textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes were needed for this forecast package. Showers have started developing over the mountains a little bit earlier than initially expected with only a couple of lightning strikes thus far.

KEY MESSAGES

1. A slow moving upper low will bring isolated to scattered showers and storms to the region this afternoon and Tuesday afternoon. A few strong storms are possible this afternoon.

2. Dry weather midweek will give way to a weak system late Thursday into Friday. Following this system, mostly dry weather returns for the weekend into early next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A closed upper low continues to slowly drift north northeast over Quebec this afternoon with a surface trough extending south into New Hampshire. This boundary will be focus for scattered showers and isolated thunderstorms this afternoon. Instability will be limited this afternoon with SPC mesoanalysis trends suggesting 100 to 250 J/kg of MLCAPE of most of the area, but a pocket of 500 J/kg will be possible over portions of the western Maine mountains by mid to late afternoon. Effective shear is currently in the 40 to 50 knot range over much of the forecast area, but should start to gradually decrease through the afternoon as the surface trough pushes east. That being said, it will not take much instability for storms to produce some hail with the cold mid-level temperatures in place overhead. The latest RAP analysis suggests Donovan heights are anywhere from 20 kft to 23 kft across the north where the best environment is forecast (freezing levels are also relatively low) so a marginally severe hail report is not out of the question under the strongest storms. Gusty winds will also be possible given steep low-level lapse rates.

Showers and storms should dissipate rather quickly this evening as we lose daytime heating. Another foggy night is then possible, especially in the valleys and over the Midcoast of Maine.

The upper low drifts further northeast towards the Gulf of Saint Lawrence but another surface trough will rotate down into the region Tuesday afternoon, leading to some more isolated/scattered showers and storms. Instability will remain marginal and shear will be weaker so the concern for stronger storms is a bit lower than today. Once again, showers and storms will diminish with the loss of diurnal heating.

KEY MESSAGE 2 DESCRIPTION... The trough axis shifts offshore Wednesday morning with northwesterly flow advecting drier air into the region. High pressure builds in at the surface so high temperatures should be able to climb into the low 80s south of the mountains under mostly clear skies and downslope flow. North of the mountains it may take a little longer to clear out so would expect the ceiling for temperatures to be more in the way of mid to upper 70s. Surface return flow begins to advect moisture back into the region Wednesday night making for a cloudier and warmer night with low temperatures in the mid to upper 50s south of the mountains, and in the low 60s right along the coast. North of the mountains temperatures fall into the low 50s. High pressure departs to the east Thursday as a trough begins to approach from the Great Lakes Region. Most of the day looks to remain dry with partly cloudy skies as southwesterly flow continues. Temperture-wise Thursday looks a few degrees warmer than Wednesday, but the difference will be felt as dewpoints climb into the mid-60s south of the mountains.

Looking at the tail end of the 24.12Z NAM and RRFS, the bulk of the precipitation looks to hold off until the evening hours Thursday with a chance for some earlier afternoon showers in New Hampshire as the warm front approaches. The main show looks to come ahead of the cold front Thursday night into Friday. NBM probabilities of greater than a half inch rainfall have already trended down since yesterday with most of the area being given a 20 to 40 percent chance. However, this may be owing to the hi- res models starting to be ingested and depicting the more convective nature where higher rainfall totals will be isolated. Speaking of convection CSU's machine learning model does have some low probabilities of severe weather for this period, but around here if the cold front is coming through too late or too early it does so without much fanfare. Trends will continue to be monitored and we should be able to start fleshing out these details better with consecutive runs of the CAMs. The aforementioned series of fronts is progged to be through the area by Saturday morning, and with broad troughing remaining over the region temperatures remain seasonable. High pressure slides eastward from the Great Lakes Region through the weekend keeping us dry despite an upper level trough. Global models suggest forcing from an upper level low sinking southward out of Quebec may reach our region Sunday night into next Monday, but at this time range there is still much uncertainty. I am fine with the NBM throwing low chance PoPs into that time window for now, but I would not be surprised if this eventually changes.

AVIATION /18Z MONDAY THROUGH SATURDAY/

VFR conditions are mainly expected the rest of the afternoon and into the evening but some isolated to scattered showers and thunderstorms may lead to brief MVFR or lower restrictions. High uncertainty regarding storm coverage and timing preclude mention at any specific TAF site. More fog is possible tonight which will lead to MVFR visibilities or lower until fog lifts by mid morning Tuesday.

Outlook:

Tuesday: Generally VFR conditions expected with local MVFR or lower restrictions under isolated showers and thunderstorms.

Tuesday night: Generally VFR outside of any valley fog.

Wednesday-Thursday: Generally VFR conditions expected

Thursday night-Friday: Generally MVFR conditions expected with widespread showers. IFR conditions possible in fog at night.

Saturday: Any early MVFR/IFR conditions should quickly improve to VFR for the remainder of the day.

MARINE

Conditions remain below SCA levels for at least the next few days, with a cold front passing through the waters this afternoon.

Generally sub-SCA conditions expected through the weekend as the Gulf of Maine remains on the periphery of Atlantic high pressure.

GYX WATCHES/WARNINGS/ADVISORIES

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