textproduct: Boston / Norton

WHAT HAS CHANGED

Gale Watches continue for Tuesday afternoon through Wednesday evening. High Surf Advisory issued for the south-facing coasts from Wednesday morning through Thursday evening.

KEY MESSAGES

- Chances for soaking downpours and severe weather return Tuesday evening through early Wednesday morning.

- Limited risk for severe weather Wednesday.

- Drying out Thursday into the latter part of the week with seasonable temperatures and lower humidity into the weekend.

DISCUSSION

KEY MESSAGE 1...Chances for soaking downpours and severe weather return Tuesday evening through early Wednesday morning.

As the dominate high pressure system pushes offshore, a low pressure system pushes east through Ontario and the trailing fronts sweep through New England Tuesday into Wednesday. A near 60kt LLJ develops Tuesday into Wednesday, ushering in dewpoints near or slightly above 70F and high PWATs on the order of 2.0"-2.25" across much of southern New England.

As the frontal system approaches Tuesday, showers and thunderstorms will begin to form primarily later in the afternoon/evening hours. While the greatest risk for severe thunderstorms continues to be to our west, a surge of SBCAPE 1200-1500 J/kg combined with increasing K-indices and 0-3km helicity values will support some risk for straight-line winds within any thunderstorms that do form. Can't rule out a few brief spin-ups, but the risk remains on the lower side.

The main concern continues to be torrential rainfall given how water- loaded these storms could be, especially along the South Coast, Cape, and Islands. While model soundings indicate mid- level winds around 30-40kts, some training of showers and thunderstorms cannot be ruled out. The latest ensemble guidance has nudged up the 24 hour QPF probs of 1"+ to near 80% along the South Coast, with 2"+ probs remaining near 40-50% (especially along the Cape and Islands). Latest Flash Flood and experimental Urban Rain Rate guidances are showing increasing probabilities that Providence and surrounding urban areas could approach the 10-year ARI early Wednesday morning... near 2.3" in 2 hours or 2.75" in 3 hours. After working with neighboring offices, highest confidence in heavier rain bands setting up remains over the Mid-Atlantic/NYC locations, prompting a Flood Watch for those areas; however, later shifts may need to expand that Flood Watch further up the I-95 corridor.

KEY MESSAGE 2...Limited risk for severe weather Wednesday.

Rain continues into Wednesday, with the higher chances and totals towards the Cape and Islands Wednesday morning into the afternoon. Ensembles continue to have higher probabilities for 1"+ totals towards the Cape and Islands and along the south coast heading into Wednesday afternoon and evening, with low probs of 2"+ over the same areas though with more discrepancies among the guidance.

The main thing to keep an eye on will be a limited severe weather risk over southern New England Wednesday given the frontal passage. Guidance shows the cold front not pushing offshore until Thursday morning. By 00z Thursday, the front may be draped over the I-95 corridor. If anything, this would aid in an increased severe risk for the interior. Global guidance indicates the midlevels drying out relatively quickly behind the front while the latest NAM is slower to clear that moisture out. Something to monitor will be how much instability can be developed behind the front. NAM guidance has around 2100 J/kg of MLCAPE over much of the interior Wednesday afternoon and evening (other guidance has values around 1500-2000 J/kg as well), and dewpoints increase to the low 70s. Given these conditions, there could be a risk for severe thunderstorms for Wednesday afternoon. Will continue to monitor this risk in coming updates.

KEY MESSAGE 3...Drying out Thursday into the latter part of the week with seasonable temperatures and lower humidity into the weekend.

The cold front exits the region by Thursday morning, leaving drier and seasonable conditions in its wake. With the front, the upper level trough centered over Quebec also moves off to the northeast. High pressure starts to build in over southern New England from the Great Lakes, continuing the drier conditions into the weekend. Temperatures aloft fall slightly, with 925 mb temperatures falling to around 16C for much of the second half of the week. Notably, Wednesday night will likely be cooler as 925 mb temperatures may fall to as low as 10C over the interior. Lows there could get into the low 50s. 850 mb temperatures are expected to mostly be around 10C as well through the back half of the week. Even with these reduced temperatures, the continued high sun angle will keep temperatures from falling too low, keeping them seasonable. Conditions remain dry and seasonable until around Sunday when the flow shifts more S again, increasing humidity. A surface low may also pass just offshore, but any impacts from it -- if any at all -- are not certain at this time.

AVIATION /20Z MONDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

18Z TAF Update:

Through Tonight: Moderate confidence.

Mainly VFR; Sea breezes along the south shore and north coast, but in-and-out around Boston Harbor. Chances for fog increase more during the early morning in interior Southern New England and mentioned MVFR mist as a starting point. Winds become light SW.

Tuesday: Moderate confidence.

General deterioration to MVFR-IFR ceilings by afternoon, with risk for showers and possible storms after 18z. Better potential for storms Tue night. Winds become SE to S and increase to 10-15 kt with gusts 20-25 kt.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Wednesday through Saturday/...

Wednesday: Mainly VFR, with local MVFR possible. Windy with gusts up to 30 kt. Chance SHRA, slight chance TSRA.

Wednesday Night: VFR. Windy with local gusts up to 30 kt. Chance SHRA.

Thursday through Saturday: VFR.

MARINE

Forecaster Confidence Levels...

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

As the frontal system approach and the 60 kt LLJ form over the Cape/Islands Tuesday, SSW-ly winds will pick up to near Gale force. Gale Watches continue Tuesday evening into Wednesday, especially across the southern and southeastern waters, and later shifts may need to upgrade to Gale Warnings. Latest round of guidance is indicating marginal Gales, but rough seas 10-12 ft. Near-shore waves along the southern coast, Islands, and outer Cape are likely in the 7-10 ft, and warranted High Surf Advisories starting early Wednesday and lingering late into Thursday. Strong Rip Currents and waves near 6 ft are possible, especially across the outer Cape, Thursday as the cold front stalls out near the coast, prompting the later ending to the High Surf Advisory.

Outlook /Wednesday through Saturday/...

Wednesday: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 13 ft. Chance of rain showers, slight chance of thunderstorms.

Wednesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 12 ft. Rain showers, slight chance of thunderstorms. Local visibility 1 to 3 nm.

Thursday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers, slight chance of thunderstorms.

Thursday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers.

Friday through Friday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Surf Advisory from 6 AM Wednesday to 8 PM EDT Thursday for MAZ020>024. RI...High Surf Advisory from 6 AM Wednesday to 8 PM EDT Thursday for RIZ006>008. MARINE...Gale Watch from Tuesday afternoon through Wednesday evening for ANZ231>235-250-254>256-280>283.