textproduct: Gray - Portland

WHAT HAS CHANGED

Loaded in the latest surface observations and adjusted the aviation section for the 12Z TAF package. Otherwise, no significant changes were made at this time.

KEY MESSAGES

1. There is a Slight Risk for severe storms across SW New Hampshire with a Marginal Risk for the rest of New Hampshire extending into western Maine today.

2. A cold front crosses New England on Sunday, bringing showers/isolated thunder and a relatively cooler airmass.

3. High pressure provides fair weather Monday into mid week with a warming trend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Mid level flow pattern will amplify this morning as a trough sharpens over the eastern Great Lakes and a surface low deepens over southern Quebec. A warm front will extend southeast from the low across Maine providing a focus for clouds and showers this morning into early afternoon. Latest available CAM guidance suggests this front will make little progress eastward through the afternoon limiting high temperatures to the upper 60s and low 70s. This will also limit instability east of Cumberland and Oxford County.

The low will track along the St Lawrence Valley this afternoon with the surface cold front lagging back through the eastern Great Lakes. Moisture advection ahead of the cold front will push dewpoints into the low to mid 60s across New Hampshire into far western Maine. The question still remains how much clearing occurs early this afternoon before a pre frontal trough provides forcing for storms. The HREF mean SB CAPE tops out around 1000 J/kg across SW NH while the max tops out around 2000 J/kg with this better instability extending into western Maine. The approaching trough will aid in favorable deep layer shear up to 40 kts that will support organized updrafts. Overall, no notable trends have emerged from previous forecast in the severe threat. CAMs suggest semi discrete cells will be possible across NH by around 4 PM transitioning to small line segments that will push into far western Maine this evening. Damaging winds will be the primary threat while any strong updrafts will also be capable of hail as mid level lapse rates approach 6.5C/km. Heights will continue to fall into tonight that may allow for convection to continue after the loss of surface heating while the severe threat will gradually diminish late this evening.

KEY MESSAGE 2 DESCRIPTION...

By Sunday morning, the surface low will be centered on the Maine/Quebec border, then gradually drift southeast into the Gulf of Maine. This drags a surface cold front through the region by afternoon, with winds turning northerly behind it and ushering in cold air advection. As such we should see highs late morning/midday (60s north, 70s south), then falling during the afternoon hours.

Still keeping an eye on the thunder potential, but since yesterday hi-res guidance has backed off in coverage, with most guidance indicating just showers/light rain over the region. Do have a pocket of colder air aloft, but does not look nearly as robust as last Sunday's cold pool aloft event. For example, freezing levels will be closer to 700 mb than the 850 mb we saw a week prior. Lapse rates through the column will be hovering around 6.0 to 7.5 C/Km, which should be enough to generate a few hundred J/Kg of CAPE. However, shear profiles will be rather wonky along/behind the front, which should limit storm organization. Therefore, just expecting scattered garden variety downpours/thunder, with the greatest spatial coverage during the afternoon hours.

KEY MESSAGE 3 DESCRIPTION...

Height rises begin on Monday with surface high pressure cresting over the region. Steady offshore flow may limit any sea breeze potential with temperatures rather uniform across the area in the upper 70s to low 80s. Heights continue to build over the Northeast Tuesday into Wednesday for a warming trend with Wednesday looking like much of the area will see highs in the 90s.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Through 12Z Sunday...Mainly VFR through this morning. Clouds thicken and lower later this morning into the afternoon. MVFR cigs are possible at PWM, RKD, AUG, and HIE in the early afternoon with SHRA. Shower coverage increases through the afternoon into the evening with scattered TSRA. Locally strong thunderstorms are likely between 20Z-02Z Sunday with strong winds and GR. The highest threat for this is at KLEB, KMHT, KCON, and KPSM. Conditions will improve later tonight but patchy FG may bring additional restrictions of IFR or worse.

Outlook:

Sunday: Scattered showers and a few thunderstorms with periods of MVFR to IFR possible.

Sunday night - Thursday: Mainly VFR prevails, but some nighttime valley fog will be possible at HIE and LEB each night.

MARINE

Winds and seas remain below SCA thresholds through this evening. Thunderstorms will develop late afternoon through the evening and will be capable of tracking into the waters with gusty winds or small hail possible near shore. The threat for thunder decreases north of Casco Bay.

A cold front approaches late tonight through Sunday morning. South to SW flow will freshen ahead of the front with gusts around 20 kts. Winds turn NW behind the front Sunday afternoon and N Sunday night with gusts 20-25 kts. High pressure builds in Monday through the middle of next week for quiet conditions on the waters.

GYX WATCHES/WARNINGS/ADVISORIES

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