textproduct: Gray - Portland

WHAT HAS CHANGED

750 PM Update...Have issued a dense fog advisory for the immediate coastal zones. May have to expand a bit inland in a few hours. Will watch trends. Otherwise, widely sct convection will continue for several more hours with the threat for very heavy rainfall rates.

Previously...

No significant changes with the main focus through this evening being the threat for isolated to scattered storms producing torrential rainfall and gusty winds.

KEY MESSAGES

1. Isolated to scattered thunderstorms will bring torrential rainfall through this evening. Slow moving and repeat rounds of storms will bring a localized risk for considerable flash flooding for areas within the Flood Watch. The threat for severe storms is Marginal while some strong to damaging winds cannot be ruled out.

2. Humidity abates this weekend while temperatures run warm with highs in the 80s to low 90s Saturday and Sunday.

3. A cold front will cross the region Sunday and bring the threat for showers and isolated strong thunderstorms.

4. High astronomical tides this weekend. A return to seasonable weather is expected for much of the work week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

An anomalously moist air mass is in place as this morning's KGYX sounding set a new PWAT record for the date at 1.91 inches. Latest SPC mesoanalysis brings an axis of SB CAPE of 2000-2500 J/kg across much of the area away from the coast with this CAPE building to 3000 J/kg through early this evening. The lack of upper level support and weak shear has led to mainly small isolated thunderstorms along the foothills and far northwest Maine. CAMs continue to highlight a SW to NE area of convergence within the Flood Watch as focus for continued thunderstorm development through this evening. These storms will be slow moving and will be efficient rain producers with rainfall rates nearing 3 inches per hour. This will bring the risk for flash flooding and isolated incidents of considerable flash flooding as the latest HREF max shows 3-hour rainfall amounts of 3-5 inches. While the lack of shear will mainly keep these storms sub-severe any core collapses could lead to isolated incidents of strong down burst winds. Convection along the area of convergence will gradually wane late this evening as a cold front approaches from the west.

The incoming cold front will bring additional chances for thunderstorms as it crosses from 8 PM to 2 AM. The lack of shear and diminishing CAPE will limit the threat of severe storms with SPC having a Marginal Risk for severe storms across much of NH and NW Maine. A few strong to severe storms remain possible bringing the potential for damaging winds. The main concern with these storms will be repeat rounds of torrential rainfall over areas that received heavy rainfall earlier in the evening.

KEY MESSAGE 2 DESCRIPTION...

A cold front will push offshore Saturday morning with steady NW winds winds ushering in a drier air mass. While dewpoints will drop into the 50s, downsloping winds will aid in another very warm day with highs in the upper 80s to low 90s south of the mountains. A Beach Hazard Statement has been issued to highlight cold water safety as ocean temperatures remain in the 50s while air temperatures at the beaches will be near 90F. Very warm conditions continue Sunday with highs in 80s to low 90s while dew points remain in the 50s to low 60s.

KEY MESSAGE 3 DESCRIPTION...

One more hot day is expected on Sunday ahead of a surface cold front that will be moving across western/central NY state. High temperatures will range from the middle to upper 80s across interior western ME with upper 80s/low 90s across NH. Cooler temperatures will be found along the beaches due to increasing southerly flow. Surface dewpoints will largely be 60 degrees or below, and therefore heat index values should be around actual air temperatures.

Scattered showers will become increasingly likely across the north during the afternoon before sinking south to the coastline by the evening hours. There is the potential for isolated strong thunderstorms during this time period with gusty winds being the primary threat. Machine learning maintains a low risk for this potential, mainly across NH and interior western ME. The latest SPC D3 outlook has trended northward with a MRGL risk across portions of south-central NH and a SLGT risk nudging into Cheshire County. Showers will persist overnight Sunday before ending Monday morning. Low temperatures will mainly be into the 50s.

KEY MESSAGE 4 DESCRIPTION...

Mean trofing looks to setup over the Northeast for much of next week, allowing for a return to more seasonable temperatures along with scattered showers at times. After mainly dry days on Monday and Tuesday, shower chances look to increase mid to late week as a series of frontal boundaries cross near or over the region.

High astronomical tides combined with storm surge up to around 1/2 foot may bring total water levels to near minor flood stage on Saturday night, especially from Portland, ME and points south.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

Through Saturday: Lowering cigs and patchy for likely brings widespread IFR/LIFR early this morning while CON and MHT may stay VFR. Conditions improve away from the coast this morning while onshore flow may keep low stratus and fog near PWM, AUG, and RKD. Showers and thunderstorms this afternoon through this evening will bring potential for TEMPO restrictions across the area. Fog can be expected at coastal terminals tonight after 22z or so. Drier air arriving Sat morning should lead to improving conditions.

Outlook:

Saturday/Saturday Night: VFR conditions expected with W-NW winds of 10-15 kts.

Sunday/Sunday Night: Late day and evening SHRA and TSRA will likely bring localized IFR or worse restrictions at times. S-SE winds of 10-20 kts.

Monday-Tuesday: Mainly VFR conditions likely with isolated -SHRA possible at times.

MARINE

Winds and seas remain below SCA thresholds into Saturday. A moist airmass over the waters will allow for areas of dense fog lasting through tonight before a cold front crosses late tonight and improves vsbys dramatically for Sat.

S-SE winds on Sunday evening and night may approach SCA criteria ahead of a cold front, which will cross over the waters early on Monday. Seas up to 5 ft will also be possible outside of the bays. Otherwise, winds and seas will largely remain below criteria through the period.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Flood Watch until 3 AM EDT Saturday for MEZ007>009-012>014-033. Dense Fog Advisory until 5 AM EDT Saturday for MEZ023>028. Beach Hazards Statement from Saturday morning through Saturday evening for MEZ023>028. NH...Flood Watch until 3 AM EDT Saturday for NHZ004-006. Beach Hazards Statement from Saturday morning through Saturday evening for NHZ014. MARINE...None.