textproduct: Gray - Portland

WHAT HAS CHANGED

The Flood Watch for flash flooding has been expanded to include all of south-central NH.

KEY MESSAGES

1. A cutoff low will bring moderate to heavy rainfall through Thursday across southern areas, resulting in isolated to scattered flash flooding, especially in southwestern New Hampshire.

2. The cutoff low is slow to depart on Friday, with gradually improving conditions through most of the weekend. A semi-unsettled pattern returns late Sunday and into the next work week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Surface low pressure is currently located across southwestern CT as of 18Z this afternoon with the associated rain shield extending northward through southern and central New England. Rainfall so far has been confined to south-central NH with amounts up to around an inch across southwestern NE including the Monadnocks. Latest CAM guidance suggests that this rain shield will continue its northward trajectory through the remainder of the afternoon and evening but it should dissipate some as it moves north of the mountains given that most forcing for ascent will remain further to the south.

Cutoff low pressure will continue to bring periods of moderate to locally heavy rainfall through tonight as the SE LLJ increases, allowing for training of convective showers within a favorable environment for heavy precipitation. The area of greatest concern for flash flooding continues to be across southern NH, particularly in urban areas where there is less permeable surfaces as well as across the hilly terrain of southwestern NH including the Monadnocks. Flash flooding is also possible across the Whites where the SE flow will assist in upslope precipitation enhancement. Low temperatures tonight will be into the 50s.

Convective showers will persist through the day on Thursday with some breaks mixed in between. Given that soils will be saturated in much of southern NH and portions of southwestern ME, will need to monitor for any additional heavy rainfall as that may result in more hydro concerns. Forecast rainfall amounts remain largely unchanged with generally between 1-3" expected across south- central NH with the latest 12Z HREF showing the potential for localized amounts of up to around 5". Rainfall amounts are then expected to drop off quite rapidly north of the mountains as well as across much of western ME with some northern locations likely to receive little to no rainfall. High temperatures on Thursday will be limited to the upper 60s/lower 70s with lows into the 50s.

KEY MESSAGE 2 DESCRIPTION...

The upper low will be slow to depart on Friday as it moves across northern New England through the day, gradually meandering east. This pattern will lead to some isolated/scattered showers, especially over New Hampshire, the western Maine mountains, and coastal areas. 12z NAM Nest soundings also suggest that we may have 200 to 500 J/kg of MUCAPE floating around so some rumbles or thunder are not out of the question. Shear will be weak, so this activity should be fairly tame. The upper low finally kicks out in the evening/overnight hours. After a cool Thursday, Friday will start a warming trend into the weekend with highs expected to mainly range from the lower 70s north, to the lower 80s south.

An upper level ridge axis will then cross the region on Saturday which will keep the area mostly dry, but a few isolated mountain showers/storms will be possible. Temperatures will increase again on Saturday with the ridge moving overhead, with highs expected to range from the mid/upper 70s north, to the low/mid 80s elsewhere.

On Sunday, we transition back into southwest flow aloft as the next trough approaches from the west. This will keep temperatures similar to Saturday but it could be just a touch cooler as clouds stream back in. The next chance for widespread rainfall will then move in Sunday evening, potentially sticking around through Monday evening/night. As of now, this system doesn't look to be a prolific rainmaker with NBM probabilities of a half inch or greater only maxing out in the 20 to 40 percent range, but most will see at least some rain with the best chances in the foothill and mountains. After this system exits, the rest of the week looks fairly quiet with normal to above normal temperatures.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Through 18Z Thursday: Periods of RA along with low ceilings will persist through Thursday, bringing IFR to LIFR restrictions. Fog is also likely tonight into Thursday morning. Winds will be out of the SE at 10-15 kts with gusts up to around 25 kts. LLWS is not anticipated through the period. Winds will become northerly on Thursday night and diminish to 5-10 kts with gradually improving flight conditions overnight.

Outlook:

Friday-Sunday: Expect conditions to slowly improve on Friday as low pressure moves away from the region though a few scattered showers and a thunderstorm are possible.

Saturday-Sunday: VFR conditions should dominate outside of an isolated afternoon shower or storm. Winds will mainly be at or below 10 kts.

Sunday night-Monday: Increasing chances for showers and storms may lead to MVFR/IFR restrictions Sunday evening through Monday.

MARINE

East-southeasterly winds will persist through Thursday before becoming NNE by early Thursday evening while diminishing. A SCA is in effect for Casco Bay along with the near shore waters from 03Z Thu through 00Z Fri for ESE wind gusts up to 25 kts with seas of 3-6 ft.

We remain below SCA criteria through the weekend and into next week, but we will see increasing onshore winds Sunday afternoon and into the overnight hours. Southerly winds with gusts up to 20 knots are possible during this timeframe.

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.