textproduct: Gray - Portland

WHAT HAS CHANGED

Little change to the going forecast as the various sources of guidance have come into better agreement. Did decrease temperatures across the higher summits Friday afternoon and overnight to introduce the chance for snow to emphasize the potential impacts of late season wintry conditions.

KEY MESSAGES

1. Cooler weather arrives today, with scattered showers and perhaps a thunderstorm. Fair weather for Friday daytime with increasing clouds.

2. Widespread rainfall and high elevation snow late Friday through Saturday. Hikers should prepare for winter conditions above 3000 feet.

3. Unsettled pattern persists into at least the first half of next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A short wave trough with associated 500mb temps of below -20c will swing southward across the forecast area today. Scattered convective showers are expected today owing to modest SFC heating working in tandem with the cold air aloft. A few hundred joules of CAPE should develop which could be enough instability for a few rumbles of thunder. There could be brief gusty winds and perhaps some small hail. NW wind gusts around 25 MPH are expected today outside of any convection. The shower activity wanes early in the evening with the loss of daytime heating. There may be some leftover light showers in the upslope regime in the mountains Thursday night but most areas should be dry with lows in the 40s.

Friday looks to be a fair weather day as we will be in an area of short wave ridging. Increasing clouds will be the theme with highs mainly in the 60s.

KEY MESSAGE 2 DESCRIPTION...

Models remain in good agreement in a compact and closed off upper- level low diving south across northern New England Friday night through Saturday with a surface low developing in the Gulf of Maine. A notable trend amongst available deterministic and ensemble guidance has been for a more progressive system with the bulk of precipitation occuring Friday night into Saturday morning.

This event will be driven by anomalously cold air aloft. Ensembles show 850 mb temperatures dropping as low as -4C. Aloft, 500 mb temperatures drop to -20 to -25C. For context, these temperatures will be at the 5th percentile or lower of climatology for late May, indicating the potential for a highly unusual event. BUFKIT profiles show wet bulb zero heights may fall as low as 2000 feet early Saturday morning while the NBM shows snow levels as low as 3000 feet. While there remains some variability in the location of the coldest air aloft, it remains likely that the highest peaks will see some accumulating snow with winter conditions possible above 3000 feet centered on Friday night into Saturday morning.

At lower elevations, a cold rain is expected Friday night into Saturday morning with the latest NBM carrying 90 percent PoPs Friday night with PoPs decreasing to 20-40 percent Saturday evening. An axis of rainfall to around 1-2 inches is likely, although the placement of this axis shows some variability with going forecast aligning it from the mountains to the Mid Coast. Rainfall amounts decrease to the southwest, although this could change if the axis of greatest rainfall shifts. While not expecting widespread or strong thunderstorms, steep lapse rates and the cold pool aloft may provide marginal instability, which may allow for some convectively-enhanced precipitation. Finally, low pressure forming in the Gulf of Maine will also bring northeast winds gusting 25 to 35 mph along the coastal plain with higher gusts along the immediate coast. It will be a cold and raw day with highs mainly in the 40s to near 50 at low elevations, but windchills likely in the 30s.

KEY MESSAGE 3 DESCRIPTION...

As one upper level low exits Saturday another shortwave will drop south through eastern Canada Sunday. The approaching wave will bring increasing chances for showers Sunday afternoon with widespread chances for showers persisting through Monday. With the cold pool still hanging around (850 mb temperatures 0-5C), we once again may have some convective augmentation to any precipitation that forms.

Uncertainty remains on how long troughing persists over the Northeast next week. In any case, near to below normal temperatures are expected to continue as long as the omega block remains firm, keeping the Northeast U.S. under persistent troughing. Some ensembles suggest flattening of the ridge by mid-late week, which could open the door for "warmer" temperatures to return.

AVIATION /10Z THURSDAY THROUGH MONDAY/

Through 12z Friday...Mainly VFR is expected through this morning. There is a MVFR/IFR deck arriving from eastern ME that could impact RKD and AUG through early morning, but may be more scattered. Scattered showers and a few thunderstorms this afternoon may allow for localized flight restrictions, these appear most likely for far southern ME terminals and PSM...as well as towards LEB/HIE. NW wind gusts of 20 to 25 kt are expected today. Tonight looks to be mainly VFR with winds generally under 10 knots.

Outlook:

Friday: MVFR cigs invade from the NW as afternoon SHRA arrive. Coverage is expected to increase as the afternoon progresses, with IFR cigs arriving by early evening.

Friday night-Saturday: Widespread rain and low cigs likely bring at least IFR. Some improvement from NW to SE Saturday afternoon. NE winds gust up to 30 kts along the coastal plain. Mountain snows above 3000 ft in elevation likely Friday night and Saturday morning.

Saturday night-Sunday: Mainly VFR.

Sunday night-Monday: Showers bring potential for restrictions.

MARINE

NW winds will generally be under 15 knots today. However, there may be a few nearshore gusts 20 to 25 kt for a time this afternoon. Relatively tranquil conditions are expected Thursday night into Friday.

Low pressure crosses northern New England Friday night into the Gulf of Maine Saturday. Strengthening northeast winds will bring potential Gales through the day Saturday and seas quickly building 6-9 feet. Winds shift northerly Saturday night and gradually drop below 25 kts while seas remain elevated through Sunday. Winds and seas likely drop below SCA thresholds Sunday night into Monday.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Gale Watch from late Friday night through Saturday afternoon for ANZ150-152>154. Gale Watch from late Friday night through Saturday evening for ANZ180-182-184.