textproduct: Boston / Norton

WHAT HAS CHANGED

A Gale Watch has been issued for all of the coastal and outer waters for Saturday. Gusty northerly winds are possible with the passage of a strong low pressure system.

KEY MESSAGES

- Cooler today with scattered showers developing in the afternoon, especially across eastern MA.

- Unsettled conditions Friday through early next week with periods of showers. Gusty northerly winds possible Saturday.

DISCUSSION

KEY MESSAGE 1...Cooler today with scattered showers developing in the afternoon, especially across eastern MA.

Cooler temperatures are expected today within the post-frontal airmass. 925mb temperatures range 11-13C, supporting highs in the low to mid 70s for most while onshore flow will hold highs in the upper 60s along the east MA coast. A shortwave trough drops southward across the region accompanied by a slight uptick in moisture. Weak ascent with the wave should be sufficient enough to trigger light scattered showers after late morning through the evening. Surface flow fields show indications of a coastal front off the eastern MA coast extending southward with E/NE flow offshore and N/NNW winds not far inland. This should act to enhance moisture convergence along the coastal and near-coastal areas thus favoring those areas for the greater risk for scattered showers.

KEY MESSAGE 2...Unsettled conditions Friday through early next week with periods of showers. Gusty northerly winds possible Saturday.

There is a good consensus among ensemble guidance for a blocky upper level pattern to set up over the northeast featuring upper troughing Friday through early next week. Several shortwaves moves through the broad cyclonic flow aloft bringing periodic chances for showers/rain.

The first wave features a closed upper low that drops south across southern New England late Friday into Saturday. There is still some spread amongst guidance on the exact timing of the wave and arrival of showers. Some guidance (GFS/AIGFS) indicates showers arriving as early as late Friday afternoon, while others lean later (Fri night). Showers continue through Saturday, with showers gradually diminishing in the afternoon/evening. With limited moisture, rain amounts aren't expected to be heavy with ensembles generally showing a more likely range of 0.20" to 0.80" with the higher amounts favored in northeast MA. Temperatures trend below normal Saturday in the upper 50s to low 60s. The pressure gradient tightens as a surface low tracks across the region which will promote a period of gusty N-NE winds Saturday. The expected gusts will depend on the track of the low, but if the low tracks nearby gusts 30-40 mph are possible, isolated 45 mph can't be ruled out. The ECMWF ensemble leans toward widespread gusty winds with moderate to high probs for gusts greater than 40 mph. However, the GEFS shows weaker winds overall and confines low-moderate probabilities for 40 mph+ to the Cape and waters. Something we will keep monitoring.

Sunday trends drier and more seasonable with a mix of sun and clouds.

A few more waves move through the flow beyond Sunday into early next week. There is still spread in the details of the waves such as the timing, but they may support periodic chances for showers. There is a better consensus for a round of showers sometime Monday and another Tuesday. 850mb temperatures anomalies lean below normal early next week likely yielding temperatures in the 60s and low 70s through Wednesday. There are signs of the blocky pattern breaking down later next week which should help warm temperatures back up.

AVIATION /06Z THURSDAY THROUGH MONDAY/

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.

IFR stratus and fog over the Islands clearing out and becoming VFR for a period toward 06z. Patchy BR possible closer to 12Z for Cape/Islands. VFR elsewhere. N winds 6-12 kts.

Today...High Confidence. Moderate (Ceilings ACK).

VFR for most. Potential for periods of MVFR/IFR ceilings at ACK; however, lower confidence. Scattered showers developing in the afternoon, especially eastern MA/RI with mainly VFR cigs. NW winds 10-15 kts with gusts 20 kts for the interior terminals and N-NE 8-12 kts near the eastern MA coast with an occasional gust to 20 kts possible.

Thursday night...High confidence.

VFR. Light N-NW wind.

KBOS...High confidence in TAF.

KBDL...High confidence in TAF. Low risk for a brief shower after 20z.

Outlook /Friday through Monday/...

Friday: VFR. Breezy. Chance SHRA.

Friday Night: Mainly VFR, with local MVFR possible. Windy with local gusts up to 35 kt. Chance SHRA, patchy BR.

Saturday: Mainly VFR, with local MVFR possible. Windy with gusts up to 35 kt. Chance SHRA.

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

Sunday: VFR.

Sunday Night: VFR. Slight chance SHRA.

Monday: VFR. Chance SHRA, slight chance TSRA.

MARINE

Forecaster Confidence Levels...

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

Today and Tonight...

Winds and seas below SCA thresholds. N winds tonight. N-NE winds 5-15 kts today becoming N-NW tonight. Scattered showers developing Thu afternoon. Low chance of an isolated storm.

We issued a Gale Watch for all the waters for Saturday due to the potential for a stronger low pressure to track across the region bringing gusty N-NE winds. Confidence is moderate as the Gales will depend on the track of the low.

Outlook /Friday through Monday/...

Friday: Winds less than 25 kt. Chance of rain showers.

Friday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of seas approaching 5 ft. Rain showers, slight chance of thunderstorms, patchy fog.

Saturday: Strong winds with gusts up to 40 kt. Areas of rough seas. Chance of rain showers.

Saturday Night: Low risk for Small Craft Advisory winds with areas of gusts up to 30 kt. Rough seas up to 11 ft.

Sunday: Winds less than 25 kt. Local rough seas.

Sunday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers.

Monday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers, slight chance of thunderstorms.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Gale Watch from late Friday night through Saturday evening for ANZ230-250-251-280. Gale Watch from Saturday morning through Saturday evening for ANZ231>237-254>256-281>283.