textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes were needed for this forecast package.

KEY MESSAGES

1. We should remain seasonable and mostly dry through at least Saturday with only a few isolated showers and thunderstorms.

2. Widespread rainfall is becoming more likely Monday into Tuesday, with a brief period of heavier rainfall possible. No flooding is anticipated at this time.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A surface low continues to move east, away from the area and out to sea. An inverted trough will extend to the west of the low and will be the focus of some isolated shower activity that will move from north to south through the afternoon into the early evening. RAP forecasts suggest that we could see upwards of 500 to 1000 J/kg MUCAPE this afternoon but with very weak shear. Thus a few rumbles of thunder may be possible, but the threat for strong convection appears low. This activity will diminish with the loss of daytime heating, giving way to only a few clouds overnight. Overnight valley and coastal fog seems likely but density and coverage remains uncertain. For now, will broad brush patchy fog over the valleys and along the entire coast in the 06z to 13z timeframe tonight into Saturday morning. Forecast lows will mainly be in the 50s.

Surface high pressure and ridging aloft will lead to a mostly dry Saturday but a few more isolated showers will be possible in the afternoon over the higher terrain. Temperatures will be a little bit warmer on Saturday than today with the ridge axis moving overhead, but still around normal or slightly above normal for this time of year. Highs are forecast to mainly range from the mid 70s to mid 80s. Clouds will be on the increase late in the day Saturday and overnight as we transition back into southwest flow aloft and the next trough approaches from the west. Saturday night lows will then be a bit more mild than tonight, expected to range from the upper 50s to the mid 60s.

KEY MESSAGE 2 DESCRIPTION...

A trough will be located across the Great Lakes Sunday morning, with increasing SW flow into New England. Will expect increasing cloudiness for the day as moisture increases from the west. Can't rule out the development of some afternoon showers across the mountains, but low level moisture and the increasing cloudiness limiting instability should keep these isolated to scattered.

Into Monday, cluster analysis shows different flavors of the 500mb trough amplitude and timing in the global ensembles. The favored solution in the latest guidance, weighted mostly with the GEPS and GEFS, is the least amplified. This leads to a drier solution early Monday, but may allude to a later passage (rain chances continuing Monday night/Tuesday morning).

While still fresh on our minds, there are a couple factors that are more impressive with this setup than the recent broad closed low...but ultimately they are different animals. The first was slow moving and brought the threat for multi-day precipitation, while this prospective event looks to occur in less than 24 hours. However, in that brief window the moisture ceiling is higher. During the last event, IVT into northern New England was routed around the base of the incoming low before wrapping in from a SE direction off the Gulf of Maine towards the coast. This event has a very direct SW injection along the East Coast that is favorably channeled between low pres in the Ohio Valley and high pressure over the Atlantic Ocean. This is noted in likely GEFS and ENS IVT probabilities of exceeding 500 kg/m/s. ECMWF comparison to climo also indicates PWAT values around the 99th percentile along much of the Eastern Seaboard.

Thus, would expect excellent warm rain processes with efficient rainfall, but shorter duration. The general pattern supports this return of moisture, but yet to be resolved is timing of these ingredients. This can make the difference between taking advantage of the complete system vs. only seeing a couple components come in line before merging offshore. At this point, either the disconnect of ingredients or brevity prevents a notable signal for excessive climo QPF based on the EFI.

After this frontal system pushes through, the closed low may remain to the west advecting moisture through the Mid-Atlantic. At this time, most guidance keeps deeper moisture south of our area, but there may be enough to provide the chance of showers through midweek amid a warming trend.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Through 18z Saturday: Mainly VFR the rest of today with a sea breeze likely to develop near the coast. Some isolated showers or thunderstorms are possible in the afternoon, but only local restrictions are possible. Do to the uncertainty in coverage and timing, left mention out for any site specific forecast. Patchy fog is possible across the area tonight with highest confidence at LEB, HIE, and CON but all sites could see some lower VIS in the 06z to 13z timeframe. Any fog should lift after 13z or so.

Outlook:

Sunday and Sunday Night: Increasing cloudiness. SHRA possible for terminals near the mountains of NH and western ME, with morning low stratus possible towards AUG/RKD. MVFR to IFR cigs possible overnight.

Monday and Monday night: More areawide SHRA and embedded TS. MVFR/IFR cigs with MVFR vis restriction possible.

Tuesday: Improvement to VFR for at least the northern terminals, some cig restrictions still possible for southern NH terminals.

Wednesday: Low confidence in restrictions, but SHRA possible for south and western areas.

MARINE

Some fog will be possible over the waters late tonight into Sat morning. Winds and seas are expected to remain below SCA thresholds thru the extended. As a front approaches Sun, southerly winds will increase. A few gusts near 25 kt are possible well outside of the bays, but guidance does not point towards a high chance of SCA conditions. It is also possible that increasing moisture ahead of the front may lead to areas of fog developing Sun night.

GYX WATCHES/WARNINGS/ADVISORIES

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