textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast update.

KEY MESSAGES

1. Cold front slips south of the area tonight returning us to seasonable weather. Patchy dense fog will be possible in the valleys tonight.

2. A polar air mass arrives behind a strong cold front early next week. Many locations south of the mountains may see their first hard freeze.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Cold front continues to slip thru the forecast area. Rain showers at this point are largely anafrontal, with a fairly well defined wind shift boundary pushing southward on radar. So very warm temps will get knocked back to the low 70s as that passes, and then rain cooled air will drop things a bit more into the 60s thru the early evening hours. Overall not expecting much rain still, with amounts at or less than a tenth of an inch.

Once the front clears there will be some high clouds to contend with early, but generally I anticipate that we will be able to radiate down into the 30s and 40s. Any location that saw some rain will be a decent candidate for radiation fog, and I have maintained fog in the major river valleys.

Sat will be a seasonable day with little in the way of weather impacts expected. Overnight however will be one of the cooler nights so far this season. Still at this point I do not anticipate 30s reaching too far into the active growing season zones and thus a need for frost headlines.

KEY MESSAGE 2 DESCRIPTION... High pressure exits the region on Sunday as an upper level trough swings in from the Great Lakes Region. Clouds are going to build in early which is going to keep afternoon temperatures in the low to mid-60s. These clouds are also going to make for a more mild night as temperatures only fall into the low to mid 40s.

A strong cold front crosses on Monday and it continues to look like it is going to be mostly dry. The latest NBM probabilities of measurable precipitation remain less than 30% south of the mountains. The immediate coast has a slightly higher chance (30-40%) as guidance starts to show a weak coastal low generating at the base of the trough. Otherwise, any meaningful chances are confined to the International Boarder where rain showers may briefly transition to light snow if they linger longer enough into Monday night. The bigger impact from this frontal passage is going to be the cold, dry air that fills in behind it. 850 mb temperatures drop to -7 to 0 C by early Tuesday so I would anticipate it being a cold night, but as the low pressure waves phase over the Canadian Maritimes the pressure gradient tightens and the resulting breezy winds should keep the area from decoupling. North of the mountains temperatures fall into the upper 20s and low 30s, and to the south they fall into the mid to upper 30s. The breezy winds carry through the day Tuesday, so despite a sunny day it is likely going to be one of the coldest days of the season so far. With 850 mb temperatures as cold as they are high temperatures top out in the upper 40s and low 50s in the north, and in the mid to upper 50s to the south. Depending on how the wind gusts pan out it could feel a bit colder. Tuesday night looks like the coldest night of the season so far and certainly will be if the pressure gradient slackens enough for winds to go calm. The latest NBM is hedging that way, bringing near freezing temperatures all the way down the coast. Many areas south of the mountains may see their first hard freeze (at the very least first frost), but most of these zones also ended their window for Frost/Freeze products yesterday. The coastal plain is the last area that can still have them issued and if favorable radiational cooling pans out, I can see that being necessary.

High pressure slides just south of the region Wednesday which should keep us sunny and dry, but precipitation chances may return late in the week as long term guidance suggests another trough with low pressure somewhere in the vicinity.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Cold front slips thru the area this evening, but generally expecting conditions to remain VFR for most locations. Local MVFR is possible in the heaviest rain showers. Widespread VFR conditions tonight thru Sat. Patchy valley fog may bring local LIFR to LEB and HIE along with other major river valleys. I could see it sneaking into CON by sunrise, but unless measurable rain falls in the Merrimack Valley I do not anticipate it there.

Outlook... Sunday: Generally VFR conditions are expected as high pressure moves through the region.

Monday: Generally VFR with some brief MVFR restrictions possible in light rain and/or snow in the mountains.

Tuesday-Wednesday: VFR prevails once again as high pressure builds back into the region.

MARINE

Cold front will cross the waters tonight but not much expected beyond a wind shift to offshore. Generally sub-SCA conditions are expected as much of the forecast period is dominated by high pressure settling over the waters. However, a strong cold front crosses the waters Monday night and may elevate seas and wind gusts to SCA criteria.

GYX WATCHES/WARNINGS/ADVISORIES

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