textproduct: Gray - Portland

WHAT HAS CHANGED

Increased some rainfall amounts for Wed afternoon due to expected convection enhancement. Precip exit timing has been increased, with much of the precip done early Thursday morning.

KEY MESSAGES

1. A slow moving front will finally push across the region during the daytime today, exiting off the coast by Thursday morning. A soaking rain will accompany the front, but the threat for flooding remains low.

2. On and off precipitation chances continue with slightly below average temperatures.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Stalled front to the west overnight tonight will continue to channel showers and stratiform rain across NY/VT/Quebec and far northern ME. Its slow trek east will bring showers to the northern NH and western ME mountains tonight, gradually expanding into the foothills and interior Wednesday afternoon.

A wave traveling north as the front pushes east is set to arrive Wednesday afternoon as net lift peaks across northern New England. This should provide good dynamics for synoptic rain process including a strong low level jet, PWATs climbing above 1 inch, and a deep moisture profile. Completing the heavy precip checklist would be a deep warm cloud layer, but at around 8.5 kft, it falls short of being a critical value.

Guidance has trended slower for the front to arrive, but quicker to press through the CWA. Better handling of the incoming wave to ride along the front likely contributes to this. This leads to precip tapering off west to east Wed night, with just a few showers remaining towards the Midcoast come Thursday morning.

The slower arrival time does allow some daytime heating to build across the interior and foothills. This will add some instability ahead of the front and thunder may result. HREF and REFS runs have depicted better rain rates, likely from convection, occuring downstream from the Whites in the ME foothills and towards central ME. HREF 6hr QPF is much higher than the comparative NBM period, again likely due to the convection. HREF IQR spread of around 0.75" over a 6 hour period likely means isolated locations in this area could see over an inch on the day.

Just as the low helped increase dynamics along the front, it will also aid in whisking the front east of the CWA overnight and into Thursday morning. Winds become NW into Thursday bringing drier conditions and temperatures in the lower 60s.

KEY MESSAGE 2 DESCRIPTION...

A drier stretch of weather to close out the week with some clearing. Behind the cold front, a plume of anomalously cold temperatures aloft swings in later Thursday, peaking Friday. 500 mb temperatures will be closing in on -30 C, with freezing levels at or below 850 mb. Strong surface heating with the May sun angle should allow for mixing up to 850 mb however, which will help take the edge of the cold temperatures aloft, as surface highs will still reach the 50s outside of the higher terrain. Could see a few diurnally-driven showers Friday afternoon given the cold pool aloft, but coverage looks isolated (10-25% POPs) at any one location, with the best odds for a brief shower up along the Canadian border.

Additional, more widespread precipitation chances continue this weekend as the long-range pattern supports extended troughing in the Northeast U.S. Downstream blocking over Greenland will continue to result in an overall stagnant pattern. Temperatures will trend slightly below average through the weekend but it will not be notably cold.

A few progressive shortwaves moving through the zonal flow are progged to impact the region. One during the day on Saturday, then a second later on Sunday and lasting into Monday. Saturday's system will be fighting with high pressure to the north. Therefore, the exact track remains uncertain, but there is the chance it could be suppressed south of the region. The highest chance for precipitation looks to be across southern Maine/NH. The second system will have a larger precipitation shield and currently looks likely to bring measurable rainfall to the entire region. Neither system looks particularly dynamic however, with guidance largely suggesting below an inch of rainfall with either system. Regardless, it appears we will have continued opportunities to put a dent in the rainfall deficit.

AVIATION /03Z WEDNESDAY THROUGH SUNDAY/

Through 06z Thursday...Weak LLWS possible overnight as surface winds remain light but 20-35 kt jet remains 1-2kft. This falls below criteria for mention in TAFs. MVFR cigs begin towards HIE late tonight and expand into the morning hours toward LEB and AUG. May also see IFR cigs move into the Midcoast for RKD and IWI, uncertain if vis will be impacted here.

Shower coverage increases west to east Wednesday. Expect MVFR to expand towards coastal ME and southern NH by Wed afternoon. Can't rule out some CB/TS early afternoon from a AUG to LEW to CON corridor, but confidence in positioning and timing is low. Will see some pockets of IFR in the mountains and Midcoast (as far south as PWM), but less continuous in between, favoring MVFR. Ceiling restrictions then look to improve late evening as a cold front slides off the coast.

Outlook:

Wednesday Night... Improving from W to E with most areas becoming VFR. Winds becoming W/NW 5-10 kts.

Thursday to Saturday... Mainly VFR with a chance of MVFR cigs towards the mountains Thursday night. Generally W winds, 5-15 kts.

MARINE

SCA conditions continue through Wed evening as a cold front slowly approaches the waters through the day. A low will ride inland along this front Wed afternoon, helping to push the front off the coast Wed night and by Thursday morning. Winds and seas will gradually decrease into the day Thursday. Into late week, high pressure will build across the eastern CONUS Friday with another low and associated frontal system approaching New England from Hudson Bay late this weekend.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 6 AM EDT Thursday for ANZ150-152- 154. Small Craft Advisory until 2 AM EDT Wednesday for ANZ151-153. Gale Warning until 2 AM EDT Wednesday for ANZ180-182.