textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast update.

KEY MESSAGES

1. High astronomical tides continue through tonight. While wave action is expected to continue to diminish, high water levels and even some minor flooding remains possible.

2. Weather turns unsettled after midweek with a seasonably strong storm expected to cross the region Thursday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

High astronomical tides continue thru the middle of the week. Wave action is not expected to be an issue until at least the latter half of the work week, and by then tides will be decreasing in magnitude. However after the high tide early this morning, we will have one more very high tide at 11.6 ft at the Portland gauge just after midnight tonight. If residual surge remains in excess of 0.5 ft that will mean another cycle at minor flood stage.

Otherwise fair weather and seasonable temps are expected thru mid week.

KEY MESSAGE 2 DESCRIPTION...

Afternoon showers and thunderstorms will become more likely Wed and the upper trof approaches and begins to influence the local weather more.

However the real attention is on the upper trof axis/upper low as it swings thru the Northeast on Thu. Surface low pressure for this time of year is outside of the model climatology and that is going to drive strong synoptics across the region. For instance, the wind fields are quite strong and something on the order of at least a 5 if not 10 year return period at 850 mb. At this time it looks like the strong wind field will bring strong moisture transport and a solid soaking rainfall thru the forecast area early in the day. As mentioned last night, I am wary of strong synoptic forcing during the warm season when instability is available. This can often lead to extreme rainfall rates that global models struggle to capture at this range. Then the question becomes whether or not we can clear out sufficiently for destabilization and a round of afternoon convection. That will occur somewhere in the region and convection looks like it could be robust. SPC already has a Day 4 convective outlook nearby, and various CAM machine learning guidance is highlighting the region too. Southwest NH will be on the fringes of those risk areas, but close enough that I do not want to rule out the threat of severe weather entirely. As things stand currently there will be extreme shear during the afternoon hours, the question remains instability. Forecast soundings are showing that we may be able to pop some 0-3 km CAPE in the 100 to 150 J/kg range, which is more than enough for severe weather locally. However in this case the magnitude of shear could be too much for sustained updrafts given the meager instability forecast. I will be very interested to see how CAMs begin to handle this system over next couple of days as it comes into range.

The weather may remain unsettled into the weekend, with an extended period of shower and thunderstorm chances each day.

AVIATION /08Z TUESDAY THROUGH SATURDAY/

Through 12z Wednesday...VFR expected with NW breeze 10 to 15 kts. Diurnal cu develops this afternoon. Could see a wind shift late this afternoon along the coast as weak seabreeze tries to push inland. Patchy valley fog is possible again tonight and may actually be more favorable without the light breeze and dry advection. This would be most likely for LEB and HIE.

Outlook:

Wednesday: VFR conditions generally expected. Local MVFR or lower possible in the afternoon or early evening in isolated showers and thunderstorms.

Wednesday Night: VFR conditions expected.

Thursday - Thursday night: Areas of MVFR or lower possible under showers and thunderstorms and low CIGs. Two rounds of precip possible, one widespread early in the day and another more scattered in the afternoon.

Friday: Low CIGs may linger into Fri before clearing during the afternoon.

Friday Night: VFR conditions expected.

Saturday - Sunday: VFR conditions generally expected. Local MVFR or lower possible in isolated showers and thunderstorms.

MARINE

Winds and seas expected to remain below SCA thresholds into Wed. By Thu southeast winds increase and eventually turn southwesterly. At this time gusts up to 30 kt look possible along with seas building to 5+ ft outside of the bays.

GYX WATCHES/WARNINGS/ADVISORIES

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