textproduct: Gray - Portland

WHAT HAS CHANGED

Have increased wind gusts during the day Tuesday but no major changes to the forecast regarding the increasing chances for showers and storms Tuesday afternoon.

KEY MESSAGES

1. Warmer and very breezy on Tuesday with increasing chances for showers and thunderstorms. Some storms could be strong to severe in the afternoon and early evening with damaging wind gusts as the primary hazard.

2. Widespread soaking rainfall Wednesday afternoon into Thursday morning.

3. Cooler with occasional chances for showers Thursday through the weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Light S/SW flow and partly cloudy skies will keep temperatures milder overnight and mostly in the 40s. Precipitation is not expected overnight, but there may be a couple light showers approaching NH toward daybreak.

A slow-moving cold front approaches from the west on Tuesday. Ahead of the front, deep layer SW flow will bring even warmer air into New England, and with very good mixing, high temperatures are expected to reach the 70s for most, possibly near 80 degrees in southern NH. The deep mixing will also bring down strong winds from aloft, resulting in a very breezy days. Based on forecast soundings wind gusts of 30-40 mph look likely, and with the top of the mixed layer 40-45 kt, some gusts 40-45 mph are possible. The exception will be east of Portland toward the Midcoast, where the southwest winds will bring in the more stable marine layer, likely keeping it cooler and less breezy there.

Lift from WAA may produce a few showers in the morning across NH, but the main focus is on the afternoon to early evening timeframe. The atmosphere will destabilize during the afternoon with the warm temperatures, and models generally agree around 500-750 J/kg MUCAPE to work with as lift increasing with the front approaching. So an uptick in showers and thunderstorms is expected, and deep layer shear is progged to be around 40 kt, which leads to the chance of some storms becoming strong to severe. Damaging winds will be the primary hazard, but there may be hail as well. The best potential for strong to severe storms will be across central and northern NH and from interior western ME extending northward to the western ME mountains, where there is denoted by a Marginal Risk from SPC. Strong storms are less of a concern roughly east of Portland to Augusta where the marine layer should keep things more stable.

Thunderstorm potential diminishes in the evening as the atmosphere stabilizes, but waves of low pressure riding along the boundary will keep a chance of showers in the evening and overnight, especially in central and northern NH and from the foothills northward in western ME. Will have to watch for patchy fog overnight, especially if marine/fog stratus develops and move onshore in western ME.

KEY MESSAGE 2 DESCRIPTION...

A stalled cold front will be draped southwest to northeast from Upstate NY through northern Maine Wednesday morning. Embedded waves within cyclonic flow aloft will allow for areas of low pressure to form along the front and track across northern New England Wednesday afternoon through Wednesday night bringing widespread rainfall. Steady moisture advection ahead of the front will allow for PWATs to climb around 1 to 1.25 inches, which will be +1 to +2 STD above normal for this time of year. These PWATs combined with some elevated instability will allow for moderate bouts of rainfall and some embedded thunder. This will be a beneficial rainfall event with amounts ranging from 0.5 to 1 inch south of the mountains and 1+ inches across the mountains and north before steady precipitation comes to an end early Thursday morning. Highs on Wednesday will range from the 50s north to low 70s across southern New Hampshire with SSW winds gusting 25 to 30 MPH along the coastal plain before the rain arrives Wednesday afternoon.

KEY MESSAGE 3 DESCRIPTION...

The waves of low pressure that track along a cold front Wednesday night will help shift the front offshore Thursday for a drying trend. Highs on Thursday will range from the 50s to 60s north to south with a mix of sun and clouds. Cyclonic flow aloft will persist through the weekend with embedded waves bringing occasional chances for showers while the timing and placement of these waves vary model to model and run to run. The latest NBM maintains low chance PoPs (20-40 percent) Friday through the weekend with variable cloud cover. More in the way of clouds are likely during the afternoons with the day time heating cycle and the higher chances for showers will be over the mountains and north. Highs will generally be in the 50s to 60s and lows in the 30s to 40s north to south. Across the higher terrain, winter conditions will be present at times with highs in the 30s and lows in the 20s and chances for snow showers, mainly in the Presidential Mountains.

AVIATION /01Z TUESDAY THROUGH SATURDAY/

Through 00Z Wednesday...VFR overnight. LLWS is likely between 04Z-13Z Tuesday at all terminals as a 40kt jet moves across the forecast area between 1-2kft. These winds will be focused above a strengthening surface inversion overnight. Its possible RKD sees a longer period of LLWS straight into Tuesday evening, with only slight mixing during the daytime.

Tuesday morning will see increasing chances for showers, but these will be mainly in the vicinity of LEB and HIE. Otherwise VFR with wind gusts out of the SW 25-30kt. A few gusts 35-40 kt possible, especially during the afternoon. Expect an onshore wind north of Casco Bay (towards IWI/RKD) with gusts not as strong.

Showers become likely in the afternoon to early evening with thunderstorms also possible. These will will be capable of producing IFR to MVFR restrictions as well as strong wind gusts with the highest potential for SHRA/TSRA will be across NH.

Outlook:

Tuesday evening/tonight: Some showers continue into the evening with an going chance at HIE and LEB overnight. Chances for MVFR ceilings will also increase for all sites. Will also have to watch for marine stratus/fog that could produce IFR to LIFR conditions at RKD and PWM. If this occurs, IFR ceilings may make it to AUG. LLWS is again likely Tuesday night.

Wednesday-Wednesday night: CIGs lower through the day Wednesday with rain Wednesday afternoon through Wednesday night. MVFR likely with periods of IFR overnight.

Thursday-Thursday night: Drying trend Thursday brings improving conditions with VFR likely.

Friday-Saturday: Mainly VFR. Low chances for -SHRA could bring brief restrictions at times.

MARINE

Through Monday night...SCA conditions persist over the coastal waters through at least Tuesday night as southwest winds generally remain above 25 to 30 kt with seas continuing to build. For the Bays, seas may reach SCA levels on Tuesday, but winds look more marginal. For the outer waters, winds gusts approach gale force Tuesday afternoon into Tuesday night, but low level stability may limit Gale frequency.

Persistent SSW flow continues Wednesday into Wednesday night with SCAs likely needed and wave heights 6-10 feet. Winds shift offshore Thursday and will drop below 25 kts while seas will be close to 5 feet. Winds generally stay below 25 kts Friday into Saturday with seas 3-5 feet.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ150-152- 154. Gale Warning from 2 PM Tuesday to 2 AM EDT Wednesday for ANZ180-182.