textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes have been made to the forecast.

KEY MESSAGES

- Showers with a few embedded thunderstorms overspread the region this morning. Pockets of torrential rainfall bring the potential for localized flash flooding with the greatest risk near the south coast.

- Overall the weather looks rather quiet and mainly dry for the upcoming work week with just the risk for a few showers sometime Thu and/or Fri. Seasonable temps Mon and Tue with a brief bout of summer warmth possible Wed and/or Thu ahead of a cold front.

DISCUSSION

KEY MESSAGE 1...Showers with a few embedded thunderstorms overspread the region this morning. Pockets of torrential rainfall bring the potential for localized flash flooding with the greatest risk near the south coast.

A fairly strong shortwave and associated mid-level warm front will approach the region this morning. In response...the atmosphere will undergo quite an instability burst with the Showalter indices dropping nearly 20 units inside 12 hours. This is often a very strong signal for a cluster of showers and a few embedded thunderstorms along with the potential for localized torrential rainfall. Pwats on the NAM/GFS climb to near 2.25 inches across RI/SE MA today...which is extremely high for this part of the country. In fact...the EPS situational awareness table has those Pwat values outside the model/s climatology for this time of year. In addition...the HREF/s worst case scenario indicates modest probabilities for 6 HR rainfall to exceed the 10 year ARI/s for RI/SE MA. Lastly...the HREF continues to indicate a 30 percent probability for 3"+ of rain inside 3 hours across RI/SE MA. That is often another signal to look for in the potential for flash flooding. Keep in mind though...these are just favorable ingredients and mesoscale factors will determine the outcome.

The above ingredients are favorable to produce a narrow swath of torrential rainfall and potentially localized flash flooding...But only if the mesoscale environment cooperates. That is often very hard to forecast even in the near term. In this case...the track of a meso-low and its surface boundary will likely be the main player. If it tracks near the south coast and a surface boundary enhances convergence with the extreme Pwat plume in place...the risk for torrential rainfall and localized flash flooding will increase. Current guidance is still favoring RI/SE MA and especially areas near the south coast at the greatest risk for this occurrence. However...the current state of the science still prevents us to pin an exact area so we can not rule out this risk further north even into the Boston and Worcester metro areas. And this is still only a watch...so if the mesoscale environment does not setup the band of torrential rainfall and localized flash flooding will not occur.

Lastly the overall severe weather threat is quite low today given most of the instability will be elevated. However...there is a very low risk for a brief spin-up/waterspout in the vicinity of the mesoscale low and its associated triple point. Again...it is a rather low probability but something we need to watch in these setups.

So to sum up...The Flood Watch continues for the BOS-ORH corridor points south and east with the highest risk near the south coast. The potential exists for 2-4" of rain inside 3 hours within the Flood Watch. Outside that region...rainfall amounts of 0.50" to 1.50" seem reasonable.

The system itself is fairly progressive...so expect the bulk of the steady showers to come to an end from northwest to southeast roughly between 2 and 4 pm. However...some of the steady activity may linger until 5-6 pm towards the Cape and Islands. A cold front will likely bring a round of brief scattered showers very late tonight into early Mon morning...but not the widespread heavy rainfall we will be dealing with today.

KEY MESSAGE 2... Overall the weather looks rather quiet and mainly dry for the upcoming work week with just the risk for a few showers sometime Thu and/or Fri. Seasonable temps Mon and Tue with a brief bout of summer warmth possible Wed and/or Thu ahead of a cold front.

Any lingering scattered showers depart the southeast New England coast by mid-morning Monday behind the cold front. Otherwise...high pressure builds back into the region Mon and Tue bringing dry and seasonable weather. High temps will mainly be in the upper 60s to the middle 70s with lows in the 40s and 50s both days. This high pressure system moves east of the region by Wed. This may allow for a brief return of summerlike warmth Wed and/or Thu. Much of the upcoming work week looks dry...but the next cold front may bring a few showers to the region sometime Thu and/or Fri but it does not look significant at this time.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

12z TAF Update...

Today...Moderate confidence.

Widespread showers and potentially a few embedded thunderstorms will overspread the region from west to east roughly through 15z. This will result in low-end MVFR to IFR conditions developing with even brief LIFR conditions in narrow bands of torrential rainfall. The bulk of the heaviest showers should be exiting the southeast New England coast by 21z/22z. Winds will be light but changeable in direction with weak boundaries in the vicinity. Some LLWS will be possible at times too.

Tonight...Moderate confidence.

Lingering MVFR-IFR with localized LIFR will gradually improve from west to east tonight behind a cold front. However...some areas that see low clouds scour out may temporarily fog in before the winds shift to the NW and bring drier air into the region. There also will likely be a brief band of scattered showers associated with the cold front overnight. Light and variable winds will shift to the NW at 5- 10 knots toward daybreak behind the cold front.

Monday...High confidence.

Any lingering lower cigs across the Cape/Islands should improve to VFR by mid morning. Otherwise...VFR conditions on Monday with sustained NW winds of 10 to 15 knots and gusts of 20 to 25 knots.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Monday Night through Thursday/...

Monday Night through Tuesday: VFR. Breezy.

Tuesday Night through Thursday: VFR.

MARINE

Forecaster Confidence Levels...

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

Today...High confidence.

A warm front will cross the waters today. The main concern for mariners will be heavy showers and potentially a few embedded thunderstorms with reduced visibility. There is a very low probability for a water spout or two mainly across the southern waters. Winds/seas though should remain below small craft advisory thresholds.

Tonight and Monday...High confidence.

Southwest winds will shift to the northwest overnight and continue from that direction on Monday behind the cold front. Winds may gust to between 20 and 25 knots at times on Monday...perhaps flirting with marginal small craft advisory criteria. Highest risk for small craft wind gusts may actually be Monday night in the cool air advection pattern. We will let the next shift take a look at these potential headlines.

Outlook /Monday Night through Thursday/...

Monday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of seas approaching 5 ft.

Tuesday through Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft.

Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft.

Thursday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Flood Watch until 6 PM EDT this evening for CTZ004. MA...Flood Watch until 6 PM EDT this evening for MAZ012-013-015>024. RI...Flood Watch until 6 PM EDT this evening for RIZ001>008. MARINE...None.