textproduct: Gray - Portland

WHAT HAS CHANGED

Confidence is increasing in locally heavy rain occuring as a slow moving low pressure system moves through the area Tuesday through Thursday. The Flash Flood threat is low, but localized flooding in urban areas and areas of complex terrain is possible.

KEY MESSAGES

1. Showers and thunderstorms develop across New Hampshire Tuesday. Some of these may contain lingering heavy downpours through early evening.

2. Slow moving low pressure crosses the area Wednesday through Friday with locally heavy rainfall possible. The threat for Flash Flooding is low.

3. August starts dry, but shower and thunderstorm chances return early next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Showers with some thunder will tend to decrease in coverage across the mountains of Maine and northern New Hampshire this evening. These have been somewhat slow moving, with low environmental shear preventing these from sustaining an updraft. Other than some low stratus or fog development along the coast and into the interior this evening, tonight should remain fairly quiet.

Tuesday begins mostly dry as low pressure begins to close off arriving from the Great Lakes. The same low shear environment will exist, but additional forcing and moisture will be arriving across the western portion of the forecast area (New Hampshire primarily). Hires guidance, including the HREF and REFS, has continued to show the potential for isolated heavy rainfall amounts greater than an inch across central and southern NH. These are likely individual storms that can sustain themselves, but are moving slow in limited flow magnitude. In a precip efficient environment (PWAT greater than 1.25 inch, warm cloud depth approaching 12kft, and mean RH greater than 90 percent) the WPC's Marginal risk of excessive rainfall fits well for isolated instances of flash flooding.

The caveat is this occuring over a portion of the forecast area that has been very dry and has flash flood guidance values of 2 to 3.5 inch per one hour. If anything, the main concern may be tracking areas that see decent rainfall Tuesday, ahead of the more widespread rainfall into the remainder of the week.

Showers/thunder Tuesday should retreat west with the loss of daytime heating, but can't rule out a few more showers rotating in as the primary circulation pivots just to the west in NY.

KEY MESSAGE 2 DESCRIPTION... Global models are slowly coming into agreement on the evolution of this weeks system, and one of those aspects is the surface low track. All camps now depict the center of the surface low, and for the most part the upper low, passing south of our area midweek. How exactly this impacts rainfall amounts seems to remain a point of contention, but one thing that can be said with confidence is that this will eliminate much of the influence from the low level jet. The spread in strength of the moisture transport has decreased with the ECMWF Ensemble, GEFS, and NAMNest all depicting PWAT values in the ballpark of 1.50 to 1.75 inches south of the mountains during the day Wednesday, with values 1.25 to 1.50 inches to the north. BUFKIT soundings also suggest warm cloud depths 10-12kft, so even without favorable jet dynamics rainfall is going to be efficient. That being said the difference in PWAT values is still causing some large spread in the modeled QPF. The Euro ensemble remains on the high end of the aforementioned range and maintains it's moderate to high confidence (50-70% probabilities) of a widespread 1-2 inch rainfall. Meanwhile, the GEFS is more in the middle of the PWAT range and maintains its low confidence (10-30% probabilities) in rainfall exceeding an inch. This spread remains a forecast challenge, but I think at this point it is fair to say that Wednesday into Thursday is going to feature a more stratiform steady rain, and less in the way of convective features. Regardless, as mentioned earlier rain will be moderate to heavy at times and may pose an isolated urban and complex terrain flooding threat. Elsewhere, recent dry conditions have left soils with high infiltration capacity and ample subsurface storage, which should absorb the bulk of initial rainfall. The exception would be locations that see heavy rainfall on Tuesday and are saturated going into the stratiform rain event. Bottom line: Confidence is increasing, albeit slowly, in a decent rain event Wednesday into Thurday, but confidence in amounts remains low and will hopefully be assisted by the CAMs as this event begins to shift into their window. Some showers linger through the day Friday, before tapering off in the evening.

KEY MESSAGE 3 DESCRIPTION... Global models are in surprisingly good agreement on the 500 mb pattern going into next week. Low pressure exits to the east on Saturday and a ridge builds making for a mostly dry day with temperatures in the low to mid-80s. Shower and thunderstorm chances return Sunday as a trough begins to approach, as usual its broad brush at this time range, but it likely won't be a washout. Monday may be a different story as all the models have an upper low somewhere in the vicinity, but of course it is to early to say for sure. Bottom line: We may only get a brief break in the wetter weather as the pattern looks to largely remain unsettled.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Through 18Z Tuesday...Conditions trend SKC this evening after SHRA/TS dissipate across the ME/NH mountains. Model guidance has been advertising the formation of low stratus and some BR/FG developing later this evening and overnight. This could bring low-MVFR to IFR conditions, especially where fog develops (this may begin inland, then build down towards portions of the ME coast). Improvement is expected after sunrise Tuesday for southern NH, but these ceilings may continue for AUG/RKD/LEW/PWM.

Outlook... Tuesday: SHRA/TS develop across NH during the afternoon. These will be capable of heavy downpours, potentially bringing vis to IFR for points CON and north. Most of this activity should remain west of ME TAF sites, which will continue to see low cigs.

Wednesday-Thursday: Conditions will generally be MVFR during the daylight hours with brief IFR possible if heavier showers move over a terminal. The nighttime hours may feature more IFR as fog will fill in anywhere where rain slows or comes to an end, this will likely be most prevalent Thursday night.

Friday-Saturday: Conditions return to VFR on Friday as rain tapers off, with VFR likely prevailing through Saturday.

MARINE

Sub-SCA conditions continue early this week as low pressure closes off entering the Northeast. Some fog or marine stratus development is likely tonight, and may continue into Tuesday.

Low pressure then moves through the region Wednesday and Thursday, which may bring wind gusts near 25kts and seas to around 5ft. Conditions should ease back below SCA criteria Friday as low pressure exits.

GYX WATCHES/WARNINGS/ADVISORIES

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