Area Forecast Discussion
Issued by NWS Boston, MA
Issued by NWS Boston, MA
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249 FXUS61 KBOX 252357 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 757 PM EDT Wed Sep 25 2024 .SYNOPSIS... An approaching frontal system will bring showers with moderate heavy downpours and few embedded thunderstorms mainly Thursday into Thursday evening. High pressure builds back in Friday through Monday and will bring dry and very pleasant weather. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... Surface high pressure over the Canadian Maritimes gradually shifts northeast tonight. Moisture increases this evening with precipitable water values increasing toward 1.3-1.4 inches. Weak synoptic ascent and warm air advection will support the risk for a few showers may increase across the distant interior this evening. Elsewhere, expect cloudy skies overnight. Temperatures drop into the mid to upper 50s. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY/... Thursday: Thursday, the strongest forcing arrives with the main upper trough and coupled jet max. This will provide increased ascent to support rainfall. Thursday morning through the afternoon, light rain spreads west to east, becoming moderate to heavy by late morning. HREF means show precipitable water values increasing into the 1.5-1.75 inch range with high end potential for values closer to 2 inches. A cold front is expected to move across SNE late in the Thursday afternoon to early evening time frame. In this time frame, we will see our highest chances for moderate to locally heavy rainfall with potential for embedded weak thunderstorms/rumbles thunder. High-res guidance shows of marginal instability with MUCAPE values < 500 J/kg. Most also reflect the passage of a line of higher reflectivity close to the frontal timing. Given weak instability and lapse rates as well as cooler temperatures, severe weather low probability. However, localized heavier rainfall is possible. Model soundings show sufficient warm cloud depths combined with plentiful moisture. The potential for brief heavier rain will accompany the passage of the main line. Rainfall rates will generally stay closer to .25"/hr with a low chance for localized .50"/hr or greater, likely associated with the heavier more convective showers. The HREF LPMM QPF continues to highlight potential for higher amounts with a swath of 1-2" totals oriented across north/central MA. Other areas, amounts are in the 0.5-1" range. Thursday Night: The main batch of rain/embedded storms moves offshore later Thursday night as the main cold front moves through. Conditions dry out behind it overnight. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Key Points... * Dry & very pleasant weather Fri into early next week * Next threat for some showers not until later Tue and/or Wed Details... Tranquil and generally dry weather is expected Friday into early next week...although a few showers may linger into early Friday morning towards the Cape/Islands but that would be short-lived. This in response to closed low shifting east of the Canadian Maritimes and a mid level ridge axis building to our northwest. There will be a surface high pressure system moving across Quebec coupled with a distant offshore low pressure system. So we will need to watch for periods of clouds as we have seen of late with the onshore flow. Overall...expect pleasant cool nights and mild days from Friday into early next week. Overnight low temps should bottom out mainly in the upper 40s to the middle 50s with the coolest readings in the typical low-lying locations. High temperatures will mainly be in the lower to middle 70s except 65 to 70 degrees near the immediate coast and parts of the high terrain. Mid level ridge axis will keep our weather dry into early next week. How quickly this mid level ridge axis shifts east of the region...but there will be some shortwave energy approaching from the west. This may bring us a period of showers...but not until later Tue and/or Wed. && .AVIATION /00Z THURSDAY THROUGH MONDAY/... Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight...Moderate confidence. Scattered showers around the terminals this evening. MVFR/IFR CIGS move in from the west overnight into early Thursday morning. Thursday...Moderate confidence Moderate to heavy rain with MVFR/IFR CIGS mainly. There could be fine line of embedded convection (thunder) that crosses the region from west to east in the late afternoon-early evening time frame. Less confidence on exact timing of this line and probability of thunder. Thursday Night...High Confidence CIGS will likely clear quickly to MVFR/VFR behind the line. A cold front will push through in the 01-04Z time frame shifting winds NW behind it. BOS TAF...Moderate Confidence in TAF. Tonight: Few showers possible this evening. CIGS range around 030-035 dropping toward MVFR early Thursday morning. Thursday: MVFR with a few showers in the morning. SE winds becoming SSE in the afternoon at 8-12 kts. Showers become more likely by the afternoon. A line of rain and embedded convection moves through late afternoon-early evening. Less certainty on exact timing, but will more likely be in the 21-03Z time frame. Front arrives 01-03Z shifting winds NW. BDL TAF...Moderate Confidence in TAF. Tonight: VFR/MVFR with showers possible tonight. Thursday: MVFR. SHRA in the morning. With increasing chances for rain and embedded thunder by the afternoon. Front moves through in the 00-03Z timeframe shifting winds NW. Outlook /Friday Night through Monday/... VFR. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Today through Thursday...High Confidence. Small Craft Advisories remain in effect for all outer open water zones along with the northern near coastal waters for seas of 5-7 feet. Winds remain east at 10-15 knots, turning SSE overnight into Thursday at 15-20 knots. Rip Current Statement remains in effect through this 8 PM this evening for eastern shorelines as large easterly swells continue to pose a high risk for rip currents. Outlook /Friday Night through Monday/... Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers. Saturday: Winds less than 25 kt. Slight chance of rain showers. Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Sunday through Monday: Winds less than 25 kt. Areas of seas approaching 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...High Risk for Rip Currents until 8 PM EDT this evening for MAZ007- 019-022-024. RI...None. MARINE...Small Craft Advisory until 8 AM EDT Friday for ANZ250-254. Small Craft Advisory until 11 PM EDT Thursday for ANZ251. Small Craft Advisory until 8 PM EDT Thursday for ANZ255-256. && $$ SYNOPSIS...Frank/Mensch NEAR TERM...Mensch SHORT TERM...Mensch LONG TERM...Frank AVIATION...Frank/Mensch MARINE...Frank/Mensch