textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes were needed for this morning's update. A little bit of light rain will continue to linger over portions of Maine for the next couple of hours. Some fog may hang around some locations into the afternoon hours.

KEY MESSAGES

1. Showers and thunderstorms are expected this evening with southern New Hampshire having the highest chance of seeing a strong to severe storm. The primary threat in the stronger storms will be strong to damaging winds.

2. Above normal temperatures and shower chances continue into Friday, with some uncertainty in extent of shower coverage.

3. A damp and cloudy weekend, with a stronger cold front on Sunday bringing widespread precipitation

4. Crisp and dry conditions to start next week, but the long- range pattern remains unsettled deeper into the week

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Low pressure tracking along a quasi-stationary boundary pushes a warm front into the area today. A seabreeze will be pushing back at it, but at least some of the area, most likely New Hampshire, will end up in the warm sector. Ridging aloft will help to allow this area to clear out a bit while clouds and showers from convection in the Great Lakes Region roll over the ridge and into northern zones keeping their potential instability development lower. The latest CAMs continue to show a swath of 1000-1500 J/kg of CAPE across southern New Hampshire and far southwestern Maine. With 500-1000 J/kg inland into the foothills. Mid-level lapse rates look to be lacking, but low level lapse rates do get up to 7-7.5 C/km in southwestern New Hampshire around the time convection is expected to be moving in (around 6 PM). Bulk deep layer shear around 50 knots favors cellular structure and the CAMs are showing this initially as well. This will be the best chance to see hail and/or a brief tornado with 25-30 knots of low layer shear present, but this will be isolated. Storm mode looks to become more multicellular and maybe even linear as the event progresses and this is where the primary threat of damaging wind gusts will be highest. Overall the threat window looks to be about 5PM to 11PM with all hazards possible initially, but quickly progressing to a primary wind threat. Southern New Hampshire will have the greatest chance of seeing strong to severe storms with the rest of the area seeing more run of the mill type thunderstorms as they will weaken moving toward the coast and more northern inland zones are likely to remain cloudier. The bulk of the showers and storms push off the coast with more scattered showers lingering overnight as the upper trough moves through.

KEY MESSAGE 2 DESCRIPTION... An upper trough moves through the area as a quasi- stationary front finally pushes southward on Friday. There is still some uncertainty between hi-res guidance on how exactly this pans out. Some runs have had the front clearing early which would cause little fanfare as the trough crosses in the afternoon, but recent runs are a little slower to clear it and result in a more widespread area of showers and possibly thunderstorms. There will be time to iron out the details with more runs, but for now a widespread chance of showers seems reasonable. Drier air will follow the front so when ever it does push out we can expect a break in this showery pattern even if it's brief.

KEY MESSAGE 3 DESCRIPTION...

The warm sector will be squashed south Friday night and that is likely it for "warm" temperatures for the foreseeable future. On Saturday a 1030 mb high develops near Newfoundland. While cool and moist conditions have been common along the immediate coast all week due to onshore flow and seabreezes, a stronger push of easterly onshore flow on Saturday is expected to result in these conditions extending farther inland. The 25th-75th percentile temperature spread peaks at 10 degrees Fahrenheit for interior zones, but remains below 5 degrees on the immediate coast, suggesting a bit more uncertainty further inland, but local experience shows cold air usually wins in these setups. Measurable precipitation chances look nil on Saturday with a dry layer aloft, but moist low-levels suggest there could be some mist/drizzle near the coast.

25th-75th percentile temperature spread is also on the high side Sunday, indicating timing differences with a strong cold front that is expected to cross the region. Unlike the previous week, dynamics will be on the strong side, owing to a 500 mb jet streak nosing into the region and rather robust height falls aloft. Current NBM mean QPF offers 0.30-0.50", with the higher totals focused near the White Mountains where some upslope enhancement is likely. The 90th percentile is below an inch regionwide, owing to the progressive nature of the front. PWATs surge to over an inch on Sunday which suggests the potential for heavy downpours. Crashing temperatures Sunday afternoon suggest rain may change to snow in the mountains and near the Canadian border.

KEY MESSAGE 4 DESCRIPTION...

Cold and crisp to start next week as high pressure crests over the region. Both Monday and Tuesday offer high temperatures near 10 degrees below average, with a hard freeze looking likely Tuesday morning. Long story short, it will feel more like March than April. Generally sunny skies should take the edge off the cold though, thanks to the high sun angle.

A rather complex upper-level pattern becomes evident later into next week. An anomalous block develops downstream from Greenland to northern Quebec, effectively forcing this weekend's trough to retrograde west and detach from jet stream flow. It is too early to speculate on details, but deterministic and ensemble guidance is in agreement on troughing returning to the Northeast by mid-to-late week with a rather blocky pattern. Chilly and wet conditions are suggested with such a pattern.

AVIATION /12Z THURSDAY THROUGH MONDAY/

Through 12z Friday...IFR/LIFR in low ceilings and coastal fog start the TAF period and persist through about 15Z. Inland terminals should start to see some improvement to at least MVFR after this, but lower ceilings are likely to persist longer at coastal terminals. Closer to 00Z Friday showers and thunderstorms enter the region which may reintroduce restrictions. Confidence in LEB, MHT, and CON seeing thunderstorms in their vicinity is moderate, with lower confidence elsewhere. Thunderstorms should be exiting the area by 06Z Friday with just lingering showers and likely low clouds and fog.

Outlook...

Friday: MVFR improving to VFR through the day. Showers and thunderstorms are possible again in the afternoon, but there is uncertainty in coverage.

Friday night: VFR to start, with IFR to LIFR possible late with marine fog pushing inland.

Saturday-Saturday night: IFR to LIFR restrictions possible due to marine fog and stratus at the coastal sites, but fog may also occur inland. VFR conditions most likely during the afternoon.

Sunday...Widespread restrictions and rain likely.

Sunday night: Conditions return to VFR, except MVFR remains possible at HIE.

Monday: Mainly VFR with gusty northwest winds.

MARINE

Generally tranquil conditions are expected to continue on the waters through at least Friday as wind gusts remain 15 knots or less and wave heights remain 3 feet or less. Winds start light from the northeast on Thursday, but will quickly turn onshore as a sea breeze develops. Winds shift back to the northeast Thursday night as and showers and thunderstorms push off the coast. They will stay there through Friday and be a bit breezier as a front crosses the waters.

More fog and stratus are likely from early Saturday through Sunday as easterly winds and moisture increase ahead of a cold front that will also bring widespread precipitation on Sunday. SCA conditions are likely with gusty post-frontal winds Sunday night and Monday. High pressure should then allow conditions to fall below SCA levels by Tuesday.

GYX WATCHES/WARNINGS/ADVISORIES

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