textproduct: Boston / Norton

WHAT HAS CHANGED

Inner southern waters added to the Gale Watch.

KEY MESSAGES

- Light showers exit early this morning, pushing offshore of the Cape and Islands by late morning. Breezy and cool this afternoon.

- Dry and cool conditions persist through mid-week. Monitoring potential late-week coastal low that has potential to bring rain, strong winds, and coastal impacts depending on the track.

DISCUSSION

KEY MESSAGE 1...Light showers exit early this morning, pushing offshore of the Cape and Islands by late morning. Breezy and cool this afternoon.

Light showers shift eastward across the eastern parts of the region early this morning. The remaining showers likely push of the Cape and Island by late-morning. Expect areas of patchy fog early this morning. A strong surface shifts east across the upper Great Lakes pushing high pressure in behind the exited system today. This will bring in a dry and cooler than normal airmass into the region starting today. Mid-level dry air pushes in quickly within the northerly flow, with some drying in the lower levels through the morning based on model soundings. This will result in gradual improvements in the low clouds and eventual some breaks in the clouds mid morning into the afternoon. Low clouds may linger longer for the Cape and Islands. Another cooler day expected today with 850mb temperatures 10-13C yielding highs in the low to mid 60s. Some spots may reach upper 60s in the valleys. Expect northeasterly onshore flow today. As a strong high builds in to the northwest the pressure gradient tightens supporting breezy N/NE winds for the south coast extending to the Cape/Islands with gusts 20-25 mph.

Mostly cloudy conditions tonight. Despite the limited radiational cooling, temperatures should fall into the 40s across most of the interior. Urban areas and areas near the coast will trend warmer in the 50s.

KEY MESSAGE 2...Mainly dry and cool conditions persist through mid- week. Monitoring potential late-week coastal low that has potential to bring rain, strong winds, and coastal impacts depending on the track.

Ensemble guidance remains in good agreement that an anomalously strong surface high will set up over Quebec and persist through late week. This will advect in a below normal airmass, yielding high temperatures in the upper 50s and 60s alongside a prolonged period of northeasterly flow through the week. A weak low pressure system moves through the Mid-Atlantic on Tuesday. There remains uncertainty regarding its northern extent; however, if it tracks north enough, a few light showers are possible. The higher probabilities for any showers will be for CT, south coast, and Cape/Islands. Otherwise, expect mainly dry conditions through mid-week.

Attention turns to a developing coastal low late in the week. Deterministic and ensemble guidance continues to highlight a slow-moving low developing and tracking northward toward the northeast coast by late-week. However, considerable spread remains in the exact track and strength of the low. Ensemble means have largely settled on a middle-ground solutions of the low developing off the Mid-Atlantic coast Wednesday, followed by a northward track into closer proximity of southern New England Thursday and Friday. Looking at individual members, tracks range from as far west as Long Island to near Cape Cod, to further offshore southeast or east of southern New England. Recent ECMWF ensemble runs have exhibited subtle shifts east/west in the cluster of low tracks, while the GEFS has show a subtle but consistent trend west. Regardless of the exact track, ensemble means bring the low close enough to bring strong gusts, rain, and coastal impacts(erosion/coastal flooding). It is important to note that the exact track and low strength will strongly influence the magnitude of gusts, QPF, and how far inland the rain extends.

While the center of the low likely holds off until late week, marine and coastal impacts may begin as early as Wednesday. As the low deepens in the Mid-Atlantic, the robust high to the north will aid in tightening the regional pressure gradient, particularly across the southern coastal waters. This will support development of strong winds by Wednesday AM, well ahead of the primary low. The exact positioning and strength the associated areas of high/low pressure will dictate how strong the winds could be. There is growing confidence in Gale force(40mph+) for the southern coastal waters which has been heavily supported by the ECMWF and its ensemble, though the GEFS/GFS remain slightly weaker. A Gale Watch has been issued in these areas. As the system lifts northward expect strong gusts to continue if not strengthen and expand across portions of the waters. The ECMWF ensemble even shows low probabilities (10-25%) for greater than 45 mph gusts Thursday into Friday. Inland winds will also be gusty, though lighter than over the waters with gusts 25-35 mph, strongest along the immediate coast.

This prolonged period of gusty northeasterly winds will build rough seas and elevate the threat for coastal erosion and flooding depending on the track. Given the aforementioned spread in track and intensity across models, rain amounts have also remained uncertain. Most guidance suggests the rain shield will clip at least the far eastern portions of MA with the 50th and upper percentiles shifting the shield further west. Some deterministic run and individual ensemble members indicate amounts > 1.0" which will require monitoring given this is a slow moving system.

In summary, confidence is steadily increasing that a coastal will at least impact the coastal waters and coastal areas late this week. However, this is still far enough out that the details (track, strength, upper level pattern), so the exact impacts and magnitude are still uncertain at this time. Stay tuned.

AVIATION /08Z MONDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

06Z TAF Update:

Through 12Z...High confidence in trends, moderate confidence in ceilings.

IFR with localize LIFR ceilings early this morning with visibilities generally mainly between 2-5SM. Patchy fog possible through 12Z. - SHRA slowly exits the eastern terminals early this morning and should be offshore the Cape and Islands by 15Z. Winds shift to the N at 8-12 kt.

Today...High confidence in trends, moderate confidence in timing and ceilings.

IFR/MVFR improving to VFR through the day, though exact timing is lower confidence. General trend is ceilings improving to VFR 09-14Z for interior terminals and 13-16Z for more east terminals (BOS, PVD). Improvement trend will be likely slow for Cape/Islands potentially not VFR until 18Z-21Z. NNE to NE winds increase to 10-15 kt, with gusts up to 25 kt possible across the Cape/Islands. Gusts diminish by 00Z, although sustained winds 10-12 kts possible for Cape/Islands early evening.

Tonight...Moderate confidence.

VFR. Lower-end VFR ceilings are possible near the southern coast and Islands closer to 12Z. NNE winds 8-12 kt, increasing to 10-15 kt across the Cape and Islands.

KBOS Terminal...Moderate confidence in TAF.

Ceilings improving to VFR 12-15Z. NNE winds 8-12 kts with gusts up to 20 kts developing after 15Z through around 21Z.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday through Friday/...

Tuesday: VFR. Breezy.

Tuesday Night: VFR. Windy with local gusts up to 30 kt.

Wednesday through Wednesday Night: VFR. Windy with gusts up to 30 kt.

Thursday: VFR. Windy with gusts up to 35 kt.

Thursday Night: VFR. Strong winds with local gusts up to 40 kt. Slight chance SHRA.

Friday: VFR. Strong winds with gusts up to 45 kt. Chance SHRA.

MARINE

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

Through Monday Night...High Confidence.

Any remaining showers end mid to late morning across the coastal waters. Winds N/NE today with gusts prompting Small Craft headlines for all southern waters and Cape Cod Bay through the afternoon. Seas build to 3-5 ft, with the highest seas across the outer waters. Winds subside Monday evening, but only for a brief time as winds increase Tuesday with rougher seas (4-7 ft). Winds continue to strength into mid-week and furthermore into late week ahead of an approaching coastal low. This has promoted a Gale Watch for the southern coastal waters. The inner southern waters have been added into the Watch in the most recent update. There is still uncertainty in the timing of the Gale force gusts and how widespread they will be. Strong winds and rough seas continue through late week.

Outlook /Tuesday through Friday/...

Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Slight chance of rain showers.

Tuesday Night: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 10 ft.

Wednesday: Strong winds with gusts up to 35 kt. Rough seas up to 13 ft.

Wednesday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 14 ft.

Thursday: Strong winds with gusts up to 45 kt. Rough seas up to 15 ft. Slight chance of rain showers.

Thursday Night: Moderate risk for gale force winds with areas of gusts up to 45 kt. Rough seas up to 16 ft. Chance of rain showers, slight chance of thunderstorms.

Friday: gale force winds with gusts up to 45 kt. Rough seas up to 18 ft. Rain showers likely, slight chance of thunderstorms. Local visibility 1 to 3 nm.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 8 AM this morning to 5 PM EDT this afternoon for ANZ231>235-237. Gale Watch from Tuesday afternoon through Thursday afternoon for ANZ235-237-255-256-282-283. Small Craft Advisory from 8 AM this morning to 3 AM EDT Tuesday for ANZ254>256.