textproduct: Gray - Portland

WHAT HAS CHANGED

Heat advisory in southern NH has been cancelled. Isolated showers and perhaps a thunderstorm remain possible early this evening but should dissipate around sunset. A much drier and cooler night is expected.

KEY MESSAGES

1. Showers and thunderstorms exit to the south this evening with more comfortable overnight low temperatures expected areawide.

2. An area of rain will slowly move northward Monday afternoon through Tuesday. In addition, this will also bring cooler temperatures to begin the week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A cold front continues to push through the forecast area north to south this afternoon. Ahead, RAP sfc CAPE 500 to 1500 j/kg is fueling showers and thunderstorms. The overlap of moisture and forcing is greatest across southern NH, with rounds of showers and storms expected to continue through early evening. A few storms may be strong, with gusty outflow winds and heavy downpours. One such storm went through Concord and then the NH Seacoast with gusts to 45 mph.

Behind the cold front, more stable and cooler air arrives, shutting off convection. Overnight lows dip into the upper 50s to lower 60s tonight, welcome relief after this stretch of heat. Could see some pockets of fog develop overnight, especially where rain fell through the afternoon. Otherwise, the lower dewpoints may inhibit much widespread fog development.

Sunday will be a couple degrees cooler than Saturday, with highs in the low to mid 80s. RH values will run about 30 to 40 percent, so it will feel less humid. Clouds begin to thicken in the evening and overnight into Monday as the next rain system approaches.

KEY MESSAGE 2 DESCRIPTION...

Low pressure shifts east towards the Mid-Atlantic Monday, advecting moisture back into central to northern New England. The low is broad, and not very deep. This results in a wide influence amid otherwise stagnant moisture. Guidance tries to take the center through the Delmarva region and south of Cape Cod through Tuesday.

This track would support a rain shield pushing north into southern NH and southern ME Monday afternoon and much of Tuesday. Expansive and thick cloud cover would also support mild temperatures perhaps with highs only around the upper 70s to around 80 Mon/Tues.

There remains some timing differences as well as coverage amid guidance flavors, but consensus was enough to include likely rain mention for southern NH. If some of the further south solutions verify, that could mean points towards central ME see a bump in temperatures Mon/Tues under thinner clouds and less precipitation.

NBM 50th percentile QPF shows a tight precip gradient, focused across southern to central NH. This indicates some of the spread still in contention, but also confidence in that region being the only spot to see much measurable precipitation. A half to three quarters of an inch of rain is possible for the southern tier of NH counties through Tuesday evening.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

Through 18z Sunday...SHRA with occasional TS continues for southern NH terminals through 00 to 02z. Occasional gusts 35 to 40 kt have been measured out of the stronger cells, but these should remain fairly isolated. These heavy downpours have produced occasional IFR vis. Areas north of SFM to LCI seem to be in more stable air now, and significant convection here has ended. VFR is expected tonight, but could see some FG develop in valley locations, especially where rain fell this afternoon (LEB in mind). VFR Sunday, with slight chance of -SHRA towards AUG in the afternoon.

Outlook:

Monday: Increasing rain chances and clouds south to north in the afternoon. Patchy MVFR cigs across southern NH possible, otherwise VFR.

Monday Night: Continued rain chances across southern NH. MVFR possible.

Tuesday: Rain departs east late, with improving cig in the late afternoon. VFR likely.

Wednesday: VFR likely under high pressure in the region.

Thursday: SHRA arrives from the NW, otherwise VFR.

MARINE

Below SCA conditions are anticipated through midweek. A storm or two could bring locally gusty winds this afternoon ahead of a cold front. This front will pass tonight, shifting winds offshore. Weak wind fields will promote some onshore breeze Sunday and Monday as broad low pressure moves east from the Ohio Valley and passes towards the Delmarva region Tues. Winds then shift east Tues, with building waves into Wed 4 to 5 ft.

GYX WATCHES/WARNINGS/ADVISORIES

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