textproduct: Caribou

WHAT HAS CHANGED

- Slight shift in convective mode thinking for Tuesday storms to more discrete threat. Uncertainty remains in timing of severe threat, but increased chance for isolated strong to severe storms early Tuesday afternoon before the main event Tuesday night

KEY MESSAGES

1) Severe thunderstorms are possible late Tuesday into Tuesday night areawide. The greatest chance for severe winds, hail, and isolated tornadoes occurs around sunset through midnight.

2) Very warm to hot temperatures Tuesday. The highest heat indices of the period should be over southern Piscatiquis County and the Bangor region, with heat indices into the mid 90s.

3) Cooler Wednesday through the weekend, with a chance for showers and storms mainly Wednesday and Thursday.

DISCUSSION

KEY MESSAGE 1...Severe thunderstorms are possible late Tuesday into Tuesday night areawide. The greatest chance for severe winds, hail, and isolated tornadoes occurs around sunset through midnight.

KEY MESSAGE 1 DESCRIPTION... The Storm Prediction Center continues their enhanced risk (level 3 of 5) for severe thunderstorm development across our forecast area. This is the first time an enhanced risk has been issued for our CWA. Ingredients are favorable for severe thunderstorm development late Tuesday into Tuesday night.

Synoptic overview: A broad upper level ridge will begin to slide eastward through the early week, wit a jet streak cresting over the Great Lakes region and the right exit region of the jet streak moving across northern Maine late on Tuesday. Though broadly speaking the subsidence under the right exit region of the jet should limit vertical motion, multiple days of increasing heat and humidity within the low to mid levels across Northern and Eastern Maine have set up favorable ingredients to feed convection this week.

Mesoscale support: With temperatures lifting into the mid 80s to lower 90s and dewpoints approaching 70 across much of the forecast area, warm moisture will lead to unstable conditions through Tuesday afternoon. At the surface, several surface troughs will move through, one early Tuesday afternoon and another overnight Tuesday night, which will provide the lift necessary to initiate convection given the warm soup providing the fuel. A tightening pressure gradient surrounding the low pressure system passing well to the north and a ridge of high pressure approaching from the west will deliver west to northwest winds aloft increasing in strength through time, which could generate 0-6 km shear of at least 40 kts. Especially for convection in the Central Highlands down through the Bangor region, these winds aloft may also be perpendicular to any convective lines, creating bowing segments and generating supercell structures in any discrete cells.

Main threats: With substantial shear of 40 to 50 kts, and strong support as CAMs indicating a healthy 2000 to 3000 J/kg of CAPE, large hail 1 to 2 inches in size remains possible with this system. Additionally, damaging winds are likely with the stronger winds aloft that could efficiently mix to the surface with low level lapse rates around 8 C/km and mid level lapse rates around 7 C/km, and with drier mid level RH. Finally, several runs from a variety of convective allowing models have indicated 0-1 km SRH values exceeding 200 m2/s2, and 0-1 km shear over 30 kts. These values indicate strong helicity that could be supportive of tornadic development, particularly within any well-developed supercells in the area. The tornado risk is limited, but present enough to warrant extra caution. With the greatest chance for severe storms occurring after sunset, lack of ambient light will present a threat of its own, as incoming storms may not be as visible. Stay weather aware and have a way to receive critical warning information overnight.

The uncertainty: A shortwave earlier in the day on Tuesday will bring showers and possibly some storms throughout the forecast area. This first shortwave may limit the diurnal surface heating through the day on Tuesday and could cap how much fuel ends up being available for the main convective threat later Tuesday night. That said, there remains timing uncertainty in this first wave of showers and storms. If it moves through earlier, there is more time for the skies to clear and surface heating to resume to build CAPE before sunset. If it comes in later, these storms may take advantage of the previous heating and generate a smaller severe risk earlier in the day, sometime through the early afternoon by most guidance giving this solution. Early afternoon storms would likely limit the areal extent of overnight convection.

KEY MESSAGE 2...Very warm to hot temperatures Tuesday. The highest heat indices of the period should be over southern Piscatiquis County and the Bangor region, with heat indices into the mid 90s.

KEY MESSAGE 2 DESCRIPTION... Southwest winds will continue to advect warm moist air into the forecast area tonight through Tuesday and into Tuesday night. Tuesday will be the hottest day of the week, with max temperatures lifting into the mid 80s to upper 80s in the north and into the lower 90s in the Bangor and Downeast regions. Combined with increased relative humidity, apparent temperatures could run into the mid to upper 90s for the Central Highlands down through the Bangor region.

However, if showers and storms develop through the late morning to early afternoon hours on Tuesday, these values could be significantly cooler. With that being said, confidence is not high enough for any heat headlines at this time. Criteria is borderline and likely only will be held briefly before conditions shift in the convective environment through Tuesday afternoon.

KEY MESSAGE 3...Cooler Wednesday through the weekend, with a chance for showers and storms mainly Wednesday and Thursday.

KEY MESSAGE 3 DESCRIPTION... Cooler weather pattern setting up mid to late week, with a deep, cool upper level low pressure centered north of the area. We could see some primarily afternoon showers and storms Wednesday and Thursday, with the better chance being Thursday. However, we don't anticipate severe storms. Can't rule out some small hail Thursday though with the cold air aloft. More uncertainty from Friday onward, but tentatively looking fairly dry. High temperatures will be fairly close to average, with low temperatures a bit cooler than average. The coolest day will likely be Friday, when highs may struggle to reach 70 in the north. Some 40s at night in cooler northern valleys are likely from Friday night through Saturday night.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Tonight...KHUL and northwards: Cigs decreasing to MVFR with scattered rain showers moving through. SW winds around 5 kts.

KGNR through KBGR/interior Downeast: VFR conditions continue, with rain showers possible late. SW winds around 5 kts.

KBHB/coastal terminals: Mainly VFR with brief MVFR/IFR possible in patchy dense fog. 40 kt LLWS from around sunset to around midnight. SW winds around 5 kts.

Tuesday - Tuesday night...Mainly VFR cigs across all terminals, though brief IFR likely in any TSRA. Increasing SW winds 10 to 15 kts with gusts increasing to 20 to 25 kts. Strong gusts of at least 40 to 50 kts possible in any +TSRA. Most likely timeframe for thunderstorms will be after 20z Tuesday.

Wednesday to Saturday: Mainly VFR with possible MVFR at times mainly in the north due to either cigs or afternoon showers/storms. Better chance for afternoon storms is Wednesday and Thursday. in afternoon showers and TS. W winds 10-15 kts with gusts up to 25 kts.

MARINE

SW winds will increase overnight tonight, with gusts increasing to 20 to 25 kts and seas gradually responding to 3 to 5 ft through the night. Cold sea surface temps will help keep boundary layer stable and limit wind gusts overnight. Swell will continue to increase through the day on Tuesday, and seas will lift to 4 to 8 ft across all waters by Tuesday night. Gusts will remain marginal for SCA at around 20 to 25 kts. Strong thunderstorms may move over the waters late Tuesday night, which could bring localized gusts over 40 kts possible.

W winds generally 10-20 kts Wednesday through Saturday with minimal fog threat. Seas 2-5 ft Wed to Sat, generally on the lower side of the range toward Saturday.

CAR WATCHES/WARNINGS/ADVISORIES

ME...None. MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ050-051. Small Craft Advisory from 6 AM Tuesday to 6 AM EDT Wednesday for ANZ052.