Area Forecast Discussion
Issued by NWS Boston, MA

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918 FXUS61 KBOX 260248 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 948 PM EST Wed Nov 25 2020 .SYNOPSIS... Low pressure approaching from the west will bring showers late tonight through Thanksgiving with pockets of heavy rainfall and a few thunderstorms possible. The showers exit the region Thursday evening followed by gradually improving conditions. Generally dry and quiet weather expected Friday through the weekend with near to above seasonable temperatures. Impactful weather possible early next week with heavy rain and strong winds possible, however it is still too early for specific details. Temperatures remaining well above normal early next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... 945 PM Update... A bit of a lull in the showers as the first batch lifts north into southern Vermont and southern New Hampshire. Overall the forecast is still on track for showers to return into western areas between 06z and 08z and steadily filling in during Thanksgiving. Only chances made were to the near term temperatures and winds as cooler air has nudged into northeastern MA. Local observations show temperatures in the low 30s in southern NH on the MA state line. As of 02z BED temperature is 37F, BOS 40F, and OWD 50F. The NAMNEST seems to handle this situation well, we did the 10pm update with this in mind. Additionally, the NAMNEST has represented the wind conditions the best and we did update the nearterm winds as well. Previous discussion from 4 PM... Area of showers lifting NE from central NY and NE PA assocd with a shortwave and W-E axis of deeper moisture which moves into SNE early this evening. This will bring a few showers to mainly northern MA this evening but more widespread showers will hold off until late tonight as potent upper low and shortwave approaches from the west. Deepening moisture ahead of the system with increasing low level jet will result in more widespread showers moving into the interior well after midnight, likely reaching BOS-PVD corridor toward daybreak. Low temps will range from upper 30s across portions of interior northern MA to around 50 along the south coast. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... * Thanksgiving will feature periods of showers with pockets of heavy rainfall and perhaps a few embedded t-storms Thanksgiving... While the approaching upper low is de-amplifying as it moves into New Eng, it is a strong system with a modest low level jet transporting PWATs up to 1.25" into SNE. The anomalous moisture combined with increasing forcing for ascent will bring widespread showers to SNE, moving west to east. Locally heavy rainfall is possible as showers will become convective as temps aloft cool with marginal instability developing and mid level lapse rates increasing to 6.5-7 C/km. HREF indicates MUCAPES 100-300 J/kg so a few t-storms will be possible from late morning through the afternoon. The system is progressive which will limit duration of heavy rainfall so rainfall amounts will average 0.50" to 1" with locally heavier amounts in any convection. Not expecting a fine line of convection as forcing along the cold front does not appear to be strong enough so looking more like a band of heavy showers and embedded t-storms moving west to east across the region late morning through the afternoon. Temps will be tricky as guidance is indicating a cool air damming signature which may result in colder air jammed up across portions of interior northern MA with temps in the 40s while temps reach 60 degrees across RI and SE MA. Areas of fog expected to develop which may be locally dense in the interior. Thursday night... Any showers and embedded t-storms will be exiting eastern New Eng during the evening as the low level jet moves to the east followed by a wind shift to the west and drier air behind the cold front. However, areas of low clouds and patchy fog may linger as low level moisture persists. Lows will be mostly in the 40s. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Highlights... * Generally dry and quiet weather through the weekend. Though cannot rule out spotty showers on Saturday. * Better chances of rain across southern New England early next week. Strong winds and building seas are also possible. * Temperatures remain near to above normal throughout the extended. Friday through Sunday... Expect brief ridging on Friday across southern New England, while a positively tilted trough digs from the Upper Mississippi River Valley into the central Great Lakes. The ridge builds into Nova Scotia Friday night while the trough digs into the eastern Great Lakes by Saturday morning. The trough then lifts offshore by Saturday evening. Another ridge will build into the central/eastern Great Lakes on Sunday. Expect a weak high to build over southern New England on Friday and a subtle cold front slides through on Saturday. High pressure will build into southern New England from Ohio and the Mid Atlantic on Sunday. Generally expecting dry and quiet weather through this period. Do anticipate it to be a bit unsettled on Friday and Saturday as westerly flow will advect lake effect clouds into southern New England. Could see some spotty showers/drizzle across the interior on Saturday as a subtle cold front moves through. This is a result of 500 hPa temperatures in the -20 to -25 degree Celsius range in combination with diurnal heating and the front moving through. Not expecting much QPF from any showers that develop. High pressure builds overhead on Sunday bringing plenty of sunshine. Temperatures trending downward through the weekend as westerly to northwesterly flow at 925 hPa advects colder air in. Despite this downward trend in temperatures, readings will remain seasonable to above normal. Highs on Friday will be in the mid to upper 50s. By Sunday highs cool into the 40s across much of the region, though there may be some low 50 degree readings along the coastline. Monday and Tuesday... Complex and highly amplified weather pattern for early next week. It is still too early to pinpoint the specific details, but confidence in increasing in widespread rainfall and strong winds during this period. The synoptic setup is a positively tilted northern stream trough situated over the Mid to Upper Mississippi River Valley on late Sunday that extends into the Central Plains. A southern stream cutoff will be located over the Lower Mississippi River Valley. Deterministic guidance phases these two features as the northern stream digs into the Central Great Lakes/Ohio River Valley on Monday and cutoff gets absorbed in. This results in a full-latitude 500 hPa trough, but a large cutoff develops as a result of the interaction over the Great Lakes/Mid Atlantic into Tuesday. A surface low develops early on Monday across the Southeastern US and rapidly occludes as it lifts into the Mid Atlantic/Ohio River Valley. The occluded front/cold front lifts into southern New England for later on Monday or into Tuesday. Has not been much of a shift from the previous forecast where the majority of international guidance, ie the UKMET, ECMWF, KMA, ICON and CMC feature a 60 to 70+ kt 850 hPa low level jet. The GFS is not as intense with a 30-50 kt low level jet at the same level - with the differences likely due to the GFS being not as amplified as the international guidance. Suspect there will be some convective elements at play give how dynamic the system is. There will be plenty of moisture available as southerly flow will tap into tropical moisture. GEFS and EPS models showing high probabilities of 1"+ of PWATs with some guidance even getting close to the 1.5" range. This is AOA the 90th percentile for this time of year for both CHH and OKX per SPC Sounding Climatology. NAEFS guidance shows PWATS around 1-2 STD above normal for this timerange. Anticipate that heavy rainfall will be possible given this setup. Given the current setup by guidance do not anticipate Ptype issues as the bulk of the cold air remains underneath the upper cyclone, which is to the W/SW of New England - so precipitation should fall mainly as rain unless there are significant shifts in the track. This is still possible given we are still more than 120+ hours out from the event and models tend to struggle with phasing even in the day 3-5 window. So, confidence is high in precipitation and strong winds, but low in the specific locations at this point in time. Given the setup have kept chance to likely PoPs and bumped up winds and gusts above the NBM guidance given the signal being shown by models. Did not make any adjustments to temperatures as the NBM guidance looked reasonable at this point in time. Potential still exists for impactful system early next week that could bring significant precipitation, strong winds and building seas. Cannot rule out coastal flooding as well, but this will largely depend on the timing, orientation and strength of the winds coming in. Stay tuned for future updates. && .AVIATION /03Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight...High confidence in trends, but moderate in timing. Borderline VFR to MVFR ceilings to start. Will see conditions deteriorating to MVFR/IFR as the night progresses. Should see IFR develop across the interior MA and northern CT between 06-12Z. These ceilings will spread eastward into PVD/BOS for the AM push. Showers overspread the region from west to east especially after 06Z. Remaining VFR for Cape Cod and the Islands. Light W/SW winds shifting to the S/SE. Thursday...Moderate confidence. Widespread IFR/LIFR conditions with showers and fog. Showers may be locally heavy and a few embedded thunderstorms are possible late morning into the afternoon. Have held off from adding a thunder mention in the latest forecast, but will likely need to be added in future updates. Cape and Islands will see conditions deteriorate to MVFR/IFR and possibly some LIFR as showers move in. Thursday night...High confidence in trends, but moderate in timing. Showers exit eastern MA during the evening. MVFR to IFR conditions with stratus and patchy fog in the evening. Should see improvement to MVFR or VFR late. Light SW winds shift to the W/NW. KBOS TAF...Moderate Confidence in TAF. KBDL TAF...Moderate Confidence in TAF. Outlook /Friday through Monday/... Friday: Mainly VFR, with local MVFR possible. Friday Night: VFR. Saturday through Sunday: VFR. Breezy. && .MARINE... Tonight through Thursday night...High Confidence. While a bit of a lull in the winds/seas is expected tonight, another pulse of 20-25 kt gusts and building seas expected on Thanksgiving. Winds shift to W/NW Thu night and diminish but 5 ft seas will linger over southern waters. Showers and fog will reduce vsbys on Thanksgiving. A few embedded t-storms will also be possible especially during the afternoon and evening. Outlook /Friday through Monday/... Friday: Winds less than 25 kt. Seas up to 5 ft. Friday Night through Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Saturday Night: Winds less than 25 kt. Seas up to 5 ft. Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 10 PM EST this evening for ANZ250- 254>256. && $$ SYNOPSIS...KJC/BL NEAR TERM...KJC/BL/Gaucher SHORT TERM...KJC LONG TERM...BL AVIATION...BL MARINE...KJC/BL is the U.S. government's official web portal to all federal, state and local government web resources and services.