textproduct: Gray - Portland

WHAT HAS CHANGED

Quick update to the aviation section for the 12Z TAFs. Otherwise, just loaded in the latest surface observations.

Rainfall amounts have increased for today into Friday morning across SW Maine and SE NH, while decreased near the Mid Coast into central Maine. There is a signal for a band of moderate to heavy rainfall to pivot across southern NH into western Maine this afternoon and evening. This band of rainfall could produce poor drainage issues and ponding of water on roadways during the evening commute.

KEY MESSAGES

1. Moderate to locally heavy rainfall overspreads the area today and last into Friday morning. While the flood risk is low due to ongoing drought conditions, heavier rainfall rates occuring this afternoon and evening could bring impacts to the evening commute.

2. Warmer than normal temperatures arrive beginning this weekend, and may be much above normal parts of next week. Rainfall chances will be few and few between.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Rainfall will remain showery in nature early this morning as low pressure over Ontario transfers to a secondary low forming near southern New England around mid day. All the while an upper trough will take on a negative tilt spreading strong forcing for ascent over New England. Latest available NWP guidance is honing in a north-south axis of moderate rain forming over western or southern New England and pushing into NH around mid day and early afternoon. However, there are some model solutions that keep this axis of rainfall south and west of NH, which will have implications for total rainfall amounts.

Upper dynamics will continue to strengthen through the afternoon into this evening with the secondary low tracking near Cape Cod. The 850 mb low will slide south of the area Thursday afternoon aiding in the development of a SE low level jet around 40 kts. This LLJ will advect moisture into the region bringing PWATs up over an 1 inch while warm cloud depths remain below thresholds for efficient rainfall processes. However, strong dynamics may help compensate allowing periods of heavy rainfall.

Models are in better agreement in another axis of moderate to heavy rainfall developing in the presence of this low level jet. There continues to be some run to run and model to model variability in the placement and residence time of this rainfall axis. A general consensus puts it between Casco Bay and SE NH extending northwestward into the mountains. It is this area where total rainfall amounts may exceed 2 inches with moderate to heavy rainfall bringing potential for poor drainage issues and ponding of water on roadways during the evening commute. Due to ongoing drought conditions, 3-hour flash flood guidance is greater than 2-3 inches. The latest HREF max 3-hour rainfall rates come up short of this criteria suggesting the risk for flash flooding is low. This axis of rain will pivot through the coastal plain Thursday night with rainfall rates diminishing into Friday morning as the upper low shifts southeast of the area.

Chances for rain will diminish from NW to SE Friday as the surface low and upper low continue to pull east of the area. Total rainfall from tonight through Friday from the Mid Coast into Somerset County and north of the mountains will be on the order of 0.5 inches. Along and south of the mountains towards the Maine coast amounts are generally 1-2 inches with amounts pushing 2 to 3 inches across far SW Maine into the NH Seacoast.

KEY MESSAGE 2 DESCRIPTION... The upper-level trough exits the region Friday night, with height rises quickly building in behind it. A strengthening ridge will be building over New England, peaking early next week. On face value, a much anticipated pattern change to warm and dry is in the cards, with sunny skies most days and only widely scattered precipitation chances.

A cold front will approach Sat and warm air advection will strengthen ahead of it. This will transport much warmer air aloft into the forecast area and temps should climb well into the 60s with some isolated 70s. WSW flow should preclude any cool onshore flow. There does not appear to be much a strong push to drive the cold front entirely thru the forecast area. This keeps a low (10-30%) chance of precipitation, most likely across the northern half of the region. There could be an isolated downpour/thunderstorm late Saturday, but the better chance likely arrives overnight to early Sunday morning. Subsidence behind the front should allow for conditions to dry out Sunday afternoon, combined with dry northwest surface winds. Depending on the timing of the front, it could be quite warm across S NH/SW Maine, as westerly winds may allow a downslope warming boost. Some warm spots may make a run at 80 degrees if deep mixing and strong surface heating occurs.

Flow returns to the south on Monday as a trough lifts out of the central CONUS, with downstream ridging amplifying further. Some guidance has been hinting at a more onshore (easterly) component to the surface wind, so will have to watch closely to see if cooler temperatures occur, primarily across Maine. Models have backed off on an actual backdoor cold front signal for now, so thinking more likely 70s in interior NH and 60s elsewhere, but still an outside shot for locally cooler along the immediate coast.

The real push of warmer air is expected to arrive by Tuesday. There is strong ensemble agreement in temps climbing much above normal. Highs in 80s and maybe even a few 90s are possible across parts of the area. If that comes to pass there will be the typical early heat concerns for sensitive populations, but also the hazard of recreation while the inland and ocean waters remain very cold. As this round of heat breaks midweek there will be another chance for thunderstorms and long range machine learning severe weather guidance suggests this as another period to keep an eye on.

AVIATION /10Z THURSDAY THROUGH MONDAY/

Through 12Z Friday...There will be a window of 30-40kt LLWS into early this AM. More RA/SHRA today, mainly focused south and west of AUG/RKD. Would expect trend to MVFR early morning, with IFR into the daytime for most terminals. RA may be heavy at times towards southern coastal ME and southern NH terminals tonight into Friday morning. Would expect IFR vis/cig, with LIFR possible.

Outlook:

Friday: Becoming VFR from west to east. NNE 5-15 kts.

Friday Night: VFR conditions expected.

Saturday: VFR conditions expected.

Saturday Night: VFR conditions expected.

Sunday: VFR conditions expected. Some local MVFR or lower possible in showers.

Sunday Night: VFR conditions expected.

Monday: VFR conditions expected.

MARINE

Southerly winds relax early this morning as they shift easterly. Another period of SCA conditions is expected this afternoon through into Friday in strengthening easterly flow that shifts northeasterly tonight with wave heights building to 4 to 6 ft Friday. Seas remain elevated into the weekend.

An approaching cold front will bring strengthening SW flow Saturday with gusts approaching SCA thresholds Saturday afternoon into the overnight. Winds and seas will drop below SCA thresholds Sunday into Monday.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory from 4 PM this afternoon to 8 AM EDT Saturday for ANZ150-152-154.