textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes with this forecast update.

KEY MESSAGES

1. A cold front will bring chances for showers this morning followed by breezy west winds.

2. Gusty WNW winds on Friday will blow 20 to 30 mph followed by high pressure bringing fair weather Saturday.

3. The remnant moisture from Topical Cyclone Isaias combined with an approaching frontal boundary to our west will bring an increasing chance for rainfall late Sunday into Columbus Day. A cold front will cross Tuesday night, bringing another chance for showers along with cooler temperatures for the middle of next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Cyclonic flow aloft will prevail today with a cold front pushing offshore this morning. A broken line of showers and perhaps a rumble will push southeast of the mountains through day break and then offshore between 7 am and 9 am. QPF with these showers will be light and generally less than a tenth of an inch. Winds will turn westerly behind the front and will increase late this morning into the afternoon when gusts will approach 25-30 mph. Deep mixing to 800 mb along with some downsloping component will allow highs to climb into the upper 60s to low 70s south of the mountains. Subtle height falls combined with surface heating will allow for MU CAPE to approach 500 J/kg with most CAMs suggesting an isolated thunderstorm will be possible across the interior this afternoon. Gusty winds will subside this evening as a surface inversion develops with the loss of heating.

KEY MESSAGE 2 DESCRIPTION...

Cyclonic flow aloft continues on Friday while the pressure gradient strengthens over the area between incoming high pressure and low pressure deepening over Atlantic Canada. A surface trough crossing the area will bring chances for showers and perhaps a thunderstorm, mainly across the north and during the afternoon. The main story on Friday will be the gusty WNW from mid morning into the afternoon. BUFKIT profiles show mixing to 700 mb with winds at the top of the mixed layer around 40 kts while mean layer momentum transfer is 25-30 kts. This should lead to persistent surface gusts up to 30 mph with potential for occasional stronger gusts. Otherwise, skies will be mostly sunny south of the mountains with highs ranging from near 50F across the north to mid 60s along the coast and southern NH.

KEY MESSAGE 3 DESCRIPTION...

Latest forecast guidance continues to indicate that remnant moisture from Tropical Cyclone Isaias combined with an approaching front to our west will bring a chance for rainfall to the area late Sunday into Columbus Day. There remains rather significant uncertainty in terms of rainfall timing along with rainfall amounts though, with some guidance bringing meaningful rainfall across much of our area while other guidance keeps it mostly to our south. Latest NAEFS guidance brings precipitable water values (PWATs) up to around 1.50" by Monday morning, which is ~+2 standard deviation anomaly. That being said, latest AI and ensemble probabilities for 24-hr QPF of >=0.50" are generally 25% or less (highest across southern areas) and therefore any rain that we manage to get from this system looks very manageable and beneficial.

Any lingering rain should end Monday evening with mainly dry conditions through the first half of Tuesday likely before another cold front and trof of low pressure crosses late day/night and brings renewed shower chances. Falling temperatures aloft may allow some of this to fall as snow across the higher terrain and towards the Canadian Border as low temperatures fall into the 30s to near 40 degrees. Colder temperatures look to follow behind this front for the middle to end of next week.

AVIATION /01Z THURSDAY THROUGH MONDAY/

Mainly VFR will prevail through early this morning. Scattered showers across the north and will push to the coast around 12Z. These showers could bring brief MVFR conditions while confidence is not high enough to put in the TAF. West winds behind the front will gust 20-25 kts starting around 16-18Z today. Winds relax tonight with mainly VFR, although there is potential for valley fog at LEB and HIE if winds can go calm.

Outlook:

Friday: Mainly VFR with WNW winds gusting up to 30 kts mid morning into the afternoon.

Saturday-Sunday: Mainly VFR. Clouds and shower chances increase Sunday afternoon.

Monday: Low cigs and potential for rain will bring potential for at least MVFR.

MARINE

SCA conditions continue through today with SW winds gusting in excess of 25 kts and seas in excess of 5 ft. Winds and seas drop below SCA thresholds tonight before ramping up again Friday. SCAs will likely be needed Friday with winds gusting in excess of 25 kts.

High pressure builds in Saturday for quiet conditions on the waters. High pressure shifts east of the waters Sunday with increasing SW flow likely bringing another period of SCA conditions. Low pressure may pass near the waters late Sunday into Monday.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ150>152. Small Craft Advisory until 2 PM EDT this afternoon for ANZ153- 154.