textproduct: Gray - Portland

WHAT HAS CHANGED

Strong to severe storms didn't really materialize last night and this morning like some of the short term models were suggesting. There will be another chance late today and into Thursday morning, but the short term models have started backing off on that potential as well. We will have to see if that trend holds while we also deal with the heatwave.

KEY MESSAGES

1. Today will be the start of a prolonged heat event through at least Friday across northern New England. The heat peaks on Thursday, with only a gradual cool down expected into the weekend. Nighttime lows in the 70s offer little relief, and result in accumulated heat stress.

2. Heat and humidity will provide fuel for thunderstorms through Thursday with storms possible during the day and overnight. Placement, timing, and coverage of storms remains variable while SPC has a Slight Risk across portions of NH and a Marginal Risk elsewhere for severe storms through tonight. There is a Marginal Risk for severe storms on Thursday.

3. The ridge responsible for the Extreme Heat breaks down late Friday through the weekend allowing for down trend in temperatures. Chances for showers and storms remain in the forecast through the weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

There have been no significant changes to forecast thinking for the prolonged heat event that begins today and lasts through at least Friday. All Heat Headlines remain in tack from previous forecast.

Areas of showers and storms moving across the area through this morning will give way to a mix of sun and clouds. Most, but not all 00Z CAM solutions keep the area largely free of showers and storms through the peak heating of the day. However, the 00Z HRRR does show some elevated wild fire smoke moving across the area late afternoon into the evening that may act to dim solar insolation. Nevertheless, hot and humid conditions are still expected with highs in the 90s and dew points in the low to mid 70s. A southerly onshore wind component will allow for some cooling marine influences near the coast, mainly from Cape Porpoise and points northward.

CAMs generally show limited convection during the day Thursday, so again no significant changes to forecast thinking for temperatures, dew points, and heat indices. Thursday still looks to be the peak of the heat with heat indices climbing in excess of 110F. Winds will have a more southwesterly component leading to little relief from any sea breezes.

The ridge starts to flatten late Thursday into Friday that may allow for a front to sink south from Canada. This front won't bring any real change in temperatures, although a westerly gradient would allow for better mixing and a slight down trend in dew points. Still, it will be hot and humid and the west winds will inhibit and sea breezes.

Overnight lows in the mid to upper 70s are expected across southern New Hampshire Wednesday night through Friday night, with lows still in the 70s elsewhere. However, with any convection that does pass through, a quick drop into the upper 60s to low 70s would be expected. The long duration of the heat event makes it more impactful than some of the single hot days we've seen in recent years. A gradual decrease in temps is expected through the weekend and early next week as a trough begins to have more influence on northern New England.

KEY MESSAGE 2 DESCRIPTION...

Latest satellite presentation shows multiple convective complexes rounding a ridge stretching from the Great Lakes into the Northeast. CAMs have been struggling to resolve these features and their evolution with even some substantial run to run variability in the HRRR for a MCS moving into Upstate NY early this morning. CAMs generally suggest some convection will cross the area through around 8-10 AM this morning, mainly from the White Mountains towards coastal southwest Maine. The overall severe threat from this activity looks low while some early morning thunder will be possible.

The main story remains the same today through Thursday. There will be more than enough CAPE with pockets of decent shear that would support strong to severe storms. The lack of surface forcing and mid level support continues to yield little in the way of coverage amongst CAM guidance, for at least the day time hours today. There are again hints amongst CAM guidance that an MCS may approach from the northwest later this evening, but given the poor handling of features out there now confidence is low. SPC has a Slight Risk across Upstate NY into NH and a Marginal Risk elsewhere for this potential convection that could arrive late this evening into tonight.

Sufficient CAPE and shear will continue to be present Thursday while CAMs generally depict little in the way of storms. A front sagging south through Canada could bring better forcing for storms late Thursday with 00Z CAMs catching on to this idea. SPC has a Marginal Risk for storms across much of the area for Thursday into Thursday night.

KEY MESSAGE 3 DESCRIPTION... Height falls Saturday through Sunday will bring about a down trend in temperatures into Monday. Saturday and Sunday are forecast to remain quite warm with highs in upper 80s to 90s with dew points in the 60s. The height falls will also support showers and storms while the timing and details are hard to pin down at this time range. By Monday, relief from the heat appears likely with highs in the 80s.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

Through tonight...

Some brief restrictions with showers and fog are possible for another hour or two this morning. Mainly VFR the rest of today although isolated showers and storms are possible late this afternoon with better chances at night. This activity could lead to brief IFR or MVFR restrictions but specific locations are uncertain at this time. More widespread fog is possible tonight.

Outlook... Thursday-Sunday: Generally VFR conditions are expected during daylight hours with TEMPO MVFR conditions possible each afternoon in showers and thunderstorms. IFR conditions will be possible each night in fog, but how widespread it gets is uncertain.

MARINE

High pressure remains south of the waters through the end of the week with a southerly flow regime today gradually transitioning to westerly by Friday. Winds generally stay below 25 kts while a few waves to around 5ft are possible across the far eastern waters late today. Winds and seas the stay below SCA thresholds through the end of the week.

CLIMATE

High temperature records at long term climate sites...

July 1st 2nd 3rd

AUG 92 (1971) 93 (1963) 94 (2002)

PWM 93 (1971) 98 (1941) 95 (2002) CON 99 (1913) 98 (1966) 102 (1966)

GYX WATCHES/WARNINGS/ADVISORIES

ME...Heat Advisory from 11 AM this morning to 8 PM EDT Thursday for MEZ007>009. Extreme Heat Warning from 11 AM this morning to 8 PM EDT Friday for MEZ012-018>020-023-033. Heat Advisory from 11 AM this morning to 8 PM EDT this evening for MEZ013-014-021-022-024>026. Extreme Heat Watch from Thursday morning through Thursday evening for MEZ013-014-021-022-024>028. Air Quality Alert from 10 AM this morning to 11 PM EDT this evening for MEZ023>028. NH...Heat Advisory from 11 AM this morning to 8 PM EDT Thursday for NHZ001>003. Extreme Heat Warning from 11 AM this morning to 8 PM EDT Friday for NHZ004>015. Air Quality Alert from 10 AM this morning to 10 PM EDT this evening for NHZ013-014. MARINE...None.