textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes were made to the forecast at this time. Loaded in the latest surface observations and adjusted the aviation section for the 12Z TAFs.

KEY MESSAGES

1. Rain will spread from west to east through the morning. After a break in precipitation, chances for showers and thunderstorms increase this afternoon and early evening, some possibly strong to severe.

2. Temperatures will take on an upward trend this weekend with heat and humidity building towards the middle of next week. There will be chances for showers and storms most days, mainly in the afternoons, while no one day looks like a washout.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A low will approach from the Great Lakes this morning with a warm front extending to the east southeast and a cold front extending south southwest. As this warm front lifts north across the region, we will see extensive rainfall spread from west to east over New Hampshire and western Maine this morning. While we could see some pockets of locally heavy rainfall and some orographic influences, the progressive nature of this system precludes any major flooding concerns for now. Most locations will likely see a quarter of an inch to three quarters of an inch, but some isolated pockets in the one to two inch range aren't out of the question. The latest HREF 6-hour max QPF suggests areas that could see some of these higher totals will be along the Connecticut River Valley and southwest New Hampshire.

This activity will gradually weaken as it moves away from the better synoptic forcing over western Maine later this morning. While some clouds and light showers/mist may hang around some locations in the early to mid afternoon, we should see some peaks of sunshine break out across portions of New Hampshire and western Maine. The more sunshine we see, the more surface based instability by the afternoon hours. Forecast soundings from various recent CAMs suggest MLCAPE up to 1000 J/kg and SBCAPE up to 1500 J/kg. Effective layer shear is generally forecast to be around 40 knots with long straight hodographs. Forecast 0-6 km shear vectors still have a significant perpendicular component to the cold front, so think that storms will start out as discrete/semi-discrete but could get messy rather quickly as the unidirectional profile aloft suggests splitting storms and several interactions. Even with a messy storm mode, the parameter space will be sufficient for at least some isolated strong to severe storms. Large hail and damaging winds will the primary threats with any sustained rotating updrafts, but a conditional weak tornado threat may not be out of the question near the southern New Hampshire/Maine border where low level shear is forecast to be better with some backed surface winds. Quite a bit of uncertainty remains at how far east strong/severe storms will make it as we stabilize towards the coast and towards sunset but showers and storms will diminish in the evening, giving way to a mostly dry overnight period.

For temperatures today, expect afternoon highs ranging from the mid to upper 60s across the north country, to the low/mid 70s most elsewhere. The one exception will be southern New Hampshire where we will see some more sunshine and highs closer to the upper 70s and lower 80s. Tonight's lows will mainly be in the 50s.

KEY MESSAGE 2 DESCRIPTION...

Cyclonic flow aloft will persist over northern New England Saturday and Sunday before ridging starts to push into the Ohio Valley allowing flow aloft to become more northwesterly. The cyclonic flow combined with surface heating will allow for chances for showers and storms to increase into both Saturday and Sunday afternoons. Latest NAM and RRFS suggest there may be enough shear and instability for a few strong to severe storms both afternoons, while machine learning guidance out of CSU does not of a signal for severe storms. Highs on Saturday will mainly be in the upper 70s climbing into the 80s on Sunday.

Northwest flow aloft will promote high pressure at the surface over the Northeast for a mostly dry day with highs well into the 80s. Global models and their ensembles suggest ridging will persist over the Ohio Valley with building heat and humidity beneath the ridge. It remains in question how much heat and humidity can push into northern New England with the axis remaining to the west. There is a consistent signal that remnant convection will roll over the top of the ridge bringing a return to chances for showers and storms towards mid week. Overall, ensembles favor building heat and humidity and potential for heat risk as temperatures could climb into the 90s with dewpoints near 70 degrees.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

Through 12Z Saturday...For this morning, expect lowering ceilings as a round of rain moves through. Ceilings could range from LIFR to MVFR with visibility from rain mainly MVFR. Outside of rain, fog could produce locally lower visibilities, primarily at RKD and AUG.

Rain gradually ends from west to east this morning with improving visibilities. Ceilings will also lift, especially for the NH terminals with a return to VFR possible. IFR to MVFR restrictions will remain possible through the morning hours at the Maine terminals before lifting in the afternoon, although KRKD may remain at MVFR-IFR through much of the day.

Isolated to scattered thunderstorms will then possible this afternoon at all terminals with the exception of KRKD. A few strong to severe thunderstorms will be possible with gusty/erratic winds and potentially some hail under the heaviest cores. Storms may lead to brief MVFR/IFR restrictions. Conditions should start to improve this evening and into the night time hours. Patchy FG will be possible later tonight, bringing additional restrictions.

Outlook:

Saturday through Sunday night: Mainly VFR outside of scattered afternoon showers and storms. Valley fog at night will be possible.

Monday through Tuesday night: VFR with no sig wx.

Wednesday: Chances for showers and storms will bring potential for restrictions.

MARINE

SCA conditions are not expected through tonight. High pressure to the east of the waters and approaching low pressure will keep southerly flow in place across the waters through the day. Flow then shifts more northwesterly or northerly tonight.

As the low approaches, fog will be possible over parts of the waters through the daytime hours today, possibly lingering into tonight. Rain is likely this morning, and then offshore- moving thunderstorms are possible late this afternoon and into the early evening hours, especially from Casco Bay southward to the Seacoast.

Winds and seas likely remain below SCA thresholds this weekend into the middle of next week.

GYX WATCHES/WARNINGS/ADVISORIES

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