textproduct: Gray - Portland

WHAT HAS CHANGED

Progression of the frontal passage has slowed for Monday.

KEY MESSAGES

1.Showers and thunderstorms increase in coverage later tonight and persist into Monday as a cold front crosses the region.

2. Humid conditions persist through the remainder of the week with chances for scattered afternoon thunderstorms.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

An upper-level trough over eastern Canada will deepen and carve south into New England tonight into Monday. Guidance continues to favor a slightly slower eastward progression, with the trough potentially closing off. Deep southerly to southwesterly flow will advect a plume of moisture rich air into the region, driving PWATs values around 2.0 inches, within an environment with deep warm-cloud depths (>13kft). The overnight timing will limit overall intensity, and should start Monday off with mostly cloudy conditions. The cloud cover will be a defining driver on intensity of any storms throughout Monday as upper levels become more favorable with ascent increasing throughout the day. Increasing large-scale ascent on Monday will interact with modest instability (CAPE 500-1000j/kg) will produce multiple rounds of heavy showers and thunderstorms. As things currently look, we can expect tall, skinny CAPE profiles, with a high moisture content, and efficient warm rain collision-coalescence processes. Translation, when it rains it will pour. The storms should be progressive, which should keep most storm total accumulations around 0.5-1. However, the storm movements will be running parallel to the frontal boundary, increasing the potential for storm training and backbuilding with FROPA. Dry antecedent conditions with ample runoff storage reduces the widespread flood risk, however localized areas could quickly accumulate 3+, so we cant rule out an isolated risk of flash flooding. Higher terrain may experience localized upslope ascent, further elevating rainfall efficiency and flash flood threats. WPC has the region in a level 1 of 4, a marginal risk for localized flash flooding. At this time it is hard to pin point exactly where the highest risk for flooding would occur, and given dry antecedent conditions, have held off on a flood watch.

In addition to the excessive rainfall threat, SPC maintains a Marginal Risk (Level 1/5) for severe weather for Monday afternoon; the primary hazard will be localized damaging wind gusts from water- loaded downbursts. CAMs continue to suggest that storms will develop along the cold front moving west to east across the area, strengthening in New Hampshire towards peak heating. As mentioned above, we will see some modest instability with MLCAPE in the 500 to 1000 J/kg range. The instability along the front will be collocated with 40 to 45 knots of effective layer shear. Thus, the more established updrafts will have the potential to produce some isolated damaging wind gusts. As is often the case around here, much of the severe potential will depend on how much New Hampshire can clear out after the morning convection moves through. If we get some more breaks, we could see pockets of MLCAPE closer to that 1000 J/kg value but if we remain socked in, instability will be more limited.

KEY MESSAGE 2 DESCRIPTION...

Tuesday through Friday... The frontal boundary will weaken south of the region on Tuesday, providing fair conditions and a brief reduction in the heat/humidity. By Wednesday through the weekend, the broad upper level trough shifts back over the Great Lakes, and with the Bermuda High offshore establishing deep southwesterly flow across the area. This will support warm and humid conditions, with shower and thunderstorm chances (2535%) diurnally driven. Some guidance is showing embedded shortwave energy Thursday that could enhance coverage, so have highest pops at that point in the extended period. While exact timing and intensity remain uncertain this far out, diurnal heating will favor the highest storm coverage during the afternoon and evening hours.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Through Monday: Expect VFR conditions through the afternoon but we could see a passing shower in the mountains, potentially impacting HIE. An approaching system will lead to lowering ceilings, fog, and increasing precipitation chances overnight into Monday. All sites should dip into IFR categories or lower overnight before gradually improving later in the day. Expect widespread showers tonight through Monday morning with some scattered showers and thunderstorms along a cold front Monday afternoon. A few stronger storms with gusty winds will be possible in the afternoon over New Hampshire, potentially impacting HIE, LEB, CON, and MHT.

Outlook...

Monday night-Wednesday night: Generally VFR conditions during the day with potential for IFR fog at some terminals during the night as we enter a period of increasing humidity through next week.

Thursday-Friday: MVFR or lower restrictions possible under isolated to scattered showers and storms.

MARINE

Winds and seas expected to remain below SCA thresholds today. However, an increase in winds and seas will likely lead to SCA conditions developing by late Monday morning through Monday night. Areas of fog will develop again tonight, locally dense at times. Additionally, chances for widespread showers and storms increase overnight into Monday.

After seas and wind relax Tuesday morning, we should remain below SCA criteria through the extended.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory from 10 AM Monday to 8 AM EDT Tuesday for ANZ150-152-154. Small Craft Advisory from 2 PM Monday to 8 AM EDT Tuesday for ANZ151-153.