textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes were needed for this forecast package.

KEY MESSAGES

1. A frontal system will lead to increasing chances for some showers and storms late this afternoon into tonight, especially across the north.

2. Cooler weather and relatively low dew points are expected from Tuesday through the end of the the week. The best chance of showers and thunderstorms will be late Wednesday into Thursday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A warm front will lift north into the forecast area today, chasing out transient high pressure to the east. A surface low will then eventually move into south central Quebec by this evening with a cold front extending to it's southwest. Most guidance suggests that we will see isolated to scattered showers and storms along and ahead of this frontal boundary as it approaches.

Most of the CAMs are suggesting that storms will be ongoing along the front across Vermont in the afternoon. Chances for showers and thunderstorms will then be on the increase over New Hampshire late afternoon and into the evening as this activity approaches. While we will have some instability to work with, timing will not be diurnally optimal and storms may be decaying as they approach. Still, one or two storms could be on the stronger side with gusty winds and locally heavy downpours. The best synoptic forcing for ascent will generally be across northern portions of our forecast area so northern New Hampshire, the western Maine mountains, and the Kennebec River Valley are most favored for the best coverage of showers and storms late today/tonight.

The front should then move out fairly quickly tonight with only some lingering showers over the far north through early Tuesday morning.

Regarding temperatures in the short term, it will be another very warm day today with highs ranging from the upper 70s/lower 80s north, to the low to mid 80s elsewhere. A few upper 80s will also be possible across southeast New Hampshire and coastal Maine. That being said, humidity will take another step down so the heat should be a bit more comfortable. Tonight's lows will mainly range from the mid/upper 50s north, to the low/mid 60s south.

KEY MESSAGE 2 DESCRIPTION... Broad troughing dominates the 500 mb pattern into the first part of the weekend with the main forecast challenge being the track of low pressure, and potentially an upper low, late in the week.

Flow becomes more zonal Tuesday into Wednesday as a broad trough sets up over the Northeast. This will help to cool dewpoints, but high temperatures are still going to be able to climb into the mid to upper 80s south of the mountains as 850 mb temperatures remain around +15 to 16C. North of the mountains should begin to see some of the cooler air advection with temperatures topping out in the mid to upper 70s. These days should be mostly dry as well with the exception being the possibility of upslope showers Tuesday as low pressure forms over the Gulf of St. Lawrence.

Precipitation chances than increase again Wednesday night into Thursday as the trough axis begins to approach the region. This is going to be the next best chance for widespread showers and thunderstorms, but the finer details are uncertain at this time. The Euro Ensemble suite has a tight cluster suggesting surface low pressure forming over Quebec in the St. Lawrence River Region, while the GFS and Canadian are more clustered over the Gulf of St. Lawrence. The Euro also has an attendant upper low which would help to intensify things, where as the the GFS and Canadian currently keep an open wave. The bottom line is that all these solutions at least suggest showers and storms, so that is high confidence. The lower confidence is in coverage and magnitude of the showers and storms. This will have to continue to be refined.

There is some uncertainty going into the first part of the weekend as depending on how the midweek event plays out there could still be low pressure in the vicinity, but there is higher confidence in general improvement as the week comes to a close. Global models are in agreement that high pressure begins to build in by Saturday and with the trough axis situated to the east, northwesterly flow will be bringing a cooler and drier airmass so the weekend could be quite a stunner with highs in the 70s and low 80s under mostly clear skies.

AVIATION /06Z MONDAY THROUGH FRIDAY/

Through 12z Tuesday...VFR prevails at most terminals through this afternoon, except HIE and LEB where we could see some valley fog develop this morning and again tonight. Fog should lift my mid morning each day. Showers and storms likely bring brief restrictions late this afternoon into tonight, with the best chance for these at LEB, HIE, and AUG. VFR then returns by late tonight outside of any valley fog.

Outlook: Tuesday-Tuesday night: VFR conditions with some late night valley fog possible.

Wednesday-Thursday: Cigs and vsbys VFR Wednesday possibly lowering to MVFR in showers and thunderstorms Wednesday night into Thursday.

Friday: VFR expected.

MARINE

Another front brings a chance for storms this evening and into the overnight hours. Fair winds and seas continue outside of any storms through tonight. Winds and seas will largely remain below SCA criteria through the week as a series of generally weak cold fronts cross the waters.

GYX WATCHES/WARNINGS/ADVISORIES

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