textproduct: Gray - Portland

WHAT HAS CHANGED

For this update, just blended in the latest observations to the going forecast and updated the aviation discussion below. No major changes needed.

KEY MESSAGES

1. Heat and oppressive humidity arrives Wednesday and lasts through at least Friday, possibly into Saturday. To be completed ahead of Wednesday's heat, all AC installations and preparations should be completed by today.

2. Starting late today through the end of the week the ingredients will be there for thunderstorms, some of which could be strong to severe. SPC has a Marginal Risk for severe storms late today and again Wednesday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Today will be very warm with highs in the 80s to low 90s. Dew points will mainly be in the 50s, except near the coast where onshore winds and marine influence will bring dew points into the 60s while cooling ambient air temperatures. This will lead to heat indices being close to ambient air temperatures.

The building heat and humidity beneath an anomalous ridge over the Ohio Valley will push into the area on Wednesday. Temperatures at 850 mb are still forecast to exceed +20C which would support highs in the upper mid to upper 90s. Winds near the coast will have some onshore component that will allow for some relief during the afternoon. Dew points are still forecast to climb into the low to mid 70s that will bring heat indices to 105 to 110 degrees for areas under the Extreme Heat Watch and well into the 90s elsewhere. There continues to be uncertainty with the timing and coverage potential rounds convection that could impact the area from late Tuesday through the end of the week. Any convection would act to reduce temperatures locally while also acting to increase dew points. More details on convection is discussed in Key Message 2.

Thursday continues to look to be the peak of the heat as temperatures at 850 mb climb to +22C to +23C. This will support high temperatures either side of 100 degrees for much of the interior, and especially for southern New Hampshire. There remains some question as to whether the coast will see a sea breeze develop of not, but this will also be influenced by the possible convection. Dew points look to push into the low to mid 70s again. This pushes heat indices to near 110 degrees anywhere the air temps reach the upper 90s to low 100s.

The heat and humidity will continue into Friday and possibly Saturday but will be on a slight downward trend. By Sunday models are in agreement that heights will be lowering as a trough moves through Atlantic Canada. This will help knock temperatures down while the latest NBM still brings highs into the low 90s for southwest Maine and southern NH.

Nighttime low temperature forecasts will be tricky tonight through Friday night. Without storms, lows only making it down into the mid to upper 70s is likely across southern and central locations. However, with any thunderstorms temperatures would quickly drop into the upper 60s to low 70s. So the low temperature forecast tries to split the difference between these two outcomes, but acknowledges that there will be variability across the area. Without storms, lows around 80 degrees are likely through the Merrimack River Valley Wednesday and Thursday nights.

KEY MESSAGE 2 DESCRIPTION...

There is the potential for multiple rounds of convection starting late today lasting through the end of the week while confidence is low on timing and coverage, especially beyond Wednesday. As mentioned in previous discussions, CAPE and shear will be sufficient for severe storms while ridging dominating the pattern will not be favorable for synoptic ascent. CAMs have been showing potential for a couple of ridge rolling MCSs or convectively induced short waves from upstream convection with one crossing the Northeast late today/tonight and another Wednesday or Wednesday night. However, placement, strength, and coverage is variable amongst CAM solutions with some keeping the strongest convection to the west over NY and Vermont. SPC has a Marginal Risk for severe storms today with a Slight Risk just to the west over NY/VT. Will have to keep a close eye on trends if this MCS can materialize and where it tracks this evening as it would bring a damaging wind threat. Latest round of CAM guidance does have some solutions bringing it into portions of NH. SPC also has a Marginal Risk for severe storms Wednesday as another MCS or convectively induced short wave is modeled to approach the Northeast. Outside of these rounds of convection there remains potential for isolated storms to form over the higher terrain or along any sea breeze convergence while CAMs do not have much of a signal for these outcomes.

The persistent heat and humidity will continue to supply fuel for any storms Thursday into the weekend. Height falls Saturday into Sunday would lead to better synoptic support for storms, but this time range details remain unclear.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Through Wednesday... Mainly VFR through today. Showers and storms become possible late today through tonight. Due to timing and location uncertainty, utilized PROB30 groups for this activity and held off on any thunder mention at any specific site for now. These showers and storms could bring localized restrictions. Increasing low level moisture will also bring potential restrictions from low cigs and/or fog late tonight into Wednesday morning. Mainly VFR Wednesday outside of isolated thunderstorms.

Outlook:

Wednesday Night: Conditions may begin to deteriorate as marine fog/stratus becomes more likely. Confidence is low at this time in MVFR or lower conditions. Local MVFR or lower also possible in showers/thunderstorms.

Thursday - Saturday: Local MVFR or lower possible in showers/thunderstorms. Areas of IFR or lower possible at night with marine fog/stratus. Saturday Night - Sunday: Some clearing possible from the north as the heat and humidity gets pushed south.

MARINE

Winds and seas remain below SCA thresholds through the holiday weekend. The main hazards to watch for will be developing marine fog, especially as winds become onshore ahead of any low pressure approaching, and then thunderstorms which will be possible most days into the weekend.

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...Extreme Heat Watch from Wednesday afternoon through Friday evening for MEZ012-013-018>021-023>026-033. NH...Extreme Heat Watch from Wednesday afternoon through Friday evening for NHZ004>015. MARINE...None.