textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes needed for this update. Still expecting a drier rest of the day and Wednesday, with a more active Thursday afternoon and Friday.

KEY MESSAGES

1. Low pressure continues to exit the region this afternoon, with overcast skies and an isolated shower possible in the evening. Skies clear for Wedensday morning with dry weather likely to persist through Thursday afternoon.

2. Next chance for widespread rainfall arrives Thursday night followed by a chance of thunderstorms Friday afternoon.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Satellite and radar imagery this afternoon shows two low pressure systems over the east coast. Both of these systems have moved offshore along with their respective quasi-strateform rain shields. Pressure will remain low this afternoon, allowing for some very weak instability over the interior. This environment could allow for the development of a couple small showers over the region, though it is unlikely they would develop into thunderstorms. This is because despite modest instability at the surface, low-level and mid- level lapse rates teeter between unstable and conditionally unstable according to forecast soundings. This in turn will limit the upward ascent of air parcels over the region making it fairly difficult for any shower to strengthen further.

Wednesday will feature a similar setup convectively to this afternoon with one key difference: skies will clear in the morning and allow for better diurnal heating and mixing. The extra diurnal heating is likely to allow for a more unstable environment Wednesday afternoon, with CAPE of 1000J possible. Despite a more robust convective environment, any forcing that would allow for sufficient ascent is still with the aforementioned low, progged to be over northern New Brunswick by this time. So the forecast challenge for tomorrow boils down to how far southwest some of the wraparound showers make it. CAMS suggest that sufficient forcing makes it southward and just crosses the Canadian border by Wednesday afternoon. With this in mind, it can not be ruled out that some campers in the Rangeley lakes region could see a storm or two but storms will likely lose steam quickly after they cross into Western Maine from Quebec.

Wednesday night and into Thursday, high pressure moves in and will allow for dry and tranquil conditions through Thursday afternoon. A low pressure system approaching from the west may allow for the development of some scattered thunderstorms in the afternoon.

KEY MESSAGE 2 DESCRIPTION...

Global models are in good agreement with a 500mb shortwave and associated surface low pressure approaching New England Thursday night. Increasing moisture and lift associated with these features is expected to allow a steadier rain to overspread the area from west to east overnight (with fog also possible) that will last into Friday morning or afternoon. Once this rainfall exits, there will be an opportunity for some degree of clearing and temperatures to warm up to bring enough instability for thunderstorms to develop ahead of the cold front. Right now, the higher chances will be across NH and into far western ME with onshore flow keeping things more stable farther east into Maine. Shear also looks sufficient enough to support strong to severe storms, but timing of the front is key as well along with the amount of instability. Ensemble means suggest 0.50" to 0.75" of rain is possible, but thunderstorms could push those amounts higher Friday afternoon. Flow doesn't look overly strong behind the front, so there will probably be fog in some areas Friday night.

Another low pressure passes to the south on Saturday as surface high pressure begins to build toward New England this weekend. This will bring overall drier conditions for the weekend, but a couple of waves aloft passing through in the northwest flow aloft could bring scattered showers and isolated storms as daytime instability builds during the afternoon to early evening hours each day.

Upper-level ridging strengthens to our west over the Great Lakes early next week as surface high pressure continuing to build into New England. This leaves us in a northwest flow pattern aloft, it's possible waves riding along the periphery of the ridge bring additional chances for showers and storms into early next week. These waves are very difficult with both timing and location, so the low NBM PoPs of 20-30% for Mon-Tues will suffice for now. Otherwise, there's a signal in the ensembles for warmer temperatures mid 80s to near 90 possible.

AVIATION /18Z TUESDAY THROUGH SATURDAY/

Restrictions are expected to generally improve through the evening with most sites returning to VFR. However, some fog may allow for restrictions to return overnight, especially along the coast. Fog lifts out Wednesday morning with VFR expected through the remainder of the day. VFR continues through Wednesday night and into the first half of the day Thursday. Scattered thunderstorms will allow for MVFR restrictions Thursday afternoon.

Outlook: Thursday night-Friday: Restrictions ranging from MVFR to LIFR are possible as low pressure and a cold front approach the region. Thursday night into the first part of Friday could me more of a steady rain with fog and low ceilings, and precipitation may become more showery in nature Friday afternoon with a few storms also possible.

Friday night: Patchy fog may bring additional IFR restrictions but confidence is low.

Saturday-Sunday...Mainly VFR. Afternoon scattered showers and isolated storms may bring TEMPO MVFR restrictions each day.

MARINE

Light and variable winds are expected. Winds will generally be out of the west tonight through Thursday morning. Seas of 2- 4ft expected through this timeframe.

Thursday night-Tuesday...Low pressure approaches Thursday night and Friday with a cold front either crossing late Friday or Friday night. Ahead of this front, there may be fog over the waters as well as increasing chances for rain showers. However, SCA conditions are not expected. Another low pressure is expected to pass south of the waters Saturday into Saturday night, but otherwise high pressure builds across the waters later this weekend into early next week, likely keeping conditions under SCA levels.

GYX WATCHES/WARNINGS/ADVISORIES

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