textproduct: Gray - Portland

WHAT HAS CHANGED

Little change to the going forecast this Saturday morning. A couple of nice days weatherwise expected this weekend with dry weather.

KEY MESSAGES

1. Fair weather is expected through Sunday as high pressure moves overhead and settles across the region.

2. Hot and humid conditions return by Tuesday, accompanied by a chance for severe weather. Above average temperatures likely continue the rest of the week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

With the passage of a weakening cold front early this morning, broad and strong high pressure takes up residence across the Great Lakes and the Northeast through the weekend. The 1020 to 1023 reading is strong, but won't be setting climo records for this time of year.

The high's presence will bring fair conditions to the region with plenty of sun and daytime cumulus. Despite the mixed layer deepening to around 750mb, there is little momentum to mix to the surface, so days should feature gentle breeze. This will likely result in a daily sea breeze in the early afternoon for points along the coast.

Forecast highs will run at or within a couple degrees of normal for the weekend, mainly in the low to mid 80s. The lack of moisture advection into the region will provide dry low levels and large daytime dewpoint depressions. Thus while it will be warm, higher humidity values should hold off until early next week.

Overnight lows in the 50s to around 60 will bring overnight relief, with the chance some areas dip into the upper 40s. The lack of much cloud cover and light winds should bring these chances from the Whites to parts of the foothills.

KEY MESSAGE 2 DESCRIPTION...

Will start the week with an anomalous 500 mb ridge over the Upper Midwest/Northern Plains. By midweek, a trough originating near Hudson Bay will strengthen while dropping south towards Quebec. This places New England on the northeast periphery of the ridge. The gradient between the trough and ridge is also likely to induce a jet streak and surface trough development by Tuesday-Wednesday. The orientation of these features suggests a period of northwest flow aloft.

The first half of the week is looking quite hot, with Tuesday coming into focus as the day of greatest heat impact. Seeing some signs of an anomalously hot airmass: The NAEFS situational awareness table is indicating both 700 mb and 850 mb temperatures will be near the 99th percentile. This translates to 850 mb temperatures of 20-23 C, which is advertised on both deterministic and ensemble guidance. Right now the forecast is for highs in the upper 80s to mid 90s from Monday- Wednesday, but there is certainly the potential for even hotter, possible near record breaking temperatures Tuesday if we can achieve deep mixing with such an anomalously warm column. A southwest wind should preclude cooler onshore flow outside of the Midcoast. The one saving grace will be more tolerable dew points, which are likely to be in the 60s rather than 70s based on current guidance.

The other headline for early next week will be a severe thunderstorm risk as the attendant trough swings through our area, which right now is timed to be late Tuesday afternoon-Tuesday night (still subject to change given we are four days out). The feature of particular interest for this setup is a potential elevated mixed layer (EML) moving over the region; steep mid- level lapse rates on the order of 7-8 C/Km. In addition, guidance is in agreement on rather robust wind fields/shear, both across the 0-1 km and 0-6 km levels. Will have to see how it all times out but the ingredients are certainly on the table for a potential higher-end severe weather event Tuesday evening/night. SPC has introduced a Day 5 15% severe risk for the the majority of New Hampshire and Maine, generally north of a Concord-Portland-Rockland line.

Guidance is in agreement on the front clearing south of our region by Wednesday morning. This brings a sharp drop in dew points into the 40s-50s through the second half of the week. However, with a still warm airmass aloft and some downsloping northwest winds, temperatures still look above average, possibly near 90 in the warm spots.

AVIATION /10Z SATURDAY THROUGH WEDNESDAY/

Through 6z Sunday...VFR is expected to prevail today. Other than early morning VCFG near LEB, only expect a light cu field and weak background flow giving way to local wind influences. This is mainly via a seabreeze along the ME/NH coast. This should result in a SSE wind shift early afternoon for PSM/PWM/RKD.

Outlook:

Tonight night-Sunday night: VFR other than possible late night or early morning valley fog at HIE and LEB.

Monday: VFR expected.

Tuesday-Wednesday: A cold front Tues night into Wed AM brings increasing chances for showers and thunderstorms. Timing and extent of restrictions is uncertain, but MVFR or IFR will be possible over parts of the region in TS.

MARINE

High pressure building over the waters will keep conditions below SCA criteria through the weekend. Winds remain generally northerly through Sunday, but afternoon sea breezes are anticipated as well.

Monday-Friday...High pressure moves out in the western Atlantic early next week as a cold front approaches. South to southwest flow increases Monday and Tuesday, potentially bringing SCA conditions. The front looks to cross the waters sometime in the late Tuesday to early Wednesday timeframe with winds turning west to northwest. Low pressure then looks to stay in the vicinity of the Canadian Maritimes, keeping winds mainly out of the west through late next week, although a couple of weak front could cross the waters during that time.

GYX WATCHES/WARNINGS/ADVISORIES

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