textproduct: Gray - Portland

WHAT HAS CHANGED

A Dense Fog Advisory has been issued through 8am this morning for the coast along with portions of interior western ME. Otherwise, no significant changes with this forecast update with the main focus being the potential for extreme heat by the middle of the week.

KEY MESSAGES

1. Seasonable temperatures continue today with scattered showers and isolated storms this afternoon.

2. Heat and humidity will build through mid week bringing the potential for extreme heat Wednesday and Thursday for interior/coastal Maine and the southern half of New Hampshire. Prepare cooling systems early this week, and if plans include strenuous outdoor activities, consider rescheduling for the morning when heat isn't as intense.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Cyclonic flow aloft will persist through today before the trough axis moves east tonight. Surface heating today will again allow for an axis of SB CAPE to build around 1000 J/kg away from the coast. This instability combined with the cyclonic flow will allow for showers and perhaps a few thunderstorms to develop in the afternoon while the lack of any surface forcing will limit coverage. Deep layer shear around 40 kts would suggest that a strong thunderstorm cannot be ruled out. High temperatures will generally be in the mid to upper 70s across the north and along the coast, and low 80s elsewhere.

Showers and any storms dissipate this evening with the loss of heating. A light west is expected tonight that will likely limit any fog to valley locations.

KEY MESSAGE 2 DESCRIPTION...

By several indicators ensembles continue to portend to a period of extreme heat pushing into at least south-central New Hampshire and southwest Maine Wednesday and Thursday with hot and humid conditions potentially lasting into Friday. Ensembles show anomalous deep layer ridging exceeding 99th percentile at 500 mb pressing into the Northeast Wednesday and Thursday. This ridge will be accompanied by anomalous moisture and heat with 850 mb temperatures also around the 99th percentile. Therefore, confidence remains high for a period of cumulative heat stress Wednesday into Friday with day time heat indices in the 100s and overnight lows struggling to drop below 70 degrees.

Before the heat and oppressive humidity arrives, northwest flow aloft and high pressure will bring fair weather Monday. Low pressure tracking across James Bay will lift a warm front through northern New England late Tuesday bringing increasing chances for showers and storms that will persist into Tuesday night. Behind the warm front temperatures at 850 mb will climb to +20C to +23C Wednesday and Thursday that will support hot surface temperatures in the mid 90s and approaching the low 100s Thursday. All the while, increasing moisture will bring dew points to oppressive levels into the 70s. Winds may have a sturdy onshore component Wednesday that may allow for some relief near the coast. On Thursday, steady west winds may keep the sea breeze at bay allow for hot temperatures to the coastline. The latest NBM continues the heat and humidity Friday with highs in the upper 90s and dew points in the 70s so precautions should be taken for a prolonged heat event. Ensembles suggest the ridge will weaken or retrograde west into next weekend that would allow for the extreme heat to abate.

A factor that may limit temperatures from realizing their full potential will be any ridge rolling convection that brings clouds and chances for showers and storms. There is a modest signal for a remnant elevated mixed layer (EML) over the Upper Midwest to round the ridge and be in the vicinity of northern New England Wednesday into Wednesday night. Were a remnant EML move atop the hot and humid airmass, CAPE would be anomously high for this part of the country and would support severe storms. Machine learning guidance out of CSU has a rather robust signal for severe storms Wednesday and Thursday while any finer details will remain unclear for a couple more days.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

Through 06Z Monday: Fog and low ceilings will result in IFR-LIFR restrictions through 12-14Z this morning, especially at coastal and valley TAF sites. VFR conditions are then expected later today with light and variable winds. A few SHRA are possible this afternoon, perhaps bringing localized brief restrictions. Additional FG is possible again tonight, although coverage is expected to be lower.

Outlook:

Monday: Lower ceilings/vis should consolidate to the coastal waters, with inland terminals becoming VFR.

Monday night: Return of lowered ceilings possible overnight for coastal ME. MVFR to IFR possible.

Tuesday and Tuesday night: Improvement to VFR in the morning, but SHRA arrive from the west in the afternoon, these may continue into the overnight for all terminals.

Wednesday and Wednesday night: SHRA with some TS possibly develop inland through the afternoon. These should reduce in coverage overnight.

MARINE

Patchy dense fog lingers through the day today, and then dissipates with a west wind tonight. Otherwise, fair conditions prevail with broad high pressure slowly building across the waters.

High pressure continues to reside overhead through Tuesday morning. A weak warm front may drape over the waters through midweek with strong high pressure remaining over the Deep South.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...None.