textproduct: Gray - Portland

WHAT HAS CHANGED

A flash flood watch is being issued for the mountains and foothills for today.

KEY MESSAGES

1. An unseasonably strong storm is forecast to cross the region Thursday, leading to a chance of both severe thunderstorms and heavy rainfall.

2. The weather pattern remains unsettled Friday through the weekend with daily chances for afternoon showers and a few storms.

3. Low pressure could bring more widespread rainfall early next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

An anomalously strong system for this time of year moves into our region from the west today. Chances for widespread rain will increase this morning with the heaviest likely being during the afternoon hours. Have increased forecast rainfall in the mountains due to an anomalously strong low level jet for this time of year coupled with relatively high PWAT values. Have heavily blended in the latest HREF and HRRR models as they will likely capture mesoscale effects to QPF more readily. Given that we have had some flooding problems over the last week or two and that mountain and foothills 1 and 3 hour FFG is relatively low, will go ahead and issue a flash flood watch for them today.

In addition there could be a few strong to severe thunderstorms in the afternoon after the larger shield of rain moves out. The issue is that instability should be lacking. However, a few hundred joules of CAPE coupled with unusually strong wind shear for this time of year we can't rule out a few rotating updrafts or some damaging line segments.

With the anomalous low level jet in place today winds could gust up to 30 MPH outside of convection.

Drier air eventually moves in tonight in the wake of the system with some upslope rain showers possibly remaining in the mountains.

KEY MESSAGE 2 DESCRIPTION... Model trends continue to hold steady in regards to broad cyclonic flow prevailing through the extended forecast period and creating unsettled conditions through the weekend.

Vertically stacked low pressure moves north of the area on Friday and into the Canadian Maritimes Saturday allowing shortwaves to rotate through the broader trough and provide the forcing for afternoon showers and storms both days (mainly in the mountains and foothills). Forecast soundings suggest freezing levels low enough to support small hail as well as 850 mb winds in the 25 to 30 kt range that could mix down to the surface. Otherwise, seasonable temperatures in the mid to upper 70s south of the mountains (60s to the north where clouds will be stubborn) should make for a pleasant start to the weekend. By Sunday the upper low opens up to a positively tilted trough, with the surface low now well of the northeast. There will still be a chance for afternoon showers and storms, but wind gusts should be noticeably more relaxed. Something else to mention for the weekend timeframe, is the potential for the remnants of what is currently Tropical Cyclone Arthur to re-develop off the Mid-Atlantic coast. If this occurs it won't have a direct impact to New England as model solutions keep it well offshore, but it could result in elevated wave heights and rip current risk along the coast.

KEY MESSAGE 3 DESCRIPTION... Global models continue to show agreement on another upper low diving into the region early next week, with continued lesser agreement on the surface low out ahead of it (and thusly our next chance for widespread rainfall). Most ensemble solutions are favoring a coastal track, but timing is all over the place with the GFS camp being the fastest by far, with rain starting Sunday night and having the low in and out on Monday. The Euro camp lags the low until Monday night and moves it up the coast during the day Tuesday. The Canadian is more in agreement with the Euro as far as timing, but by far has the weakest low of the three. Certainly a good time window to watch trends as coastal systems usually lend themselves to enhanced rainfall, but as you can see there is much uncertainty at this time. The models then all begin to trend drier toward the middle of the week as they suggest an upper level ridge building in.

AVIATION /01Z THURSDAY THROUGH MONDAY/

A strong low pressure system approaches this morning which will lead to deteriorating conditions and widespread IFR or lower categories through most of the day on today in widespread rain and thunderstorms. A few strong to severe thunderstorms will be possible, especially over southern New Hampshire. Damaging winds will be the most likely threat. Gusty southerly winds will be possible by Thursday afternoon across all terminals ahead of an approaching cold front that should cross from west to east. Winds will shift to out of the west Thursday night as they gradually diminish.

Outlook:

Friday-Saturday: Mainly VFR, but scattered afternoon/early evening showers and isolated storms could produce brief MVFR restrictions. W/WNW winds could gust 25-30 kt both days during the daytime hours, possibly more in the 30-35 kt range on Friday.

Sunday: Mainly VFR and less breezy. Similar to Friday and Saturday, afternoon/early evening showers and isolated storms could produce brief MVFR restrictions.

Sunday night and/or Monday: Low pressure may bring more widespread IFR and rain showers.

MARINE

Southeast winds begin to increase today in response to the strong storm moving out of the Great Lakes. A strong low level jet is forecast to develop in response to this storm, and while mixing will not be strong it does appear that a period of gale force southerly wind gusts is likely ahead of the front. For this reason, a gale warning has been issued outside of Penobscot Bay for this afternoon and evening.

Elevated wave heights may still be present Friday and Saturday as low pressure slowly departs the area. Will also have to keep an eye on the remnants of what is currently Tropical Cyclone Arthur as some models suggest they may redevelop off the Mid- Atlantic coast in this timeframe. This may result in elevated swell reaching the Gulf of Maine. There should be some window of sub-SCA conditions late in the weekend, but will have to monitor early next week for elevated wind gusts and seas with another low pressure system modeled to be in the vicinity of the Gulf of Maine.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Flood Watch from noon EDT today through this evening for MEZ012-013. NH...Flood Watch from noon EDT today through this evening for NHZ002>006. MARINE...Gale Warning from noon today to 8 PM EDT this evening for ANZ150-152>154-180-182-184. Small Craft Advisory from noon today to 8 AM EDT Friday for ANZ151.