textproduct: Gray - Portland

WHAT HAS CHANGED

7 PM Update...Have added slightly higher PoPs for this evening into the overnight as CAMs are starting to support a solution of thunderstorms currently over Upstate NY and VT moving into western NH in a few hours. Given the instability in place, if storms do indeed make it may produce gusty winds and heavy rainfall. Will continue to monitor.

Previously...

Temperatures adjustments were made from Thursday through Saturday. Any watches were upgraded to extreme heat warnings or heat advisories.

KEY MESSAGES

1. Heat continues to build into Thursday, and then linger into at least Friday. Overall the heat remains on track, with minor tweaks to temperatures and heat indices over the next few days.

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. 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. The main hazards with any severe storms are damaging winds and hail.

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

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

The heat is underway across the area. Overall today's temps look likely to land a few degrees cooler than the original forecast, and this outcome was kept in mid for the next few days. The forecast was mostly just fined tuned from previous versions, but with these high end readings the minor changes can make the difference in headline decisions.

So with the forecast adjustments, an extreme heat warning has been issued into central Maine and the MidCoast for Thursday, and advisories further north. Heat advisories will likely be needed in these new warning areas for Friday, but will be addressed once the peak of the heat is dealt with tomorrow.

In general, heat indices of 105-110 are expected in the warning areas, and heat indices of 95-104 are expected in the advisory areas. Tomorrow features both the highest temperatures, and widespread dew points in the low to mid 70s. Dew points likely drop back into the upper 60s to low 70s across interior New Hampshire during the afternoon hours, which will allow temps to climb higher. Friday then looks to produce slightly lower temperatures, and lower dew points. A west wind keeps all the coastlines hot on Friday as well, with widespread highs in the 90s.

Significant changes were made to the high temperatures for Saturday, the 4th of July. Earlier NBM forecasts of highs in the upper 90s were not consistent with expected 850mb temps, and were addressed in this forecast update. This still brings one more very warm day with highs in the upper 80s to low 90s, but will mark a downward trend from the peak of the heat. Dew points will also be lower on Saturday, keeping heat indices within a few degrees of the air temps.

KEY MESSAGE 2 DESCRIPTION... Not much has changed with regards to severe potential over the next 48 hours as models continue to do a poor job of tracking the convective waves rolling the ridge. The latest runs of the CAMs aren't convinced in much developing this afternoon with lack of forcing in the area, but also develop 2000-3000 J/kg of CAPE over the area. As mentioned previously, anything that does develop (think terrain forced convection) would have the ingredients to quickly strengthen. The other reason this is significant is because that means that the convective systems developing along the front draped over the Great Lakes Region have a supportive environment to sustain themselves as they journey into our area. At the time of this writing there is robust convection northeast of Ottawa (that you won't find on any of the current CAM runs) that may reach New Hampshire later this afternoon and may very well be severe if it does. A perfect example of the analogy I used yesterday where very often CAMs struggle to capture these features in these setups and we have to wait to see what actually forms before we can start talking about it with confidence.

Speaking of confidence, the majority of CAMs once again have convection (currently located over Wisconsin) making the journey to western New Hampshire around day break, but after their whiff last night I have my doubts. By then stable air near the surface would likely lessen the severe threat anyhow. The majority of CAMs maintain a severe threat Thursday afternoon/evening as well, but that is another low confidence, wait and see, situation.

KEY MESSAGE 3 DESCRIPTION... The ridge begins to break down Friday, but not before one more very warm day of temperatures in the upper 90s. Unsurprisingly, models have backed off on the progression of the cold front until later on Friday of even Saturday but regardless we should begin heading in a more comfortable direction. The holiday weekend will feature summer like temperatures, but the lower dewpoints behind the front should make it less sticky. Global ensembles continue to trend toward the transition to a broad trough following this ridge, which would continue the cooling trend early next week, as well as keeping showers and storms in the forecast. We then may see another warm up midweek as there is surprising agreement already of another ridge taking shape.

AVIATION /00Z THURSDAY THROUGH MONDAY/

Mainly VFR prevails at most terminals through Thursday night, but some brief restrictions are possible with any showers or storms from late this afternoon through Thursday night. Valley fog or low stratus is also likely at LEB and HIE late tonight. Some shallow marine fog is also possible at RKD tonight.

Outlook... Friday-Monday: Generally VFR conditions are expected during daylight hours with TEMPO MVFR conditions possible each afternoon in showers and thunderstorms.

MARINE

High pressure remains centered south of New England for the next couple days. Seas up to 5ft are possible across the far eastern waters late tonight in southwest flow. Some patches of marine fog are possible near the MidCoast shoreline tonight. High pressure settles south of the waters late week keeping winds and seas below SCA levels into midweek next week.

CLIMATE

High temperature records at long term climate sites...

July 2nd 3rd

AUG 93 (1963) 94 (2002)

PWM 98 (1941) 95 (2002) CON 98 (1966) 102 (1966)

GYX WATCHES/WARNINGS/ADVISORIES

ME...Heat Advisory until 8 PM EDT Thursday for MEZ007>009. Extreme Heat Warning until 8 PM EDT Friday for MEZ012-018>020- 023-033. Heat Advisory until 8 PM EDT this evening for MEZ013-014-021- 022-024>026. Heat Advisory from 11 AM Thursday to 8 PM EDT Friday for MEZ013-014. Extreme Heat Warning from 11 AM to 8 PM EDT Thursday for MEZ021-022-024>028. Air Quality Alert until 11 PM EDT this evening for MEZ023>028. NH...Heat Advisory until 8 PM EDT Thursday for NHZ001>003. Extreme Heat Warning until 8 PM EDT Friday for NHZ004>015. Air Quality Alert until 10 PM EDT this evening for NHZ013-014. MARINE...None.