textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast update.

KEY MESSAGES

1. Low pressure crosses Friday through Friday night bringing beneficial rainfall.

2. Cold temperatures and strong winds arrive for Saturday, with winter-like wind chills. Moderating temperatures from Sunday into next week with rain-free conditions.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

High pressure races across New England today, exiting to the southeast this evening. The timing will bring one more dry and mild day today before conditions turn more damp and cold into the weekend.

Rain remains on schedule for Friday, pushing across the area west to east in the morning. This will be associated with low pressure that most model guidance agrees will pass along or just NW of the US/CAN border. The positively tilted trough across the Great Lakes will ensure the low is progressive, pushing into the Canadian Maritimes shortly after midnight Friday night.

Rain continues to be the primary precip type for the forecast area as the system passes to the north. There is the chance for some flurries to mix in across the higher western ME mountains early Friday morning, but this should quickly transition to rain for the day. Ensemble IVT values are impressive, albeit briefly, Friday afternoon. Greater than 750 kg/m/s is likely per both the GEFS and ENS. Combined with the jet dynamics and PWATs that near the 90th percentile for early/mid Oct, can't rule out a period of moderate to heavy rainfall rates. The tricky part is location, as global models poorly represent how rain transits the region as far as intensity goes. It is also possible a dry slot bisects periods of rain or showers.

For now this seems like the better solution given QPF totals that have only slightly increased through the week, totaling 0.5 to 1 inch. Without better instability to take advantage of the plume of moisture, precip may be focused along features associated with the low (fronts, fgen, etc). A better grasp on precip presentation will come with the short term guidance window arriving.

It then becomes more showery Friday night with cold air moving in allowing for a transition to snow showers in the mountains. As temperatures continue to cool into Saturday morning, snow will likely make it to the valley floors across the north.

KEY MESSAGE 2 DESCRIPTION...

Strong cold air advection arrives on Saturday with plummeting temperatures. Very confident we will see 850 mb temperatures drop to -7 to -9 C. Even with mixed low-levels, this translates to surface temperatures only in the 40s-near 50 for the coastal plain, with 30s in the higher terrain. Combined with strong winds during the day on Saturday, wind chills will be in the 20s-30s regionwide! Those with outdoor plans this weekend should dress for winter conditions. Conditions will be potentially hazardous for hikers in high terrain.

Strong northwest winds, combined with lingering boundary layer moisture and steep low-level lapse rates, will provide a setup for upslope snow showers in the mountains. A few flakes are possible in the foothills too.

The other headline will be strong post frontal winds Saturday. Wind speeds at 850 mb are on the order of 30-40 kts. Mixing should be rather deep, so despite not particularly intense wind fields aloft, should see a good bit of this reach the surface, especially in the higher terrain. Gusts to 30 mph can be expected region-wide with gusts to 40 mph in the foothills, and much stronger winds above treeline.

The weather turns far more tranquil from Sunday onward. After a very cold Saturday night (with lows bottoming out in the 20s-low 30s regionwide), temperatures start a steady moderating trend. High pressure remains in control of the weather pattern for at least early to midweek next week, but models are indicating that low pressure likely begins to organize to our south during this timeframe, and will be worth monitoring through the forecast period. Temperatures look to be seasonable to potentially mild for this stretch with daytime 50s-60s.

AVIATION /12Z THURSDAY THROUGH MONDAY/

Through tonight...Occasionally gusty northwest winds continue behind a cold front thru midday then winds diminishing. Upslope clouds will linger around MVFR thresholds north of the mtns for the next hour or so then VFR conditions expected thru tonight. Towards Fri morning some MVFR may begin to work in as rain approaches the area.

Outlook...

Friday-Friday night: Widespread IFR and rain are likely. Rain should taper off Friday night with improvement to MVFR or VFR. HIE may see lingering rain showers switch to snow showers with IFR or MVFR.

Saturday: Rain and snow showers likely at HIE to bring MVFR/IFR. Elsewhere VFR, although it's possible a few showers make it downwind of the mountains. It also looks breezy with west to northwest winds of 30 kts possible.

Sunday-Monday: Mainly VFR with daytime wind gusts 10 to 15 kts. No sig wx.

MARINE

SCA conditions continue for the waters early this morning. A period of Gales will also be slackening for the outer waters off Penobscot Bay. Offshore winds should help to decrease wave heights as well, falling to 3 to 4 ft by this afternoon.

High pressure quickly slides over the waters today, before inland low pressure tracks along the St Lawrence Valley Friday. Increasing onshore winds are expected Friday morning with gusts to 30 kts through the afternoon and building waves 3 to 5 ft. Winds become NW Friday night, increasing to around Gale for at least the outer waters through Saturday. Winds and wave heights will slowly subside Saturday night into Sunday, returning to below SCA conditions for the beginning of next week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 10 AM EDT this morning for ANZ150>152-154. Gale Warning until 10 AM EDT this morning for ANZ180.