textproduct: Gray - Portland

WHAT HAS CHANGED

Introduced some low chance PoPs in southern New Hampshire and far southwestern Maine as some showers may spark as the front clears the coast tonight, but these should be very light if they materialize as dry air will be quickly filling in.

KEY MESSAGES

1. After a brief taste of summer, temperatures will be returning to normal Thursday and a bit below normal Thursday night. A Frost advisory will likely be needed Thursday night away from the coast.

2. High pressure provides mostly dry weather Friday into Saturday. Moisture attempts to push into New England early next week while high pressure may suppress and wet weather until sometime Sunday night or Monday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... A cold front is crossing the area at the time of this writing with some scattered showers. So far they have been just been brief downpours, but with SPC mesoanalysis showing 500-1000 J/kg out there, a rumble of thunder is not out of the question. Behind the front will be quite the airmass change from what we have observed over the past two afternoons. Mean troughing overhead will likely keep clouds lingering through tonight, and may even keep some lingering showers in southern New Hampshire and far southern Maine the first part of the night, but 850 mb temperatures crashing back down to the freezing mark will still make for a cool night. North of the mountains where clouds clear out first temperatures should be able to radiate down into the mid- to upper 30s. In the foothills and on the coastal plain where clouds and slightly warmer 850 mb temperatures will be overhead, surface temperatures bottom out in the mid-40s to low 50s.

Northwesterly flow than increases during the day Thursday keeping 850 mb temperatures cool, but with high pressure clearing skies and the high May sun angle we should still be able to warm up into 60s south of the mountains, and into the 50s to the north. High pressure slides overhead Thursday night which is going to lead to efficient radiational cooling, tapping into 850 mb temperatures in the range of -2 to 0C. As usual in these setups, I have blended in cooler MOS guidance to the NBM to reflect this. As a result, there is a possibility that northern New Hampshire and portions of the western Maine Mountains may find themselves in a Freeze Warning (although currently it looks like time below freezing may only last an hour or two), as those forecast zones become eligible beginning tonight. At the very least a frost advisory will be needed along with interior zones as widespread temperatures in the mid-30s seems most likely which will be conducive to frost development. There is still time so I will let the next shift take a look and decide where to hoist the advisories. The coastal plain will largely end up in the upper 30s and low 40s, precluding a frost threat but some of the normally cooler spots could see patchy frost. Bottom line if you have sensitive plants you'd like to keep safe, Thursday night will be a good night to take protective measures.

KEY MESSAGE 2 DESCRIPTION...

High pressure will crest over the region Friday providing fair weather with highs in the 60s to low 70s. A short wave diving southeast across Atlantic Canada will reinforce high pressure over the region Saturday as another trough lifts into the Great Lakes Region. Latest available ensemble guidance suggests high pressure will hold off any moisture from the wave moving through the Great Lakes Saturday. This will result in a mostly dry day with temperatures in the 60s.

Deterministic and ensemble guidance suggest that additional waves moving into the Great Lakes will help build a mid level ridge over New England Sunday with surface high pressure centered offshore. Moisture will be on the increase Sunday while the bulk of any meaningful rainfall may remain south and west of the forecast area. More in the way of clouds and steady easterly winds will keep Sunday cooler with highs in the upper 50s to low 60s. Ensembles then suggest the mid level ridge will shift east Sunday night into Monday with highest chances (40-60 percent) for rain during the holiday weekend occuring on Monday. Chances for showers will linger into Tuesday while a general drying and warming trend seem likely into the middle of next week.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Through 18Z Thursday...Wind gusts ease after 00Z Thursday with conditions remaining VFR through Thursday afternoon.

Outlook... Thursday night: VFR conditions expected to prevail.

Friday: VFR.

Saturday and Saturday night: Mainly VFR Saturday. Clouds increase from the southwest and may lower to MVFR across southern NH with light rain Saturday night.

Sunday and Monday: Periods of MVFR with low cigs and occasional rain.

MARINE

SCA wind gusts and waves in the coastal waters subside this evening as a front crosses. High pressure builds in behind the front making for tranquil conditions on the waters through Thursday night. Winds shift around to northwesterly behind the front

High pressure slides over the waters Friday and holds through Saturday. Seas of 2-4ft and light, variable winds are expected Friday into Sunday morning. Easterly winds prevail Sunday into Monday while winds and seas likely remain below SCA thresholds.

GYX WATCHES/WARNINGS/ADVISORIES

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