textproduct: Gray - Portland

WHAT HAS CHANGED

Have increased PoPs over the next couple of hours in western NH to account for showers developing there.

KEY MESSAGES

1. Humid conditions persist into today bringing at least a moderate heat risk. Heat Advisories are in effect this afternoon across south-central NH and interior western Maine.

2. Scattered showers and thunderstorms this afternoon and evening may be strong.

3. A cold front will cross the region Sunday and bring the threat for some strong thunderstorms.

4. High astronomical tides this weekend. A return to seasonable weather is expected for much of the work week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A humid air mass will remain in place through this evening with dewpoints in the upper 60s to low 70s. There have been no changes to the heat advisory that was issued yesterday. There is some uncertainty in cloud cover, especially on the coastal plain which along with developing east winds could knock temperatures down to levels lower than forecast. Early morning fog could be locally dense as well, and could affect the morning commute in some areas.

KEY MESSAGE 2 DESCRIPTION...

As per latest CAMs, widely scattered thunderstorms are expected to develop during the afternoon today, with a somewhat greater threat for more widespread activity during the early evening hours when modest forcing for ascent arrives in association with an approaching cold front. This may aid in better coverage of thunderstorms in the midst of an environment characterized my marginally moderate CAPE and deep layer shear that will be fairly weak. Not expecting widespread strong to severe thunderstorms, but one or two cannot be ruled out late in the day or early in the evening with torrential downpours and gusty winds being the main threats. The threat should end by around midnight. The coast should begin to experience fog during the evening which could be locally dense.

The cold front ushers in much drier air for Saturday. While highs are expected to be well into the 80s (90 is possible far south) the heat index will be lower than the actual temperatures. Dry weather is expected.

KEY MESSAGE 3 DESCRIPTION...

Sunday will be the last of the hot days for parts of the forecast area. We should be able to make another run at the low 90s for southern parts of the forecast area. As things stand currently, there is enough dry air aloft and enough mixing that dewpoints look to remain around 60. So I do not anticipate significantly higher heat index values.

By Sun afternoon though models are already hinting at a pre-frontal trof locally, while the main trof/cold front hangs back across western/central NY. So from about midday onward, I cannot really rule out convection. Machine learning severe weather guidance indicates that something akin to a marginal risk is possible. Coupled with a slight risk from SPC lurking nearby, I certainly have my eyes on the possibility of severe weather somewhere in the forecast area. Sampling a few forecast soundings, there may be pretty healthy low level CAPE, which is typically all we need to produce isolated severe weather. Especially considering that shear should be adequate for storm organization. I'm comfortable leaving whatever thunderstorms chances the NBM guidance has, with details to be refined once the event moves into the hi-res CAM window by this afternoon.

KEY MESSAGE 4 DESCRIPTION...

Fairly good model consensus on temps returning to near normal following the cold frontal passage on Sun. Largely looking at highs ranging 75 to 85 degrees, with nighttime temps falling back into the 50s to low 60s most of the time. Broad high pressure will be centered well southwest of the region, and it likely will not be enough to rule out rain showers completely. Especially towards the mid to latter half of the week diurnal convection will become more likely, but certainly not organized. That will have to wait until Fri or Sat, when a large upper low/trof is forecast to cross the Northeast. Showers/thunderstorms along that cold front should be fairly numerous, and will represent our next chance for widespread rainfall.

High astronomical tides combined with storm surge up to around 1/2 foot may bring total water levels to near minor flood stage on Saturday night, especially from Portland, ME and points south.

AVIATION /09Z FRIDAY THROUGH TUESDAY/

Through Saturday: Lowering cigs and patchy for likely brings widespread IFR/LIFR early this morning while CON and MHT may stay VFR. Conditions improve away from the coast this morning while onshore flow may keep low stratus and fog near PWM, AUG, and RKD. Showers and thunderstorms this afternoon through this evening will bring potential for TEMPO restrictions across the area. Fog can be expected at coastal terminals tonight after 22z or so. Drier air arriving Sat morning should lead to improving conditions.

Outlook:

Saturday/Saturday Night: VFR conditions expected with W-NW winds of 10-15 kts.

Sunday/Sunday Night: Late day and evening SHRA and TSRA will likely bring localized IFR or worse restrictions at times. S-SE winds of 10-20 kts.

Monday-Tuesday: Mainly VFR conditions likely with isolated -SHRA possible at times.

MARINE

Winds and seas remain below SCA thresholds into Saturday. A moist airmass over the waters will allow for areas of dense fog lasting through tonight before a cold front crosses late tonight and improves vsbys dramatically for Sat.

S-SE winds on Sunday evening and night may approach SCA criteria ahead of a cold front, which will cross over the waters early on Monday. Seas up to 5 ft will also be possible outside of the bays. Otherwise, winds and seas will largely remain below criteria through the period.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Heat Advisory from noon today to 8 PM EDT this evening for MEZ012-018-019-033. NH...Heat Advisory from noon today to 8 PM EDT this evening for NHZ004>013-015. MARINE...None.