textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes were needed as a couple of strong to severe storms remain possible through early this evening.

KEY MESSAGES

1. A couple of strong to severe storms are possible through early this evening before high pressure brings dry conditions Tuesday.

2. Warming Trend Starts For the later half of the week

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... A cold front is right on our doorstep to the west as of early this afternoon with showers and storms developing along and just ahead of the boundary, which are currently nearing the CT River. The environment this front will encounter across northern New England has effective shear of 40-50 kt over most of the area, per latest SPC mesoanalysis. In addition to this, around 500 J/kg of SBCAPE is being analyzed across most of NH and far western ME and the western ME mountains for the next few hours, which will be the primary areas to watch for a few strong to severe storms through early this evening due to the better instability/shear overlap, especially where more breaks in the clouds occur that could allow more localized CAPE upwards of 1000 J/kg. Damaging winds, mainly from wet microbursts from the juicy airmass will be the primary hazard, but lowering freezing levels may also allow for hail to the size of quarters, although lapse rates aloft aren't particularly favorable. Enough directional shear will also be present to support a brief tornado or two.

Showers and storms could produce hefty rain rates but should be fast- moving enough to where the risk of flash flooding is very low. The exceptions are around the Nashua/Manchester corridor and also across portions of Carroll and Oxford counties where earlier heavy rains fell, where a closer eye will need to be kept.

Once the front passes, chances for showers and storms will quickly diminish as the atmosphere quickly dries out. Based on latest CAM guidance, this should be by 6 to 7 pm for NH (although earlier for western NH) and then maybe an hour or so after that in far western ME. There will be probably be showers and a few storms continuing to advance along the front farther east into Maine this evening, but convection is expected to be on a weakening trend as the environment will be less favorable.

Skies will continue to clear out from west to east this evening and overnight behind the front with mostly dry conditions other than a shower or two in the mountains. Patchy fog is possible where winds become light enough, but this should be mainly limited to river valleys.

High pressure returns for Tuesday, bringing much quieter weather with plenty of sunshine. 850mb temps of +15C to +17C support highs reaching the mid to upper 80s south of the mountains with lower humidity as drier air aloft mixes down. Highs in and around the mountains will likely be a bit cooler and in the upper 70s to low 80s.

Mostly clear skies are expected Tuesday night with light winds, bringing a good opportunity for radiational cooling. Most of the area will see lows in the 50s, but some of the northern valleys could see lows in the mid-upper 40s. So I have gone a bit cooler than the NBM on lows.

KEY MESSAGE 2 DESCRIPTION...Warming Trend Starts For the later half of the week

A warming trend will take hold starting Wed and last through the extended forecast. Sunny skies and mainly dry conditions will prevail as surface high pressure settles over the region and deep layer ridging builds overhead. H850 temps will rise from around 12C Tue up to 18C Wed-Fri (potentially higher by Fri). Wednesday will be slightly warmer than Tue as the ridge axis shifts overhead, accompanied by increasing dewpoints. The warm advection accompanied by one or two weak waves could trigger a few scattered showers offshore that could clip the coast, but not going to pin point it this far out. By Thursday and Friday, our region will become sandwiched between a Bermuda High shifting to our east and west, and a nearly stationary mid- level trough over the Great Lakes. Broad and persistent sw flow will drive dew points into the mid 60s to 70s by Friday. High temperatures will reach the 80s to near 90F each afternoon, creating muggy conditions with heat index values reaching or exceeding 90F in lower elevations. Nighttime temperatures will offer little relief, dropping only into the mid to upper 60s. The main chances for rain and thunderstorms will focus on Thursday through Friday, driven by a combination of a weakening cutoff low drifting northeast, a stalled or slow- moving frontal boundary approaching from the west. Additional afternoon shower activity may develop along local sea breezes, however warm air aloft (cap) will keep the activity spotty. While weak overall forcing should keep the severe weather threat low, any well timed diurnal shortwaves could be sufficient to trigger a few stronger thunderstorms. Any shower development during that period will have high rainfall rates,so locally heavy rainfall is possible. Overall the later half of the forecast will feature mostly dry weather with plenty of sunshine and heat.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Prevailing ceilings are more likely to be MVFR or VFR for the NH terminals today, but TEMPO IFR conditions are possible in SHRA or TSRA through early this evening as a cold approaches. The western ME terminals are more likely to hold onto the IFR or even LIFR ceilings, but visibility reductions are also possible with FG/BR or SHRA/TSRA. Mainly VFR tonight, but fog may develop overnight if winds become light enough, especially in valleys.

Outlook... Tuesday-Saturday: Generally VFR conditions as high pressure settles over the area. Can't rule out diurnal IFR fog as we enter a period of increasing humidity through the week. Shower chances increase Friday into Saturday.

MARINE

Through Tuesday night...SCA conditions continue over the waters through at least early Tuesday morning. Southerly flow steadily increases this afternoon ahead of a cold front, and then westerly this evening and overnight behind the front. Winds will be mainly out of the south to southwest below SCA levels Tuesday and Tuesday night, but seas could stay at or above 5 ft over portions of the nearshore waters during the day Tuesday, so the current SCA may need to be extended.

Expect benign winds and seas from Wed onwards (5-10 kt winds and 2-4 ft seas) as weak high pressure builds over the area.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 8 AM EDT Tuesday for ANZ150-152-154. Small Craft Advisory until 8 AM EDT Tuesday for ANZ151-153.