textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes were needed for the morning forecast update. Other than some high clouds across the area, we continue to see some low stratus and fog along the coast this morning. This area of clouds/fog should move inland a little bit more this morning. Satellite imagery also shows some valley fog.

KEY MESSAGES

1. Onshore flow develops today bringing cool temperatures and fog, possibly dense, into the forecast. High astronomical tides combined with some surge may bring water levels near minor flood stage during tonight's high tide cycle.

2. Widespread rain, briefly heavy at times, on Sunday will transition to light snow showers Sunday night. Light snow accumulations possible across the mountains and far north.

3. Cool and dry conditions for the majority of next week, but the pattern remains somewhat unsettled.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Vertically stacked low pressure develops offshore today which will start a feed a cooler moist air into the region. However high pressure stays nosed in through at least the afternoon keeping the area mostly dry. Temperatures only top out in the 50s as clouds become abundant south of the mountains. As this moist flow continues Saturday night dense marine fog will push onshore, with drizzle and patchy fog making it inland as well as the column continues to moisten. Temperatures fall into the upper 30s and low 40s.

Looking at coastal flood guidance there is some suggestion that with this low in the position south of the Gulf of Maine Saturday night, we could see around a half foot of surge which combined with the 11.3 astrotide could bring areas from Portland south near their minor flood stages during the Saturday night high tide cycle. This flood guidance has seemed to run hot during recent events so my current thinking is that we wouldn't quite get there, but since there is still time I will let the next shift review subsequent guidance cycles and make a decision if a product is necessary.

KEY MESSAGE 2 DESCRIPTION... Low pressure tracks just to our southeast on Sunday as a cold front is progged to move through the area. This combination brings widespread rainfall to many locations. Even though convection won't be much of a factor all this aforementioned onshore flow is going to surge PWATs into the 1-1.25" range, so I would anticipate embedded heavier showers developing within the lighter stratiform band. The QPF trend has been holding steady with amounts generally in the 0.25-0.75 inch range, with locally higher amounts in the mountains where upslope snow will be contributing. Speaking of this, rain showers will be transitioning to snow showers from northwest to southeast beginning Sunday afternoon as cold air rapidly moves in behind the front. The totals are going to be higher in the mountains and near the international border, but with about half of the event occuring early enough that surface temperatures won't be conducive to accumulation a slushy 1-2 inches is about all it is going to amount to, with locally higher amounts near 3 inches at higher elevations. By the time any snow showers get south of the mountains Sunday night, forcing will be weakening as ridging begins to build aloft. This combined with surface temperatures still above freezing means accumulation in these locations is not likely. Some stray showers may linger into the later hours Sunday, but with the aforementioned height rises aloft and high pressure beginning to nose in at the surface, it looks like the area could be mostly dry not long after sunset.

KEY MESSAGE 3 DESCRIPTION...

The upper-level trough that brings Sunday's precipitation chances will stall out overhead for most of next week due to downstream ridging over the northern Atlantic. It will be quite cold aloft initially with 850 mb temperature anomalies peaking at 10C below climatology, but a moderating trend is expected deeper into next week. On face value there is a signal for cooler than normal surface temperatures, which is in the forecast.

Monday and Tuesday both look chilly but bright with highs largely in the 40s outside of the mountains. A moderating trend will commence for the second half of the week owing to height rises aloft as the trough finally moves out. The pattern generally looks drier than previously thought, owing to prevailing westerly flow, which will keep things quite dry despite upper level troughing. There may be clouds and isolated precipitation in the mountains with an upslope wind component. One chance for more widespread precipitation is on Wednesday, but overall a weak signal at this range with probabilities generally 20-50% of measurable precipitation at this range. Given the cold airmass, there is a chance of snowflakes if any precipitation chance comes to fruition.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Through 12Z Sunday...Marine fog and stratus will continue to be possible this morning along coastal terminals. Patchy fog has also made it inland MHT and CON. LIFR to IFR conditions will remain possible this morning with a brief improvement to MVFR or VFR this afternoon before more stratus and fog settles in once again tonight. HIE and LEB are expected to remain generally VFR until some MVFR ceilings tonight. Widespread LIFR to IFR conditions again tonight.

Outlook...

Sunday: Some improvement to ceilings and visibilities as rain enters the region, but only MVFR at best. Rain transitions to light snow showers Sunday evening.

Sunday night: Precipitation tapers off and drier air moves in rapidly improving conditions to VFR.

Monday: Largely VFR conditions with NW winds at 10-15 kts during the daytime before becoming light and variable at night. MVFR or lower possible only at HIE.

Tuesday: VFR conditions with light winds. Lowering ceilings possible at night.

Wednesday: Precipitation is possible (20-50%), resulting in MVFR or lower, otherwise VFR conditions continue.

MARINE

Conditions remain generally below SCA levels in the coastal waters through Sunday. Low pressure crossing east of the Gulf of Maine may bring some 5ft seas in briefly, but this may not require an SCA as spatial extent looks marginal. Similarly, a few wind gusts around 25 knots are possible Sunday night as a strong cold front crosses the waters, but mainly in the area bordering the outer waters.

From Monday afternoon onward, wind and seas will relax and are expected to remain below SCA criteria through the period. High pressure should then allow conditions to fall below SCA levels by Tuesday. The only possible exception is on Wednesday where higher waves are possible with briefly unsettled conditions.

GYX WATCHES/WARNINGS/ADVISORIES

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