textproduct: Gray - Portland

WHAT HAS CHANGED

The Heat Advisory for Tuesday has been expanded to include coastal Cumberland county. Confidence continues to increase with the threat for severe weather Tuesday evening into Tuesday night.

KEY MESSAGES

1. Hot and humid Tuesday with a Heat Advisory in effect for much of northern New England.

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

3. Humidity abates Wednesday while hot conditions continue with breezy winds.

4. Cold front crosses Thursday bringing chances for showers and storms along with a slight cooling trend into the weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Latest RAP analysis indicates a 500mb shortwave moving across Quebec this afternoon with satellite and radar mosaic an area of increased cloud cover and showers. This wave will cross northern New England later today bringing a round of mostly light showers, mainly over the northern half of NH and into portions western ME later this afternoon and evening. Can't completely rule out a rumble of thunder, but based on forecast soundings the potential looks very low. These showers should push off to the east and out of the area by around midnight or so with dry and mild conditions overnight. However, will have to watch for shower or storms dropping south out of Canada late and into the early part of Tuesday morning. There is good agreement that these will be on a weakening trend as they approach the International Border, but various runs/solutions indicate the possibility of holding together long enough to cross into northern NH or northwestern ME.

For Tuesday, warmer and more humid air continue to advect into the area ahead of a cold, resulting in a hot and humid day. 850mb temperatures of +19C to +22C support high temperatures in the 90s for most of Northern New England, and dewpoints are expected to increase to the mid-upper 60s, bringing heat indices in the mid 90s to low 100s, which is a solid Heat Advisory. The only change made to the existing Heat Advisory was expanding it to include Coastal Cumberland County.

KEY MESSAGE 2 DESCRIPTION...

Now to the increasing threat for severe weather, which again could be potentially significant. An Enhanced Risk (Level 3/5) of Severe Weather is in place for northern NH and the western ME mountains,a Slight Risk (Level 2/5) for central NH and the Maine interior, and a Marginal Risk (Level 1/5) for coastal ME and approaching southern NH.

A potent 500mb shortwave will cross Quebec in the afternoon and evening, with its associated cold front approaching the International Border from the northwest early Tuesday evening. Increasing forced ascent, significant height falls, and strengthening wind fields aloft will set the stage for severe thunderstorms to develop with the hot and humid air mass in place, further enhanced by an elevated mixed layer that will steepen lapse rates aloft.

Today's 12Z model suite, including CAMs, continue to show deep layer shear of 50-60 kt, MUCAPE in the 2000-2500 J/kg range, and high values of DCAPE (800-1000 J/kg) brought on by some dry air/high dewpoint depressions in the low levels. This supports damaging wind gusts, and given the parameters, there's certainly potential for these to be significant on the order of 75+ mph. In addition, the instability profiles and steep mid-level lapse rates on forecast soundings depict large hail could be to the size of golf balls or even larger. There is also enough directional shear to support a tornado threat with mean 0-1km SRH of 100-200 m^2/s^2. These hazards are all possible within discrete or semi-discrete storm mode, but the primary hazard will become damaging winds when the dominant mode becomes more linear, although a tornado within the line(s) will remain possible. For timing: there's strong consensus among the CAMs with multiple clusters or semi-discrete developing across southern Quebec in the late afternoon to early evening with rapid upscale growth in coverage and intensity as storms approach northern NH and northwestern ME in the 6pm to 8pm time frame. Storms are expected to quickly progress to the south the rest of the evening into the early overnight hours. However, it should be noted there are hints of isolated discrete cells developing in the 4pm to 6pm timeframe that may approach from the north. Based on CAM solutions, the threat for severe weather should diminish after 2 AM, but some development of showers and a few storms is possible overnight.

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...

The cold front responsible for severe storms will be pushing offshore Wednesday morning. Drier air moving into the region along with deep mixing will allow for dew points to drop into the 50s. Temperatures aloft will remain warm and with deep mixing highs will climb into the upper 80s to low 90s. Winds at the top of the mixed layer will be around 30 kts which will result in surface gusts 25 to 30 mph from late morning through the afternoon.

KEY MESSAGE 4 DESCRIPTION...

A troughing pattern will linger over the Northeast through the end of the week. A short wave embedded within the trough will send another cold front through the region Thursday evening. Dew points in the 50s will limit instability ahead of the front while MU CAPE may still build to around 1000 J/kg. The 12Z model suite is in decent agreement that the front will bring chances for showers and storms Thursday afternoon and evening, with the best chances along and north of the mountains. Prior to the arrival of the front breezy west winds will continue hot and dry conditions south of the mountains with highs in the upper 80s and low 90s. 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 /18Z MONDAY THROUGH SATURDAY/

Through 18Z Tuesday...Mainly VFR this afternoon and evening, but a round of showers could bring MVFR visibilities, mainly to AUG, RKD, and HIE. For tonight, patchy fog may bring IFR restrictions to AUG and RKD but otherwise VFR. Will also have to see about any showers or a storm moving in from the north, but chances appear too low. VFR is like the rest of Tuesday morning into early afternoon.

Outlook:

Tuesday afternoon-Tuesday night: Showers and thunderstorms are likely with potential for IFR restrictions. Some storms to severe with large hail, wind gusts over 50 kt, and even a tornado. All terminals are in play, but highest chances are at HIE and AUG with the lower of the chances at MHT and PSM. LLWS may also be needed for Tuesday night.

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

Through Tuesday night...South to southwest increasing today through Tuesday night as a cold front approaches the waters. Periods of SCA conditions are likely, especially over the waters east of Portland, where wind gusts will be around 25 kt at times with seas building to 5-6kt. For the nearshore waters south of Portland, a short duration of wind gusts up to 25 kt is possible into this evening. Strong to severe offshore-moving thunderstorms are possible Tuesday evening into Tuesday night.

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 from 11 AM to 8 PM EDT Tuesday for MEZ012>014- 018>021-023-024-033. NH...Heat Advisory from 11 AM to 8 PM EDT Tuesday for NHZ002>015. MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ150-152. Small Craft Advisory from 6 AM Tuesday to 6 AM EDT Wednesday for ANZ151. Small Craft Advisory until midnight EDT tonight for ANZ154.