textproduct: Gray - Portland

WHAT HAS CHANGED

Extreme heat warnings and heat advisories have been issued for most of the forecast area. The heat and humidity remains on track for Wednesday through at least Friday.

KEY MESSAGES

1. The hot and humid conditions begin on Wednesday, and continue 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. 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 for at least a portion of our area late today through Thursday.

3. Chances for afternoon showers and thunderstorms will remain in the forecast through the holiday weekend and into early next week, but extreme heat looks to break with the passage of a cold front late week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

The heat and humidity remains on track for the next few days. Watches have been upgraded to warnings and advisories for most areas. A heat advisory is in effect for parts of Maine on Wednesday, and an extreme heat watch is in effect for Thursday when the highest temps are expected.

Overall thinking hasn't changed from the last few days. There remains some uncertainty on the timing of cloud cover and ridge rollers' effect on temperatures through the next few days, which is discussed in more detail in Key Message 2. These hold the potential to keep temps lower than expected should they arrive in the late morning to early afternoon timeframe, but without them highs are likely to reach they're forecasted levels.

One other factor we're watching that could limit temperatures is some wildfire smoke from the western US, which would lessen the amount of solar radiation reaching the surface. Almost of this smoke will remain elevated, but some of it could make its way to the surface around clusters of showers and thunderstorms.

Much work was done from the NBM starting point to keep forecast temps and heat indices within reasonable ranges given the 850mb temperature forecasts and surface dew points. Wednesday features a sea breeze along the Maine coast, offering relief after a warm morning to the immediate coast. A seabreeze looks unlikely away from the MidCoast on Thursday, and least likely on Friday with a west wind.

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.

So at this stage, the headlines primarily tell the story on the heat's progression. Near record high temperatures are expected each day, with widespread heat indices in the 105-110 range in the warning areas, and up to 104 in the advisory areas. 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... For the next several days extreme heat (detailed in key message 1) is going to aid in providing the ingredients for severe weather. Each afternoon CAMs show ample CAPE development (greater 1000 J/kg), steep low level lapse rates (7 to 8 C/km), and bulk shear 35 to 50 knots. Not to mention the increasing dewpoints providing plenty of buoyancy. However, what the models are lacking is much in the way of lift, a key component for convective initiation. Models also have some thoughts on later timing of the convective waves as well, which would lower the severe risk. The bottom line is that while lack of lift may keep coverage low, anything that gets itself together certainly has the potential to quickly become severe. I will break down the daily details in the next paragraphs.

This afternoon, two distinct MCSs are visible on radar moving through Ontario downstream of the ridge. At the time of this writing the leading MCS is just about to the U.S. border and should be crossing into Upstate New York shortly. The last few runs of the CAMs have trended toward the bulk of this staying to our west with maybe a few showers and storms able to sneak into Western New Hampshire. The other thing we will have to watch this afternoon and evening is the seabreeze as that can be a good source of lift, but again would most likely not be a source of widespread convective initiation and only result in just a few storms that may quickly become strong to severe. The second MCS, just now cresting the ridge at the time of this writing, looks like it will arrive in our area as the ridge axis slowly shifts eastward. However, timing on the latest CAMs suggest this occurs in the very early hours of Wednesday morning. So, while it is possible some locations, especially along the international border get woken up by some impressive nocturnal convection, that time of the day is not likely to lend itself to severe weather.

Parameter-wise Wednesday looks like the best chance for severe weather on paper with MUCAPE values on the CAMs coming in at a very respectable 2000-3000 J/kg along with the aforementioned lapse rates and shear that are more than sufficient as well. Currently what Wednesday lacks is a reliable source of lift and this is very apparent on the latest runs of the CAM which show very little in the way of showers and storms present. The NAMNest is currently latching on to another late arriving shortwave, but it is still too early to say for sure what might be out there, and we may have to wait to see the whites of its eyes this time tomorrow if more models don't trend that way beforehand. This looks like a very touch and go time period so anyone with plans to be outdoors Wednesday afternoon should monitor the forecast and conditions closely.

Ironically Thursday may be the day with the best lift as models prog a cold front to sweep through, but it also looks like severe parameters take a downward turn as the ridge axis moves directly overhead. The 30/12Z NAMNest is not depicting much more than a few isolated storms, but this is also getting near the end of its run. I wouldn't be surprised to see activity trend upward with subsequent runs, but also wouldn't be surprised if it ends up being a similar day with low coverage, but plenty of room for any organized convection to quickly strengthen.

KEY MESSAGE 3 DESCRIPTION... The 500 mb pattern transitions to zonal flow after the ridge axis slides east late week. However, high pressure stays firmly planted over the East Coast so Friday could still be a very warm day in southern zones. Northern zones are more likely to see the influence of the zonal flow as showers and thunderstorms remain in the forecast there. The holiday weekend still looks warm, but the continued northwesterly flow on the back side of the cold front should start to bring high temperatures below Extreme Heat thresholds. The continued northwesterly flow means that showers and thunderstorms remain a possibility so keep an eye and an ear to the sky during any outdoor festivities. The pattern dosen't change much going into early next week other than some models transitioning the zonal flow into a trough, but either way this would result in unsettled weather, so I will just leave it at that.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Through 18Z Wednesday...

Mainly VFR prevails, but some brief periods of restrictions with showers and storms are possible tonight, and then again late Wednesday afternoon and Wednesday night. Some marine fog is also possible at RKD 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 settles south of the waters tonight. Some patchy marine fog is possible at times east of Casco Bay tonight and into Wednesday. A few waves to around 5ft are possible across the far eastern waters in southwest flow late tonight, and then again late tomorrow. A ridge of high pressure settles over the waters from midweek onward which should keep waves and wind gusts primarily below SCA conditions.

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