textproduct: Gray - Portland

WHAT HAS CHANGED

A Heat Advisory has been issued for south-central NH and interior western Maine for Friday afternoon.

KEY MESSAGES

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

2. Isolated thunderstorms will be capable of localized heavy rainfall through this evening. Scattered thunderstorms Friday will bring the threat for localized flash flooding across ME and NH. Storms across NH may become strong to severe Friday evening.

3. A cold front moves through the area Sunday night, bringing another round of showers and thunderstorms. More seasonable temperatures will return next week with scattered showers possible at times.

4. High astronomical tides this weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A humid air mass will remain in place through Friday evening with dewpoints in the upper 60s to low 70s. Temperatures aloft will increase slightly from today with T8s approaching +20C. A surface trough through the area will provide a focus for early afternoon clouds that may limit high temperatures from reaching their full potential. The slight increase in temperatures aloft should allow for most spots in south-central NH and interior western Maine to reach the low 90s for highs with heat indices of 95-100F. Therefore, have issued a Heat Advisory for these areas while much of the area will still experience very humid conditions.

KEY MESSAGE 2 DESCRIPTION...

A moisture rich airmass is in place this afternoon with dew points in the upper 60s to low 70s and PWATs nearing 2 inches. Weak waves moving through a mid level ridge over the Northeast combined with a surface trough will allow for continued chances for scattered showers and isolated thunderstorms into this evening. The high PWATs combined with warm cloud depths around 12kFT will allow for heavy rainfall rates in excess of 1 inch per hour in the stronger cells. Recent runs of hi res guidance have been focusing activity across the western Maine mountains through central Maine and a second area across SW NH. Latest 1-hour flash flood guidance in these areas is generally around 2 inches with 3-hour flash flood guidance around 2.5-3 inches. Flow aloft is weak, although guidance suggest storms will somewhat transient enough that the risk for flash flooding is low but cannot be ruled out. Showers and storms will dissipate this evening with patchy fog likely tonight.

A hot and humid airmass will remain in place through Friday as a cold front approaches from the west Friday evening. The 12Z HREF brings an axis of MU CAPE of 1500-2000 J/kj across NH into western Maine while deep layer shear will be somewhat lacking around 25 kts. HREF memebers have been suggesting storms could initiate Friday afternoon along a surface trough from the Lakes Region of NH through the western Maine mountains into central Maine. The lack of shear with these storms suggest they will struggle to reach severe thresholds while high PWATs and deep warm cloud depths will support heavy rainfall rates. Low MBE Velocity Vectors over central Maine also suggest there will be potential for backbuilding storms. The 12Z HREF max 3-hour rates does bring upwards of 3-4 inches across Carrol County and central Maine... both of which may be more sensitive to heavy rainfall from last night's heavy rain in Carrol County and today's rain in central Maine. Therefore these areas will need to be closely watched for potential flash flooding Friday afternoon. This activity along the surface trough should wane into the evening when attention then turns to convection ariving with the approaching cold front.

CAMs are in general agreement that storms associated with the approaching cold front will reach the CT Valley around 8 PM Friday. Height falls along with a moist boundary layer will still allow for MU CAPE to hang around 1000-1500 J/kg per the HREF mean with deep layer shear increasing to around 30 kts. This will allow for storms to approach severe thresholds bringing mainly a damaging wind threat while steepening mid level lapse rates suggest severe hail cannot be ruled out. These storms will push across NH into western Maine through midnight and will likely weaken as they do so. Will need to watch for these storms impacting areas that received heavy rainfall during the afternoon as PWATs remain high and heavy rainfall rates will be possible. The cold front pushes through the area after midnight for drying trend west to east into Saturday morning.

KEY MESSAGE 3 DESCRIPTION...

Surface high pressure will be located to our southwest on Saturday, allowing for mainly sunny skies with west-northwesterly winds. High temperatures will range from the lower 80s across the north with upper 80s to near 90 degrees south. Dewpoints will fall during the afternoon and therefore heat indices should not differ too significantly from actual air temperatures. Clouds will gradually increase Saturday night with low temperatures into the 50s to lower 60s.

Skies will become mainly cloudy by late Sunday ahead of a cold front that will be approaching from the west. A few scattered showers and thunderstorms are possible late in the day, especially across northern locations as well as across western NH. Depending on thermodynamic profiles, a few storms could be strong with gusty winds and hail. Highs will be into the 80s to lower 90s from north to south. The cold front will then cross Sunday night, bringing widespread showers and thunderstorms. Monday and Tuesday look to feature mainly dry conditions along with lowering dewpoints and near seasonable temperatures. The middle to end of next week looks to be more unsettled with nearby frontal boundaries combined with daytime heating resulting in increased shower and thunderstorm probabilities.

KEY MESSAGE 4 DESCRIPTION...

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 /18Z THURSDAY THROUGH TUESDAY/

Through Friday night: Cloud bases this afternoon around may bring occasional MVFR while VFR likely prevails into the evening. Lowering cigs and patchy for likely brings widespread IFR/LIFR tonight while CON and MHT may stay VFR. Conditions improve away from the coast Friday morning while onshore flow may keep low stratus and fog near PWM, AUG, and RKD. Showers and thunderstorms Friday afternoon through Friday evening will bring potential for TEMPO restrictions across the area. Fog cannot be ruled out Friday night while drier air arriving overnight should lead to improving conditions into Saturday morning.

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 tonight lasting through Friday evening before a cold front crosses Friday night.

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 to 8 PM EDT Friday for MEZ012-018-019- 033. NH...Heat Advisory from noon to 8 PM EDT Friday for NHZ004>013-015. Heat Advisory until 8 PM EDT this evening for NHZ005-007>013- 015. Air Quality Alert until 10 PM EDT this evening for NHZ014. MARINE...None.