textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes were made to the forecast at this time. Still monitoring ongoing convection for both severe as well as hydrological threats.

KEY MESSAGES

1. A cold front will cross the region this evening, bringing a threat for strong to severe thunderstorms as well as localized flash flooding.

2. High astronomical tides will continue into early this week.

3. Seasonable temperatures will return for much of the week. An unsettled second half of the week is likely with multiple rounds of showers in the forecast.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... A surface warm front is currently located near the Crown of ME with a trailing cold front moving eastward across the eastern Great Lakes region. New England is sitting within the warm sector with southerly flow at the surface.

Convection is ongoing across the northern portion of the CWA with a noticeable increase in lightning activity over the past hour or so. The 18Z KGYX RAOB report shows significant deep layer shear over the region with 0-6km shear at 52 kts along with 0-3km of 50 kts. Surface based CAPE was sampled to be at around 1400 J/KG with steep low-level lapse rates at ~8C/km. Mid-level lapse rates are on the weaker side but given the plethora of shear in place along with growing instability and lift, expecting storm coverage to increase over the next few hours. This is generally supported by latest CAM guidance, though with differing solutions between individual models and model runs. The main threat continues to be wind, both from straight line winds and potential downbursts. A brief tornado also remains possible.

The other concern is for possible localized flash flooding, especially where storms train over the same locations and where heavy rainfall was experienced a few days ago. Observed rainfall rates of around 1"/hr have been observed so far and therefore localized rainfall amounts of between 2-3" are likely with isolated amounts of up to 4" possible.

The severe threat will diminish overnight tonight but the threat from locally heavy rainfall and flash flooding may persist until early Monday morning.

KEY MESSAGE 2 DESCRIPTION...

High astronomical tides continue into Monday. The evening tides are of greatest concern for Minor flooding, with the best chance for these levels to be met for Portland and Hampton this evening and Monday evening.

This evening's tide will align with increasing southerly flow as a front approaches from the west. This will result in growing sea state, but wave heights of 2 to 4 ft remain more tame vs. winter systems. A Coastal Flood Advisory remains in effect for Portland through the NH Seacoast for this evening's tide after 9pm.

Subsequent tides will feature lower wave heights, but high still water will still rise into action stage, and approach minor flood stage.

KEY MESSAGE 3 DESCRIPTION...

Good model consensus remains for seasonable temps to return for much of the week. Widespread 70s and 80s are anticipated during the day and overnight dropping back into the 50s to lower 60s, with 40s across the north. The humidity will start to creep back up towards the latter half of the work week with the approach of the next chance of rain.

That will come as an upper-level low moves eastward across Ontario and Quebec. This upper-level low will allow for cooler temperatures and more unsettled weather. The first of three disturbances arrives on Wednesday evening as a decaying low brings scattered showers and storms. A more robust system then looks to overspread the northeast on Thursday, with more widespread rain in the morning followed by thunderstorms in the evening. Some wraparound showers from the aforementioned low may persist, with somewhat unsettled weather likely continuing through the weekend. High temperatures are expected to be in the upper 70s and lower 80s during this timeframe. Unsettled weather may continue through the weekend.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Through 18Z Monday: SHRA and TSRA will persist through tonight along with lowering ceilings to MVFR or worse. Any TSRA will be capable of producing strong wind gusts and possible hail, especially aloft. The greatest threat for TSRA is at KLEB, KHIE, and KAUG. S winds will persist through the night with gusts up to 25 kts. Conditions will improve back to VFR from west to east after 12Z Monday with NW winds at 10-20 kts. LLWS is possible overnight tonight at around 1k ft.

Outlook: Monday night: VFR conditions expected.

Tuesday-Wednesday: VFR conditions likely, possible restrictions late day Wednesday and Wednesday night with SHRA and lowering ceilings.

Thursday-Thursday night: Restrictions possible due to RA and increasing SE winds.

Friday: Possible restrictions due to lingering SHRA.

MARINE

Southerly wind gusts between 25-30 kts are expected tonight into early Monday morning with seas of 3-6 ft outside of the bays. Winds will become northwesterly on Monday but gusts should largely remain below 20 kts. Southeasterly winds will increase late week as another storm system crosses west of the waters, possibly bringing SCA to gale force gusts.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Flood Watch through Monday morning for MEZ007>009-012>014-033. Beach Hazards Statement until 8 PM EDT this evening for MEZ023>028. Coastal Flood Advisory from 9 PM this evening to 1 AM EDT Monday for MEZ023-024. NH...Flood Watch through Monday morning for NHZ001>004. Beach Hazards Statement until 8 PM EDT this evening for NHZ014. Coastal Flood Advisory from 9 PM this evening to 1 AM EDT Monday for NHZ014. MARINE...Small Craft Advisory until 8 AM EDT Monday for ANZ150>154.