textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast package.

KEY MESSAGES

1. Scattered showers and thunderstorms today.

2. Low pressure moves in late Monday, exiting east Tuesday morning. This will bring a soaking rain to the southern half of the forecast area, with lighter rain elsewhere. Will want to monitor locations that see rainfall from slow moving showers this weekend for repeat moderate to heavy rainfall come Monday and Tuesday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A weak short wave trough will pass through the region during the early afternoon hours today. Cold temperatures aloft are expected which will aid in relatively steep SFC and mid level lapse rates. This coupled with a modest increase in SFC moisture will allow for at least 1000 J/KG SBCAPE to develop rather quickly by noontime. However, wind shear is pretty much non- existent as flow aloft is less than 20 KT all the way up to the upper troposphere.

A weak SFC trough aided by the weak approaching short wave trough will aid in the development of scattered showers and thunderstorms, likely starting by noon as there's no appreciable CINH. A few of these storms could produce small hail and gusty SFC winds. But with minimal wind shear there is no mechanism for storms to organize. Therefore, any one storm should be short- lived with the collapsing updrafts and resulting outflow likely initiating other showers and short-lived storms nearby.

The coverage of showers and any thunderstorms will wane quickly in the later afternoon hours as we lose heating and forcing for ascent as the short wave trough moves east.

Winds will be much lighter today than they were on Saturday. High temperatures should reach well into the 70s at most lower elevations.

KEY MESSAGE 2 DESCRIPTION... Guidance spread has tightened somewhat for rainfall expected Monday through Monday night. Low pressure exiting the Ohio Valley will nudge a warm front through southern New England, with the wave riding this slow to advance boundary. The wave itself is moving quite fast, with a range of solutions either keeping the precip shield tight and compact, or with a little more areal coverage of light rainfall.

Euro ensemble did drift south for recent guidance yesterday's assessment at this time, and this becomes the dominant solution in the global suite. Predominant rain process appears to be from fgen banding in the precip shield, almost similar to a coiled up winter storm. As the developing low is ushered across southern New England, it will then transit northeast across the Gulf of Maine. This drags the precip shield along with it, keeping rain chances going Monday evening for much of southern NH into the coast and interior of Maine into early Tuesday morning.

This is a good tracer for the primary lift for this system, with instability held to the south of the forecast area. It will have a moist environment to take advantage of where this forcing exists, with 90th percentile PWATs, warm cloud depth of around 11 to 12kft, and high mean RH below 600mb. So rainfall efficiency should be well represented in the precip shield. Outside of this, dry air takes a toll on efficiency. While the corridor of greatest rainfall is narrowing in on southern NH and up the southern ME coast, any further shift could place this axis further north. Given the potential for slow moving showers/storms on Sunday preceding this event, will need to monitor any localized locations that take on appreciable rainfall, leading to a greater flood risk Mon/Tues.

The area will see a lapse in widespread rainfall for Wednesday, but remaining moisture in the low levels will drive daytime cumulus and scattered showers across the mountains and portions of the interior. Thursday may play out much the same way, both days seeing a quick warmup before the next synoptic system late week. This could bring another wave of steady rainfall and/or thunderstorms to the area, but would like to see better agreement in guidance to mention further impacts.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

Through Sunday night: VFR early this morning, while patchy fog cannot be ruled out at LEB and HIE through 13z. Mainly VFR during the day today with scattered TSRA developing around 16Z and last until around 00Z. Scattered TSRA will bring the potential for localized restrictions. Depending on rainfall Sunday some patchy fog may be possible Sunday night. Otherwise, VFR.

Outlook:

Monday: AM VFR, MVFR in the evening due to rain and thunderstorms, IFR cig possible for southern NH terminals and later south coastal ME.

Tuesday: AM IFR to MVFR in RA, improving to VFR by the end of the day.

Wednesday: VFR likely with some SHRA across the interior.

Thursday-Friday: MVFR likely due to showers and thunderstorms.

MARINE

Winds and seas will remain below SCA thresholds Sunday into Sunday night.

Light, variable winds and seas of around 2-4ft are expected through all of next week. One potential exception to this may be late Monday into Tuesday. This is due to low pressure moving across the area, and some southeasterly SCA gusts are possible especially over the open waters. Seas also could be a little bit elevated on Tuesday as well, with occasional 3-5ft seas possible as well. Conditions return to baseline by the end of the day Tuesday.

GYX WATCHES/WARNINGS/ADVISORIES

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