textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast update.

KEY MESSAGES

1. Showers and thunderstorms bring the threat of locally heavy rainfall late today through tonight.

2. A few strong to severe thunderstorms are possible Wednesday afternoon. High pressure then brings a period of mostly dry weather through the weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Low pressure tracking east into Quebec will lift a warm towards the forecast area late this morning into the afternoon. As a result clouds will thicken and lower from SW to NE with mostly cloudy skies across much of the area by this evening. Latest available CAM guidance suggest the warm front will stay south of the area through peak heating limiting the atmosphere's ability to destabilize. As stronger winds at 850 mb arrive from the SW late this afternoon, CAMs suggest the warm front make will some progress through SW NH where upwards of 500 J/kg of MU CAPE may develop. Shower chances will increase from west to east this afternoon with thunderstorm chances increasing across SW NH this evening. CAMs are bearish on any storms developing robust updrafts while a few storms bringing the potential for gusty winds cannot be ruled out.

The warm front will make northward progress through tonight with a moisture rich airmass overspreading the area. PWATs will climb to 1.75 to 2.0 inches with portions of southern NH and coastal Maine possibly seeing PWATs over 2.0 inches. These high PWATs combined with deep warm cloud depths and pockets of elevated instability will bring potential for torrential downpours. CAMs show swift moving storms capable of rainfall rates of 1-2 inches per hour with localized total amounts in excess of 3 inches. However, CAMs are variable in the location of these higher amounts and are generally favored south NH/MA border with much of the area likely to see 0.5 inches or less. At this time southern NH into coastal Maine see some potential for isolated instances of flash flooding overnight with the flood risk lowering to the north. The time window for locally heavy rainfall will be from 8 PM until 4 AM Wednesday morning with rain tapering off west to east into daybreak.

KEY MESSAGE 2 DESCRIPTION...

There will be additional chances for thunderstorms Wednesday. New England will remain in the warm sector. The upper- level low over Quebec will begin to vertically stack, bringing height falls and cooling temperatures aloft. As such, a bit of a "cold pool aloft" type of setup as thermodynamics show low freezing levels and plentiful 3CAPE.

It remains to be seen how much instability we can generate; which will be contingent on getting some sunny breaks during the morning hours. Some guidance shows an abundance of morning storms, which could still be strong given the cold pool aloft, but would likely result in lower peak instability. 18Z/0Z hi-res guidance is advertising 1000-3000 J/Kg of SBCAPE by early afternoon, depending on model of choice. Consensus on dew points holding in the upper 60s/low 70s adds confidence in an unstable environment Wednesday.

Expecting weak 0-1 km shear but strong 0-6 km shear, as the 850 mb low-level jet will have already pushed offshore. Very limited directional shear is noted.

One of the biggest uncertainties (and likely limitations) from a higher-end event will be a dry punch aloft, which may serve hostile for updrafts. Combined with a cellular storm mode (owing to a lack of strong convergence), likely will see storms posing a damaging wind and hail risk, but also the potential for some areas to see next to nothing for various caveats mentioned above. For what it is worth, both CSU and CIPS are rather bullish for Wednesday, with a 15% severe risk highlighted on both products. Current day 3 Marginal from SPC.

Should generally have near seasonable temperatures this upcoming week. Strong cold front Wednesday night should really dry things out with dew points in the 40s-50s for the remainder of the week and rain-free conditions as the cold front clears offshore.

AVIATION /01Z TUESDAY THROUGH SATURDAY/

Through Tonight...VFR prevails this morning with clouds increasing SW to NE this afternoon. MVFR cigs likely develop across southern NH early this evening and spread northeastward through AUG by 00Z. Cigs continue to lower tonight to IFR with visibility restrictions likely in rain, locally heavy at times. LLWS likely on the coastal plain and across southern ME during the evening hours.

Outlook...

Wednesday: Low cigs likely lift and scattered mid morning into the afternoon. Scattered TSRA bring potential for localized restrictions during the afternoon and evening.

Wednesday Night-Saturday: Mainly VFR, no sig wx.

MARINE

A warm front approaches from the SW today with SE winds increasing this afternoon with a few gusts to 25 kts. Winds and seas will bring SCA conditions tonight with localized bouts of torrential rainfall possible over the waters. South winds and elevated seas will continue SCA conditions through Wednesday. Winds ease Wednesday night while seas remain elevated into Thursday. Highs pressure allows for light winds and lowering seas Friday into the weekend.

GYX WATCHES/WARNINGS/ADVISORIES

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