textproduct: Gray - Portland

WHAT HAS CHANGED

Minor changes to the Aviation section for the 00Z TAFs.

KEY MESSAGES

1. A cold front will bring showers to the region on Thursday, with very light rainfall expected. A stray thunderstorm can not be ruled out across the interior Thursday afternoon.

2. Gusty northwest winds on Friday could down a few branches before sunny and dry weather expected through the weekend.

3. An area of low pressure will bring the potential for widespread rain by Monday, amounts look light at this time.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A cold front moves through the area on Friday morning. Ahead of the front, the environment is stable and convection is unlikely to build along the front. Recent CAMs indicate showers with the aforementioned front will likely be more scattered with light rainfall accumulations. Behind the front, skies clear and quickly destabilize the environment during the mid-day. Forecast soundings show an inverted-V sounding across the interior, with up to 300J of CAPE to accompany. This setup could allow for the development of a stray isolated low-top thunderstorm, with embedded gusty winds possible if storms develop. Friday's storm setup looks similar, with CAMs continuing to suggest decent mixing conditions and an isolated storm threat.

KEY MESSAGE 2 DESCRIPTION...

Even if the isolated storm threat does not come to fruition on Thursday or Friday, low-level mixing could still allow for gusty westerly winds and potentially some marginal impacts across the interior. With gusts up to 30mph possible Thursday and Friday afternoon, some loose tree branches could come down and unrestricted items like empty garbage cans and tents could get blown away.

The airmass quickly warms through the weekend as flow turns southerly with some brief East Coast ridging developing. 850 mb temperatures rise to around 15C by Sunday with southerly flow arriving. If mixing can extend up to 925 mb, this would spell a mild day with surface temps reaching the 70s inland.

KEY MESSAGE 3 DESCRIPTION...

Quite a few moving parts with remnant moisture from newly formed tropical cyclone Isaias which is expected to make landfall on the Gulf Coast this weekend, then quickly lift northeast towards our region by Sunday night-Monday in much weekend form. In fact, the only real sensible impact locally is looking to be the potential for a rainy period, which is overall a best case scenario as the region still needs rain due to ongoing deficits. Right now the NBM mean is under half an inch regionwide, but should note there is still quite a large spread in the guidance, as some deterministic outcomes bring in a soaking rainfall with quite a juicy environment that would support efficient rainfall, while other outcomes suppress the system south of our region with high pressure and ridging over the region holding firm.

The bottom line is this aspect of the forecast is still low confidence at this range, with impacts raining from just a cloudy/dry period to a period of soaking rainfall on the table. Do expect the greater chances for rain to be across southern zones regardless of how it exactly plays out.

Some vigorous northern stream energy with a positively tilted trough will swing through in the later Monday-Tuesday timeframe. Right now it appears this feature will be separate, and not phase with the remnants of Isaias. A period of rain/unsettled conditions may continue with a cold frontal passage around this time, and cooler temperatures arriving towards midweek.

AVIATION /00Z THURSDAY THROUGH MONDAY/

Mainly VFR will prevail tonight through Thursday. Scattered showers will be possible tonight across the north and will push to the coast around 12Z. These showers could bring brief MVFR conditions while confidence is not high enough to put in the TAF. West winds behind the front will gust 20-25 kts starting around 16-18Z Thursday. Winds relax Thursday night with mainly VFR, although there is potential for valley fog at LEB and HIE if winds can go calm.

Outlook:

Friday-Sunday: VFR Expected.

Monday: MVFR possible due to showery weather as a potential low tracks near the region.

MARINE

SCA level southwesterly winds and 3-6ft seas will commence this evening, and will continue through the end of the day on Thursday. Winds shift to westerlies and lighten Thursday night. Seas of 2-4ft are expected through the weekend.

Outlook: Southwesterly winds may gust up to around 30 kts late Sunday into Sunday night with seas building to 4-7 ft, highest outside of the bays.

GYX WATCHES/WARNINGS/ADVISORIES

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