textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes since the previous forecast cycle. Continuing to watch for localized flash flooding today with very heavy downpurs.

KEY MESSAGES

1. Showers and thunderstorms will increase in coverage through today as a cold front slowly 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 H5 trof axis will move across the region through today from west to east while becoming negatively tiled as a sfc low tracks to our west over Quebec. A sfc warm front will lift northward this morning, placing much of the region within a broad warm sector with deep S-SW flow. Forcing for ascent will increase through today with precipitable water increasing up to around 2.00", which is around +2 standard deviations. This combined with relatively deep warm cloud depths to around 13 kft will allow for embedded downpours within any showers and thunderstorms. Some backbuilding/training is also possible with relatively short corfidi vectors. Despite this, the rainfall will be beneficial in most locations as outside of portions of extreme southern NH, most areas remain quite dry. Given the large storage capacity of area rivers, river flooding is not expected but some localized nuisance poor drainage flooding cannot be ruled out where heavy rainfall does occur. There is also a nonzero threat for an isolated flash flood but coverage should be limited in scope. Generally expecting up to around 2" of rainfall on localized scales but locally higher amounts are possible.

The other thing to watch will be for possible strong to marginally severe thunderstorms during the afternoon as the cold front crosses. The main question is whether earlier day rain is able to clear in time for enough destabilization to occur but most CAMs do show modest MLCAPE to develop of between 500-1000 J/kg, which will be collocated with up to 45 kts of 0-6 km bulk shear. The greatest threat for a strong storm will be across NH with locally damaging winds the primary threat.

It will otherwise be a muggy day with high temperatures into the 70s to near 80 degrees. Low temperatures tonight will mainly be into the 50s with patchy fog possible.

KEY MESSAGE 2 DESCRIPTION...

Tuesday looks mostly dry as a ridge builds in behind a departing low pressure system. Temperatures should be able to climb into the low to mid 80s south of the mountains, with mid to upper 70s to the north, as skies quickly clear out. Mostly clear skies persist through the evening with calming wind, so we should see a cooler night with temperatures falling into the mid to upper 50s. Wednesday should be similar as the ridge axis moves overhead with slightly warmer temperatures. The ridge axis moves off to the east Wednesday night with a trough becoming established over the Great Lakes Region. This is going to re- establish the southwesterly flow and once again advect more moisture into the region. However, this time the strengthening Bermuda High is going to largely keep the lower levels dry, so this likely only results in increased humidity. The ridge axis stays off to the east so the building heat won't get too out of hand, but the latter half of the week could see temperatures approaching the 90F degree mark each afternoon and with elevated dewpoints it is going to feel a bit warmer. This is going to make nighttime temperatures warm too as they can only cool to the dewpoints which look to be in the mid to upper 60s. Global models are in good agreement that the 500 mb flow becomes more zonal Sunday which may be a turning point for the heat and humidity, but that being at the end of the forecast we will have to watch the trends.

Precipitation chances won't be nil, but they are looking like they will be sparse with not much in the way of forcing. Global models suggest there could be some shortwave energy moving through the broader trough late week, but this signal has been inconsistent as of late so it is hard to say if we will see anything more than just spotty diurnal showers and thunderstorms at this point. Many locations are likely to see dry conditions continue.

AVIATION /12Z MONDAY THROUGH FRIDAY/

Through Monday Night: SHRA and low ceilings will result in IFR or worse restrictions through this evening with S-SW wind gusts up to 25 kts. There is the potential for TSRA this afternoon, which may contain locally stronger wind gusts. The greatest potential for a TSRA will be at LEB, HIE, MHT, and CON. A period of LLWS possible between roughly 16Z-00Z Tue at PSM, PWM, and RKD. SHRA will end tonight but lingering low ceilings and possible FG will likely result in continued flight restrictions.

Outlook... Tuesday-Friday: 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 the week.

MARINE

Low pressure crossing west of the waters will bring a period of SW wind gusts up to 30 kts with seas of 4-7 ft. An SCA is in effect through early Tue morning.

The Bermuda High strengthens Tuesday making for sub-SCA conditions into at least the first part of the weekend.

GYX WATCHES/WARNINGS/ADVISORIES

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