textproduct: Gray - Portland

WHAT HAS CHANGED

Latest CAM runs show a growing threat for 3-5" of rain across much of southern NH with locally higher amounts possible. As a result, went ahead and expanded the Flood Watch to include all of south-central NH. Otherwise, just made some minor adjustments to PoPs at this time.

KEY MESSAGES

1. A closed low develops over or just west of our region today and tonight. This will bring multiple rounds of rainfall with locally heavy rainfall possible resulting in isolated to scattered flash flooding, especially in southwestern New Hampshire.

2. The cutoff low is slow to depart, with gradually improving conditions through the end of the week. A semi-unsettled pattern remains into next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Not much change to earlier forecast philosophies. However, based on the infusion of latest CAM and ensemble output, we feel a bit more confidence in at least localized flash flooding across southwestern NH today and tonight. A flash flood watch has been issued to cover this possible threat.

Overall, large scale mass fields remain largely unchanged in latest model output for the next two days or so. The main issue has been finer scale feature such as where training convection will take place. Right now the most concern is for the eastern slopes of the southwestern NH hills and mountains. This is where the highest PWATs will coincide with instability and an increasing southeasterly LLJ (helps the upslope process). Overall, looking like basin averages of 1-2 inches there but moderate chances for localized amounts up to 5 inches in persistent convective showers and thunderstorms. This will likely lead to at least a localized flash flood threat, mainly this afternoon and evening.

As for the rest of the the forecast area, fog is likely this morning on the coastal plain. For this afternoon, expect increasing chances for showers and perhaps a few thunderstorms with locally heavy rainfall. However, the flood threat is much lower than southwestern NH. In place, many areas in central and mountainous ME may be dry for much of the day except the aforementioned fog and possible drizzle.

KEY MESSAGE 2 DESCRIPTION...

It will be a slow rebound to warmer, drier conditions with the upper -level low still hanging overhead Thursday, but eventually it is ejected out to sea by a 500 mb jet streak over the Gulf Stream. There will at least be a chance for nuisance rain showers until the low fully exits our region sometime Friday/Friday night, but with the upper-level forcing collapsing as the surface low occludes/vertically stacks, the chances for more heavy rain will decrease as time goes on. Thursday may bring some locally heavy rain early, especially focused along the coast, but more widespread will be areas of drizzle/mist, and generally raw conditions for July. Some mixed signals Friday, as there may be enough cold air and moisture aloft with the remnants of the trough to fuel some diurnal showers, but there is also the chance subtle height rises will result in subsidence and dry conditions prevailing. Cool temperatures in the 60s on Thursday will increase to more seasonable 70s-low 80s by Friday.

Saturday looks to generally be a rinse and repeat of Friday, with only a isolated chance of diurnal/upslope-enhanced showers, and near seasonable temperatures. The calm weather is short lived, however, as sizable rain chances return by Sunday, especially for the interior parts of the forecast area. Model spread begins to increase by this range, but there is agreement regardless that another slow- moving, moisture-rich shortwave trough will impact the Northeast U.S. to start next week. The biggest disparity at this point is the ECMWF indicating a full cutoff low over the Ohio Valley, while the GFS shows more of an open wave (but still slow moving) 500 mb trough. Interests with outdoor activities should plan for rain in the forecast Sunday-Monday.

AVIATION /14Z WEDNESDAY THROUGH SUNDAY/

Through tonight...A mix of MVFR to IFR to LIFR exists this morning in low cigs and fog. Some improvement is expected through mid morning into early afternoon before rain, heavy at times moves into southern NH and expands into Maine tonight. Some thunder will also be possible across southern NH. HIE likely stays mainly VFR through today. Winds will increase out of the SSE at 5-15 kts, with gusts to 25 kts bringing low cigs and fog into coastal TAF sites later this evening. Low cigs, fog, and periods rain likely brings widespread IFR to LIFR tonight.

Thursday: IFR cigs much of the day, with some breaks possible in the afternoon.

Friday-Sunday: Expect conditions to slowly improve on Friday as low pressure moves away from the region though scattered showers and a thunderstorm are possible. VFR conditions should dominate Saturday outside of an isolated afternoon shower/storm. Winds at or below 10kts.

MARINE

Broad low pressure moves east into New England. Onshore flow will result in abundant marine stratus and vis reducing fog today and tonight.

SCA conditions likely this evening through early Fri with S-SE wind gusts southwest of Pen Bay up to 25 kts with seas of 3-7 ft.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...Flood Watch through Thursday morning for NHZ005>015. MARINE...Small Craft Advisory from 11 PM this evening to 8 PM EDT Thursday for ANZ150-152>154.