textproduct: Gray - Portland

WHAT HAS CHANGED

Added fog to most of the river valleys that received rain overnight. That will only last another hour or so at most.

KEY MESSAGES

1. Cooler weather and relatively low dew points are expected from today into early next week. The best chance of showers and thunderstorms will be late Wednesday into Thursday. High astronomical tides are expected through Friday night, but impacts should remain minimal.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Broad troughing begins to take hold today which is going to kick off our transition toward cooler, more seasonable temperatures. Northwest flow quickly drops 850 mb temperatures down to +12-14C by the afternoon meaning the ceiling for surface temperatures is only in the low to mid-80s south of the mountains, and in the mid to upper 70s to the north. CAMs are hinting at some isolated showers near the International Boarder during the day, but with drier air also being advected in I imagine that is the worst case scenario. Wednesday look like a similar day temperature-wise with more zonal flow aloft, and the drier airmass firmly in place. Again CAMs are trying to hint at isolated afternoon showers in the mountains, but they may have trouble overcoming the dry air.

Precipitation chances increase late Wednesday night into Thursday as a trough axis approaches the area. As of now models are suggesting the brief period of southwesterly flow leading into Thursday afternoon is not going to do much to increase moisture with long range ensembles and the NAM only suggesting PWATS climb into the 1 to 1.25 inch range (a trend downward from earlier). Dewpoints also look to end up in the low to mid-60s, and long range ensembles suggest that around 500 J/kg of CAPE could form (with 25-30 kts of shear), so a strong storm is possible (and CSUs machine learning model has some low probabilities) but it is too early to say for certain what the parameter space is going to look like. The other point of uncertainty is going to be coverage in general as the ECMWF and GFS ensembles have come into better agreement on the surface low pressure forming over the St. Lawrence River and ultimately tracking east of Maine. This may keep coverage on the lower end, but I am fine with the blanket 30-50% PoPs the NBM is advertising for most of the area until we can get this event in the CAM window.

One thing the aforementioned low track would certainly portend to is clearing out on Friday. Some showers would still be possible as the forcing from the trough axis moves through Friday afternoon, but for the most part high pressure could start building in fairly quickly afterward. This may make for breezy conditions Friday and Saturday as the pressure gradient tightens as a result, but the chance for a nice weekend is looking better.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Through tonight...Outbreak of valley fog overnight thanks to rainfall. That will lift in short order this morning, but may linger thru 12z at LEB and HIE especially. Otherwise VFR conditions expected today, with a steady west breeze occasionally gusting to 20 or 25 kt. Cannot rule out an isolated shower or storm today, but confidence is too low to include in any TAF currently. Valley fog will be possible again tonight once winds diminish.

Outlook:

Wednesday: VFR expected.

Thursday-Friday: At least MVFR restrictions are likely due to trough axis bringing rain and thunderstorms. Conditions improve to VFR after the aforementioned trough axis exits the area.

Saturday: VFR expected.

MARINE

Showers and thunderstorms are possible overnight near the coast as a warm front pushes through the area.

Broad high pressure gradually builds in today, keeping fair conditions ongoing through at least tonight. Winds and seas will largely remain below SCA criteria through the week as a series of generally weak cold fronts cross the waters.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...None.