textproduct: Gray - Portland

WHAT HAS CHANGED

Chances for light rain showers have been increased slightly across southern NH for this evening since the previous forecast. However, any accumulations are still expected to remain light.

KEY MESSAGES

1. There will be a few chances for light showers through Friday night, mainly across southern NH, but most of northern New England will remain dry with seasonable temperatures.

2. The chance for showers headlines a majority of the weekend, first along the coast and interior, then more widespread as a frontal system passes through the region.

3. Drier conditions arrive for coastal counties early next week, although clouds and showers remain possible for the foothills and mountains. The next disturbance approaches our area by Thursday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... A weak front will cross Northern New England this afternoon with clouds continuing to increase as it does so. Forecast soundings remain quite dry in the low levels, which should keep most dry, other than maybe a few sprinkles or light showers here and there. The exception will be across southern NH and maybe as far north as Portland where a wave crossing southern New England will bring in more low-level moisture and a slightly better chance for light showers and measurable precip this evening. These same areas may see a couple of additional light showers overnight, but accumulations are likely to remain light and less than a tenth of an inch by daybreak Friday. Elsewhere, skies should thin out behind the front overnight with temperatures a bit cooler than last night. This puts lows mostly in the 50s, but some northern valley could dip into the 40s. More valley fog is expected, especially along the CT River.

High pressure builds in behind the front for Friday, keeping things mostly dry, although an occasional light shower or two can't be discounted along the coast or southern NH during the day and again Friday night due to be in closer proximity to a weak low/surface trough developing south of Long Island. Highs should away from the coast should be in the mid 70s to low 80s, except onshore flow and more clouds could keep southern NH and coastal spots a bit cooler. Lows again are forecast to be mostly in the 50s but could again be in the 40s across the north depending on the clouds.

KEY MESSAGE 2 DESCRIPTION...

A front that initially appears stalled will begin moving northward as a warm front on Saturday, bringing unsettled weather with multiple chances for precipitation as multiple waves of low pressure ride along it. A 500 mb trough approaching from the west is likely to interact with another wave arriving from the south on a low-level southerly flow, likely driving surface low development. Models highlight two potential outcomes for this interaction: either a full phase of the two waves resulting in a potential locally heavy rainfall event, or a split setup with two distinct rainfall maxima that could shift the heaviest totals outside of interior portions of our CWA. While timing discrepancies persist between short-range models showing early arrival and outliers suggesting a much slower progression, most models are anticipating up to three quarters of an inch towards the coast, with localized totals potentially nearing an inch. Temperatures throughout the period are expected to remain right around seasonal for this time of year with mild overnight lows.

As the trough moves across the Great Lakes, SW flow aids in moisture advection into the Northeast. Ensemble probabilities place the greatest IVT just off the New England coast where around 2 inches of PWAT resides. Values aren't expected to be this high over land. Low level moisture will be the most ready to advect into the coast and interior as a disturbance moves into the Gulf of Maine, but residual mid and upper level dry air could limit coverage of showers Saturday. So while forecast rain chances encompass much of the forecast area, the morning and early afternoon are currently not expected be a washout for interior and mountain locations. Dry air will ultimately be fighting a losing battle as the trough moves east. IVT improves, as does PWAT across the interior overnight into Sunday. As clouds thicken, upper low from the Great Lakes will have a broad precip field that moves into NH and western ME Sunday morning. This should again bring intermittent periods of showers, but rainfall totals may be limited by escaping PWATs.

KEY MESSAGE 3 DESCRIPTION...

The 500 mb trough develops a closed low and will keep high- elevation shower chances going into the start of the week. This eventually departs northward into the eastern Canadian Shield by Wednesday.

Temperatures and nighttime lows will remain seasonal for this time of year as precipitation chances wane through the middle of the week from the return of high pressure. The next chance for widespread rainfall returns towards the end of next week.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Through 18Z Friday...Mostly VFR. MHT, CON, and PSM could see -SHRA this evening and maybe into tonight, but these are unlikely to produce any restrictions. Valley fog is again likely to produce IFR or LIFR at LEB and HIE overnight, which is likely to clear by 13Z Friday. VFR Friday morning.

Outlook:

Rest of Friday-Friday night: Mainly VFR. A few light showers possible in southeast NH and the Maine coast, but these are unlikely to produce restrictions. More IFR to LIFR valley fog is possible at HIE and LEB Friday night.

Saturday and Saturday night: MVFR to IFR cigs through the day as SHRA focuses along the coast and portions of the interior. Fog could also develop into the evening and overnight hours, resulting in lowered vis.

Sunday-Sunday night: MVFR to IFR conditions with SHRA expected. Rain may taper off Sunday night giving way to areas of fog that would likely bring about IFR conditions.

Monday: Conditions return to VFR, but there may still be some MVFR and/or IFR conditions along the US/CAN border.

MARINE

Through Friday night...Conditions are expected to remain below SCA levels. A weak front crosses the water this evening, switching winds around to light and mostly northerly. High pressure builds across New England Friday, and then winds become more east to northeast late Friday into Friday night as a weak low or surface trough starts to lift north from the DELMARVA region, shifting the high pressure center northeast.

Weak low pressure may track into the Gulf of Maine Saturday, with fog developing overnight into Sunday. Broad southerly flow continues into Sunday as large low pressure exits the Great Lakes and begins to move north into Quebec. Winds are expected to remain below SCA criteria, but wind wave slowly increases into the beginning of next week 3 to 4 ft.

GYX WATCHES/WARNINGS/ADVISORIES

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