, with SW breeze continuing overnight. No sig wx.

Wednesday: Scattered showers and TS may bring localized restrictions late morning early afternoon. Otherwise mainly VFR.

Wednesday night: VFR with NW winds gusting up to 20 kts.

Thursday through Friday: Mainly VFR.

Friday night through Saturday: Clouds may increase and lower across southern NH and Coastal Maine. Low confidence in height and extent, but could see periods of MVFR south of the mountains.

MARINE

Onshore winds advect moisture over the coastal waters tonight, with areas of fog possible. This may continue into Tues morning. Showers and storms develop inland Tues afternoon and may track towards the NH/ME coast by evening. SW winds continue overnight, nearing 25 kt but confidence isn't high enough to issue a SCA at this time.

Southwest winds turn westerly Wednesday as a cold front approaches and crosses the waters during the afternoon. Winds may gust up to 25 kts with seas approaching 5 feet. Winds and seas are forecast to remain below SCA thresholds Thursday into Saturday as high pressure builds over the Northeast.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Air Quality Alert from 10 AM to 10 PM EDT Tuesday for MEZ023>028. NH...Air Quality Alert from 10 AM to 10 PM EDT Tuesday for NHZ013- 014. Heat Advisory from 11 AM to 8 PM EDT Tuesday for NHZ012-013- 015. MARINE...None.