textproduct: Gray - Portland

WHAT HAS CHANGED

QPF has trended downward a hair with models suggesting a more progressive system, but a widespread soaking rain is still expected.

KEY MESSAGES

1. A widespread soaking rain is expected Thursday with much of the area receiving 1-2 inches of rainfall before it comes to an end early on Friday. Due to antecedent dry conditions and low stream flows, flood risk is low.

2. A cooler than normal and showery pattern is expected for the weekend through early next week though no day looks to be a washout at this time.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Moisture continues to be funneled into the region this evening ahead of a frontal boundary approaching from the west. CAMs continue to suggest some scattered showers may develop and if they do they may be briefly heavy. However, there is still plenty of dry air aloft and depending on how much the column can saturate by then, they may just end up being a continuation of the drizzle. Temperatures tonight are going to be moderated by the climbing dewpoints which are already in the 40s, so anticipate a milder night with patchy fog developing as well.

The bulk of the rain begins to move into New Hampshire overnight and spread eastward across the area by the early morning hours Thursday as the region becomes sandwiched between two low pressure systems. The one to our north drags the cold front through while the one to the south is going to provide the continued moisture feed for some enhancement of rain rates as the front pushes east. With that being said PWATs are generally only 0.75-1" across the area with the higher end of that being along the coast. This makes me think that, while some locally heavier rain is possible, the event looks more like a drawn out steady rain. This will be great for two reasons, it should help us make a dent in our drought and any risk of flooding is low since we have been dry and the rivers still have plenty of storage capacity. QPF has trended down ever so slightly owing to the more progressive nature of this system, but a soaking rain is still expected. The QPF gradient looks like it could be split between the two states with New Hampshire not seeing the enhanced rainfall rates and ending up more in the 0.5-1" range (NBM probabilities of more than an inch are largely less than 30%). The White Mountains should see some upslope enhancement and therefore locally higher amounts. This runoff will raise the levels of some of the more sensitive rivers in this area, but again flooding is not expected. Western Maine is more likely to see amounts in the 1-2" as low pressure moves into the Gulf of Maine Thursday afternoon adding some additional moisture into the mix, but NBM probabilities of amounts exceeding 2 inches are low (20-30%). The only exception is right along the coast from Casco Bay northward where there is a narrow band of probabilities in the 50-60% range, but with the "low" PWATs we are probably only talking about a 10th or 2 over.

Precipitation begins to taper off overnight Thursday/early Friday morning as the surface lows make their exit, but an upper low will be moving into the region which will keep some clouds and showers hanging around during the day Friday. South of the mountains where skies may be able to clear more, temperatures rebound into the 50s, but to the north where clouds will be more abundant temperatures likely get stunted in the 40s.

KEY MESSAGE 2 DESCRIPTION...

Teleconnection indices show -EPO/-NAO through the next week which matches well with longwave trough centered over eastern North America through the period. With H5 trough axis /and associated cool temperatures aloft/ centered overhead through the weekend expect temperatures to remain a few degrees below normal for the beginning of May with morning sun giving way to afternoon showers. Wet bulb zero heights fall to below 2kft so expect snow showers in the mountains with some graupel likely in spots to the south and east. Troughing briefly lifts on Monday before recharging for Tuesday/Wednesday. Frontal passage late Tuesday to push cooler air back into the region brings with it another threat for showers with some potential for a thunderstorm given timing and some weak instability ahead of the front.

Frost/freeze program begins May 1 for coastal areas and the lower Merrimack Valley in NH. Upper 30s are likely in these areas for the nights beginning Friday night and continuing through the weekend...so we/ll have to watch esp away from the coastal zones where some patchy frost may be possible.

Otherwise, no widespread high impact weather is expected through the period.

AVIATION /18Z WEDNESDAY THROUGH SUNDAY/

Through 18Z Thursday...IFR continues at coastal terminals through at least 18Z Thursday, with the remainder of terminals expected to come down to IFR around 00Z Thursday (LEB and HIE may not come down until 09Z) with fog and drizzle developing before the onset of rain. Rain may improve visibilities some, but low ceilings are going to persist into the afternoon hours. Some heavier rain is possible at times Thursday and convection can not be ruled out, especially near the coast.

Outlook: Thursday night: IFR improving to MVFR as rain begins to come to an end overnight.

Friday: MVFR improving to VFR as ceilings lift and scatter.

Saturday-Sunday:VFR mornings giving way to showers /rain with some snow- graupel possible/ with scattered MVFR restrictions this weekend.

MARINE

SCAs have been extended into Friday as swell continues to build, peaking at 6-9 ft on Thursday, and then will be slow to fall below 5ft. Wind gusts will start to become a factor as well Thursday as low pressure moves into the Gulf of Maine. The coastal waters off central Maine and Penobscot bay will see more frequent easterly gusts 25-30 kts. The outer waters (25-60 nm) from Stonington to Port Clyde may even see a few Gale Force Gusts.

Residual SCAs due to waves are likely to open the period Friday night into early Saturday with a break in winds through the remainder of the weekend into Monday of next week likely bringing headline-free conditions. Southerly winds strengthen ahead of a cold front on Tuesday with SCA conditions returning.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 8 AM EDT Thursday for ANZ150-152- 154. Small Craft Advisory from 2 PM Wednesday to 8 AM EDT Thursday for ANZ153.