textproduct: Gray - Portland

WHAT HAS CHANGED

Minor update to Aviation section for the 12Z TAFs.

KEY MESSAGES

1. Pleasant weather continues through today under high pressure.

2. A low pressure system stalls out over the Northeast from Tuesday through the end of the week. Expect a wet middle-late week sequence, with locally heavy rainfall possible.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A cold front begins moving eastward toward the region today, but it doesn't look like it is going to make much progress as an area of short wave ridging aloft moves eastward as well. There is a low probability that one or two showers or thunderstorms cross the CT River by early evening but they won't last long as they'll run out of solar insolation and run into an area with warning mid level temperatures.

KEY MESSAGE 2 DESCRIPTION...

Closed low down to 995 mb will slam on the breaks as it fully occludes and cuts off from the upper-level flow. The result will be a showery period later Tuesday onward. Will break down the details below, but in a nutshell, expect scattered-numerous showers and embedded thunderstorms each day through at least Friday. Falling heights and surface-500 mb thickness, combined with abundant cloud cover, suggests below average daytime temperatures, however the humidity will be back as dew points surge into the 60s.

Tuesday's precipitation chances will be in the west - primarily across New Hampshire. Marginal daytime instability likely develops with morning sunshine, but weak shear (0-6 km BWD < 20 kts), will hold back a formidable chance for organized/long-lived convection. Likely some topographically-enhanced pulse storms across the higher terrain. Should see these weaken around sundown given the limited shear.

The surface low marches closer on Wednesday. A TROWAL-enhanced band likely lifts onshore around this time, but guidance remains undecided whether central New England is more directly impacted, or if it primarily stays further south. Thus, Wednesday has a range of outcomes from a washout to mostly dry, with the best odds for the latter nearest to the Canadian border. What is more likely however is with the low center spinning to our southwest, increasingly onshore develops, which may usher in some rather cool conditions for July, especially across Maine.

From Thursday onward, impacts become less clear, especially outside of the hi-res guidance window, but the GFS and ECMWF are in rather strong agreement through Friday the surface low slowly meanders/dissipates in the vicinity of southern New England. With the look of a cold pool aloft, likely we see diurnally/topographically forced downpours/thunderstorms Thu-Fri, focused each afternoon. Moist columns with marginal MUCAPE and weak shear suggest pulse storms primarily posing a heavy rain risk rather than severe hazards. There also remains uncertainty how long the surface low meanders, with the GFS suggesting improving conditions by Saturday, while the ECMWF is less optimistic. Current mean ensemble QPF outputs are for 1-2" regionwide, with a 90th percentile of 2-4", with the gradient featuring the highest totals across SW New Hampshire and the lowest in central Maine. Keep in mind however these are 3-4 day totals, so not overly concerned about excessive rainfall at a glance, but still will monitor closely.

AVIATION /10Z MONDAY THROUGH FRIDAY/

Through Tonight....VFR prevails today into tonight. Patchy valley fog cannot be ruled out tonight while more in the way of clouds may limit its formation in the CT Valley.

Outlook... Tuesday: SHRA develop across NH in the afternoon with MVFR cigs. TS possible.

Tuesday Night: MVFR to IFR cigs possible as clouds thicken and lower. Fog development possible across southern ME with MVFR/IFR vis restrictions.

Wednesday thru Thursday: Some vis recovery towards VFR, but ceilings remain MVFR with IFR possible for much of NH and far southern ME with RA. Heavy rainfall rates may cause lower restrictions.

Friday: Improvement expected, but there is uncertainty on coverage and intensity of restrictions at this time. RA should be decreasing in coverage, meaning greater chances of VFR/MVFR vs. MVFR/IFR.

MARINE

Sub-SCA conditions continue through at least early Wednesday. Thereafter the flow over the waters picks up a bit which may result in marginal SCA conditions Wednesday night through Thursday.

GYX WATCHES/WARNINGS/ADVISORIES

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